Follow-up observations of SGR 1935+2154 with NICER

ATel #14916; Tolga Guver (Istanbul Univ.), Chin-Ping Hu (NCUE), George Younes (NASA/GSFC/USRA), Zorawar Wadiasingh (NASA/GSFC/University of Maryland College Park), Chryssa Kouveliotou (George Washington University), K. C. Gendreau, Z. Arzoumanian (NASA/GSFC)
on 14 Sep 2021; 09:18 UT
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Credential Certification: Tolga Guver (tolga.guver@istanbul.edu.tr)

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Following the reports of bursting activity from SGR 1935+2154 (ATel #14907, GCN 30806, 30804, 30803, 30797), we observed the source with The Neutron star Interior Composition Explorer (NICER) on 2021 September 11 and 12 with a total exposure time of 4009 s.

In these observations, we detected 20 burst candidates with significance higher than 5 sigma through the Bayesian block technique. The durations are between milliseconds to ~2 seconds. A burst with the highest fluence is detected at 2021-09-11 23:55:45.872 (UTC). It has a peak count rate of 2.6E4 counts/s and a fluence of (3.9 +/-0.1) x 10^-8 erg/cm^2.

We also extracted an X-ray spectrum of the persistent emission from the source, after filtering out the burst intervals. The resulting spectrum (exposure time of 3750 s) was binned to 50 counts per channel and fit in the 1-6 keV range with an absorbed blackbody plus a power-law model (chi2/d.o.f. = 0.7 for 104 d.o.f.). Interstellar absorption is taken into account using the ‘tbabs’ model with ISM abundances (Wilms et al. 2000; ApJ, 542, 914) and is fixed to N_H = 2.4E22 cm^-2 following Younes et al. (2020; ApJ, 904L, 21Y). The best fit blackbody temperature and emitting area are found to be kT = 0.51 +/- 0.02 keV and R ~ 2.6 km, respectively with a power-law photon index of \Gamma=0.98+/-0.29. The unabsorbed 0.5 - 10 keV flux of the source is 1.75 +/- 0.05 x10^-11 ergs/cm^2/s, which is about an order of magnitude larger than the source quiescent flux level. This implies that the source is undergoing yet another outburst, with a flux enhancement comparable to the one measured after the April 2020 major outburst.

SGR 1935+2154 continues to be active and showing bursts at various intensities; further NICER observations are planned.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.