AstroNote 2020-221

We report observations of the Fast Blue Optical Transient (FBOT) AT2020xnd (ATel #14105, AstroNote #2020-199) with the Nuclear Spectroscopic Telescope Array (NuSTAR). We observed AT2020xnd on November 4th 2020 (t=21-25 days since explosion) for a total exposure of 29 ks (OBS ID: 80701407002, Proposal 22500192 PI: D. Matthews).

We do not find evidence for X-ray emission from the location of FBOT AT2020xnd and we place a 3 sigma c.l. flux limit of ~3.7E-13 erg/s/cm^2 (3-79 keV) and ~2.1E-13 erg/s/cm^2 (3-30 keV) for an assumed power law spectrum with index Γ = 1.5 (similar to what was found for AT2018cow, Margutti+ 2019). At a redshift of z=0.2442 (luminosity distance ~ 1.24 Gpc), this corresponds to a luminosity of ~ 6.9E43 erg/s (3-79 keV) and ~ 3.9E43 erg/s (3-30 keV). As a comparison, the only other FBOT with hard X-ray detection is AT2018cow, which showed fainter emission than our current NuSTAR limits for AT2020xnd at a similar time (Margutti+ 2019). Similar to AT2018cow, AT2020xnd could have harbored a brighter hard X-ray hump of emission at earlier times (t< 20 days) that had faded by the time of our current observations. 

Despite the hard X-ray non-detection, we have detected soft X-ray emission at the location of AT2020xnd with our coordinated Chandra monitoring campaign (AstroNote #2020-218, Atel #14154). Further NuSTAR observations can help in constraining the temporal evolution of the X-ray emission and its origins. 

We thank the entire NuSTAR team for carrying out these observations.