While repeating fast radio bursts ( FRBs ) remain scarce in number , they provide a unique opportunity for follow-up observations that enhance our knowledge of their sources and potentially of the FRB population as a whole .
Attaining more burst spectra could lead to a better understanding of the origin of these bright , millisecond-duration radio pulses .
We therefore performed \sim 20 hr of simultaneous observations on FRB 121102 with the Effelsberg 100-m radio telescope and the Low Frequency Array ( LOFAR ) to constrain the spectral behaviour of bursts from FRB 121102 at 1.4 GHz and 150 MHz .
This campaign resulted in the detection of nine new bursts at 1.4 GHz but no simultaneous detections with LOFAR .
Assuming that the ratio of the fluence at two frequencies scales as a power law , we placed a lower limit of \alpha > -1.2 \pm 0.4 on the spectral index for the fluence of the instantaneous broad band emission of FRB 121102 .
For the derivation of this limit , a realistic fluence detection threshold for LOFAR was determined empirically assuming a burst would be scattered as predicted by the NE2001 model .
A significant variation was observed in the burst repeat rate R at L-band .
During observations in September 2016 , nine bursts were detected , giving R = 1.1 \pm 0.4 hr ^ { -1 } , while in November no bursts were detected , yielding R < 0.3 hr ^ { -1 } ( 95 % confidence limit ) .
This variation is consistent with earlier seen episodic emission of FRB 121102 .
In a blind and targeted search , no bursts were found with LOFAR at 150 MHz , resulting in a repeat rate limit of R < 0.16 hr ^ { -1 } ( 95 % confidence limit ) .
Burst repeat rate ratios of FRB 121102 at 3 , 2 , 1.4 , and 0.15 GHz are consistent within the uncertainties with a flattening of its spectrum below 1 GHz .