- ID:
- ivo://CDS.VizieR/J/ApJ/846/34
- Title:
- Flaring gamma-ray sources; LAT 7.4yr (2FAV)
- Short Name:
- J/ApJ/846/34
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the second catalog of flaring gamma-ray sources (2FAV) detected with the Fermi All-sky Variability Analysis (FAVA), a tool that blindly searches for transients over the entire sky observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. With respect to the first FAVA catalog, this catalog benefits from a larger data set, the latest LAT data release (Pass 8), as well as from an improved analysis that includes likelihood techniques for a more precise localization of the transients. Applying this analysis to the first 7.4 years of Fermi observations, and in two separate energy bands 0.1-0.8GeV and 0.8-300GeV, a total of 4547 flares were detected with significance greater than 6{sigma} (before trials), on the timescale of one week. Through spatial clustering of these flares, 518 variable gamma-ray sources were identified. Based on positional coincidence, likely counterparts have been found for 441 sources, mostly among the blazar class of active galactic nuclei. For 77 2FAV sources, no likely gamma-ray counterpart has been found. For each source in the catalog, we provide the time, location, and spectrum of each flaring episode. Studying the spectra of the flares, we observe a harder-when-brighter behavior for flares associated with blazars, with the exception of BL Lac flares detected in the low-energy band. The photon indexes of the flares are never significantly smaller than 1.5. For a leptonic model, and under the assumption of isotropy, this limit suggests that the spectrum of freshly accelerated electrons is never harder than p~2.
Number of results to display per page
Search Results
- ID:
- ivo://CDS.VizieR/J/AZh/89/388
- Title:
- Flat-spectrum radio sources long-term variability
- Short Name:
- J/AZh/89/388
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The paper reports the results of ten-year centimeter-wavelength observations with the RATAN-600 radio telescope of a complete sample of 83 flat-spectrum sources from the GB6 catalog of the MGB Survey (Cat. VIII/40), with S_4.85_>200mJy at declinations 10{deg}-12{deg}30'. Starting in 2000, the observations were conducted simultaneously at six frequencies in the range 0.97-21.7GHz. Seventy-six sources (including 54 quasars) have been identified with optical objects, which have redshifts in the range z=0.331-3.601.
- ID:
- ivo://CDS.VizieR/J/ApJ/874/8
- Title:
- Follow-up spectroscopy of SDSS changing-look QSOs
- Short Name:
- J/ApJ/874/8
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Active galactic nuclei (AGNs) that show strong rest-frame optical/UV variability in their blue continuum and broad line emission are classified as changing-look AGN, or at higher luminosities, changing-look quasars (CLQs). These surprisingly large and sometimes rapid transitions challenge accepted models of quasar physics and duty cycles, offer several new avenues for study of quasar host galaxies, and open a wider interpretation of the cause of differences between broad and narrow-line AGN. To better characterize extreme quasar variability, we present follow-up spectroscopy as part of a comprehensive search for CLQs across the full Sloan Digital Sky Survey (SDSS) footprint using spectroscopically confirmed quasars from the SDSS DR7 catalog. Our primary selection requires large-amplitude (|{Delta}g|>1mag, |{Delta}r|>0.5mag) variability over any of the available time baselines probed by the SDSS and Pan-STARRS 1 surveys. We employ photometry from the Catalina Sky Survey to verify variability behavior in CLQ candidates where available, and confirm CLQs using optical spectroscopy from the William Herschel, MMT, Magellan, and Palomar telescopes. For our adopted signal-to-noise ratio threshold on variability of broad H{beta} emission, we find 17 new CLQs, yielding a confirmation rate of >~20%. These candidates are at lower Eddington ratio relative to the overall quasar population, which supports a disk-wind model for the broad line region. Based on our sample, the CLQ fraction increases from 10% to roughly half as the continuum flux ratio between repeat spectra at 3420{AA} increases from 1.5 to 6. We release a catalog of more than 200 highly variable candidates to facilitate future CLQ searches.
- ID:
- ivo://CDS.VizieR/J/ApJ/892/105
- Title:
- Fourth catalog of Fermi LAT-detected AGNs (4LAC)
- Short Name:
- J/ApJ/892/105
- Date:
- 17 Jan 2022 00:31:09
- Publisher:
- CDS
- Description:
- The fourth catalog of active galactic nuclei (AGNs) detected by the Fermi Gamma-ray Space Telescope Large Area Telescope (4LAC) between 2008 August 4 and 2016 August 2 contains 2863 objects located at high Galactic latitudes (|b|>10{deg}). It includes 85% more sources than the previous 3LAC catalog based on 4yr of data. AGNs represent at least 79% of the high-latitude sources in the fourth Fermi-Large Area Telescope Source Catalog (4FGL), which covers the energy range from 50MeV to 1TeV. In addition, 344 gamma-ray AGNs are found at low Galactic latitudes. Most of the 4LAC AGNs are blazars (98%), while the remainder are other types of AGNs. The blazar population consists of 24% Flat Spectrum Radio Quasars (FSRQs), 38% BL Lac-type objects, and 38% blazar candidates of unknown types (BCUs). On average, FSRQs display softer spectra and stronger variability in the gamma-ray band than BL Lacs do, confirming previous findings. All AGNs detected by ground-based atmospheric Cerenkov telescopes are also found in the 4LAC.
- ID:
- ivo://CDS.VizieR/J/other/ApSS/365.89
- Title:
- Gaia Alerts with LAMOST and SDSS
- Short Name:
- J/other/ApSS/365
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The ESA-Gaia satellite is regularly producing Alerts on objects where photometric variability has been detected after several passages over the same region of the sky. The physical nature of these objects has often to be determined with the help of complementary observations from ground-based facilities. We have compared the list of Gaia Alerts (from the beginning in 2014 to Nov. 1st, 2018) with archival LAMOST and SDSS spectroscopic data. A search radius of 3" has been adopted. In using survey data, the date of the ground-based observation rarely corresponds to the date of the Alert, but this allows at least the identification of the source if it is persistent, or the host galaxy if the object was only transient like a supernova (SN). Some of the objects have several LAMOST observations, and we complemented this search by adding also SDSS DR15 data in order to look for long-term variability. A list of Gaia Nuclear Transients (GNT) from Kostrzewa-Rutkowska et al. (2018. 2018MNRAS.481..307K, Cat. J/MNRAS/481/307), has been included in this search also. We found 26 Gaia Alerts with spectra in LAMOST+SDSS labelled as stars, among which 12 have multi-epoch spectra. A majority of them are Cataclysmic Variables (CVs). Similarly, 206 Gaia Alerts have associated spectra labelled as galaxies, among which 49 have multi-epoch spectra. Those spectra were generally obtained on a date widely different from the Alert date, and are mostly emission-line galaxies with no particularity (except a few Seyferts), leading to the suspicion that most of the Alerts were due to a SN. As for the GNT list, we found 55 associated spectra labelled as galaxies, among them 13 with multi-epoch spectra. In these two galaxy samples, in only two cases, Gaia17aal and GNTJ170213+2543, was the date of the spectroscopic observation close enough to the Alert date: we find a trace of the SN itself in their LAMOST spectrum, both being now classified here as a type Ia SN. Compared to the galaxy sample from the Gaia alerts, the GNT sample has a higher proportion of AGNs, suggesting that some of the detected variations are also due to the AGN itself. Similarly for Quasars, we found only 30 Gaia Alerts but 68 GNT cases associated with single epoch quasar spectra in the databases. In addition to those, 12 plus 23 are quasars where multi-epoch spectra are available. For ten out of these 35, their multi-epoch spectra show appearance or disappearance of the broad Balmer lines and also variations in the continuum, qualifying them as "Changing Look Quasars" and therefore significantly increasing the available sample of such objects.
- ID:
- ivo://CDS.VizieR/J/A+A/611/A52
- Title:
- Gaia DR1 QSO magnitude
- Short Name:
- J/A+A/611/A52
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The first release of the Gaia catalog is available since 14 September 2016. It is a first step in the realization of the future Gaia reference frame. This reference frame will be materialized by the optical positions of the sources and will be compared with and linked to the International Celestial Reference Frame, materialized by the radio position of extragalactic sources. As in the radio domain, it can be reasonably postulated that quasar optical flux variations can alert us to potential changes in the source structure. These changes could have important implications for the position of the target photocenters (together with the evolution in time of these centers) and in parallel have consequences for the link of the reference systems. A set of nine optical telescopes was used to monitor the magnitude variations, often at the same time as Gaia, thanks to the Gaia Observation Forecast Tool. The Allan variances, which are statistical tools widely used in the atomic time and frequency community, are introduced.
- ID:
- ivo://CDS.VizieR/VII/285
- Title:
- Gaia DR2 quasar and galaxy classification
- Short Name:
- VII/285
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We provide probabilistic quasar and galaxy classifications for 2.7 million sources in Gaia Data Release 2. This has been achieved using a supervised classification method (Gaussian Mixture Models) based only on photometric and astrometric data (8 features) in Gaia-DR2. The model is trained empirically to classify objects into three classes - star, quasar, galaxy - for all objects with G>=14.5mag down to the Gaia magnitude limit of G=21.0mag. We provide the probabilities for being a quasar (pqso) and a galaxy (pgal); the probability of being a star is pstar = 1-(pqso+pgal), and all other Gaia data can be obtained by cross-matching Gaia-DR2 using the source identifier. As our main goal is to identify extragalactic objects, we only report objects with pqso+pgal>0.5. These probabilities incorporate a sensible class prior, namely that quasars are 500 times rarer than stars, and that galaxies 7500 times rarer than stars. See the paper for details of the purity and completeness of samples drawn from this catalogue, and for more details of its construction, contents, and validation.
- ID:
- ivo://CDS.VizieR/J/ApJS/105/369
- Title:
- Galactic H I column densities
- Short Name:
- J/ApJS/105/369
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have determined accurate values of the Galactic neutral hydrogen column density, N_H_, toward 220 quasars and active galactic nuclei from 21cm H I measurements made on the 130 Foot Telescope (42.7m). Accurate values of N_H_ have now been obtained for the whole PG bright quasar sample and most quasars that have been observed by ROSAT and the Hubble Space Telescope through mid-1993. The spectra were corrected for stray 21cm radiation yielding values of N_H_ with a typical uncertainty of 1x10^19cm^-2 for high Galactic latitude directions. The H I column densities will be useful for correcting for interstellar opacity at UV and soft X-ray wavelengths, and for estimating the reddening and extinction toward these objects.
- ID:
- ivo://CDS.VizieR/J/A+A/591/A136
- Title:
- Galaxies and QSOs FIR size and surface brightness
- Short Name:
- J/A+A/591/A136
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We use Herschel 70 to 160{mu}m images to study the size of the far-infrared emitting region in about 400 local galaxies and quasar (QSO) hosts. The sample includes normal "main-sequence" star-forming galaxies, as well as infrared luminous galaxies and Palomar-Green QSOs, with different levels and structures of star formation. Assuming Gaussian spatial distribution of the far-infrared (FIR) emission, the excellent stability of the Herschel point spread function (PSF) enables us to measure sizes well below the PSF width, by subtracting widths in quadrature. We derive scalings of FIR size and surface brightness of local galaxies with FIR luminosity, with distance from the star-forming main-sequence, and with FIR color. Luminosities L_FIR_~10^11^L_{sun}_ can be reached with a variety of structures spanning 2dex in size. Ultraluminous L_FIR_~10^12^L_{sun}_ galaxies far above the main-sequence inevitably have small R_e,70_~0.5kpc FIR emitting regions with large surface brightness, and can be close to optically thick in the FIR on average over these regions. Compared to these local relations, first ALMA sizes for the dust emission regions in high redshift galaxies, measured at somewhat longer rest wavelengths, suggest larger sizes at the same IR luminosity. We report a remarkably tight relation with 0.15dex scatter between FIR surface brightness and the ratio of [CII] 158{mu}m emission and FIR emission - the so-called [CII]-deficit is more tightly linked to surface brightness than to FIR luminosity or FIR color. Among 33 z<=0.1 PG QSOs with typical L_FIR_/L_Bol,AGN_~0.1, 19 have a measured 70{mu}m half light radius, with median R_e,70_=1.1kpc. This is consistent with the FIR size for galaxies with similar L_FIR_ but lacking a QSO, in accordance with a scenario where the rest FIR emission of these types of QSOs is, in most cases, due to host star formation.
- ID:
- ivo://CDS.VizieR/J/ApJ/774/40
- Title:
- Galaxy clusters aligned with quasars. III.
- Short Name:
- J/ApJ/774/40
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present Gemini/GMOS-S multi-object spectroscopy of 31 galaxy cluster candidates at redshifts between 0.2 and 1.0 and centered on QSO sight lines taken from Lopez et al. (Paper I, J/ApJ/679/1144). The targets were selected based on the presence of an intervening MgII absorption system at a similar redshift to that of a galaxy cluster candidate lying at a projected distance <2h_71_^-1^Mpc from the QSO sight line (a "photometric hit"). The absorption systems span rest-frame equivalent widths between 0.015 and 2.028{AA}. The general population of our confirmed absorbing galaxies have luminosities L_B_~L_B_^*^ and mean rest-frame colors (R_c_-z') typical of S_cd_ galaxies.
