Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998astl...24...16t&link_type=abstract
Astronomy Letters, Volume 24, Issue 1, January 1998, pp.16-21; Pis'ma v Astronomicheskii Zhurnal, Vol. 24, p. 19
Astronomy and Astrophysics
Astronomy
5
Scientific paper
Results of daily observations of the binary system Cyg X-3 using the RATAN-600 radio telescope from May 21 to July 20, 1997 are presented. These observations were part of a patrol program to study the flare variability of the radio emission of X-ray sources. Two short, moderate (~1 Jy), optically thick flares were detected on May 22 and May 26 at six frequencies (0.96, 2.3, 3.9, 7.7, 11.2, and 21.7 GHz). The flux decay of the second flare was exponential (S_v = S_0e^{t/t}) with t = 1.03 days. After this flare, there was a sharp decrease in the radio intensity on May 29-31, to 15-20 mJy at 4-11 GHz, 10-12 days before a powerful flare on June 12 (~ 4 Jy). In this flare, the flux density of Cyg X-3 rose from the quiescent level by a factor of 20-30 over 12 days. This was the only flare in our observing period which was optically thin, with spectral index alpha = 0.34 to -0.50 at high frequencies during the first four days. Four days after the onset of the June 12 flare, its flux density began to decay exponentially at all frequencies, and the characteristic decay time decreased with frequency according to the power law tau = 11.5(days)nu^{0.29+/-0.03}, where nu is in GHz. Four days later, the decay changed to a power law at all frequencies except for 21.7 GHz, where a new outburst began. The average behavior for all the frequencies for the final stage of the flare was a decay according to the power law S_nu = S_1t^{4.0+/-0.2}. Later, the spectral index alpha = +0.64 was observed for two, short, optically thick flares on June 22 and 29. The June 29 flare displayed a power-law decay S_nu = S_0t^{2.0+/-0.1} to the same very low flux level from which it began, ~35 mJy. During the deep pre-flare minimum of May 29-31, the spectral index was = +1.23 +/- 0.05, indicating that the radio source was strongly absorbed in this period, possibly due to synchrotron or thermal self-absorption of the central source of Cyg X-3. At frequencies higher than 2.3 GHz, the delay in the flare flux-density maxima was substantially smaller than the interval between our observations (one day); this delay was ~1.01.5 days at 0.96 GHz for local maxima over the entire observation period.
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