A Flat-spectrum Radio Transient at 122 Mpc Consistent with an Emerging Pulsar Wind Nebula

Abstract

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 Very Large Array Sky Survey. It is located in the brightest region of a dwarf starburst galaxy at a luminosity distance of 121.6 Mpc. Its 3 GHz luminosity is comparable to luminous radio supernovae associated with dense circumstellar interaction and relativistic outflows. However, its broadband radio spectrum—proportional to ν −0.35 over a range of ≳10× in frequency and fading at a rate of 5% yr–1—cannot be directly explained by the shock of a stellar explosion. Jets launched by various classes of accreting black holes also struggle to account for VT 1137–0337's combination of observational properties. Instead, we propose that VT 1137–0337 is a decades-old pulsar wind nebula that has recently emerged from within the free–free opacity of its surrounding supernova ejecta. If the nebula is powered by spin-down, the central neutron star should have a surface dipole field of ∼1013–1014 G and a present-day spin period of ∼10–100 ms. Alternatively, the nebula may be powered by the release of magnetic energy from a magnetar. Magnetar nebulae have been proposed to explain the persistent radio sources associated with the repeating fast radio bursts FRB 121102 and FRB 190520B. These FRB persistent sources have not previously been observed as transients but do bear a striking resemblance to VT 1137–0337 in their radio luminosity, spectral index, and host galaxy properties.

Constraints on the Persistent Radio Source Associated with FRB 20190520B Using the European VLBI Network

“…Motivated by the size constraints and luminosities of different source types and how they compare to FRB PRSs (see Figure 5), we investigate the phase space of radio luminosity and physical size of sources such as the known PRSs associated with FRBs; wandering black hole (BH) candidates in dwarf galaxies (Reines et al 2020;Sargent et al 2022); the compact radio source emerging from SN1986J's SNR at the epoch of maximum radio luminosity (Bietenholz & Bartel 2017); Galactic SNRs and PWNe 14 (Green 2019;Ranasinghe & Leahy 2023); SNRs in nearby galaxies with distances ranging from 0.055-14.5 Mpc (Urošević et al 2005); transient sources such as the PWN candidate VT 1137-0337 (Dong & Hallinan 2023) and the orphan GRB afterglow FIRST J141918.9 +394036 (Law et al 2018;Marcote et al 2019); PRS-like candidates identified in a low-frequency survey (Vohl et al 2023); and low-luminosity AGNe detected with the VLBA spanning a redshift range of z = 0.3-3.4 (Radcliffe et al 2018). We note that the sample of Galactic SNRs is nearly complete for remnant ages <2 kyr and shows a mean Galactic SNR diameter of 30.5 pc (Ranasinghe & Leahy 2023).…”