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. et al. 2016. "Slow-blue nuclear hypervariables in PanSTARRS-1." Monthly Notices of the Royal Astronomical Society, 463 296–331. https://doi.org/10.1093/mnras/stw1963.
We discuss 76 large amplitude transients (?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) 3p 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.
