TITLE:   GCN CIRCULAR
NUMBER:  31641
SUBJECT: ZTF22aaajecp/AT2022cmc: Swift Observation and a UV Detection
DATE:    22/02/23 19:00:52 GMT
FROM:    Yuhan Yao at Caltech  <yyao@astro.caltech.edu>

Yuhan Yao (Caltech), Igor Andreoni (JSI), Sergio Campana (INAF-OAB), Brad Cenko (GSFC)

AT2022cmc was discovered by the Zwicky Transient Facility (ZTF) on 2022 Feb 11 as a fast and red optical transient (GCN 31590).  At z=1.193 (GCN 31602, 31612), its luminous radio, millimeter, and X-ray emission (GCN 31592, 31627, 31601) distinguish AT2022cmc from most extragalactic transients. The highly variable X-ray emission, the hardness evolution, and the power-law energy break revealed by NICER+NuSTAR observations make it similar to the relativistic TDE Swift J1644+57 (ATel 15230, 15232).

On Feb 23, the Neil Gehrels Swift Observatory conducted a 2.6-ks observation at rest-frame 5.3 days after the first ZTF detection (i.e., 2022-02-11T10:42:40).

AT2022cmc was detected by the X-Ray Telescope (XRT) at a count rate of 0.197+/- 0.0091 count/s. The Swift/XRT image shows just one source coincident with the radio position (GCN 31592), within the errors.
We grouped the XRT spectrum to have at least 20 counts per bin and fitted the 0.3–6 keV spectrum with tbabs*ztbabs*zashift*powerlaw. We fixed the column density nH in tbabs at the Galactic value of 8.88 E+19 cm^-2 (HI4PI Collaboration et al. 2016). We obtained a good fit with a chi-square/degrees of freedom (chi2/dof) of 20.54/15. The best-fit power law index Gamma = 1.41 +0.26 -0.22, and host galaxy nH = 0.21 +0.43 -0.21 E+22 cm^-2 (errors are 90% confidence level for one parameter of interest). We note that the host nH is consistent with 0. The observed 0.3–6 keV flux is 5.4 +/- 0.6 E-12 erg/s/cm^2, which is similar to the NICER 0.3–6 keV flux of 8.7 +/- 0.1 E-12 erg/s/cm^2 on Feb 19 (Atel 15232).

Photometry with the UltraViolet/Optical Telescope (UVOT) gives the following magnitudes (5” extraction region)
Filter exptime (s) AB mag
V       83.44 > 18.70
B 83.44 > 19.63
U 83.44 > 20.44
UVW2 419.06 > 22.09
UVM2 634.64 21.30 +/- 0.22 (stat) +/- 0.03 (sys)
UVW1 251.73 > 21.35
Upper limits are reported in 3-sigma.

The detection in the M2 band testifies for a low intrinsic absorbing column density.