Abstract
We study X-ray bright tidal disruption events (TDE), close to the peak of their emission, with the intention of understanding the evolution of their light curves and spectra. Candidate TDE are identified by searching for soft X-ray flares from non-active galaxies in recent XMM-Newton slew data. In April 2014, X-ray emission was detected from the galaxy XMMSL1 J074008.2-853927 (a.k.a. 2MASX 07400785-8539307), a factor 20 times higher than an upper limit from 20 years earlier. Both the X-ray and UV flux subsequently fell, by factors of 70 and 12 respectively. The bolometric luminosity peaked at Lbol~2E44 ergs/s with a spectrum that may be modelled with thermal emission in the UV band, a power-law with slope~2 dominating in the X-ray band above 2 keV and a soft X-ray excess with an effective temperature of ~86 eV. Rapid variability locates the X-ray emission to within <73 Rg of the nuclear black hole. Radio emission of flux density ~1 mJy, peaking at 1.5 GHz was detected 21 months after discovery. Optical spectra indicate that the galaxy, at a distance of 73 Mpc (z=0.0173), underwent a starburst 2 Gyr ago and is now quiescent. We consider a tidal disruption event to be the most likely cause of the flare. If this proves to be correct then this is a very clean example of a disruption exhibiting both thermal and non-thermal radiation.
Highlights
When a star approaches very close to a supermassive black hole (SMBH) the gravitational shear can overcome the selfgravitation of the star causing it to be torn apart (Hills 1975) in what is known as a tidal disruption event (TDE)
A flare was detected from the galaxy XMMSL1 J0740-85 on April 1st 2014, reaching a bolometric luminosity, Lbol = 2 × 1044 ergs s−1
From the rapid X-ray variability we calculate the mass of the black hole to be 1 × < MBH
Summary
When a star approaches very close to a supermassive black hole (SMBH) the gravitational shear can overcome the selfgravitation of the star causing it to be torn apart (Hills 1975) in what is known as a tidal disruption event (TDE). About half of the stellar debris remains gravitationally bound to the hole and is accreted on return, causing a flare of thermal radiation which peaks in the EUV band (Rees 1988) This radiation signature has been observed in the soft X-ray (Komossa et al 2004; Bade et al 1996; Komossa & Greiner 1999b; Esquej et al 2007; Saxton et al 2012; Maksym et al 2010; Lin et al 2015) and UV (Gezari et al 2006, 2008, 2009) bands. Transient radio emission was seen in ASSASN14li (Alexander et al 2016; van Velzen et al 2016) but at a level which may be consistent with a non-relativistic outflow rather than a jet It is an open, and interesting, question as to why these events should show a hard X-ray component, compatible with power-law emission, while many TDE show only thermal X-ray emission
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