Giant radio flare of Cyg X-3 observed with Medicina and SRT in April 2024

ATel #16574; E. Egron (INAF-OAC, Italy), M. N. Iacolina (ASI, Italy), A. Pellizzoni, M. Pilia (INAF-OAC), S. Righini (INAF-IRA, Italy), G. Carboni, M. Marongiu (INAF-OAC), A. Zanichelli, A. Orlati (INAF-IRA), S. Trushkin (SAO RAS, Russia)
on 5 Apr 2024; 18:28 UT
Credential Certification: Elise Egron (elise.egron@inaf.it)

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The microquasar Cyg X-3 entered in the hyper-soft X-ray state in mid January 2024 (Swift/BAT monitor). Bright radio flares of different intensity were observed by the Nasu telescope array (ATels #16455, #16520) and with the Allen Telescope Array (ATA; ATel #16466) in February 2024. Then Cyg X-3 came back into the quenched radio state.

A new radio activity was detected above 1 Jy by the RATAN-600 (S. Trushkin) on 2024 March 28. We triggered multi-frequency observations with the Italian radio telescopes: Medicina (32m) and the Sardinia Radio Telescope (SRT; 64m) at 4.8 GHz, 8.3 GHz and 18.5 GHz. On-The-Fly cross-scans were performed on the target and on the flux calibrator NGC7027 using the TP backend with 250 MHz of bandwidth at 4.8 GHz and 8.3 GHz, and 680 MHz of bandwidth at 18.5 GHz in order to avoid the RFI.

A first radio flare was detected on 2024 March 29 with a flux density of 2.8 +/- 0.08 Jy (06:12 UT) at 4.8 GHz, 2.0 +/- 0.06 Jy (08:17 UT) at 8.3 GHz and 1.2 +/- 0.1 Jy (09:29 UT) at 18.5 GHz but the flux decreased the day after: 0.58 +/- 0.02 Jy (03:30 UT) at 8.5 GHz and 0.33 +/- 0.02 Jy (09:22 UT) at 18.5 GHz.

In April 2024, the radio flux considerably increased. On April 2, its intensity reached 9.1 +/- 0.3 Jy (13:06 UT) at 4.8 GHz, 8.1 +/- 0.4 Jy (12:35 UT) at 8.3 GHz and 5.9 +/- 0.5 GHz (12:11 UT) at 18.5 GHz. The peak was observed by the RATAN-600 the following day indicating a giant flare emission. Then the radio flux decreased (2.3 +/- 0.2 Jy at 8.3 GHz on April 5 at 02:37 UT).

Radio light-curve obtained with Medicina and SRT