A Flat-Spectrum Radio Transient at 122 Mpc consistent with an Emerging Pulsar Wind Nebula

Understand

We report the discovery and follow-up observations of VT 1137-0337: an unusual radio transient found in our systematic search for extragalactic explosions in the VLA Sky Survey (VLASS).

• VT 1137-0337 is located in the brightest region of a dwarf starburst galaxy (stellar mass $\sim 10^{8.3} M_{\odot}$, star formation rate $\sim 0.5 M_{\odot}$ yr$^{-1}$) at a luminosity distance of 121.6 Mpc.
• Its 3 GHz luminosity of $\sim 2.5 \times 10^{28}$ erg s$^{-1}$ Hz$^{-1}$ is comparable to luminous radio supernovae associated with dense circumstellar interaction and relativistic outflows.
• However, its broadband radio spectrum - a featureless power law $\propto \nu^{-0.35 \pm 0.02}$ over a range of $\gtrsim$10$\times$ in frequency and fading at a rate of $\sim$ 5% per year over 4 years - cannot be directly explained by the shock of a stellar explosion.