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 VLA Sky Survey (VLASS). VT 1137-0337 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% per year -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 spindown, the central neutron star should have a surface dipole field of ∼ 10 13 − 10 14 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.

Radio Nebulae from Hyperaccreting X-Ray Binaries as Common-envelope Precursors and Persistent Counterparts of Fast Radio Bursts

“…The PRSs spatially coincident with the engines of repeating FRB are compact sources of incoherent synchrotron radiation that are too luminous (L ν  10 29 erg s −1 Hz −1 ) to be explained by star formation activity in the host galaxy, or individual supernova remnants (e.g., Law et al 2022). A transient radio source, with luminosity and spectral properties similar to FRB PRS, was recently discovered in VLASS (Dong & Hallinan 2022). In this section, we apply our model of ULX hypernebulae to explain the observed properties of some PRSs from some of the brightest FRBs (see Section 4.3 for more discussion on our model's relevance to fainter FRBs and XRBs).…”

Radio Nebulæ from Hyper-Accreting X-ray Binaries as Common Envelope Precursors and Persistent Counterparts of Fast Radio Bursts

“…The PRSs spatially coincident with the engines of repeating FRB are compact sources of incoherent synchrotron radiation that are too luminous (L ν 10 29 erg s −1 Hz −1 ) to be explained by star formation activity in the host galaxy, or individual supernova remnants (e.g., Law et al 2022). A transient radio source, with a luminosity and spectral properties similar to FRB PRS, was recently discovered in VLASS (Dong & Hallinan 2022). In this section, we apply our model of ULX hyper-nebulae to explain the observed properties of some of PRSs from some of the brightest FRBs (see Sec.…”

Evidence for Extended Hydrogen-Poor CSM in the Three-Peaked Light Curve of Stripped Envelope Ib Supernova

“…The first possibility is that the radio emitting region consists of a localized overdense "knot" of CSM (as it was proposed for SN 1986J, e.g.,Bietenholz & Bartel 2017a,b), or, alternatively, a disk of material in the environment. The second possibility includes the emergence of radiation from a newly-formed compact object, for example in the form of a pulsar wind nebula (PWN, see review bySlane (2017);Dong & Hallinan (2022)). While we leave a detailed study of the radio emission from SN 2019tsf (and its temporal evolution) to future work, here we note that radio observations are consistent with a disk-like geometry of the CSM that we propose in Sec 3.…”