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Stars that pass too close to a super-massive black hole may be disrupted bystrong tidal forces. OGLE16aaa is one such tidal disruption event (TDE) whichrapidly brightened and peaked in the optical/UV bands in early 2016 andsubsequently decayed over the rest of the year. OGLE16aaa was detected in anXMM-Newton X-ray observation on June 9, 2016 with a flux slightly below theSwift/XRT upper limits obtained during the optical light curve peak. BetweenJune 16-21, 2016, Swift/XRT also detected OGLE16aaa and based on the stackedspectrum, we could infer that the X-ray luminosity had jumped up by more than afactor of ten in just one week. No brightening signal was seen in thesimultaneous optical/UV data to cause the X-ray luminosity to exceed theoptical/UV one. A further XMM-Newton observation on November 30, 2016 showedthat almost a year after the optical/UV peak, the X-ray emission was still atan elevated level, while the optical/UV flux decay had already leveled off tovalues comparable to those of the host galaxy. In all X-ray observations, thespectra were nicely modeled with a 50-70 eV thermal component with no intrinsicabsorption, with a weak X-ray tail seen only in the November 30 XMM-Newtonobservation. The late-time X-ray behavior of OGLE16aaa strongly resembles thetidal disruption events ASASSN-15oi and AT2019azh. We were able to pinpoint thetime delay between the initial optical TDE onset and the X-ray brightening to$182 \pm 5$ days, which may possibly represent the timescale between theinitial circularization of the disrupted star around the super-massive blackhole and the subsequent delayed accretion. Alternatively, the delayed X-raybrightening could be related to a rapid clearing of a thick envelope thatcovers the central X-ray engine during the first six months.

Rapid late-time X-ray brightening of the tidal disruption event OGLE16aaa