CygX-3 multi-band radio observations of the new flaring activity

ATel #1839; S. Migliari (ESAC, Spain), J. Tomsick (SSL/UC Berkeley, USA), G. Pooley (Cavendish Lab, UK), S. Corbel (CEA Saclay, France), J. Rodriguez (CEA Saclay, France), K. Pottschmidt (NASA/GSFC, USA), J. Wilms (ECAP, Germany)
on 11 Nov 2008; 16:03 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Simone Migliari (migliari@science.uva.nl)

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We report the results of the radio weekly monitoring of the X-ray binary Cyg X-3 at 9 GHz and 5 GHz with the Green Bank Telescope during the recent radio flare (Pooley; ATel #1828). Tavani et al. (ATel #1827) reported the detection of a gamma-ray transient near the Cygnus Region in an observation taken on 2008 November 2-3. The positional error box of the source includes Cyg X-3 and therefore it is possible that the gamma-ray emission is associated with the new flaring activity of Cyg X-3. However, given the large positional error box, which is almost 1 degree in radius at a 95% confidence level, a confirmation of the association between the two sources is still required. The flux densities of Cyg X-3 on 2008 October 16 are 132+\-4 mJy at 5GHz and 52+\-3 mJy at 9 GHz and increase on 2008 October 22 to 206+\-3 mJy at 5GHz and 306+\-6 mJy at 9 GHz. The errors quoted are 1 sigma rms. At the beginning of the flare, the radio spectral index changes from optically thin on October 16 to optically thick on October 22, which is the opposite of what is expected in a transient radio flare. The spectrum is again optically thin at the peak of the flare on 2008 October 29 with a flux density at 9 GHz reaching 526+\-7 mJy and a spectral index of -0.4 (using the measurement at 15 GHz from Pooley, ATel #1828). The dual-band spectral index measured on October 16 is -1.6 and it is +0.7 on October 22. Synchrotron emission from radio jets show, depending on the nature of the jet, optically thin and optically thick emission that is often inverted, but a spectral index with an absolute value as large as 1.6 is unusual. Since the observations at 9GHz are usually taken almost 1 hr after those at 5 GHz, the large absolute numbers of the spectral indexes very likely reflect a variability of the source on times-scales less than 2 hr. Indeed, variability of the flux density by a factor of 2 on time-scales of minutes has been observed on October 16 (from 35+\-5 mJy to 68+\-4 mJy within 20 min) and October 29 (from 526+\-7 mJy to 210+\-3 mJy within 15 min). The unusual spectral behaviour observed might also be related to a multiple flaring activity. Indeed, we also confirm a new radio flare of Cyg X-3 reported by Fujisawa et al. (ATel 1838), detected after the period of quiescence reported on 2008 November 4-5 (Pooley ATel #1830). A new 15 GHz observation with the AMI Large Array on 2008 November 11 shows a flux density of 300 mJy. A plot with the flux densities of the last month of radio monitoring is available at ftp://ssh.sciops.esa.int/pub/smigliari/cygx3_radio_lc.eps