A matched-filter approach to radio variability and transients: searching for orphan afterglows in the VAST Pilot Survey

Abstract

Radio transient searches using traditional variability metrics struggle to recover sources whose evolution timescale is significantly longer than the survey cadence. Motivated by the recent observations of slowly evolving radio afterglows at gigahertz frequency, we present the results of a search for radio variables and transients using an alternative matched-filter approach. We designed our matched-filter to recover sources with radio light curves that have a high-significance fit to power-law and smoothly broken power-law functions; light curves following these functions are characteristic of synchrotron transients, including "orphan" gamma-ray burst afterglows, which were the primary targets of our search. Applying this matched-filter approach to data from Variables and Slow Transients Pilot Survey conducted using the Australian SKA Pathfinder, we produced five candidates in our search. Subsequent Australia Telescope Compact Array observations and analysis revealed that: one is likely a synchrotron transient; one is likely a flaring active galactic nucleus, exhibiting a flat-to-steep spectral transition over $4$months; one is associated with a starburst galaxy, with the radio emission originating from either star formation or an underlying slowly evolving transient; and the remaining two are likely extrinsic variables caused by interstellar scintillation. The synchrotron transient, VAST J175036.1$-$181454, has a multi-frequency light curve, peak spectral luminosity, and volumetric rate that is consistent with both an off-axis afterglow and an off-axis tidal disruption event; interpreted as an off-axis afterglow would imply an average inverse beaming factor $⟨f_{\text{b}}^{-1}⟩=860_{-710}^{+1980}$, or equivalently, an average jet opening angle of $⟨{\theta }_{\text{j}}⟩=3_{-1}^{+4}$deg.

20 pages, 6 figures; accepted for publication in MNRAS

References in corpus (21)

• The LOFAR Two-metre Sky Survey - I. Survey Description and Preliminary Data Release
• GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey I: A low-frequency extragalactic catalogue
• The third realization of the International Celestial Reference Frame by very long baseline interferometry
• The Rapid ASKAP Continuum Survey I: Design and First Results
• Science With The Australian Square Kilometre Array Pathfinder
• Radio Properties of Tidal Disruption Events
• Signatures of a jet cocoon in early spectra of a supernova associated with a $\gamma$-ray burst
• The Koala: A Fast Blue Optical Transient with Luminous Radio Emission from a Starburst Dwarf Galaxy at $z=0.27$
• A unifying view of Gamma Ray Burst Afterglows
• Delayed Radio Flares from a Tidal Disruption Event
• Electrons' energy in GRB afterglows implied by radio peaks
• Gamma-ray Urgent Archiver for Novel Opportunities (GUANO): Swift/BAT event data dumps on demand to enable sensitive sub-threshold GRB searches
• Rapid compact jet quenching in the Galactic black hole candidate X-ray binary MAXI J1535-571
• Probing dust-obscured star formation in the most massive Gamma-Ray Burst host galaxies
• "Orphan" afterglows in the Universal Structured Jet Model for gamma-ray bursts
• Discovery of ASKAP J173608.2-321635 as a Highly-Polarized Transient Point Source with the Australian SKA Pathfinder
• Resolving the decades-long transient FIRST J141918.9+394036: an orphan long gamma-ray burst or a young magnetar nebula?
• 1000 days of lowest frequency emission from the low-luminosity GRB 171205A
• A search for radio afterglows from gamma-ray bursts with the Australian Square Kilometre Array Pathfinder
• Constraining the Radio Properties of the $z$=6.44 QSO VIK J2318$-$3113
• A Matched Filter Technique For Slow Radio Transient Detection And First Demonstration With The Murchison Widefield Array

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