Discovery and follow-up of ASASSN-19dj: an X-ray and UV luminous TDE in an extreme post-starburst galaxy

• Published in: Monthly notices of the Royal Astronomical Society Vol. 500; no. 2; pp. 1673 - 1696
• Main Authors: Hinkle, Jason T, Holoien, T W-S, Auchettl, K, Shappee, B J, Neustadt, J M M, Payne, A V, Brown, J S, Kochanek, C S, Stanek, K Z, Graham, M J, Tucker, M A, Do, A, Anderson, J P, Bose, S, Chen, P, Coulter, D A, Dimitriadis, G, Dong, Subo, Foley, R J, Huber, M E, Hung, T, Kilpatrick, C D, Pignata, G, Piro, A L, Rojas-Bravo, C, Siebert, M R, Stalder, B, Thompson, Todd A, Tonry, J L, Vallely, P J, Wisniewski, J P
• Format: Journal Article
• Language: English
• Published: United States Oxford University Press 01-01-2021
• Subjects: accretion; accretion discs; Astronomy & Astrophysics; ASTRONOMY AND ASTROPHYSICS; black hole physics; galaxies: nuclei; black hole physics; galaxies: nuclei; accretion, accretion discs
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/staa3170

ABSTRACT
We present observations of ASASSN-19dj, a nearby tidal disruption event (TDE) discovered in the post-starburst galaxy KUG 0810+227 by the All-Sky Automated Survey for Supernovae (ASAS-SN) at a distance of d ≃ 98 Mpc. We observed ASASSN-19dj from −21 to 392 d relative to peak ultraviolet (UV)/optical emission using high-cadence, multiwavelength spectroscopy and photometry. From the ASAS-SN g-band data, we determine that the TDE began to brighten on 2019 February 6.8 and for the first 16 d the rise was consistent with a flux ∝t2 power law. ASASSN-19dj peaked in the UV/optical on 2019 March 6.5 (MJD = 58548.5) at a bolometric luminosity of L = (6.2 ± 0.2) × 1044 erg s−1. Initially remaining roughly constant in X-rays and slowly fading in the UV/optical, the X-ray flux increased by over an order of magnitude ∼225 d after peak, resulting from the expansion of the X-ray emitting region. The late-time X-ray emission is well fitted by a blackbody with an effective radius of ∼1 × 1012 cm and a temperature of ∼6 × 105 K. The X-ray hardness ratio becomes softer after brightening and then returns to a harder state as the X-rays fade. Analysis of Catalina Real-Time Transient Survey images reveals a nuclear outburst roughly 14.5 yr earlier with a smooth decline and a luminosity of LV ≥ 1.4 × 1043 erg s−1, although the nature of the flare is unknown. ASASSN-19dj occurred in the most extreme post-starburst galaxy yet to host a TDE, with Lick HδA = 7.67 ± 0.17 Å.