ASKAP and VLASS Search for a Radio-continuum Counterpart of Ultra-high-energy Neutrino Event KM3–230213A

Abstract We present the results of an Australian Square Kilometre Array Pathfinder (ASKAP) 944 MHz and Very Large Array Sky Survey (VLASS) 3 GHz search for a radio-continuum counterpart of the recent ultra-high-energy neutrino event, KM3–230213A. Using ASKAP, we catalog 1052 radio sources within the 1 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>°</mml:mi> </mml:mrow> </mml:mover> </mml:math> 5 radius search area (68% certainty region) around the particle’s calculated origin, 10 of which we classify as blazar candidates based on their radio spectra. The most prominent radio source in the search area is the nearby spiral galaxy UGCA 127 (nicknamed Phaedra, from Greek ϕαiδρα , a Cretan princess of Greek mythology, derived from Phaidros, Greek φα ι δρoς , meaning “bright”). Its nonthermal radio spectrum classifies it as a nonblazar active galactic nucleus. We also present an extended radio source, WISEA J061715.89–075455.4 (nicknamed Hebe, from Greek H βη , the Greek goddess of youth), located only ∼7 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>′</mml:mi> </mml:math> from the geometric center of the search area, with a very unusual, highly polarized compact component. Finally, we present a strong radio source, EMU J062248–072246 (nicknamed Narcissus, from Greek Nαρκ ι σσoζ , a self-absorbed hunter from Thespiae in Boeotia), which can be modeled either with a synchrotron self-absorbed spectrum or synchrotron emission from thermal electrons. It exhibits ∼25% flux density variability over the ∼5 yr VLASS 3 GHz survey.