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31. Blazars with {gamma}-ray counterparts. II.
ID:
ivo://CDS.VizieR/J/ApJS/206/13
Title:
Blazars with {gamma}-ray counterparts. II.
Short Name:
J/ApJS/206/13
Date:
21 Oct 2021
Publisher:
CDS
Description:
A significant fraction (~30%) of the high-energy gamma-ray sources listed in the second Fermi Large Area Telescope catalog (2FGL) are still of unknown origin, being not yet associated with counterparts at low energies. We recently developed a new association method to identify if there is a {gamma}-ray blazar candidate within the positional uncertainty region of a generic 2FGL source. This method is entirely based on the discovery that blazars have distinct infrared colors with respect to other extragalactic sources found, thanks to the Wide-field Infrared Survey Explorer (WISE) all-sky observations. Several improvements have also been performed to increase the efficiency of our method in recognizing {gamma}-ray blazar candidates. In this paper we applied our method to two different samples, the first constituted by unidentified {gamma}-ray sources (UGSs), and the second by active galaxies of uncertain type, both listed in the 2FGL. We present a catalog of IR counterparts for ~20% of the UGSs investigated. Then, we also compare our results for the associated sources with those present in the literature. In addition, we illustrate the extensive archival research carried out to identify the radio, infrared, optical, and X-ray counterparts of the WISE-selected, {gamma}-ray blazar candidates. Finally, we discuss the future developments of our method based on ground-based follow-up observations.
32. BL Lac candidates for TeV observations
ID:
ivo://CDS.VizieR/J/ApJS/207/16
Title:
BL Lac candidates for TeV observations
Short Name:
J/ApJS/207/16
Date:
21 Oct 2021
Publisher:
CDS
Description:
BL Lac objects are the most numerous class of extragalactic TeV-detected sources. One of the biggest difficulties in investigating their TeV emission is due to their limited number, since only 47 BL Lac objects are known to be TeV emitters. In this paper, we propose new criteria to select TeV BL Lac candidates based on infrared and X-ray observations. We apply our selection criteria to the BL Lac objects listed in the ROMA-BZCAT catalog (Cat. J/A+A/495/691), thereby identifying 41 potential TeV emitters. We then perform a search over a more extended sample combining the ROSAT bright source catalog (Cat. IX/10) and the WISE all-sky survey (Cat. II/311), revealing 54 additional candidates for TeV observations. Our investigation also led to a tentative classification of 16 unidentified X-ray sources as BL Lac candidates. This analysis provides new interesting BL Lac targets for future observations with ground-based Cherenkov telescopes.
33. Bulk Lorentz factors of gamma-ray bursts
ID:
ivo://CDS.VizieR/J/A+A/609/A112
Title:
Bulk Lorentz factors of gamma-ray bursts
Short Name:
J/A+A/609/A112
Date:
21 Oct 2021
Publisher:
CDS
Description:
Knowledge of the bulk Lorentz factor {Gamma}_0_ of gamma-ray bursts (GRBs) allows us to compute their comoving frame properties shedding light on their physics. Upon collisions with the circumburst matter, the fireball of a GRB starts to decelerate, producing a peak or a break (depending on the circumburst density profile) in the light curve of the afterglow. Considering all bursts with known redshift and with an early coverage of their emission, we find 67 GRBs (including one short event) with a peak in their optical or GeV light curves at a time t_p_. For another 106 GRBs we set an upper limit t_p_^UL^. The measure of t_p_ provides the bulk Lorentz factor {Gamma}_0_ of the fireball before deceleration. We show that t_p_ is due to the dynamics of the fireball deceleration and not to the passage of a characteristic frequency of the synchrotron spectrum across the optical band. Considering the t_p_ of 66 long GRBs and the 85 most constraining upper limits, we estimate {Gamma}_0_ or a lower limit {Gamma}_0_^LL^. Using censored data analysis methods, we reconstruct the most likely distribution of t_p_. All t_p_ are larger than the time T_p,{gamma}_ when the prompt {gamma}-ray emission peaks, and are much larger than the time T_ph_ when the fireball becomes transparent, that is, t_p_>T_p,{gamma}_>T_ph_. The reconstructed distribution of {Gamma}_0_ has median value ~300 (150) for a uniform (wind) circumburst density profile. In the comoving frame, long GRBs have typical isotropic energy, luminosity, and peak energy <E_iso_>=3(8)x10^50^erg, <L_iso_>=3(15)x10^47^erg/s, and <E_peak_>=1(2)keV in the homogeneous (wind) case. We confirm that the significant correlations between {Gamma}_0_ and the rest frame isotropic energy (E_iso_), luminosity (L_iso_), and peak energy (E_p_) are not due to selection effects. When combined, they lead to the observed E_p_-E_iso_ and E_p_-L_iso_ correlations. Finally, assuming a typical opening angle of 5 degrees, we derive the distribution of the jet baryon loading which is centered around a few 10^-6^M_{\sun}_.
34. Burst duration measurements for a GRB sample
ID:
ivo://CDS.VizieR/J/ApJ/787/66
Title:
Burst duration measurements for a GRB sample
Short Name:
J/ApJ/787/66
Date:
21 Oct 2021
Publisher:
CDS
Description:
Several gamma-ray bursts (GRBs) last much longer (~hours) in {gamma}-rays than typical long GRBs (~minutes), and it has recently been proposed that these "ultra-long GRBs" may form a distinct population, probably with a different (e.g., blue supergiant) progenitor than typical GRBs. However, Swift observations suggest that many GRBs have extended central engine activities manifested as flares and internal plateaus in X-rays. We perform a comprehensive study on a large sample of Swift GRBs with X-Ray Telescope observations to investigate GRB central engine activity duration and to determine whether ultra-long GRBs are unusual events. We define burst duration t_burst_ based on both {gamma}-ray and X-ray light curves rather than using {gamma}-ray observations alone. We find that t_burst_can be reliably measured in 343 GRBs. Within this "good" sample, 21.9% GRBs have t_burst_>~10^3^ s and 11.5% GRBs have t_burst_>~10^4^ s. There is an apparent bimodal distribution of t_burst_ in this sample. However, when we consider an "undetermined" sample (304 GRBs) with t_burst_ possibly falling in the gap between GRB duration T_90_ and the first X-ray observational time, as well as a selection effect against t_burst_ falling into the first Swift orbital "dead zone" due to observation constraints, the intrinsic underlying t_burst_ distribution is consistent with being a single component distribution. We found that the existing evidence for a separate ultra-long GRB population is inconclusive, and further multi-wavelength observations are needed to draw a firmer conclusion. We also discuss the theoretical implications of our results. In particular, the central engine activity duration of GRBs is generally much longer than the {gamma}-ray T_90_ duration and it does not even correlate with T_90_. It would be premature to make a direct connection between T_90_ and the size of the progenitor star.
35. Catalog of short gamma-ray transients
ID:
ivo://CDS.VizieR/J/PAZh/40/271
Title:
Catalog of short gamma-ray transients
Short Name:
J/PAZh/40/271
Date:
21 Oct 2021
Publisher:
CDS
Description:
We analyzed the data obtained by the SPI telescope onboard the INTEGRAL observatory to search for short transient events with a duration from 1 ms to a few tens of seconds. An algorithm for identifying gamma-ray events against the background of a large number of charged particle interactions with the detector has been developed. The classification of events was made. Apart from the events associated with cosmic gamma-ray bursts (GRBs) confirmed by other space experiments and the activity of known soft gamma repeaters (for example, SGR 1806-20), previously unreported GRBs have been found. GRB candidates and short gamma-ray events probably associated with the activity of known SGRs and AXPs have been selected. The spectral evolution of 28 bright GRBs from the catalog has been studied extensively. A new method for investigating the spectral evolution is proposed. The energy dependence of the spectral lag for bursts with a simple structure of their light curves and for individual pulses of multipulse events is shown to be described by a logarithmic function, lag~Alog(E). It has been established that the parameter A depends on the pulse duration, with the dependence being universal for all of the investigated GRBs. No negative spectral lags have been detected for bursts with a simple structure of their light curves.
36. CGRaBS: survey of {gamma}-ray blazar candidates
ID:
ivo://CDS.VizieR/J/ApJS/175/97
Title:
CGRaBS: survey of {gamma}-ray blazar candidates
Short Name:
J/ApJS/175/97
Date:
21 Oct 2021
Publisher:
CDS
Description:
We describe a uniform all-sky survey of bright blazars, selected primarily by their flat radio spectra, that is designed to provide a large catalog of likely {gamma}-ray active galactic nuclei (AGNs). The defined sample has 1625 targets with radio and X-ray properties similar to those of the EGRET blazars, spread uniformly across the |^b^|>10{deg} sky. We also report progress toward optical characterization of the sample; of objects with known R<23, 85% have been classified and 81% have measured redshifts. One goal of this program is to focus attention on the most interesting (e.g., high-redshift, high-luminosity, ...) sources for intensive multiwavelength study during the observations by the Large Area Telescope (LAT) on GLAST.
37. Chandra observations of 8 INTEGRAL sources
ID:
ivo://CDS.VizieR/J/ApJ/816/38
Title:
Chandra observations of 8 INTEGRAL sources
Short Name:
J/ApJ/816/38
Date:
21 Oct 2021
Publisher:
CDS
Description:
We report on 0.3-10keV observations with the Chandra X-ray Observatory of eight hard X-ray sources discovered within 8{deg} of the Galactic plane by the International Gamma-ray Astrophysics Laboratory (INTEGRAL) satellite. The short (~5ks) Chandra observations of the IGR source fields have yielded very likely identifications of X-ray counterparts for three of the IGR sources: IGRJ14091-6108, IGRJ18088-2741, and IGRJ18381-0924. The first two have very hard spectra in the Chandra band that can be described by a power law with photon indices of {Gamma}=0.6+/-0.4 and -0.7_-0.3_^+0.4^, respectively (90% confidence errors are given), and both have a unique near-IR counterpart consistent with the Chandra position. IGR J14091-6108 also displays a strong iron line and a relatively low X-ray luminosity, and we argue that the most likely source type is a cataclysmic variable (CV), although we do not completely rule out the possibility of a high mass X-ray binary. IGR J18088-2741 has an optical counterpart with a previously measured 6.84hr periodicity, which may be the binary orbital period. We also detect five cycles of a possible 800-950s period in the Chandra light curve, which may be the compact object spin period. We suggest that IGR J18088-2741 is also most likely a CV. For IGR J18381-0924, the spectrum is intrinsically softer with {Gamma}=1.5_-0.4_^+0.5^, and it is moderately absorbed, N_H_=(4+/-1)x10^22^/cm2. There are two near-IR sources consistent with the Chandra position, and they are both classified as galaxies, making it likely that IGR J18381-0924 is an active galactic nucleus. For the other five IGR sources, we provide lists of nearby Chandra sources, which may be used along with further observations to identify the correct counterparts, and we discuss the implications of the low inferred Chandra count rates for these five sources.
38. Chandra X-Ray sources in the {gamma} Cyg Field
ID:
ivo://CDS.VizieR/J/ApJ/615/897
Title:
Chandra X-Ray sources in the {gamma} Cyg Field
Short Name:
J/ApJ/615/897
Date:
21 Oct 2021
Publisher:
CDS
Description:
In search of the counterpart to the brightest unidentified gamma-ray source, 3EG J2020+4017 (2CG 078+2), we report on new X-ray and radio observations of the {gamma} Cygni field with the Chandra X-Ray Observatory and with the Green Bank Telescope (GBT). We also report on reanalysis of archival ROSAT data.
39. Classification of Fermi blazar cand. from the 4FGL
ID:
ivo://CDS.VizieR/J/ApJ/887/134
Title:
Classification of Fermi blazar cand. from the 4FGL
Short Name:
J/ApJ/887/134
Date:
21 Oct 2021
Publisher:
CDS
Description:
The recently published fourth Fermi Large Area Telescope source catalog (4FGL) reports 5065 gamma-ray sources in terms of direct observational gamma-ray properties. Among the sources, the largest population is the active galactic nuclei (AGNs), which consists of 3137 blazars, 42 radio galaxies, and 28 other AGNs. The blazar sample comprises 694 flat-spectrum radio quasars (FSRQs), 1131 BL Lac-type objects (BL Lacs), and 1312 blazar candidates of an unknown type (BCUs). The classification of blazars is difficult using optical spectroscopy given the limited knowledge with respect to their intrinsic properties, and the limited availability of astronomical observations. To overcome these challenges, machine-learning algorithms are being investigated as alternative approaches. Using the 4FGL catalog, a sample of 3137 Fermi blazars with 23 parameters is systematically selected. Three established supervised machine-learning algorithms (random forests (RFs), support vector machines (SVMs), artificial neural networks (ANNs)) are employed to general predictive models to classify the BCUs. We analyze the results for all of the different combinations of parameters. Interestingly, a previously reported trend the use of more parameters leading to higher accuracy is not found. Considering the least number of parameters used, combinations of eight, 12 or 10 parameters in the SVM, ANN, or RF generated models achieve the highest accuracy (Accuracy ~91.8%, or ~92.9%). Using the combined classification results from the optimal combinations of parameters, 724 BL Lac type candidates and 332 FSRQ type candidates are predicted; however, 256 remain without a clear prediction.
40. Classification of X-ray counterparts of 3FGL sources
ID:
ivo://CDS.VizieR/J/ApJ/887/18
Title:
Classification of X-ray counterparts of 3FGL sources
Short Name:
J/ApJ/887/18
Date:
21 Oct 2021
Publisher:
CDS
Description:
Approximately one-third of the gamma-ray sources in the third Fermi-LAT catalog are unidentified or unassociated with objects at other wavelengths. Observations with the X-Ray Telescope on the Neil Gehrels Swift Observatory (Swift-XRT) have yielded possible counterparts in ~30% of these source regions. The objective of this work is to identify the nature of these possible counterparts, utilizing their gamma-ray properties coupled with the Swift derived X-ray properties. The majority of the known sources in the Fermi catalogs are blazars, which constitute the bulk of the extragalactic gamma-ray source population. The galactic population on the other hand is dominated by pulsars. Overall, these two categories constitute the majority of all gamma-ray objects. Blazars and pulsars occupy different parameter space when X-ray fluxes are compared with various gamma-ray properties. In this work, we utilize the X-ray observations performed with the Swift-XRT for the unknown Fermi sources and compare their X-ray and gamma-ray properties to differentiate between the two source classes. We employ two machine-learning algorithms, decision tree and random forest (RF) classifier, to our high signal-to-noise ratio sample of 217 sources, each of which corresponds to Fermi unassociated regions. The accuracy scores for both methods were found to be 97% and 99%, respectively. The RF classifier, which is based on the application of a multitude of decision trees, associated a probability value (P_bzr_) for each source to be a blazar. This yielded 173 blazar candidates from this source sample, with P_bzr_>=90% for each of these sources, and 134 of these possible blazar source associations had P_bzr_>=99%. The results yielded 13 sources with P_bzr_<=10%, which we deemed as reasonable candidates for pulsars, seven of which result with P_bzr_<=1%. There were 31 sources that exhibited intermediate probabilities and were termed ambiguous due to their unclear characterization as a pulsar or a blazar.

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