The Rise and Fall of ASASSN-18pg : Following a TDE from Early to Late Times

Holoien, Thomas W.S. and Auchettl, Katie and Tucker, Michael A. and Shappee, Benjamin J. and Patel, Shannon G. and Miller-Jones, James C.A. and Mockler, Brenna and Groenewald, Dani l.N. and Hinkle, Jason T. and Brown, Jonathan S. and Kochanek, Christopher S. and Stanek, K. Z. and Chen, Ping and Dong, Subo and Prieto, Jose L. and Thompson, Todd A. and Beaton, Rachael L. and Connor, Thomas and Cowperthwaite, Philip S. and Dahmen, Linnea and French, K. Decker and Morrell, Nidia and Buckley, David A.H. and Gromadzki, Mariusz and Roy, Rupak and Coulter, David A. and Dimitriadis, Georgios and Foley, Ryan J. and Kilpatrick, Charles D. and Piro, Anthony L. and Rojas-Bravo, César and Siebert, Matthew R. and Velzen, Sjoert van (2020) The Rise and Fall of ASASSN-18pg : Following a TDE from Early to Late Times. Astrophysical Journal, 898 (2): 161. ISSN 0004-637X

Abstract

We present nearly 500 days of observations of the tidal disruption event (TDE) ASASSN-18pg, spanning from 54 days before peak light to 441 days after peak light. Our data set includes X-ray, UV, and optical photometry, optical spectroscopy, radio observations, and the first published spectropolarimetric observations of a TDE. ASASSN-18pg was discovered on 2018 July 11 by the All-Sky Automated Survey for Supernovae (ASAS-SN) at a distance of d = 78.6 Mpc; with a peak UV magnitude of m ≃ 14, it is both one of the nearest and brightest TDEs discovered to-date. The photometric data allow us to track both the rise to peak and the long-term evolution of the TDE. ASASSN-18pg peaked at a luminosity of L ≃ 2.4 1044 erg s-1, and its late-time evolution is shallower than a flux ∝t -5/3 power-law model, similar to what has been seen in other TDEs. ASASSN-18pg exhibited Balmer lines and spectroscopic features consistent with Bowen fluorescence prior to peak, which remained detectable for roughly 225 days after peak. Analysis of the two-component Hα profile indicates that, if they are the result of reprocessing of emission from the accretion disk, the different spectroscopic lines may be coming from regions between ∼10 and ∼60 lt-days from the black hole. No X-ray emission is detected from the TDE, and there is no evidence of a jet or strong outflow detected in the radio. Our spectropolarimetric observations indicate that the projected emission region is likely not significantly aspherical, with the projected emission region having an axis ratio of ⪆0.65.