A tidal disruption-like X-ray flare from the quiescent galaxy SDSS J120136.02+300305.5^⋆

¹ XMM SOC, ESAC, Apartado 78, 28691 Villanueva de la Cañada, Madrid, Spain
e-mail: richard.saxton@sciops.esa.int
² Dept. of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
³ Technische Universität München, Fakultät für Physik, James-Franck-Strasse 1/I, 85748 Garching, Germany
⁴ Excellence Cluster Universe, TUM, Boltzmannstrasse 2, 85748 Garching, Germany
⁵ Max Planck Institut für Plasmaphysik, Boltzmannstrasse 2, 85748 Garching, Germany
⁶ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, 100012 Beijing, PR China
⁷ National Research Council, Herzberg Institute of Astrophysics, DRAO, Box 248, Penticton BC, V2A 6J9, Canada
⁸ Dept of Physics and Astronomy, University of Calgary, 2500 University Dr. NW., Calgary, AB T2N 1N4, Canada
⁹ Centro de Astrobiología Depto. Astrofisica (INTA-CSIC), ESAC campus, Apartado 78, 28691 Villanueva de la Cañada, Spain
¹⁰ Calar Alto Observatory, Centro Astronómico Hispano Alemán, Almería, Spain

Received: 31 October 2011
Accepted: 25 February 2012

Abstract

Aims. The study of tidal disruption flares from galactic nuclei has historically been hampered by a lack of high quality spectral observations taken around the peak of the outburst. Here we introduce the first results from a program designed to identify tidal disruption events at their peak by making near-real-time comparisons of the flux seen in XMM-Newton slew sources with that seen in ROSAT.

Methods. Flaring extragalactic sources, which do not appear to be AGN, are monitored with Swift and XMM-Newton to track their temporal and spectral evolution. Timely optical observations are made to monitor the reaction of circumnuclear material to the X-ray flare.

Results. SDSS J120136.02+300305.5 was detected in an XMM-Newton slew from June 2010 with a flux 56 times higher than an upper limit from ROSAT, corresponding to L_X ~ 3 × 10⁴⁴ erg s^-1. It has the optical spectrum of a quiescent galaxy (z = 0.146). Overall the X-ray flux has evolved consistently with the canonical t^−5/3 model, expected for returning stellar debris, fading by a factor  ~300 over 300 days. In detail the source is very variable and became invisible to Swift between 27 and 48 days after discovery, perhaps due to self-absorption. The X-ray spectrum is soft but is not the expected tail of optically thick thermal emission. It may be fit with a Bremsstrahlung or double-power-law model and is seen to soften with time and declining flux. Optical spectra taken 12 days and 11 months after discovery indicate a deficit of material in the broad line and coronal line regions of this galaxy, while a deep radio non-detection implies that a jet was not launched during this event.