Radio and X-ray monitoring of Cygnus X-1 during the recent state change

ATel #2734; A. Rushton (ESO), Y. Evangelista (INAF/IASF-Rm), Z. Paragi (JIVE), J. Miller-Jones (NRAO), P. Casella (Uni. Southampton), T. Maccarone (Uni. Southampton), V. Tudose (ASTRON), G. Pooley (Uni. Cambridge), R. Fender (Uni. Southampton), R. Spencer (Uni. Manchester) V. Dhawan (NRAO), M. Garrett (ASTRON), R. Campana (INAF/IASF-Rm) & E. Del Monte (INAF/IASF-Rm)
on 12 Jul 2010; 17:24 UT
Credential Certification: Anthony Rushton (Anthony.Rushton@Manchester.ac.uk)

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Following the reports of a state transition in the BHC Cygnus X-1 around 28 June (Atels #2711, #2714 and #2715), we triggered multiple X-ray and radio observations to observe a possible spectral state change. Further evidence of a spectral state change also appeared in the Fermi-GBM observations, showing a rapid fall in hard X-rays around 29 June - 01 July 2010 (Atel #2721).

High-resolution radio observations were first taken by MERLIN at 6cm on:
2010 July 02 20:00 – 2010 July 03 10:00 UT
2010 July 03 16:00 – 2010 July 04 10:00 UT
2010 July 04 16:00 – 2010 July 05 10:00 UT

Results showed an unresolved source with a slightly variable peak brightness of about 10 mJy/bm (beam size of about 44 x 50 mas). SWIFT-XRT observations taken on July 02 (Atel #2724), simultaneous with the MERLIN run, showed that the source appeared to be in an intermediate-soft state (Belloni et al., ApJ, 472, L107, 1996).

After confirmation that the source had left the hard state, two e-VLBI observations were scheduled with the European VLBI Network (EVN) also at 6cm:
2010 July 08 18:30 – 2010 July 09 06:30 UT
2010 July 10 18:20 – 2010 July 11 06:20 UT

Preliminary analysis has shown the source to be unresolved, with a beam of about 8 x 5.5 mas, during both e-VLBI epochs; however, a fast quenching of the radio emission from 10 to 3 mJy/bm was discovered in the source between epochs. Moreover, simultaneous SWIFT-XRT observations (PI: Evangelista) were taken during the two e-VLBI observations and by fitting the XRT spectrum with an absorbed disk blackbody plus a powerlaw we obtain a spectral index of 2.64 for the last epoch (2010 July 11 06:15:00 – 06:33:00 UT). We therefore suggest that the source has finally reached the soft state.

A more detailed analysis of the data is required, but the current results tentatively suggest that the ejection of rapid jet components occurs relatively far along in the spectral state transition, rather than at the start of the intermediate state. A detection of a rapidly moving jet component in our upcoming VLBA observations would provide additional support for this idea. Four further VLBA observations have therefore been scheduled starting from 2010 July 15 06:00 – 12:00 UT and we encourage multi-wavelength observations to continue monitoring this object (please contact us if you would like to be further updated to the VLBA schedule).

e-VLBI developments in Europe were supported by the EC DG-INFSO funded Communication Network Developments project 'EXPReS' (http://www.expres-eu.org). The European VLBI Network (http://www.evlbi.org) is a joint facility of European, Chinese, South African and other radio astronomy institutes funded by their national research councils. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. We also thank the Swift team for the XRT observations.