A. Pramesh Rao

ON THE ORIGIN OF THE VARIOUS TYPES OF RADIO EMISSION IN GRS 1915+105

We investigate the association between the radio plateau states and the large superluminal flares in GRS 1915+105 and propose a qualitative scenario to explain this association. To investigate the properties of the source during a superluminal flare, we present Giant Metrewave Radio Telescope observations during a radio flare that turned out to be a preplateau flare, as shown by the contemporaneous Ryle Telescope observations. A major superluminal ejection was observed at the end of this plateau state (as described by V. Dhawan et al.), associated with highly variable X-ray emission showing X-ray soft dips. This episode thus has all three types of radio emission: a preplateau flare, a plateau state, and superluminal jets. We analyze all the available Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array data during this episode and show that (1) the preflare plateau state consists of a three-component X-ray spectrum that includes a multicolor disk-blackbody, a Comptonized component, and a power law and (2) the Compton cloud, which is responsible for the Comptonizing component, is ejected away during the X-ray soft dips. We investigate all the available monitoring data on this source, identify several candidate superluminal flare events, and analyze the contemporaneous RXTE pointed observations. We detect a strong correlation between the average X-ray flux during the plateau state and the total energy emitted in radio during the subsequent radio flare. We find that the sequence of events is similar for all large radio flares, with a fast rise and exponential decay morphology. Based on these results, we propose a qualitative scenario in which the separating ejecta during the superluminal flares are the interaction of the matter blob ejected during the X-ray soft dips with the steady jet already established during the plateau state. This picture can explain all the types of radio emission observed from this source in terms of its X-ray-emission characteristics.

GMRT Observations of Microquasar V4641 Sgr

We report the GMRT observations of V4641 Sgr during the May 2002 outburst at radio frequencies of 610 and 244 MHz. This is the lowest frequency radio detection of this source. The present low frequency radio observations clearly showed spectral evolution from the optically thick to thin state. This behavior is broadly consistent with the expanding bubble model. However, the flux densities observed at lower frequencies are much higher than predicted by this model. In the conical jet model, this discrepancy could be reconciled.

Low Frequency Radio Observations of GRS 1915+105 with GMRT

We present the first detailed low frequency radio measurements of the galactic microquasar GRS 1915+105 with GMRT. Simultaneous observations were carried out at 610 and 244 MHz. Our data does not show any signature of spectral turn over even at low radio frequency of 244 MHz. We propose that while the radio emission at high radio frequencies could predominantly come from compact jets, the emission at lower frequency originates in the lobes at the end of the jet which acts like a reservoir of low energy electrons.