Upper limits on the radio fluence of the most recent X-ray bursts from SGR1935+2154

ATel #14382; Franz Kirsten (Chalmers/OSO, Sweden), Jakob van den Eijnden (U. of Oxford, England), Mark Snelders (U. of Amsterdam, The Netherlands), Kenzie Nimmo (ASTRON, U. of Amsterdam, The Netherlands), Jason Hessels (ASTRON, U. of Amsterdam, The Netherlands), Marcin Gawronski (NCU, Torun, Poland), Jun Yang (Chalmers/OSO, Sweden), Roman Feiler (NCU, Torun, Poland)
on 9 Feb 2021; 20:50 UT
Credential Certification: Franz Kirsten (franz.kirsten@chalmers.se)

Tweet

All of Fermi GBM, Konus Wind, GECAM, and Swift-BAT reported several bright bursts during the recent X-ray activity of SGR1935+2154 (GCN circulars GCN #29363, #29365, #29373, #29374, #29388 and ATel #14359). We were observing the source at multiple radio frequencies during the following bursts:

29 Jan 2021 07:00:00.99 UTC (ATel #14359, GCN #29373)
29 Jan 2021 10:35:39.94 UTC (ATel #14359)
29 Jan 2021 15:23:29.92 UTC (ATel #14359)
02 Feb 2021 12:54:27.00 UTC (GCN #29388)

During the three bursts on 29 Jan 2021 we observed with the Westerbork (Wb) single 25-m dish RT1 (P-band, 300.0-364.0 MHz) and the Onsala (O8) 25-m telescope (L-band, 1360-1488 MHz). On 2 Feb 2021 we observed also with the Torun 32-m telescope (C-band, 4550-4806 MHz) in addition to Wb and O8.

We used a custom built pipeline to search the baseband recordings. This pipeline first creates channelised filterbank files (Stokes I) which are then searched for bursts with Heimdall (using the DM-range 332.7 +/- 50 pc/cm^3). Potential candidates are classified with FETCH (Agarwal et al. 2020). Neither the automated pipeline nor a manual search around -10/+30 seconds of the reported X-ray burst times lead to a detection. Thus we can constrain the fluence of any potential bursts coincident with the X-ray bursts to be lower than 78 Jy ms, 8 Jy ms and 3 Jy ms at P-, L-, and C-band, respectively (7-sigma detection threshold for the automated pipeline).

We also performed a preliminary analysis of the overlapping Fermi/GBM bursts on 29 Jan 2021. We find that the spectral parameters, when fitting each burst with a double blackbody model, are consistent with typical X-ray bursts seen from SGR 1935+2154 (Lin et al. 2020). They are, on the contrary, not consistent with the spectrum seen during the anomalous, hard X-ray burst with contemporaneous radio burst emission (Mereghetti et al. 2020). The same holds for the spectral parameters reported for the 02 Feb 2021 Fermi/GBM bursts (GCN #29388). The fluences of all four X-ray bursts are similar to that of the anomalous, hard X-ray burst; if a contemporaneous radio burst of similar fluence (1.5 MJy ms; Bochenek et al. 2020) had occurred, it would have been detected given our sensitivity.

We also observed the source during the following times
30 Jan 2021 0530-1245 UTC
31 Jan 2021 0530-1045 UTC
01 Feb 2021 0515-1345 UTC
02 Feb 2021 0430-1800 UTC
03 Feb 2021 0430-1800 UTC

No radio bursts have been found as of yet. We will continue to monitor the source regularly.