- Email: [email protected]
Discovery of six high-redshift radio-quiet, X-ray luminous quasars
CHINESE A!STRONOMY AND AsTRoPI-IYsIcs PERGAMON
Chinese Astronomy and Astrophysics 23 (1999) l-5
Discovery of six high-redshift radio-quiet, X-ray luminous quasarA* WU Xue-bing112 ‘Beijing
As tn, n omica1 Observatory, 2CAS-PKU
Norbert Bade3
Chinese
Academy
of Sciences,
Beijing
100080
Joint Beijing Astrophysical Center, Beijing 100871,
3Hamburger
China
Sternw arte, Gojenbeqaweg
112,
D-21029 Hamburg, Germany
Abstract We report the discovery of 6 radio-quiet, X-ray luminous quasars with redshifts larger than 1.3. 3 of them have redshifts larger than 2. They were selected from Hamburg objective prism plates and identified by optical spectroscopic observations with the 2.16-m telescope of Beijing Astronomical Observatory. This discovery substantially increases the number of high-redshift
radio-quiet,
X-ray luminous
Key words:
galaxies:
quasars
detected
active - quasars:
in the ROSAT
general-X-ray:
All Sky Survey.
galaxies
1. INTRODUCTION
Although more than 50% of the X-ray sources found in the ROSAT All Sky Survey (RASS) are expected to be active galactic nuclei (AGNs), the overwhelming majority of them have redshifts less than IIll. High-redshift quasars in the RASS are very rare, especially radioquiet objects. A comparison of the radio-loud and radio-quiet quasars detected in the RASS indicated that the X-ray detection rate of radio-quiet quasars drops much faster towards From the catalogue of ROSAT detected higher redshift than that of radio-loud quasars. radioquiet quasars121, we found 18 quasars with z > 2 and 55 quasars with z > 1.3. In the northern sky with galactic latitude b > 35’, we found only 20 quasars with z > 1.3 and 4 quasars with z > 2. No radio-quiet quasars have been found at z > 3 in this sky area by the RASS so far. t Supported by National Natural Science Foundation Received 1998-O-21;
revised version 199&11-12
*CAA LETTER 0275_1062/99/$
- see front matter Q 1999 Elsevier
PII: SO275-1062(99)00018-l
Science
B.V.
All rights reserved.
Wu Xue-bing, N. Bade / Chinese Astronomy and Astrophysics 23 (1999) l-5
2
High-redshift, radio-quiet,
X-ray luminous quasars have some interesting properties.
For example, at high redshifts the rest frame energy range is shifted to higher energies, e.g. z = 2.5 means the ROSAT/PSPC ( sensitive between 0.1 and 2.4 keV) receives photons emitted between 0.35 and 8.4keV. This enables us to study the spectra of quasars in the higher frequencies and gives us some information on their wide band X-ray spectral properties. Moreover, the higher resolution X-ray spectroscopy of these quasars from future X-ray telescopes like XMM and AXAF will allow us to study the intervening absorption material between the quasars and us. In addition, the high-redshift X-ray luminous, radio-quiet objects have high probability of being gravitationally lensed. Two lensed radio-quiet quasars, namely HE1104-1805 (with z = 2.4) and RX JO911.4+0551 (with z = 2.8), have been found by the Hamburg optical identification program of ROSAT X-ray sources 13141. However, due to the limited number of high-redshift radio-quiet, X-ray luminous quasars, the studies we mentioned above were based on with only a few objects. The statistics of this class of quasars is still very weak 151. In order to enlarge the current sample, we have devised a plan of selecting new high-redshift X-ray luminous, radioquiet RASS by optical methods.
2.
CANDIDATE
SELECTION
AND
SPECTROSCOPIC
quasars from the
OBSERVATIONS
We used the digitized objective prism spectra from the Hamburg Quasar Survey (HQS)161for the selection of quasars with high redshifts. Currently the Hamburg identification project of RASS sources covers about 10200 deg2 of the northern high galactic latitude sky (lb] > 20)o PI . The strongest quasar emission line, Ly-a, can be detected between 1.8 < z < 3.2 provided the lines arc above a minimal signal to noise. If the CIV 1549 emission line is discernible in the objective prism spectrum, then the reliability of the classification is very high (> 90%). If only one strong emission line is visible, the other lines could be the source (e.g. CIII 1909, MgII 2798, [0111]5007) and smaller redshifts are possible. Follow-up observations of the quasar candidates are thus necessary. Table
1 Spectroscopic
Object
Observation Results of 6 New High-redshift Quasars
Opt. coordinates R.A. (2000.0)
Offset
Decl.
(“)
f,’
B
Date
Exposure
Redshift
8
RX Jo959.7+0049
09 59 46.9
00 49 16
33
4.8
19.0
97/12/07
3600
RX 31059.8+0909
10 59 51.0
09 09 05
13
4.0
17.1
98/03 JO5
3000
1.683
RX J1259.8+3423
12 59 48.9
34 23 19
4
6.0
16.9
98/05/24
600
1.376
2.243
RX 51425.0+2749
14 25 02.6
2749 10
12
2.6
18.8
98/03/05
1800
2.346
RX J1541.2+7125
15 41 15.2
71 25 58
9
3.9
18.0
98/03/05
900
1.418
RX J1701.4+3511
17 01 24.6
35 11 56
10
4.7
18.0
98/03/05
900
2.115
Note: fr is in unit of lo-l3
ergcmm2 s-l
The classification of AGN candidates in the Hamburg identification project of RASS sources is based mainly on their blue continuum which can be well described by a power law. For the selection of quasars with higher redshifts we inspected by eye the objective prism spectra and looked for strong emission lines. Many AGN candidates have brightnesses near
Wa Xae-bing, N. Bade I Chinese Astronomy and Astrophysics
23 (1999) 1-s
3
the plate limit. It is then difficult to distinguish emission lines from noise. Another problem for number counts is the varying detection lit
of the objective prism plates. As a working
figure we used B = 18 as detection limit for strong emission lines. We mainly selected radioquiet sources with radio to optical flux desity ratios fs~n~/f~ < 10. Following describes our observation of the targets in the area b > 35’. Our follow-up observations were undertaken in December 1997, and March And May 1998. We used the CCD detector TEK1024 mounted on the 2.16-m telescope at the Xinglong station of Beijing Astronomical
Observatory.
The spectra dispersion was 400A/mm
and the exposure time was from 10 to 60 minutes depending on the weather status and the magnitude of the object. In Table 1 we summarize our identification results of 6 high-redshift radio-quiet, X-ray luminous quasars. From column 1 to 8 in Table 1 we give their ROSAT names, optical coordinates, offsets between the optical and X-ray positions, X-ray fluxes, B magnitudes, observation dates, exposure times and redshifts, respectively. The X-ray fluxes
JO95946.9-tOQ4plB .
.’ I
.
.
..z
‘
J 105951.0+990963
J 125948.9+342319
J154115.2+712558
J1’70124.6+351156
‘.
. , .
;
.
‘.
’
‘.
.. . .
!‘
*
.
..
“,a.
.
-/
..‘..
. * ..
-’
*
.
*_ l ’
*
*. . ’ .
?
.f
,-
l
. .
.
.. .I
.
-.
J142502.6+274919 .
“
_-.*
“X
Fig. 1
.**
The 8’ x 8’ POSS-I images of six new radio-quiet, X-ray luminous quasars with redshift larger than 1.3. The arrow points to the new quasar in each image.
Wu Xue-bing, N. Bade / Chinese Astronomy and Astrophysics 23 (1999) l-5
Fig. 2
Spectra of 6 new high-redshift radio-quiet, X-ray luminous quasars. Three most significant emission lines in each spectrum are labeled.
Wu Xue-bing, N. Bade / Chinese Astronomy and Astrophysics 2.3 (1999) 1-5
between
0.1 and 2.4 keV are calculated
index -1.8 and low energy
absorption
under
the assumption
due to neutral
5
of a power law with photon
hydrogen
only from our own Galaxy.
We give the finding charts and spectra of these 6 new quasars in Figure 1 and Figure 2. The finding charts were extracted from the Palomar Digitized Sky-Survey (POSS-I). The spectra were obtained after the standard sky light substraction using the MIDAS software developed by the ESO. Other results of identifications of lower redshift quasars in our sample will be published
elsewhere.
3. DISCUSSION Our discovery of 6 new high-redshift radio-quiet, X-ray luminous quasars substantially increases the number of this class of quasars. For the sky area of high galactic latitude (b > 35’) from which our candidates were selected, we have made a correlation between the internal catalogue of identifications of RASS sources from the Hamburg Quasar Survey Schmidt plates and the Veron’s catalogue of quasars and AGNs[‘l. We found that in this area only 18 previously known radio-quiet quasars have redshifts larger than 1.3 and only 4 of them with redshifts larger than 2. These numbers are nearly the same as those we have found in the catalogue of Yuan et al. I21 Among the new quasars discovered by us, 6 have redshifts larger than 1.3 and 3 of them have redshifts larger than 2. Therefore our discovery has nearly doubled the present number of radio-quiet, X-ray luminous quasars with z > 2 in the northern sky area with b > 35’. Further study of high-redshift radio-quiet, provide us more information about the nature
X-ray luminous quasars will undoubtedly of these objects. For example, detailed
investigation of their broad X-ray spectra using XMM and AXAF and future high-resolution observations made by larger optical telescopes will enable US to study the spectral flattening, the intervening absorbing material and the probability of gravitational lensing. The study of the central engine of these special quasars using multiband observational data may also lead to some important progress on the physics of accretion disks around supermassive black holes. ACKNOWLEDGEMENTS
X. -B. Wu acknowledges
Funds of Beijing Astronomical Observatory. by the 2.16 m telescope at Xinglong station
the support
References
[II I21 131 [4J 151 [61 171 [81
from the Director
This work is based on the observations of Beijing Astronomical Observatory.
Voges W., Aschenbach B., Boiler Th., et al., 1996, IAU C&c. 6420 W., Siebert J., Voges W., 1998, A&A 330, 108 Yuan W.,B . ’ Wisotzki L., Kiihler T., Kayser R., Reimers D., 1993, A&A 278, L15 Bade N., Siebert J., Lopez S., et al., A&A, 1997, 317, L13 Bade N., Engels D., Voges W., et al., A&AS, 1998, 127, 145 Hagen H.-J., Groote D., Engels D., Reimers D., A&AS, 1995, 111, 195 Fiore F., Elvis M.m Giomi P., Padovani P., ApJ, 1998, 492, 79 V&on-Cetty, M.-P., V&on, P., 1996, ESO, Scientifk Report, No. 17
made
Chinese Astronomy and Astrophysics 23 (1999) l-5
Discovery of six high-redshift radio-quiet, X-ray luminous quasarA* WU Xue-bing112 ‘Beijing
As tn, n omica1 Observatory, 2CAS-PKU
Norbert Bade3
Chinese
Academy
of Sciences,
Beijing
100080
Joint Beijing Astrophysical Center, Beijing 100871,
3Hamburger
China
Sternw arte, Gojenbeqaweg
112,
D-21029 Hamburg, Germany
Abstract We report the discovery of 6 radio-quiet, X-ray luminous quasars with redshifts larger than 1.3. 3 of them have redshifts larger than 2. They were selected from Hamburg objective prism plates and identified by optical spectroscopic observations with the 2.16-m telescope of Beijing Astronomical Observatory. This discovery substantially increases the number of high-redshift
radio-quiet,
X-ray luminous
Key words:
galaxies:
quasars
detected
active - quasars:
in the ROSAT
general-X-ray:
All Sky Survey.
galaxies
1. INTRODUCTION
Although more than 50% of the X-ray sources found in the ROSAT All Sky Survey (RASS) are expected to be active galactic nuclei (AGNs), the overwhelming majority of them have redshifts less than IIll. High-redshift quasars in the RASS are very rare, especially radioquiet objects. A comparison of the radio-loud and radio-quiet quasars detected in the RASS indicated that the X-ray detection rate of radio-quiet quasars drops much faster towards From the catalogue of ROSAT detected higher redshift than that of radio-loud quasars. radioquiet quasars121, we found 18 quasars with z > 2 and 55 quasars with z > 1.3. In the northern sky with galactic latitude b > 35’, we found only 20 quasars with z > 1.3 and 4 quasars with z > 2. No radio-quiet quasars have been found at z > 3 in this sky area by the RASS so far. t Supported by National Natural Science Foundation Received 1998-O-21;
revised version 199&11-12
*CAA LETTER 0275_1062/99/$
- see front matter Q 1999 Elsevier
PII: SO275-1062(99)00018-l
Science
B.V.
All rights reserved.
Wu Xue-bing, N. Bade / Chinese Astronomy and Astrophysics 23 (1999) l-5
2
High-redshift, radio-quiet,
X-ray luminous quasars have some interesting properties.
For example, at high redshifts the rest frame energy range is shifted to higher energies, e.g. z = 2.5 means the ROSAT/PSPC ( sensitive between 0.1 and 2.4 keV) receives photons emitted between 0.35 and 8.4keV. This enables us to study the spectra of quasars in the higher frequencies and gives us some information on their wide band X-ray spectral properties. Moreover, the higher resolution X-ray spectroscopy of these quasars from future X-ray telescopes like XMM and AXAF will allow us to study the intervening absorption material between the quasars and us. In addition, the high-redshift X-ray luminous, radio-quiet objects have high probability of being gravitationally lensed. Two lensed radio-quiet quasars, namely HE1104-1805 (with z = 2.4) and RX JO911.4+0551 (with z = 2.8), have been found by the Hamburg optical identification program of ROSAT X-ray sources 13141. However, due to the limited number of high-redshift radio-quiet, X-ray luminous quasars, the studies we mentioned above were based on with only a few objects. The statistics of this class of quasars is still very weak 151. In order to enlarge the current sample, we have devised a plan of selecting new high-redshift X-ray luminous, radioquiet RASS by optical methods.
2.
CANDIDATE
SELECTION
AND
SPECTROSCOPIC
quasars from the
OBSERVATIONS
We used the digitized objective prism spectra from the Hamburg Quasar Survey (HQS)161for the selection of quasars with high redshifts. Currently the Hamburg identification project of RASS sources covers about 10200 deg2 of the northern high galactic latitude sky (lb] > 20)o PI . The strongest quasar emission line, Ly-a, can be detected between 1.8 < z < 3.2 provided the lines arc above a minimal signal to noise. If the CIV 1549 emission line is discernible in the objective prism spectrum, then the reliability of the classification is very high (> 90%). If only one strong emission line is visible, the other lines could be the source (e.g. CIII 1909, MgII 2798, [0111]5007) and smaller redshifts are possible. Follow-up observations of the quasar candidates are thus necessary. Table
1 Spectroscopic
Object
Observation Results of 6 New High-redshift Quasars
Opt. coordinates R.A. (2000.0)
Offset
Decl.
(“)
f,’
B
Date
Exposure
Redshift
8
RX Jo959.7+0049
09 59 46.9
00 49 16
33
4.8
19.0
97/12/07
3600
RX 31059.8+0909
10 59 51.0
09 09 05
13
4.0
17.1
98/03 JO5
3000
1.683
RX J1259.8+3423
12 59 48.9
34 23 19
4
6.0
16.9
98/05/24
600
1.376
2.243
RX 51425.0+2749
14 25 02.6
2749 10
12
2.6
18.8
98/03/05
1800
2.346
RX J1541.2+7125
15 41 15.2
71 25 58
9
3.9
18.0
98/03/05
900
1.418
RX J1701.4+3511
17 01 24.6
35 11 56
10
4.7
18.0
98/03/05
900
2.115
Note: fr is in unit of lo-l3
ergcmm2 s-l
The classification of AGN candidates in the Hamburg identification project of RASS sources is based mainly on their blue continuum which can be well described by a power law. For the selection of quasars with higher redshifts we inspected by eye the objective prism spectra and looked for strong emission lines. Many AGN candidates have brightnesses near
Wa Xae-bing, N. Bade I Chinese Astronomy and Astrophysics
23 (1999) 1-s
3
the plate limit. It is then difficult to distinguish emission lines from noise. Another problem for number counts is the varying detection lit
of the objective prism plates. As a working
figure we used B = 18 as detection limit for strong emission lines. We mainly selected radioquiet sources with radio to optical flux desity ratios fs~n~/f~ < 10. Following describes our observation of the targets in the area b > 35’. Our follow-up observations were undertaken in December 1997, and March And May 1998. We used the CCD detector TEK1024 mounted on the 2.16-m telescope at the Xinglong station of Beijing Astronomical
Observatory.
The spectra dispersion was 400A/mm
and the exposure time was from 10 to 60 minutes depending on the weather status and the magnitude of the object. In Table 1 we summarize our identification results of 6 high-redshift radio-quiet, X-ray luminous quasars. From column 1 to 8 in Table 1 we give their ROSAT names, optical coordinates, offsets between the optical and X-ray positions, X-ray fluxes, B magnitudes, observation dates, exposure times and redshifts, respectively. The X-ray fluxes
JO95946.9-tOQ4plB .
.’ I
.
.
..z
‘
J 105951.0+990963
J 125948.9+342319
J154115.2+712558
J1’70124.6+351156
‘.
. , .
;
.
‘.
’
‘.
.. . .
!‘
*
.
..
“,a.
.
-/
..‘..
. * ..
-’
*
.
*_ l ’
*
*. . ’ .
?
.f
,-
l
. .
.
.. .I
.
-.
J142502.6+274919 .
“
_-.*
“X
Fig. 1
.**
The 8’ x 8’ POSS-I images of six new radio-quiet, X-ray luminous quasars with redshift larger than 1.3. The arrow points to the new quasar in each image.
Wu Xue-bing, N. Bade / Chinese Astronomy and Astrophysics 23 (1999) l-5
Fig. 2
Spectra of 6 new high-redshift radio-quiet, X-ray luminous quasars. Three most significant emission lines in each spectrum are labeled.
Wu Xue-bing, N. Bade / Chinese Astronomy and Astrophysics 2.3 (1999) 1-5
between
0.1 and 2.4 keV are calculated
index -1.8 and low energy
absorption
under
the assumption
due to neutral
5
of a power law with photon
hydrogen
only from our own Galaxy.
We give the finding charts and spectra of these 6 new quasars in Figure 1 and Figure 2. The finding charts were extracted from the Palomar Digitized Sky-Survey (POSS-I). The spectra were obtained after the standard sky light substraction using the MIDAS software developed by the ESO. Other results of identifications of lower redshift quasars in our sample will be published
elsewhere.
3. DISCUSSION Our discovery of 6 new high-redshift radio-quiet, X-ray luminous quasars substantially increases the number of this class of quasars. For the sky area of high galactic latitude (b > 35’) from which our candidates were selected, we have made a correlation between the internal catalogue of identifications of RASS sources from the Hamburg Quasar Survey Schmidt plates and the Veron’s catalogue of quasars and AGNs[‘l. We found that in this area only 18 previously known radio-quiet quasars have redshifts larger than 1.3 and only 4 of them with redshifts larger than 2. These numbers are nearly the same as those we have found in the catalogue of Yuan et al. I21 Among the new quasars discovered by us, 6 have redshifts larger than 1.3 and 3 of them have redshifts larger than 2. Therefore our discovery has nearly doubled the present number of radio-quiet, X-ray luminous quasars with z > 2 in the northern sky area with b > 35’. Further study of high-redshift radio-quiet, provide us more information about the nature
X-ray luminous quasars will undoubtedly of these objects. For example, detailed
investigation of their broad X-ray spectra using XMM and AXAF and future high-resolution observations made by larger optical telescopes will enable US to study the spectral flattening, the intervening absorbing material and the probability of gravitational lensing. The study of the central engine of these special quasars using multiband observational data may also lead to some important progress on the physics of accretion disks around supermassive black holes. ACKNOWLEDGEMENTS
X. -B. Wu acknowledges
Funds of Beijing Astronomical Observatory. by the 2.16 m telescope at Xinglong station
the support
References
[II I21 131 [4J 151 [61 171 [81
from the Director
This work is based on the observations of Beijing Astronomical Observatory.
Voges W., Aschenbach B., Boiler Th., et al., 1996, IAU C&c. 6420 W., Siebert J., Voges W., 1998, A&A 330, 108 Yuan W.,B . ’ Wisotzki L., Kiihler T., Kayser R., Reimers D., 1993, A&A 278, L15 Bade N., Siebert J., Lopez S., et al., A&A, 1997, 317, L13 Bade N., Engels D., Voges W., et al., A&AS, 1998, 127, 145 Hagen H.-J., Groote D., Engels D., Reimers D., A&AS, 1995, 111, 195 Fiore F., Elvis M.m Giomi P., Padovani P., ApJ, 1998, 492, 79 V&on-Cetty, M.-P., V&on, P., 1996, ESO, Scientifk Report, No. 17
made