Radio observations of the TeV source HESS J1943+213: a new case of a pulsar wind nebula?

Abstract

Context. Recently, the H.E.S.S. Collaboration discovered a very high energy gamma-ray point source close to the Galactic plane. They offered three possible explanations for the nature of the sourc e: a gamma-ray binary, a pulsar wind nebula, or a BL Lacertae object. They concluded that the observations favoured an extreme BL Lacertae object interpretation. Aims. We investigated the nature of the radio source reported as the counterpart of the very high energy gamma-ray source. Methods. We performed high-resolution radio interferometric observations with the European Very Long Baseline Interferometry Network (EVN) at a frequency of 1.6 GHz on 2011 May 18. We also reanalysed archival 1.4-GHz radio continuum and HI spectral line data taken with the Very Large Array (VLA). Results. The accurate position of the radio source, as observed with EVN, is∼ 4 ′′ off from the one obtained in the NRAO VLA Sky Survey (NVSS). The new position is in excellent agreement with that of the proposed X-ray counterpart of the TeV source. The brightness temperature of the compact radio source is∼8× 10 7 K. From HI absorption data, a distance of about 11.5± 1.5 kpc can be inferred for this source. The large-scale HI data unveiled t he presence of a shell-like feature with the radio/X-ray/TeV point source in its interior. We interpret this shell as the last vestige of a very old supernova that exploded in a tenuous environment created by the stellar wind of its massive stellar precursor. Conclusions. The estimated brightness temperature of the radio point source counterpart of HESS J1943+213 is well below the value expected from the Doppler-boosted radio emission of a BL Lacertae object. This fact and the discovery of traces of a dista nt supernova explosion around the location of the TeV source lead us to conclude that the most likely origin of the high-energy emission is a remote pulsar wind nebula. If this scenario is true, then the HI shel l around HESS J1943+213 may represent a population of hitherto missing Galactic supernova remnants.