NuSTAR observation of AT2022cmc, and joint spectral fitting with NICER

ATel #15230; Yuhan Yao (Caltech), Dheeraj R. Pasham (MIT), Keith C. Gendreau (GSFC)
on 21 Feb 2022; 02:40 UT
Credential Certification: Yuhan Yao (yyao@astro.caltech.edu)

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AT2022cmc/ZTF22aaajecp is a fast and red optical transient discovered by ZTF (Andreoni et al, GCN 31590) at a redshift of 1.193 (Tanvir et al, GCN 31602; Lundquist et al. GCN 31612). Hereafter we use t to denote rest-frame time since the first ZTF detection (i.e., 2022-02-11T10:42:40).

The observed 0.3--6 keV X-ray luminosity at t = 2.44 days is 2.46 +/- 0.04 E+47 erg/s (Pasham et al, GCN 31601), making this object much brighter than the brightest GRB afterglow at a similar phase (see, e.g., Figure 5 of Margutti et al. 2019, ApJ, 872, 18). Its X-ray luminosity is comparable to the jetted tidal disruption event (TDE) Swift J1644 at a similar phase.

A NuSTAR ToO observation of AT2022cmc was triggered under our Cycle-7 GO program (proposal 07112; PI Yao). The observation was conducted from 2022-02-18T18:15:14 to 2022-02-19T18:35:00 (i.e., t = 3.34--3.80 days), with an on-source time of 47.9 ks. Hour-timescale variability was observed. The 3--30 keV source net count rate (FPMA+FPMB) varied between 0.08 +/- 0.01 cnt/s and 0.43 +/- 0.05 cnt/s. The trend of flux evolution agrees with simultaneous NICER (0.3--6 keV) observations (Pasham et al., ATel, in prep).

We first modeled the 3--30 keV NuSTAR spectrum using tbabs*ztbabs*zashift*powerlaw, with the column density nH in tbabs fixed at the Galactic value of 8.88 E+19 cm^-2 (HI4PI Collaboration et al. 2016). This gives a good fit with a chi-square/degrees of freedom (chi2/dof) of 179/161. The best-fit power law index Gamma = 2.06 +/- 0.07, and host galaxy nH = 11.9 +/- 5.2 E+22 cm^-2. However, we note that the best-fit Gamma is significantly greater than the photon index obtained from NICER spectral fitting (Pasham et al, ATel in prep).

Next, we produced a NICER spectrum using events within the NuSTAR time window of a longer monitoring campaign (Pasham et al., ATel, in prep). We jointly fit the 3--30 keV NuSTAR and 0.3--5 keV NICER spectra with a broken power-law tbabs*ztbabs*zashift*bknpower. The best-fit model has chi2/dof = 279/230. The rest-frame energy break point is at E_bk = 11.1 +/- 1.6 keV (i.e., 5.1 +/- 0.7 keV in observer-frame). The photon index at EE_bk is Gamma_2 = 1.99 +/- 0.06. The host galaxy nH = 6.4 +/- 1.7 E+20 cm^-2. The 0.3--30 keV observed flux = 1.47+/-0.02 E-11 erg/s/cm^2.

If AT2022cmc belongs to the class of relativistic TDEs, it will be the first such member uncovered in over a decade. Further multi-wavelength observations are strongly encouraged.