The newly discovered z=5.8 quasar SDSSp J104433.04-012502.2 was recently
detected in X-rays and found to be extremely X-ray weak. Here we present the
hardness ratio analysis of the XMM-Newton observation. We consider various
models to explain the detection in the soft X-ray band and non-detection in the
hard band, together with its X-ray weakness. We show that the source may have a
steep power-law slope, with an absorber partially covering the continuum. This
may be X-ray evidence to support the argument of Mathur (2000) that narrow line
Seyfert 1 galaxies, which show steep power-law slopes, might be the low
redshift, low luminosity analogues of the high redshift quasars. Heavily
shrouded and steep X-ray spectrum quasars may indeed represent the early stages
of quasar evolution (Mathur 2000, Fabian 1999) and SDSSp J104433.04-012502.2 is
possibly giving us a first glimpse of the physical evolution of quasar
properties.Comment: To appear in A

Brandt W. N.

Maiolino R.

Smita Mathur

The Astronomical Journal

English

arXiv

Crossref

The [ITAL][CLC]z[/CLC][/ITAL] = 5.8 Quasar [CLC]SDSSp[/CLC] J1044−0125: A Peek at Quasar Evolution?

The newly discovered z = 5.8 quasar SDSSp J104433.04-012502.2 was recently detected in X-rays and found to be extremely X-ray weak. Here we present the hardness ratio analysis of the XMM-Newton observation. We consider various models to explain the detection in the soft X-ray band and nondetection in the hard band, together with its X-ray weakness. We show that the source may have a steep power-law slope, with an absorber partially covering the continuum. This may be X-ray evidence to support the recent argument of Mathur that narrow-line Seyfert 1 galaxies, which show steep power-law slopes, might be the low-redshift, low-luminosity analogs of the high-redshift quasars. Heavily shrouded and steep X-ray spectrum quasars may indeed represent the early stages of quasar evolution, and SDSSp J104433.04-012502.2 is possibly giving us a first glimpse of the physical evolution of quasar properties

Mathur, Smita

KnowledgeBank at OSU

THE ASTRONOMICAL JOURNAL, 122 :1688È1691, 2001 October( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.THE z\ 5.8 QUASAR SDSSp J1044[0125 : A PEEK AT QUASAR EVOLUTION?SMITA MATHURDepartment of Astronomy, 140 West 18th Avenue, Ohio State University, Columbus, OH 43210-1173 ;smita=astronomy.ohio-state.eduReceived 2001 May 30; accepted 2001 July 9ABSTRACTThe newly discovered z\ 5.8 quasar SDSSp J104433.04[012502.2 was recently detected in X-raysand found to be extremely X-ray weak. Here we present the hardness ratio analysis of the XMM-Newton observation. We consider various models to explain the detection in the soft X-ray band andnondetection in the hard band, together with its X-ray weakness. We show that the source may have asteep power-law slope, with an absorber partially covering the continuum. This may be X-ray evidenceto support the recent argument of Mathur that narrow-line Seyfert 1 galaxies, which show steep power-law slopes, might be the low-redshift, low-luminosity analogs of the high-redshift quasars. Heavilyshrouded and steep X-ray spectrum quasars may indeed represent the early stages of quasar evolution,and SDSSp J104433.04[012502.2 is possibly giving us a Ðrst glimpse of the physical evolution of quasarproperties.Key words : galaxies : active È galaxies : evolution È galaxies : nuclei È quasars : general Èquasars : individual (SDSSp J104433.04[012502.2) È X-rays1. INTRODUCTIONThe discovery of a quasar at redshift z\ 5.8 (Fan et al.2000) was remarkable as it showed that luminous quasarswith massive black holes had formed when the universewas less than 1 Gyr old. The quasar SDSSpJ104433.04[012502.2 (hereafter SDSS 1044[0125) wasdiscovered as part of the Sloan Digital Sky Survey imagingmulticolor observations. The follow-up spectrum with Keckrevealed a rich spectrum of a quasar at z\ 5.8 with strongLya forest. Brandt et al. (2001, hereafter Paper I) thenobserved the quasar in X-rays with XMM-Newton. Thequasar was clearly detected but found to be unusually X-rayweak. The source was too faint to do meaningful spectralanalysis.To extract spectral information from only a few tens ofcounts, X-ray astronomers have traditionally used a hard-ness ratio analysis (Maccacaro et al. 1988). The observedphotons are divided into two energy bands, soft (S) andhard (H), for low energy and high energy, respectively. Thehardness ratio HR \ H[S/H]S then gives a measure ofthe shape of the X-ray spectrum. A more sophisticatedversion of the hardness ratio method has been used byvarious authors to extract valuable information on high-redshift quasars : Bechtold et al. (1994) found that the radio-quiet high-redshift quasars either have steeper power-lawslopes and large absorbing columns or no absorption andpower-law slopes similar to their lower redshift cousins.Based on a detection with Einstein, Mathur & Elvis (1995)concluded that the high-redshift radio-loud quasar GB1508]5714 either has an unusually hard power-law slopeor is highly absorbed. Later observations have conÐrmedthe unusually hard slopes of high-redshift radio-loudquasars (e.g., Moran & Helfand 1997 ; Reeves et al. 2001).The hardness ratio analysis thus provides us with a power-ful tool to extract spectral information when the totalobserved counts are too small to perform detailed spectralanalysis.In this paper we perform the hardness ratio analysis onthe highest redshift quasar, SDSS 1044[0125, to expand onthe work already reported in Paper I.2. XMM OBSERVATIONS OF SDSS 1044[0125SDSS 1044[0125 was detected with XMM EPIC-pn.The total counts observed in the 0.5È2.0 keV band were32 ^ 9. This corresponds to the rest-frame energy range3.4È13.6 keV (see Paper I for the details of observation).Based on this observed count rate, SDSS 1044[0125 isunusually X-ray weak ; it has about a factor of 10 smallerX-ray Ñux for its optical observed Ñux relative to otherluminous optically selected quasars (Paper I). These obser-vations imply that either (1) the object is intrinsically X-rayweak, and so is a very unusual quasar, or (2) it is a normalquasar that is heavily absorbed, with a column densitycm~2, as seen in some broad absorption lineNH Z 1024quasars (BALQSOs). Brandt et al. have discussed both ofthese possibilities, though the latter was their preferredinterpretation.Note also that SDSS 1044[0125 was not detected in the2.0È7.0 keV band (rest-frame energy range 13.6È47.6 keV).Since hard X-rays are less a†ected by photoelectric absorp-tion, the nondetection in the hard band was rather sur-prising if heavy absorption was indeed the cause of X-rayweakness of the object.1 In most BALQSOs, for example,X-ray detections are preferentially in the hard band(Mathur et al. 2000 and references therein). Another pointto note is that the low spectral resolution X-ray obser-vations cannot determine the redshift of the absorber.Absorption at lower redshift with smaller column densitymay mimic the absorption at high redshift with largercolumn density of gas. Given that SDSS 1044[0125 is atvery high redshift and that there is a lot of absorption alongthe line of sight, including that due to a Lyman limit systemand a Mg II system (Fan et al. 2000), the X-ray absorptionmay take place in the intervening systems. To study thesedi†erent possibilities and to extract further spectral infor-mation, we perform a hardness ratio analysis of SDSS1044[0125.ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ1 We note that Brandt et al. have mentioned that perhaps partial cover-ing of the X-ray continuum source may explain the photons detected atkeV.[81688QUASAR SDSSp J1044[0125 16892.1. Hardness Ratio AnalysisFor SDSS 1044[0125, the total counts detected in thesoft band are 31.7 ^ 8.5, and the upper limit in the hardband is 12.3 counts (95% conÐdence ; Paper I). This resultsin the hardness ratio HR \ [0.44. The maximum HR(assuming minimum soft counts, [1 p) is [0.31. In Figure1 we have plotted the expected HR as a function of theX-ray power-law slope, ! (1] a for for a range offlP l~a),absorbing column densities. We have used the 0.5È2.0 keVrange for the soft band and the 2.0È7.0 keV range for thehard band as in Brandt et al. All the HR calculations wereperformed using the Portable, Interactive, Multi-MissionSimulator (PIMMS, Version 3.0 ; Mukai 2000), and XSPEC(Arnaud 1996) was used to deÐne the spectral models (seePaper I for the appropriateness of using PIMMS). Thelowest curve corresponds to Galactic column density of4.6] 1020 cm~2 toward SDSS 1044[0125, the next curvecorresponds to column density 10 times Galactic, and thetop one corresponds to column density of 1.7 ] 1022 cm~2.These are all column densities at z\ 0.0. The highestcolumn density plotted in Figure 1 is that required for thesuppression of the soft Ñux by a factor of 10 and wouldcorrespond to cm~2 at z\ 5.8. If the quasar is notZ1024intrinsically X-ray faint, then this much absorption isrequired to suppress the total Ñux to the observed value.What is immediately apparent from Figure 1 is that therequired excess absorption is inconsistent with the observedhardness ratio for any reasonable value of !.What, then, is the possible spectral shape of SDSS1044[0125 consistent with the observed hardness ratio? Asimple power law with just Galactic absorption and slope inFIG. 1.ÈCalculated hardness ratio plotted as a function of X-raypower-law photon index for a range of absorbing column densities atz\ 0. The horizontal line represents the observed upper limit for SDSS1044[0125. The top curve corresponds to the column density required tosuppress the soft X-ray Ñux by a factor of 10. This model is inconsistentwith observed HR for any reasonable value !. The observed HR is consis-tent with only Galactic absorption, but that will make the quasarextremely X-ray weak.the normal range !B 2 is consistent with observations(also noted by Brandt et al.). This, however, makes thequasar unusually X-ray weak. There are a few ways to makethe heavy absorption scenario consistent with the observedhardness ratio : (1) a high-energy cuto† in the observedband (this will bring down the number of hard X-rayphotons), (2) a completely Compton thick X-ray absorbersuppressing both soft and hard X-ray Ñux, but partiallycovering the continuum source and leaking some softphotons, and (3) a partially covering absorber and a steepX-ray spectrum. Below we consider each of these pos-sibilities in turn. One immediate conclusion of the aboveexercise is that the absorber must be close to the continuumsource for partial covering to work. So it is highly unlikelythat the X-ray absorption is caused by the interveningmatter.2.1.1. High-Energy Cuto†High-energy cuto†s to the power-law spectra areexpected from the thermal Comptonization models and arerequired to explain the cosmic hard X-ray background.There is no universal value for a cuto† energy, but generallyit is assumed to be B270 keV (e.g., Matt 1998). This is toohigh to have any e†ect on the observed hard band (rest-frame energy range 13.6È47.6 keV). To our knowledge, thelowest value of high-energy cuto† observed so far is that forthe highly absorbed maser source ESO 103-G35 (Wilkes etal. 2001). It may have a cuto† energy as low as 30 ^ 10 keV.We calculated hardness ratios for cuto† energies of 20, 30,and 40 keV for !\ 2.0 and the e-folding energy of 20 keV.The resulting HR values are ]0.22, ]0.31, and ]0.34,respectively. There values are well above the observed HRfor SDSS 1044[0125. We then repeated the above experi-ment for steeper power-law slopes. The results indicate thatif the high-energy cuto† model is to account for theobserved HR, then the cuto† energy has to be smaller than20 keV and ! has to be steeper than 3. This, again, isextremely unusual.2.1.2. Compton T hick Absorber with Partial CoveringIf an absorber near the source is completely Comptonthick (say, cm~2), then the continuum is com-NH \ 1025pletely suppressed, even in the high-energy band. Thishappens because the high-energy photons are down-scattered to lower energy, at which they are e†ectivelyabsorbed by photoelectric absorption (Matt 1998). If suchan absorber does not cover the source completely, with 10%of the continuum leaking through, then the observed X-rayÑux is naturally 1/10 of the intrinsic Ñux. This can happeneither as a result of the geometry of the absorber or as aresult of scattering toward our line of sight. Such a scenariowith a normal power-law slope of !\ 2 then explains theobservations, including the hardness ratio (° 2.1). We haveveriÐed with explicit simulations that indeed for a modelwith !\ 2 and for an absorber with 90% covering, acolumn density of as much as cm~2 is requiredNH \ 1025to suppress the total Ñux by a factor of 10.Such a model, however, has other consequences. Theabsorber in this case is not only thick to Compton scat-tering, it is also thick to Thomson scattering. Thomsonscattering is achromatic (up to and would result inmec2)attenuation at all wavelengths, including optical and UV(unless the absorber covers the whole sky as seen by thesource). For the column density quoted above, the Thomsonoptical depth, with attenuation factor ofqTh \ 6.65,1690 MATHUR Vol. 122773. Thus the source would be heavily attenuated, not onlyin the X-rays, but also in UV. This is not unusual. Broadabsorption line quasars, which show strong X-ray absorp-tion, also su†er from attenuation in the opticalÈUV by anamount appropriate for their observed column densities(e.g., Mathur et al. 2000 and references therein). In NGC1068, a classic example of Seyfert 2 galaxies, in which theX-ray absorber is believed to be Compton thick (e.g.,Guainazzi, Matt, & Fiore 2000), the optical and UV contin-uum is also completely obscured.On the other hand, SDSS 1044[0125 is luminous inoptical (rest-frame UV). Even at the observed Ñux levels, theluminosity of the object requires a black hole (BH) of3 ] 109 accreting at the Eddington rate (Fan et al.M_2000). If the intrinsic luminosity of the source were signiÐ-cantly higher, then it would push the required BH mass tosigniÐcantly higher values. In the framework of hierarchicalmodels of structure formation, Haiman & Loeb (2001) havecalculated the maximum allowed BH mass as a function ofredshift that could be found in a given survey. At z\ 5.8and BH mass of 3 ] 109 SDSS 1044[0125 is alreadyM_,at a model limit (see their Fig. 3). While not impossible, anincrease of orders of magnitude in the implied BH masswould be stretching the limits. Moreover, if the observedoptical Ñux of SDSS 1044[0125 is also signiÐcantly attenu-ated, then it does not solve the problem of X-ray weaknessof the object.One way out of this problem is to place the Comptonthick X-ray absorber outside the X-rayÈemitting region butinside the region emitting UV. This would be extremelydifficult given that the X-rayÈUV emission arises from a fewto a few tens of Schwarzschild radii away from the nuclearblack hole, and especially if UV photons provide the seedpopulation for Compton up-scattering to produce X-rays(e.g., Nandra 2001).2.1.3. Steep X-Ray Spectrum and a Partially Covering AbsorberTo alleviate the above problem, we considered a model inwhich the X-ray continuum of the source has a steep spec-trum and, again, a partially covering absorber along ourline of sight. A steeper continuum would reduce the hardX-ray Ñux for a given normalization, while the absorberwould suppress the soft X-ray Ñux. The required columndensity of the absorber then may not be excessive. Such amodel is physically motivated. SDSS 1044[0125 seems tobe a broad absorption line quasar (Maiolino et al. 2001).There is some evidence that the X-ray power-law slopes ofBALQSOs may be steep (Mathur et al. 2001 ; although seeGreen et al. 2001).So we constructed a model with !\ 2.5 and the coveringfactor of the absorber 90%. We then varied the columndensity of the absorber from to 1025 cm~2. InNH \ 1022each case we noted the factor by which the intrinsic Ñux wassuppressed and calculated the hardness ratio. A factor of 10smaller Ñux (compared with an unobscured source with!\ 2) is obtained for cm~2. The hardnessNH D 7 ] 1023ratio in this case is between [0.47 and [0.58, consistentwith the observed value. Thus such a model satisÐes con-straints from both Ñux and HR. The implied Thomsonopacity in this case is B0.5, resulting in no signiÐcantattenuation in opticalÈUV. The derived column density isalso consistent with that observed in other BALQSOs(Mathur et al. 2000, 2001 ; Green et al. 2001 ; Gallagher et al.2001).3. DISCUSSIONWe are aware that only 33 counts were detected in theXMM observation of SDSS 1044[0125. Trying to extractmuch more information from this observation might seemlike overinterpreting the data. However, the hardness ratioanalysis has been used in situations like this. This paper notonly shows the power of this technique, but it also drawsimportant conclusions from its use.We show that the X-ray properties of this z\ 5.8 quasarare unlikely to be exactly like its low-redshift cousins. It iseither extremely X-ray weak (Paper I) or it has a steeppower-law slope. Since we are looking at an object at thetime when the structures in the universe were very young, itmay be even more of a surprise if it looked exactly likenearby quasars. Quasars must evolve, and the evolution ofquasar luminosity function has been observed. However, westill do not know how quasars evolve. With the observationof SDSS 1044[0125 we might be getting a Ðrst look at theevolution of quasar spectra with time. Heavily shroudedand steep hard X-ray spectrum quasars may represent anearly evolutionary stage (Fabian 1999 ; Mathur 2000).Given the extremely large luminosity of the object (Fan etal. 2000), it is most likely accreting at an Eddington, or evensuper-Eddington rate. The narrow-line Seyfert 1 galaxies(NLS1s), nearby active galactic nuclei believed to be accret-ing at close to the Eddington rate, also have steep hardX-ray spectra (Brandt, Mathur, & Elvis 1997). Mathur(2000) has argued that the NLS1 galaxies might be low-redshift, low-luminosity analogs of the high-redshiftquasars. Here we might be seeing X-ray evidence in supportof that argument. But can accretion onto a 109 blackM_hole produce X-rays with such steep power-law slopes? Itwould challenge current accretion disk theories. Clearly,good X-ray spectra of radio-quiet high-redshift quasars areneeded to conÐrm this hypothesis.It is my pleasure to thank David Weinberg, Niel Brandt,and Giorgio Matt for useful discussions, Koji Mukai forupdating PIMMS Version 3.0, and Giorgio Matt for the useof his Monte Carlo code. This work is supported in part byNASA grant NAG 5-8913 (LTSA).REFERENCESArnaud, K. 1996, in ASP Conference Series 101, Astronomical DataAnalysis Software and Systems V, ed. G. 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T. 1988,ApJ, 326, 680Maiolino, R., Mannucci, F., Ba†a, C., Gennari, S., & Oliva, E. 2001, A&A,371, L5No. 4, 2001 QUASAR SDSSp J1044[0125 1691Mathur, S. 2000, MNRAS, 314, 17Mathur, S., & Elvis, M. 1995, AJ, 110, 1551Mathur, S., et al. 2000, ApJ, 533, L79Mathur, S., Matt, G., Green, P. J., Elvis, M., & Singh, K. P. 2001, ApJ, 551,L13Matt, G. 1998, preprint (astro-ph/9811053)Moran, E., & Helfand, D. 1997, ApJ, 484, L95Mukai, K. 2000, PIMMS UsersÏ Guide, Version 3.0 (Greenbelt : NASA)Nandra, K. 2001, in ASP Conf. Ser. 224, Probing The Physics of ActiveGalactic Nuclei by Multiwavelength Monitoring, ed. B. M. Peterson,R. S. Polidan, & R. W. Pogge (San Francisco : ASP), 167Reeves, J. N., et al. 2001, A&A, 365, L116Wilkes, B. J., Mathur, S., Fiore, F., Antonelli, A., & Nicastro, F. 2001, ApJ,549, 248

The z = 5.8 Quasar SDSSp J1044−0125: A Peek at Quasar Evolution?

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The z=5.8 Quasar SDSSp J1044-0125: A Peek at Quasar Evolution?

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