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Start Over Subject gamma-ray astronomy Validation Level 2
421. VLBA 5GHz observations of Fermi/LAT AGNs
ID:
ivo://CDS.VizieR/J/ApJ/744/177
Title:
VLBA 5GHz observations of Fermi/LAT AGNs
Short Name:
J/ApJ/744/177
Date:
21 Oct 2021
Publisher:
CDS
Description:
The radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been observed contemporaneously by the Very Long Baseline Array (VLBA). In total, 232 sources were observed with the VLBA. Ninety sources that were previously observed as part of the VLBA Imaging and Polarimetry Survey (VIPS) have been included in the sample, as well as 142 sources not found in VIPS. This very large, 5GHz flux-limited sample of active galactic nuclei (AGNs) provides insights into the mechanism that produces strong {gamma}-ray emission. In particular, we see that {gamma}-ray emission is related to strong, uniform magnetic fields in the cores of the host AGN. Included in this sample are non-blazar AGNs such as 3C84, M82, and NGC 6251. For the blazars, the total VLBA radio flux density at 5GHz correlates strongly with {gamma}-ray flux. The LAT BL Lac objects tend to be similar to the non-LAT BL Lac objects, but the LAT flat-spectrum radio quasars (FSRQs) are significantly different from the non-LAT FSRQs. Strong core polarization is significantly more common among the LAT sources, and core fractional polarization appears to increase during LAT detection.
422. VLBI and bright Fermi/LAT sources
ID:
ivo://CDS.VizieR/J/ApJ/707/L56
Title:
VLBI and bright Fermi/LAT sources
Short Name:
J/ApJ/707/L56
Date:
21 Oct 2021
Publisher:
CDS
Description:
A list of 205 {gamma}-ray strong objects was reported recently as a result of a three-month integration with the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. We attempted identification of these objects, cross-correlating the {gamma}-ray positions with very long baseline interferometry (VLBI) positions of a large all-sky sample of extragalactic radio sources selected on the basis of their parsec-scale flux density. The original associations reported by the Fermi team are confirmed, and six new identifications are suggested. A Monte Carlo analysis shows that the fraction of chance associations in our analysis is less than 5%, and confirms that the vast majority of {gamma}-ray bright extragalactic sources are radio-loud blazars with strong parsec-scale jets. A correlation between the parsec-scale radio and {gamma}-ray flux is supported by our analysis of a complete VLBI flux-density-limited sample of extragalactic jets. The effectiveness of using a VLBI catalog to find associations between {gamma}-ray detections and compact extragalactic radio sources, especially near the Galactic plane, is demonstrated. It is suggested that VLBI catalogs should be used for future identification of Fermi/LAT objects.
423. VLBI images of gamma-ray nova V407 Cygni
ID:
ivo://CDS.VizieR/J/A+A/638/A130
Title:
VLBI images of gamma-ray nova V407 Cygni
Short Name:
J/A+A/638/A130
Date:
21 Oct 2021
Publisher:
CDS
Description:
In 2010 March, the Large Area Telescope on board Fermi revealed a transient gamma-ray source that is positionally coincident with the optical nova in the symbiotic binary, V407 Cyg. This event marked the first discovery of gamma-ray emission from a nova. We aim to obtain resolved radio imaging of the material involved in the nova event, to determine the ejecta geometry and advance velocity directly in the image plane, and to constrain the physical conditions of the system. We observed the source with the European VLBI (Very Long Baseline Interferometry) Network in real time mode, at 1.6 and 5GHz, and the Very Long Baseline Array at 1.6, 5, and 8.4GHz. In total, we observed the source over 16 epochs, starting 20 days after the optical discovery and continuing for over six months. Milliarcsecond-scale radio emission is detected in 10/16 epochs of observations. The source is initially very dim but it later shows a substantial increase in brightness and a resolved shell-like structure 40-90 days after the optical event. The shell has a projected elliptical shape and is asymmetric in brightness and spectral index, being brighter and characterised by a rising spectrum at the south-eastern edge. We determine a projected expansion velocity of ~3500km/s in the initial phase (for an adopted 2.7kpc distance), and ~2100km/s between day 20 and 91. We also found an emitting feature about 350 mas (940AU) to the north-west, advancing at a projected velocity of ~700km/s along the polar axis of the binary. The total flux density in the VLBI images is significantly lower than that previously reported at similar epochs and over much wider angular scales with the VLA. Optical spectra convincingly demonstrated that in 2010 we were viewing V407 Cyg along the equatorial plane and from behind the Mira. Our radio observations image the bipolar flow of the ejecta perpendicular to the orbital plane, where deceleration is much lower than through the equatorial plane probed by the truncated profile of optical emission lines. The separated polar knot at 350 mas and the bipolar flow strictly resemble a similar arrangement seen in Hen 2-104, another symbiotic Mira seen equator-on that went through a large outburst ~5700yrs ago. The observed ~700km/s expansion constrains the launch date of the polar knot around 2004, during the accretion-fed active phase preceding the 2010 nova outburst.
424. W49B with H.E.S.S. and Fermi-LAT
ID:
ivo://CDS.VizieR/J/A+A/612/A5
Title:
W49B with H.E.S.S. and Fermi-LAT
Short Name:
J/A+A/612/A5
Date:
21 Oct 2021
Publisher:
CDS
Description:
The supernova remnant (SNR) W49B originated from a core-collapse supernova that occurred between one and four thousand years ago, and subsequently evolved into a mixed-morphology remnant, which is interacting with molecular clouds (MC). Gamma-ray observations of SNR-MC associations are a powerful tool to constrain the origin of Galactic cosmic rays, as they can probe the acceleration of hadrons through their interaction with the surrounding medium and subsequent emission of non-thermal photons. We report the detection of a gamma-ray source coincident with W49B at very high energies (VHE; E>100GeV) with the H.E.S.S. Cherenkov telescopes together with a study of the source with five years of Fermi-LAT high-energy gamma-ray (0.06-300GeV) data. The smoothly connected, combined source spectrum, measured from 60MeV to multi-TeV energies, shows two significant spectral breaks at 304+/-20MeV and 8.4_-2.5_^+2.2^GeV; the latter is constrained by the joint fit from the two instruments. The detected spectral features are similar to those observed in several other SNR-MC associations and are found to be indicative of gamma-ray emission produced through neutral-pion decay.
425. 1WHSP: VHE {gamma}-ray blazar candidates
ID:
ivo://CDS.VizieR/J/A+A/579/A34
Title:
1WHSP: VHE {gamma}-ray blazar candidates
Short Name:
J/A+A/579/A34
Date:
21 Oct 2021
Publisher:
CDS
Description:
Blazars are the dominant type of extragalactic sources at microwave and at {gamma}-ray energies. In the most energetic part of the electromagnetic spectrum (E>~100GeV) a high fraction of high Galactic latitude sources are blazars of the high synchrotron peaked (HSP) type, that is BL Lac objects with synchrotron power peaking in the UV or in the X-ray band. Building new large samples of HSP blazars is key to understand the properties of jets under extreme conditions, and to study the demographics and the peculiar cosmological evolution of these sources. High synchrotron peaked blazars are remarkably rare, with only a few hundreds of them expected to be above the sensitivity limits of currently available surveys, some of which include hundreds of millions of sources. To find these very uncommon objects, we have devised a method that combines ALLWISE survey data with multi-frequency selection criteria. The sample was defined starting from a primary list of infrared colour-colour selected sources from the ALLWISE all sky survey database, and applying further restrictions on IR-radio and IR-X-ray flux ratios. Using a polynomial fit to the multi-frequency data (radio to X-ray), we estimated synchrotron peak frequencies and fluxes of each object.
426. WISE IR colors of gamma-ray blazars
ID:
ivo://CDS.VizieR/J/ApJ/748/68
Title:
WISE IR colors of gamma-ray blazars
Short Name:
J/ApJ/748/68
Date:
21 Oct 2021
Publisher:
CDS
Description:
Blazars constitute the most enigmatic class of extragalactic {gamma}-ray sources, and their observational features have been ascribed to a relativistic jet closely aligned to the line of sight. They are generally divided in two main classes: the BL Lac objects (BL Lacs) and the flat-spectrum radio quasars (FSRQs). In the case of BL Lacs the double-bumped spectral energy distribution (SED) is generally described by the synchrotron self-Compton (SSC) emission, while for the FSRQs it is interpreted as due to external Compton (EC) emission. Recently, we showed that in the [3.4]-[4.6]-[12]{mu}m color-color diagram the blazar population covers a distinct region (i.e., the WISE blazar Strip (WBS)) clearly separated from the other extragalactic sources that are dominated by thermal emission. In this paper, we investigate the relation between the infrared and {gamma}-ray emission for a subset of confirmed blazars from the literature, associated with Fermi sources, for which WISE archival observations are available. This sample is a proper subset of the sample of sources used previously, and the availability of Fermi data is critical to constrain the models on the emission mechanisms for the blazars.
427. WISE MIR variability in gamma-ray Seyfert 1 gal.
ID:
ivo://CDS.VizieR/J/ApJS/255/10
Title:
WISE MIR variability in gamma-ray Seyfert 1 gal.
Short Name:
J/ApJS/255/10
Date:
11 Mar 2022
Publisher:
CDS
Description:
Fermi-LAT's detection of {gamma}-rays from narrow-line Seyfert 1 galaxies (NLS1s) has received increasing attention. Understanding these {gamma}-NLS1s is of interest because they have some properties similar to blazars, which are known to show rapid and large-amplitude variability. Based on the largest sample of {gamma}-NLS1s (25 sources), we carried out a systematic search for rapid mid-infrared (MIR, 3.4 and 4.6{mu}m) variability using the multiepoch data of the Wide-field Infrared Survey Explorer (WISE). We also compared a few variability properties between {gamma}-NLS1s and {gamma}-ray blazars. Our main results are as follows. (1) Thirteen {gamma}-NLS1s showed significant (>3{sigma}) rapid variability in at least one of the two MIR bands. The MIR emission of these sources is dominated by the synchrotron emission of relativistic electrons in the jet. (2) The {gamma}-NLS1s with flat radio spectra are more variable than those not detected in {gamma}-rays. (3) The {gamma}-NLS1s tend to show smaller amplitude of variability as well as lower duty cycle relative to {gamma}-ray blazars. (4) The {gamma}-NLS1s tend to show a trend of bluer-when-brighter on both intraday and long timescales, similar to {gamma}-ray blazars. (5) The {gamma}-NLS1s that are more variable on long timescales have larger amplitudes of variability and higher duty cycles on intraday timescales. (6) In the majority of cases, the {gamma}-NLS1s fall in the WISE Gamma-ray Blazar Strip (WGS). However, we noted migrations outside of the WGS due to significant variability.
428. X- and gamma-ray fluxes of {gamma}-ray-loud blazars
ID:
ivo://CDS.VizieR/J/A+A/436/799
Title:
X- and gamma-ray fluxes of {gamma}-ray-loud blazars
Short Name:
J/A+A/436/799
Date:
21 Oct 2021
Publisher:
CDS
Description:
We determined the basic parameters, such as the central black hole mass (M), the boosting factor (or Doppler factor) ({delta}) the propagation angle ({Phi}) and the distance along the axis to the site of {gamma}-ray production (d) for 23 {gamma}-ray-loud blazars using their available variability timescales.
429. X and {gamma} spectral indexes of Fermi blazars
ID:
ivo://CDS.VizieR/J/ApJ/761/125
Title:
X and {gamma} spectral indexes of Fermi blazars
Short Name:
J/ApJ/761/125
Date:
21 Oct 2021
Publisher:
CDS
Description:
In this paper, a sample of 451 blazars (193 flat spectrum radio quasars (FSRQs), 258 BL Lacertae objects) with corresponding X-ray and Fermi {gamma}-ray data is compiled to investigate the correlation both between the X-ray spectral index and the {gamma}-ray spectral index and between the spectral index and the luminosity, and to compare the spectral indexes {alpha}_X_, {alpha}_{gamma}_, {alpha}_X{gamma}_, and {alpha}_{gamma}X{gamma}_ for different subclasses. We also investigated the correlation between the X-ray and the {gamma}-ray luminosity.
430. X-ray and radio images of SNR G351.0-5.4
ID:
ivo://CDS.VizieR/J/A+A/568/A107
Title:
X-ray and radio images of SNR G351.0-5.4
Short Name:
J/A+A/568/A107
Date:
21 Oct 2021
Publisher:
CDS
Description:
While searching the NRAO VLA Sky Survey (NVSS) for diffuse radio emission, we have serendipitously discovered extended radio emission close to the Galactic plane. The radio morphology suggests the presence of a previously unknown Galactic supernova remnant. An unclassified gamma-ray source detected by EGRET (3EG J1744-3934) is present in the same location and may stem from the interaction between high-speed particles escaping the remnant and the surrounding interstellar medium. Our aim is to confirm the presence of a previously unknown supernova remnant and to determine a possible association with the gamma-ray emission 3EG J1744-3934. We have conducted optical and radio follow-ups of the target using the Dark Energy Camera (DECam) on the Blanco telescope at Cerro Tololo Inter-American Observatory (CTIO) and the Giant Meterwave Radio Telescope (GMRT). We then combined these data with archival radio and gamma-ray observations.