- ID:
- ivo://CDS.VizieR/J/A+A/466/823
- Title:
- Hard 2-10kev X-ray selected sources
- Short Name:
- J/A+A/466/823
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present Chandra and Spitzer data for the 186, extragalactic, hard 2-10keV X-ray selected sources, which lie in the central part of the Chandra Deep Field South (CDFS). For the vast majority of sources (99.5%), there is a spectroscopic or photometric redshift available. We classify 17 sources as X-ray obscured QSOs, strictly according to X-ray criteria, i.e. defined as having large hydrogen column densities (NH>10^22cm^-2^) and luminosities (LX>10^44^erg/s). The surface density of X-ray obscured QSOs is ~210{deg}^-2^. We find 18 candidate Compton-thick NH>10^24^cm^-2^ sources, of which three have QSO luminosities (LX>10^44^erg/s). The X-ray obscured QSO comprise a mixed bag of objects, covering the redshift range z=1.3-4.3. Eight of these show narrow-line optical spectra, two show no obscuration in their optical spectra that present broad lines, while for the other seven there is only a photometric redshift available. About half of the X-ray obscured QSOs show high X-ray to optical flux ratios, X/O>1, and red colours, I-3.6{mu}m>4. Combining the X-ray with the mid-IR 8{mu}m or 24{mu}m flux can be used as an additional diagnostic to sift out the heavily obscured AGN. All X-ray selected QSOs present red mid-IR colours and can be easily separated among mid-IR sources, demonstrating that mid-IR selection provides a powerful tool for detecting obscured QSOs.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/618/123
- Title:
- Hard X-ray-emitting AGNs
- Short Name:
- J/ApJ/618/123
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present X-ray and optical analysis of 188 active galactic nuclei (AGN) identified from 497 hard X-ray (f_2.0-8.0keV_>2.7x10-15ergs/cm^2^/s) sources in 20 Chandra fields (1.5{deg}^2^) forming part of the Chandra Multiwavelength Project (ChaMP). These medium depth X-ray observations enable us to detect a representative subset of those sources responsible for the bulk of the 28keV cosmic X-ray background. Brighter than our optical spectroscopic limit, we achieve a reasonable degree of completeness (77% of X-ray sources with counterparts r'<22.5 have been classified): broad emission-line AGNs (62%), narrow emission-line galaxies (24%), absorption line galaxies (7%), stars (5%), or clusters (2%).
- ID:
- ivo://CDS.VizieR/J/A+A/375/739
- Title:
- Hard X-ray properties of blazars
- Short Name:
- J/A+A/375/739
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have considered all blazars observed in the X-ray band and for which the slope of the X-ray spectrum is available. We have collected 421 spectra of 268 blazars, including 12 archival unpublished ASCA spectra of 7 blazars whose analysis is presented here. The X-ray spectra of blazars show trends as a function of their power, confirming that the blazar overall energy distribution can be parameterized on the basis of one parameter only, i.e. the bolometric luminosity. This is confirmed by the relatively new hard (2-10 keV) X-ray data. Our results confirm the idea that in low power objects the X-ray emission mechanism is the synchrotron process, dominating both the soft and the hard X-ray emissions. Low energy peaked BL Lac objects are intermediate, often showing harder spectra in the hard X-ray band, suggesting that the synchrotron process dominates in the soft band, with the inverse Compton process dominating at high energies. The most powerful objects have X-ray spectra that are flat both in the soft and in the hard band, consistent with a dominating inverse Compton component.
- ID:
- ivo://CDS.VizieR/J/ApJ/798/95
- Title:
- HE1104-1805 BVRIJ light curves
- Short Name:
- J/ApJ/798/95
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The gravitationally lensed quasar HE 1104-1805 has been observed at a variety of wavelengths ranging from the mid-infrared to X-ray for nearly 20yr. We combine flux ratios from the literature, including recent Chandra data, with new observations from the SMARTS telescope and Hubble Space Telescope, and use them to investigate the spatial structure of the central regions using a Bayesian Monte Carlo analysis of the microlensing variability. The wide wavelength coverage allows us to constrain not only the accretion disk half-light radius r_1/2_, but the power-law slope {xi} of the size-wavelength relation r_1/2_{prop.to}{lambda}^{xi}^. With a logarithmic prior on the source size, the (observed-frame) R-band half-light radius log (r_1/2_/cm) is 16.0_-0.4_^+0.3^, and the slope {xi} is 1.0_-0.56_^+0.30^. We put upper limits on the source size in soft (0.4-1.2 keV) and hard (1.2-8keV) X-ray bands, finding 95% upper limits on log (r_1/2_/cm) of 15.33 in both bands. A linear prior yields somewhat larger sizes, particularly in the X-ray bands. For comparison, the gravitational radius, using a black hole mass estimated using the H{beta} line, is log(r_g_/cm)=13.94. We find that the accretion disk is probably close to face-on, with cos(i)=1.0 being four times more likely than cos(i)=0.5. We also find probability distributions for the mean mass of the stars in the foreground lensing galaxy, the direction of the transverse peculiar velocity of the lens, and the position angle of the projected accretion disk's major axis (if not face-on).
- ID:
- ivo://CDS.VizieR/J/ApJ/660/146
- Title:
- HE 1104-1805 differential light curves
- Short Name:
- J/ApJ/660/146
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The mid-IR flux ratios FA/FB=2.84+/-0.06 of the two images of the gravitationally lensed quasar HE 1104-1805 show no wavelength dependence to within 3% across 3.6-8.0um, show no time dependence over 6 months, and agree with the broad emission-line flux ratios. This indicates that the mid-IR emission likely comes from scales large enough to be little affected by microlensing and that there is little differential extinction between the images. We measure a revised time delay between these two images of 152.2^+2.8^_-3.0_ (1sigma) days from R- and V-band data covering the years 1997-2006. This time delay indicates that the lens has an approximately flat rotation curve over scales of 1-2Re. We also observed uncorrelated variations of ~0.05mag/yr, which we attribute to microlensing of the optical emission from the accretion disk. The optical colors have also changed significantly in the sense that image A is now redder than image B, rather than bluer, as it was in 1993.
- ID:
- ivo://CDS.VizieR/J/ApJS/217/11
- Title:
- HeI* in broad absorption line QSOs
- Short Name:
- J/ApJS/217/11
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Neutral helium multiplets, HeI*{lambda}{lambda}3189, 3889, 10830, are very useful diagnostics for the geometry and physical conditions of the absorbing gas in quasars. So far only a handful of HeI* detections have been reported. Using a newly developed method, we detected the HeI*{lambda}3889 absorption line in 101 sources of a well-defined sample of 285 MgII broad absorption line (BAL) quasars selected from SDSS DR5. This has increased the number of HeI* BAL quasars by more than one order of magnitude. We further detected HeI*{lambda}3189 in 50% (52/101) of the quasars in the sample. The detection fraction of HeI* BALs in MgII BAL quasars is ~35% as a whole, and it increases dramatically with increasing spectral signal-to-noise ratio (S/N), from ~18% at S/N<=10 to ~93% at S/N>=35. This suggests that HeI* BALs could be detected in most MgII LoBAL quasars, provided the spectra S/N is high enough. Such a surprisingly high HeI* BAL fraction is actually predicted from photoionization calculations based on a simple BAL model. The result indicates that HeI* absorption lines can be used to search for BAL quasars at low z, which cannot be identified by ground-based optical spectroscopic surveys with commonly seen UV absorption lines. Using HeI*{lambda}3889, we discovered 19 BAL quasars at z<0.3 from the available SDSS spectral database. The fraction of HeI* BAL quasars is similar to that of LoBAL objects.
- ID:
- ivo://CDS.VizieR/J/ApJ/847/81
- Title:
- HeII photoionization rates of quasars
- Short Name:
- J/ApJ/847/81
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The HeII transverse proximity effect-enhanced HeII Ly{alpha} transmission in a background sightline caused by the ionizing radiation of a foreground quasar-offers a unique opportunity to probe the morphology of quasar-driven HeII reionization. We conduct a comprehensive spectroscopic survey to find z~3 quasars in the foreground of 22 background quasar sightlines with Hubble Space Telescope/COS HeII Ly{alpha} transmission spectra. With our two-tiered survey strategy, consisting of a deep pencil-beam survey and a shallow wide-field survey, we discover 131 new quasars, which we complement with known SDSS/BOSS quasars in our fields. Using a restricted sample of 66 foreground quasars with inferred HeII photoionization rates greater than the expected UV background at these redshifts ({Gamma}_QSO_^HeII^>5x10^-16^s^-1^) we perform the first statistical analysis of the HeII transverse proximity effect. Our results show qualitative evidence for a large object-to-object variance: among the four foreground quasars with the highest {Gamma}_QSO_^HeII^ only one (previously known) quasar is associated with a significant HeII transmission spike. We perform a stacking analysis to average down these fluctuations, and detect an excess in the average HeII transmission near the foreground quasars at 3{sigma} significance. This statistical evidence for the transverse proximity effect is corroborated by a clear dependence of the signal strength on {Gamma}_QSO_^HeII^. Our detection places a purely geometrical lower limit on the quasar lifetime of t_Q_>25Myr. Improved modeling would additionally constrain quasar obscuration and the mean free path of HeII-ionizing photons.
- ID:
- ivo://CDS.VizieR/J/A+A/409/79
- Title:
- HELLAS2XMM survey. IV.
- Short Name:
- J/A+A/409/79
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present results from the photometric and spectroscopic identification of 122 X-ray sources recently discovered by XMM-Newton in the 2-10keV band (the HELLAS2XMM 1dF sample). Their flux cover the range 8x10^-15^-4x10^-13^erg/cm^2^/s and the total area surveyed is 0.9 square degrees. One of the most interesting results (which is found also in deeper surveys) is that about 20% of the hard X-ray selected sources have an X-ray to optical flux ratio (X/O) ten times or more higher than that of optically selected AGN. Unlike the faint sources found in the ultra-deep Chandra and XMM-Newton surveys, which reach X-ray (and optical) fluxes more than one order of magnitude lower than the HELLAS2XMM survey sources, many of the extreme X/O sources in our sample have R<~25 and are therefore accessible to optical spectroscopy. We report the identification of 13 sources with X/O>~10 (to be compared with 9 sources known from the deeper, pencil-beam surveys). Eight of them are narrow line QSO (seemingly the extension to very high luminosity of the type 2 Seyfert galaxies), four are broad line QSO. The results from our survey are also used to make reliable predictions about the luminosity of the sources not yet spectroscopically identified, both in our sample and in deeper Chandra and XMM-Newton samples. We then use a combined sample of 317 hard X-ray selected sources (HELLAS2XMM 1dF, Chandra Deep Field North 1Msec, Chandra SSA13 and XMM-Newton Lockman Hole flux limited samples), 221 with measured redshifts, to evaluate the cosmological evolution of the hard X-ray source's number and luminosity densities.
- ID:
- ivo://CDS.VizieR/J/MNRAS/462/4197
- Title:
- HI 21-cm absorption in redshifted galaxies
- Short Name:
- J/MNRAS/462/4197
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The star-forming reservoir in the distant Universe can be detected through HI 21-cm absorption arising from either cool gas associated with a radio source or from within a galaxy intervening the sightline to the continuum source. In order to test whether the nature of the absorber can be predicted from the profile shape, we have compiled and analysed all of the known redshifted (z>=0.1) HI 21-cm absorption profiles. Although between individual spectra there is too much variation to assign a typical spectral profile, we confirm that associated absorption profiles are, on average, wider than their intervening counterparts. It is widely hypothesized that this is due to high-velocity nuclear gas feeding the central engine, absent in the more quiescent intervening absorbers. Modelling the column density distribution of the mean associated and intervening spectra, we confirm that the additional low optical depth, wide dispersion component, typical of associated absorbers, arises from gas within the inner parsec. With regard to the potential of predicting the absorber type in the absence of optical spectroscopy, we have implemented machine learning techniques to the 55 associated and 43 intervening spectra, with each of the tested models giving a >=80 per cent accuracy in the prediction of the absorber type. Given the impracticability of follow-up optical spectroscopy of the large number of 21-cm detections expected from the next generation of large radio telescopes, this could provide a powerful new technique with which to determine the nature of the absorbing galaxy.
- ID:
- ivo://CDS.VizieR/J/MNRAS/465/588
- Title:
- HI 21-cm absorption survey of QSO-galaxy pairs
- Short Name:
- J/MNRAS/465/588
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results from our survey of HI 21-cm absorption, using Giant Metrewave Radio Telescope, Very Large Array and Westerbork Radio Synthesis Telescope, in a sample of 55 z<0.4 galaxies towards radio sources with impact parameters (b) in the range ~0-35kpc. In our primary sample (defined for statistical analyses) of 40 quasar-galaxy pairs, probed by 45 sightlines, we have found seven HI 21-cm absorption detections, two of which are reported here for the first time. Combining our primary sample with measurements having similar optical depth sensitivity ({int}{tau}dv<=0.3km/s) from the literature, we find a weak anti-correlation (rank correlation coefficient =-0.20 at 2.42{sigma} level) between {int}{tau}dv and b, consistent with previous literature results. The covering factor of HI 21-cm absorbers (C21) is estimated to be 0.24^+0.12^_-0.08_ at b<=15kpc and 0.06^+0.09^_-0.04_ at b=15-35kpc. {int}{tau}dv and C_21_ show similar declining trend with radial distance along the galaxy's major axis and distances scaled with the effective HI radius. There is also tentative indication that most of the HI 21-cm absorbers could be co-planar with the extended HI discs. No significant dependence of {int}{tau}dv and C_21_ on galaxy luminosity, stellar mass, colour and star formation rate is found, though the HI 21-cm absorbing gas cross-section may be larger for the luminous galaxies. The higher detection rate (by a factor of ~4) of HI 21-cm absorption in z<1 damped Lyman-{alpha} systems compared to the quasar-galaxy pairs indicates towards small covering factor and patchy distribution of cold gas clouds around low-z galaxies.
