Slow-blue nuclear hypervariables in PanSTARRS-1
Bruce, A. G.
Smartt, S. J.
Smith, K. W.
Chen, T. W.
Monthly Notices of the Royal Astronomical Society
Oxford University Press
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Lawrence, A., Bruce, A. G., MacLeod, C., Gezari, S., Elvis, M., Ward, M., Smartt, S. J., et al. (2016). Slow-blue nuclear hypervariables in PanSTARRS-1. Monthly Notices of the Royal Astronomical Society, 463 (1), 296-331. https://doi.org/10.1093/mnras/stw1963
This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw1963
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) 3$\pi$ 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 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.
accretion, accretion discs, gravitational lensing: micro, galaxies: active, galaxies: nuclei, quasars: general
European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013) (Grant IDs: 291222, 320360)
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External DOI: https://doi.org/10.1093/mnras/stw1963
This record's URL: https://www.repository.cam.ac.uk/handle/1810/260416
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