SIMBAD references

The behaviour of fast radio bursts (FRBs) at radio frequencies $< 400$ MHz is poorly understood, with only two sources detected below 300 MHz. We robustly characterize the 150-MHz activity of CHIME-detected FRB sources relative to their 600-MHz activity - using their non-detection in 473 h of archival observations from the LOFAR Tied-Array All-Sky Survey (LOTAAS), and in 252 h of LOFAR observations of 14 repeating FRB sources - the largest sub-300 MHz targeted FRB campaign to date. We search the LOTAAS data for repeat bursts from 33 CHIME/FRB repeaters, 10 candidate repeaters, and 430 apparent non-repeaters. Their non-detection yields a population-level statistical spectral index constraint of $\alpha _{s, 135\, \rm {MHz}/600\, \rm {MHz}}>-0.9$, indicating that FRB spectral indices are, on average, flatter than those of pulsars. From the targeted campaign, the prolific repeater FRB 20201124A shows $\alpha _s>0.55$, implying reduced low-frequency activity, unlike the typically negative $\alpha _ \rm {s}$ seen from FRBs at higher frequency bands. We explore free-free absorption in its circumburst environment as a cause of the non-detection at 150 MHz, and find that it is consistent with either a very young $\sim 10$ yr old supernova remnant; or a typical H II region. Our simulations indicate that LOFAR2.0 can detect 0.3-9 FRBs per week, with up to 4 FRBs originating from redshifts $1< z< 3$. Such detections will provide robust constraints on cosmological parameters due to their clean environments. Our results thus inform future low-frequency FRB searches through the limits we place on repetition rates and show how even non-detections can place meaningful constraints on FRB circumburst environments.