The Enigmatic compact radio source coincident with the energetic X-ray pulsar PSR J1813$-$1749 and HESS J1813$-$178

TL;DR

This paper reports new VLA observations of a variable, non-thermal radio source associated with the energetic X-ray pulsar PSR J1813-1749 and HESS J1813-178, revealing it as the pulsar's time-averaged emission despite previous non-detections of pulses.

Contribution

It demonstrates that the radio source is the pulsar's time-averaged emission, resolving previous ambiguities caused by scattering effects.

Findings

Radio source has high polarization and negative spectral index.

Pulses were not detected in initial observations due to scattering.

Higher frequency observations confirmed the source as the pulsar's average emission.

Abstract

New VLA detections of the variable radio continuum source VLA J181335.1$-$174957, associated with the energetic X-ray pulsar PSR~J1813$-$1749 and the TeV source HESS J1813-178, are presented. The radio source has a right circular polarization of $\sim50\%$, and a negative spectral index of $-1.3\pm0.1$, which show that it is non-thermal. The radio pulses of the pulsar are not detected from additional Effelsberg observations at 1.4~GHz made within one week of a VLA detection. This result would appear to support the idea that the continuum radio emission detected with the VLA does not trace the time-averaged emission pulses, as had previously been suggested. We discuss other possible origins for the radio source, such as a pulsar wind, magnetospheric emission, and a low-mass star companion. However, observations made at higher frequencies by Camilo et al. (in preparation) show that the VLA source is in fact the time-averaged pulsed emission and that the detection of the pulses had not been achieved because this is the most scattered pulsar known.