The first interferometric detections of Fast Radio Bursts (1703.10173v1)

Abstract: We present the first interferometric detections of Fast Radio Bursts (FRBs), an enigmatic new class of astrophysical transient. In a 180-day survey of the Southern sky we discovered 3 FRBs at 843 MHz with the UTMOST array, as part of commissioning science during a major ongoing upgrade. The wide field of view of UTMOST ($\approx 9$ deg$^{2}$) is well suited to FRB searches. The primary beam is covered by 352 partially overlapping fan-beams, each of which is searched for FRBs in real time with pulse widths in the range 0.655 to 42 ms, and dispersion measures $\leq$2000 pc cm$^{-3}$. Detections of FRBs with the UTMOST array places a lower limit on their distances of $\approx 10^4$ km (limit of the telescope near-field) supporting the case for an astronomical origin. Repeating FRBs at UTMOST or an FRB detected simultaneously with the Parkes radio telescope and UTMOST, would allow a few arcsec localisation, thereby providing an excellent means of identifying FRB host galaxies, if present. Up to 100 hours of follow-up for each FRB has been carried out with the UTMOST, with no repeating bursts seen. From the detected position, we present 3$\sigma$ error ellipses of 15 arcsec x 8.4 deg on the sky for the point of origin for the FRBs. We estimate an all-sky FRB rate at 843 MHz above a fluence $\cal F_\mathrm{lim}$ of 11 Jy ms of $\sim 78$ events sky$^{-1}$ d$^{-1}$ at the 95 percent confidence level. The measured rate of FRBs at 843 MHz is of order two times higher than we had expected, scaling from the FRB rate at the Parkes radio telescope, assuming that FRBs have a flat spectral index and a uniform distribution in Euclidean space. We examine how this can be explained by FRBs having a steeper spectral index and/or a flatter log$N$-log$\mathcal{F}$ distribution than expected for a Euclidean Universe.

• The paper demonstrates the first successful interferometric detection of FRBs, decisively ruling out a terrestrial origin.
• It employs a 180-day UTMOST survey with overlapping fan-beams to capture pulses ranging from 0.655 to 42 ms and dispersion measures up to 2000 pc cm⁻³.
• The study estimates an FRB rate of about 78 events sky⁻¹ day⁻¹ and underscores the need for multi-frequency observations to localize host galaxies.

An Analysis of Interferometric Detections of Fast Radio Bursts

The paper titled "The first interferometric detections of Fast Radio Bursts," presented by Caleb et al., introduces a significant advancement in the observation and understanding of Fast Radio Bursts (FRBs) through the use of interferometry. This paper details the pioneering detection of FRBs using the UTMOST array, marking a substantial step in FRB research by affirmatively ruling out a local terrestrial origin for these enigmatic signals.

Methodology and Observations

Caleb et al. conducted a comprehensive 180-day survey of the Southern sky using the UTMOST array, which is undergoing significant upgrades to enhance its capabilities as an FRB detection instrument. This survey yielded the detection of three FRBs at a frequency of 843 MHz, a noteworthy achievement given the challenges associated with identifying these transients. The UTMOST array's wide field of view and the innovative use of partially overlapping fan-beams were instrumental in these detections, enabling real-time searches for pulses with widths ranging from 0.655 to 42 ms and dispersion measures (DMs) of up to 2000 pc cm$^{-3}$.

From a methodological perspective, the use of an interferometric array like UTMOST represents a departure from traditional single dish observations, which typically suffer from poor angular resolution. By achieving interferometric detections, the research solidifies the astronomical origins of FRBs, showing they occur at distances exceeding the telescope's near-field limit of approximately 10,000 km. These results support the cosmological distance hypothesis, reinforcing theories that suggest the large DMs are due to propagation through the Intergalactic Medium (IGM).

Results and Implications

The FRBs detected—160317, 160410, and 160608—exhibit DMs exceeding those accounted for by the Galactic Interstellar Medium alone, which supports their extragalactic origins. In particular, FRB 160410 has the lowest DM excess of any published FRB to date, making it a prime target for future observations to investigate potential repeating behavior.

The research provides a robust measure of the all-sky FRB rate at 843 MHz, estimating about 78 events sky$^{-1}$ d$^{-1}$ above a fluence limit of 11 Jy ms. This finding indicates an event rate nearly two times higher than previously expected, suggesting the possibility of steeper spectral indices or flatter log$N$-log$\cal{F}$ distributions for FRBs than those anticipated in a Euclidean Universe. Such results have implications for our understanding of the intrinsic properties and distribution of FRBs, potentially refining current progenitor models and informing the design of future surveys.

Future Directions

By utilizing interferometric techniques, the UTMOST array sets a precedent for future FRB detection and localization efforts. A repeated FRB observation at a different hour angle could significantly refine positional constraints, potentially isolating the bursts to a few arcseconds, paving the way for identifying host galaxies and further understanding the extragalactic phenomena.

The paper also points towards the necessity for multi-frequency observations, as simultaneous detections across a broad frequency range could constrain the spectral characteristics of FRBs more effectively. Upcoming surveys with instruments like CHIME and HIRAX, which offer broad frequency coverage and high sensitivity, are expected to leverage these findings, enhancing detection rates and providing deeper insights into FRB characteristics.

In conclusion, Caleb et al.'s work represents a foundational step forward in FRB research, employing interferometric techniques which have further substantiated the extragalactic nature of FRBs and charting a course for future explorations in this dynamic field of astrophysics.