Spectral differences between the radio-loud and radio-quiet low-hard states of GRS 1915+105: Possible detection of synchrotron radiation in X-rays

The Galactic microquasar GRS 1915+105 exhibits several episodes of steady X -ray emission characterized by a hard power-law spectrum and intense Quasi Periodic Oscillations. It is known that there are two types of such low-har d states, one with steady radio emission and the other without any signific ant radio emission. We present the results of a detailed X-ray spectroscopi c study of GRS 1915+105, using data from the Rossi X-ray Timing Explorer ob tained during various episodes of the low-hard states of the source. We sho w that there are distinct X-ray spectral differences between the radio-quie t and radio-loud low-hard states of the source. The X-ray spectra of the ra dio-quiet low-hard state is best described by a model consisting of a multi color disk-blackbody and a Comptonized component, whereas the X-ray spectra of radio-loud low-hard state requires a model consisting of three componen ts: a multicolor disk-blackbody, a Comptonized component and a power-law, f or statistically and physically acceptable fits. We attempt to model the pr esence of this additional power-law component as due to synchrotron radiati on which is responsible for the radio and infrared radiation from the sourc e. We show that a simple adiabatically expanding jet model for the synchrot ron radiation can account for the observed X-ray flux for reasonable values of the magnetic field and the mass out ow rate. This is the first report o f detection of the synchrotron radiation in the X-ray band for this source.