Synchrotron radiation emitted from one relativistic electron population with density Nrel gyrating along a magnetic field B produces a power-law radio spectrum (S ∝ να) with spectral slope αthin 0 above a critical frequency νbreak that depends on B and Nrel. Below this critical frequency self-absorption effects become important and the emission becomes optically thick with a spectral slope αthick = 2.5 (e.g. Kaiser 2006). For instance, the spectrum of the radio outburst in the pulsar system PSR B1259−63 has a spectral index α = −0.7, which is consistent with optically thin synchrotron emission (Fig. 3 in Connors et al. 2002). In contrast, the jets of microquasars often show flat/inverted radio spectra as demonstrated by Fender (2001). Plasma and magnetic field variations along the jet in fact may create regions with different B and Nrel values resulting in spectral components with different turn-over frequencies νbreak. The overlap of the optically thin part of one spectral component with the self-absorbed part of the adjacent one will result in a flat spectrum when observed with a spatial resolution insufficient to resolve the jet (Kaiser 2006). In the microquasar Cygnus X-1, radio emission has been measured with a flat spectrum, i.e. α = −0.06 ± 0.05 and α = 0.07 ± 0.04 (Fender et al. 2000). In Cygnus X-1 the black hole is powered by accretion of the stellar wind of its supergiant companion star. Since the companion is close to filling its Roche lobe, the wind is not symmetric but strongly focused towards the black hole (Miškovičová et al. 2011, and references therein). In contrast, as discussed above in the context of an accretion scenario, accretion in LS I +61°303  occurs in two particular orbital phases due to the eccentric orbit around the Be star. Is the accretion phase and subsequent ejection in LS I +61°303  able to develop a microquasar jet with a flat spectrum? As shown by Corbel et al. (2013), in the early phase of jet formation the low density particles in the jet produce an optically thin synchrotron power-law spectrum at GHz frequencies. Only when the density of the jet plasma increases, a transition to higher optical depth occurs resulting in flat/inverted radio spectra (α ≥ 0). In LS I +61°303 strong evidence for a flat radio spectrum does exist. Early VLA observations at four epochs revealed a flat spectrum between 1.5 and 22 GHz (Gregory et al. 1979). Furthermore, Strickman et al. (1998) carried out multi-frequency VLA observations sparsely covering one orbit and found deviations in the radio spectrum from a simple power law during the outburst. Finally, Massi & Kaufman Bernadó (2009) measured even inverted, optically thick spectra between 2.2 GHz and 8.3 GHz. The primary aim of the current work is to extend previous radio observations of LS I +61°303 and systematically study – for the first time – broad-band cm/mm (2.6–32 GHz) radio spectra and their evolution during a complete outburst. The densely sampled observations of LS I +61°303 were performed over a period of about three weeks (a total of 24 days) using the Effelsberg 100 m telescope at a total of seven frequency bands. The paper is structured as follows. In Sect. 2 we briefly present the observations and data reduction procedures. In Sect. 3 we present the results. Section 4 provides a short discussion and our conclusions.
Citation: Fender et al. 2000
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