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

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. Deployed at the University of California, Berkeley's Leuschner Radio Observatory, LIMBO employs a $4.3\mathrm{m}$ dish with a dual-polarization feed to continuously monitor a $250\mathrm{MHz}$ band centred at $1475\mathrm{MHz}$. 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 $\ge 43\mathrm{Jy}\cdot \mathrm{ms}$. 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 $R(\ge 65\mathrm{Jy}\cdot \mathrm{ms})=112.3_{-54.5}^{+81.3}{\mathrm{yr}}^{-1}$ and $R(\ge 130\mathrm{Jy}\cdot \mathrm{ms})=17.7_{-15.1}^{+40.8}{\mathrm{yr}}^{-1}$. Combining these results with previously reported FRBs from SGR 1935+2154, we infer a cumulative rate-fluence power-law slope of $\hat{I}\pm =-0.60_{-0.28}^{+0.24}$ in the fluence range between $10$ and $10^6\mathrm{Jy}\cdot \mathrm{ms}$. These observations demonstrate the capability of continuous, real-time monitoring of Galactic magnetars and establish LIMBO as an effective instrument for detecting Galactic FRBs.

16 pages, 15 figures, submitting to RASTI