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Register a new userX-ray Observations of Luminous Dusty Quasars at z > 2
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We present new X-ray observations of luminous heavily dust-reddened quasars (HRQs) selected from infrared sky surveys. HRQs appear to be a dominant population at high redshifts and the highest luminosities, and may be associated with a transitional blowout phase of black hole and galaxy co-evolution models. Despite this, their high-energy properties have been poorly known. We use the overall sample of $10$ objects with XMM-Newton coverage to study the high-energy properties of HRQs at $left< L_{rm bol} right> = 10^{47.5}$ erg/s and $left< z right>= 2.5$. For the seven sources with strong X-ray detections, we perform spectral analyses. These find a median X-ray luminosity of $left< L_{rm 2-10,keV} right> = 10^{45.1}$ erg/s, comparable to the most powerful X-ray quasars known. The gas column densities are $N_{rm H}=(1$-$8)times 10^{22}$ cm$^{-2}$, in agreement with the amount of dust extinction observed. The dust to gas ratios are sub-Galactic, but are higher than found in local AGN. The intrinsic X-ray luminosities of HRQs are weak compared to the mid-infrared ($L_{rm 6mu m}$) and bolometric luminosities ($L_{rm bol}$), in agreement with findings for other luminous quasar samples. For instance, the X-ray to bolometric corrections range from $kappa_{rm bol}approx 50$-$3000$. The moderate absorption levels and accretion rates close to the Eddington limit ($left< lambda_{rm Edd} right>=1.06$) are in agreement with a quasar blowout phase. Indeed, we find that the HRQs lie in the forbidden region of the $N_{rm H}$-$lambda_{rm Edd}$ plane, and therefore that radiation pressure feedback on the dusty interstellar medium may be driving a phase of blowout that has been ongoing for a few $10^{5}$ years. The wider properties, including [OIII] narrow-line region kinematics, broadly agree with this interpretation.
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We present NuSTAR hard X-ray (3-79 keV) observations of three Type 2 quasars at z ~ 0.4-0.5, optically selected from the Sloan Digital Sky Survey (SDSS). Although the quasars show evidence for being heavily obscured Compton-thick systems on the basis of the 2-10 keV to [OIII] luminosity ratio and multiwavelength diagnostics, their X-ray absorbing column densities (N_H) are poorly known. In this analysis: (1) we study X-ray emission at >10 keV, where X-rays from the central black hole are relatively unabsorbed, in order to better constrain N_H; (2) we further characterize the physical properties of the sources through broad-band near-UV to mid-IR spectral energy distribution (SED) analyses. One of the quasars is detected with NuSTAR at >8 keV with a no-source probability of <0.1%, and its X-ray band ratio suggests near Compton-thick absorption with N_H gtrsim 5 x 10^23 cm^-2. The other two quasars are undetected, and have low X-ray to mid-IR luminosity ratios in both the low energy (2-10 keV) and high energy (10-40 keV) X-ray regimes that are consistent with extreme, Compton-thick absorption (N_H gtrsim 10^24 cm^-2). We find that for quasars at z ~ 0.5, NuSTAR provides a significant improvement compared to lower energy (<10 keV) Chandra and XMM-Newton observations alone, as higher column densities can now be directly constrained.
We report on the second installment of an X-ray monitoring project of seven luminous radio-quiet quasars (RQQs). New {sl Chandra} observations of four of these, at $4.10leq zleq4.35$, yield a total of six X-ray epochs, per source, with temporal baselines of $sim850-1600$ days in the rest frame. These data provide the best X-ray light curves for RQQs at $z>4$, to date, enabling qualitative investigations of the X-ray variability behavior of such sources for the first time. On average, these sources follow the trend of decreasing variability amplitude with increasing luminosity, and there is no evidence for X-ray variability increasing toward higher redshifts, in contrast with earlier predictions of potential evolutionary scenarios. An ensemble variability structure function reveals that their variability level remains relatively flat across $approx20 - 1000$ days in the rest frame and it is generally lower than that of three similarly luminous RQQs at $1.33leq zleq 2.74$ over the same temporal range. We discuss possible explanations for the increased variability of the lower-redshift subsample and, in particular, whether higher accretion rates play a leading role. Near-simultaneous optical monitoring of the sources at $4.10leq zleq 4.35$ indicates that none is variable on $approx1$-day timescales, although flux variations of up to $sim25$% are observed on $approx100$-day timescales, typical of RQQs at similar redshifts. Significant optical-X-ray spectral slope variations observed in two of these sources are consistent with the levels observed in luminous RQQs and are dominated by X-ray variations.
We present results from a 20 ks XMM-Newton DDT observation of the radio-load quasar CFHQS J142952+544717 at z=6.18, whose extreme X-ray luminosity was recently revealed by the SRG/eROSITA telescope in the course of its first all-sky survey. The quasar has been confidently detected with a total of $sim 1400$ net counts in the 0.2-10 keV energy band (1.4 to 72 keV in the objects rest frame). Its measured spectrum is unusually soft and can be described by an absorbed power-law model with a photon index of $Gamma = 2.5pm0.2$. There are no signs of a high-energy cutoff or reflected component, with an 90 % upper limit on the fluorescence iron K$alpha$ equivalent width of $approx 290$ eV and the corresponding upper limit on the iron K-edge absorption depth of 0.6. We have detected, at the $> 95%$ confidence level, an excess absorption above the Galactic value, corresponding to a column density $N_H= 3pm2 times 10^{22}$ cm$^{-2}$ of material located at z=6.18. The intrinsic luminosity of CFHQS J142952+544717 in the 1.4 to 72 keV energy band is found to be $5.5_{-0.6}^{+0.8} times 10^{46}$ erg s$^{-1}$. We did not detect statistically significant flux changes between two SRG scans and the XMM-Newton observation, spanning over $sim 7.5$ months, implying that the quasar remained at this extremely high luminosity level for at least a month in its rest frame. We put forward the hypothesis that the extreme X-ray properties of CFHQS J142952+544717 are associated with inverse Compton scattering of cosmic microwave background photons (at z=6.18) in its relativistic jets.
We investigate the relation between star formation rates ($dot{M}_{s}$) and AGN properties in optically selected type 1 quasars at $2<z<3$ using data from Herschel and the SDSS. We find that $dot{rm{M}}_s$ remains approximately constant with redshift, at $300pm100~rm{M}_{odot}$yr$^{-1}$. Conversely, $dot{rm{M}}_s$ increases with AGN luminosity, up to a maximum of $sim600~rm{M}_{odot}$yr$^{-1}$, and with CIV FWHM. In context with previous results, this is consistent with a relation between $dot{rm{M}}_s$ and black hole accretion rate ($dot{rm{M}}_{bh}$) existing in only parts of the $z-dot{rm{M}}_{s}-dot{rm{M}}_{bh}$ plane, dependent on the free gas fraction, the trigger for activity, and the processes that may quench star formation. The relations between $dot{rm{M}}_s$ and both AGN luminosity and CIV FWHM are consistent with star formation rates in quasars scaling with black hole mass, though we cannot rule out a separate relation with black hole accretion rate. Star formation rates are observed to decline with increasing CIV equivalent width. This decline can be partially explained via the Baldwin effect, but may have an additional contribution from one or more of three factors; $M_i$ is not a linear tracer of L$_{2500}$, the Baldwin effect changes form at high AGN luminosities, and high CIV EW values signpost a change in the relation between $dot{rm{M}}_s$ and $dot{rm{M}}_{bh}$. Finally, there is no strong relation between $dot{rm{M}}_s$ and Eddington ratio, or the asymmetry of the CIV line. The former suggests that star formation rates do not scale with how efficiently the black hole is accreting, while the latter is consistent with CIV asymmetries arising from orientation effects.
SRGE J170245.3+130104 was discovered by the eROSITA telescope aboard the SRG space observatory on March 13-15, 2020 during the first half-year scan of its all-sky X-ray survey. The optical counterpart of the X-ray source was photometrically identified as a distant quasar candidate at $zapprox5.5$. Follow-up spectroscopic observations, done in August/September 2020 with the SCORPIO-II instrument at the BTA 6-m telescope, confirmed that SRGE J170245.3+130104 is a distant quasar at redshift z=5.466. The X-ray luminosity of the quasar during the first half-year scan of the eROSITA all-sky survey was $3.6^{+2.1}_{-1.5}times 10^{46}$ erg/s (in the 2-10 keV energy range), whereas its X-ray spectrum could be described by a power law with a slope of $Gamma=1.8^{+0.9}_{-0.8}$. Six months later (September 13-14, 2020), during the second half-year scan of the eROSITA all-sky survey, the quasar was detected again and its X-ray luminosity had decreased by a factor of 2 (at the $approx 1.9sigma$ confidence level). The SRGE J170245.3+130104 proves to be the most X-ray luminous among all known X-ray quasars at $z>5$. It is also one of the radio-loudest distant quasars (with radio-loudness $Rsim10^3$), which may imply that it is a blazar. In the Appendix, we present the list of all $z>5$ quasars detected in X-rays to date.
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