Slow-blue nuclear hypervariables in PanSTARRS-1

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

Abstract

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 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.

Citation

Lawrence, A., Bruce, A., MacLeod, C., Gezari, S., Elvis, M., Ward, M., …Soderberg, A. (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

• Journal Article Type: Article
• Acceptance Date: Aug 4, 2016
• Online Publication Date: Aug 9, 2016
• Publication Date: Nov 21, 2016
• Deposit Date: Sep 28, 2016
• Publicly Available Date: Oct 6, 2016
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 463
• Issue: 1
• Pages: 296-331
• DOI: https://doi.org/10.1093/mnras/stw1963