X-Ray Brightening and UV Fading of Tidal Disruption Event ASASSN-15oi

S. Gezari, S. B. Cenko, I. Arcavi

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We present late-time observations by Swift and XMM-Newton of the tidal disruption event (TDE) ASASSN-15oi that reveal that the source brightened in the X-rays by a factor of ∼10 one year after its discovery, while it faded in the UV/optical by a factor of ∼100. The XMM-Newton observations measure a soft X-ray blackbody component with kTbb ∼ 45 eV, corresponding to radiation from several gravitational radii of a central ∼106 M black hole. The last Swift epoch taken almost 600 days after discovery shows that the X-ray source has faded back to its levels during the UV/optical peak. The timescale of the X-ray brightening suggests that the X-ray emission could be coming from delayed accretion through a newly forming debris disk and that the prompt UV/optical emission is from the prior circularization of the disk through stream-stream collisions. The lack of spectral evolution during the X-ray brightening disfavors ionization breakout of a TDE "veiled" by obscuring material. This is the first time a TDE has been shown to have a delayed peak in soft X-rays relative to the UV/optical peak, which may be the first clear signature of the real-time assembly of a nascent accretion disk, and provides strong evidence for the origin of the UV/optical emission from circularization, as opposed to reprocessed emission of accretion radiation.

Original languageEnglish
Article numberL47
JournalAstrophysical Journal Letters
Issue number2
StatePublished - 20 Dec 2017
Externally publishedYes


FundersFunder number
National Science Foundation
Directorate for Mathematical and Physical Sciences1454816
Directorate for Mathematical and Physical Sciences
National Aeronautics and Space AdministrationPF6-170148
National Aeronautics and Space Administration


    • accretion, accretion disks
    • black hole physics
    • galaxies: nuclei


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