We report on a 20-ks observation of the z = 0.93 hyperluminous galaxy IRAS F15307+3252 with the ROSAT HRI. No X-ray source is detected at the position of F15307+3252 at an upper limit of ∼4 × 10⁴³ erg s⁻¹. This is less than 2 × 10⁻⁴ of the bolometric luminosity of the object, and indicates either that the nucleus emits an unusually small fraction of its total power in X-rays, or that little of the nuclear X-ray flux is scattered into our line of sight by electrons. The lack of an X-ray detection around F15307+3252 also rules out it being at the centre of a cluster, such as is observed for IRAS P09104+4109. A weak, possibly extended, X-ray source is detected 13 arcsec south of the galaxy, spatially coincident with a clump of faint objects visible in a Keck K_s-band image of the field. This may be the core of a cluster near the line of sight to F15307+3252

Brandt, W. N.

Crawford, C. S.

Cutri, R. M.

Fabian, A. C.

Smith, H. E.

English

Caltech Authors - Main

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Mon. Not. R. Astron. Soc. 283, L95-L98 (1996) 
X-ray emission from the field of the hyperluminous lRAS galaxy 
lRAS F15307+3252 
A. C. Fabian,i R. M. Cutri,2 H. E. Smith,2,3 C. S. Crawford 1 and W. N. Brandt1* 
1 Institute of Astronomy, Madingley Road, Cambridge CB3 OHA 
2IPAC, Caltech, MS 100-22, Pasadena, CA 91125, USA 
3UCSD, Center for Astrophysics & Space Science 0111, La Jolla, CA 92093, USA 
Accepted 1996 September 23. Received 1996 July 22 
ABSTRACT 
We report on a 20-ks observation of the z = 0.93 hypeduminous galaxy IRAS F15307+3252 
with the ROSATHRI. No X-ray source is detected at the position ofFI5307+3252 at an upper 
limit of -4 x 1043 erg S-l • This is less than 2 x 10-4 of the bolometric luminosity of the object, 
and indicates either that the nucleus emits an unusually small fraction of its total power in 
X-rays, or that little of the nuclear X-ray flux is scattered into our line of sight by electrons. The 
lack of an X-ray detection around FI5307+3252 also rules out it being at the centre of a cluster, 
such as is observed for IRAS P09104+4109. A weak, possibly extended, X-ray source is 
detected 13 arcsec south of the galaxy, spatially coincident with a clump of faint objects visible 
in a Keck Ks -band image of the field. This may be the core of a cluster near the line of sight to 
FI5307+3252. 
Key words: galaxies: individual: IRAS F15307+3252 - infrared: galaxies - X-rays: galaxies. 
1 INTRODUCTION 
Several galaxies found from the lRAS catalogues are among the 
most luminous objects in the Universe. They include 
P09104+4109 (Kleinmann et al. 1988), F10214+4724 (Rowan-
Robinson et al. 1991) and FI5307+3252 (Cutri et al. 1994). If the 
emission is isotropic, they all ex~d 1013~ in bolometric 
luminosity, most of which is emitted in the mid- to far-infrared. 
The origin of the high luminosity is unclear, but it is probably due 
to either a massive burst of star formation or an embedded active 
nucleus (see e.g. Rowan-Robinson et al. 1993). In the case of 
F10214+4724 the luminosity appears to be greatly enhanced by 
gravitational lensing (e.g. Eisenhardt et al. 1996), so it is no 
longer considered to be such an extreme object, although it still 
appears to be a type 2 quasar (Goodrich et al. 1996). The situation 
with FI5307+3252 is ambiguous. Liu, Graham & Wright (1996) 
have recently suggested that it too may be lensed, due to its 
complex morphology on arcsecond scales, although there is no 
other clear evidence for this possibility. 
The high-ionization optical emission-line spectra and the dis-
covery of a strong polarized component to the optical flux suggest 
that the hyperluminous galaxies host active nuclei. As is the case for 
Seyfert 2 galaxies, some of which are similar to hyperluminous 
IRAS galaxies in optical appearance, X-ray observations may be a 
useful discriminant between starburst and active galaxy, especially 
if they enable the detection of either an absorbed active nucleus or a 
*Present address: Harvard-Smithsonian Center for Astrophysics, 60 
Garden Street, Cambridge, MA 02138, USA 
© 1996RAS 
scattered component. In the case of F10214+4724, Lawrence et al. 
(1993) made only a marginal X-ray detection. We have begun a 
programme to observe other hyperluminous galaxies in X-rays. A 
source coincident with P09104+4109 was detected with ASCA 
(Fabian et al. 1994) and resolved with the ROSAT HRI (Fabian & 
Crawford 1995). Most of the X-ray emission from that object comes 
from a surrounding cluster. Here we report on a ROSAT HRI 
observation of FI5307+3252. 
2 THE ROSAT HRI IMAGE OF F15307+3252 
IRAS FI5307+3252 was observed with the ROSAT (friimper 1983) 
High Resolution Imager (HRI) (David et al. 1995) for 20749 s 
during 1995 February 17-19. No source is detected at the position 
of FI5307+3252 with a 3<1 upper limit of 3 x 10-4 counts-1• A 
weak source at ISh 32m 44",320 42' 33", J2000 (with a count rate of 
4 x 10-4 count s-l and a probability of 0.0016 = 3.2<1 of being a 
chance fluctuation in the background), lies about 13 arcsec south of 
FI5307+3252 (Fig. 1). The upper limit at the position of 
FI5307+3252 (determined as 3<1 of the background predicted at 
that position) corresponds to a source flux in the 0.1-2.4keV band 
of8 x 10-15 ergcm-2 s-l or a luminosity of3.5 x 1043 erg S-l at the 
redshift of 0.93, assuming a power-law continuum with a photon 
index of 2, the Galactic column density of 2.1 x 1<fO cm-2, 
Ho = 50kms-1 Mpc-l and qo = 0.5. 
A moderately bright source in the field, at ISh 31m 59!'2, +320 48' 
25:'7 (Jiooo) with 4 x 10-3 count S-l, falls within 3 arcsec of the 
centroid of a bright object on the Digitized Sky Survey (Fig. 2). We 
therefore conclude that the pointing of ROSAT was good to that 
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.." "~ ~:.:~ 
>::' .. :<. 
; . 
;.:-:- . . :
.s. ._ 
.... 
50" 
40" 
320 42' 30" 
445 
Right Ascension (2000) 
Figure 1. Contours of the adaptively smoothed ROSAT HRI image around F15307+3252 (bright object with double structure near the centre) overlaid on a 
K,,-band image from the Keck Telescope. The contours are linearly spaced from 66 to 100 per cent of the peak flux (see text for normalization). 
accuracy. This object does not appear to be catalogued by either 
NED or SIMBAD. 
The weak: X-ray source near F15307+3252 lies close to a clump 
offaint objects seen in aK,.-band mosaic image. This image, which 
was obtained in moderate seeing (FWHM 1.0 arcsec) with the Near 
IR Camera at the Keck Telescope on 1995 April 9, is the average of 
15 shifted and registered 60-s exposures. Although it is centred 
slightly south of the near-infrared image presented by tiu et al .. 
(1996), our image is otherwise consistent with their data. The X-ray 
image in Fig. 1 was originally binned on a scale of 2 arcsec, then 
adaptively smoothed for 9 counts per bin using the algorithm of 
Ebeling, White & Rangarajan (1996). The appearance of the X-ray 
source suggests that it might be extended, although it is too weak: to 
quantify any extent. Given the proximity of the X-ray source to the 
faintK,.-band objects, it is plausible that the core of a poor cluster of 
galaxies lies about 13 arcsec south of F15307+3252. If this is the 
case then the X-ray luminosity of the cluster is at least 
5 x 1043 erg S-l if at the redshift of F15307+3252, and less, of 
course, at lower redshifts (the upward correction factor if the source 
is extended is probably 2-3). 
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X-ray emission from the field oflRAS F15307+3252 L97 
40" I······ .. ·············· .. ········ .. ····· .. ·············i·········· ...................................... _ .................. ···········f·· .. ···_·_······················ .. ··········'': 
35" 1····· .. ···••· .. ·············••·••··· .. •·•··• .. ·······+··· ................... _ ........ _ ............ _ ................. "-.. ···If .. ·• .. •· .. ························ .. ···•············ .. ························+············1 
6" 30" ~ 
§ 
';:1 
~ 25" 
a3 
0 
20" 
15" t····· .. ············ .. ·····························+···· ................................. _ ............... _ ............. ·······-il····· .... ·-··································································+-·········· .. 1 
32°48'10"r-----------~------------~~~r------------------r--1 
59' 
Right Ascension (2000) 
Figure 2. Contours of the brightest source in the ROSAT HRI image overlaid on the Digiti-red Sky Survey (copyright National Geographic Society). 
3 DISCUSSION 
Our ROSAT limit of -4 x 1043 ergs-1 to the (0.1-2.0keV) X-ray 
luminosity of F15307+3252 is about 4 times lower than the 2-
lOkeV limit from ASCA (Ogasaka et al. 1996). The bolometric 
luminosity of the object -1.6 x 1047 ergs-I, making it among the 
most luminous galaxies known (Cutri I;t al. 1994). The 0.1-2.4 ke V 
luminosity is less than 2 x 10-4 of the bolometric luminosity. 
Superficially this is as expected if the object is a Type IT quasar 
(i.e. a direct line of sight to the nucleus is blocked for us by a high, 
X-ray absorbing, column density of matter which is Thomson thick, 
NH > 1024 cm-2). 
F15307+3252 is polarized in the (observed) optical band at the 
level of 16 per cent and shows broad Mg II in the polarized 
continuum, indicating the presence of an active nucleus (Hines et 
al. 1995). The polarized component is accounted for by scattering. 
If the scattering medium consists of free electrons then we expect a 
significant scattered X-ray flux. Using the quasars common to the 
bolometric luminosity LBol list of Padovani & Rafenelli (1988) and 
the 0.3-3.5 keY X-ray luminosity Lx list of Wilkes & Elvis (1987), 
we find a mean ratio for Lx/LBol of 4.8 per cent, with a range from 
2.4"to 7.3 per cent (3C273 is close to the mean at 4.9 per cent). 
Adopting 5 per cent, our ROSAT limit implies that less than 0.4 per 
cent of the X-ray luminosity of the nucleus is scattered into our line 
of sight. This is considerably below the 10 per cent scattering 
fraction in the optical band inferred by Hines et al. (1995) from 
considerations of the equivalent widths of the narrow emission lines 
seen in F15307+3252 and the strength of the scattered nuclear 
continuum. 
© 1996 RAS, MNRAS 283, L95-L98 
This mismatch leads us to conclude that either the nucleus emits 
less than about 0.2 per cent of its power in X-rays or the scattering 
mirror is composed of dust (which does not scatter X-rays through 
large angles), not electrons. We note that there must then be little 
ionized gas capable of scattering X-rays into our line of sight, which 
is surprising for such a powerful object. If the result is general, then 
it may be difficult to detect any Thomson-thick Type IT quasars in 
the X-ray band. 
Alternatively, the unusually weak X-ray flux can be explained if 
the active nucleus accounts for only about 5 per cent of the 
bolometric flux. This is doubtful, given the optical magnitude and 
polarized fraction ofF15307+3252, and the scaling of the polarized 
optical spectrum carried out by Hines et al. (1995). 
From deep counts of sources in high Galactic latitude fields 
(e.g. Barcons et al. 1995), we estimate that there are about 100 
sources deg-2 at the level of the weak X-ray source to the south 
of F15307+3252. The probability that it lies by chance within 13 
arcsec of F15307+3252 is then about 0.4 per cent, and even less 
that it lies so close to the cluster. It is therefore likely that the X-
ray source is in some way related to both: perhaps it is the core 
of a cluster, of which F15307+3252 is a member, or it is a 
foreground cluster which has some gravitational lensing effect 
on F15307+3252. It is most unlikely from the richness inferred 
from its X-ray luminosity that the cluster potential is the sole 
lensing object; the morphology' of F15307+3252 is not obviously 
similar to that of an object lensed by a cluster core (tiu et al. 
1996). 
We note that our strong limit on X-rays from immediately around 
F15307+3252 rules out it being at the centre of a cluster, as is the 
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case for P09104+4109. Also it argues against the host object being a 
cD galaxy - at the present epoch these are generally found in cluster 
cores. Where they occur in poor clusters there is still a moderate 
cluster X-ray luminosity (e.g. MKW3s). 
4 CONCLUSIONS 
Along our line of sight, the quasar nucleus of F15307+3252 emits 
in X-rays less than 0.02 per cent of the bolometric luminosity of 
F15307+3252 and, through scattering, about 10 per cent of its 
optical luminosity. This indicates that either the scattering 
medium is dust, not electrons, or the quasar is anomalously 
weak: in X-rays. 
ACKNOWLEDGMENTS 
ACF thanks the Royal Society for support, and esc acknowledges 
support from the PP ARC. RMC acknowledges the support of the Jet 
Propulsion Laboratory, Caltech, which is operated under contract 
with NASA WNB thanks the United States National Science 
Foundation. We acknowledge the use of SkyView at Goddard 
Space Science Center. 
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X-ray emission from the field of the hyperluminous IRAS galaxy IRASF15307+3252

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X-ray emission from the field of the hyperluminous IRAS galaxy IRASF15307+3252

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