- ID:
- ivo://CDS.VizieR/J/MNRAS/504/5556
- Title:
- X-ray properties of 26 high-luminosity quasars
- Short Name:
- J/MNRAS/504/5556
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an investigation of the interdependence of the optical-to-X-ray spectral slope (aox), the HeII equivalent-width (EW), and the monochromatic luminosity at 2500{AA} (L2500). The values of {alpha}_ox_ and HeII EW are indicators of the strength/shape of the quasar ionizing continuum, from the ultraviolet (UV; 1500-2500{AA}), through the extreme ultraviolet (EUV;300-50{AA}), to the X-ray (2keV) regime. For this investigation, we measure the HeII EW of 206 radio-quiet quasars devoid of broad absorption lines that have high-quality spectral observations of the UV and 2keV X-rays. The sample spans wide redshift (z~0.13-3.5) and luminosity (logL_2500_~29.2-32.5erg/s/Hz) ranges. We recover the well-known {alpha}_ox_-L_2500_ and HeII EW-L_2500_ anti-correlations, and we find a similarly strong correlation between {alpha}_ox_ and HeII EW, and thus the overall spectral shape from the UV, through the EUV, to the X-ray regime is largely set by luminosity. A significant {alpha}_ox_-HeII EW correlation remains after removing the contribution of L_2500_ from each quantity, and thus the emission in the EUV and the X-rays are also directly tied. This set of relations is surprising, since the UV, EUV, and X-ray emission are expected to be formed in three physically distinct regions. Our results indicate the presence of a redshift-independent physical mechanism that couples the continuum emission from these three different regions, and thus controls the overall continuum shape from the UV to the X-ray regime.
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- ID:
- ivo://CDS.VizieR/J/ApJ/805/122
- Title:
- X-ray properties of PHL1811 analogs and WLQs
- Short Name:
- J/ApJ/805/122
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present an X-ray and multiwavelength study of 33 weak emission-line quasars (WLQs) and 18 quasars that are analogs of the extreme WLQ, PHL1811, at z~0.5-2.9. New Chandra 1.5-9.5ks exploratory observations were obtained for 32 objects while the others have archival X-ray observations. Significant fractions of these luminous type 1 quasars are distinctly X-ray weak compared to typical quasars, including 16 (48%) of the WLQs and 17 (94%) of the PHL 1811 analogs with average X-ray weakness factors of 17 and 39, respectively. We measure a relatively hard ({Gamma}=1.16^+0.37^_-0.32_) effective power-law photon index for a stack of the X-ray weak subsample, suggesting X-ray absorption, and spectral analysis of one PHL 1811 analog, J1521+5202, also indicates significant intrinsic X-ray absorption. We compare composite Sloan Digital Sky Survey spectra for the X-ray weak and X-ray normal populations and find several optical-UV tracers of X-ray weakness, e.g., FeII rest-frame equivalent width (REW) and relative color. We describe how orientation effects under our previously proposed "shielding-gas" scenario can likely unify the X-ray weak and X-ray normal populations. We suggest that the shielding gas may naturally be understood as a geometrically thick inner accretion disk that shields the broad line region from the ionizing continuum. If WLQs and PHL 1811 analogs have very high Eddington ratios, the inner disk could be significantly puffed up (e.g., a slim disk). Shielding of the broad emission-line region by a geometrically thick disk may have a significant role in setting the broad distributions of CIV REW and blueshift for quasars more generally.
- ID:
- ivo://CDS.VizieR/J/A+A/434/385
- Title:
- X-ray/radio data of high energy peaked BL Lacs
- Short Name:
- J/A+A/434/385
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The fully identified complete sample including 150 extreme HBL BL Lacs is presented in Table 1 where we give the source name built with the catalog identification code SHBL (where S stands for "Sedentary" survey and HBL for High energy peaked BL Lacs) and the arcsecond precision optical coordinates of the source taken from the APM and COSMOS on-line services; we give also the RASS name, the X-ray flux (0.1-2.4keV), the radio flux (20cm, from the NVSS survey), and the optical apparent V magnitude (from APM and COSMOS) respectively; we give the redshift when available and the reference for the optical identification. In Table 2 we report properties for five bright elliptical galaxies/low luminosity HBLs in the original sample of high fX/fr sources of Paper I (Giommi et al., 1999MNRAS.310..465G). In Table 3 the 19 rejected emission line AGNs together with their properties are presented.
- ID:
- ivo://CDS.VizieR/J/A+A/434/801
- Title:
- X-ray sources near Lockman Hole
- Short Name:
- J/A+A/434/801
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have identified a large-scale structure traced by galaxies at z=0.8, within the Lockman Hole, by means of multi-object spectroscopic observations. By using deep XMM images we have investigated the soft X-ray emission from the Warm-Hot Intergalactic Medium (WHIM) expected to be associated with this large-scale structure and we set a tight upper limit to its flux in the very soft 0.2-0.4keV band. The non-detection requires the WHIM at these redshifts to be cooler than 0.1keV. Combined with the WHIM emission detections at lower redshift, our result indicates that the WHIM temperature rapidly decreases with redshift, as expected in popular cosmological models.
- ID:
- ivo://CDS.VizieR/J/ApJ/763/111
- Title:
- X-ray spectral analysis of Swift/BAT AGNs
- Short Name:
- J/ApJ/763/111
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a detailed X-ray spectral analysis of the non-beamed, hard X-ray selected active galactic nuclei (AGNs) in the northern Galactic cap of the 58 month Swift Burst Alert Telescope (Swift/BAT) catalog, consisting of 100 AGNs with b>50{deg}. This sky area has excellent potential for further dedicated study due to a wide range of multi-wavelength data that are already available, and we propose it as a low-redshift analog to the "deep field" observations of AGNs at higher redshifts (e.g., CDFN/S, COSMOS, Lockman Hole). We present distributions of luminosity, absorbing column density, and other key quantities for the catalog. We use a consistent approach to fit new and archival X-ray data gathered from XMM-Newton, Swift/XRT, ASCA, and Swift/BAT. We probe to deeper redshifts than the 9 month BAT catalog (<z>=0.043 compared to <z>=0.03 for the 9 month catalog), and uncover a broader absorbing column density distribution. The fraction of obscured (logN_H_>=22) objects in the sample is ~60%, and 43%-56% of the sample exhibits "complex" 0.4-10keV spectra. We present the average 1-10keV spectrum for the sample, which reproduces the 1-10keV X-ray background slope as found for the brighter 9 month BAT AGN sample.
- ID:
- ivo://CDS.VizieR/J/ApJ/807/129
- Title:
- X-ray to MIR luminosities relation of AGNs
- Short Name:
- J/ApJ/807/129
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The X-ray and mid-IR emission from active galactic nuclei (AGNs) are strongly correlated. However, while various published parameterizations of this correlation are consistent with the low-redshift, local Seyfert galaxy population, extrapolations of these relations to high luminosity differ by an order of magnitude at {nu}L_{nu}_(6{mu}m)~10^47^erg/s. Using data from the Wide-field Infrared Survey Explorer, we determine the mid-IR luminosities of the most luminous quasars from the Sloan Digital Sky Survey and present a revised formulation of the X-ray to mid-IR relation of AGNs which is appropriate from the Seyfert regime to the powerful quasar regime.
- ID:
- ivo://CDS.VizieR/J/ApJ/685/773
- Title:
- X-ray-UV relations in SDSS DR5 QSOs
- Short Name:
- J/ApJ/685/773
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We analyze archived Chandra and XMM-Newton X-ray observations of 536 Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) quasars (QSOs) at 1.7<=z<=2.7 in order to characterize the relative UV and X-ray spectral properties of QSOs that do not have broad UV absorption lines (BALs). We constrain the fraction of X-ray-weak, non-BAL QSOs and find that such objects are rare; for example, sources underluminous by a factor of 10 comprise <~2% of optically selected SDSS QSOs. X-ray luminosities vary with respect to UV emission by a factor of <~2 over several years for most sources. UV continuum reddening and the presence of narrow-line absorbing systems are not strongly associated with X-ray weakness in our sample. X-ray brightness is significantly correlated with UV emission-line properties, so that relatively X-ray-weak, non-BAL QSOs generally have weaker, blueshifted CIV {lambda}1549 emission and broader CIII] {lambda}1909 lines. The CIV emission-line strength depends on both UV and X-ray luminosity, suggesting that the physical mechanism driving the global Baldwin effect is also associated with X-ray emission.
- ID:
- ivo://CDS.VizieR/J/A+A/487/475
- Title:
- X-ray variability of AGNs in Lockman Hole
- Short Name:
- J/A+A/487/475
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results from a detailed X-ray variability analysis of 66 AGN in the Lockman Hole, which have optical spectroscopic identifications. We compare, quantitatively, their variability properties with the properties of local AGN, and we study the "variability - luminosity" relation as a function of redshift, and the "variability - redshift" relation in two luminosity bins. We use archival data from the last 10 XMM-Newton observations of the Lockman Hole field to extract light curves in the rest frame, 2-10keV band. We use the "normalized excess variance" to quantify the variability amplitude. Using the latest results regarding the AGN power spectral shape and its dependence on black hole mass and accretion rate, we are able to compute model "variability - luminosity" curves, which we compare with the relations we observe. When we consider all the sources in our sample, we find that their variability amplitude decreases with increasing redshift and luminosity.
- ID:
- ivo://CDS.VizieR/J/ApJ/900/141
- Title:
- X-ray weak quasars from SDSS & Chandra
- Short Name:
- J/ApJ/900/141
- Date:
- 02 Feb 2022 13:37:44
- Publisher:
- CDS
- Description:
- We investigate systematically the X-ray emission from type 1 quasars using a sample of 1825 Sloan Digital Sky Survey 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}_OX_) and the 2500{AA} monochromatic luminosity (L_2500{AA}_), and the X-ray weakness of a quasar is assessed via the deviation of its {alpha}_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 >=6 and >=10 are 5.8%+/-0.7% and 2.7%+/-0.5%, respectively. We classify X-ray-weak quasars (X-ray weak by factors of >=6) into three categories based on their optical spectral features: weak emission-line quasars (WLQs; CIV rest-frame equivalent width <16{AA}), 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.
- ID:
- ivo://CDS.VizieR/J/ApJS/245/17
- Title:
- X-shaped radio galaxies from FIRST
- Short Name:
- J/ApJS/245/17
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a catalog of 290 "winged" or X-shaped radio galaxies (XRGs) extracted from the latest (2014 December 17) data release of the "Very Large Array Faint Images of the Radio Sky at Twenty centimeter". We have combined these radio images with their counterparts in the TIFR GMRT sky survey at 150MHz, in an attempt to identify any low surface brightness radio emission present in these sources. This has enabled us to assemble a sample of 106 "strong" XRG candidates and 184 "probable" XRG candidates whose XRG designation needs to be verified by further observations. The present sample of 290 XRG candidates is almost twice as large as the number of XRGs currently known. Twenty-five of our 290 XRG candidates (9 "strong" and 16 "probable") are identified as quasars. Double-peaked narrow emission lines are seen in the optical spectra of three of the XRG candidates (two "strong" and one "probable"). Nearly 90% of the sample is located in the FR II domain of the Owen-Ledlow diagram. A few of the strong XRG candidates have a rather flat radio spectrum (spectral index {alpha} flatter than -0.3) between 150MHz and 1.4GHz, or between 1.4 and 5GHz. Since this is not expected for lobe-dominated extragalactic radio sources (like nearly all known XRGs), these sources are particularly suited for follow-up radio imaging and near-simultaneous measurement of the radio spectrum.