Non-detection of radio pulsations from SGR1935+2154 by CHIME/Pulsar

ATel #13838; Chia Min Tan (McGill) on behalf of the CHIME/Pulsar Collaboration
on 29 Jun 2020; 15:25 UT
Credential Certification: Paul Scholz (paul.scholz@dunlap.utoronto.ca)

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We report on radio observations of the magnetar SGR 1935+2154 with the CHIME/Pulsar instrument, made over the span from 29 April 2020 to 25 May 2020, and again on 30 May 2020. These observations were prompted by the discovery of bright radio bursts from the magnetar by the CHIME/FRB and STARE2 at 28 April 2020 (The CHIME/FRB Collaboration 2020, arXiv:2005.10324, Bochenek et al 2020, arXiv:2005.10828), and we note the report of radio pulsations by the Medicina Northern Cross on 30 May 2020 (ATel #13783).

The CHIME FX correlator (Denman et al 2020, arXiv:2005.09481) forms a tied-array beam at the position of SGR 1935+2154 when the source is transiting the CHIME sky. The data are then streamed to the CHIME/Pulsar instrument for processing. The magnetar was tracked for 15 minutes daily at a central frequency of 600 MHz and a bandwidth of 400 MHz. The data were recorded in filterbank mode with 327.68 us time resolution and coherently dedispersed to the known DM of 332.8 pc/cc.

As the magnetar is known to show periodic pulsations in X-rays (Israel et al. 2016, MNRAS 457, 3448, ATel #13720) of 3.245-3.247s, we conducted periodicity searches on the filterbank data. The data were searched at trial DMs between 310 and 350 pc/cc using a Fast Folding Algorithm (Parent et al 2018). No radio pulsations at the X-ray period were found in these searches.

Following the Medicina Northern Cross report of radio pulsations on 30 May 2020, we analysed the CHIME/Pulsar filterbank data from the magnetar on the same day, between UT 10:52 and 11:05. While the data was recorded during a test run after CHIME software upgrades, other observations from the same run indicate the data quality was comparable to that obtained pre-upgrade. We folded the filterbank data at the period detected by the Medicina Northern Cross but detected no pulsation. A periodicity search also yielded no candidates near that period.

The non-detection of radio pulsations from SGR1935+2154 by CHIME suggests an upper limit in flux density of ~0.2 mJy at 600 MHz for any single 15 minute observation, given a detection signal-to-noise ratio of 10, the SEFD of the telescope of ~70 Jy towards the direction of SGR 1935+2154, usable bandwidth of 200 MHz and assuming a pulse width of 100 ms. Our upper limit is ~20 times lower than the reported detection by the Medicina Northern Cross. If both CHIME and the Medicina Northern Cross reported measurements are robust, this suggests that the radio pulsations may be highly intermittent with a short active window of less than a day. We are continuing to monitor SGR 1935+2154 with CHIME/Pulsar for the foreseeable future.