High-cadence NICER X-ray observations of AT2022cmc/ZTF22aaajecpc: flux variability and spectral evolution suggest it could be a relativistic tidal disruption event

ATel #15232; Dheeraj Pasham (MIT), Yuhan Yao (Caltech), Keith Gendreau (NASA/GSFC), Dan Perley (Liverpool John Moores University), Brad Cenko (NASA/GSFC), Zaven Arzoumanian (NASA/GSFC), and Elizabeth Ferrara (NASA/GSFC)
on 21 Feb 2022; 21:08 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)

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AT2022cmc/ZTF22aaajecp was identified by the Zwicky Transient Facility (ZTF) on 11 February 2022 10:42:40 UT as a fast and red transient (Andreoni et al, GCN 31590). A follow-up optical spectrum taken roughly six days after discovery revealed its redshift to be 1.193 (Tanvir et al, GCN 31602). This was independently confirmed by Lundquist et al. GCN 31612 using Keck observations on 17 February 2022. AT2022cmc has several intriguing properties including rapidly fading optical emission and red optical color (Pankov et al., GCN 31593; Perley et al., GCN 31594; Kumar et al., GCN 31597), luminous radio emission (Perley et al. GCN 31592) and an extremely luminous and long-lived (compared to GRB afterglows) X-ray emission (Pasham et al., GCN 31601).

Encouraged by NICER detection of strong X-ray emission on 16 February 2022 (Pasham et al., GCN 31601) we continued to perform high-cadence monitoring observations (one visit every few hours) through 21 February 2022. Here we report on data collected until 19 February. NICER detected the source all throughout between 0.3-6 keV with an observed 0.3-6 keV count rate varying between 3.3 and 15.6 counts/sec. While the X-ray emission is variable on hours timescale the overall observed 0.3-6 keV flux [luminosity] decreased from a mean value of 3.19(+0.02,-0.02)E-11 erg/s/cm^2 [1.52(+0.01,-0.01)E47 erg/s] on 16 Feb to 8.66(+0.11,-0.13)E-12 erg/s/cm^2 [4.68(+0.05,-0.05)E46 erg/s] on 19 February. The best-fit unabsorbed 0.3-6 keV luminosities on the 16th and 19th were (6.2+-0.2)E47 erg/s and (2.3+-0.1)E47 erg/s, respectively.

We divided the NICER data on a per day basis and extracted four spectra corresponding to 16 through 19 February 2022. All the spectra can be well-fit with a powerlaw modified by Galactic and host absorption. The X-ray spectral powerlaw index is variable with time and appears to be anticorrelated with the observed luminosity, i.e., the powerlaw index has a higher value at lower fluxes. Interestingly, this is similar to the behavior of the relativistic tidal disruption event SwJ1644+57 (e.g., Burrows et al., 2011), but atypical for GRB afterglows.

Based on hours timescale variability in both soft (0.3-6 keV) and hard (3-30 keV) X-rays (Yao et al., ATEL#15230) and spectral variations, we suggest that AT2022cmc may be an analog of SwJ1644+57. This would make it the first relativistic TDE identified in more than a decade. Further panchromatic monitoring is necessary for a robust classification. We strongly encourage multi-wavelength observations of this extreme event especially at radio and high energies.

NICER is planning to continue to monitor AT2022cmc as part of a GO program from cycle 3 (PI: Pasham) and with additional DDT observations. A weekly updated schedule can be found here: https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html. To coordinate multi-wavelength observations please contact drreddy@mit.edu

NICER carries out prompt follow-up observations of X-ray-bright extragalactic transients and tracks alerts from LIGO/VIRGO and other facilities. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.