Archival XMM and latest Swift/XRT X-ray observations of the candidate tidal disruption event AT2019aalc/ZTF19aaejtoy

ATel #16118; Dheeraj Pasham (MIT)
on 4 Jul 2023; 20:23 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)

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The optical transient AT2019aalc/ZTF19aaejtoy has been suggested to be an AGN tidal disruption event candidate and also potentially the host of the IceCube neutrino event IC191119A (van Velzen et al. 2021). Recently, it was reported to re-brighten in the optical band 4 years after its first flare in April 2019 (Veres et al. 2023).

We started by analyzing the archival XMM-Newton observation (obsID: 0862900501) from 20 February 2021 during optical quiescence. After screening for background/particle flaring we recovered about 15 ks of exposure (3650 counts). We extracted an EPIC-pn spectrum and fit it in the 0.2-10 keV band using the C-statistic with tbabs*zashift(powlaw) and tbabs*zashift(diskbb). The resulting C-stat/degrees of freedom were 160/168 and 312/168, respectively, suggesting that a powerlaw is a better model. The best-fit powerlaw index and the observed 0.3-8 keV flux were 2.61(+0.08,-0.08) and 5.1(+0.1,-0.2)e-13 erg/s/cm^2 (68% confidence), respectively.

We then analyzed the two recent Swift/XRT datasets (obsIDs: 00016080001 and 000160800012) which had a total exposure time of 2138 s. These were taken on the 23 and the 29 of June 2023, respectively. We extracted a combined X-ray spectrum which yielded 59 source counts in the 0.3-8.0 keV band. Modeling the spectrum with a powerlaw (tbabs*zashift(pow) in XSPEC) yields a best-fit powerlaw index of 3.03(+0.43,-0.40) and an observed 0.3-8.0 flux of 8.4(+1.2,-1.2)e-13 erg/s/cm^2 (68% confidence). The best-fit C-stat/degrees of freedom was 14/15.

The most recent powerlaw index is consistent with the value from more than 2 years ago and the X-ray flux is marginally elevated, which is not surprising for a variable AGN. We have requested for NICER observations which should be able to constrain the recent spectrum better.

References: van Velzen et al., arXiv e-prints, arXiv:2111.09391 (2021); Veres et al., astronote, 2023-194