J/ApJ/836/25 Swift UVOT light curves of ASASSN-15lh (Margutti+, 2017) ================================================================================ X-rays from the location of the double-humped transient ASASSN-15lh. Margutti R., Metzger B.D., Chornock R., Milisavljevic D., Berger E., Blanchard P.K., Guidorzi C., Migliori G., Kamble A., Lunnan R., Nicholl M., Coppejans D.L., Dall'Osso S., Drout M.R., Perna R., Sbarufatti B. =2017ApJ...836...25M (SIMBAD/NED BibCode) ================================================================================ ADC_Keywords: Photometry, ultraviolet ; Photometry, UBV ; Supernovae Keywords: supernovae: individual: ASASSN-15lh Abstract: We present the detection of persistent soft X-ray radiation with L_x_~10^41^-10^42^erg/s at the location of the extremely luminous, double-humped transient ASASSN-15lh as revealed by Chandra and Swift. We interpret this finding in the context of observations from our multiwavelength campaign, which revealed the presence of weak narrow nebular emission features from the host-galaxy nucleus and clear differences with respect to superluminous supernova optical spectra. Significant UV flux variability on short timescales detected at the time of the rebrightening disfavors the shock interaction scenario as the source of energy powering the long-lived UV emission, while deep radio limits exclude the presence of relativistic jets propagating into a low-density environment. We propose a model where the extreme luminosity and double-peaked temporal structure of ASASSN-15lh is powered by a central source of ionizing radiation that produces a sudden change in the ejecta opacity at later times. As a result, UV radiation can more easily escape, producing the second bump in the light curve. We discuss different interpretations for the intrinsic nature of the ionizing source. We conclude that, if the X-ray source is physically associated with the optical-UV transient, then ASASSN-15lh most likely represents the tidal disruption of a main-sequence star by the most massive spinning black hole detected to date. In this case, ASASSN-15lh and similar events discovered in the future would constitute the most direct probes of very massive, dormant, spinning, supermassive black holes in galaxies. Future monitoring of the X-rays may allow us to distinguish between the supernova hypothesis and the hypothesis of a tidal disruption event. Description: We obtained four epochs of deep X-ray observations of ASASSN-15lh with the Chandra X-ray Observatory (CXO) on 2015 November 12 (exposure of 10ks), 2015 December 13 (10ks), 2016 February 20 (40ks), and 2016 August 19 (30 ks, PI Margutti), corresponding to {delta}t=129.4d, {delta}t=154.6, {delta}t=210.5d, and {delta}t=357.8d rest-frame since optical maximum light, which occurred on 2015 June 5. We reprocessed all the X-ray data collected by the Swift-XRT and all the Swift-UVOT observations obtained from 2015 June 24 until 2016 July 22. A partial collection of the Swift-UVOT photometry of ASASSN-15lh has already been presented by Dong+ (2016Sci...351..257D), Godoy-Rivera+ (2017MNRAS.466.1428G), Brown+ (2016, J/ApJ/828/3), and Leloudas+ (2016NatAs...1E...2L). Here we update the observations and focus on the presence of significant temporal variability that appears at the time of the rebrightening. Objects: ---------------------------------------------------------- RA (ICRS) DE Designation(s) ---------------------------------------------------------- 22 02 15.45 -61 39 34.6 ASASSN-15lh = SN 2015L ---------------------------------------------------------- File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table2.dat 119 75 Swift UVOT photometry -------------------------------------------------------------------------------- See also: B/chandra : The Chandra Archive Log (CXC, 1999-2014) B/xmm : XMM-Newton Log (XMM-Newton Science Operation Center, 2012) B/sn : Asiago Supernova Catalogue (Barbon et al., 1999-) VIII/76 : Leiden/Argentine/Bonn (LAB) Survey of Galactic HI (Kalberla+ 2005) J/AJ/137/4517 : UVOT light curves of supernovae (Brown+, 2009) J/MNRAS/412/1419 : Nearby supernova rates (Leaman+, 2011) J/ApJ/746/156 : Radio afterglow observations of GRBs (Chandra+, 2012) J/MNRAS/428/729 : GRB Swift X-ray light curves analysis (Margutti+, 2013) J/ApJ/794/23 : Pan-STARRS1 transients optical photometry (Drout+, 2014) J/ApJ/828/3 : Swift obs. of the superluminous SNI ASASSN-15lh (Brown+, 2016) J/ApJ/817/103 : Swift/XRT LC of Swift J164449.3+573451 (Mangano+, 2016) J/ApJ/830/13 : Host-galaxy NUV-NIR data of superluminous SNe (Perley+, 2016) Byte-by-byte Description of file: table2.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 8 F8.2 d MJD-v ? Observation date, Modified Julian Date, vmag 10- 14 F5.2 mag vmag [16.8/19]? Swift/UVOT v band Vega magnitude 16- 19 F4.2 mag e_vmag [0.07/0.6]? Uncertainty in vmag 21- 28 F8.2 d MJD-b ? Observation date, Modified Julian Date, bmag 30- 34 F5.2 mag bmag [16.7/19.5]? Swift/UVOT b band Vega magnitude 36- 39 F4.2 mag e_bmag [0.04/0.3]? Uncertainty in bmag 41- 48 F8.2 d MJD-u ? Observation date, Modified Julian Date, umag 50- 54 F5.2 mag umag [15.3/18]? Swift/UVOT u band Vega magnitude 56- 59 F4.2 mag e_umag [0.03/0.2]? Uncertainty in umag 61- 68 F8.2 d MJD-w1 ? Observation date, Modified Julian Date, w1mag 70- 74 F5.2 mag w1mag [15.2/18]? Swift/UVOT w1 band Vega magnitude 76- 79 F4.2 mag e_w1mag [0.04/0.2]? Uncertainty in w1mag 81- 88 F8.2 d MJD-w2 ? Observation date, Modified Julian Date, w2mag 90- 94 F5.2 mag w2mag [15.6/18.5]? Swift/UVOT w2 band Vega magnitude 96- 99 F4.2 mag e_w2mag [0.04/0.2]? Uncertainty in w2mag 101-108 F8.2 d MJD-m2 ? Observation date, Modified Julian Date, m2mag 110-114 F5.2 mag m2mag [15.2/18.3]? Swift/UVOT m2 band Vega magnitude 116-119 F4.2 mag e_m2mag [0.04/0.2]? Uncertainty in m2mag -------------------------------------------------------------------------------- History: From electronic version of the journal ================================================================================ (End) Prepared by [AAS], Emmanuelle Perret [CDS] 07-Sep-2017