Publication Date:
2016-10-09
Description:
We present ground-based and Swift photometric and spectroscopic observations of the tidal disruption event (TDE) ASASSN-15oi, discovered at the centre of 2MASX J20390918–3045201 ( d ~= 216 Mpc) by the All-Sky Automated Survey for SuperNovae. The source peaked at a bolometric luminosity of L ~= 1.3 x 10 44 erg s –1 and radiated a total energy of E ~= 6.6 x 10 50 erg over the first ~3.5 months of observations. The early optical/UV emission of the source can be fit by a blackbody with temperature increasing from T ~ 2 x 10 4 K to T ~ 4 x 10 4 K while the luminosity declines from L ~= 1.3 x 10 44 erg s –1 to L ~= 2.3 x 10 43 erg s –1 , requiring the photosphere to be shrinking rapidly. The optical/UV luminosity decline during this period is most consistent with an exponential decline, $L\propto {\rm e}^{-(t-t_0)/\tau }$ , with ~= 46.5 d for t 0 ~= 57241.6 (MJD), while a power-law decline of L ( t – t 0 ) –α with t 0 ~= 57 212.3 and α = 1.62 provides a moderately worse fit. ASASSN-15oi also exhibits roughly constant soft X-ray emission that is significantly weaker than the optical/UV emission. Spectra of the source show broad helium emission lines and strong blue continuum emission in early epochs, although these features fade rapidly and are not present ~3 months after discovery. The early spectroscopic features and colour evolution of ASASSN-15oi are consistent with a TDE, but the rapid spectral evolution is unique among optically selected TDEs.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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