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Title: Slow-blue nuclear hypervariables in PanSTARRS-1

We discuss 76 large amplitude transients (Δm > 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) 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 ~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 explanationsmore » 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. As a result, both hold promise of considerable new insight into the AGN phenomenon.« less
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  1. Univ. of Edinburgh (United Kingdom)
  2. Univ. of Maryland, College Park, MD (United States)
  3. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
  4. Durham Univ. (United Kingdom)
  5. Queen's Univ., Belfast, Northern Ireland (United Kingdom)
  6. Queen's Univ., Belfast, Northern Ireland (United Kingdom); Univ. of Cambridge (United Kingdom)
  7. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  8. Univ. of Hawaii, Honolulu, HI (United States)
  9. Princeton Univ., Princeton, NJ (United States)
Publication Date:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Monthly Notices of the Royal Astronomical Society
Additional Journal Information:
Journal Volume: 463; Journal Issue: 1; Journal ID: ISSN 0035-8711
Royal Astronomical Society
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
Country of Publication:
United States
79 ASTRONOMY AND ASTROPHYSICS; accretion; accretion discs; gravitational lensing: micro; galaxies: active; galaxies: nuclei; quasars: general
OSTI Identifier: