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 $\geq 43~\mathrm{Jy \cdot ms}$. 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 $R(\geq 65~\mathrm{Jy \cdot ms}) = 112.3^{+81.3}_{-54.5}~\mathrm{yr}^{-1}$ and $R(\geq 130~\mathrm{Jy \cdot ms}) = 17.7^{+40.8}_{-15.1}~\mathrm{yr}^{-1}$. Combining these results with previously reported FRBs from SGR 1935+2154, we infer a cumulative rate-fluence power-law slope of $α=-0.60^{+0.24}_{-0.28}$ 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.

Figures (15)

• Figure 1: Minimum FRB burst energy versus follow-up observation time for a range of FRB projects. For extragalactic sources, each symbol represents a different FRB source as observed by a given instrument; the symbols outside the "extragalactic detections" region represent FRBs originating from the Galactic magnetar SGR 1935+2154 but detected by different instruments. For comparison, we show LIMBO’s minimum detectable burst energy in both the Galactic and extragalactic (assumed redshift of $z=0.1$) regimes.
• Figure 2: The 4.3-m radio telescope located at the UC Berkeley Leuschner Radio Observatory, equipped with the LIMBO FRB detection pipeline.
• Figure 3: Schematic of the LIMBO analogue system. A dual-polarization feed horn captures signals which are amplified and filtered before being transmitted to an indoor processing rack via fibre-optic cabling. Inside, signals are heterodyne mixed, filtered, and further amplified before being digitally sampled on a SNAP board and sent to a local server over a 10-Gb Ethernet connection. The two linear polarizations follow parallel, matched paths on their way to the server.
• Figure 4: Schematic of the LIMBO digital signal processing system. Analogue signals are digitized by dual 8-bit, 500 MHz ADCs on a SNAP board and processed in real time by the onboard Kintex-7 FPGA, the design of which is based upon that of the DSA-10 2019MNRAS.489..919K. A polyphase filter bank converts time-domain data into 2048 frequency channels, which are split into parallel power and voltage streams. Power spectra are integrated and down-sampled before transmission to a local server, while voltage spectra are quantized and streamed with high time resolution. Both streams use 10-Gb Ethernet with the HASHPIPE protocol.
• Figure 5: Modelling of Leuschner's bandpass using DPSS filters. The black curve is an uncalibrated Cygnus A spectrum with arbitrary units, time-averaged over $\sim28$ s. A DPSS filter excises clock-lines, RFI, and narrow spectral features, yielding a smoothed passband model (dashed yellow).