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- ID:
- ivo://CDS.VizieR/J/MNRAS/498/4033
- Title:
- Extreme quasar X-ray variability
- Short Name:
- J/MNRAS/498/4033
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We analyze 1598 serendipitous Chandra X-ray observations of 462 radio-quiet quasars to constrain the frequency of extreme amplitude X-ray variability that is intrinsic to the quasar corona and innermost accretion flow. The quasars in this investigation are all spectroscopically confirmed, optically bright (m_i_<=20.2), and contain no identifiable broad absorption lines in their optical/ultraviolet spectra. This sample includes quasars spanning z~0.1-4 and probes X-ray variability on timescales of up to ~12 rest-frame years. Variability amplitudes are computed between every epoch of observation for each quasar and are analyzed as a function of timescale and luminosity. The tail-heavy distributions of variability amplitudes at all timescales indicate that extreme X-ray variations are driven by an additional physical mechanism and not just typical random fluctuations of the coronal emission. Similarly, extreme X-ray variations of low-luminosity quasars seem to be driven by an additional physical mechanism, whereas high-luminosity quasars seem more consistent with random fluctuations. The amplitude at which an X-ray variability event can be considered extreme is quantified for different timescales and luminosities. Extreme X-ray variations occur more frequently at long timescales ({DELTA}t>=300-days) than at shorter timescales, and in low-luminosity quasars compared to high-luminosity quasars over a similar timescale. A binomial analysis indicates that extreme intrinsic X-ray variations are rare, with a maximum occurrence rate of <2.4% of observations. Finally, we present X-ray variability and basic optical emission-line properties of three archival quasars that have been newly discovered to exhibit extreme X-ray variability.
- ID:
- ivo://CDS.VizieR/J/AJ/129/2074
- Title:
- Faint Chandra sources in SWIRE
- Short Name:
- J/AJ/129/2074
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We exploit deep combined observations with Spitzer and Chandra of the Spitzer Wide-Area Infrared Extragalactic Survey (SWIRE) in the ELAIS N1 region to investigate the nature of the faint X-ray and IR sources in common, to identify active galactic nucleus (AGN)/starburst diagnostics, and to study the sources of the X-ray and IR cosmic backgrounds (XRB and CIRB).
- ID:
- ivo://CDS.VizieR/J/AJ/131/1216
- Title:
- Faint radio sources in the CDF-S ACS field
- Short Name:
- J/AJ/131/1216
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present optical and X-ray identifications for the 64 radio sources in the Great Observatories Origins Deep Survey (GOODS) in Chandra Deep Field-South Advanced Camera for Surveys (ACS) field revealed in the Australia Telescope Compact Array 1.4GHz survey of the Chandra Deep Field-South. Optical identifications are made using the ACS images and catalogs, while the X-ray view is provided by the Chandra X-Ray Observatory 1Ms observations. Redshifts for the identified sources are drawn from publicly available catalogs of spectroscopic observations and multiband photometric-based estimates. Using this multiwavelength information we provide a first characterization of the faint radio source population in this region.
- ID:
- ivo://CDS.VizieR/J/ApJ/651/1098
- Title:
- Faint X-ray sources in Terzan 5
- Short Name:
- J/ApJ/651/1098
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report our analysis of a Chandra X-ray observation of the rich globular cluster Terzan 5, in which we detect 50 sources to a limiting 1.0-6keV X-ray luminosity of 3x10^31^ergs/s within the half-mass radius of the cluster. Thirty-three of these have LX>10^32^ergs/s, the largest number yet seen in any globular cluster. In addition to the quiescent low-mass X-ray binary (LMXB; identified by Wijnands et al., 2005ApJ...618..883W), another 12 relatively soft sources may be quiescent LMXBs. We compare the X-ray colors of the harder sources in Terzan 5 to the Galactic center sources studied by Muno and collaborators (2003, Cat. <J/ApJ/589/225>) and find the Galactic center sources to have harder X-ray colors, indicating a possible difference in the populations. We cannot clearly identify a metallicity dependence in the production of low-luminosity X-ray binaries in Galactic globular clusters, but a metallicity dependence of the form suggested by Jordan et al. (2004, Cat. <J/ApJ/613/270>) for extragalactic LMXBs is consistent with our data.
- ID:
- ivo://CDS.VizieR/J/ApJ/744/121
- Title:
- Far-UV spectroscopy of T Tau stars
- Short Name:
- J/ApJ/744/121
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a far-ultraviolet (FUV) spectral atlas consisting of spectra of 91 pre-main-sequence stars. Most stars in this sample were observed with the Space Telescope Imaging Spectrograph and Advanced Camera for Surveys on the Hubble Space Telescope (HST). A few archival spectra from the International Ultraviolet Explorer and the Goddard High Resolution Spectrograph on HST are included for completeness. We find strong correlations among the OI{lambda}1304 triplet, the SiIV {lambda}{lambda}1394/1403 doublet, the CIV{lambda}1549 doublet, and the HeII {lambda}1640 line luminosities. For classical T Tauri stars (CTTSs), we also find strong correlations between these lines and the accretion luminosity, suggesting that these lines form in processes related to accretion. These FUV line fluxes and X-ray luminosity correlate loosely with large scatters. The FUV emission also correlates well with H{alpha}, H{beta}, and CaII K line luminosities. These correlations between FUV and optical diagnostics can be used to obtain rough estimates of FUV line fluxes from optical observations. Molecular hydrogen (H_2_) emission is generally present in the spectra of actively accreting CTTSs but not the weak-lined T Tauri stars that are not accreting. The presence of H_2_ emission in the spectrum of HD 98800N suggests that the disk should be classified as actively accreting rather than a debris disk.
- ID:
- ivo://CDS.VizieR/J/ApJ/888/40
- Title:
- Fast radio bursts with AstroSat/CZTI
- Short Name:
- J/ApJ/888/40
- Date:
- 07 Dec 2021
- Publisher:
- CDS
- Description:
- Fast radio bursts (FRBs) are short-lived (~ms), energetic transients (having a peak flux density of ~Jy) with no known prompt emission in other energy bands. We present results of a search for prompt X-ray emissions from 41 FRBs using the Cadmium Zinc Telluride Imager (CZTI) on AstroSat, which continuously monitors ~70% of the sky. Our searches on various timescales in the 20-200keV range, did not yield any counterparts in this hard X-ray band. We calculate upper limits on hard X-ray flux, in the same energy range and convert them to upper bounds for {eta}: the ratio of X-ray to radio fluence of FRBs. We find {eta}<=10^8-10^ for hard X-ray emission. Our results will help constrain the theoretical models of FRBs as the models become more quantitative and nearer, brighter FRBs are discovered.
- ID:
- ivo://CDS.VizieR/J/MNRAS/411/2099
- Title:
- Fast-rotating M-dwarf stars in NGC 2547
- Short Name:
- J/MNRAS/411/2099
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- At fast rotation rates, the coronal activity of G- and K-type stars has been observed to "saturate" and then decline again at even faster rotation rates - a phenomenon dubbed "supersaturation". In this paper, we investigate coronal activity in fast-rotating M-dwarfs using deep XMM-Newton observations of 97 low-mass stars of known rotation period in the young open cluster NGC 2547 and combine these with published X-ray surveys of low-mass field and cluster stars of known rotation period.
- ID:
- ivo://CDS.VizieR/J/A+A/527/A134
- Title:
- Fe abundances for 28 XMM galaxy clusters
- Short Name:
- J/A+A/527/A134
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The abundances of Fe in the intracluster medium of nearby (z<0.08) clusters were measured up to 0.3~0.5r_180_. We analyzed 28 clusters of galaxies observed with XMM-Newton. We derived Fe abundances from the flux ratios of Fe lines to the continuum within an energy range of 3.5-6keV to minimize and evaluate systematic uncertainties.
- ID:
- ivo://CDS.VizieR/J/ApJ/835/L38
- Title:
- Fermi blazars with Doppler factors
- Short Name:
- J/ApJ/835/L38
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Blazars are an extreme subclass of active galactic nuclei. Their rapid variability, luminous brightness, superluminal motion, and high and variable polarization are probably due to a beaming effect. However, this beaming factor (or Doppler factor) is very difficult to measure. Currently, a good way to estimate it is to use the timescale of their radio flares. In this Letter, we use multiwavelength data and Doppler factors reported in the literature for a sample of 86 flaring blazars detected by Fermi to compute their intrinsic multiwavelength data and intrinsic spectral energy distributions and investigate the correlations among observed and intrinsic data. Quite interestingly, intrinsic data show a positive correlation between luminosity and peak frequency, in contrast with the behavior of observed data, and a tighter correlation between {gamma}-ray luminosity and the lower-energy ones. For flaring blazars detected by Fermi, we conclude that (1) observed emissions are strongly beamed; (2) the anti-correlation between luminosity and peak frequency from the observed data is an apparent result, the correlation between intrinsic data being positive; and (3) intrinsic {gamma}-ray luminosity is strongly correlated with other intrinsic luminosities.
