A highly polarised radio burst detected from SGR 1935+2154 by FAST

ATel #13699; C. F. Zhang (PKU, NAOC), J. C. Jiang (PKU, NAOC), Y. P. Men (PKU, NAOC), B. J. Wang (PKU, NAOC), H. Xu (PKU, NAOC), J. W. Xu (PKU, NAOC), C. H. Niu (NAOC), D. J. Zhou (NAOC), X. Guan (NAOC), J. L. Han (NAOC), P. Jiang (NAOC), K. J. Lee (*PKU, NAOC), D. Li (NAOC), L. Lin (BNU), J. R. Niu (NAOC), P. Wang (NAOC),Z. L. Wang (NAOC), R. X.Xu (PKU), W. Yu (NAOC), B. Zhang (UNLV), W. W. Zhu (*NAOC)
on 3 May 2020; 02:32 UT
Credential Certification: K.J. Lee (kjlee007@gmail.com)

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Radio observations using Five-hundred-metre Aperture Spherical radio telescope (FAST) detected a highly polarised radio burst from SGR 1935+2154 at MJD 58969.9048669008. The observation session started at 21:18:00 UTC and stopped at to 22:20:00 on 30th April 2020. The 19 beam receiver with the system temperature of 20-25 K mounted on the telescope was used for observations at the center frequency of 1.25 GHz with a bandwidth of 460MHz. Polarisation data of all beams were recorded. The burst was clearly detected (see Figure.1) using the software package BEAR. The estimated flux is 30 mJy and fluence is 60 mJy ms. The DM value is 332.9 pc cm^-3, consistent with previous reported DM value of the source (Atel No.13681,13684)

The linear polarisation of the burst is up to 90% on some parts of the pulse (Figure.2), and the polarisation angles stay at a constant value if the uncertainties are considered. No circular polarisation is detected. These polarisation features of this burst resemble very much radio pulses from magnetars.

The Faraday rotation measure (RM) derived from this polarised pulse by using the Bayesian method is +112.3 rad m^-2, consistent with typical values of pulsars at a distance of 9 kpc or with the DM in this sky region of the Milky Way (see Han et al. 2018, ApJS 234, 11). This low RM probably originates from the interstellar medium between SGR 1935+2154 and us, with possible cancellation of RMs due to the magnetic field reversals between spiral arms. The contribution from the local magnetoionic environment is apparently very low, opposite to the case of repeating FRBs.

In connection with radio detection of an FRB from SGR J1935+2154 (Atel No. 13681, 13684), the low flux burst reported here and the high polarisation may suggest that the SGR J1935+2154 is turning to a phase of radio active magnetar.

Figure 1 A radio burst detected at the location of SGR 1935+2154 by using the software-package BEAR.
Download here: https://psr.pku.edu.cn/wp-content/uploads/2020/05/sgr_burst.png
Figure 2 Faraday rotation corrected (via RM = 112.3 rad m^-2) polarisation profile shows a highly linear polarisation.
Download here: https://psr.pku.edu.cn/sgr.pdf