NuSTAR observations of AT2018cow reveal a hard X-ray component of emission above 15 keV
ATel #11775; R. Margutti (Northwestern University), R. Chornock (Ohio University), T. Laskar (NRAO/Berkeley), W. Fong (Northwestern University), B. Grefenstette (Caltech), G. Risaliti (UniFi) on behalf of a larger collaboration
on 24 Jun 2018; 18:58 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Raffaella Margutti (rmargutti@cfa.harvard.edu)
Subjects: X-ray, Supernovae
NuSTAR started observing the transient AT2018cow (ATel #11727) on June 23 17:31:09 UT under Director Discretionary Time (DDT, PI Margutti). From the analysis of 28 ks of data, we find the following preliminary results:
AT2018cow is very well detected with signal in excess of the background up to 60 keV. NuSTAR data in the 3-60 keV range can be fit with a simple power-law spectral model with best fitting photon index Gamma=1.20 +\- 0.03 (1 sigma c.l.). This is significantly harder than inferred from Swift-XRT observations (Gamma~1.6; 0.3-10 keV) obtained around the same time.
From our analysis of Swift-XRT observations of
AT2018cow obtained so far (Swift ID 10724, segments 2-14), we find no evidence for statistically significant spectral evolution of the source. The combined 0.3-10 keV spectrum is well described by a simple power-law with Gamma=1.62 +\- 0.03 (1 sigma c.l.) and no evidence for intrinsic neutral hydrogen absorption in excess to the Galactic component along the line of sight (NH_gal=4.95E+20 cm-2; Kalberla et al., 2005). The temporal evolution of the 0.3-10 keV flux is consistent with a ~t^-1 decay, assuming t0 to be near the initial optical detection (ATel #
11727). These results are in broad agreement with results from previous Swift-XRT analysis ( ATels #
11739 and 11761). We do not confirm the soft photon index (Gamma~2.2; 0.5-10.0 keV) and intrinsic absorption NH~1d21 cm-2 inferred from NICER data (ATel #
11773).
Fitting the NuSTAR data with a broken power-law model we find a low-energy photon index Gamma1=1.53 +\- 0.03 (1 sigma c.l.), with break energy E~15 keV, consistent with the results from the Swift-XRT data at 0.3-10 keV. Above 15 keV NuSTAR data show evidence for an additional hard X-ray component with index Gamma2=0.50 +\- 0.05 (1 sigma c.l.). The inferred flux of the hard X-ray component between 15-79 keV is ~3e-11 ergs/cm^2/s. Further investigation is needed to constrain the physical origin of this hard X-ray component.
We thank the entire NuSTAR team for arranging these observations.