VLBI observations of a flared optical quasar CGRaBS J0809+5341

TL;DR

This study used VLBI observations to investigate structural changes in the high-redshift quasar CGRaBS J0809+5341 following a significant optical flare, finding no detectable jet activity or structural evolution in the radio core.

Contribution

First VLBI follow-up of a highly flaring quasar revealing no new jet components or structural changes post-flare.

Findings

No extended jet structures detected after the flare.

The radio core remained unchanged in size and flux.

Possible short-lived or unresolved jet activity.

Abstract

A bright optical flare was detected in the high-redshift ($z=2.133$) quasar CGRaBS J0809+5341 on 2014 April 13. The absolute magnitude of the object reached $-30.0$ during the flare, making it the brightest one (in flaring stage) among all known quasars so far. The 15 GHz flux density of CGRaBS J0809+5341 monitored in the period from 2008 to 2016 also reached its peak at the same time. To reveal any structural change possibly associated with the flare in the innermost radio structure of the quasar, we conducted a pilot very long baseline interferometry (VLBI) observation of CGRaBS J0809+5341 using the European VLBI Network (EVN) at 5 GHz on 2014 November 18, about seven months after the prominent optical flare. Three epochs of follow-up KaVA (Korean VLBI Network and VLBI Exploration of Radio Astrometry Array) observations were carried out at 22 and 43 GHz frequencies from 2015 February 25 to June 4, with the intention of exploring a possibly emerging new radio jet component associated with the optical flare. However, these high-resolution VLBI observations revealed only the milliarcsecond-scale compact "core" that was known in the quasar from earlier VLBI images, and showed no sign of any extended jet structure. Neither the size, nor the flux density of the "core" changed considerably after the flare according to our VLBI monitoring. The results suggest that any putative radio ejecta associated with the major optical and radio flare could not yet be separated from the "core" component, or the newly-born jet was short-lived.