GBM detection of a faint magnetar-like burst temporally coincident with a CHIME/FRB radio burst
ATel #15794; G. Younes (NASA GSFC), E. Burns (LSU), O. J. Roberts (USRA), J. Wood (NASA/MSFC), P. Veres (UAH), C. Kouveliotou (GWU) On behalf of a larger collaboration
on 6 Dec 2022; 16:24 UT
Credential Certification: George Younes (gyounes@email.gwu.edu)
Subjects: X-ray, Gamma Ray, Neutron Star, Soft Gamma-ray Repeater, Pulsar, Fast Radio Burst, Magnetar
Following the recent report of another bright radio burst from the magnetar SGR 1935+2154 (ATEL #15792), we searched the Fermi Gamma-ray Burst Monitor (GBM) Continuous TIME Tagged Event (CTTE) data for any untriggered enhanced emission at the time of the CHIME burst (2022-12-01 22:06:59.079 UTC, topocentric arrival time at infinite frequency). Bayesian Block and Poisson likelihood searches in the 10-100 keV range both resulted in the detection of a faint signal at 2022-12-01 22:06:58 (T0 in UTC, TT), i.e., 1 second preceding the CHIME burst. This delay is larger than the 50 ms maximum expected light travel time from CHIME/FRB site to GBM.
We then utilized the Fermi-GBM Targeted Search (arXiv:1612.02395) to scan for short transients with soft, normal, and hard gamma-ray burst spectra within +/- 30 s around the FRB time. The same signal is identified with the soft spectral template beginning ~1.5 s before the FRB with a duration of approximately 0.25-0.5 s. The significance of a chance coincidence within the 1 minute search window is approximately 3 sigma. The best-fit location is RA = 287.2, Dec = +1.8 (J2000 degrees, equivalent to J2000 19h 08m, +01d 48') with a 21.5 deg error radius (68% containment, statistical + systematic uncertainty). The spatial association probability is 90%. We note that the spatial association calculation assumes an isotropic distribution of subthreshold transients, which is invalid for soft events since they preferentially arise from the Galactic plane.
Thes spectrum of the GBM source from T0-0.72 s to T0-0.21 s is best fit by a blackbody with temperature kT = 12.0+/-2.0 keV. The fluence in the 10-100 keV energy range is (4.65 +/- 0.92)E-8 erg/cm^2.
We caution that several other magnetars are in the GBM localization uncertainty region, and we cannot rule them out, or a new magnetar, as the source of this burst. The X-ray flare may also have a non-magnetar origin. A better localization, e.g., through triangulation, may exclude this association or increase the confidence of association. We encourage searches of data in other high energy monitors.
Detailed spectral and temporal analysis of this event will be reported elsewhere.
