The Broad Absorption Line Tidal Disruption Event iPTF15af: Optical and Ultraviolet Evolution

Creators: Blagorodnova, N.; Cenko, S. B.; Kulkarni, S. R.; Arcavi, I.; Bloom, J. S.; Duggan, G.; Filippenko, A. V.; Fremling, C.; Horesh, A.; Hosseinzadeh, G.; Karamehmetoglu, E.; Levan, A.; Masci, F. J.; Nugent, P. E.; Pasham, D. R.; Veilleux, S.; Walters, R.; Yan, L.; Zheng, W.

Abstract

We present multiwavelength observations of the tidal disruption event (TDE) iPTF15af, discovered by the intermediate Palomar Transient Factory survey at redshift z = 0.07897. The optical and ultraviolet (UV) light curves of the transient show a slow decay over 5 months, in agreement with previous optically discovered TDEs. It also has a comparable blackbody peak luminosity of L_(peak) ≈ 1.2 × 10^(44) erg s^(-1). The inferred temperature from the optical and UV data shows a value of (3–5) × 10^4 K. The transient is not detected in X-rays up to L_X < 3 × 10^(42) erg s^(-1) within the first 5 months after discovery. The optical spectra exhibit two distinct broad emission lines in the He ii region, and at later times also Hα emission. Additionally, emission from [N iii] and [O iii] is detected, likely produced by the Bowen fluorescence effect. UV spectra reveal broad emission and absorption lines associated with high-ionization states of N v, C iv, Si iv, and possibly P v. These features, analogous to those of broad absorption line quasars (BAL QSOs), require an absorber with column densities N_H > 10^(23) cm^(−2). This optically thick gas would also explain the nondetection in soft X-rays. The profile of the absorption lines with the highest column density material at the largest velocity is opposite that of BAL QSOs. We suggest that radiation pressure generated by the TDE flare at early times could have provided the initial acceleration mechanism for this gas. Spectral UV line monitoring of future TDEs could test this proposal.

Additional Information

© 2019. The American Astronomical Society. Received 2018 September 19; revised 2019 January 23; accepted 2019 February 1; published 2019 March 7. We thank the anonymous referee, whose comments and suggestions helped to improve the paper. We are grateful to Peter Maksym, Sterl Phinney, Eliot Quataert, Clément Bonnerot, Tiara Hung, and Sjoert Van Velzen for valuable discussions and comments on this work. We are grateful to Eric Bellm, Melissa Graham, Anna Ho, and Daniel Perley for carrying out some of the spectroscopic observations and/or reductions, and Linda Strubbe for her support with the HST proposal. We thank the HST staff for the prompt scheduling of these ToO observations, as well as the PI Neil Gehrels and the Swift ToO team for the timely execution of our observations. This work was supported by the GROWTH project funded by the National Science Foundation (NSF) under grant AST-1545949. This work is part of the research program VENI, with project no. 016.192.277, which is (partly) financed by the Netherlands Organisation for Scientific Research (NWO). Support for I.A. was provided by NASA through the Einstein Fellowship Program, grant PF6-170148. A.H. acknowledges support by the I-Core Program of the Planning and Budgeting Committee and the Israel Science Foundation. This research was supported by a Grant from the GIF, the German-Israeli Foundation for Scientific Research and Development. A.V.F.'s group at UC Berkeley is grateful for financial assistance from the TABASGO Foundation, the Christopher R. Redlich Fund, and the Miller Institute for Basic Research in Science (UC Berkeley). Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration (NASA); the observatory was made possible by the generous financial support of the W. M. Keck Foundation. This work makes use of observations from Las Cumbres Observatory. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the NSF. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by NASA. Based on observations made with the NASA Galaxy Evolution Explorer. GALEX is operated for NASA by the California Institute of Technology under NASA contract NAS5-98034. Facilities: HST (STIS) - , Karl G. Jansky Very Large Array - , Keck:I (LRIS) - , PO:1.2 m - , PO:1.5 m - , Swift - , WISE. - Software: Astropy (The Astropy Collaboration et al. 2018), emcee (Foreman-Mackey et al. 2013), FPipe (Fremling et al. 2016), lcogtsnpipe (Valenti et al. 2016), lmfit (Newville et al. 2014), lpipe http://www.astro.caltech.edu/~dperley/programs/lpipe.html, MOSFIT (Guillochon et al. 2018), NumPy (van der Walt et al. 2011), PTFIDE (Masci et al. 2017), SciPy (Jones et al. 2001).

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