NICER and NuSTAR observations of SGR 1935+2154 bracketing the time of the bright radio burst

ATel #15690; Teruaki Enoto (RIKEN), Chin-Ping Hu (NCUE), Tolga Guver (Istanbul Univ.), George Younes (NASA GSFC/GWU), Keith C. Gendreau, Z. Arzoumanian (NASA GSFC), Zorawar Wadiasingh (UMD, NASA/GSFC), Walid A. Majid (JPL, Caltech), Aaron B. Pearlman (McGill Univ., McGill Space Institute, Caltech), and S. Guillot (IRAP/CNRS)
on 18 Oct 2022; 06:35 UT
Credential Certification: Teruaki Enoto (teruaki.enoto@gmail.com)

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Magnetar SGR 1935+2154, the only known Galactic emitter of fast radio bursts (FRB), entered a burst active state on 2022 October 10 (ATel #15667, GCN 32698, 32706, and 32708). We have continued our high-cadence monitoring campaign of the source with NICER (ATel #15674). We also performed ToO observations with NuSTAR starting from 2022-10-14 02:00 UTC with an exposure totalling 50 ks (NICER Proposal 5076).

NuSTAR was observing SGR 1935+2154 during the radio burst which occurred on 2022-10-14 at 19:21:47 UTC as reported by the CHIME/FRB collaboration (ATel #15681). Unfortunately, at the arrival time of the burst, the source was Earth-occulted for NuSTAR. The closest on-target good time interval (GTI) ended approximately 12 minutes prior to the radio burst.

During the NuSTAR observation, SGR 1935+2154 exhibited a high bursting rate, most notably during the two GTIs prior to the radio burst, starting at 2022-10-14 16:53 and ending on 2022-10-14 19:10. During the span of about 4 ks of livetime, we detect over a hundred bursts, sitting on a bed of enhanced baseline emission.

We extracted the NuSTAR hard X-ray persistent spectrum from a few GTIs at the end of the observation which showed the lowest level of bursting activity, after eliminating the burst intervals. The background-subtracted source count rate was 0.3 cps in the 3-70 keV band. We also extracted a burst-free NICER spectrum from data that were simultaneous to NuSTAR. A preliminary analysis of the 1-70 keV broad-band spectrum indicates that it is best fit with an absorbed 2 blackbody and power-law model, "tbabs * (bbodyrad + bbodyrad + powerlaw)" in Xspec terminology. We derive an absorption-corrected flux of about 2.0e-11 erg/s/cm2 and 1.0e-11 erg/s/cm2 in the 0.5-10 keV and 10-70 keV ranges, respectively.

Through our continued NICER monitoring, we report that SGR 1935+2154 bursting activity has decreased. From the time of the radio burst and up to 2022-10-17, we detected 14 burst candidates with significance above 7 sigma, corresponding to an occurrence rate of 0.7 bursts/ks.

Further NICER monitoring is planned, and the schedule can be found on the website (https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html). Another NuSTAR observation is also planned. Detailed analysis of these observations, including the persistent temporal and spectral evolution of the source, is ongoing. We encourage contemporaneous multiwavelength observations.

We are grateful to the NuSTAR and NICER teams for the rapid scheduling of our observations.

The Neutron star Interior Composition Explorer (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.