- Email: [email protected]
The ROSAT view of the cataclysmic variable sky
Adv. Space Res. Vol. 13,All No.rights 12, pp. (12)115—(12)124, Printed in Great Britain. reserved.
0273T~77~ $6.00 + 0.00 C~ynght@ 1993 COSPAR
1993
THE ROSAT VIEW OF THE CATACLYSMIC VARIABLE SKY K. Beuermann,*,** and H.-C. Thomas*** * Universitats-Sternwarte Gottingen, Geismarlandstr. 11, D-34(X) Gottingen, Germany ** Max Planck Institutflir Extraterrestrische Physilc D-8046 Garciung b. Mtinchen, Germany *** Max Planck InstitutflirAstrophysik Garching b. Mlinchen, Germany
ABSTRACT The ROSAT All-Sky Survey has for the first time permitted a synoptic view of the soft X-ray sky with high sensitivity. In this paper, we discuss the X-ray properties of known cataclysmic variables (CVs) as observed in the Survey and present a status report on programs to identify CVs among the newly discovered ROSAT X-ray sources. Of 170 CVs with known orbital period, 92 were detected in the Survey and 22 of these fall in the brightsource category with more than 0.5 PSPC cts/s. Among the new bright sources, so far 19 have been identified as CVs and 3 as CV candidates, about doubling the census. We present spectra and light curves of known and new systems and discuss the origin of the X-rayemission in the different subclasses of CVs.
THE CATACLYSMIC-VARIABLE SKY The ROSAT All-Sky X-ray Survey /1, 2/ presents a breakthrough for studies of cataclysmic variables (CVs) because it provided for the first time a synoptic view of the entire sky with high sensitivity and substantial energy resolution in the soft X-rayband. In addition, it covered any individual source for > 2 days, sufficient to provide information on variabilities. E.g., for most CVs which are not dominated by aperiodic variations, mean orbital or spin light curves can be constructed from the data. At the MPE, the ROSAT Survey data were searched for X-ray sources coinciding with positions of known CVs. We used the most recent version of Ritter’s catalogue of cataclysmic variables /3/ which contains 170 objects, supplemented by a list of 260 further CVs and CV candidates drawn from several catalogues. As of this writing, 92 objects from Ritter’s catalogue and 30 objects from the additional lists were positively detected as X-ray sources. Hence, for the CVs with known orbital period, the success rate is 54 %. Broken up according to subclass, there are significant differences: 23 of 30 magnetic CVs (AM Her, DQ Her, IP), 55 of 83 dwarf novae (DN, SU UMa, Z Cam), but only 13 of 54 old novae and novalike variables with optically thick accretion disks, or 77 %, 66 %, and 24 %, respectively, were detected. These percentages may still rise somewhat when reprocessing of the initial CV non-detections is complete (Bunk, private communication). Among the 22 ‘bright sources’ with PSPC countrates S > 0.5 PSPC cts/s, we fmd 13 magnetic systems and 9 dwarf novae but no novalikevariable. To facilitate comparison with the EINSTEIN observations /4, 5/, we compared PSPC and IPC countrates for the mutually observed objects. Discounting those observed in outburst or found in a low state, the countrates are well correlated with a mean ratio PSPCIIPC = 2.5. A scatter of up to a factor of 3 is superimposed and is likely caused by the different spectral sensitivities ofthe two instruments and the intrinsic variability of the sources. Fig. la shows the distribution of known CVs (filled circles) in galactic coordinates, with the diameter of the symbol scaled as log S. Fig. lb gives the latitude distributions for three brightness intervals. As expected, the distribution of the nearby bright sources is consistent with being isotropic, whereas the faint distant sources are concentrated towards the galactic plane with a median sin b = 0.3. For a scale height of the galactic zdistribution of CVs of h = 150 Pc /6/, the faint systems (S < 0.05 cts/s) are typically at a distance of d = h/sin b = 500 pc. Preliminary results from several programs to identify newly discovered ROSAT X-ray sources show that our census of X-ray emitting CVs in the wider solar neighbourhood is still utterly incomplete. Below (see Table 1), we comment on 22 new CVs and CV candidates which fall in the ‘bright-source’ category with S > 0.5 cts/s. Since even for the bright sources, identificationis not yet complete it is clear that in the end the number of (12)115
(12)116
K. BeuennannandH.-C. ‘l’homas
0
-1
-5
0 sin b
.5
l-l
-.5
0 sin b
.5
l-3
-.5
0
.5
1
sin b
Fig. 1. (a, top) Distribution of X-ray emitting cataclysmic variables in galactic coordinates with symbol size representing brightness. Solid circles: previously known CVs, open circles: CVS with S > 0.5 PSPC cts/s, newly identified from the ROSAT Soft X-ray All Sky Survey. (b, bottom) Distribution of known (solid lines) and newly identified (dashed line) CVs in galactic latitude for three intervals of the PSPC countrate.
known X-ray emitting CVs may be more than twice that of pre-ROSAT times. The new CVs and CV candidates of Table 1 have been included in Fig. la as open circles and in Fig. lb as a dashed line. They are included according to type (if already known) in Figs. 2 and 3. The individual subclasses of CVs differ appreciably in their average X-ray vs. visual flux ratios Fz/Fv. For optically selected sources, the X-ray detection probability will obviously drop with decreasing Fz/Fv. The low detection probability of novalike systems is a consequence of their low Fz/Fv values. In Fig. 2, we show the PSPC countrate S vs. visual magnitude V for CVs of known type. Whenever possible, we have attempted to use the magnitude at the time of observation (e.g. from AAVSO and RSNZ reports). Also shown are lines of constant Fz/Pv calculated with the following conversion fat!??. As a rul of thumb, theI conversion factor from countrate S to Fz for the 0.07 - 2.4 keV band is C = 1.0 x 10 ergs cm -3 PSPC count valid both for thermal spectra of ahnost any temperature and also for a 25 eV blackbody with NH = 5 x 1019 H-atoms cms2. Such value of N is a reasonable estimate for CVs within the local bubble of low interstellar density. The visual flux is defined Por a bandwidth of 1000 A as log Fv = -0.4 V - 5.44. The scatter in Fig. 2 is tremendous if all sources are considered but is considerably reduced for the individual subclasses. AM Her stars are the top runners with Fz/Fv = 3 300, dwarf novae and DQ Her stars (or intermediate polars) follow with Fx/Fv = 0.1 - 10, and the systems with optically thick disks including the systems caught in outburst have Fz/Fv = 0.003 - 0.3. For disk systems, this dependence of Fx/Fv on type is synonymous with Patterson and Raymond’s /7/ statement that FzlFv decreases with increasing accretion rate. The high ratio in AM Her stars is due to the absence of a bright accretion disk which radiates 50 % of the accretion luminority in the optical and UV by virtue of the virial theorem.
(12)117
Ca*aclysmic Variable Sky
Given that the ROSAT Survey becomes incomplete for S < 0.05 cts/s, we conclude that AM Her stars should typically be detectable as X-ray sources down to V > 20, dwarf novae and DO Her stars to V = 17, and the optically thick disk accretors only to V = 13. For an optically selected flux-limited sample, the X-ray detection probability should, therefore, be lowest for the novalikes. That some well-known AM Her stars (as e.g. ST LMi) were not seen in the Survey is due to the occurrence of low states in these systems with accretion effectively switched off. SOFT AND HARD X-RAY EMISSION The X-ray emission in CVs is almost entirely of accretion origin /4, 5, 6/. In magnetic systems, the matter follows the field lines and is heated in shocks which either stand above the white-dwarf surface or may be buried in the photosphere. The emitted spectrum then consists of a hard bremsstrahlung component from the optically thin plasma and a blackbody component from the heated photosphere. Photoabsorption in the pre-shock stream or in the interstellar medium may render the soft X-ray component unobservable. This is likely to occur in DO Her stars (or intermediate polars) which accrete from an accretion disk via an azimuthally extended funnel geometry but not in AM Her stars which are fed by a relatively narrow pencil stream. There is general agreement now that the strong soft X-ray emission in the high states of AM Her stars is due to non-radiative heating of the photosphere /e.g. 8/. One of the open questions is whether in DO Her stars this component is lacking or is present but internally absorbed. The origin of the X-ray emission from non-magnetic disk accretors is still a debated issue. The most likely site seems to be the boundary layer between the Ke~pleriandisk and the (slowly rotating) white dwarf /7, 9/. The boundary laye~will be optically thin and hot (10°K) if the accretion rate M is low and become optically thick and cool (10 K) if M is high. Within the framework of their simple model, P~btersonand Raymond /9/ 2~M , consistent with the prepredicted the soft X-ray flux to scale with M and white-dwarf mass M as M EINSTEIN result that strong soft X-ray emission during outburst was only observed in SS Cyg and U Gem which both contain a massive white dwarf. One may caution, again, that photoabsorption within the binary system may reduce the soft X-ray flux, particularly if the inclination is high and puffed-up disk matter intervenes. If the column density in the line of sight would exceed about 1024 H-atoms cm2 all X-rays in the ROSAT band would effectively be occulted.
,.
S AM Her
100
F ‘F —100
o
DQ Her o DN, SIJ. Z Cam
XI
,. ,.‘.
/
/
.,
,.
.‘
+ uncertain
~
•
a) .,.,l~
~ 1
•
•
,~/
,‘ •
S
.~
.~
U •
vu
$
.,
1,~ S
5 0 +0
/
+
.01
20
°°~
~
H
/
~‘~°°
0
•
on
•±/
_JI
~,.
__-._t_~,
,.
-...--
on’~
0
‘-~_)
o
~
Q
,. .... •
a
#
—,~,. ./
w v•w
‘4
0
*
~
0.01,
*...
S j9 0~a
~
/
9’
o-~7~
th0,b ,.‘~
/
/
5
,.
~
fr SS
/.,
~ S
.,
0
.1
,.
.,
v DD 10
•
V
~ systems in outburst •N,NL
~‘
• ,./~
4.
• *
10
Fig. 2. Mean PSPC countrate vs. visual magnitude for CVs of different subclasses as observed in the ROSAT X-ray Survey. Lines of constant F,/Fv (see text) are indicated by dot-dashed lines. Dashed lines indicate range observed for SS Cyg and VW Hyi in outburst. AM Her (110 cts/s) has been included for reference purposes although it was not in its high state during the Survey.
(12)118
K. Beuermann and H.-C.Thomas
What can ROSAT Survey data add to our understanding of X-ray generation in CVs? Some general aspects become apparent already if we consider the two-point spectral information provided by the ‘hardness ratio 1’ which is defined as HR1 = (hard - soft)/(hard + soft), where ‘soft’ and ‘hard’ refer to the counts in the soft and hard X-ray bands, E = 0.07-0.40 keV and E = 0.40-2.40 keV, respectively. Fig. 3 shows the PSPC countrate (hard + soft) vs. HR1 with individual sources characterized according to subtype. The left of the diagrams is dominated by AM Her stars with low interstellar absorption, including the new AM Her candidates down to a present limit of S = 0.5 cts/s. Most of the other sources are residing in the right of the diagram at HR1 > - 0.1. This dichotomy is easily understood if we consider one-component Raymond-Smith or blackbody spectra ~ keep in mind 2that we are biased towards the detection oflittle-absorbed sources with typically NH < 5 x law H-atoms cm . For standard Raymond-Smith thermal spectra of about any tempera~urekT> 0.fleV, HR1 is near zero for NH = 0 and increases to 0.85 for NH = ~ x io20 H-atoms cm . Ultrasoft spectra, either Raym~d-Smithwith kT 0.1dichotomy keV or softinblackbodies, havetherefore, HR1 nearthat -1 for NH = 0 spectra and reach -0.5afor NH 2. < The Fig. 3 implies, composite with moderate 5blackbody x 10 H-atoms cm are rare. All spectra in the left part feature strong blackbody contributions while those contribution in the right part are dominated by optically thin thermal radiation. While the objects on the left are mostly AM Her stars, this group contains also the newly identified EUV-bright intermediate polar RXJO751 + 14 (Table 1 and /10/) and SS Cyg in outburst. The discovery of RXJO751 +14 will spur the search for ultrasoft emission from the heated photospheres in other intermediate polars. While some such systems have absorbed spectra as AO Psc and V1223 Sgr with HR1 = + 0.71 and + 0.66, respectively, others do not as the newly established 529-s rotator DO Dra (/11/ and next section) and the DO Her star AE Aqr with HR1 = + 0.14 and -0.04, respectively. For the disk accretors, SS Cyg and U Gem in outburst are still the only systems for which soft X-ray emission from the boundary layer has been detected. During the ROSAT Survey, SS Cyg was caught in the decline from outburst with the soft component disappearing rapidly (/11/, and dashed lines in Figs.2 and 3). Besides SS Cyg, one U Gem star (RU Peg), five SU UMa stars (YZ Cnc, VW Hyi, AQ Er V436 Cen, V2051 Oph) and two Z Cam stars (Z Cam, HL CMa) were encountered in outburst while RX And was in standstill. None has displayed convincing evidence for soft X-ray emission from an optically thick boundary layer. VW Hyl received the best coverage and was observed throughout a normal outburst. Its behaviour differed drastically from that of SS Cyg in that the X-ray flux dropped in outburst without change of HR1 (/12/, and dashed lines in Figs. 2 and 3). The spectrum is that of a moderately hot optically thin boundary layer /13/ with no evidence for a soft blackbody component. Superficially, the reduction in flux is suggestive of partial occultation of the optically thin boundary layer (and any optically thick section), perhaps by an inflated inner disk. The fact, however, that all dwarf novae observed in outburst except SS Cyg and YZ Cnc have very low FXIFY suggests that the behaviour of VW Hyi is not atypical and any explanation should also be applicable to low-inclinationsystems as, e.g., RU Peg.
lao
•
•AMHer 0 DQ Her o DN, SU. Z Cam * systems in outburst
• N, ML SS*-.. 10
VDD + uncertain
•______-.
-.
0
•~+•.•~ 5
11
0
-_ —
~
.1
•0
*
H
0
* +°*
~
a
~ 8*0~~~0
•
*
.01
•
a I
—1
—.5
0 HR1
.5
1
Fig. 3. Mean PSPC countrate vs. hardness ratio HR1 (see text) for CVs ofdifferent subclasses.
Cataclysmic Variable Sky
100
.,,,~
..I..,~
.:..i.j
IX Vel xlO
0.01
iiiiriii~
r
I
blIlli
0.1
I
1
—
11111
0.1
I
I
I
I~IIIII
111111
QQ Vul
i
iii:IIiIIiiIi
BL Hyi quiet
AM Her xlO
,...I
I
BY Cam
ii~
10
•
.1111
~.
IIITIIIIIII
(12)119
.......I.__
BL Hyi flare
11111
1
0.1
I
I 111111
I
1
Energy (keV) Fig. 4. Sample PSPC spectra from the Survey data for one novalike variable and four AM Her stars. Also shown are the results of two-component spectral fits (see text).
The other type of disk accretors which are expected to develop optically thick boundary layers are the stationary high-M systems of which IX Vel, V3885 Sgr, and RR Pic display the softest spectra (HR1 = ~0.07, 0.04, and -0.31, respectively. These hardness ratios suggest that a weak soft component may be present. To illustrate the prospects of isolating such component, we consider the PSPC Survey spectra of IX Vel (0.4 cts/s, HR1 = -0.07) and of two AM Her stars, BY Cam (1.4 cts/s, HR1 = -0.09) and AM Her itself in its low state (0.13 cts/s, HR1 = + 0.06). Fig. 4 shows the PSPC spectra along with the results from two-component spectral fitting. For BY Cam and AM Her, the sum of a 20 keV bremsstrahlung spectrum and a 25 eV blackbody provides an adequate fit. For IX Vel, the more peaky spectrum at E > 0.4 keV requires a low-temperature standard Raymond-Smith 2 /14/, an additional soft component seems to be spectrum of kT = 1 keV. With NH = 2 x iol9 H-atoms cm indicated. The same holds for PR Plc. In both cases, however, higher quality spectra are needed to unequivocallyestablish the presence of soft components. Finally, we note that the spectra of BY Cam and AM Her in its low state are somewhat atypical of AM Her stars. In its high state, AM Her is dominated by soft X-ray emission and its spectrum is similar to that of 00 Vul (Fig. 4). In the low state, its spectrum has apparently lost the soft X-ray component, probably because the reduced temperature of the heated photosphere has shifted it out of the PSPC bandpass. BY Cam is atypical as a comparatively hard and yet bright source. Our tw 9-component fit2.suggests Hence, intrinsically that it contains is does a substantial not differ blackbody component butHer is absorbed from the other bright AM stars. by about 5 x 10 0 H-atoms cm LIGHT CURVES During the Survey, ROSAT has viewed a source in the field of view every % minutes for about 25 seconds. Despite this undersampling, periodic phenomena can be studied provided the period is not commensurate with the satellite period. In fact, in some cases the rather complete phase coverage obtained in the Survey is superior to that achieved in subsequent pointed observations. Of course, the Survey data are not well suited for the study oftransient phenomena on time scales of minutes like, e.g. eclipses or flaring activity. In this section, we present selected results on periodic phenomena in different subclasses of CVs: (1) orbital light curves of non-magnetic CVs (2) spin light curves of intermediate polars, (3) orbital light curves of known AM Her stars, and (4) light curves ofnewly identified CVs.
(12)120
K. Beuermann and H.-C. Thomas
SU UMa 10
_‘
I
I
I
I
I
I
I
+
00
-
1.0
-
+
I
I
I
I
I
I
0.07—0.4 keV
0.5 U
I
GP Corn
~
‘
I_
_I
I
I
++~L ‘+~k+ ++ T ‘
I
I
+ ++
I
I
I
I
1H1752+08 I
I
I
I
0.07—0.4 keV
j~Tk+
+ ++
I_
_1
I
I
~-5
I
I
I
I
~
+q
I
I
-
__
I
Arb. Orbital Phase
Arb. Orbital Phase
I I I I I I 0.07—2.4 keY
..
T T r ~T iIHIIIHlHIiIIHiiIHIiHIIiIHiiiH~~HiIUtIiUi -
I
I
iIiiiiII
++
+q
414
~
I
I
4I$ -
IIIIII
Eclipse Phase
Fig. 5. Orbital light curves of SU UMa, the double degenerate GP Corn, and the 1.88-hour binary H1752 + 08 along with the hardness ratio HR1.
In disk accretors, there is no a-priori reason why the rapidly rotating inner disk and the boundary layer between disk and non-magnetic white dwarf should display any azimuthal asymmetry which could lead to orbital modulation of the emitted X-rays. It is well-known, however, that the distribution of cold matter in the outer accretion disk may well display such azimuthal dependence which can cause phase-dependent photoabsorption in high-inclination systems. The periodic dips in low-mass X-ray binaries and the similar dips in the soft X-ray light curve of U Gem /15/ (inclination i = 70°)indicate that an inflated rim or bulge forms where the accretion stream from the inner Lagrangian point hits the outer disk. We have produced orbital light curves for a number of CVs which received sufficient coverage in the Survey and have found several good examples for modulation produced by phase-dependent photoabsorption. Fig. 5 shows the phase-folded soft X-ray light curves of the prototype SU UMa star and of the double degenerate GP Com. In both cases, the modulation is more pronounced below than above 0.4 keV as is indicated by the systematic variation 8f the 2. hardness Two other ratio. systems The hydrogen column densityorbital required to produce modulation is about cm~variable IX Vel. The which display a similar modulation are the the observed dwarf nova El UMa and the io2 novalike inclination of IX Vel is near 60°/16/, those of SU UMa, GP Corn, and El UMa are unknown but cannot exceed some 750 because the systems are not eclipsing. The presence of phase-dependent internal absorption in several systems complicates the interpretation of their X-ray spectra and, in particular, the search for soft X-ray emission from their boundary layers. Information on internal photoabsorption can also be obtained by confronting NH as obtained from spectral fitting (or simply from HR1) with NH as estimated from interstellar reddening by the depth of the 2200 A feature in IUE spectra /17/. With NH = 6.6 x lO~EBV, we fmd that Z Cha (HR1 = + 1.00), 0623 + 71 (+ 0.98), MV Lyr (+ 0.90), and possibly TT An (+ 0.76), TV Col (+ 0.64), and RW Sex (+ 0.59) are harder than expected from the values of or uper limits on EBV /14, 17/. Internal absorption by matter in the outer accretion disk would not be reconcilable with the low inclination of TT An and RW Sex but would be quite natural for the eclipsing system Z Cha. The phase dependent study of eclipsing disk accretors does not only allow to map the cold absorbing matter in the system. More importantly, the shape ofthe eclipse (or non-eclipse) is the only means to directly measure the location and extent of the X-ray emitting boundary-layer gas. Unfortunately, the known eclipsing systems (like Z Cha) are too faint for this purpose with the one exception of 1H1752 + 08, studied recently by Silber /18/. This short-period binary (P = 1.88 h) and possible SW Sex star is a hard X-ray source (HR1 = + 0.77). The X-ray orbital light curve from the Survey (Fig. 5) is 100 % modulated and probably shaped by absorption or partial occultation. Unfortunately, none of the individual satellite passes falls into the narrow eclipse and a dedicated pointed observation is needed.
Cataclysmic VariableSky I
I
I
I
I
I
I
I
1.0
-
0.5
~++4~4~~4-+
I
DO Dra
I
I
I I
I
0.07—2.4 keY
++ I I I
I I I I
V1223 Sgr .~
_I
I
I
I
I
I
I
I
I
EK UMa
-
I
I
I
I
I
I
I
I
0.07—0.4 keV
-1
++~+~f+++. I I I I I I I I 1
+ ±+
1-+
I_
(12)121
5
.4
-
.4
20
0.07—2.4 keY
BL Hyi
0.07—0.4 keV
fs
.~1o-
±-
U
+ -
U
O_IIIIIIIIIIIIIIIIIII 1.0
-
AO Psc
0.07—2.4 keY
:1 0.5
+1 + T
-T ++
:± 0.0 I 0.0
0
I
~II
111111
0.5
+
IIiIr~i1°lI
~l~Ir~I~0II BY Cam
-
0.07—0.4 keV
-
2
-
-
41
++ 111111
1.0 1.5 Spin Phase
I
2.0
0 1~I 0.0
¶1
4 l0I10I
0.5
41
4
~I
1.0 Orbital Phase
1.5
2.0
Fig.6. (a) Phase-binned spin light curves of the intermediate polar DO Dra (= YY Dra), V1223 Sgr, and AO Psc. (b) Soft X-ray orbital light curves of three AM Herculis binaries. In BL Hyi and BY Cam, all high data points are produced by soft X-ray flaring. Phase-folding of the Survey data is possible also for the spin and orbital periods of intermediate polars although care has to be taken to avoid interference of the two modulations. In Fig. 6a, we show the spin light curves of V1223 Sgr, AO Psc, and DO Dra. For all three systems, orbital modulation was not obiously present and periodogram analysis readily yielded the spin period. For the newly recognized intermediate polar DO Dra (= YY Dna) /11/, the spin period is 529.2 s. No signal was found at the optical period of 550 s or near P/2. In intermediate polars or DO Her stars, the spin modulation is thought to arise from the varying aspect of the emission region on the white dwarf and/or from phase-dependent photoabsorption in the accretion funneL This is supported by (subtle) phase-dependent HR1 variations in EX Hya and in the three sources of Fig. 6. AM Hercules binaries are among the brightest objects in the soft X-ray sky and the ROSAT Survey allowed, therefore, to construct light curves for all known systems which did not happen to be in a low state of switchedoff accretion. Of the 17 known AM Her stars /3/, 9 had more than 0.5 cts/s at the time they were covered during the Survey (V834 Cen, 00 Vul, EF Er VV Pup, BL Hyi, AN UMa, BY Cam, EK UMa, UZ For), 6 systems were in low states (AM Her, MR Ser, WW Hor, ST LMi, DP Leo, EX00329-26) and 2 sources not previously detected in X-rays (V1500 Cyg, Grus Vi) were not seen in the Survey either. A good example of the light curves that may be obtained from Survey data under favourable conditions is that of EK UMa (Fig. 6 b) of which only a short EINSTEIN exposure existed so far /19/. Fig. 6 reveals a bright phase which lasts for half of the orbital period and is interrupted by a pronounced absorption dip of 0.07 in phase. The dip is likely to be produced by the accretion stream crossing the line of sight, similar to the well-studied dip in EF En /20, 21/. For most of the bright AM Her stars, the light curves present themselves in a recognizable shape relating to the accretion geometry these systems had assumed at the time of the Survey. Two systems, however, BL Hyi and BY Cam, display a rather chaotic behaviour (Fig. 6b). BL Hyi previously produced X-rays at its main pole on the far hemisphere, visible in the phase interval 0.0-0.4 /22/. In the Survey, BL Hyi had brightened considerably and showed extremely soft X-rayflaring at odd phases, apparently due to intermittent accretion at its near pole. This chaotic behaviour was later confirmed by a pointed observation: stable accretion to its ‘main’ pole for a few satellite passes alternated with chaotic aperiodic very soft X-ray flaring /23/. BY Cam was shown by Ishida et al. /24/to possess a state of stable accretion at one pole which changed over to a flaring state with accretion at both poles. During the Survey, it was observed in what seems to be the flaring state. In Fig. 6 b, all high data points for BL Hyi and BY Cam are due to soft X-ray flaring. For the case of BL Hyi, the mean PSPC spectra of the small flares (low countrate) and huge flares (high countrate) are separately depicted in Fig. 4. For the case of BY Cam, Silber et al. /25/ suggested the chaotic behaviour to be related to a slight deviation from synchronism.
K.Beuermann and
(12)122
RX 11149+28 10
=
— 11111111
H.-C. Thomas
RX 11938—46
RE 1149+284 I I I I
I_
1111
RE 1938—481 141 I I I
=
411111111
I
I:
20-
-
.4,
~io-
~5. 0 U
$
0
~
I
4 III~~*I*$~
UT(days)
—
I
4
•
0
0 I11
~
2448220.
_I
I
I
I
I
I
I
I
~
I
I
=
1
140 mm)
RX 10203+29 I
II
I
I
I
I
I
I
I
I
1~
I
lI~
Phase (P
RX 11002—19 2
1
•
I
UT(days)
II~I
—
I
I
II
I_
2
_IIIIIII
-
1
-
~i~I
2448210.
0~L~ ~
I
I
Phase (P
I
1I
=
I
I
I
I
I
~
~I~U
I
1142
227 mm)
Fig. 7. PSPC light curves of 4 bright sources from the ROSAT soft X-ray All sky Survey identified as new or problable new AM Herculis binaries.
NEW CATACLYSMIC VARIABLES Besides providing a synoptic view of the known CVs, the ROSAT Survey has led to the discovery of numerous new X-ray sources, the identification of which represents a tremendous task. To some extent, fmding new CVs among these sources is possible by using the X-ray data alone. Fig. 7 depicts, in the two left panels, the Survey light curves of two sources with mean PSPC countrates of 2.2 and 0.6 cts/s. These light curves in which each data point represents one satellite pass clearly show that the sources are periodic. Between two successive satellite orbits (96 mm), the phase progresses (or recedes) by 0.064 and 0.085 for the two systems. The implied (orbital) periods are 102.5 or 90 mm and 105 or 885 mm, respectively, of which 90 mm is the correct orbital period for the first object /26/. The picture is less clear if the phase shift is larger as, e.g., 0.686 for RXJ1938-46 (Fig. 7, upper right). Period search, however, immediately reveals that P = 140 mm (or shorter than 66 mm): The light curve is clearly that of a self eclipsing AM Her star like VV Pup. Fig. 7, lower right, shows a case where period search goes astray and the correct period could only be found by optical astronomy /23/. All four systems ofFig. 7 are new AM Her stars or AM Her candidates. RXJO2O3 + 29 belongs to the class of flaring sources like BL Hyi or BY Cam. The ultimate identification of a new X-ray source will normally require optical spectroscopy, photometry, and/or polarimetry. At the MPE, a number of long-range collaborative identification programs is being coordinated. For CV’s, the most important are the galactic-plane Survey (GPS) /27/ and a program to identify bright high galactic-latitude sources (HGLS) of which the latter is so far restricted to bright soft sources (S > 05 PSPC cts/s, HR1 <0). The objects of Fig. 7 are from this program. To date, a total of 22 new bright CVs and CV candidates has been identified, mostly by optical spectroscopy (Table 1). For, some of these, detailed follow-up observations have already been performed. It is noteworthy that 15 of the 17 HGLS sources are AM Her stars or AM Her candidates while the 3 new intermediate polars are all from the GPS. At this early stage, orbital periods are already known for 12 of the new confirmed and candidate AM Her stars and, surprisingly, none falls in the famous 114-mm spike which contains 6 of 17 previously known AM Her stars /3/. On the other hand, the periods of two new AM Her stars, 1255 mm and 140 mm, place them in the period gap 012-3 hours. At least the latter of these must be born inside the gap. Our search for new AM Her stars is so far restricted to sources with countrate S > 05 cts/s (Fig. 3). If extended down to 0.2 cts/s, we expect to find about 20 further AM Her stars which would bring up the census to beyond 50, a sound basis for statistical and evolutionary studies. To be sure, synchronous rotation of the magnetic white dwarf, the ultimate signature of an AM Her star, still remains to be proven for most of the new systems, requiring a substantial amount of telescope time.
Cataclysmic VariableSky
(12)123
The ROSAT Survey has also led to the discovery of a number of peculiar sources which do not fit into the known CV-subtype pattern. We mentioned already RXJO751 + 14/10, 26/ which represents a further connecting link between the intermediate polars and the AM Her stars (polars). Another peculiar source is the long-period ecipsing binary RXJ0515 +01 (Table 1) which is very soft and has a CV-like spectrum with strong He114686 emission, yet does not seem to be an AM Her star /28/. Finally, we mention a supersoft X-ray source (not listed in Table 1) which resides in a binary with 16 hours orbital period and is of as yet indeterniinate nature. It bears similarities to the cataclysmic variable V Sge but also to the more luminous supersoft X-ray sources in the Magellanic Clouds./29/ which possibly contain accreating and hydrogen-burning white dwarfs /30/. These examples show that our knowledge of certain aspects of the nature and evolution of CVs is still fragmentary. CONCLUSIONS The ROSAT X-ray Survey has allowed a synoptic view of the CV sky. Full exploitation of the available material will allow an improved understanding of the class as a whole. The Survey has also led to the discovery of 22 new bright CVs with S > 0.5 PSPC cts/s, which doubles the number of bright CVs from 22 to 44. These sources are important as individuals with their own idiosyncrasies but the increased number also provides a much improved basis for statistical and evolutionary studies. We anticipate that the information obtained from the ROSAT Xray Survey and from follow-up observations of new sources will significantly change our understanding of cataclysmic variables and related objects.
TABLE 1
New CVs and CV candidates with PSPC countrate
>
0.5 cts/s
*
RXJ
A)
B)
cts/s
HR1
WFC (y yes)
Mag
Type
y
19 17 19
AM: AM: AM
15
CV:
y
17
AM:
**
Period (mm)
Soft bright high-galactic-latitude sources 0132-65
0.6
-0.90
0203 + 29 0453 -42 0515+01
0.5
-
0.67
1.7
-
0.89
0.5
-0.82
0531 - 46 1002 - 19 1007-20
1.2 0.6
-0.70 0.84
17
2.1
1015+09
13 2.2
y y
18 17 17 17
0.57
y
16 18
AM AM
y
-
0.94 0.89 0.91
14 17 15
AM AM: AM
140 99 125
-
0.74
18
AM
199:
-
0.97 033
14 13
DQ ?
15
DN
14 14 14
DO DO AM?
-
32
2.1
0.91 -0.74 - 0.95 - 0.85 - 0.79
1844 - 74
1.4
-
1938 -46
8.9
-
1957 - 57 2107 -05 2316 - 05
1.2 1.3 1.5
1149 + 28 1307 + 53
2.5
1313
-
-
-
y
227: 95 480
sp ph ph
AM:
105/89
X
AM AM: AM AM
208:
sp
90 + 80 + 252:
X
90
sp
+
X, sp sp ph X, sp
Galactic-plane sources 0028 + 59 0558 + 53 0640- 24 0751+ 14
1712 + 24 1802+18
0.5
1.6
1.1
0.01
5.3 0.6 2.4
-0.69 0.93 - 0.85
y
339k
AM, AM Her star. DQ, DQ Her star, intermediate polar. DN, dwarf nova ph, photometric. sp, spectroscopic. X, from ROSAT data. + from WFC identifications /10, 26/ JASS i3:12-J
(12)124
K. Beuemm andH.-C. ‘homas
ACKNOWLEDGEMENTS We thank the ROSAT team for making the X-ray All-Sky Survey a success and permitting studies like this one. We aIs0 thank all the individuals at the MPE who helped us with the data analysis. We thank the members of the CV working group at the MPE, in particular W. Bunk, for contributions and discussions and V. Burwitz, Gettingen, for analysing part of the data and producing the figures.
REFERENCES 1. J. Triimper, The ROSAT mission&v.
Space Res. # 4,241(1983).
2. W. Voges, The ROSAT All-Sky Survey, Proc. ZSYSymp. ‘Space Science with particular emphasis on High Energy Astrophysics’, Munich (1992). 3.
H. Ritter,Astron. Astrophys. Suppl. Ser. 851179 (1990) and private communication.
4.
F.A. Cordova and K.O. Mason, MNRAS 206,879 (1984).
5.
M. Eracleous, J. Halpern and J. Patterson, Ap. J. 382,290 (1991).
6. J. Patterson, Ap.J.SuppZ. 54,443 (1984). 7.
J. Patterson and J.C. Raymond, ApJ. 292,535 (1985).
8.
K. Beuermann, X-ray Properties of AM Herculis binaries, A&Space
Res. 8, # 2,283 (1988).
9. J. Patterson and J.C. Raymond, Ap.J. 292,550 (1985). 10. K.O. Mason, M.G. Watson, T.J. Ponman, P.A. Charles, S.R. Duck, B.J.M. Has&, D.H.P. Jones and J.P.D. Mittaz, MNRAS 258,749 (1992).
S.B. Howell, M. Ishida,
11. J. Patterson, D.A. Schwartz, J.-P. Pye, W.P. Blair, G.A. Williams and J.-P. CaiIIault, ApJ. 392,233 (1992). 12. T. BelIoni, F. Verbunt and W. Bunk, Soft X-ray Emission from Boundary Layers in Dwarf Novae, Ringberg Meeting March 1992, MPE Report (1992), and private communication. 13. T. BelIoni, F. Verbunt, K. Beuermann, W. Bunk, C. Izzo, W. KIey, W. Pietsch, H. Ritter, H.C. Thomas, and W. Voges, AstronAstrophys. 246, L44 (1991). 14. Ch. Mauche, J.C. Raymond and F.A. Cordova, ApJ
335,829 (1988).
15. K.O. Mason, F.A. Cordova, M.G. Watson and A.R. King, MNRAS 232,779 (1988). 16. K. Beuermaun and H.-C. Thomas, Astmulstrophys.
230,326 (1990).
17. F. Verbunt, AstromAstrophys.Suppl.Ser. 71,339 (1987). 18. A. SiIber, Thesis, MIT (1992). 19. S.L. Morris, G.D. Schmidt, J. Liebert, J. Stocke, I. Gioia and T. Maccacaro, Ap..!. 314,641(1987). 2Q. K. Beuermann, J. Patterson and L. Stella,Ap.J. 316,360 (1987).
21. M.G. Watson, A.R. King, M.H. Jones and C. Match, MNZUS 237,299 (1989). 22. K. Beuermann and A.D. Schwope, Astrordstrophys.
223,179 (1989).
23. A.D. Schwope, private communication. 24. M. Ishida, A. Silber, H.V. Bradt, R.A. Remillard, K. Makishiia,
and T. Ohashi, Ap.J. 367,270 (1991).
25. A. SiIber, H.V. Bradt, M. Ishida, T. Ohashi and R.A. RemiIIard, Ap.J. 389,704 (1992).
26. M.G. Watson, this issue. 27. C. Match, T. BeIIoni, D. Buckley, M. Gottwald, G. Hasinger, M.W. PakuII, W. Pietsch,, K. Reinsch, R.A. Remillard, J.H.M.M. Schmitt, J.Triimper and H.-U. Zimmermann, AsfromWrophys. 246, L 24 WV, ad Ringberg Meeting March 1992, MPE Report (1992). 28. A. Shafter, private communication. 29. J. Greiner, G. Hasinger and P. Kahabka, Ashonsistrophys. 246, L17 (1991). 30. E.J.P. van den Heuvel, D. Bhattacharya, K. Nomoto and S.A. Rappaport,Asrro~l.As@op~ly~. x2,97
(1992).
0273T~77~ $6.00 + 0.00 C~ynght@ 1993 COSPAR
1993
THE ROSAT VIEW OF THE CATACLYSMIC VARIABLE SKY K. Beuermann,*,** and H.-C. Thomas*** * Universitats-Sternwarte Gottingen, Geismarlandstr. 11, D-34(X) Gottingen, Germany ** Max Planck Institutflir Extraterrestrische Physilc D-8046 Garciung b. Mtinchen, Germany *** Max Planck InstitutflirAstrophysik Garching b. Mlinchen, Germany
ABSTRACT The ROSAT All-Sky Survey has for the first time permitted a synoptic view of the soft X-ray sky with high sensitivity. In this paper, we discuss the X-ray properties of known cataclysmic variables (CVs) as observed in the Survey and present a status report on programs to identify CVs among the newly discovered ROSAT X-ray sources. Of 170 CVs with known orbital period, 92 were detected in the Survey and 22 of these fall in the brightsource category with more than 0.5 PSPC cts/s. Among the new bright sources, so far 19 have been identified as CVs and 3 as CV candidates, about doubling the census. We present spectra and light curves of known and new systems and discuss the origin of the X-rayemission in the different subclasses of CVs.
THE CATACLYSMIC-VARIABLE SKY The ROSAT All-Sky X-ray Survey /1, 2/ presents a breakthrough for studies of cataclysmic variables (CVs) because it provided for the first time a synoptic view of the entire sky with high sensitivity and substantial energy resolution in the soft X-rayband. In addition, it covered any individual source for > 2 days, sufficient to provide information on variabilities. E.g., for most CVs which are not dominated by aperiodic variations, mean orbital or spin light curves can be constructed from the data. At the MPE, the ROSAT Survey data were searched for X-ray sources coinciding with positions of known CVs. We used the most recent version of Ritter’s catalogue of cataclysmic variables /3/ which contains 170 objects, supplemented by a list of 260 further CVs and CV candidates drawn from several catalogues. As of this writing, 92 objects from Ritter’s catalogue and 30 objects from the additional lists were positively detected as X-ray sources. Hence, for the CVs with known orbital period, the success rate is 54 %. Broken up according to subclass, there are significant differences: 23 of 30 magnetic CVs (AM Her, DQ Her, IP), 55 of 83 dwarf novae (DN, SU UMa, Z Cam), but only 13 of 54 old novae and novalike variables with optically thick accretion disks, or 77 %, 66 %, and 24 %, respectively, were detected. These percentages may still rise somewhat when reprocessing of the initial CV non-detections is complete (Bunk, private communication). Among the 22 ‘bright sources’ with PSPC countrates S > 0.5 PSPC cts/s, we fmd 13 magnetic systems and 9 dwarf novae but no novalikevariable. To facilitate comparison with the EINSTEIN observations /4, 5/, we compared PSPC and IPC countrates for the mutually observed objects. Discounting those observed in outburst or found in a low state, the countrates are well correlated with a mean ratio PSPCIIPC = 2.5. A scatter of up to a factor of 3 is superimposed and is likely caused by the different spectral sensitivities ofthe two instruments and the intrinsic variability of the sources. Fig. la shows the distribution of known CVs (filled circles) in galactic coordinates, with the diameter of the symbol scaled as log S. Fig. lb gives the latitude distributions for three brightness intervals. As expected, the distribution of the nearby bright sources is consistent with being isotropic, whereas the faint distant sources are concentrated towards the galactic plane with a median sin b = 0.3. For a scale height of the galactic zdistribution of CVs of h = 150 Pc /6/, the faint systems (S < 0.05 cts/s) are typically at a distance of d = h/sin b = 500 pc. Preliminary results from several programs to identify newly discovered ROSAT X-ray sources show that our census of X-ray emitting CVs in the wider solar neighbourhood is still utterly incomplete. Below (see Table 1), we comment on 22 new CVs and CV candidates which fall in the ‘bright-source’ category with S > 0.5 cts/s. Since even for the bright sources, identificationis not yet complete it is clear that in the end the number of (12)115
(12)116
K. BeuennannandH.-C. ‘l’homas
0
-1
-5
0 sin b
.5
l-l
-.5
0 sin b
.5
l-3
-.5
0
.5
1
sin b
Fig. 1. (a, top) Distribution of X-ray emitting cataclysmic variables in galactic coordinates with symbol size representing brightness. Solid circles: previously known CVs, open circles: CVS with S > 0.5 PSPC cts/s, newly identified from the ROSAT Soft X-ray All Sky Survey. (b, bottom) Distribution of known (solid lines) and newly identified (dashed line) CVs in galactic latitude for three intervals of the PSPC countrate.
known X-ray emitting CVs may be more than twice that of pre-ROSAT times. The new CVs and CV candidates of Table 1 have been included in Fig. la as open circles and in Fig. lb as a dashed line. They are included according to type (if already known) in Figs. 2 and 3. The individual subclasses of CVs differ appreciably in their average X-ray vs. visual flux ratios Fz/Fv. For optically selected sources, the X-ray detection probability will obviously drop with decreasing Fz/Fv. The low detection probability of novalike systems is a consequence of their low Fz/Fv values. In Fig. 2, we show the PSPC countrate S vs. visual magnitude V for CVs of known type. Whenever possible, we have attempted to use the magnitude at the time of observation (e.g. from AAVSO and RSNZ reports). Also shown are lines of constant Fz/Pv calculated with the following conversion fat!??. As a rul of thumb, theI conversion factor from countrate S to Fz for the 0.07 - 2.4 keV band is C = 1.0 x 10 ergs cm -3 PSPC count valid both for thermal spectra of ahnost any temperature and also for a 25 eV blackbody with NH = 5 x 1019 H-atoms cms2. Such value of N is a reasonable estimate for CVs within the local bubble of low interstellar density. The visual flux is defined Por a bandwidth of 1000 A as log Fv = -0.4 V - 5.44. The scatter in Fig. 2 is tremendous if all sources are considered but is considerably reduced for the individual subclasses. AM Her stars are the top runners with Fz/Fv = 3 300, dwarf novae and DQ Her stars (or intermediate polars) follow with Fx/Fv = 0.1 - 10, and the systems with optically thick disks including the systems caught in outburst have Fz/Fv = 0.003 - 0.3. For disk systems, this dependence of Fx/Fv on type is synonymous with Patterson and Raymond’s /7/ statement that FzlFv decreases with increasing accretion rate. The high ratio in AM Her stars is due to the absence of a bright accretion disk which radiates 50 % of the accretion luminority in the optical and UV by virtue of the virial theorem.
(12)117
Ca*aclysmic Variable Sky
Given that the ROSAT Survey becomes incomplete for S < 0.05 cts/s, we conclude that AM Her stars should typically be detectable as X-ray sources down to V > 20, dwarf novae and DO Her stars to V = 17, and the optically thick disk accretors only to V = 13. For an optically selected flux-limited sample, the X-ray detection probability should, therefore, be lowest for the novalikes. That some well-known AM Her stars (as e.g. ST LMi) were not seen in the Survey is due to the occurrence of low states in these systems with accretion effectively switched off. SOFT AND HARD X-RAY EMISSION The X-ray emission in CVs is almost entirely of accretion origin /4, 5, 6/. In magnetic systems, the matter follows the field lines and is heated in shocks which either stand above the white-dwarf surface or may be buried in the photosphere. The emitted spectrum then consists of a hard bremsstrahlung component from the optically thin plasma and a blackbody component from the heated photosphere. Photoabsorption in the pre-shock stream or in the interstellar medium may render the soft X-ray component unobservable. This is likely to occur in DO Her stars (or intermediate polars) which accrete from an accretion disk via an azimuthally extended funnel geometry but not in AM Her stars which are fed by a relatively narrow pencil stream. There is general agreement now that the strong soft X-ray emission in the high states of AM Her stars is due to non-radiative heating of the photosphere /e.g. 8/. One of the open questions is whether in DO Her stars this component is lacking or is present but internally absorbed. The origin of the X-ray emission from non-magnetic disk accretors is still a debated issue. The most likely site seems to be the boundary layer between the Ke~pleriandisk and the (slowly rotating) white dwarf /7, 9/. The boundary laye~will be optically thin and hot (10°K) if the accretion rate M is low and become optically thick and cool (10 K) if M is high. Within the framework of their simple model, P~btersonand Raymond /9/ 2~M , consistent with the prepredicted the soft X-ray flux to scale with M and white-dwarf mass M as M EINSTEIN result that strong soft X-ray emission during outburst was only observed in SS Cyg and U Gem which both contain a massive white dwarf. One may caution, again, that photoabsorption within the binary system may reduce the soft X-ray flux, particularly if the inclination is high and puffed-up disk matter intervenes. If the column density in the line of sight would exceed about 1024 H-atoms cm2 all X-rays in the ROSAT band would effectively be occulted.
,.
S AM Her
100
F ‘F —100
o
DQ Her o DN, SIJ. Z Cam
XI
,. ,.‘.
/
/
.,
,.
.‘
+ uncertain
~
•
a) .,.,l~
~ 1
•
•
,~/
,‘ •
S
.~
.~
U •
vu
$
.,
1,~ S
5 0 +0
/
+
.01
20
°°~
~
H
/
~‘~°°
0
•
on
•±/
_JI
~,.
__-._t_~,
,.
-...--
on’~
0
‘-~_)
o
~
Q
,. .... •
a
#
—,~,. ./
w v•w
‘4
0
*
~
0.01,
*...
S j9 0~a
~
/
9’
o-~7~
th0,b ,.‘~
/
/
5
,.
~
fr SS
/.,
~ S
.,
0
.1
,.
.,
v DD 10
•
V
~ systems in outburst •N,NL
~‘
• ,./~
4.
• *
10
Fig. 2. Mean PSPC countrate vs. visual magnitude for CVs of different subclasses as observed in the ROSAT X-ray Survey. Lines of constant F,/Fv (see text) are indicated by dot-dashed lines. Dashed lines indicate range observed for SS Cyg and VW Hyi in outburst. AM Her (110 cts/s) has been included for reference purposes although it was not in its high state during the Survey.
(12)118
K. Beuermann and H.-C.Thomas
What can ROSAT Survey data add to our understanding of X-ray generation in CVs? Some general aspects become apparent already if we consider the two-point spectral information provided by the ‘hardness ratio 1’ which is defined as HR1 = (hard - soft)/(hard + soft), where ‘soft’ and ‘hard’ refer to the counts in the soft and hard X-ray bands, E = 0.07-0.40 keV and E = 0.40-2.40 keV, respectively. Fig. 3 shows the PSPC countrate (hard + soft) vs. HR1 with individual sources characterized according to subtype. The left of the diagrams is dominated by AM Her stars with low interstellar absorption, including the new AM Her candidates down to a present limit of S = 0.5 cts/s. Most of the other sources are residing in the right of the diagram at HR1 > - 0.1. This dichotomy is easily understood if we consider one-component Raymond-Smith or blackbody spectra ~ keep in mind 2that we are biased towards the detection oflittle-absorbed sources with typically NH < 5 x law H-atoms cm . For standard Raymond-Smith thermal spectra of about any tempera~urekT> 0.fleV, HR1 is near zero for NH = 0 and increases to 0.85 for NH = ~ x io20 H-atoms cm . Ultrasoft spectra, either Raym~d-Smithwith kT 0.1dichotomy keV or softinblackbodies, havetherefore, HR1 nearthat -1 for NH = 0 spectra and reach -0.5afor NH 2. < The Fig. 3 implies, composite with moderate 5blackbody x 10 H-atoms cm are rare. All spectra in the left part feature strong blackbody contributions while those contribution in the right part are dominated by optically thin thermal radiation. While the objects on the left are mostly AM Her stars, this group contains also the newly identified EUV-bright intermediate polar RXJO751 + 14 (Table 1 and /10/) and SS Cyg in outburst. The discovery of RXJO751 +14 will spur the search for ultrasoft emission from the heated photospheres in other intermediate polars. While some such systems have absorbed spectra as AO Psc and V1223 Sgr with HR1 = + 0.71 and + 0.66, respectively, others do not as the newly established 529-s rotator DO Dra (/11/ and next section) and the DO Her star AE Aqr with HR1 = + 0.14 and -0.04, respectively. For the disk accretors, SS Cyg and U Gem in outburst are still the only systems for which soft X-ray emission from the boundary layer has been detected. During the ROSAT Survey, SS Cyg was caught in the decline from outburst with the soft component disappearing rapidly (/11/, and dashed lines in Figs.2 and 3). Besides SS Cyg, one U Gem star (RU Peg), five SU UMa stars (YZ Cnc, VW Hyi, AQ Er V436 Cen, V2051 Oph) and two Z Cam stars (Z Cam, HL CMa) were encountered in outburst while RX And was in standstill. None has displayed convincing evidence for soft X-ray emission from an optically thick boundary layer. VW Hyl received the best coverage and was observed throughout a normal outburst. Its behaviour differed drastically from that of SS Cyg in that the X-ray flux dropped in outburst without change of HR1 (/12/, and dashed lines in Figs. 2 and 3). The spectrum is that of a moderately hot optically thin boundary layer /13/ with no evidence for a soft blackbody component. Superficially, the reduction in flux is suggestive of partial occultation of the optically thin boundary layer (and any optically thick section), perhaps by an inflated inner disk. The fact, however, that all dwarf novae observed in outburst except SS Cyg and YZ Cnc have very low FXIFY suggests that the behaviour of VW Hyi is not atypical and any explanation should also be applicable to low-inclinationsystems as, e.g., RU Peg.
lao
•
•AMHer 0 DQ Her o DN, SU. Z Cam * systems in outburst
• N, ML SS*-.. 10
VDD + uncertain
•______-.
-.
0
•~+•.•~ 5
11
0
-_ —
~
.1
•0
*
H
0
* +°*
~
a
~ 8*0~~~0
•
*
.01
•
a I
—1
—.5
0 HR1
.5
1
Fig. 3. Mean PSPC countrate vs. hardness ratio HR1 (see text) for CVs ofdifferent subclasses.
Cataclysmic Variable Sky
100
.,,,~
..I..,~
.:..i.j
IX Vel xlO
0.01
iiiiriii~
r
I
blIlli
0.1
I
1
—
11111
0.1
I
I
I
I~IIIII
111111
QQ Vul
i
iii:IIiIIiiIi
BL Hyi quiet
AM Her xlO
,...I
I
BY Cam
ii~
10
•
.1111
~.
IIITIIIIIII
(12)119
.......I.__
BL Hyi flare
11111
1
0.1
I
I 111111
I
1
Energy (keV) Fig. 4. Sample PSPC spectra from the Survey data for one novalike variable and four AM Her stars. Also shown are the results of two-component spectral fits (see text).
The other type of disk accretors which are expected to develop optically thick boundary layers are the stationary high-M systems of which IX Vel, V3885 Sgr, and RR Pic display the softest spectra (HR1 = ~0.07, 0.04, and -0.31, respectively. These hardness ratios suggest that a weak soft component may be present. To illustrate the prospects of isolating such component, we consider the PSPC Survey spectra of IX Vel (0.4 cts/s, HR1 = -0.07) and of two AM Her stars, BY Cam (1.4 cts/s, HR1 = -0.09) and AM Her itself in its low state (0.13 cts/s, HR1 = + 0.06). Fig. 4 shows the PSPC spectra along with the results from two-component spectral fitting. For BY Cam and AM Her, the sum of a 20 keV bremsstrahlung spectrum and a 25 eV blackbody provides an adequate fit. For IX Vel, the more peaky spectrum at E > 0.4 keV requires a low-temperature standard Raymond-Smith 2 /14/, an additional soft component seems to be spectrum of kT = 1 keV. With NH = 2 x iol9 H-atoms cm indicated. The same holds for PR Plc. In both cases, however, higher quality spectra are needed to unequivocallyestablish the presence of soft components. Finally, we note that the spectra of BY Cam and AM Her in its low state are somewhat atypical of AM Her stars. In its high state, AM Her is dominated by soft X-ray emission and its spectrum is similar to that of 00 Vul (Fig. 4). In the low state, its spectrum has apparently lost the soft X-ray component, probably because the reduced temperature of the heated photosphere has shifted it out of the PSPC bandpass. BY Cam is atypical as a comparatively hard and yet bright source. Our tw 9-component fit2.suggests Hence, intrinsically that it contains is does a substantial not differ blackbody component butHer is absorbed from the other bright AM stars. by about 5 x 10 0 H-atoms cm LIGHT CURVES During the Survey, ROSAT has viewed a source in the field of view every % minutes for about 25 seconds. Despite this undersampling, periodic phenomena can be studied provided the period is not commensurate with the satellite period. In fact, in some cases the rather complete phase coverage obtained in the Survey is superior to that achieved in subsequent pointed observations. Of course, the Survey data are not well suited for the study oftransient phenomena on time scales of minutes like, e.g. eclipses or flaring activity. In this section, we present selected results on periodic phenomena in different subclasses of CVs: (1) orbital light curves of non-magnetic CVs (2) spin light curves of intermediate polars, (3) orbital light curves of known AM Her stars, and (4) light curves ofnewly identified CVs.
(12)120
K. Beuermann and H.-C. Thomas
SU UMa 10
_‘
I
I
I
I
I
I
I
+
00
-
1.0
-
+
I
I
I
I
I
I
0.07—0.4 keV
0.5 U
I
GP Corn
~
‘
I_
_I
I
I
++~L ‘+~k+ ++ T ‘
I
I
+ ++
I
I
I
I
1H1752+08 I
I
I
I
0.07—0.4 keV
j~Tk+
+ ++
I_
_1
I
I
~-5
I
I
I
I
~
+q
I
I
-
__
I
Arb. Orbital Phase
Arb. Orbital Phase
I I I I I I 0.07—2.4 keY
..
T T r ~T iIHIIIHlHIiIIHiiIHIiHIIiIHiiiH~~HiIUtIiUi -
I
I
iIiiiiII
++
+q
414
~
I
I
4I$ -
IIIIII
Eclipse Phase
Fig. 5. Orbital light curves of SU UMa, the double degenerate GP Corn, and the 1.88-hour binary H1752 + 08 along with the hardness ratio HR1.
In disk accretors, there is no a-priori reason why the rapidly rotating inner disk and the boundary layer between disk and non-magnetic white dwarf should display any azimuthal asymmetry which could lead to orbital modulation of the emitted X-rays. It is well-known, however, that the distribution of cold matter in the outer accretion disk may well display such azimuthal dependence which can cause phase-dependent photoabsorption in high-inclination systems. The periodic dips in low-mass X-ray binaries and the similar dips in the soft X-ray light curve of U Gem /15/ (inclination i = 70°)indicate that an inflated rim or bulge forms where the accretion stream from the inner Lagrangian point hits the outer disk. We have produced orbital light curves for a number of CVs which received sufficient coverage in the Survey and have found several good examples for modulation produced by phase-dependent photoabsorption. Fig. 5 shows the phase-folded soft X-ray light curves of the prototype SU UMa star and of the double degenerate GP Com. In both cases, the modulation is more pronounced below than above 0.4 keV as is indicated by the systematic variation 8f the 2. hardness Two other ratio. systems The hydrogen column densityorbital required to produce modulation is about cm~variable IX Vel. The which display a similar modulation are the the observed dwarf nova El UMa and the io2 novalike inclination of IX Vel is near 60°/16/, those of SU UMa, GP Corn, and El UMa are unknown but cannot exceed some 750 because the systems are not eclipsing. The presence of phase-dependent internal absorption in several systems complicates the interpretation of their X-ray spectra and, in particular, the search for soft X-ray emission from their boundary layers. Information on internal photoabsorption can also be obtained by confronting NH as obtained from spectral fitting (or simply from HR1) with NH as estimated from interstellar reddening by the depth of the 2200 A feature in IUE spectra /17/. With NH = 6.6 x lO~EBV, we fmd that Z Cha (HR1 = + 1.00), 0623 + 71 (+ 0.98), MV Lyr (+ 0.90), and possibly TT An (+ 0.76), TV Col (+ 0.64), and RW Sex (+ 0.59) are harder than expected from the values of or uper limits on EBV /14, 17/. Internal absorption by matter in the outer accretion disk would not be reconcilable with the low inclination of TT An and RW Sex but would be quite natural for the eclipsing system Z Cha. The phase dependent study of eclipsing disk accretors does not only allow to map the cold absorbing matter in the system. More importantly, the shape ofthe eclipse (or non-eclipse) is the only means to directly measure the location and extent of the X-ray emitting boundary-layer gas. Unfortunately, the known eclipsing systems (like Z Cha) are too faint for this purpose with the one exception of 1H1752 + 08, studied recently by Silber /18/. This short-period binary (P = 1.88 h) and possible SW Sex star is a hard X-ray source (HR1 = + 0.77). The X-ray orbital light curve from the Survey (Fig. 5) is 100 % modulated and probably shaped by absorption or partial occultation. Unfortunately, none of the individual satellite passes falls into the narrow eclipse and a dedicated pointed observation is needed.
Cataclysmic VariableSky I
I
I
I
I
I
I
I
1.0
-
0.5
~++4~4~~4-+
I
DO Dra
I
I
I I
I
0.07—2.4 keY
++ I I I
I I I I
V1223 Sgr .~
_I
I
I
I
I
I
I
I
I
EK UMa
-
I
I
I
I
I
I
I
I
0.07—0.4 keV
-1
++~+~f+++. I I I I I I I I 1
+ ±+
1-+
I_
(12)121
5
.4
-
.4
20
0.07—2.4 keY
BL Hyi
0.07—0.4 keV
fs
.~1o-
±-
U
+ -
U
O_IIIIIIIIIIIIIIIIIII 1.0
-
AO Psc
0.07—2.4 keY
:1 0.5
+1 + T
-T ++
:± 0.0 I 0.0
0
I
~II
111111
0.5
+
IIiIr~i1°lI
~l~Ir~I~0II BY Cam
-
0.07—0.4 keV
-
2
-
-
41
++ 111111
1.0 1.5 Spin Phase
I
2.0
0 1~I 0.0
¶1
4 l0I10I
0.5
41
4
~I
1.0 Orbital Phase
1.5
2.0
Fig.6. (a) Phase-binned spin light curves of the intermediate polar DO Dra (= YY Dra), V1223 Sgr, and AO Psc. (b) Soft X-ray orbital light curves of three AM Herculis binaries. In BL Hyi and BY Cam, all high data points are produced by soft X-ray flaring. Phase-folding of the Survey data is possible also for the spin and orbital periods of intermediate polars although care has to be taken to avoid interference of the two modulations. In Fig. 6a, we show the spin light curves of V1223 Sgr, AO Psc, and DO Dra. For all three systems, orbital modulation was not obiously present and periodogram analysis readily yielded the spin period. For the newly recognized intermediate polar DO Dra (= YY Dna) /11/, the spin period is 529.2 s. No signal was found at the optical period of 550 s or near P/2. In intermediate polars or DO Her stars, the spin modulation is thought to arise from the varying aspect of the emission region on the white dwarf and/or from phase-dependent photoabsorption in the accretion funneL This is supported by (subtle) phase-dependent HR1 variations in EX Hya and in the three sources of Fig. 6. AM Hercules binaries are among the brightest objects in the soft X-ray sky and the ROSAT Survey allowed, therefore, to construct light curves for all known systems which did not happen to be in a low state of switchedoff accretion. Of the 17 known AM Her stars /3/, 9 had more than 0.5 cts/s at the time they were covered during the Survey (V834 Cen, 00 Vul, EF Er VV Pup, BL Hyi, AN UMa, BY Cam, EK UMa, UZ For), 6 systems were in low states (AM Her, MR Ser, WW Hor, ST LMi, DP Leo, EX00329-26) and 2 sources not previously detected in X-rays (V1500 Cyg, Grus Vi) were not seen in the Survey either. A good example of the light curves that may be obtained from Survey data under favourable conditions is that of EK UMa (Fig. 6 b) of which only a short EINSTEIN exposure existed so far /19/. Fig. 6 reveals a bright phase which lasts for half of the orbital period and is interrupted by a pronounced absorption dip of 0.07 in phase. The dip is likely to be produced by the accretion stream crossing the line of sight, similar to the well-studied dip in EF En /20, 21/. For most of the bright AM Her stars, the light curves present themselves in a recognizable shape relating to the accretion geometry these systems had assumed at the time of the Survey. Two systems, however, BL Hyi and BY Cam, display a rather chaotic behaviour (Fig. 6b). BL Hyi previously produced X-rays at its main pole on the far hemisphere, visible in the phase interval 0.0-0.4 /22/. In the Survey, BL Hyi had brightened considerably and showed extremely soft X-rayflaring at odd phases, apparently due to intermittent accretion at its near pole. This chaotic behaviour was later confirmed by a pointed observation: stable accretion to its ‘main’ pole for a few satellite passes alternated with chaotic aperiodic very soft X-ray flaring /23/. BY Cam was shown by Ishida et al. /24/to possess a state of stable accretion at one pole which changed over to a flaring state with accretion at both poles. During the Survey, it was observed in what seems to be the flaring state. In Fig. 6 b, all high data points for BL Hyi and BY Cam are due to soft X-ray flaring. For the case of BL Hyi, the mean PSPC spectra of the small flares (low countrate) and huge flares (high countrate) are separately depicted in Fig. 4. For the case of BY Cam, Silber et al. /25/ suggested the chaotic behaviour to be related to a slight deviation from synchronism.
K.Beuermann and
(12)122
RX 11149+28 10
=
— 11111111
H.-C. Thomas
RX 11938—46
RE 1149+284 I I I I
I_
1111
RE 1938—481 141 I I I
=
411111111
I
I:
20-
-
.4,
~io-
~5. 0 U
$
0
~
I
4 III~~*I*$~
UT(days)
—
I
4
•
0
0 I11
~
2448220.
_I
I
I
I
I
I
I
I
~
I
I
=
1
140 mm)
RX 10203+29 I
II
I
I
I
I
I
I
I
I
1~
I
lI~
Phase (P
RX 11002—19 2
1
•
I
UT(days)
II~I
—
I
I
II
I_
2
_IIIIIII
-
1
-
~i~I
2448210.
0~L~ ~
I
I
Phase (P
I
1I
=
I
I
I
I
I
~
~I~U
I
1142
227 mm)
Fig. 7. PSPC light curves of 4 bright sources from the ROSAT soft X-ray All sky Survey identified as new or problable new AM Herculis binaries.
NEW CATACLYSMIC VARIABLES Besides providing a synoptic view of the known CVs, the ROSAT Survey has led to the discovery of numerous new X-ray sources, the identification of which represents a tremendous task. To some extent, fmding new CVs among these sources is possible by using the X-ray data alone. Fig. 7 depicts, in the two left panels, the Survey light curves of two sources with mean PSPC countrates of 2.2 and 0.6 cts/s. These light curves in which each data point represents one satellite pass clearly show that the sources are periodic. Between two successive satellite orbits (96 mm), the phase progresses (or recedes) by 0.064 and 0.085 for the two systems. The implied (orbital) periods are 102.5 or 90 mm and 105 or 885 mm, respectively, of which 90 mm is the correct orbital period for the first object /26/. The picture is less clear if the phase shift is larger as, e.g., 0.686 for RXJ1938-46 (Fig. 7, upper right). Period search, however, immediately reveals that P = 140 mm (or shorter than 66 mm): The light curve is clearly that of a self eclipsing AM Her star like VV Pup. Fig. 7, lower right, shows a case where period search goes astray and the correct period could only be found by optical astronomy /23/. All four systems ofFig. 7 are new AM Her stars or AM Her candidates. RXJO2O3 + 29 belongs to the class of flaring sources like BL Hyi or BY Cam. The ultimate identification of a new X-ray source will normally require optical spectroscopy, photometry, and/or polarimetry. At the MPE, a number of long-range collaborative identification programs is being coordinated. For CV’s, the most important are the galactic-plane Survey (GPS) /27/ and a program to identify bright high galactic-latitude sources (HGLS) of which the latter is so far restricted to bright soft sources (S > 05 PSPC cts/s, HR1 <0). The objects of Fig. 7 are from this program. To date, a total of 22 new bright CVs and CV candidates has been identified, mostly by optical spectroscopy (Table 1). For, some of these, detailed follow-up observations have already been performed. It is noteworthy that 15 of the 17 HGLS sources are AM Her stars or AM Her candidates while the 3 new intermediate polars are all from the GPS. At this early stage, orbital periods are already known for 12 of the new confirmed and candidate AM Her stars and, surprisingly, none falls in the famous 114-mm spike which contains 6 of 17 previously known AM Her stars /3/. On the other hand, the periods of two new AM Her stars, 1255 mm and 140 mm, place them in the period gap 012-3 hours. At least the latter of these must be born inside the gap. Our search for new AM Her stars is so far restricted to sources with countrate S > 05 cts/s (Fig. 3). If extended down to 0.2 cts/s, we expect to find about 20 further AM Her stars which would bring up the census to beyond 50, a sound basis for statistical and evolutionary studies. To be sure, synchronous rotation of the magnetic white dwarf, the ultimate signature of an AM Her star, still remains to be proven for most of the new systems, requiring a substantial amount of telescope time.
Cataclysmic VariableSky
(12)123
The ROSAT Survey has also led to the discovery of a number of peculiar sources which do not fit into the known CV-subtype pattern. We mentioned already RXJO751 + 14/10, 26/ which represents a further connecting link between the intermediate polars and the AM Her stars (polars). Another peculiar source is the long-period ecipsing binary RXJ0515 +01 (Table 1) which is very soft and has a CV-like spectrum with strong He114686 emission, yet does not seem to be an AM Her star /28/. Finally, we mention a supersoft X-ray source (not listed in Table 1) which resides in a binary with 16 hours orbital period and is of as yet indeterniinate nature. It bears similarities to the cataclysmic variable V Sge but also to the more luminous supersoft X-ray sources in the Magellanic Clouds./29/ which possibly contain accreating and hydrogen-burning white dwarfs /30/. These examples show that our knowledge of certain aspects of the nature and evolution of CVs is still fragmentary. CONCLUSIONS The ROSAT X-ray Survey has allowed a synoptic view of the CV sky. Full exploitation of the available material will allow an improved understanding of the class as a whole. The Survey has also led to the discovery of 22 new bright CVs with S > 0.5 PSPC cts/s, which doubles the number of bright CVs from 22 to 44. These sources are important as individuals with their own idiosyncrasies but the increased number also provides a much improved basis for statistical and evolutionary studies. We anticipate that the information obtained from the ROSAT Xray Survey and from follow-up observations of new sources will significantly change our understanding of cataclysmic variables and related objects.
TABLE 1
New CVs and CV candidates with PSPC countrate
>
0.5 cts/s
*
RXJ
A)
B)
cts/s
HR1
WFC (y yes)
Mag
Type
y
19 17 19
AM: AM: AM
15
CV:
y
17
AM:
**
Period (mm)
Soft bright high-galactic-latitude sources 0132-65
0.6
-0.90
0203 + 29 0453 -42 0515+01
0.5
-
0.67
1.7
-
0.89
0.5
-0.82
0531 - 46 1002 - 19 1007-20
1.2 0.6
-0.70 0.84
17
2.1
1015+09
13 2.2
y y
18 17 17 17
0.57
y
16 18
AM AM
y
-
0.94 0.89 0.91
14 17 15
AM AM: AM
140 99 125
-
0.74
18
AM
199:
-
0.97 033
14 13
DQ ?
15
DN
14 14 14
DO DO AM?
-
32
2.1
0.91 -0.74 - 0.95 - 0.85 - 0.79
1844 - 74
1.4
-
1938 -46
8.9
-
1957 - 57 2107 -05 2316 - 05
1.2 1.3 1.5
1149 + 28 1307 + 53
2.5
1313
-
-
-
y
227: 95 480
sp ph ph
AM:
105/89
X
AM AM: AM AM
208:
sp
90 + 80 + 252:
X
90
sp
+
X, sp sp ph X, sp
Galactic-plane sources 0028 + 59 0558 + 53 0640- 24 0751+ 14
1712 + 24 1802+18
0.5
1.6
1.1
0.01
5.3 0.6 2.4
-0.69 0.93 - 0.85
y
339k
AM, AM Her star. DQ, DQ Her star, intermediate polar. DN, dwarf nova ph, photometric. sp, spectroscopic. X, from ROSAT data. + from WFC identifications /10, 26/ JASS i3:12-J
(12)124
K. Beuemm andH.-C. ‘homas
ACKNOWLEDGEMENTS We thank the ROSAT team for making the X-ray All-Sky Survey a success and permitting studies like this one. We aIs0 thank all the individuals at the MPE who helped us with the data analysis. We thank the members of the CV working group at the MPE, in particular W. Bunk, for contributions and discussions and V. Burwitz, Gettingen, for analysing part of the data and producing the figures.
REFERENCES 1. J. Triimper, The ROSAT mission&v.
Space Res. # 4,241(1983).
2. W. Voges, The ROSAT All-Sky Survey, Proc. ZSYSymp. ‘Space Science with particular emphasis on High Energy Astrophysics’, Munich (1992). 3.
H. Ritter,Astron. Astrophys. Suppl. Ser. 851179 (1990) and private communication.
4.
F.A. Cordova and K.O. Mason, MNRAS 206,879 (1984).
5.
M. Eracleous, J. Halpern and J. Patterson, Ap. J. 382,290 (1991).
6. J. Patterson, Ap.J.SuppZ. 54,443 (1984). 7.
J. Patterson and J.C. Raymond, ApJ. 292,535 (1985).
8.
K. Beuermann, X-ray Properties of AM Herculis binaries, A&Space
Res. 8, # 2,283 (1988).
9. J. Patterson and J.C. Raymond, Ap.J. 292,550 (1985). 10. K.O. Mason, M.G. Watson, T.J. Ponman, P.A. Charles, S.R. Duck, B.J.M. Has&, D.H.P. Jones and J.P.D. Mittaz, MNRAS 258,749 (1992).
S.B. Howell, M. Ishida,
11. J. Patterson, D.A. Schwartz, J.-P. Pye, W.P. Blair, G.A. Williams and J.-P. CaiIIault, ApJ. 392,233 (1992). 12. T. BelIoni, F. Verbunt and W. Bunk, Soft X-ray Emission from Boundary Layers in Dwarf Novae, Ringberg Meeting March 1992, MPE Report (1992), and private communication. 13. T. BelIoni, F. Verbunt, K. Beuermann, W. Bunk, C. Izzo, W. KIey, W. Pietsch, H. Ritter, H.C. Thomas, and W. Voges, AstronAstrophys. 246, L44 (1991). 14. Ch. Mauche, J.C. Raymond and F.A. Cordova, ApJ
335,829 (1988).
15. K.O. Mason, F.A. Cordova, M.G. Watson and A.R. King, MNRAS 232,779 (1988). 16. K. Beuermaun and H.-C. Thomas, Astmulstrophys.
230,326 (1990).
17. F. Verbunt, AstromAstrophys.Suppl.Ser. 71,339 (1987). 18. A. SiIber, Thesis, MIT (1992). 19. S.L. Morris, G.D. Schmidt, J. Liebert, J. Stocke, I. Gioia and T. Maccacaro, Ap..!. 314,641(1987). 2Q. K. Beuermann, J. Patterson and L. Stella,Ap.J. 316,360 (1987).
21. M.G. Watson, A.R. King, M.H. Jones and C. Match, MNZUS 237,299 (1989). 22. K. Beuermann and A.D. Schwope, Astrordstrophys.
223,179 (1989).
23. A.D. Schwope, private communication. 24. M. Ishida, A. Silber, H.V. Bradt, R.A. Remillard, K. Makishiia,
and T. Ohashi, Ap.J. 367,270 (1991).
25. A. SiIber, H.V. Bradt, M. Ishida, T. Ohashi and R.A. RemiIIard, Ap.J. 389,704 (1992).
26. M.G. Watson, this issue. 27. C. Match, T. BeIIoni, D. Buckley, M. Gottwald, G. Hasinger, M.W. PakuII, W. Pietsch,, K. Reinsch, R.A. Remillard, J.H.M.M. Schmitt, J.Triimper and H.-U. Zimmermann, AsfromWrophys. 246, L 24 WV, ad Ringberg Meeting March 1992, MPE Report (1992). 28. A. Shafter, private communication. 29. J. Greiner, G. Hasinger and P. Kahabka, Ashonsistrophys. 246, L17 (1991). 30. E.J.P. van den Heuvel, D. Bhattacharya, K. Nomoto and S.A. Rappaport,Asrro~l.As@op~ly~. x2,97
(1992).