Evidence for fading of the hard X-ray emission from AT2018cow

ATel #11813; B. Grefenstette (Caltech), R. Margutti (Northwestern University), R. Chornock (Ohio University), B. Metzger (Columbia), G. Migliori (INAF-Bologna) on behalf of a larger collaboration
on 4 Jul 2018; 13:44 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Brian Grefenstette (Bwgref@srl.caltech.edu)

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NuSTAR performed a follow-up ToO for the transient AT2018cow (ATel #11727) starting on 2018-07-02 14:01:09 UTC under Director's Discretionary Time (DDT, PI Margutti). During this observation we obtained roughly 30 ks of exposure time spanning from roughly 16.2 to 16.8 days post discovery. AT2018cow is still clearly detected in excess of background up to ~50 keV, although we see a drop in the total flux compared to the previous NuSTAR observation (ATEL #11775). The spectrum can be described by a single simple power-law fit across the 3-50 keV bandpass with a power-law index of 1.39 +/- 0.02 and a 3-79 keV flux of 3.2e-11 ergs / cm2 / sec. This is slightly softer than we observed during the first NuSTAR epoch. For comparison with our previous results, we find a 15-79 keV flux for the model describe above to be ~2.4e-11 ergs / cm2 / sec, or roughly 20% fainter than we previously measured. From our analysis of 3.6 ks of Swift XRT observations obtained during the second NuSTAR epoch we find that the spectrum can be well fit with a single power-law model with a power-law index of 1.42 +\- 0.07 (all errors are 1-sigma). We note that this is harder than the Swift XRT spectrum we reported during the observation obtained during the first seven days, which had a power-law index of 1.60 +/- 0.03. A joint Swift/NuSTAR fit shows that the spectrum can now be modeled by a single power-law component across the 0.3-50 keV band-pass. We no longer see any evidence requiring a second, prominent, hard power-law component at high energies, which was previously detected by NuSTAR and INTEGRAL (ATel #11775 and #11788). We thank the entire NuSTAR team for arranging these observations.