X-ray emission of SN 2017eaw and its progenitor

ATel #10380; A. K. H. Kong (NTHU & Oxford), K. L. Li (Michigan State)
on 15 May 2017; 20:32 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)

Subjects: X-ray, Supernovae, Transient

Referred to by ATel #: 10381

Following the discovery of the Type II supernova 2017eaw in the nearby galaxy NGC 6946 (ATel #10372,#10374,#10376,#10377), we have searched for X-ray emission of the supernova and its progenitor with archival data from the Swift X-ray Telescope and the Chandra X-ray Observatory. Swift TOO observations were triggered shortly after the discovery. The first Swift observation was performed on 2017 May 14, 14:09 UT for 1.95 ks. SN 2017eaw is marginally seen in the 0.3-10 keV image. There are 7 counts in a circular region of 25 arcsec in radius centered at the SN, corresponding to 0.02 counts per pixel. With 10 counts in a source-free region covering 2025 pixels, the background is about 0.005 counts per pixel. Using a binomial distribution, the chance probability that all 7 counts or more are background is 0.2%. With 5.6+/-2.5 background subtracted counts and assuming an absorbed power-law of photon index of 2 and a Galactic N_H of 2e21 cm^-2, the 0.3-10 keV unabsorbed flux is 1.69e-13 erg/s/cm^2 corresponding to 6.1e38 erg/s (d=5.5 Mpc).

The second Swift observation was taken on 2017 May 14, 22:00 UT for 980 s. In a 25 arcsec circular region, we obtained 8.8+/-3.0 background subtracted counts. Using the above spectral model, the 0.3-10 keV luminosity is 1.9e39 erg/s. This indicates that the X-ray brightness of the SN is increasing.

We also examined archival Chandra images taken before the onset of the SN to look for X-ray emission from the progenitor (ATel #10373,#10378). Six observations were taken between 2001 and 2012 and we combined all the observations together. At the position of the SN, the effective exposure time is about 162 ks. No source is visible at the SN position. Assuming the same spectral model, we set a 3-sigma 0.3-10 keV luminosity limit of 5.4e36 erg/s.