Gaia16ada: the most recent outburst of the supernova impostor in NGC 4559

ATel #8755; Dominik J. Bomans, Ancla Mueller, Alexander Becker, Kerstin Weis (all Astronomical Institute, Ruhr-University Bochum), Thomas Granzer (Leibnitz Institute for Astrophysics, Potsdam)
on 29 Feb 2016; 15:13 UT
Credential Certification: Dominik Bomans (bomans@astro.rub.de)

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On February 11, 2016 the Gaia Photometric Alerts page (http://gsaweb.ast.cam.ac.uk/alerts) reported a possible transient near NGC 4559 at RA=12:35:52.28 DEC=+27:55:55.5, spatially coincident with a supernova impostor, with 3 previous recorded outbursts. The transient was measured by Gaia reaching a maximum of brightness of 17.72 (2016/02/09.01) in the Gaia G band (which can be approximated with SDSS g' for blue sources (Jordi et al. 2010)). We observed the transient in the night of February 23, 2016 with one of the 1.2m telescopes of the STELLA robotic observatory, using the WiFSIP imager in the g', r', i' filter bands. Despite strong scattered light due to full moon during the observation, the transient is detected at a position RA=12:35:52.26 DEC=+27:55:55.3, coincident with the positions and errors given in the Gaia alert and in Atels #3865, #6303 which reported the previous (2012/01/11.250; 2014/02/15.722) outbursts. Another outburst (2014/06/22.89) at the same position was reported by G. Contini (https://www.flickr.com/photos/snimages/14298380558/). The measured (2016/02/24.25) magnitude g' = 20.0 +- 0.2 from the STELLA data implies that the transient faded already more than 2 mag, being only ~ 0.5 mag above its brightness in apparent quiescence of g' = 20.56 (based on HST HRC data taken 2005/03/08). Additional STELLA observations of Gaia16ada on 2016/02/26 and 2016/02/29 show a slight further decline in brightness. The current STELLA g' band brightness is also near the lowest reported Gaia brightness of G = 20.22, indicating a fast rise of 1.8 mag in less than 2.5 days and fall by >2 mag in less than 12 days. Such flickering in brightness appears similar to the one observed in SN imposter SN2000ch (Pastorello et al. 2010). Gaia16ada is apparently also the first SN impostor eruption measured by Gaia. Observations and detailed analysis are ongoing.