Context : Aims : New high variabililty extragalactic sources may be identified by comparing the flux of sources seen in the XMM- Newton Slew Survey with detections and upper limits from the ROSAT All Sky Survey . Methods : Detected flaring extragalactic sources , are subsequently monitored with Swift and XMM- Newton to track their temporal and spectral evolution . Optical and radio observations are 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 . Results : 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 L _ { X } \sim 8 \times 10 ^ { 43 } ergs s ^ { -1 } with a typical Sy I-like power-law X-ray spectrum of \Gamma \sim 2 . The flare has either been caused by a tidal disruption event or by an increase in the accretion rate of a persistent AGN . Conclusions :