Search for radio pulsations from SGR 1935+2154 with the uGMRT

ATel #13778; Jayanta Roy (NCRA-TIFR), Shriharsh Tendulkar (McGill University), Poonam Chandra (NCRA-TIFR), Apurba Bera (NCRA-TIFR), Surajit Mondal (NCRA-TIFR), Sonalika Purkayastha (NCRA-TIFR), Ujjwal Panda (BITS Pilani)
on 4 Jun 2020; 01:46 UT
Credential Certification: Poonam Chandra (poonam@ncra.tifr.res.in)

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SGR 1935+2154 is a Galactic magnetar currently in active state showing a series of X-ray bursts (ATel #13675; ATel #13678; ATel #13682; ATel #13685; ATel #13687). During this X-ray flaring state, CHIME, STARE2 and later FAST detected bright millisecond bursts from this source (ATel #13681; ATel #13684; ATel #13699). We observed SGR 1935+2154 with the upgraded Giant Metrewave Radio Telescope (uGMRT) using the band-4 (550-750 MHz) and band-5 (1260-1460 MHz). During these observations we recorded phased array beam along wth interferometric visibilities. The beam data for the target source were recorded on 26th May, 22:50 UT to 27th May 2020, 02:32 UT in band-4 and on 28th May, 23:30 UT to 29th May, 02:19 UT in band-5 with approximately 2.5-3 hours of on-source time in each observing session. The band-4 filterbank was with 4096 channels at 327.68 micro-second time-resolution. A coherently dispersed (at DM of 332.8 pc cm^-3) filterbank with 1024 channels at 40.96 micro-second time-resolution was recorded for band-5. In both these epoch we observed PSR J1939+2134 to validate the data quality and to estimate the sensitivity of the array. In ATel #13773, we have reported the limits from the imaging analysis of this data while looking for afterglow or an associated persistent compact radio source. For the time-domain search, we dedispersed the band-4 and band-5 data over a dispersion measure (DM) range of 200-450 pc cm^-3 with a DM step of 0.5 and carried out search for millisecond bursts using single-pulse detection routine in PRESTO (Ransom 2001). We have also manually inspected the dedispersed time-series around a DM of 333 pc cm^-3. No radio busts were detected above signal-to-noise (S/N) ratio of 10 on these epochs. We achieved a 10-sigma fluence upper limit for millisecond bursts as ~0.7 Jy-ms at 0.65 GHz and ~3 Jy-ms at 1.36 GHz. The dedispersed data were also searched for periodic pulsations. We used a Fast Folding Algorithm (FFA) based pipeline (RIPTIDE; Morello et al. 2020) to search for pulsations over a period range between 200 ms and 100 s. No periodic pulsations were seen above signal-to-noise (S/N) ratio of 10. We also manually looked for radio pulsations around the 3.2 s rotational period (Israel et al. 2016). Considering pulse duty-cycle of 10%, the 10-sigma upper limits for radio pulsation searches are ~0.2 mJy at 0.65 GHz and ~0.6 mJy at 1.36 GHz. We thank the staff of the GMRT who have made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research.