Powerful young pulsar wind nebulae as extragalactic radio transients: the case of VLASS Transient J1137-0337

After decades of improvements in the sensitivity and areal coverage of time-domain radio surveys, modern all-sky surveys such as the VLA Sky Survey (VLASS) have finally reached the scale where new populations of radio-selected transients are beginning to emerge. One of the most unusual new transients detected in VLASS is VT 1137-0337, a source that brightened by at least 5x between a nondetection in the FIRST survey (September 1998) and detection in VLASS (Jan 2018). VT 1137-0337 is located in a massive star forming region of a starbursting ~SMC mass galaxy at a distance of 121.6 Mpc. Follow-up observations taken over 4 years with the VLA reveal that it has a slowly fading radio spectrum (5% per year) that is 7σ too flat (spectral index α = 0.35 ± 0.02) to be explained by diffusive shock acceleration: the mechanism likely responsible for all other known radio transients of remotely comparable luminosity and timescale. Jets launched by a black hole could in principle produce such a flat spectrum, but (1) the black hole would likely have to be an IMBH, and (2) the slow fading and constant spectral index would require an unreasonably stable jet. Instead, we show that all of VT 1137-0337’s unusual properties can be explained by the emergence of a ~10 - 100 year old pulsar wind nebula, produced by the spindown of a neutron star with an initial surface dipole field of B₀ ~ 10¹³-10¹⁴ G, and an initial period of P₀ ~ 10 - 100 ms. Though such objects are not presently found in the Milky Way, they can reasonably be expected to exist in local-universe galaxies. VT 1137-0337 highlights the emerging ability of time-domain radio surveys to uncover the population of young and energetic neutron stars in the local universe.