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1997MNRAS.291L...5F

Mon. Not. R. Astron. Soc. 291, LS-L7 (1997)

The extreme X-ray luminosity of the z = 4.72 radio-loud quasar

GB 1428+4217

A. C. Fabian,1 W. N. Brandt,I,2 R. G. McMahon

1 and I. M. Hook 3

1 Institute of Astronomy, Madingley Road, Cambridge CB3 OHA

2 High Energy Astrophysics Division. Mail Stop 6, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

3 European Southern Observatory, D-85748 Garching, Germany

Accepted 1997 July 8. Received 1997 July 8; in original form 1997 April 29

ABSTRACT

We report on a ROSAT High Resolution Imager observation of the radio-loud quasar GB 1428+4217, which is at a redshift z = 4.72. The quasar was well detected, with a flux in the observed

0.1-2.4 keVband of _10-

12 ergcm- 2

S-1 • Our result confirms that the object

in the catalogue of serendipitous X-ray sources, WGACAT, is indeed the quasar. GB 1428+4217 is therefore the brightest X-ray source above a redshift of 4 and has an extremely high isotropic X-ray luminosity, which exceeds that in other (observed) bands. The observed spectral energy distribution of the object therefore peaks at X-ray, or even shorter, wavelengths. We speculate that the emission in the object is beamed toward us. Key words: quasars: general-quasars: individual: GB 1428+4217 -X-rays: galaxies.

1 INTRODUCTION

Quasars above a redshift of 4 have wide cosmological import ance since they must be associated with deep and massive potential wells in the earliest collapsed structures (see

Efstathiou

& Rees 1988; Thmer 1991). Those detected so far are generally among the most luminous objects known and, from the Eddington limit, require billion solar mass black holes.

Despite the high luminosity, only

a. few have been detected in X-rays (McMahon et al., in preparation). Only one (the radio loud

GB 1508+5714 at z = 4.30: Hook et al. 1994)

is bright enough (Mathur & Elvis 1995) for broad-band X-ray spectro scopy to be possible with current instrumentation such as ASCA (Moran & Helfand 1997). Hook & McMahon (1997) have recently discovered the radio loud object with the highest redshift known: the quasar

GB 1428+4217 at

z = 4.72. In the radio band it is core-dominated and has a flat spectrum. Such quasars are typically X-ray-brighter (by up to a factor of iO) than radio-quiet ones. A source with a position consistent with that of the quasar is in the ROSAT archive

WGACAT.

It is, however, about 40 arcmin off-axis in the image and the positional error box is large. Here we report on an observation of the GB 1428+4217 field with the ROSAT High Resolution Imager (HRI) which confirms that the X-ray source is indeed the quasar. GB 1428+4217 is therefore currently the most distant X-ray detected object.

We find that the isotropic X-ray luminosity

ofGB 1428+4217 is very high, indicating that either it is one of the most luminous objects in the Universe, or its apparent luminosity is boosted by gravitational lensing, or intrinsic beaming (say, in a jet oriented towards us).

© 1997RAS

2 THE ROSAT OBSERVATIONS

GB 1428+4217 lies -40 arcmin from the centre of an archival ROSAT Position Sensitive Proportional Counter (pSPC) field (RP700535 of 9593 s taken on 1992 January 24) at a position consistent with an X-ray source of 0.02 count S-1 (to within the positional uncertainty of -1.5 arcmin). The vignetting, exposure map and background corrected count rate of the source is

0.05 ::!:: 0.02 count S-I. It was close to a detector rib so a semicir

cular acceptance region for the source counts was used and reflec tion symmetry assumed for the image of the source. The position also complicates spectral analysis of the source. We find that the spectrum is hard and is consistent with a power-law model over a range of photon index r from less than 1.5 to above 2.5. The absorption is consistent with the Galactic value of 1.4 x loW cm- 2 for the lower values of r and exceeds 5 x 1020 cm- 2 at the upper end. The field was observed with the ROSAT HRI on 1996 July 30.

77 counts were detected in 2894 s from a position consistent with

GB 1428+4217 (Fig. 1). The source centroid of 14h 30m +42° 04' (J2ooo) is about 4 arcsec from the optical position of
the quasar, which is well within the typical ROSAT pointing uncertainty. This converts to a flux, corrected for Galactic absorp tion, of9 x 10- 13 ergcm- 2

S-1 in the

(observed) 0.1-2.4 keV band for a photon index of 2 (using PlMMS: Mukai 1995). It is more than

10 times the flux

of all other quasars at z > 4 (McMahon et al., in preparation), apart from

GB 1508+5714 which is about half as

bright. If a photon index of 1.5 is used for the flux of GB 1428+4217, as is appropriate for a flat-spectrum radio quasar (Worrall & Wilkes

1990) and in agreement with the PSPC spectrum, we obtain

1 x 10- 12 ergcm- 2

S-1 for the unabsorbed flux in the observed

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1997MNRAS.291L...5F

L6 A. C. Fabian et al.

§ 45"

s:: 0 .;:l s:: 30"
Q 15"

04'00"

28' 26' 24' 22'

Right Ascension (2000)

Figure 1. HRI source (contours) overlaid on the Digitized Sky Survey (Copyright National Geographic Society). Compare with fig. 2 of Hook & McMahon

(1997). Table

1. Radio properties ofGB 1428+4217.

Aux Survey Ref

GHz mJy

0.151

280:t 30 7C I

0.326 24O:t 7 WENSS 2

0.365 199

:t 16 Texas 3 0.408

24O:t 50 B3 4

1.40 215
:t 7 NVSS 5

1.40 177 FIRST 6

1.4 396 GB 7

4.9 250
:t 15 87GB 8 4.9 259
:t 31 87GB 9

4.9 337

:t 33 GB6 10

8.0 220 JBVAS

II

15.2 139 :t 14 Ryle 12

References:

(I) Riley (private communication); (2) De

Bruyn (private communication); (3) Douglas

et al. (1996); (4) Ficarra, Grueff & Tomassetti (1985); (5) Condon et al. (1994); (6) White et al. (private communication); (7) White & Becker (1992); (8) Becker, White & Edwards (1991); (9) Gregory & Condon (1991); (10) Gregory etal. (1996); (II) Patnaik et al. (1992); (12) Pooley (private communication).

0.1-2.4 ke V band. The uncertainty in these fluxes from HRI photon

statistics is about

10 per cent.

The radio data on

GB 1428+4217 have been obtained from the

literature and are listed (with references) in Table

1. The object

clearly has a flat radio spectrum. There is marginal evidence for variability at 5 GHz. From the

NVSS survey (Condon et al. 1994)

we find that the fraction of the 20-cm radio flux that is polarized is

5.5 per cent.

3 THE X-RAY LUMINOSITY OF GB 1428+4217

From the foregoing, the luminosity of GB 1428+4217 is then

1.3 (2.0) x 10

47
erg s -I in the (rest frame) 0.6-11 ke V band for a photon index of 2.0 (1.5). (We assume Ho = 50kms-

1 Mpc-I and

qo = 0.5.) The most luminous quasars at z -1 such as PG 1634+706 and PG 1736+481 have X-ray luminosities in this band of -8x 10 45
ergs-I (Nandra et al. 1995), comparable to those of previously observed luminous quasars at z > 4 (McMahon et al. 1997). The high X-ray luminosity is in accord with the object being radio-loud (Wilkes & Elvis 1987; Canizares & White 1989).

The continuum

flux measured on 1996 July 19 at 1450 A in the quasar rest frame (i.e. at 8260

A in the observed spectrum) through a

wide slit was!. = 6.3 x 1O-28ergcm-2s-1 HZ-I. The rest-frame 2500
A to 2 keY, UV to X-ray spectral index is then O!ox = 0.87, assuming a UV energy spectral index of 0.7. The emitted luminosity peaks in the X-ray band, unless the optical spectrum is reddened or

© 1997 RAS, MNRAS 291, LS-L7

© Royal Astronomical Society • Provided by the NASA Astrophysics Data System Downloaded from https://academic.oup.com/mnras/article/291/1/L5/1252555 by guest on 20 October 2023

1997MNRAS.291L...5F

The extreme X-ray luminosity of GB 1428+4217 L7

i !"l i !"l i !"l i o

10· 10'0 10" 10'2 10'3 10'4 10'5 10'· 10" 10'· 10'·

Hz Figure 2. Overall observed spectrum of GB 1428+4217, in units of ergcm- 2 s- l• the object is highly variable. Note that if the X-ray spectrum is intrinsically hard (r -1.5) then the overall spectrum (Fig. 2) does not extrapolate simply from the optical to the X-ray band.

Ifr > 1.5

then the absorption required by the PSPC spectrum implies that CXox is yet smaller.

4 DISCUSSION

The total luminosity of GB 1428+4217, if emitted isotropically, is extremely high and would make it perhaps the most luminous steady object in the Universe. (Its luminosity increases if qo < 0.5, by a factor of up to 5.6 as qo goes from 0.5 to 0.) From the Eddington limit the mass of the central object must be > 1010 This must have formed in less than the approximately one billion years available before that redshift (Efstathiou & Rees 1988; Thrner

1991).

We note, however, that the quasar radiation may either be beamed along our line of sight (it has a flat radio spectrum and is polarized: Section 2) or be gravitationally lensell. The equivalent width of the C IV emission line is low which does suggest a boosted optical continuum. There is, however, no detected radio structure (patnaik et al. 1992), which argues against lensing. The lack of X-ray variability argues against microlensing dominating the flux. The simplest explanation for the extreme luminosity is that the quasar is beamed toward us. (Note that if the third most distant quasar is highly aligned with our line of sight then there are probably many more such objects beamed elsewhere.)

The X-ray dominance and value

of CXox are exceptional, even for a radio-loud quasar (see e.g. Elvis et al. 1994). Such a low value of CXox is only seen in active galactic nuclei (AGN) for X-ray-selected BL Lac objects (see e.g. Sambruna, Maraschi & Urry 1996), which are of much lower apparent luminosity. The most similar object over the optical to X-ray band is perhaps the gamma-ray blazar

© 1997 RAS, MNRAS 291, L5-L7

PKS 0528+ 134 (see the spectral energy distribution in Sambruna et al. 1997) which is at z = 2.07. GB 1428+4217 does not feature in the EGRET catalogue of GeV-emitting gamma-ray sources (Thompson et al. 1995). Nevertheless, its observed spectral energy distribution is highly suggestive of a beamed source. GB 1428+4217 is sufficiently bright that detailed X-ray spectra can be obtained first with ASCA and then at high spectral resolution with the gratings on

AXAF, after its launch in 1998. Both intrinsic

absorption (_10 22
cm -2 is typical for distant radio-loud quasars: Elvis 1996; Cappi et al. 1997) and line-of-sight absorption in the intergalactic medium can then be sought. If the object is beamed then it is likely that the X-ray spectrum is hard and the peak of its spectral energy distribution is in the gamma-ray band.

ACKNOWLEDGMENTS

We are very grateful to Professor J. Truemper for permitting

GB 1428+4217 to become a

ROSAT Target of Opportunity. ACF

and RGM thank the Royal Society for support.

REFERENCES

Becker R. H., White R. L., Edwards A L., 1991, ApIS, 75, 1

Canizares C.

R, White I. L., 1989, ApI, 339, 27

Cappi M., Matsuoka M., Comastri A., Brinkmann W., Elvis M., Polumbo

G. G. C.,

Vignali C., 1997, ApI, 478, 492

Condon

I. I., Cotton W. D., Greisen E. W., Y'm Q. F., Perley R A., Broderick I. J., 1994, in Crabtree D. R, Hanisch R 1., Barnes J., eds, ASP Conf.

Ser., Astronomical

Data Analysis Software & Systems m. Astron. Soc.

Pac., San Francisco, p. 155

DouglasJ.N.,BashF.N.,BozyenF.A, TorrenceG. W., Wolfe C., 1996,AI,

111,1945

Efstathiou G., Rees M. I., 1988, MNRAS, 230, 5p

Elvis M.

I., 1996, in Proc. Wurzburg Meeting, MPE Report 263, p. 409

Elvis M.

I. et al. 1994, ApIS, 95, 1

Ficarra A., Grueff G., Tomassetti G., 1985, A&AS, 59, 255

Gregory

P. C., Condon I. I., 1991, ApJS, 75, lOll

Gregory P. C., Scott W. K., Douglas K., Condon I. I., 1996, ApIS, 103,427 Hook I. M., McMahon R. G., Patnaik A. R., Browne I. W. A., Wilkinson P. N., Irwin M. I., Hazard C., 1994, MNRAS, 273, L63

Hook I. M., McMahon R G., 1997, MNRAS, submitted

Mathur S., Elvis M., 1995, AI, 110, 1551

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