NICER detection of over 100 bursts and enhanced persistent emission from SGR 1935+2154

ATel #15674; George Younes (NASA GSFC/GWU), Teruaki Enoto (RIKEN), Chin-Ping Hu (NCUE), Keith C. Gendreau, Z. Arzoumanian (NASA GSFC), S. Guillot (IRAP/CNRS), Zorawar Wadiasingh (UMD, NASA/GSFC), Walid A. Majid (JPL, Caltech), Chryssa Kouveliotou (GWU)
on 13 Oct 2022; 16:46 UT
Credential Certification: George Younes (gyounes@email.gwu.edu)

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Following the reports of bursting activity from SGR 1935+2154 (ATel #15667, GCN 32698, 32706, 32708), we observed the source with the Neutron star Interior Composition Explorer (NICER) on 2022 October 12 starting at 17:32:40 UTC for a total exposure time of 8.5 ks spanning 15.8 hours.

Through preliminary analysis of these observations, we detect 112 burst candidates with significance higher than 7 sigma. This implies a burst rate of about 0.013 per second, an order of magnitude lower than what NICER recorded during the 2020 April burst storm from the source (Younes et al. 2020 ApJ, 904L, 21). The burst that reached the highest count rate is recorded at 17:43:11 (UTC); it has a duration of about 0.3 s and reaches a peak rate of 4.2e4 counts/s at 16 ms resolution. Assuming a blackbody model with a temperature of 3 keV, this translates to a peak flux in the 1-10 keV range of about 8.0e-7 erg/cm^2/s.

We also extracted an X-ray spectrum of the persistent emission from the source, after filtering out the burst intervals. The resulting spectrum is best fit with an absorbed blackbody plus a power-law model in the 1-8 keV range. 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. We find a best fit blackbody temperature kT = 0.70 +/- 0.03 keV and a power-law with a photon index of Gamma=1.0+/-0.2. The unabsorbed 0.5 - 10 keV flux of the source is 2.6 +/- 0.1 x10^-11 ergs/cm^2/s, which is an order of magnitude higher than the source quiescent flux level (e.g., Younes et al. 2020 ApJ, 904L, 21Y, Borghese et al. 2022, MNRAS, 516, 602B). These properties indicate that SGR 1935+2154 is undergoing yet another prolific outburst.

Further NICER observations are planned and we encourage multiwavelength observations, especially with radio facilities. The NICER schedule can be found at https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html.

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.