A wide field view of the galactic center region

ELSEVIER

Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 186-195

;;'- . . . . 1 PROCEEDINGS SUPPLEMENTS

A wide field view of the galactic center region John Heise a aSpace Research Organization Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands A long monitoring campaign is being carried out on the galactic bulge region with the Wide Field Camera's on BeppoSAX. The unique large field of view combined with arcminute resolution makes the instrument very suitable to detect fast X-ray phenomena at unpredictable times or positions on the sky, such as X-ray bursts, gamma ray bursts and flares. The diagnostic value of such phenomena can be very high. Type I X-ray bursts unambiguously identify the compact object in X-ray binaries as a neutron star. Long inactive periods may hide this evidence for a number of sources. Unpredictable bursts from such system are relatively easy detected with WEe. Therefore, the current exposures with the WFCs lead to a significant increase in the total number of neutron-star low mass X-ray binaries. They include (1) new transients, (2) sources previously thought to contain black holes, (3) sources previously thought to be a high mass X-ray binary, (4) sources in globular clusters.

1. I N T R O D U C T I O N Most of the brightest X-ray celestial point sources in the energy range 2 to 10 keV are mass accreting compact objects in binary systems. Depending on the mass of the donor star these systems are divided into low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBS). The sky distribution of b o t h classes is concentrated along the galactic plane but the LMXBs have a stronger concentration towards the galactic center t h a n the HMXBS have. A catalog compiled in 1993 (Van Paradijs [26]) lists 124 galactic LMXBS. 55% of these are within 20 ° from the galactic center, 30% are transient in nature. There are indications t h a t the fraction of LMXB transients in which the compact object is a black hole candidate (BHC) is high. Chen, Shrader & Livio [5] suggest this to be 70% (see their Table 4). Conversely, 90% of all galactic BHCS are found in LMXB transients (Tanalca & Lewin [30]). The Wide Field Cameras WFCs on board BEPPOSAX have been used for several monitoring campaigns on the region within 20 ° of the Galactic Centre, in order to address a few basic questions a b o u t the abundance of low-mass X-ray binaries in our galaxy: how m a n y quiescent LMXBs are there, and what is the relative frequency of LMXBS with a neutron star as compact componeat versus a black hole? 0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0920-5632(98)00207-2

Tanaka & Shibazaki [31] estimate from the number of bright LMXB transients ("X-ray novae") that the number of quiescent LMXB systems in our galaxy is between 200 and 1000. This assumes an X-ray nova recurrence time of 10 to 50 years and an effective coverage of 10% of the galactic population at a detection limit of 1 Crab. This detection limit arrives from typical all-sky monitor (ASM) sensitivities. The knowledge about weaker transients is much more liable to selection effects t h a n t h a t of brighter-than-lCrab-unit transients: better detection limits imply instrumentation with narrower fields of view than ASMS that view the whole sky in an unbiased manner. Some detections have been made of dim LMXB transients. Chen et al. [5] list three LMXB transients with small peak intensities: 0.05 Crab for 1918+146, 0.22 Crab for 1846-031 and 0.2 Crab for 0042+32. In 't Zand et al. [15] found the 25 m C r a b transient KS 1741-293 which is very likely a LMXB. Obviously, unbiased detections of dim LMXB transients as obtained from a regular monitoring of the galactic center region, may constrain the number of quiescent LMXBs. For m a n y LMXBs it is not known with certainty whether the compact object is a neutron star or a black hole candidate. Certain diagnostics only exist for neutron stars. The detection of type I Xray bursts or X-ray pulsations in X-ray binaries provide a reliable way of identifying the nature

187

J. Heise/Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 186-195

of the compact object as a neutron star. T y p e I bursts are thought to be thermonuclear flashes on the hard surface of a neutron star. They are characterized by a duration of usually several tens of seconds, a black body spectrum and a softening (cooling) of the emission during the decay (e.g., Lewin, Van Paradijs & T a a m [22]). On the basis of X-ray observations alone no unique criteria exist for identifying the source as a BHC. Dynamical evidence from radial velocity measurements based on optical spectroscopy (Doppler shifted lines in the optical spectrum of the companion star) so far has implicated 10 sources as BHCs, their mass is higher than the generally accepted threshold of 3 M o. Optical observations of LMXBS in the direction of the galactic (:enter are h a m p e r e d by their low optical magnitude and the large extinction in that direction. The nature of the X-ray spectrum and timing behaviour of these X-ray sources often have been taken as criteria for the black hole nature in other X-ray binaries (see e.g. White & van Paradijs [35]). The X-ray spectra of black hole candidates are characterized by two components, ultrasoft and ultrahard. The ultrasoft component (temperature or ~ 1 keV) results from the inner most optical thick regions of an accretion disk. The ultrahard component is a powerlaw extending up to several hundred keV, probably resulting from the inner most and optical thin regions of the accreting flow. In some cases sources with these black hole signatures have subsequently been discovered to be pulsating (e.g. 4U0142+61, Israel, Mereghetti & Stella 1995), so the signature is not fool proof. Hard power law spectra are also emitted by atoll sources at low accretion rates. Atoll sources are thought to contain weakly magnetized neutron stars. Two X-ray spectral characteristics give a high likelihood for a BHC identification (Tanaka & Shibazaki [al]): the simultaneous occurrence of an ultra-soft and a power-law component, or the occurrence of a single power-law component at a bolometric luminosity in excess of 10 a7 erg s -1. LMXBS are strongly clustered around the galactic center. Skinner [29] lists 60 sources within 6 ° but notes t h a t the observations are spread over a

Table 1 Main characteristics per B e p p o S A X - W F C Field of view Angular resolution Location accuracy Detector type Energy range Energy resolution Time resolution

40 ° x 40 ° (3.7% of sky) 5 arcminutes oil axis >0.6 arcmin (68% level) Multi-wire prop. counter 2 to 25 keV (in 31 chan.) 18% at 6 keV 0.5 ms

long period of time. Many of these are transients. An interesting question is if the strong clustering is simply a result of the enhanced stellar density in the core of the galaxy or if sources in the central concentration are different in nature to those elsewhere. Previous results on the population of X-ray sources from A R T - P on G r a n a t (Grebenev et al. [10]) confirm the earlier findings of a clear excess, which however still falls ~ 1.5 order of magnitude short of the number of sources per unit mass seen in globular clusters. Suggestions for an explanation are numerous and include sources resulting from globular clusters which m a y have been tidally disrupted in the core of the galaxy. Because of the clustering toward the galactic center, monitoring observations with a sufficiently wide field of view enable an unbiased and, relative to all-sky monitoring, deep view of the population of LMXBS. For this reason the WFC instrument on the BEPPOSAx satellite is an excelhmt device for carrying out such a monitoring p r o g r a m and is indeed being employed for this. In this p a p e r we discuss a few results from these observations. We remark that the study of LMXBs is not the only subject of the observations on the galactic bulge, although it is the most important one. 2. W F C INSTRUMENTATION OBSERVATIONS

AND

The Wide Field C a m e r a instrument (WFC, [20] consists of two identical coded aperture cameras pointed in opposite directions of the sky. It is placed on the Italian-Dutch BEPPOSAx satellite that further contains a set of narrow-field X-ray instruments that point perpendicular to the WFC

188

J Heise /Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 186-195

directions [6]. Table 1 presents the main characteristics for each camera. It is interesting to note t h a t the field of view (40 ° x 40 ° full width at zero response) is the largest of any flown Xray arcminute-imaging device so far. Therefore, WFC has an exceptional capability to detect short duration and weak transient events. WFC is based on the coded aperture principle. This implies a basic difference with directimaging devices such as X-ray mirror telescopes: there is cross-talk between FOV positions located much further from each other t h a n the angular resolution. In the case of WFC, there is a degradation in sensitivity to any sky position within 20 ° from a bright source. This has a relatively large impact on observations of the crowded galactic center field where the sensitivity is a b o u t two times less t h a n at high galactic latitudes far from bright sources. Detector d a t a contain an image of the sky which is coded with the aperture pattern. The reconstruction of the sky image involves an algorithm whose basic component is a cross correlation of the detector d a t a with the aperture p a t t e r n (Jager et al. [20]). This algorithm is opt i m u m for point sources but not necessarily for diffuse sources. In the case of WFC the reconstruction can be performed with an arbitrary time and photon energy resolution within the limitations given in Table 1. The accuracy to determine the celestial position of a point source is determined by a statistical and a systematic component. The dominant terms in the systematic component are the relation between the pointing of each camera and the satellite attitude system and the plate scale (determined by the distance between aperture and the top of the detector window). Both dominant terms have been calibrated in flight through observations of parts of the sky where more than 2 sources are detected in the FOV (e.g., the galactic center region, the large magellanic cloud region, the Cygnus region). This calibration is updated regularly because of the increasing observation database volume. Currently, the systematic error region is a circle with a radius of 1.41 for a confidence level of 99%. The statistical error region has the form close to an ellipse, due to the imaging effects in the detector (i.e., photon

Table 2 Log of the W F C Galactic Bulge Observations Observing dates

Exposure time

(ks) August September October October March March March April October Total

21-30 11-17 10-12 25-29 18-20 23-25 29-31 13-15 4-5

1996 1996 1996 1996 1997 1997 1997 1997 1997

340 240 100 100 100 90 90 100 60 1220

penetration in the proportional counter) with an eccentricity that increases with the off-axis angle. The size is determined by the intensity and spectrum of the point source. When error regions are shown, the statistical error (as measured from scanning the p a r a m e t e r space) and constant systematic error are added in quadrature as measured from the best fit position. WFC has so far monitored the galactic bulge region in three campaigns, see Table 2, with a total exposure time of roughly 1 Ms. Further campaigns are planned for future visibility periods during spring and a u t u m m times. During most of the observations, the pointing of either camera is positioned as close to the galactic center as possible with one constraint: Sco X-1 is kept outside the FOV. This brightest 2 to 10 keV X-ray source is regarded as a nuisance in WFC observations because of its detrimental effect on the sensitivity within 20 °. Figure 1 shows an example of a typical sky image. Of primary interest is the search for transient phenomena in the GC d a t a of each camera. The algorithms are different for phenomena with durations shorter or longer t h a n ,-~ 1 min. For short events the search is done by analyzing the time profile of the total detector in the full bandpass (Heise et al. [13]) with a time resolution of 1 s. Enhancements beyond 5a above a background modeled to vary linearly with time are searched for on time scales of up to 48 s. For the observa-

189

J. Heise/Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 186-195

BeppoSAX

-

Wide Field C a m e r a bulge in 2 fo 10 keV

C o m p o s i t e i m a g e of g a l a c t i c

0.0

I I' IIIIIIIII III1r

7.5 Sianal-to-noise ratio

....

15.0

Figure 1. A sky image of the WFC observations on the galactic bulge, a few point sources are labeled with their c o m m o n name. Above a signal to noise ratio of 15 all source are displayed similarly.

190

J. Heise/Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 186-195

1200 ooo 8OO 6OO

2x10 4

4x10 4 6x10 4 time (sec)

8x10 4

Figure 2. Photon rate in 2 to 25 keV versus time for one observation on the galactic bulge in March 1997. The observation is interrupted at 110 rain intervals due to Earth occultations. This plot illustrates the large amount of bursts. The following burst sources were identified: GRO J1744-28 (26 bursts), SAX J1750-29 (4), H 1724-307 (2), GX 354-0 (4), GS 1826-24 (2), KS 1731-26 (2)

tions discussed here this implies an on-axis sensitivity of about 0.6 Crab units (2-25 keV) in 1 s to 0.1 Crab in 48 s. If a burst is found, a sky image is generated for the time interval when the burst occurs, another sky image is generated for a long time interval (usually ten times as long as the burst time interval) right before the burst time, the latter image is normalized and subtracted from the former and this image is searched for point sources with intensity increases equivalent to the increase found in the time profile of the detector. The image subtraction is not necessary but facilitates quick identification in case there are many point sources in the FOV. 3. T Y P E

I X-RAY

BURSTS

Type I bursts are attributed to thermonuclear flashes on or near a neutron star surface. Detection of type I bursts is, therefore, a strong indicator for a neutron star. There are two types of X-ray bursts with durations less than a few minutes (see review by Lewin, Van Paradijs & Taam [22]). Type II bursts have only been seen from two sources (the rapid burster and GRO J174428, Kouveliotou et al. [21]) and have spectra similar to the underlying persistent emission. They are thought to be due to accretion instabilities. Within the group of optically classified X-ray binaries, type I X-ray bursts have so far only been detected from LMXBs. Therefore, when type I

bursts are detected from an as yet unclassified system chances are far more likely for the system to contain a low-mass rather than a high-mass companion star to the compact object. The defining property for a burst to be of type I is the black body nature of the spectrum and the detection of spectral softening. The bursts reported here are identified as type I bursts. We conclude in such cases that these bursts imply the excistence for a neutron star as the nature of the compact X-ray source. The inferred size of the emitting region for the black body radiation is consistent with a neutron star interpretation. The bursts seach through the WFC data relevant to the galactic bulge observations so far has lead to the identification of more than 349 bursts (see also Cocchi et al. [8]) from 21 different sources among which are 9 newly detected burst sources. On average the WFCS observe one X-ray burst per 6000 sec within its field of view. The sources are listed in Table 3. Apart from GRO J1744-28, the two most active bursters in this observing period are GX354-0 and GS 182624. The latter source is thought to be a black hole candidate, but apparently in this period exhibits a large number of type I X-ray bursts. Figure 2 illustrates the bursting activity seen with WFC. 3.1. B u r s t s f r o m b l a c k h o l e c a n d i d a t e s Type I X-ray bursts have been observed with the WFC from X-ray sources, previously thought

I HeiselNuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 186-195

to be black hole candidates. Examples are SLX 1735-269 and GS 1826-24, see also Bazzano et al. [3] and Ubertini et al. [32]. We here further discuss the case of GS 1826-24. GS 1826-24 is a serendipitous G I N G A source (Makino et al. [23]) first detected on September 8, 1988 with an average X-ray flux of about 28 m C r a b in the 1-40 keV range. As the source was not reported and observed before, it is tentatively classified as a transient (Tanaka & Lewin [30]). The source showed rapid fluctuations on a time scale down to 2 ms with a r m s variation of 30% (Tanaka 1989). Moreover the spectrum was well fitted with a power law with photon index 1.7. The similarity with Cyg X-1 and GX 339-4 in the low (hard) state suggested the source to be considered as a black hole candidate (Tanaka 1989). The source was subsequently detected with the TTM experiment on March 17 at a flux of about 32 m C r a b (2-30 keV). ROSAT observed GS 1826-24 in October 1990 and June and October 1992 using the PSPC and the authors (Barret et al. [1]) do not report bursts during the 8 hour observing time. The spectrum is well fitted by a power law with photon index in the range 1.5-1.8 and n H ~ - - 5 X 1021cm -2. Follow-up optical studies let to the identification of a V=19.3 optical counterpart located at a = 18h28m28.2s and 5 = -23°47'49.12 ", indicating the source to be a LMXB. The absence of bursts was stressed by Barret et al. [1], to favour the black hole hypothesis. Some observations already indicated the presence of a neutron star. The source position is inside a larger error box of an unidentified X-ray burster observed with OSO-8 (Becker et al. 1976), reported in the OSO 7 Catalogue (Markert et al. [25]) and containing also 4U1831-23 (Forman et al. [9]) and the GINGA position. OSSE observed GS 1826-24 during November 1994 at higher energy at 7.5 standard deviations (Strickman et al. [28]). The spectrum is consistent with a powerlaw index -3. Assuming no strong variations with time and fitting the GINGA and OSSE d a t a together, the spectrum is well represented by a power law with an exponential cut-off energy of a b o u t 58 keV. The WFCs detected type-I X-ray bursts form

191

200

8--25

keY

11°o 14o

, ~ l t , nlq

~

120 :~ U U u ~

u

n~,J] rq_

~u ' % J U ~

....

Ljt~./ u-

100

450

2-8

keV

"5 400 "~ 350 C 2 c

650

2-25

keV

600

i

550~ 50O

450 .

.

.

.

.

,

.

.

.

.

,

,

- -

50 1 O0 Time since MJD 5 0 5 4 2 . 5 4 7 0 0

,

150 (s)

Figure 3. Time profile of one of the bursts observed in the Globular Cluster source NGC 6652, per each of two bandpasses and for the complete WFC bandpass. The bin time is 2 s. The smooth curves are exponential models for the appropriate time profiles (see text)

GS 1826-24 in every observing run on the galactic center region. In total 30 bursts exhibiting spectral softening are detected (see Table 3). After one of these burst active phases, GS 1826-24 has been observed in april 1997 as a target of opportunity with the Narrow Field Instruments on board BEPPOSAX. Two additional bursts requiring black body fit to their spectra have been observed [12]. These observations make clear t h a t the compact sources in GS 1826-24 is a neutron star. Apparently the source is another example which shows t h a t the spectral and t e m p o r a l characteristics for a black hole candidate are not always fool proof.

192

J. Heise/Nuclear Physics B (Proc. SuppL) 69/1-3 (1998) 186-195

Table 3 List of sources in the galactic bulge seen bursting with outside the galactic bulge Source name

New burster?

4U 1248-58 X 1608-522 H 1702-429 X T E J1709-267 H 1724-307 GX 354-0 KS 1731-26 SLX 1735-269 GRS 1741.9-2853 GRO J1744-28 A 1742-294

New

New

New New

Number of bursts detected 2 1 5 3 9 64 22 1 4 156 20

3.2. B u r s t s f r o m g l o b u l a r clusters In contrast to LMXBS in the galactic disk, none of the X-ray sources in globular clusters have been found to contain a black hole (Hut et al. [14], Verbunt et al. [34]). The formation of LMXBS is different in globular clusters than in the disk. Tidal capture and exchange encounters favour the capture of more massive stars, and thus of black holes above neutron stars. One may thus predict a larger presence of black holes in the X-ray binaries in globular clusters than in the galactic disk. Twelve X-ray sources in globular clusters have shown X-ray luminosities Lx _> 1036erg s -1, of which seven are permanently bright, and five transients. The nature of the accreting object in the transients in NGC6640, NGC6652 and Terzan 6 is still unknown. With the WFC two bursts were found at a position consistent with NGC 6652. There are discussed by In 't Zand et al. [16]. Both bursts are relatively weak, the signal-to-noise ratio of the image being around 6. Consequently, the extraction of meaningful time-dependent spectral decoded information is not possible. As an alternative, we analyze time profiles directly of the detector for the part illuminated from the sky po-

WFC

in the first year of operations, plus one

Source name

SLX 1744-299 SAX J1750-290 SAX J1753-238 GX 3+1 SAX J1806-222 SAX J1808-389 H 1812-121 NGC 6624 NGC 6652 GS 1826-238

New burster?

New New New New

New New

Number of bursts detected 6 9 1 6 2 2 1 4 2 30

sition of NGC 6652 in two photon energy bands. By imposing less constraints on the time profile one, on the one hand, preserves the little available statistics but, on the other hand, ends up with a time profile that includes the combined flux from all sources that illuminate the same part of the detector. We regard the latter disadvantage as not important because the bursts are a coherent and clearly recognizable signal which can only be due to a single source of emission and the identification of the whole burst with the source is unambiguous. Figure 3 present the time profile for one of the bursts. The time profiles were tested for the evidence of spectral changes. They were modeled with an exponential decay function with 4 parameters: peak intensity, onset time of exponential, e-folding decay time T and background level. In both cases the ratio of the decay time between the upper and lower energy band is 0.4+0.2. We regard this as good evidence for the presence of spectral softening in bursts from NGC 6652. The small statistical quality of the data do not permit an analysis of the time-dependent decoded (background-subtracted) spectrum. This is somewhat different for the average spectrum

J. Heise/Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 186-195

over both bursts. We fitted a number of sireple spectral models to the spectrum. The models are described by 4 p a r a m e t e r s and only the normalization was allowed to differ between both bursts. All of the models fit the d a t a equally well with a reduced X2 value of 0.9 for 58 independent P H A bins. The d a t a does not allow to single out a best fit model. Nevertheless, if we assume that a black b o d y model applies with cold absorption according to Morrison & M c C a m m o n (1983) the t e m p e r a t u r e is reasonably well constrained to 2.6 4- 0.4 keV. If the black body radiation is isotropic and the distance is 14.3 kpc, we find a bolometric luminosity averaged over the first 16 s of each burst of 2 103s ergs s -1 and a radius of the emitting region of 8 + 3 km. 3.3. B u r s t s f r o m a p r e s u m e d h i g h - m a s s Xray binary Although not in the galactic bulge, it is in the current context appropriate to mention that WFC also sheds more light on a source t h a t was previously thought to be a HMXB [33]: 4U 124858. WFC detected a strong X-ray burst from this source ([27], [7]) which makes it a better candidate LMXB because no X-ray burst has ever been recorded from an optically confirmed HMXB. 4. N E W T R A N S I E N T

350 500 250 200 150 100 7" 700

[Lff~

]~_~g~~-

193

8-25

keV I

8 keY !

600 500 400 ¢#

500 1000 90o 800 700 600 500 400 10 20 50 40 50 60 Time since MJD 50339.52850 (s)

Figure 4. Time profiles of a bright burst in the new X-ray transient SAX J1808.4-3658. The softening during burst evolution is shown here in two energy bands

X-RAY SOURCES

New X-ray transients can be observed with the WFCs down to a typical sensitivity limit of order 10 to 20 m C r a b in 105 s, depending on other sources in the field. The d a t a have been searched for transients on two different time scales: by accumulating over entire observing periods with a given pointing ( ~ 105 s) and on an orbit by orbit basis ( ~ 103 s). The first results are summarized in Table 4. This list of newly discovered sources include four new transients brighter than 50 m C r a b s (Bazzano et al. [4], In 't Zand et al. [19], Heise et al. [11]). SAX J 1808.4-3658 is a fairly bright transient (in 't Zand et al. [17]) observed during observations in September 1996. Peak intensity is 0.1 Crab in the 2 to 10 keV range and the source lasted between 6 and 40 days above a detection threshold of 2 mcrab. During these observations

the source exhibits two very bright Eddington limited type I X-ray bursts, implying a distance of 4 kpc. These bursts identify this transient as a low mass X-ray binary with a neutron star as compact object. Two transients only seen as X-ray bursters without detectable steady emission (SAX J1753.5-2349 and SAX J1806.5-2215) are reported at this conference (in 't Zand, et al. [18]). 5. C O N C L U S I O N S The WFC onboard BEPPOSAx have been designed to locate and study short lasting X-ray phenomena. These include fast transients, flare stars, G a m m a Ray Bursts and X-ray bursts. This review surveys the results on X-ray bursts

194

J. Heise/Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 186-195

Table 4 New transient X-ray sources in the WFCs

Transient SAX SAX SAX SAX

New transient X-ray sources R.A. Dec.

J1750.8-2900 J!753.5-2349 J1806.5-2215 J1808.4-3658

17h 17h 18h 18h

50m 53m 06m 08m

and new transient X-ray sources in the direction of the galactic centre. The observations with BEPPOSAX-WFC carried out during the first year of operations on this region have been successful in the discovery of new X-ray transients sources. The number of X-ray binary systems now known to contain neutron stars has increased significantly. This success mainly pertains to the detection of new X-ray burst sources. In 1993, 42 LMXBS were known to exhibit type I X-ray bursts (Van Paradijs [26]). During BEPPOSAXWFC observations in 1996 and 1997, 10 additional X-ray burst sources have been discovered (Bazzano et al. [2-4], Cocchi et al. [8], Heise et al. [11], In 't Zand et al. [16,17,19], Piro et al. [27], Ubertini et al. [32]), see Table 3. It took BEPPOSAX-WFC only 1 year to discover as many new burst sources as it took other instruments in the previous 10 years. Interestingly, the 10 new bursters include ones previously thought to be BHCs (Heise et al. [12]) or HMXBS. These results very much prove the relevance of the Xray observations with modest sensitivity but with a large field of view, such as performed with the WFC on board BEPPOSAX.

Acknowledgements I thank J. in 't Zand (SRON, Utrecht) and Angela Bazzano (IAS, Rome) for their contributions to this paper and in addition also P. Ubertini, M. Cocchi and L. Natalucci (IAS, Rome) for their contribution to the data analysis reported here. Furthermore I thank A. Klumper, M. Savenije, J. Schuurmans, G. Wiersma (SRON), J.M. Muller (SRON, SDC, Rome) for crucial s/w

48s 34s 34s 29s

-28o -23o -220 -360

59.9' 49.4' 15.1' 58.6'

bursts? yes only in 1 burst only in 2 bursts yes

support and help during the analysis, R. Jager and W. Mels (SRON) for help in understanding the physics and data of the instrument, F. Verbunt (SIU, Utrecht) for useful discussions and NWO for financial support. For their help in carrying out and processing the WFC Galactic Center observations, I thank the staff of the BEPPOSAX Satellite Operation Center and Science Data Center, in particular the Mission Planners and Duty Scientists. The BEPPOSAX satellite is a joint Italian and Dutch program.

REFERENCES 1. Barret et al 1995, A&A, 303, 526 2. Bazzano, A., et al. 1997a, Observations of Xray bursters with B e p p o S A X - W F C , in Proc. 4th Compton Symp., AIP Conf. Proc 410 (New York: AIP), 719 3. Bazzano, A., et al. 1997b, New X-ray bursters with B e p p o S A X - W F C : S A X J1750.8-2900, GS1826-24 and S L X 1735-269, in Proc. 4th Compton Symp., AIP Conf. Proc 410 (New York: AIP), 729 4. Bazzano, A., et al. 1997c, IAUC 6597 5. Chen, W., Shrader, C., Livio, M. 1997, ApJ, 491,312 6. Boella, G., Butler, R.C., Perola, G.C., Piro, L., Scarsi, L., Bleeker, J.A.M. 1997, AAPS, 122, 299 7. Boller, T., Haberl, F., Piro, L., Heise, J., 1997, IAUC 6546 8. Cocchi, M. et al. 1997, Detection of Xray bursts from X T E J1709-267, in Proc. SAX/XTE-symposium, Rome, Oct 21-25,

J. Heise/Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 186-195

1997. 9. Forman, W. et al. 1978, Ap.J. Suppl., 38, 357 10. Grebenev, S.A., et al. 1995, R6ntgenstrahlung from the universe, Wfirzburg, 1995. 11. Heise, J., et al. 1997b, S A X J1750.8-2900 and G R S 1737-31, IAUC 6606 12. Heise, J., et al. 1997c, Detection of X-ray bursts in the Mack hole candidate GS182624, A&A Lett., in preparation 13. Heise, J., et al. 1998, in preparation 14. Hut, P. et al. 1992, PASP, 104, 981 15. in 't Zand, J.J.M., et al. 1991, Adv.Sp.Sci. 11, (8) 187 16. in 't Zand, J.J.M., et al. 1998a, Detection of X-ray bursts in the globular cluster NGC6652, A&A Lett., 329, L37 17. in 't Zand, J.J.M., et al. 1998b, Discover), of the transient low-mass X-ray binary S A X J1808.4-3658, A&A Lett., 331, L25 18. in 't Zand, J.J.M., et al. 1997c, S A X J1753.52349 and S A X J1806.5-2215: two X-ray bursters without detectable steady emission,

19. 20. 21.

22.

23. 24.

25. 26.

27. 28.

in Proc. SAX/XTE-symposium, Rome, Oct 21-25, 1997. in 't Zand, J.J.M., et al. 1998d, Astron. Nachr. 319, 112 Jager, R., Mels, W., Brinkman, A., et al. 1997, A&AS, 125, 557 Kouveliotou, C., van Paradijs, J., Fishman, G., Briggs, M., Kommers, J., Harmon, B., Meegan, C., Lewin, W. 1996, Nature, 379, 799 Lewin, W., van Paradijs, J., Taam, R. 1995, in W. Lewin, J. van Paradijs, E. van den Heuvel (eds.), X-ray Binaries, Cambridge University Press, Cambridge, p. 175 Makino et al., IAUC 4653 Mardling, R., 1996, in P. Hut, J. makino (eds.), Dynamical evolution of star clusters, IAU Symp. 175, Dordrecht, p. 273 Markert, T. H. et al. 1977, Ap.J. 218,801 wm Paradijs, J. 1995, in W. Lewin, J. van Paradijs, E. van den Heuvel (eds.), X-ray Binaries, Cambridge University Press, Cambridge, p. 536 Piro, L., et al. 1997, IAUC 6538 Strickman, M. et al. 1996, A&A Suppl. Ser., 120, 217

195

29. Skinner, G.K., 1993, A&A Suppl. 97, 149 30. Tanaka, Y., Lewin, W. 1995, in X-ray binaries (CUP, eds. Lewin et al.), 126 31. Tanaka, Y., Shibazaki, N. 1996, ARAA 34, 607 32. Ubertini, P., et al. 1997, GS 1826-24, IAUC 6611, April 4 1997 33. Van Paradijs, J. 1995, in "X-ray Binaries", eds. W.H.G. Lewin, J. v. Paradijs & E.P.J. van den Heuvel (Cambridge University Press), 536 34. Verbunt, F., Bunk, W., Hasinger, G., Johnston, H. 1995, A&A, 300,732 35. White, N.E., van Paradijs, J. 1996, ApJ Letters, 473, L25