Twenty-four thousand hours of GREENBURST observations with the GBT

TL;DR

GREENBURST leverages commensal Green Bank Telescope observations to search for dispersed radio pulses, addressing the FRB population with long, uninterrupted campaigns. The study describes a data-processing pipeline with normality-based RFI excision and FETCH-based candidate classification, reporting results from 24,186 hours of data that yield 50 pulsars and 3 FRBs, plus GBP 220718 and the new pulsar PSR J0039+5407. The newly discovered PSR J0039+5407 has a ~2.2 s period and a high nulling fraction, with CHIME/Pulsar timing providing a phase-connected solution; GBP 220718 presents a DM around 146 cm^-3 pc and a narrow-band morphology that complicates origin assessment. Overall, the work demonstrates the viability of long-duration commensal FRB searches with large facilities, expands the pulsar census, and highlights the ongoing challenges in distinguishing celestial from terrestrial narrow-band signals, guiding future follow-up and methodological refinements.

Abstract

In addition to fast radio burst (FRB) searches carried out using dedicated surveys, a number of radio observatories take advantage of commensal opportunities with large facilities in which observations for other projects can be searched for FRBs and other transient sources. We present the results from one such effort, the first 24,186 hours of the GREENBURST search for dispersed radio pulses with the Green Bank Telescope (GBT). To date, GREENBURST has detected a total of 50 pulsars and three FRBs. One of the pulsars, PSR J0039+5407, has a period of 2.2 s and was previously unknown. Using follow-up observations with the Canadian Hydrogen Intensity Mapping Experiment, we found a timing solution for this pulsar which shows it to have a characteristic age of 2 Myr. Additional GBT observations show the pulsar has a very high nulling fraction ($\sim70-80\%$). All three of the FRBs are repeating sources that were previously known and were being monitored by the GBT as part of other projects. A major challenge for GREENBURST in the discovery of new FRBs is its single beam. This makes it hard to distinguish some of the pulses from sources of radio frequency interference. We highlight this problem with a case study of an FRB-like pulse that initially passed our interference filters. Upon closer inspection, the event appears to be part of a longer-duration narrow-band source of unknown origin. Further observations and monitoring are required to determine whether it is terrestrial or celestial.

Figures (11)

• Figure 1: Schematic showing the GREENBURST data acquisition and analysis pipeline. The blue input squares are data provided by the Serendip VI serendip_6 backend. Data are sent to GREENBURST via UDP packets which are buffered and written in filterbank format 2011ascl.soft07016L. The double buffer ensures continuous data coverage over discrete filterbanks. The Elasticsearch database keeps track of the telescope pointing at 8 min cadence over the lifetime of the experiment. If the data are valid (suitable receiver position, not in maintenance, etc.) they are cleaned with jess_gauss.py’s Jarque-Bera filter. These cleaned filterbank data are subsequently searched with heimdallBarsdell. Candidate pulses are cut out and classified as astrophysical or RFI by FETCH 2020MNRAS.497.1661A. Candidates classified as astrophysical are reviewed by humans on a daily basis via a dedicated Slack channel. An InfluxDB2 keeps track of the telescope pointing at a cadence of 1 s. Yellow squares show steps that leverage GPU acceleration.
• Figure 2: Sky coverage for the observation period reported in this paper. The positions are quantized into 6 deg$^2$ hexagons and colour coded into hours spent per region. Stars indicate pulsars, crosses indicate FRBs. Grey points are known objects. The blue star is represents PSR J0039+5407. The blue cross is GBP 220718.
• Figure 3: Timeline of observation showing totals for each year. The 2024 total is up to the end of July. Overall, the L-Band receiver was in focus 72% of the time. Other receivers being used by the primary observer were: X-Band 12%, C-Band 7%, Ka-Band 5% and Mustang 3%.
• Figure 4: Sample individual pulses detected from the 49 pulsars in Table 1. The numerical labels under each pulse denote time in ms with respect to the centre of the time series. For presentation purposes, each pulse has been aligned so that its peak value is located 75% along the horizontal axis.
• Figure 5: Example detections of individual pulses from the three previously known FRBs (FRB 20121102A, FRB 20190520B, and FRB 20200120E) observed during follow-up observations of these sources as well as the discovery pulse for GBP 220718. In each case the top panel shows the dedispersed pulse, the middle panel shows the 'waterfall' plot of frequency vs time, while the bottom panel shows the trial DM as a function of time.