A Catalog of Variability-selected, VLBI-detected Blazars for Construction of the ICRF

Abstract

Several studies have highlighted the presence of statistically significant offsets between Gaia and VLBI positions [1-5], and additional studies helped determine possible astrophysical causes for these differences [5-8]. As we draw nearer to the next realization of the International Celestial Reference System (ICRS), it is important to address these challenges and others, which affect not only the individual reference frames but also the multi-wavelength tie between them. This is also of particular importance as long-term maintenance of the principal radio frame at S/X-band (2.3/8.4 GHz) gives way to K or Ka (22 GHz, 32 GHz). Recently, Secrest [9] showed that offsets between source positions could be mitigated by selecting photometrically variable sources, showing that the most highly variable sources have properties consistent with blazars, such as counterparts in the Fermi catalog. Lambert & Secrest [10] then expanded upon these findings, showing photometric and astrometric variability are inversely correlated. Therefore, selecting on or weighting by photometric variability presents the clearest path to a stable and rigid reference frame. A sample of bona fide blazars selected on photometric variability may be key to the creation of a truly wavelengthindependent frame. To this end, we have created a catalog of VLBI-detected sources likely to be blazars based on variability properties derived from timedomain, photometric surveys such as ZTF and WISE. Here we present this catalog and discuss the sample and metrics used to create it.