Context :

Aims : Our aim is to explore the broad-band radio continuum spectrum of LS I +61 ^ { \circ } 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 ^ { \circ } 303 allowed observations to be scheduled covering the large radio outburst in March-April 2012 .
We observed LS I +61 ^ { \circ } 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 hours 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 ( \alpha = 0.00 \pm 0.07 , S \propto \nu ^ { \alpha } ) 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 \alpha \sim - 0.5 to - 0.7 .

Conclusions : The radio characteristics of LS I +61 ^ { \circ } 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 .