ASKAP low-band interferometric localisation of the FRB 20201124A source

ATel #14592; C. K. Day (Swinburne University of Technology, SUT; CSIRO), S. Bhandari (CSIRO), A. T. Deller (SUT), R. M. Shannon (SUT), on behalf of the ASKAP-CRAFT survey science project.
on 3 May 2021; 06:55 UT
Credential Certification: Shivani Bhandari (shivanibhandari58@gmail.com)

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The Australian Square Kilometre Array Pathfinder (ASKAP) has detected multiple bursts from the repeating FRB 20201124A source (ATel #14502, ATel #14508). We have previously reported a burst localisation using the second burst detected by ASKAP (ATel #14508).

Here we provide an update on the localisation of the FRB 20201124A source using the first burst detected at 864.5 MHz with ASKAP. The interferometric localisation of this burst proceeded identically to the previously reported burst, using methods described in Bannister et al. (2019, Science, 365, 565).

The newly analysed burst was localised to the following best-fitting position (epoch J2000):
RA: 05h08m03.475s
DEC: +26d03m38.44s
We estimate the total uncertainty ellipse to be 1.25"x0.67", PA 140 deg. We also slightly revise our high-band position originally reported in ATel #14515 (epoch J2000):
RA: 05h08m03.665s
DEC: +26d03m39.50s and make an updated (and larger) estimate of its total uncertainty ellipse to be 1.55"x0.91", PA 42 deg. The reason for this revision is detailed below.

Our low-band burst position is consistent with the localisation reported using the VLA/realfast and uGMRT observations (ATel #14526, ATel #14538). The high-band burst position remains offset by 2.6" from the best-fit location of our low-band burst and those reported by other facilities, a disagreement of up to ~1.5-2 sigma.

The total uncertainty in the burst position comprises a statistical component (dependent on the S/N of the FRB image) and a systematic component related to the uncertainty of the astrometric frame registration. The reference frame correction, and its associated uncertainty, were derived by comparing positions of background sources detected in the 3-s voltage download containing the FRB) with catalogue counterparts, in this case from the NVSS (Condon et al. 1999, AJ, 115,1693).

A detailed investigation into the discrepancies found between the two independently obtained ASKAP positions is ongoing. Below, we list relevant information already identified:

• Both this burst and the one reported in ATel #14508 were observed at low elevation due to the northerly location of FRB 20201124A (approximately 23 degrees in each case).
• This leads to a highly elongated synthesised beam and hence positional uncertainties that are correlated in right ascension and declination. As the ASKAP beam is usually approximately circular, uncertainties are normally reported projected onto the RA and Decl. axes without highlighting this correlation. For both the burst reported here and in ATel #14508, this approach leads to a misrepresentation of the positional uncertainties.
• Visible residuals were present in the high-band image (at a level higher than typically observed for other ASKAP FRBs) that may be indicative of calibration errors, possibly related to the low observing elevation.
• For both ASKAP bursts, the derived scatter in measured offsets between background radio source positions in the 3-s ASKAP image and their catalogue counterparts in NVSS is larger than expected given the statistical positional uncertainties (and larger than seen for previous FRBs), further indicating potential systematic errors.

To account for the elliptical beam shape in these two bursts, we derive the reference frame correction (and its uncertainty) in a rotated frame that is aligned with the synthesised beam major axis. We then account for the larger-than-expected scatter between the reference frame source offsets by applying a scale factor (which is close to 3 in each case) to the final systematic uncertainty.

The results of the full investigation will be reported in a forthcoming paper, once the analysis of a further 3 bursts (i.e., a total of 5) detected from this source by ASKAP is completed.

We compare ASKAP positions with the VLA/realfast and uGMRT position. The Pan-STARRS optical image of the field is shown as a background.