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Register a new userOn the Fraction of X-ray Weak Quasars from the Sloan Digital Sky Survey
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We investigate systematically the X-ray emission from type 1 quasars using a sample of 1825 Sloan Digital Sky Survey (SDSS) non-broad absorption line (non-BAL) quasars with Chandra archival observations. A significant correlation is found between the X-ray-to-optical power-law slope parameter ($alpha_{rm OX}$) and the 2500 $r{A}$ monochromatic luminosity ($L_{rm 2500~r{A}}$), and the X-ray weakness of a quasar is assessed via the deviation of its $alpha_{rm OX}$ value from that expected from this relation. We demonstrate the existence of a population of non-BAL X-ray weak quasars, and the fractions of quasars that are X-ray weak by factors of $ge6$ and $ge10$ are $5.8pm0.7%$ and $2.7pm0.5%$, respectively. We classify the X-ray weak quasars (X-ray weak by factors of $ge6$) into three categories based on their optical spectral features: weak emission-line quasars (WLQs; CIV REW $<16~r{A}$), red quasars ($Delta(g-i)>0.2$), and unclassified X-ray weak quasars. The X-ray weak fraction of $35_{- 9}^{+12}%$ within the WLQ population is significantly higher than that within non-WLQs, confirming previous findings that WLQs represent one population of X-ray weak quasars. The X-ray weak fraction of $13_{- 3}^{+ 5}%$ within the red quasar population is also considerably higher than that within the normal quasar population. The unclassified X-ray weak quasars do not have unusual optical spectral features, and their X-ray weakness may be mainly related to quasar X-ray variability.
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We perform a systematic search for long-term extreme variability quasars (EVQs) in the overlapping Sloan Digital Sky Survey (SDSS) and 3-Year Dark Energy Survey (DES) imaging, which provide light curves spanning more than 15 years. We identified ~1000 EVQs with a maximum g band magnitude change of more than 1 mag over this period, about 10% of all quasars searched. The EVQs have L_bol~10^45-10^47 erg/s and L/L_Edd~0.01-1. Accounting for selection effects, we estimate an intrinsic EVQ fraction of ~30-50% among all g<~22 quasars over a baseline of ~15 years. These EVQs are good candidates for so-called changing-look quasars, where a spectral transition between the two types of quasars (broad-line and narrow-line) is observed between the dim and bright states. We performed detailed multi-wavelength, spectral and variability analyses for the EVQs and compared to their parent quasar sample. We found that EVQs are distinct from a control sample of quasars matched in redshift and optical luminosity: (1) their UV broad emission lines have larger equivalent widths; (2) their Eddington ratios are systematically lower; and (3) they are more variable on all timescales. The intrinsic difference in quasar properties for EVQs suggest that internal processes associated with accretion are the main driver for the observed extreme long-term variability. However, despite their different properties, EVQs seem to be in the tail of a continuous distribution of quasar properties, rather than standing out as a distinct population. We speculate that EVQs are normal quasars accreting at relatively low accretion rates, where the accretion flow is more likely to experience instabilities that drive the factor of few changes in flux on multi-year timescales.
We present new and archival Chandra snapshot (10 ks each) observations of 15 optically identified (from the Sloan Digital Sky Survey, SDSS) Type 2 quasars at z=0.40-0.73. When combined with existing X-ray data, this work provides complete X-ray coverage for all 25 radio-quiet Type 2 quasars with logL_[OIII]>9.28 L_sun from Zakamska et al. (2003). Two targets out of 15 were not detected by Chandra and most of the remaining sources are X-ray weak, with nine having less than 10 counts in the 0.5-8keV band. Low-to-moderate quality spectral analysis was limited to three sources, whose properties are consistent with the presence of column densities in the range NH~10^22-10^23 cm^-2 in the source rest frame. If the [OIII] luminosity is a reliable proxy for the intrinsic X-ray luminosity, the current X-ray data indicate that Compton-thick quasars may hide among ~65 per cent of the SDSS Type 2 quasar population (L_{X, meas}/L_{X, [OIII]}<0.01); however, since the Type 2 quasar sample is selected on [OIII] luminosity, the estimated Compton-thick quasar fraction may be overestimated. Using archival Spitzer observations, we find that ~50 per cent of SDSS Type 2 quasars appear to be obscured by Compton-thick material based on both the L_{X, meas}/L_{X, mid-IR} (where mid-IR corresponds to rest-frame 12.3 micron) and L_{X, meas}/L_{X, [OIII]} ratios. We use this information to provide an estimate of the Compton-thick quasar number density at z=0.3-0.8, which we find is in broad agreement with the expectations from X-ray background models.
Utilizing 21 new Chandra observations as well as archival Chandra, ROSAT, and XMM-Newton data, we study the X-ray properties of a representative sample of 59 of the most optically luminous quasars in the Universe (M_i~~-29.3 to -30.2) spanning a redshift range of z~~1.5-4.5. Our full sample consists of 32 quasars from the Sloan Digital Sky Survey (SDSS) Data Release 3 (DR3) quasar catalog, two additional objects in the DR3 area that were missed by the SDSS selection criteria, and 25 comparably luminous quasars at z>~4. This is the largest X-ray study of such luminous quasars to date. By jointly fitting the X-ray spectra of our sample quasars, excluding radio-loud and broad absorption line (BAL) objects, we find a mean X-ray power-law photon index of Gamma=1.92^{+0.09}_{-0.08} and constrain any neutral intrinsic absorbing material to have a mean column density of N_H<~2x10^{21} cm^{-2}. We find, consistent with other studies, that Gamma does not change with redshift, and we constrain the amount of allowed Gamma evolution for the most-luminous quasars. Our sample, excluding radio-loud and BAL quasars, has a mean X-ray-to-optical spectral slope of a_ox=-1.80+/-0.02, as well as no significant evolution of a_ox with redshift. We also comment upon the X-ray properties of a number of notable quasars, including an X-ray weak quasar with several strong narrow absorption-line systems, a mildly radio-loud BAL quasar, and a well-studied gravitationally lensed quasar.
We identified a large sample of radio quasars, including those with complex radio morphology, from the Sloan Digital Sky Survey (SDSS) and the Faint Images of Radio Sky at Twenty-cm (FIRST). Using this sample, we inspect previous radio quasar samples for selection effects resulting from complex radio morphologies and adopting positional coincidence between radio and optical sources alone. We find that 13.0% and 8.1% radio quasars do not show a radio core within 1.2 and 2 arcsecs of their optical position, and thus are missed in such samples. Radio flux is under-estimated by a factor of more than 2 for an additional 8.7% radio quasars. These missing radio extended quasars are more radio loud with a typical radio-to-optical flux ratio namely radio loudness RL >100, and radio power P >10^{25} W/Hz. They account for more than one third of all quasars with RL>100. The color of radio extended quasars tends to be bluer than the radio compact quasars. This suggests that radio extended quasars are more radio powerful sources, e.g., Fanaroff-Riley type 2 (FR-II) sources, rather than the compact ones viewed at larger inclination angles. By comparison with the radio data from the NRAO VLA Sky Survey (NVSS), we find that for sources with total radio flux less than 3 mJy, low surface brightness components tend to be underestimated by FIRST, indicating that lobes in these faint radio sources are still missed.
We employ the Chandra Multiwavelength Project (ChaMP) and the Sloan Digital Sky Survey (SDSS) to study the fraction of X-ray-active galaxies in the field out to z = 0.7. We utilize spectroscopic redshifts from SDSS and ChaMP, as well as photometric redshifts from several SDSS catalogs, to compile a parent sample of more than 100,000 SDSS galaxies and nearly 1,600 Chandra X-ray detections. Detailed ChaMP volume completeness maps allow us to investigate the local fraction of active galactic nuclei (AGN), defined as those objects having broad-band X-ray luminosities L_X (0.5-8 keV) > 10^42 erg s^-1, as a function of absolute optical magnitude, X-ray luminosity, redshift, mass, and host color/morphological type. In five independent samples complete in redshift and i-band absolute magnitude, we determine the field AGN fraction to be between 0.16 +/- 0.06% (for z < 0.125 and -18 > M_i > -20) and 3.80 +/- 0.92% (for z < 0.7 and M_i < -23). We find striking agreement between our ChaMP/SDSS field AGN fraction and the Chandra cluster AGN fraction, for samples restricted to similar redshift and absolute magnitude ranges: 1.19 +/- 0.11% of ChaMP/SDSS field galaxies with 0.05 < z < 0.31 and absolute R-band magnitude more luminous than M_R < -20 are AGN. Our results are also broadly consistent with measures of the field AGN fraction in narrow, deep fields, though differences in the optical selection criteria, redshift coverage, and possible cosmic variance between fields introduce larger uncertainties in these comparisons.
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