Burst forest from SGR 1935+2154 as detected with NICER

ATel #13678; George Younes (GWU), Tolga Guver (Istanbul Univ.), Teruaki Enoto (RIKEN), Zaven Arzoumanian, Keith Gendreau (NASA/GSFC), Chin-Ping Hu (Kyoto Univ./RIKEN/JSPS Fellow), Paul S. Ray (NRL), Chryssa Kouveliotou (GWU), Sebastien Guillot (IRAP/CNRS), Wynn C. G. Ho (Haverford), Elizabeth C. Ferrara (NASA-GSFC/UMD), Christian Malacaria (NASA-MSFC/USRA), report on behalf of the NICER Team
on 28 Apr 2020; 14:34 UT
Credential Certification: George Younes (gyounes@email.gwu.edu)

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Following reports of substantial bursting activity from the magnetar SGR 1935+2154 (e.g., GCN #27657, Barthelmy et al.; GCN #27661, Nakahira et al.), we initiated a series of DDT observations with NICER. The first observation started at 00:40:58.000 UTC of 2020-04-28 UTC and lasted approximately 1 ks.

The NICER light curve (1 - 9 keV; 4 ms time resolution) shows a large number of typical magnetar short bursts. Assuming that bursts separated by more than 200 ms are distinct, we identify over 100 bursts during this observation. The burst with the brightest peak is detected at 00:46:21.2 UTC. This burst lasted approximately 200 ms, and reached a peak count rate of about 140k counts/second. Assuming a typical magnetar spectrum at soft energies consisting of a black-body with a temperature of 4 keV, and a hydrogen column density towards the source of about 3e22 /cm2 (Israel et al. 2016, MNRAS, 457, 3448; Younes et al. 2017, ApJ, 847, 85), we estimate a 4 ms peak flux for this burst of about 2.7e-6 erg/s/cm2 (1 - 9 keV). We caution that this value is affected by deadtime and other saturation effects, which may be substantial given the large count rate during burst peaks.

We also explored the NICER light curve at multiple time resolutions to identify any enhanced emission following any of the bursts. Starting at around 1 second resolution, we find that the full bursting activity is sitting on a bed of enhanced emission that begins about 100 seconds from the start of the observation. This enhanced emission peaks around 400 seconds later. The source count rate remained at a higher level towards the end of the 1 ks observation compared to the flux at the start (see linked light curves).

We also extracted the spectrum of the enhanced emission using the burst-free times of the observation, which resulted in an exposure time of 397.9 s. We found a background-subtracted count rate of about 12.0 counts/s. We modeled the spectrum in the 1-10 keV energy range with an absorbed power law model. The resulting hydrogen column density is (4.10 +/- 0.17)e22 /cm2, using ISM abundance. The photon index is 1.80 +/- 0.06. This fit results in a reduced chi2 of 1.1 for 137 degrees of freedom. The absorbed and unabsorbed flux values are 1.0e-10 and 1.62e-10 ergs/cm2/s, respectively. Assuming a distance of 12.5 kpc, as inferred from the possible association of the magnetar with the SNR G57.2+0.8 (Kothes et al. 2018, ApJ, 852, 54), the unabsorbed flux implies a source luminosity L = 3.0e36 erg/s.

We note that the subsequent observations starting at 03:53:15.0 and totaling 1200 seconds show very little bursting activity, while the count rate decreased to 5 counts/s. The spectrum shows a clear softening with a best-fit photon index of 2.12 +/- 0.07. We also note that the unabsorbed 1-10 keV flux decreased to 3.84e-11 ergs/cm2/s, corresponding to a flux decrease by a factor of 4.2.

Detailed analysis of this dataset is underway.

The 4 ms binned light curve of the first observation can be found here.
The 1 second binned light curve of the first observation can be found here.

We thank the NICER operations team for rapid scheduling and execution of these observations. 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.