Igor Andreoni (JSI/UMD/NASA-GSFC) reports on behalf of a larger collaboration
Follow-up observations of the optical fast transient ATLAS23msn/ZTF23aaoohpy/AT2023lcr (Swain et al., GCN 34022; Gompertz et al., GCN34023; Kumar et al., GCN34025; Perley et al, GCN 34031; Perley et al., GCN 34041; Fulton et al., 34042; Chen et al., GCN 34043) were carried out with the Neil Gehrels Swift Observatory starting on 2023-06-20 12:57:02 UT until 2023-06-20 14:54:56 UT. The total exposure time on target was 3ks.
A source was detected in Swift XRT data in the 0.3-10 keV energy range at a location compatible with AT2023lcr using HEASoft XIMAGE. The source has a background-subtracted count rate of (7.14 +/-1.8) * 10^(-3) ct/s.
Using WebPIMMS , we converted the count rate to flux assuming a powerlaw with photon index of 2, a redshift z=1.0272 (Perley et al, GCN 34041), and a Galactic NH of 3.16 * 10^(20) cm^(-2) . The resulting flux in the 0.3-10 keV band is (2.49 +- 0.63) * 10^(-13) erg/s/cm^2.
The detection of an X-ray counterpart and the high optical luminosity implied by the redshift measurement (Perley et al., GCN 34041) further suggest that the fast optical transient AT2023lcr is the afterglow of a relativistic event.
We thank the Swift team for approving and scheduling ToO observations of this source (target ID 16083)
 HI4PI Collaboration, N. Ben Bekhti, L. Floer, et al., 2016, Astronomy & Astrophysics, 594, A116 (HI4PI Map)