SGR 1935+2154 October 8 radio bursts: No gamma-ray counterparts in INTEGRAL/SPI-ACS and IBIS observations

ATel #14087; V. Savchenko, C. Ferrigno (ISDC/UniGE, Switzerland), J. Rodi (IAPS-Roma, Italy), S. Mereghetti (INAF IASF-Milano, Italy), D. Gotz (CEA, France), on behalf of the INTEGRAL multi-messenger collaboration: https://www. astro. unige. ch/cdci/integral-multimessenger-collaborationt
on 11 Oct 2020; 11:57 UT
Credential Certification: Volodymyr Savchenko (Volodymyr.Savchenko@unige.ch)

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Using combination of INTEGRAL all-sky detectors (following [1]): SPI/ACS, IBIS/Veto, and IBIS we have performed a search for a prompt gamma-ray counterpart of SGR 1935+2154 (Atels #14074, #14080).

At the time of the radio bursts reported in Atel #14074, SGR 1935+2154 was at an angle of 62 deg with respect to the spacecraft pointing axis. This orientation implies strongly suppressed (21% of optimal) response of ISGRI, somewhat suppressed (44% of optimal) response of IBIS/Veto, and near-optimal (87% of optimal) response of SPI-ACS.

INTEGRAL was operating in nominal mode and the background within +/-300 seconds around the event was very stable (excess variance 1.1).

We searched in the SPI-ACS, IBIS, and IBIS/Veto data for a counterpart of the radio bursts (following [1],[2]) and did not detect any. Assuming a spectrum as observed for the 2020, April 20 burst (an exponentially cut off power law with photon index 0.7 and Epeak 65 keV, as derived by [3]), we derived a 3-sigma upper limit on the 75-2000 keV fluence of 1.1 x 10^-7 erg cm^-2 for a burst lasting less than 0.5 s occurring at any time in the interval within 300 s around 2020-10-08T02:23:41.

This limit corresponds to a ratio of radio to gamma-ray fluence of 5 x 10^-8, 5 x 10^-10, and 3 x 10^-10 for the three radio bursts, based on their radio fluences reported in ATel #14080). These values are in the range of ratios of the 2020, April 28 burst (2 x 10^-5) and of the FAST limits obtained for the burst detected by Fermi/GBM (3 x 10^-14) considered in [3].

An INTEGRAL ToO observation of SGR 1935+2154 has been scheduled from 2020-10-12 17:01 UT to 2020-10-14 22:19 UT.
Simultaneous observations are encouraged.

All results quoted are preliminary.

[1] Savchenko et al. 2017, A&A 603, A46
[2] Savchenko et al. 2012, A&A 541A, 122S
[3] Mereghetti et al. 2020, ApJ 898L 29M