TITLE: GCN CIRCULAR NUMBER: 36329 SUBJECT: LIGO/Virgo/KAGRA S240422ed: Archival radio detection of EP240426a DATE: 24/04/26 23:01:02 GMT FROM: akashjanaki98@gmail.com
Akash Anumarlapudi (UWM), David Kaplan (UWM), Dougal Dobie (U. Sydney/OzGrav), Tara Murphy (U. Sydney), and Emil Lenc (CSIRO), Tomas Ahumada (Caltech).
We report the radio detection of an archival source positionally consistent with the recently discovered Einstein Probe (EP) transient EP240426a (GCN 36313). EP240426a was detected as a potential counterpart to of the LIGO/Virgo/KAGRA event S240422ed (GCN 36236, GCN 36240). A coincident optical source was also detected in the error circle of EP240426a by Palmese et al. (GCN 36317). Here we report the radio observations from the Australian SKA Pathfinder (ASKAP) Variable And Slow Transients Survey (VAST; Murphy et al. 2021, PASA, 38, e054).
The field of EP240426a was observed regularly as a part of the VAST survey with a biweekly cadence starting in December 2022. We detected a persistent faint source at the location of EP240426a in all epochs. Stacking all the observations together, we find a strong radio source with a flux density of 600+/-40 uJy at a frequency of 888 MHz at this location. We also inspect the archival data from the Very Large Array Sky Survey (VLASS) and do not find a point source at this location. Using VAST and VLASS data, we infer the power-law spectral index to be steeper than -0.7, consistent with it being an AGN. We also inspect the WISE colors and using the W4 color and radio fluxes, the estimated FIR to radio ratio is consistent with FIR/radio correlation which supports the AGN hypothesis. Additionally, we analyzed the historic WISE and NEOWISE data at the source location (within 2 arcsec), and found a variable source in the W1 and W2 passbands.
The ASKAP VAST image for this event can be obtained [here](https://www.vast-survey.org/assets/images/EP240426a.jpg).
This work uses data obtained from Inyarrimanha Ilgari Bundara / the Murchison Radio-astronomy Observatory. We acknowledge the Wajarri Yamaji People as the Traditional Owners and native title holders of the Observatory site. CSIRO’s ASKAP radio telescope is part of the Australia Telescope National Facility (https://ror.org/05qajvd42). Operation of ASKAP is funded by the Australian Government with support from the National Collaborative Research Infrastructure Strategy. ASKAP uses the resources of the Pawsey Supercomputing Research Centre. Establishment of ASKAP, Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-astronomy Observatory and the Pawsey Supercomputing Research Centre are initiatives of the Australian Government, with support from the Government of Western Australia and the Science and Industry Endowment Fund.
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