Long-Integration Magnetar Burst Observatory (LIMBO): Instrument Summary and Early FRB Rate Constraints

Abstract

Abstract The Long-Integration Magnetar Burst Observatory (LIMBO) is a real-time radio transient detection pipeline designed to search for dispersed fast radio bursts (FRBs) from Galactic magnetars. LIMBO employs a 4.3-m dish with a dual-polarization feed to continuously monitor a 125MHz effective band centred at 1460MHz. A real-time processing pipeline performs a search for dispersed transients on the summed polarizations, with detections triggering dumps of buffered voltage data to disk. Based on calibrated sensitivity measurements, synthetic signal-injection and recovery tests, and successful detection of pulses from the Crab Pulsar, we determine that LIMBO is sensitive to radio transients with fluences ≳ 43Jy · ms. Between May and August 2023, LIMBO conducted 833 hours of follow-up observations of the Galactic magnetar SGR 1935+2154, yielding 12 candidate FRB detections. If these events are true, we measure FRB-like event rates from SGR 1935+2154 of $R(\ge 65 \mathrm{Jy \cdot ms}) = 112.3^{+81.3}_{-54.5} \mathrm{yr}^{-1}$ and $R(\ge 130 \mathrm{Jy \cdot ms}) = 17.7^{+40.8}_{-15.1} \mathrm{yr}^{-1}$. Combining these results with previously reported FRBs-like from SGR 1935+2154, we infer a cumulative rate–fluence power-law slope of $\alpha =-0.60^{+0.24}_{-0.28}$ in the fluence range between 10 and $10^6\rm \, Jy \cdot ms$. These observations demonstrate the capability of continuous, real-time monitoring of Galactic magnetars and establish LIMBO as an effective instrument for detecting Galactic FRBs.