AstroNote 2022-40

ATLAS is a quadruple 0.5m telescope system with two units in Hawaii (Haleakala and Mauna Loa), and one each in Chile (El Sauce) and South Africa (Sutherland). With the installation of the two southern units, we are robotically surveying the whole sky with a cadence of 1 day (Tonry et al. 2018, PASP,130:064505). Two filters are used, cyan and orange (denoted c and o; all mags quoted are in the AB system). 

Here we report the ATLAS survey observations of the fast and red optical transient ZTF22aaajecp =AT2022cmc (see Andreoni et al. AstroNote 2022-38 for discovery details). Our one-day cadence confirms a fast-evolving transient. We detected the transient at its brightest on MJD = 59622.48 (2022-02-12.48), with an apparent magnitude of m_o = 19.06 +/- 0.11. Forced photometry over five consecutive nights (all in the o-band) are given below. The upper limits are to 3σ. We measure m_o = 19.95 +/- 0.37 on MJD = 59624.53 (2022-02-14.53) and thus a decline rate of 0.43 +/- 0.19 mag/d. The last 3σ non-detection through forced photometry was on MJD = 59621.50 (2022-02-11.50), which places a lower limit on the incline rate of 1.52 +/- 0.11 mag/d. 

There are no historical detections in our ATLAS or Pan-STARRS databases. 

On the host galaxy and redshift :

AT2022cmc is 4.3 arcsec from a faint r = 24.06 source in DECaLS (203.6794 33.2157). We confirm the reality of this galaxy detection in our deep Pan-STARRS w-band images (it is the nearest source in these images). DECaLS gives a photoz of z = 1.05 +/-0.33, which would mean a projected offset of 33kpc.

The next nearest source is the r=21.3 galaxy at RA=203.6794, DEC=33.2183, or 5.7 arcsec separation. At a photoz of z = 0.37 +/- 0.10, this would mean a projected offset of 29kpc. 

While the optical lightcurve bears quite an interesting resemblance to the kilonova AT2017gfo (and the range of decline rates from the grid of kilonova models of Nicholl et al. 2021 MNRAS, 505, 3016), in its incline rate, decline rate, it would need to be at a distance of 100-200 Mpc to be a viable kilonova candidate. The two nearest low redshift galaxies are UGC 08538 (z =0.0245) and PGC 2029069  (z =0.0403), which would mean offsets of at least 500kpc and 250kpc respectively. At these large angular separations, such coincidences are expected. 

The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for Near-Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.