FRB20221128A - corrected position and fluence

ATel #15865; A. Mandlik (Swinburne University of Technology, "SUT"), M. Bailes (SUT), A. Deller (SUT), C. Flynn (SUT), V. Gupta (CSIRO), A. Jameson (SUT), T. Bateman (SUT), M. Caleb (University of Sydney, "USyd"), D. Campbell-Wilson (USyd), C. Day (SUT), L. Dunn (University of Melbourne "UMelb"), W. Farah (University of California, Berkeley), A. J. Green (USyd), G. Howitt (UMelb), Y. S. C. Lee (UMelb), M. E. Lower (CSIRO), A. Melatos (UMelb), P. Meyers (UMelb), D. C. Price (SUT), R. Sekhri (Alphington Grammar School), A. Sutherland (USyd), G. Torr (SUT), G. Urquhart (SUT), P. Uttarkar (SUT), V. Venkatraman Krishnan (Max-Planck-Institut fuer Radioastronomie)
on 19 Jan 2023; 08:03 UT
Credential Certification: Chris Flynn (cflynn@swin.edu.au)

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In ATel #15783 , we reported the discovery of FRB20221128A with the UTMOST-NS facility. The FRB was reported at a position of RA = 07:30(10), DEC = -41:32(1) with a fluence of ~ 130 +/- 30 Jy ms, pulse width ~ 5.5 +/- 0.7 ms and with a detection signal-to-noise ratio = 9.0.

However, during the course of additional processing, we have identified that the FRB candidate that triggered the UTMOST-NS voltage download system actually originated in a side-lobe. UTMOST-NS processes 192 beams simultaneously and all candidates are logged. When a candidate is found, push notifications for additional FRB candidates are suppressed while the trigger download is in progress. The voltage download was triggered by the side-lobe detection, which led to the much brighter bore-sight FRB candidate to be overlooked. Accordingly, our early estimate of the position and brightness of FRB20221128A are now significantly revised. The detection S/N in the brightest beam is 111, leading to a revised provisional estimate (lower limit) of the FRB fluence to be 1000 +/- 250 Jy ms (corrected for attenuation of the primary beam in the NS direction, but not in the EW direction), making FRB20221128A one of the brightest FRBs detected to date. The optimal dispersion measure (DM) that maximizes the signal-to-noise ratio is very slightly revised to 503.6 pc cm^-3. Given the greatly increased S/N, we fit the FRB as a Gaussian convolved with an exponential scattering tail - the FRB is well fit by a single Gaussian component of width 1.06 +/-0.04 ms and scattering time of 3.30 +/- 0.07 ms at 831.25 MHz. The revised FRB position is RA = 07:30(10), DEC = -42:30(1) in equatorial (J2000) coordinates, which corresponds to Galactic coordinates: Gl = 255.1 deg, Gb = -11.4 deg (we additionally note that the Galactic coordinates in ATel #15783 were in error). While the right ascension is unchanged, the declination is ~1 degree from the originally estimated position.

At the revised location, the NE2001 model (Cordes and Lazio, 2002) predicts a total Milky Way DM along this sightline of ~300 pc cm^-3, while the YMW16 model (Yao, Manchester, and Wang, 2017) predicts ~250 pc cm^-3. If, in line with typical practice, we assume a contribution from the Milky Way halo of 50 pc cm^-3 and a further 50 pc cm^-3 contribution from the FRB host galaxy, the predicted intergalactic excess DM now could thus range from ~100 to ~150 pc cm^-3, making FRB20221128A still likely to originate from a relatively nearby galaxy.

The UTMOST team acknowledges the generous support of the Mount Cuba Astronomical Foundation which contributed to the UTMOST-NS upgrade. This research was supported by Australian Research Council grants CE110001020, FT150100415 and FL150100148.

References

Cordes and Lazio, 2002, arXiv:0207156
Deller and Flynn, 2020, Nature Astronomy, 4, 292
Yao, Manchester, and Wang, ApJ, 2017, 835, 29