Slow-blue nuclear hypervariables in PanSTARRS-1.

TLDR: In this paper, the authors discuss 76 large amplitude transients (Δm > 15) occurring in the nuclei of galaxies, nearly all with no previously known active galactic nucleus (AGN).

Abstract: We discuss 76 large amplitude transients (Δm > 15) 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) 3π survey, by comparison with Sloan Digital Sky Survey (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 per cent) are blue and evolve slowly, on a time-scale of years Spectroscopy shows them to be AGN at z ∼ 03 − 14, 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 recognized 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