A bright radio flare observed in GRS 1915+105 with the Medicina and RATAN radio telescopes with no X-ray counterpart in the INTEGRAL energy range

ATel #16008; E. Egron (INAF-OAC, It), J. Rodriguez (CEA/AIM, Fr), S. A. Trushkin (SAO RAS, Ru), A. Pellizzoni, M. Pilia, A. Melis (INAF-OAC, It), S. Righini (INAF-IRA, It), T. Bouchet (CEA/AIM, Fr), F. Cangemi, A. Coleiro (APC, Fr), J. Chenevez (DTU, Dk), V. Grinberg (ESA/ESTEC, NL), S. Loru (INAF-OACt, It), P. O. Petrucci (IPAG, Fr), K. Pottschmidt (GSFC, USA), J. Steiner (Cfa, USA), J. Tomsick (UC Berkeley/SSL, USA), J. Wilms (FAU Erlangen-Nuernberg, D), Shu Zhang, Shuangnan Zhang, Lian Tao (IHEP, Cn)
on 24 Apr 2023; 19:59 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Elise Egron (elise.egron@inaf.it)

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After a long period of weak radio activity during the two last years, two radio flares were recently detected in the microquasar GRS 1915+105 during the daily monitoring conducted by the RATAN-600 on 29 March 2023 (0.13 Jy at 4.7 GHz; ATel #15964) and 4 April 2023 (a giant flare of 2.23 Jy at 8.2 GHz; ATel #15974).

We report the detection of a bright radio flare with the 32m Medicina radio telescope (PI: E. Egron) and the RATAN-600 (PI: S. Trushkin) on 21-22 April 2023 during simultaneous observations performed with INTEGRAL. The radio observations were carried out at 8.3 GHz during a long session of 9 continuous hours on 21 April from 23:30 UT to 08:30 UT (22 April), then a shorter session of 1 hour on 22 April (from 23:30 UT to 00:30 UT - 23 April). We alternated On-The-Fly cross-scans centered on the target and on flux density calibrators NGC 7027, 3C48 and 3C147, using the SARDARA backend. The radio flux was 0.60 +/- 0.03 Jy on 21 April 2023 (MJD 60055.992, corresponding to the beginning of the first session) and quickly increased to reach a peak intensity of 0.77 +/- 0.04 Jy on 22 April (MJD 60056.105). The flux density then progressively decreased: 0.53 +/- 0.03 Jy (MJD 60056.349), then 0.42 +/- 0.02 Jy (MJD 60057.025). The results are clearly confirmed by the RATAN-600, which observed the target during the daily monitoring program. The flux intensity at 8.2 GHz was 0.40 +/- 0.02 Jy on 20 April (MJD 60054.109), then 0.71 +/- 0.04, 0.77 +/- 0.04 and 0.53 +/- 0.03 Jy on 22 April (MJD 60056.104, MJD 60056.105 and MJD 60056.349, respectively), 0.42 +/- 0.02 Jy on 23 April (MJD 60057.101) and 0.24 +/- 0.01 Jy on 24 April (60058.098).

As part of a long term monitoring program (PI: J. Rodriguez) INTEGRAL observed GRS 1915+105 for a total duration of 90 ks quasi continuously between MJD 60065.569 and 60056.682 with a ~1 hour gap between 60056.5162 and 60056.5545. The source is detected with IBIS in the majority of the individual pointing with averaged 30-50 keV and 50-100 keV fluxes of 2.6 +/- 0.1 cts/s (about 23 mCrab), and 0.96 +/- 0.12 cts/s (about 12 mCrab). It is not detected in the upper energy range with a 100-150 keV 3-sigma upper limit of 0.2 cts (about 18 mCrab). The source 100s 30-50 keV light curve is quite steady around a mean count rate of 2.9 cts/s with quite some dispersion but no evidence for any flaring activity above (a rough 3-sigma limit of) ~15 cts/s (~130 mCrab, 1 bin). The average JEMX 5-12 keV (12-25 keV) rate is 2.4 cts/s (45 mCrab) with a large dispersion in the 32s resolution light curve. It, however, does not show any evidence of flare, or any typical GRS 1915+105 X-ray pattern/cycles known to lead to ejections seen as radio flares (e.g. Rodriguez et al. 2008, ApJ, 675, 1436). As the JEM-X field of view is much smaller than the IBIS one, and due to the dithering pattern of INTEGRAL during its observations, we cannot exclude the occurrence of an X-ray flare during one of the JEM-X observing gaps, but the relative steadiness at higher energy does not favour such a possibility.

Neither Swift/BAT or MAXI show any flaring activity in X-rays during an observing period covering the radio flare. The occurrence of radio flare without X-ray counterpart is quite unusual.

The source will be observed again with INTEGRAL and Medicina in the coming days (26-27 April and 1-2 May 2023), and with the RATAN-600 daily.

Radio and INTEGRAL light curves