Discovery and follow up of repeating FRB 20240711A

ATel #17019; Kavya Shaji (University of Sydney), Ines Pastor-Marazuela (University of Manchester), Ben Stappers (University of Manchester), Manisha Caleb (University of Sydney), Kaustubh Rajwade (University of Oxford), Jun Tian (University of Manchester), Mayuresh Surnis (IISER Bhopal), Ewan Barr (MPIfR), on behalf of the MeerTRAP team
on 7 Feb 2025; 09:42 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Ines Pastor-Marazuela (ines.pastor.marazuela@gmail.com)

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The MeerTRAP team reports the discovery of a new repeating FRB source, FRB 20240711A. The FRB was first discovered as a single burst on 2024 July 11 at 21:51:39.138 UTC in the UHF band (544-1088 MHz). The burst showed multiple components drifting downwards in frequency (sad trombone effect, see Fig. 1), an effect typically observed in repeating FRBs (Pleunis et al. 2021). By optimising the burst structure (Seymour et al. 2019), we obtained a dispersion measure (DM) of 834.75 pc/cm^3. The region of sky where the first burst was detected is regularly monitored by the MeerKAT telescope, but no detections were made previous to that date. On 2025 January 18, the field in which the FRB was discovered was re-observed in the UHF band with MeerKAT, and a second burst was detected with a position and DM consistent (to within 6 arcsec and 2 pc/cm^3 respectively) with the first burst, thus confirming the repeating nature of the source (see Fig. 2).

The detection of both bursts triggered the storage of channelised voltage data from all MeerKAT antennas. We used the first, brighter burst to localise the source with arcsecond precision using the MeerTRAP transient buffer pipeline (Rajwade et al. 2020). The best coordinates we find are RA=19h02m08.60s DEC=-51d30m30.47s, with a 1-sigma error of 0.7 arcsec in both RA and DEC. We find two optical sources in the DESI Legacy Survey DR10 archival images (see Fig. 3) (Dey et al. 2019) at 3.4 arcsec (S1) and 4.2 arcsec (S2) respectively from the FRB location. Deep optical observations have been planned to identify any potential galaxy closer to the FRB error region.

The times of arrival of the MeerKAT bursts at the top of the UHF band (1088 MHz) are as below:
M1: 2024-07-11T21:51:39.138
M2: 2025-01-18T13:07:14.838

FRB 20240711A was followed up for repeat bursts using the ultra-wide band (UWL) receiver on the 64-m Murriyang radio telescope, covering 704-4032 MHz. Observations were conducted on January 25 and 30, 2025, for a total of two hours using available green slots. A search for bursts in Murriyang data revealed three bursts on January 25 and two on January 30, each with an S/N above 7, corresponding to a fluence completeness threshold of 0.232 Jy ms. The times of arrival of the detected bursts at 704 MHz are listed below.
P1: 2025-01-25T05:16:50.447
P2: 2025-01-25T05:34:41.774
P3: 2025-01-25T05:45:49.168
P4: 2025-01-30T05:12:32.305
P5: 2025-01-30T05:41:30.080

The dynamic spectra of the two bursts, P3 and P4, are illustrated in Figures 4 and 5, respectively. A preliminary analysis of the polarisation properties of the bursts shows that the Faraday rotation measure (RM) of all bursts remains close to zero within uncertainties. All the bursts exhibit a high linear polarisation fraction of ~80%-90%. Given the low RM and the significant degree of linear polarisation, RM scattering is unlikely to be the dominant cause of the low observed RM. Instead, the low RM may be intrinsic to the source or result from balanced positive and negative RM contributions along the propagation path. As FRB 20240711A is likely still moderately active, we strongly recommend continued observations.

The MeerKAT telescope is operated by SARAO, a facility of the National Research Foundation, an agency of the Department of Science and Innovation. Murriyang is operated by CSIRO’s Australia Telescope National Facility, which is funded by the Australian Government for use by the international scientific community. We extend our gratitude to the team at Murriyang for their support in conducting these follow-up observations.

Link to Google Drive for figures