An X-ray and UV flare from the galaxy XMMSL1 J061927.1-655311

<p><strong>Aims:</strong> New high variability extragalactic sources may be identified by comparing the flux of sources seen in the <i>XMM-Newton</i> Slew Survey with detections and upper limits from the ROSAT All Sky Survey.</p> <p><strong>Methods:</strong> A detected flaring extragalactic source was monitored with <i>Swift</i> and <i>XMM-Newton</i> to track its temporal and spectral evolution. Optical and radio observations were made to help classify the galaxy, investigate the reaction of circumnuclear material to the X-ray flare, and check for the presence of a jet.</p> <p><strong>Results:</strong> In November 2012, X-ray emission was detected from the galaxy XMMSL1 J061927.1-655311 (a.k.a. 2MASX 06192755-6553079), a factor 140 times higher than an upper limit from 20 years earlier. Both the X-ray and UV flux subsequently fell over the following year by factors of 20 and 4, respectively. Optically, the galaxy appears to be a Seyfert I with broad Balmer lines and weak, narrow, low-ionisation emission lines, at a redshift of 0.0729. The X-ray luminosity peaks at <i>L</i>_<i>X</i> ~ 8 × 10⁴³ erg s^-1 with a typical Sy I-like power-law X-ray spectrum of Γ ~ 2. The flare has either been caused by a tidal disruption event or by an increase in the accretion rate of a persistent active galactic nucleus.</p>