e-Merlin follow-up of the candidate PRS associated with FRB20240114A

ATel #16864; Gabriele Bruni (INAF-IAPS), Luigi Piro (INAF-IAPS), Luciano Nicastro (INAF-OAS), Eliana Palazzi (INAF-OAS), Yuan-Pei Yang (Yunnan University), Bing Zhang (University of Nevada)
on 12 Oct 2024; 09:03 UT
Credential Certification: Gabriele Bruni (gabriele.bruni@inaf.it)

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We performed e-Merlin follow-up observations of the candidate PRS associated with FRB20240114A, recently detected with the uGMRT (ATel #16820) and MeerKAT (Atel #16695). Observations at C band (5 GHz), under project CY18016 (PI Bruni) were carried in five ~12-hour runs between September 20 and 28, 2024, pointing at the FRB coordinates from the PRECISE collaboration (ATel #16542). Phase referencing was applied. Visibilities were calibrated through the e-Merlin pipeline (Moldon 2021, ascl:2109.006), and imaged in CASA. An RMS of 10 uJy/beam was reached, and no source was detected either within the uncertainty region of the FRB coordinates (+/-200 mas), or within the wider image field (20"x20"). This sets a 5-sigma upper limit of 50 uJy for the candidate PRS flux density at 5 GHz. The image resolution in natural weighting was 228x87 mas.

On the basis of previous detections at 650 MHz (uGMRT, 65.6 +/- 8.1 uJy) and 1.3 GHz (MeerKAT, 72 +/- 14 uJy) a flat spectral index and a spectral luminosity of (2.83+/-0.35) x 10^28 erg/s/Hz were estimated (ATel #16820). Nevertheless, in-band spectral analysis of MeerKAT data resulted in a spectral index of -1.1 +/- 0.8 between 1 and 1.5 GHz, suggesting a possible steepening of the radio emission at higher frequencies. Our e-Merlin non-detection at higher angular resolution seems to confirm such steepening, with a spectral index <-0.6 in the 1.3-5 GHz range. Alternatively, the radio emission comes from a diffuse region (>~460 mas corresponding to a physical size >~1 kpc) resolved-out in our high-resolution images, thus falling below the detection threshold. The latter scenario would exclude a possible compact nature of the emission, such as a PRS origin.

We thank the staff of the e-Merlin for their support and assistance with these observations. e-MERLIN is a National Facility operated by the University of Manchester at Jodrell Bank Observatory on behalf of STFC, part of UK Research and Innovation.