Slow-blue nuclear hypervariables in PanSTARRS-1

A. Lawrence, A. G. Bruce, C. MacLeod, S. Gezari, M. Elvis, M. Ward, S. J. Smartt, K. W. Smith, D. Wright, M. Fraser, P. Marshall, N. Kaiser, W. Burgett, E. Magnier, J. Tonry, K. Chambers, R. Wainscoat, C. Waters, P. Price, N. MetcalfeS. Valenti, R. Kotak, A. Mead, C. Inserra, T. W. Chen, A. Soderberg

Research output: Contribution to journalArticlepeer-review

25 Scopus citations

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 languageEnglish (US)
Pages (from-to)296-331
Number of pages36
JournalMonthly Notices of the Royal Astronomical Society
Volume463
Issue number1
DOIs
StatePublished - Nov 21 2016

Bibliographical note

Publisher Copyright:
© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Keywords

  • Accretion, accretion discs
  • Galaxies:active
  • Galaxies:nuclei
  • Gravitational lensing:Micro
  • Quasars:General

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