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Published March 1, 2016 | Submitted + Published
Journal Article Open

Late Time Multi-wavelength Observations of Swift J1644+5734: A Luminous Optical/IR Bump and Quiescent X-Ray Emission

Abstract

We present late time multi-wavelength observations of Swift J1644+57, suggested to be a relativistic tidal disruption flare (TDF). Our observations extend to >4 years from discovery and show that 1.4 years after outburst the relativistic jet switched off on a timescale less than tens of days, corresponding to a power-law decay faster than t−70. Beyond this point weak X-rays continue to be detected at an approximately constant luminosity of L_X ~ 5 × 10^(42) erg s^(−1) and are marginally inconsistent with a continuing decay of t^(−5/3), similar to that seen prior to the switch-off. Host photometry enables us to infer a black hole mass of M_(BH) = 3 × 10^6 M_⊙, consistent with the late time X-ray luminosity arising from sub-Eddington accretion onto the black hole in the form of either an unusually optically faint active galactic nucleus or a slowly varying phase of the transient. Optical/IR observations show a clear bump in the light curve at timescales of 30–50 days, with a peak magnitude (corrected for host galaxy extinction) of M_R ~ −22 to −23. The luminosity of the bump is significantly higher than seen in other, non-relativistic TDFs and does not match any re-brightening seen at X-ray or radio wavelengths. Its luminosity, light curve shape, and spectrum are broadly similar to those seen in superluminous supervnovae, although subject to large uncertainties in the correction of the significant host extinction. We discuss these observations in the context of both TDF and massive star origins for Swift J1644+5734 and other candidate relativistic tidal flares.

Additional Information

© 2016 The American Astronomical Society. Received 2015 September 28; accepted 2015 December 23; published 2016 February 26. We thank the referee for constructive comments on the paper. A. J. L., N. R. T., K. W., and P. T. O. thank STFC for support. K. L. P. thanks the UK Space Agency. We thank Matt Mountain, Harvey Tannenbaum, and Norbert Schartel and the teams from STScI, CXC, and ESAC for the approval and rapid scheduling of DDT observations with HST, Chandra and XMM-Newton respectively. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained [from the Data Archive] at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with HST programs GO 12447, 12378 and 12764. The scientific results reported in this article are based to a significant degree on observations made by the Chandra X-ray Observatory. The observations reported are from program numbers 12900486, 13708437, and 15700509. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil), and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester, funded by the UK Space Agency.

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Published - apj_819_1_51.pdf

Submitted - 1601.05508v2.pdf

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August 22, 2023
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