Radio study of HESS J1857+026: Gamma-rays from a superbubble?

Abstract

Aims. We provide new insights into the nature of HESS J1857+026, a very-high-energy γ-ray source whose complex morphology in the TeV band was attributed to the superposition of two distinct sources. Methods. We performed radio continuum observations to look for the pulsar wind nebula and the supernova remnant associated with the pulsar PSR J1856+0245, which might be powering part of the γ-ray emission. We observed HESS J1857+026 with the Karl G. Jansky Very Large Array (VLA) at 1.5 GHz in the C configuration. In addition, using the same array configuration, we observed a region of 0.4° × 0.4° towards PSR J1856+0245 at 6.0 GHz. We obtained complementary data for the neutral hydrogen and molecular gas emission from public surveys in order to investigate the properties of the interstellar medium in the direction of HESS J1857+026. Results. The new observations at 1.5 GHz do not show evidence of emission above the noise level of 0.7 mJy beam-1 that could be associated with either HESS J1857+026 or PSR J1856+0245. Also, in the new image at 6.0 GHz we do not detect radio emission from a pulsar wind nebula powered by PSR J1856+0245. The neutral gas analysis shows the existence of a superbubble in the direction of the γ-ray source. We suggest that this structure is located at ~5.5 kpc, compatible with the distance to the pulsar PSR J1856+0245. Conclusions. We conclude that TeV emission from HESS J1857+026 originates in a superbubble, arguing in favour of a single γ-ray source rather than the superposition of two distinct sources. The pulsar PSR J1856+0245 could also be contributing as a source of γ-rays within the bubble.