AstroNote 2021-64

We report on UV/optical and X-ray observations of ZTF21aakilyd/AT2021csp (Perley et al., Astronote #2021-62) with the Neil Gehrels Swift Observatory (Gehrels et al. 2004). Observations started on 12 February 2021 at 22:14 UT. We measured the brightness of the transient using an aperture with a radius of 3'':

The magnitudes are reported in the AB system and not corrected for reddening. The reported measurements include flux from the host galaxy. However, we expect the total host contribution to be small based on GALEX and SDSS observations obtained before the discovery of AT2021csp.

Swift/XRT observations were carried out in photon-counting mode. We analyzed the data using http://www.swift.ac.uk/user_objects/. No source was detected at the position of AT2021csp. We infer a limit of 0.014 ct/s (3 sigma confidence). Assuming a Galactic column density of n(H) 2.36E+20 cm^-2, a redshift of 0.081, and a power-law spectrum with photon index = 2, this count rate corresponds to an unabsorbed flux limit of 5.19E-13 erg cm^-2 s^-1 in the bandpass from 0.3-10 keV. At the distance of AT2021csp this corresponds to a 0.3-10 keV luminosity L < 7.1E+42 erg s^-1.

ZTF is supported by the National Science Foundation under Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW.