Abstract
We discuss 76 large amplitude transients (Δm>1.5) occurring in the nuclei of galaxies, nearly allwith 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 theWilliam Herschel Telescope (WHT). Based on colours, light-curve shape, and spectra, these transients fall into four groups.Afeware 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 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 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.
Original language | English |
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Pages (from-to) | 296-331 |
Number of pages | 36 |
Journal | Monthly Notices of the Royal Astronomical Society |
Volume | 463 |
Issue number | 1 |
DOIs | |
Publication status | Published - 08 Sep 2016 |
Keywords
- Accretion, accretion discs
- Galaxies:active
- Galaxies:nuclei
- Gravitational lensing:Micro
- Quasars:General
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science