XMMSL2 J144605.0+685735: a slow tidal disruption event

¹ Telespazio-Vega UK for ESA, European Space Astronomy Centre, Operations Department, 28691 Villanueva de la Cañada, Spain
e-mail: richard.saxton@sciops.esa.int, rsaxton@xmm.vilspa.esa.es
² Dept. of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
³ Max Planck Institut für Radioastronomie, Auf dem Huegel 69, 53121 Bonn, Germany
⁴ Universidad de Chile, Observatorio Astronomico Nacional Cerro Calan, Santiago, Chile
⁵ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
⁶ Liverpool John Moores University, Liverpool L3 5RF, UK
⁷ Dpto. de Física de la Tierra y Astrofísica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA

Received: 10 April 2019
Accepted: 15 August 2019

Abstract

Aims. We investigate the evolution of X-ray selected tidal disruption events.

Methods. New events are found in near real-time data from XMM-Newton slews, and are monitored by multi-wavelength facilities.

Results. In August 2016, X-ray emission was detected from the galaxy XMMSL2 J144605.0+685735 (also known as 2MASX 14460522+6857311), that was 20 times higher than an upper limit from 25 years earlier. The X-ray flux was flat for ∼100 days and then fell by a factor of 100 over the following 500 days. The UV flux was stable for the first 400 days before fading by a magnitude, while the optical (U,B,V) bands were roughly constant for 850 days. Optically, the galaxy appears to be quiescent, at a distance of 127 ± 4 Mpc (z = 0.029 ± 0.001) with a spectrum consisting of a young stellar population of 1–5 Gyr in age, an older population, and a total stellar mass of ∼6 × 10⁹ M_⊙. The bolometric luminosity peaked at L_bol ∼ 10⁴³ ergs s⁻¹ with an X-ray spectrum that may be modelled by a power law of Γ ∼ 2.6 or Comptonisation of a low-temperature thermal component by thermal electrons. We consider a tidal disruption event to be the most likely cause of the flare. Radio emission was absent in this event down to < 10 μJy, which limits the total energy of a hypothetical off-axis jet to E <  5 × 10⁵⁰ ergs. The independent behaviour of the optical, UV, and X-ray light curves challenges models where the UV emission is produced by reprocessing of thermal nuclear emission or by stream-stream collisions. We suggest that the observed UV emission may have been produced from a truncated accretion disc and the X-rays from Compton upscattering of these disc photons.