AstroNote 2022-207

We observed the fast transient AT2022tsd with Swift/XRT. The observation started at 9:17 UT on 4 October 2022, corresponding to 26.9 days after the first ZTF detection. In a 3.7 ks exposure (distributed over 14.3 hours), we detect X-ray emission with a count rate of 0.010 +- 0.002 ct/s. Using WebPIMMS [1] we estimate [2] that this corresponds to an unabsorbed 0.3-10 keV flux of 4.8E-13 ergs/cm^2/s. At z=0.256 (Ho et al. AstroNote 2022-199) this is a luminosity of 1E44 erg/s, typical of an X-ray afterglow of a long-duration GRB at this epoch [3], an order of magnitude more luminous than AT2018cow at the same epoch (Rivera Sandoval et al. 2018, MNRAS, 480, L146; Margutti et al. 2019, ApJ, 872, 18; Ho et al. 2019, ApJ, 871, 73), and a factor of a few more luminous than that of AT2020mrf at a similar epoch (Yao et al. 2022, ApJ, 934, 104).

We thank the Swift staff for rapidly approving and executing our observations.

[1] https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl 

[2] Using a hydrogen column density of 1.08E21/cm^2 (HI4PI Collaboration, N. Ben Bekhti, L. Floer, et al., 2016, Astronomy & Astrophysics, 594, A116) and a power-law index of 2 

[3] Evans, P. A., et al. 2007, AAP, 469, 379