AstroNote 2023-45

We observed the fast, blue, and luminous optical transient AT2023bbu (Perley et al., AstroNote 2023-41) with Swift/XRT beginning at 16:27 on 2023-02-25 UT, or 21d after the first ZTF detection. In a 3.5 ks exposure, we did not detect any X-ray emission, with a 3-sigma upper limit of 0.005 counts/sec. Using WebPIMMS [1] we estimate [2] that this corresponds to an upper limit on the unabsorbed 0.3-10 keV flux density of f_X < 1.9E-13 ergs/cm2/s, a luminosity of L_X < 2.3E42 erg/s at z=0.0689. This does not rule out X-ray emission with the luminosity of AT2018cow (which fluctuated by factors of a few on timescales of days at this epoch; [3]) but does rule out X-ray emission with the luminosity of AT2020xnd [4, 5] and AT2020mrf [6]. 

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

[1] https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl [2] Using a hydrogen column density of 1.6E20/cm2 (Willingale, R., et al. 2013, MNRAS, 431, 394) and a power-law index of 2 [3] Rivera Sandoval et al. 2018, MNRAS, 480, L146, [4] Bright et al. 2022, ApJ, 926, 112B, [5] Ho et al. 2022, ApJ, 932, 116, [6] Yao et al. 2022, ApJ, 934, 104.