Effects of observational cutoff in frequency on fast radio bursts

Hu, Chenran

Fast radio bursts (FRBs) are significantly affected by observational cutoff effects， which can be categorized into fluence-threshold-induced and bandpass-induced cutoffs， yielding puzzling incomplete observations. Previous research suggested a Gaussian spectrum for FRBs， and bandpass-induced cutoffs， when imposed on Gaussian spectra， can effectively explain the variation in spectral index between negative and positive values. Employing available data， aiming to observational cutoff effects， we performed inverse modeling of the bursts from FRB 20121102A to investigate their underlying features. We reveal that strong bursts tend to concentrate their energy in a narrow frequency range， while weak bursts are independent of spectral width. Weak bursts with broad spectra are more prone to be cutoff， leading to narrower spectra in observations. That is， the narrow spectrum featuring in repeating FRBs， as previously proposed， may be an observational selection effect. Observational cutoffs introduce substantial energy loss and distort the observed energy distribution， as well as shadow the inherent patterns in two-dimensional distribution of bandwidth and central frequency. Our findings show that， in the L-band (1-2 GHz)， bandwidth increase with central frequency， whereas in the C-band (4-8 GHz)， no such link. This indicates potential differences in the underlying physics of repeating FRBs at low and high frequencies.