Multi wavelenght radio observations of Cygnus X-3 during the giant flare of May-June 2006

Abstract

We report the multi-frequency radio observation of the Galactic microquasar Cygnus X-3 during the giant flare of 2006 May-June, which was one of the largest flare in its recorded history. We have used the Giant Metrewave Radio Telescope (GMRT) data at 244 and 614 MHz and Very Large Array (VLA) data at 8.43 and 43.3 GHz. By combining the published results from RATAN [1], we obtain the radio spectrum of the source during both rising and the fading phases of the flare. We found that the radio spectrum is showing absorption towards lower frequency and after flare the turn-over frequency is shifting towards lower frequency region with time. The two point spectral index between 244 and 614 MHz showed clear variation from positive (optically thick) and negative (optically thin) values. We showed that synchrotron self absorption is consistent as the cause for such behavior. We have also examined this evolution of low frequency spectral index of the source and propose a possible model to interpret the observed behavior in terms of expanding blobs of plasma. Applying the synchrotron self absorption model, we obtain the physical parameters such as the size of the emitting region, turn over frequency and corresponding peak flux. By obtaining the size of the emitting region at different epochs of the flare we estimate the velocity of the expansion of the blob to be in the range 0.02c to 0.17c, assuming the magnetic field strength in the range 0.1 to 1 Gauss. This is in broad agreement with the known expansion velocities of other micro-quasars.