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Start Over Subject radio astronomy Content Level Research
1781. VANDAM IV. Free-free emission from protostars
ID:
ivo://CDS.VizieR/J/ApJS/238/19
Title:
VANDAM IV. Free-free emission from protostars
Short Name:
J/ApJS/238/19
Date:
21 Oct 2021
Publisher:
CDS
Description:
Emission from protostars at centimeter radio wavelengths has been shown to trace the free-free emission arising from ionizing shocks as a result of jets and outflows driven by protostars. Therefore, measuring properties of protostars at radio frequencies can provide valuable insights into the nature of their outflows and jets. We present a C-band (4.1 and 6.4cm) survey of all known protostars (Class0 and ClassI) in Perseus as part of the VLA Nascent Disk and Multiplicity (VANDAM) Survey. We examine the known correlations between radio flux density and protostellar parameters, such as bolometric luminosity and outflow force, for our sample. We also investigate the relationship between radio flux density and far-infrared line luminosities from Herschel. We show that free-free emission most likely originates from J-type shocks; however, the large scatter indicates that those two types of emission probe different time and spatial scales. Using C-band fluxes, we removed an estimation of free-free contamination from the corresponding Ka- band (9mm) flux densities that primarily probe dust emission from embedded disks. We find that the compact (<1") dust emission is lower for Class I sources (median dust mass 96M_{Earth}_) relative to Class 0 (248M_{Earth}_), but several times higher than in Class II (5-15M_{Earth}_). If this compact dust emission is tracing primarily the embedded disk, as is likely for many sources, this result provides evidence of decreasing disk masses with protostellar evolution, with sufficient mass for forming giant planet cores primarily at early times.
1782. Variability at Frequencies 3.9 and 7.5 GHz
ID:
ivo://CDS.VizieR/J/AZh/72/291
Title:
Variability at Frequencies 3.9 and 7.5 GHz
Short Name:
J/AZh/72/291
Date:
21 Oct 2021
Publisher:
CDS
Description:
A complete sample of radio sources from the Zelenchukskii survey with fluxes S(3.9GHz)>200mJy and declination range from +4 till +6 degrees were observed at 3.9 and 7.5GHz over the course of seven years. There are one hundred and eighty radio sources in the sample.
1783. Variability monitoring of QSO B1156+295 at 4.8GHz
ID:
ivo://CDS.VizieR/J/A+A/555/A134
Title:
Variability monitoring of QSO B1156+295 at 4.8GHz
Short Name:
J/A+A/555/A134
Date:
21 Oct 2021
Publisher:
CDS
Description:
The quasar 1156+295 (4C +29.45) is one of the targets in the Urumqi monitoring program which aimed to search for evidence of annual modulation in the timescales of Intra-Day Variable (IDV) sources. The IDV observations of 1156+295 were carried out nearly monthly from October 2007 to October 2009, with the Urumqi 25m radio telescope at 4.8GHz. The source has shown prominent IDV of total flux density in most observing sessions with variability timescales of 1 day at 4.8GHz. The estimated IDV timescales seem to follow an annual cycle, which can be fitted with an anisotropic interstellar scintillation (ISS) model, suggesting that a significant part of the flux density variations is due to ISS. The source underwent a dramatic flare in 2008. We studied the possible consequences of the flare on the IDV of 1156+295, by comparing the changes in its variability characteristics with the evolution of the 43 GHz VLBA core size of the source. The quasar 1156+295 shows evidence for an annual modulation of its IDV timescales at 4.8GHz, the ISS-induced IDV timescales and variability strength might be affected by the overall activity state of the source core. More frequent IDV and VLBI measurements are required to confirm the relation between the IDV appearance and the core-size evolution of the source.
1784. Variability of 198 extragalactic radio sources
ID:
ivo://CDS.VizieR/J/A+A/553/A107
Title:
Variability of 198 extragalactic radio sources
Short Name:
J/A+A/553/A107
Date:
21 Oct 2021
Publisher:
CDS
Description:
Combining measurements taken using the Wilkinson Microwave Anisotropy Probe (WMAP) from 2001 to 2008 with measurements taken using Planck from 2009 to 2010, we investigate the long-term flux density variability of extragalactic radio sources selected from the Planck Early Release Compact Source Catalogue. The single-year, single-frequency WMAP maps are used to estimate yearly-averaged flux densities of the sources in the four WMAP bands: Ka (33GHz), Q (41GHz), V (61GHz), and W (94GHz). We identify 82, 67, 32, and 15 sources respectively as variable at greater than 99% confidence level in these four bands. The amplitudes of variation are comparable between bands, and are not correlated with either the flux densities or the spectral indices of the sources. The number counts of WMAP Ka-band sources are stable from year to year despite the fluctuation caused by individual source variability. Most of our sources show strong correlation in variability between bands. Almost all the sources that show variability are blazars. We have attempted to fit two simple, four-parameter models to the time-series of 32 sources showing correlated variability at multiple frequencies - a long-term flaring model and a rotating-jet model. We find that 19 sources (60%) can be fit with the simple rotating-jet model, and ten of these also fit the simple long-term flaring model. The remaining 13 sources (40%) show more complex variability behaviour that is not consistent with either model. Extended radio galaxies in our sample show no sign of variability, as expected, with the exception of Pictor A for which we report evidence for a millimetre flare lasting between 2002 and 2010.
1785. Variability of gamma-ray sources
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.
1786. Variability of radio-bright BL Lac objects
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.
1787. Variable and transient radio sources in FIRST
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.
1788. Variable 1.4GHz radio sources from NVSS and FIRST
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.
1789. VBLA observations of W51 Main/South
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.
1790. VCC 2062 CO(1-0) data cubes
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.