Six months of multiwavelength follow-up of the tidal disruption candidate ASASSN-14li and implied TDE rates from ASAS-SN

Abstract

We present ground-based and Swift photometric and spectroscopic observations of the candidate tidal disruption event (TDE) ASASSN-14li, found at the center of PGC 043234 (d 90 Mpc) by the All-Sky Automated Survey for SuperNovae (ASAS-SN). The source had a peak bolometric luminosity of L 10 44 ergs s −1 and a total integrated energy of E 7 × 10 50 ergs radiated over the ∼ 6 months of observations presented. The UV/optical emission of the source is well-fit by a blackbody with roughly constant temperature of T ∼ 35, 000 K, while the luminosity declines by roughly a factor of 16 over this time. The optical/UV luminosity decline is broadly consistent with an exponential decline, L ∝ e −t/t0 , with t 0 60 days. ASASSN14li also exhibits soft X-ray emission comparable in luminosity to the optical and UV emission but declining at a slower rate, and the X-ray emission now dominates. Spectra of the source show broad Balmer and helium lines in emission as well as strong blue continuum emission at all epochs. We use the discoveries of ASASSN14li and ASASSN-14ae to estimate the TDE rate implied by ASAS-SN, finding an average rate of r 4.1 × 10 −5 yr −1 per galaxy with a 90% confidence interval of (2.2 − 17.0) × 10 −5 yr −1 per galaxy. ASAS-SN found roughly 1 TDE for every 70 Type Ia supernovae in 2014, a rate that is much higher than that of other surveys.

The X-Ray Through Optical Fluxes and Line Strengths of Tidal Disruption Events

“…The radiation absorbed by photoionization (though small) is re-emitted over a wide range of longer wavelengths and significantly enhances the optical flux. The simultaneous emission of both soft X-rays and optical light with an SED that does not follow a single blackbody bears a qualitative resemblance to PTF10iya, ASASSN-14li, and ASASSN-15oi (Cenko et al 2012a(Cenko et al , 2016Miller et al 2015;Holoien et al 2016aHoloien et al , 2016b.…”

“…Observations of ASASSN-14li showed simultaneous X-ray, UV, and optical emission Cenko et al 2016;Holoien et al 2016b). The SED cannot be fit by a single blackbody, but appears consistent with the superposition of different temperature blackbodies for the X-rays and optical/ UV.…”

“…There have also been TDE candidates found in the ultraviolet (UV; Gezari et al 2006Gezari et al , 2009, and in the optical in SDSS (van Velzen et al 2011;van Velzen & Farrar 2014), Pan-STARRS1 (Gezari et al 2012;Chornock et al 2014), ASASSN (Holoien et al 2014(Holoien et al , 2016a(Holoien et al , 2016b, PTF (Cenko et al 2012a;Arcavi et al 2014), and ROTSE (Vinkó et al 2015). These events typically rise to a peak (observer frame) R-band luminosity of ∼2×10 43 erg s −1 on a timescale of ∼months (Arcavi et al 2014), with a late time fall consistent with t −5/3 .…”

“…These events typically rise to a peak (observer frame) R-band luminosity of ∼2×10 43 erg s −1 on a timescale of ∼months (Arcavi et al 2014), with a late time fall consistent with t −5/3 . Intriguingly, PTF10iya, Swift J2058.4, ASASSN14li, and ASASSN-15oi have been simultaneously observed in both optical and X-ray (Cenko et al 2012a(Cenko et al , 2012b(Cenko et al , 2016Miller et al 2015;Holoien et al 2016aHoloien et al , 2016b.…”

“…We show that escaping line photons are produced only in the outer layers of the envelope, such that the H to He line ratio is suppressed and will vary with the envelope extent. Indeed, some optical TDE candidates do show detectable, but varying, Hα emission (Arcavi et al 2014;Holoien et al 2016b).…”