Implications of the radio spectral index transition in LS I +61{\deg}303 for its INTEGRAL data analysis

Abstract

The TeV emitting X-ray binary LS I +61{\deg}303 has two radio periodicities that correspond to a large periodic outburst with the same period as the orbit, 26.5 days (phase \Phi), and a second periodicity of 1667 days (phase \Theta), which modulates the orbital phase and amplitude of the large outburst. Analyses of the radio spectral index revealed in LS I +61{\deg}303 the presence of the critical transition typical for microquasars from optically thick emission (related to a steady jet) to an optically thin outburst (related to a transient jet), and found that it occurs at \Phi_{crit}, which is modulated by \Theta: \Phi_{crit}=f(\Theta). We examine the possible implications of averaging high energy data over large \Theta and \Phi intervals in the light of puzzling published INTEGRAL results, which differ for different averaging of the data. In microquasars, a simultaneous transition between two X-ray states occurs at the switch from optically thick radio emission to an optically thin radio outburst, from the low/hard to the steep power-law state. Assuming that the same transition occurs in LS I +61{\deg}303 at \Phi_{crit}, we can show qualitatively the effect of averaging high energy data on \Theta, by analysing the effects of averaging radio spectral index data across the same \Theta interval. We then model the two X-ray states, low/hard and steep power-law state, and show quantitatively how their mixing can affect the results. When folded over too large a \Theta interval, spectral data from INTEGRAL can yield a false picture of the emission behaviour of the source along the orbit because it may be mixing two spectral states. Furthermore, averaging the data along the orbit may result in a dominant low/hard spectral state, which, for insufficiently extended sampling, might appear without a cut-off.