MWA low-frequency radio imaging of SGR 1935+2154

ATel #13816; Tao An (SHAO), Xiaoyu Hong (SHAO), Baoqiang Lao (SHAO), Prashanth Mohan (SHAO), Xiaohui Sun (YNU), Shen Wang (SHAO), Zhijun Xu (SHAO), Zhongli Zhang (SHAO)
on 19 Jun 2020; 00:30 UT
Credential Certification: Tao An (antao@shao.ac.cn)

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The Galactic magnetar hosting soft gamma-ray repeater SGR 1935+2154 was reported to be in an active state, based on the X-ray detection at 08:53:17 UT on 2020-04-22 (Hurley et al. GCN#27625), followed by a flurry of short timescale (hundreds of milliseconds) X-ray bursts from 2020-04-27 UT onwards (Palmer et al. ATel #13675; Younes et al. ATel#13678). The transient drew immense attention owing to the detection of milli-second timescale radio bursts at 400 - 800 MHz (CHIME/FRB, Scholz et al. ATel#13681) and 1.4 GHz (STARE2, Bochenek et al. ATel#13684). This invokes the interesting connection between magnetars and fast radio bursts (FRB), especially in this case, owing to its proximity. Low-frequency radio observations were mainly aimed at (i) identifying the FRB from high time resolution data or (ii) imaging the magnetar; these however resulted in a non-detection (LOFAR at 150 MHz, Bassa et al. ATel#13707; Arecibo at 327 MHz and 1.4 GHz, Palliyaguru et al. ATel#13726; Westerbork 25m single dish at 350 MHz, Onsala 25m at 1.4 GHz, Kirsten et al. ATel#13735; Arecibo at 327 MHz and GBT at 1.4 GHz, Surnis et al. ATel#13769; uGMRT at 550 - 750 MHz and 1250 - 1450 MHz, Bera et al. ATel#13773, Roy et al. ATel#13778 and Surnis et al. ATel#13777). A high linear polarization pulse was detected at 1.25 GHz by FAST (observation on 2020-04-30, Zhang et al. ATel#13699). A marginal detection of radio bursts with a period of 3.2476 s and a pulse width of ~ 100 ms was reported based on Medicina Northern Cross observations at 408 MHz (observation on 2020-04-30, Burgas et al. ATel#13783). Our Murchison Widefield Array (MWA, Tingay et al. 2013, PASA,30,7) observations at central frequencies of 154.24 MHz and 184.96 MHz were carried out on 3 May and 4 May 2020, respectively. The observations were aimed at imaging the low-frequency afterglow emission from the magnetar/FRB. Each observation session lasted 4 hr from UT 18 to 22. We report a non-detection of the magnetar at the position 19:34:55.68, +21:53:48.2 (J2000) from any of the observations. The image rms noise level is 14.4 mJy/beam on 3 May at the central frequency 154.24 MHz, and 3.8 mJy/beam on 4 May at 184.96 MHz. No radio emission was detected above the 5-sigma threshold at the position of SGR 1935+2154. Instead, we obtained high-resolution high-sensitivity images of the associated supernova remnant SNR G057.2+0.8. We thank the MWA director, Prof. Melanie Johnston-Hollitt, for kindly approving the DDT proposal and offering time on the MWA for these time-critical observations. We thank the operation team (Dr. Andrew Williams et al.) for rapid scheduling and conducting the observations in a timely manner, despite the difficult ongoing situation owing to the pandemic. The raw observational data were downloaded from the MWA ASVO and the data analysis was made on the China SKA Regional Centre prototype (An, Wu, Hong, 2019, Nat Astron 3, 1030).