2015A&A...580L...2Z

Our aim is to explore the broad-band radio continuum spectrum of LS I +61°303 during its outbursts by employing the available set of secondary focus receivers of the Effelsberg 100 m telescope. The clear periodicity of the system LS I +61°303 allowed observations to be scheduled covering the large radio outburst in March-April 2012. We observed LS I +61°303 on 14 consecutive days at 2.6, 4.85, 8.35, 10.45, 14.3, 23, and 32GHz with a cadence of about 12h followed by two additional observations several days later. Based on these observations we obtained a total of 24 quasi-simultaneous broad-band radio spectra. During onset, the main flare shows an almost flat broad-band spectrum, most prominently seen on March 27, 2012, where - for the first time - a flat spectrum (α=0.00±0.07, S∝ν^α) is observed up to 32GHz (9mm wavelength). The flare decay phase shows superimposed ``sub-flares'' with the spectral index oscillating between -0.4 and -0.1 in a quasi-regular fashion. Finally, the spectral index steepens during the decay phase, showing optically thin emission with values α~-0.5 to -0.7. The radio characteristics of LS I +61°303 compare well with those of the microquasars XTE J1752-223 and Cygnus X-3. In these systems the flaring phase is actually also composed of a sequence of outbursts with clearly different spectral characteristics: a first outburst with a flat/inverted spectrum followed by a bursting phase of optically thin emission.