Bright radio flare from the X-ray binary Swift J1727.8-1613

ATel #16289; S. A. Trushkin, N. N. Bursov, N. A. Nizhelskij, P. G. Tsybulev (SAO RAS, Russia)
on 14 Oct 2023; 19:45 UT
Credential Certification: Sergei Trushkin (satr@sao.ru)

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We have almost daily observed the black hole candidate X-ray binary Swift J1727.8-1613 on 14-30 September 2023 with the ''Southern Sector and Flat mirror'' antenna and later from 1 October 2003 with the ''Northern Sector'' antenna of the RATAN-600 radio telescope (SAO RAS) using the similar radiometers at 4.7, 8.2, 11.2 and 22.3 GHz, but also we have 2.3 GHz radiometer at the second antenna. We have detected Swift J1727.8-1613, often at three frequencies 4.7-11.2 GHz. For example on 18 September (MJD 60205.620) flaring fluxes reached 195, 195, and 150 mJy at 4.7, 8.2 and 11.2 GHz respectively with errors of 10 %. Unfortunately we met with the high level of interference and we lost many measurements. With "Northern sector" antenna we measured fluxes at 22.3 GHz with sufficient accuracy on 5 and 6 October, when the source was in a small flare with fluxes: 152, 223, 535 mJy on 5 October (MJD 60222.574) and 135, 175 and 275 mJy at 4.7, 8.2 and 22.3 GHz respectively. These inverted spectra have the positive spectral indices: 0.82 and 0.46 on 5 and 6 October respectively. After that this flare quenched to the level of 30-50 mJy. However on 14 October (MJD 60231.540) we have detected the bright flare, which fluxes are equal to 485 +/- 40, 760 +/- 30, 770 +/- 30, 660 +/- 40 and 535 +/- 40 mJy at 2.3, 4.7, 8.2, 11.2 and 22.3 GHz respectively. Its spectrum (see below) had the notable optically thick mode at 2.3 GHz, meanwhile for optically thin mode at frequencies higher 4.7 GHz the spectral index is equal to -0.24. Thus we see the the launching of relativistically-moving transient ejecta (see ATel #16271). We strongly encourage observations with the interferometric radio telescopes (VLA, eMERIL IN, MeerKAT and VLBI) to detect jets clearly. We will continue observations with RATAN-600, supported by the Ministry of Science and Higher Education of the Russian Federation (Grant No 075-15-2022-262).

The radio spectrum of Swift J1727.8-1613 on 14 October 2023