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Radio observations of Cyg X-1 in the soft X-ray state

ATel #2755; V. Tudose (ASTRON), G. Pooley (Uni. Cambridge), A. Rushton (ESO), J. Miller-Jones (ICRAR), Z. Paragi (JIVE), T. Maccarone (Uni. Southampton), R. Fender (Uni. Southampton), R. Spencer (Uni. Manchester), V. Dhawan (NRAO), M. Garrett (ASTRON), Y. Evangelista (INAF/IASF-Rm), R. Campana (INAF/IASF-Rm), E. Del Monte (INAF/IASF-Rm), S. Sabatini (INAF/IASF-Rm), P. Casella (Uni. Southampton)
on 24 Jul 2010; 10:49 UT
Credential Certification: Valeriu Tudose (vtudose@science.uva.nl)

Subjects: Radio, Binary, Black Hole, Transient

Referred to by ATel #: 3307

Cyg X-1 has been transiting towards a soft X-ray state in the last month, as revealed by a series of X-ray observations (ATel #2711, ATel #2711, ATel #2721, ATel #2724). Currently the source seems to be in a typical soft X-ray state (ATel #2751).

As part of our radio campaign (ATel #2734), we performed a WSRT observation of Cyg X-1 at 4.9 GHz on July 23rd between 00:30-05:30 UT. We detected an unresolved source (for a beam size of 16x6 arcsec^2) with a peak flux density of 3.2 +/- 0.2 mJy/beam. Simultaneous observations (02:00-03:00 UT) at 15 GHz with AMI showed a compact emitting region (at 25 arcsec resolution) with a flux density of 1.6 +/- 0.2 mJy. This suggests a radio spectrum in the GHz range with a spectral index of about -0.6 (+/- 0.2), indicative of ejecta and/or shocks.

Further multi-wavelength observations are encouraged.