The broad-band radio spectrum of LS I +61°303 in outburst

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

Received: 18 December 2014
Accepted: 27 June 2015

Abstract

Aims. 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.

Methods. 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 32 GHz with a cadence of about 12 h followed by two additional observations several days later. Based on these observations we obtained a total of 24 quasi-simultaneous broad-band radio spectra.

Results. 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 32 GHz (9 mm 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.

Conclusions. 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.