[Paper Review] Chromatic periodic activity down to 120 MHz in a Fast Radio Burst

Inés Pastor-Marazuela, Liam Connor+35 more|arXiv (Cornell University)|Dec 15, 2020

Pulsars and Gravitational Waves Research61 references21 citations

TL;DR

This study presents the first detection of chromatic periodic activity in a fast radio burst (FRB) down to 120 MHz using simultaneous Apertif and LOFAR observations of FRB 20180916B. The bursts exhibit a narrower and earlier activity window at higher frequencies, contradicting binary wind interaction models, and demonstrate that low-frequency emission can escape the local medium, indicating clean environments for some FRBs.

ABSTRACT

Fast radio bursts (FRBs) are extragalactic astrophysical transients whose brightness requires emitters that are highly energetic, yet compact enough to produce the short, millisecond-duration bursts. FRBs have thus far been detected between 300 MHz and 8 GHz, but lower-frequency emission has remained elusive. A subset of FRBs is known to repeat, and one of those sources, FRB 20180916B, does so with a 16.3 day activity period. Using simultaneous Apertif and LOFAR data, we show that FRB 20180916B emits down to 120 MHz, and that its activity window is both narrower and earlier at higher frequencies. Binary wind interaction models predict a narrower periodic activity window at lower frequencies, which is the opposite of our observations. Our detections establish that low-frequency FRB emission can escape the local medium. For bursts of the same fluence, FRB 20180916B is more active below 200 MHz than at 1.4 GHz. Combining our results with previous upper-limits on the all-sky FRB rate at 150 MHz, we find that there are 3-450 FRBs/sky/day above 50 Jy ms at 90% confidence. We are able to rule out the scenario in which companion winds cause FRB periodicity. We also demonstrate that some FRBs live in clean environments that do not absorb or scatter low-frequency radiation.

Motivation & Objective

• To detect low-frequency emission from FRB 20180916B below 300 MHz, where previous searches had failed.
• To investigate the chromatic behavior of periodic activity in FRB 20180916B across a wide frequency range (120–1520 MHz).
• To test whether binary wind interaction models can explain the observed periodicity and frequency-dependent activity windows.
• To constrain the all-sky FRB rate at 150 MHz using new detections and previous upper limits.
• To assess the ionized environment of FRB 20180916B by analyzing dispersion measures and scattering timescales at low frequencies.

Proposed method

• Conducted simultaneous observations of FRB 20180916B using the Apertif system on the Westerbork Synthesis Radio Telescope (1220–1520 MHz) and LOFAR (110–190 MHz) over multiple activity cycles.
• Used the Psrchive software to compute the dispersion measure (DM) that maximizes signal-to-noise ratio (S/N) for LOFAR bursts, with subsequent correction to align sub-bursts via a ν⁻² time delay model.
• Applied a multi-component Gaussian fitting routine to sub-burst profiles to measure drift rates, using least-squares fitting to determine sub-pulse centroids and their frequency-time evolution.
• Computed the final DM for LOFAR bursts as the average of bursts with S/N > 20, yielding DM_LOFAR = 349.00 ± 0.02 pc cm⁻³.
• Fitted observed drift rates at 400 MHz and 600 MHz to power-law and linear functions to extrapolate to LOFAR frequencies.
• Stacked LOFAR bursts after dedispersion to improve S/N and estimate the average scattering timescale.

Experimental results

Research questions

• RQ1 Can FRB emission be detected at frequencies as low as 120 MHz, and does it exhibit periodic activity?
• RQ2 How does the activity window of FRB 20180916B vary with frequency, and does it align with predictions from binary wind interaction models?
• RQ3 What is the dispersion measure of FRB 20180916B at low frequencies, and how does it compare to high-frequency measurements?
• RQ4 What is the sub-pulse drift rate of FRB 20180916B at 1.4 GHz, and how does it evolve with frequency?
• RQ5 What constraints can be placed on the all-sky FRB rate at 150 MHz based on the new detections and prior upper limits?

Key findings

• Nine bursts were detected at 120–190 MHz with LOFAR, marking the first detection of FRB emission below 300 MHz.
• The activity window for FRB 20180916B is narrower and earlier at higher frequencies, contrary to predictions from binary wind interaction models.
• The LOFAR dispersion measure was measured as 349.00 ± 0.02 pc cm⁻³, consistent with the Apertif DM of 348.75 pc cm⁻³ when corrected to the same k_DM value.
• The average sub-pulse drift rate at 1370 MHz was found to be -39 ± 7 MHz ms⁻¹, with a standard deviation of 31 MHz ms⁻¹.
• Extrapolation to 150 MHz suggests a drift rate of approximately -6 MHz ms⁻¹, though no clear sub-pulse structure was observed in LOFAR data to confirm this.
• The detection of low-frequency bursts rules out scenarios in which free-free absorption or strong scattering prevents low-frequency emission, indicating that FRB 20180916B resides in a clean environment.