- ID:
- ivo://CDS.VizieR/J/MNRAS/428/220
- Title:
- Gamma-ray AGN type determination
- Short Name:
- J/MNRAS/428/220
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Fermi Gamma-ray Space Telescope (Fermi) is producing the most detailed inventory of the gamma-ray sky to date. Despite tremendous achievements approximately 25 per cent of all Fermi extragalactic sources in the Second Fermi Large Area Telescope Catalogue (2FGL) are listed as active galactic nuclei (AGN) of uncertain type. Typically, these are suspected blazar candidates without a conclusive optical spectrum or lacking spectroscopic observations. Here, we explore the use of machine-learning algorithms - random forests and support vector machines - to predict specific AGN subclass based on observed gamma-ray spectral properties. After training and testing on identified/associated AGN from the 2FGL we find that 235 out of 269 AGN of uncertain type have properties compatible with gamma-ray BL Lacertae and flat-spectrum radio quasars with accuracy rates of 85 per cent. Additionally, direct comparison of our results with class predictions made after following the infrared colour-colour space of Massaro et al. shows that the agreement rate is over four-fifths for 54 overlapping sources, providing independent cross-validation. These results can help tailor follow-up spectroscopic programmes and inform future pointed surveys with ground-based Cherenkov telescopes.
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Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/626/95
- Title:
- Gamma-ray blazar candidates
- Short Name:
- J/ApJ/626/95
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In preparation for the Gamma-Ray Large Area Space Telescope (GLAST), we have compiled a sample of blazar candidates to increase the pool of well-studied active galactic nuclei from which GLAST counterparts will be drawn.
- ID:
- ivo://CDS.VizieR/J/ApJ/590/109
- Title:
- Gamma-ray blazar in northern sky
- Short Name:
- J/ApJ/590/109
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using survey data, we have re-evaluated the correlation of flat-spectrum radio sources with EGRET sources in the northern sky. An analysis incorporating the radio and X-ray properties and the gamma-ray source localization is used to gauge the reliability of associations and to search for counterparts of previously unidentified EGRET sources. Above |b|=10{deg}, where the classification is complete, we find that 70% of the northern EGRET sources have counterparts similar to the bright EGRET blazars. For several of these we identify known blazar counterparts more likely than the earlier proposed 3EG association; for ~20 we have new identifications. Spectroscopic confirmation of these candidates is in progress, and we have found flat-spectrum radio quasars and BL Lacertae counterparts with redshifts as high as 4. We also find strong evidence for a set of 28 objects with no plausible counterpart like the known EGRET blazars. These thus represent either a new extragalactic population or a population of Galactic objects with a large scale height. The survey has been extended into the plane, where we find several new blazar candidates; the bulk of the sources are, however, Galactic. Looking ahead to the GLAST era, we predict that several of the present 3EG sources are composite and that higher resolution data will break these into multiple blazar identifications.
- ID:
- ivo://CDS.VizieR/J/ApJ/789/135
- Title:
- Gamma-ray bright blazars spectrophotometry
- Short Name:
- J/ApJ/789/135
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present {gamma}-ray, X-ray, ultraviolet, optical, and near-infrared light curves of 33 {gamma}-ray bright blazars over 4 years that we have been monitoring since 2008 August with multiple optical, ground-based telescopes and the Swift satellite, and augmented by data from the Fermi Gamma-ray Space Telescope and other publicly available data from Swift. The sample consists of 21 flat-spectrum radio quasars (FSRQs) and 12 BL Lac objects (BL Lacs). We identify quiescent and active states of the sources based on their {gamma}-ray behavior. We derive {gamma}-ray, X-ray, and optical spectral indices, {alpha}_{gamma}_, {alpha}_X_, and {alpha}_o_, respectively (F_{nu}_{prop.to}{nu}^{alpha}^), and construct spectral energy distributions during quiescent and active states. We analyze the relationships between different spectral indices, blazar classes, and activity states. We find (1) significantly steeper {gamma}-ray spectra of FSRQs than for BL Lacs during quiescent states, but a flattening of the spectra for FSRQs during active states while the BL Lacs show no significant change; (2) a small difference of {alpha}_X_ within each class between states, with BL Lac X-ray spectra significantly steeper than in FSRQs; (3) a highly peaked distribution of X-ray spectral slopes of FSRQs at ~ -0.60, but a very broad distribution of {alpha}_X_of BL Lacs during active states; (4) flattening of the optical spectra of FSRQs during quiescent states, but no statistically significant change of {alpha}_o_ of BL Lacs between states; and (5) a positive correlation between optical and {gamma}-ray spectral slopes of BL Lacs, with similar values of the slopes. We discuss the findings with respect to the relative prominence of different components of high-energy and optical emission as the flux state changes.
- ID:
- ivo://CDS.VizieR/J/MNRAS/449/L6
- Title:
- Gamma-ray burst Eiso/Lopt
- Short Name:
- J/MNRAS/449/L6
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Gamma-ray burst (GRB) optical and X-ray afterglow luminosity is expected to correlate with the GRB isotropic equivalent kinetic energy of the outflow in the standard synchrotron model for GRB afterglows. Previous studies, using prompt GRB isotropic equivalent energy (E_iso_) as a proxy for isotropic equivalent kinetic energy, have generally confirmed a correlation between X-ray and optical afterglow luminosities. Assuming that GRB afterglow luminosity does not evolve strongly with redshift, we identify a strong Malmquist bias in GRB optical and X-ray afterglow luminosity data. We show that selection effects dominate the observed E_iso_-L_opt_, X correlations, and have likely been underestimated in other studies. The bias is strongest for a subset of optically faint bursts m>24 at 24h with z>2. After removing this optical selection bias, the E_iso_-L_opt_, X correlation for long GRBs is not statistically significant, but combining both long and short GRB luminosity data the correlation is significant. Using the median of the E_iso_ and L_opt_, X distributions, we apply the synchrotron model assuming the same power-law index for short and long GRBs, but different microphysical parameter distributions. Comparing the ratio of optical and X-ray luminosities, we find tentative evidence that the fraction of post-shock energy in magnetic fields, {epsilon}_B_, could be systematically higher in short GRBs compared to long GRBs.
- ID:
- ivo://CDS.VizieR/J/ApJ/788/30
- Title:
- Gamma-ray burst flares: X-ray flaring
- Short Name:
- J/ApJ/788/30
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a catalog of 498 flaring periods found in gamma-ray burst (GRB) light curves taken from the online Swift X-Ray Telescope GRB Catalogue (Evans et al. 2007A&A...469..379E, 2009, J/MNRAS/397/1177). We analyzed 680 individual light curves using a flare detection method developed and used on our UV/optical GRB Flare Catalog. This method makes use of the Bayesian Information Criterion to analyze the residuals of fitted GRB light curves and statistically determines the optimal fit to the light curve residuals in an attempt to identify any additional features. These features, which we classify as flares, are identified by iteratively adding additional "breaks" to the light curve. We find evidence of flaring in 326 of the analyzed light curves. For those light curves with flares, we find an average number of ~1.5 flares per GRB. As with the UV/optical, flaring in our sample is generally confined to the first 1000 s of the afterglow, but can be detected to beyond 10^5^ s. Only ~50% of the detected flares follow the "classical" definition of {Delta}t/t<=0.5, with many of the largest flares exceeding this value.
- ID:
- ivo://CDS.VizieR/J/ApJ/609/935
- Title:
- Gamma-ray burst formation rate
- Short Name:
- J/ApJ/609/935
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We estimate a gamma-ray burst (GRB) formation rate based on the new relation between the spectral peak energy (E_p_) and the peak luminosity. The new relation is derived by combining the data of E_p_ and the peak luminosities by BeppoSAX and BATSE, and it looks considerably tighter and more reliable than the relations suggested by the previous works. Using the new E_p_-luminosity relation, we estimate redshifts of the 689 GRBs without known distances in the BATSE catalog and derive a GRB formation rate as a function of the redshift.
- ID:
- ivo://CDS.VizieR/J/ApJ/563/80
- Title:
- 3906 gamma-ray bursts BATSE triggers
- Short Name:
- J/ApJ/563/80
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- During a scan of the archival BATSE daily records covering the entire 9.1yr (TJD 8369-11690) of the BATSE operation, 3906 gamma-ray bursts (GRBs) have been detected. 2068 of these GRBs are previously known BATSE triggers, while 1838 of them are new nontriggered bursts. It is important that all events were detected in the same type of data and were processed with the same procedure. Therefore these 3906 GRBs constitute a uniform sample. We have created a publicly available electronic data base (http://www.astro.su.se/groups/head/grb_archive.html) containing this sample. We describe the procedures of the data reduction, the selection of the GRB candidates, and the statistical tests for possible non-GRB contaminations. We also describe a novel test burst method used to measure the scan efficiency and the information obtained using the test bursts. Our scan decreases the BATSE detection threshold to ~0.1photon/s/cm2. As a first result, we show that the differential logN-logP distribution corrected for the detection efficiency extends to low brightnesses without any indication of a turnover. Any reasonable extrapolation of the new logN-logP to lower brightnesses imply a rate of several thousands of GRBs in the universe per year.
- ID:
- ivo://CDS.VizieR/J/MNRAS/464/4545
- Title:
- Gamma Ray Bursts detected by Swift (2004-2015)
- Short Name:
- J/MNRAS/464/4545
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- An important constraint for galaxy evolution models is how much gas resides in galaxies, in particular, at the peak of star formation z=1-3. We attempt a novel approach by letting long-duration gamma ray bursts (LGRBs) x-ray their host galaxies and deliver column densities to us. This requires a good understanding of the obscurer and biases introduced by incomplete follow-up observations. We analyse the X-ray afterglow of all 844 Swift LGRBs to date for their column density NH. To derive the population properties, we propagate all uncertainties in a consistent Bayesian methodology. The NH distribution covers the 10^20-23%cm^-2^ range and shows no evolutionary effect. Higher obscurations, e.g. Compton-thick columns, could have been detected but are not observed. The NH distribution is consistent with sources randomly populating a ellipsoidal gas cloud of major axis N_H_^major^=10^23^cm^-2^ with 0.22dex intrinsic scatter between objects. The unbiased SHOALS survey of afterglows and hosts allows us to constrain the relation between Spitzer-derived stellar masses and X-ray derived column densities NH. We find a well-constrained power-law relation of NH=10^21.7^cm^-2^x(M*/10^9.5^M_{sun}_)^1/3^, with 0.5dex intrinsic scatter between objects. The Milky Way and the Magellanic clouds also follow this relation. From the geometry of the obscurer, its stellar mass dependence and comparison with local galaxies, we conclude that LGRBs are primarily obscured by galaxy-scale gas. Ray tracing of simulated Illustris galaxies reveals a relation of the same normalization, but a steeper stellar-mass dependence and mild redshift evolution. Our new approach provides valuable insight into the gas residing in high-redshift galaxies.
- ID:
- ivo://CDS.VizieR/J/ApJ/787/90
- Title:
- Gamma-ray bursts minimum timescales
- Short Name:
- J/ApJ/787/90
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We develop a robust technique to determine the minimum variability timescale for gamma-ray burst (GRB) light curves, utilizing Haar wavelets. Our approach averages over the data for a given GRB, providing an aggregate measure of signal variation while also retaining sensitivity to narrow pulses within complicated time series. In contrast to previous studies using wavelets, which simply define the minimum timescale in reference to the measurement noise floor, our approach identifies the signature of temporally smooth features in the wavelet scaleogram and then additionally identifies a break in the scaleogram on longer timescales as a signature of a true, temporally unsmooth light curve feature or features. We apply our technique to the large sample of Swift GRB gamma-ray light curves and for the first time - due to the presence of a large number of GRBs with measured redshift - determine the distribution of minimum variability timescales in the source frame. We find a median minimum timescale for long-duration GRBs in the source frame of {Delta}t_min_=0.5 s, with the shortest timescale found being on the order of 10 ms. This short timescale suggests a compact central engine (3x10^3^ km). We discuss further implications for the GRB fireball model and present a tantalizing correlation between the minimum timescale and redshift, which may in part be due to cosmological time dilation.
