Slow blue nuclear hypervariables in PanSTARRS-1

TL;DR

This paper reports on 76 large amplitude nuclear transients discovered in galaxy centers by Pan-STARRS-1, most of which are likely hypervariable AGN showing slow evolution over years, with potential explanations including accretion changes and microlensing.

Contribution

The study identifies and characterizes a new class of slow, blue hypervariable AGN, expanding understanding of AGN variability and proposing multiple possible physical mechanisms.

Findings

Majority are likely hypervariable AGN at z~0.3-1.4.

Most objects brightened by about an order of magnitude since SDSS.

Mixture of accretion state changes and microlensing likely explains variability.

Abstract

We discuss 76 large amplitude transients (Delta-m>1.5) occurring in the nuclei of galaxies, nearly all with no previously known Active Galactic Nucleus (AGN). They have been discovered as part of the Pan-STARRS1 (PS1) 3pi survey, by comparison with SDSS photometry a decade earlier, and then monitored with the Liverpool Telescope, and studied spectroscopically with the William Herschel Telescope (WHT). Based on colours, light curve shape, and spectra, these transients fall into four groups. A few are misclassified stars or objects of unknown type. Some are red/fast transients and are known or likely nuclear supernovae. A few are either radio sources or erratic variables and so likely blazars. However the majority (~66%) are blue and evolve slowly, on a timescale of years. Spectroscopy shows them to be AGN at z~ 0.3 - 1.4, which must have brightened since the SDSS photometry by around an order of magnitude. It is likely that these objects were in fact AGN a decade ago, but too weak to be recognised by SDSS; they could then be classed as "hypervariable" AGN. By searching the SDSS Stripe 82 quasar database, we find 15 similar objects. We discuss several possible explanations for these slow blue hypervariables - (i) unusually luminous tidal disruption events; (ii) extinction events; (iii) changes in accretion state; and (iv) large amplitude microlensing by stars in foreground galaxies. A mixture of explanations (iii) and (iv) seems most likely. Both hold promise of considerable new insight into the AGN phenomenon.