arXiv subjects

L. Y. Petrov

Publications and source records attributed to L. Y. Petrov.

3 recordsLinked to original sources

arXiv2026

SKA VLBI survey of the Southern sky for astrometry, geodesy, and astrophysics

The development of SKA will open the opportunity to run dedicated VLBI surveys of the Southern sky and observe sources not visible at radio telescopes located in the Northern hemisphere. These surveys will allow for doing geodesy with a Southern hemisphere-centered network, increase the density of compact radio sources that can be used as calibrators, further extend the celestial reference frame to deep south, and facilitate high-precision differential astrometry for a wide range of applications. Achieving a deep completeness level and determining the parsec-scale properties of extragalactic radio sources is crucial for multi-wavelength and multi-messenger astrophysics. This includes supporting Cherenkov and neutrino telescope science cases, as well as joint VLBI-Gaia studies of active galaxies.

astro-ph.GA↗

arXiv2025

Dim cores of radio-bright AGN jets: VLBI and Gaia astrometry pinpoint different parsec-scale features

Astrometry with the very long baseline radio interferometry (VLBI) allows to determine the position of a point close to the source's brightest compact detail at milliarcsecond scales. For most active galactic nuclei (AGNs), this compact detail is the opaque core of the radio jet. Rare cases of sources whose brightest detail is not the core but a prominent jet feature parsecs away from the core have been reported, but such sources remained elusive. In this work, we use a novel method for a systematic search of these sources. We scrutinize the AGNs for which the offset between their coordinates determined with VLBI and Gaia is statistically significant and coincides with the vector between two dominant features in their VLBI images, using publicly available archival multi-frequency data. We find 35 sources whose VLBI coordinates are associated with a bright component of their jet separated by several to tens of mas from the radio core. Their Gaia coordinates, in turn, correspond to the jet origin close to the radio jet core. The previously published jet directions of most of them must be reversed. These sources exhibit atypically low brightness temperatures of the radio cores, down to 10^9 K in the host galaxy frame, and, at the same time, extreme brightness of the dominating jet components. We argue that these bright components are standing shock fronts and discuss possible physical explanations for the low core brightness, such as ineffective particle heating, atypical absorption, or differential Doppler boosting.

astro-ph.HE↗

arXiv2025

VLBI studies of FLASH H I 21-cm absorbers -- I

We have conducted VLBA 1.4 GHz (L-band) continuum observations towards twelve sources with HI 21-cm absorption detections at redshift $0.4<z<0.7$ in the pilot surveys of FLASH, an ongoing survey with the ASKAP radio telescope. 11 of the 12 targets are resolved in the VLBA observations. Using the parsec scale radio images, we have classified the source morphology and identified the radio core. Six of the twelve targets have core-jet morphology, four have two-sided jet morphology, one has a complex morphology, and one is unresolved. We describe a methodology to test whether the emission from the core or the total emission detected in the VLBA image has sufficient flux density to cause the entire HI 21-cm absorption, and we estimate limits on the gas covering factor and velocity-integrated optical depth (VOD). We find that for seven of the twelve sources, the core has sufficient flux density to cause all the HI 21-cm absorption detected in the ASKAP spectrum. For three other targets, with projected sizes in the range $\rm 305-409 \ pc$, a large fraction of the entire emission in the VLBA map could be occulted by the gas. For 0903+010 (NVSS J090331+010846), we estimate that at least $\approx 73\%$ of the peak absorption detected in the ASKAP spectrum could arise against the emission detected in the VLBA image. For the target 0023+010 (NVSS J002331+010114), we estimate an upper limit on the VOD of $\rm 169 \ km \ s^{-1}$, the highest in our sample. For 0903+010 (NVSS J090331+010846) we estimate a lower limit of $\rm 104 \ km \ s^{-1}$ on the VOD. We find that the distribution of HI 21-cm VODs at $0.4<z<1.0$ could increase by up to a factor of three after correction for the covering factors using our VLBA measurements.

astro-ph.GA↗