SGR 1935+2154: Swift detection of enhanced X-ray emission and dust scattered halo

ATel #13679; J. A. Kennea (PSU), A. P. Beardmore, K. L. Page (Leicester) and D. M. Palmer (LANL)
on 28 Apr 2020; 15:59 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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Following multiple BAT triggers on SGR 1935+2154 (GCN #27657, ATEL #13675), Swift performed a target-of-opportunity observation of SGR 1935+2154, beginning at 19:41 UT on April 27th, 2020. The observation exposure was 2ks and the majority of the data were collected with XRT in Photon Counting mode. The SGR is well detected in the analysis, at a brightness higher than previously seen by Swift, with a count rate of 1.74 +/- 0.09 c/s; in previous target-of-opportunity observations of SGR 1935+2154. the brightest mean count rate in an observation was 0.16 +/- 0.01 c/s, seen on Jun 23rd, 2016, shortly after a previous outburst of this source was detected by BAT (e.g. GCN #19556). Average (quiescent?) count rate from SGR 1935+2154 seen in Swift XRT monitoring is approximately 0.025 c/s, so the current outburst is ~70x brighter than that.

We note examination of the PC mode light curve shows apparent detections of SGR flares during the observation. Approximately 6 bright flares are seen, although we note that PC mode has a 2.5s frame time, meaning that bursts closer together than this will not be resolved.

Examination of the XRT PC mode data show clear evidence for a dust scattered halo. By fitting a PSF model to the XRT data, this is prominent in the residuals, with a peak approximately 85 arc-seconds from the center of the PSF (extending from 60-115 arc-seconds).

The spectrum of SGR 1935+2154 can be well fitted with an absorbed power-law model, with a photon index of 1.22 +/- 0.22, and an absorption of 2.66 +/- 0.44 x 10^22 cm^-2. A blackbody model also well fits the data, with kT= 1.65 +/- 0.15 keV. The mean flux in the observation is 6.2 +/- 0.5 x 10^-11 erg/s/cm^2 (0.5 - 10 keV).

Monitoring observations by Swift to track the evolution of the dust scattered halo are planned.