Research Note

Implications of the radio spectral index transition in LS I +61°303 for its INTEGRAL data analysis

Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: lzimmerm@mpifr-bonn.mpg.de; mmassi@mpifr-bonn.mpg.de

Received: 13 February 2011
Accepted: 17 November 2011

Abstract

Context. The TeV emitting X-ray binary LS I +61°303 has two radio periodicities that correspond to a large periodic outburst with the same period as the orbit, 26.5 days (phase Φ), and a second periodicity of 1667 days (phase Θ), which modulates the orbital phase and amplitude of the large outburst. Analyses of the radio spectral index revealed in LS I +61°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 Φ_crit, which is modulated by Θ: Φ_crit = f(Θ).

Aims. We examine the possible implications of averaging high energy data over large Θ and Φ intervals in the light of puzzling published INTEGRAL results, which differ for different averaging of the data.

Methods. 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°303 at Φ_crit, we can show qualitatively the effect of averaging high energy data on Θ, by analysing the effects of averaging radio spectral index data across the same Θ 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.

Results. When folded over too large a Θ 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.

Conclusions. The INTEGRAL results can be interpreted as two X-ray states that alternate with each other along the orbit. The implications of this analysis for HE/VHE data from LS I +61°303 and a possible connection between HE/VHE emission and the steep power-law state are discussed.