A bright millisecond-timescale radio burst from the direction of the Galactic magnetar SGR 1935+2154

ATel #13681; Paul Scholz (UToronto) on behalf of CHIME/FRB Collaboration
on 28 Apr 2020; 20:45 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Shriharsh Tendulkar (shriharsh@physics.mcgill.ca)

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At 2020-04-28 14:34:33 UTC (topocentric arrival time at 400 MHz) the CHIME/FRB backend (CHIME/FRB Collaboration et al. 2018 ApJ, 863, 48) detected a bright radio burst from the direction of SGR 1935+2154, a Galactic magnetar known to be in an active state (GCN #27657, ATels #13675, #13678, #13679).

The burst had a DM of 332.8 pc/cc. This value is consistent with the predicted DM of 530 +/- 200 pc/cc based on the NH-DM relation of He et al. (2013, ApJ, 768, 64) and the measured NH of SGR 1935+2154 of 1.6e22 cm^-2 (Israel et al. 2016, MNRAS, 457, 3448). Further, SGR 1935+2154 is potentially associated with SNR G57.2+0.8. Based on the measured rotation measure of the remnant, Kothes et al. (2018, ApJ, 852, 54) estimate a DM towards G57.2+0.8 of ~290 pc/cc. The maximum expected DM due to the Milky Way along this line-of-sight, as estimated from the NE2001 Galactic electron density model (Cordes & Lazio 2002, astro-ph/0207156), is ~540 pc/cc.

The radio burst was detected in multiple CHIME/FRB beams. This, and the frequency structure seen in the dynamic spectrum plot available at the URL provided below) indicate that the burst was detected in a far side lobe. Based on a frequency-dependent model of the synthesized CHIME/FRB beams fit to the detected spectra, we localize the event to RA (deg) = 293.9, Dec (deg) = 22.1, 0.3 deg from the position of SGR 1935+2154, which was 21 deg from the CHIME meridian at the time of the burst. This offset is within the typical systematic uncertainties of this preliminary localization method.

The burst had a double-peak structure with two components ~5 ms wide separated by ~30 ms (see URL below). The spectra of the two components show differing band-limited structure, which we caution are not corrected for the telescope sidelobe response or instrumental bandpass. However, we do not expect the telescope's spectral response to change significantly on a timescale of 30 ms (the separation of the two bursts), which suggests that the two peaks indeed had different spectra. There is clear evidence for a scattering tail of similar magnitude in the two peaks. Because of the distance of the source from the CHIME meridian, the sensitivity of CHIME to the location of SGR 1935+2154 is highly uncertain. However, accounting for our best estimate of the telescope response at that location, we estimate the 400-800 MHz radio fluence to be a few kJy ms.

If confirmed, this would be the first radio detection of SGR 1935+2154. Previous observations of this source by Surnis et al. (2014, ATel #6376) at 326.5 MHz and 610 MHz during a different active phase yielded upper limits of 0.4 mJy and 0.2 mJy, respectively. Burgay et al. (2014, ATel #6371) reported upper limits at 3 and 1.5 GHz of about 0.07 mJy and 0.1 mJy, respectively. Younes et al. (2017, ApJ, 847, 85) reported upper limits of 14 uJy and 7 uJy at 4.6 and 1.4 GHz, respectively. A preliminary search of the CHIME/FRB data since it commenced operations in late 2018 has revealed no past events from the source in spite of approximately daily ~10-min exposures. That the bright radio burst is seen at the time of particularly high X-ray activity from SGR 1935+2154 lends further credence to the association.

Additional observations are strongly encouraged at all wavelengths.

Dynamic spectrum of the burst