- ID:
- ivo://CDS.VizieR/J/A+A/370/468
- Title:
- Variability of gamma-ray sources
- Short Name:
- J/A+A/370/468
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a study of 40 low-latitude unidentified 3EG gamma-ray sources which were found to be not positionally coincident with any known class of potential gamma-ray emitters in the Galaxy (Romero et al., 1999A&A...348..868R). We have performed a variability analysis which reveals that many of these 40 sources are variable. These sources have, in addition, a steep mean value of the gamma-ray spectral index, <{Gamma}>=2.41+/-0.2, which, combined with the high level of variability, seems to rule out a pulsar origin. The positional coincidences with uncatalogued candidates to supernova remnants were also studied. Only 7 sources in the sample are spatially coincident with these candidates, a result that is shown to be consistent with the expected level of pure chance association. A complementary search for weak radio counterparts was also conducted and the results are presented as an extensive table containing all significant point-like radio sources within the 40 EGRET fields. We argue that in order to produce the high variability, steep gamma-ray spectra, and absence of strong radio counterparts observed in some of the gamma-ray sources of our sample, a new class of objects should be postulated, and we analyze a viable candidate.
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Search Results
- ID:
- ivo://CDS.VizieR/J/AJ/137/5022
- Title:
- Variability of radio-bright BL Lac objects
- Short Name:
- J/AJ/137/5022
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Radio-bright BL Lacertae objects (BLOs) are typically very variable and exhibit prominent flaring. We use a sample of 24 BLOs, regularly monitored at Metsahovi Radio Observatory, to get a clear idea of their flaring behavior in the radio domain and to find possible commonalities in their variability patterns. Our goal was to compare the results given by computational timescales and the observed variability parameters determined directly from the flux curves. Also, we wanted to find out if the BLO flares adhere to the generalized shock model, which gives a schematic explanation for the physical process giving rise to the variability. We use long-term monitoring data from 4.8, 8, 14.5, 22, 37, 90, and 230GHz, obtained mainly from the University of Michigan and Metsahovi Radio Observatories. The structure function, discrete correlation function, and Lomb-Scargle periodogram timescales, calculated in a previous study, are analyzed in more detail. Also, we determine flare durations, rise and decay times, and absolute and relative peak fluxes from the monitoring data.
- ID:
- ivo://CDS.VizieR/J/ApJ/742/49
- Title:
- Variable and transient radio sources in FIRST
- Short Name:
- J/ApJ/742/49
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A comprehensive search for variable and transient radio sources has been conducted using ~55000 snapshot images of the Faint Images of the Radio Sky at Twenty-cm survey (Cat. VIII/90). We present an analysis leading to the discovery of 1627 variable and transient objects down to mJy levels over a wide range of timescales (a few minutes to years). Variations observed range from 20% to a factor of 25. Multi-wavelength matching for counterparts reveals the diverse classes of objects exhibiting variability, ranging from nearby stars and pulsars to galaxies and distant quasars. Interestingly, more than half of the objects in the sample have either no classified counterparts or no corresponding sources at any other wavelength and require multi-wavelength follow-up observations. We discuss these classes of variables and speculate on the identity of objects that lack multi-wavelength counterparts.
- ID:
- ivo://CDS.VizieR/J/ApJ/737/45
- Title:
- Variable 1.4GHz radio sources from NVSS and FIRST
- Short Name:
- J/ApJ/737/45
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We augment the two widest/deepest 1.4GHz radio surveys, the NRAO VLA Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty centimeters (FIRST), with the mean epoch in which each source was observed. We use these catalogs to search for unresolved sources that vary between the FIRST and NVSS epochs. We find 43 variable sources (0.1% of the sources) that vary by more than 4{sigma}, and we construct the mean structure function of these objects. This enables us to explore radio variability on timescales between several months and about five years. We find that, on these timescales, the mean structure function of the variable sources is consistent with a flat structure function. A plausible explanation to these observations is that a large fraction of the variability at 1.4GHz is induced by scintillations in the interstellar medium, rather than by intrinsic variability. Finally, for a subsample of the variables for which the redshift is available, we do not find strong evidence for a correlation between the variability amplitude and the source redshift.
- ID:
- ivo://CDS.VizieR/J/ApJ/720/1055
- Title:
- VBLA observations of W51 Main/South
- Short Name:
- J/ApJ/720/1055
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report measurement of the trigonometric parallax of W51 Main/South using the Very Long Baseline Array. We measure a value of 0.185+/-0.010mas corresponding to a distance of 5.41^+0.31^_-0.28_kpc. W51 Main/South is a well-known massive star-forming region near the tangent point of the Sagittarius spiral arm of the Milky Way. Our distance to W51 yields an estimate of the distance to the Galactic center of R_0_=8.3+/-0.46(statistical)+/-1.0(systematic)kpc by simple geometry. Combining the parallax and proper motion measurements for W51, we obtained the full-space motion of this massive star-forming region. We find W51 is in a nearly circular orbit about the Galactic center. The H_2_O masers used for our parallax measurements trace four powerful bipolar outflows within a 0.4pc size region, some of which are associated with dusty molecular hot cores and/or hyper- or ultra-compact HII regions.
- ID:
- ivo://CDS.VizieR/J/A+A/590/A92
- Title:
- VCC 2062 CO(1-0) data cubes
- Short Name:
- J/A+A/590/A92
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The physical mechanisms driving star formation (SF) in galaxies are still not fully understood. Tidal dwarf galaxies (TDGs), made of gas ejected during galaxy interactions, seem to be devoid of dark matter and have a near-solar metallicity. The latter makes it possible to study molecular gas and its link to SF using standard tracers (CO, dust) in a peculiar environment. We present a detailed study of a nearby TDG in the Virgo Cluster, VCC 2062, using new high-resolution CO(1-0) data from the Plateau de Bure, deep optical imaging from the Next Generation Virgo Cluster Survey (NGVS), and complementary multiwavelength data. Until now, there was some doubt whether VCC 2062 was a true TDG, but the new deep optical images from the NGVS reveal a stellar bridge between VCC 2062 and its parent galaxy, NGC 4694, which is clear proof of its tidal origin. Several high-resolution tracers (H{alpha}, UV, 8um, and 24um) of the star formation rate (SFR) are compared to the molecular gas distribution as traced by the CO(1-0). Coupled with the SFR tracers, the NGVS data are used with the CIGALE code to model the stellar populations throughout VCC 2062, yielding a declining SFR in the recent past, consistent with the low H{alpha}/UV ratio, and a high burst strength. HI emission covers VCC 2062, whereas the CO is concentrated near the HI maxima. The CO peaks correspond to two very distinct regions: one with moderate SF to the NE and one with only slightly weaker CO emission but with nearly no SF. Even where SF is clearly present, the SFR is below the value expected from the surface density of the molecular and the total gas as compared to spiral galaxies and other TDGs. After discussing different possible explanations, we conclude that the low surface brightness is a crucial parameter to understand the low SFR.
- ID:
- ivo://CDS.VizieR/J/ApJ/582/756
- Title:
- Velocities of HII regions
- Short Name:
- J/ApJ/582/756
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Kinematic distance determinations in the inner Galaxy are hampered by the near-far kinematic distance ambiguity. Here we resolve the ambiguity for 49 HII region complexes with known recombination-line velocities in the first Galactic quadrant. We measured the 21cm HI absorption spectrum toward each source with the Very Large Array in the C array. The maximum velocity of HI absorption was used to discriminate between the near and far kinematic distances. The number ratio of far to near sources, ~3, can be entirely explained as a geometrical effect. The kinematic distances that we derive are compared with previous determinations for the same sources. Although our distance determinations are largely in agreement with previous measurements, there are 22 discrepancies that we discuss. Using our distance determinations, we create a face-on Galactic map of the HII region complexes and compare it with a kinematically derived profile of the distribution of CO-traced molecular hydrogen. The HII region complexes delineate the large-scale features seen in the molecular gas. The 5kpc molecular ring and the Sagittarius spiral arm are clearly evident, and a few HII region complexes lie in the Perseus arm.
- ID:
- ivo://CDS.VizieR/J/other/ChJAA/3.49
- Title:
- Velocity and distance of methanol maser sources
- Short Name:
- J/other/ChJAA/3.
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a statistical analysis of 482 6.7GHz methanol maser sources from the available literature, on their maser emission and the characteristics of their associated infrared sources. On the color-color diagram, more than 70% of the objects fall within a very small region (0.57<=[25-12]<=1.30 and 1.30<=[60-12]<=2.50). This suggests that 6.7GHz methanol maser emission occurs only within a very short evolutionary phase during the earliest stage of star formation. The velocity ranges of the masers belong to two main groups: one from 1 to 10km/s, and one from about 11 to 20km/s. These velocity ranges indicate that the masers are probably associated with both disks and outflows. The correlations between the maser and infrared flux densities, and between the maser and infrared luminosities, suggest that far-infrared radiation is a possible pumping mechanism for the masers which most probably originate from some outer molecular envelopes or disks.
- ID:
- ivo://CDS.VizieR/J/ApJ/585/647
- Title:
- Velocity dispersion in AGN
- Short Name:
- J/ApJ/585/647
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- It has been proposed that the width of the narrow [O III] {lambda}5007 emission line can be used as a surrogate for the stellar velocity dispersion in active galaxies. This proposition is tested using the Sloan Digital Sky Survey (SDSS) Early Data Release (EDR) spectra of 107 low-redshift radio-quiet QSOs and Seyfert 1 galaxies by investigating the correlation between black hole mass, as determined from H{beta} FWHM and optical luminosity, and [O III] FWHM. The correlation is real, but the scatter is large. Without additional information or selection criteria, the [O III] width can predict the black hole mass to a factor of 5.
- ID:
- ivo://CDS.VizieR/J/ApJ/622/187
- Title:
- Velocity of eight RASSCALS galaxy groups
- Short Name:
- J/ApJ/622/187
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We study the distribution of gas pressure and entropy in eight groups of galaxies belonging to the ROSAT All-Sky Survey/Center for Astrophysics Loose Systems (RASSCALS, Cat. <J/ApJ/534/114>). We use archival and proprietary XMM-Newton observations, supplementing the X-ray data with redshifts derived from the literature; we also list 125 new redshifts measured with the Gemini North telescope.
