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.

Energy-dependent Analyses of the Gamma-Ray Emission from HESS J1857+026 with Fermi-LAT

“…The characteristic age and spin-down luminosity of PSR J1857+0300 are τ c ∼ 4.6 × 10 6 yr and  ~´-E 2.3 10 erg s 32 1 , with a distance of ∼6.7 kpc. Meanwhile, an elliptical superbubble was detected with a neutral gas observation (Petriella et al 2021), which is also spatially coincident with HESS J1857+026. The kinematical distance of the superbubble is about 5.5 kpc and is close to the DM distance of PSR J1856+0245 (∼6.3 kpc).…”

“…For the possible origin of the gammaray emission from HESS J1857+026, MAGIC Collaboration et al (2014) suggested a PWN powered by PSR J1856+0245 with  = É 4.6 10 36 erg s −1 , and the spin-down luminosity of PSR J1857+0300 with  ~´-E 2.5 10 erg s 32 1 is too low to power a gamma-ray PWN (Abdo et al 2013;Acero et al 2013). Petriella et al (2021) revealed the existence of a superbubble with the analysis of atomic gas in this region and suggested a superbubble origin for the TeV emission from HESS J1857+026. However, the hard GeV gamma-ray spectrum of HESS J1857+026 makes it different from the typical superbubble with an index of ∼2.2, e.g., Cygnus Cocoon (Ackermann et al 2011;Aharonian et al 2019), but is similar to the typical PWNe, e.g., MSH 15-52 (Abdo et al 2010a) For PWNe, the emission from radio to X-rays is normally produced by the synchrotron emission, whereas the gamma-ray emission is explained by the IC scattering process (the leptonic process).…”

“…The distance of HESS J1857+026 is adopted to be 6.3 kpc, derived from the dispersion measurement of PSR J1856+0245 (Yao et al 2017). Considering the absence of radio detection for the PWN, we used the sensitivity of the VLA in Petriella et al (2021) to calculate the upper limits of the radio flux by assuming that the spatial extension of the radio PWN is the same as that of the gamma-ray emission with a radius of 0°.4. The radio upper limits are estimated to be 9.2 × 10 −14 erg cm −2 s −1 at 1.5 GHz and 1.5 × 10 −13 erg cm −2 s −1 at 5 GHz.…”

“…The kinematical distance of the superbubble is about 5.5 kpc and is close to the DM distance of PSR J1856+0245 (∼6.3 kpc). Petriella et al (2021) concluded that the TeV emission of HESS J1857+026 originates from the superbubble, and PSR J1856 +0245 is located inside the superbubble. In addition, they found five molecular components in the velocity interval between 78 and 90 km s −1 with 13 CO(J = 1-0) observations, which are probably associated with the superbubble.…”

“…And they favor a single gammaray source scenario instead of the superposition of two gamma-ray sources. Petriella et al (2021) also performed radio observations at 1.5 and 6.0 GHz with the Very Large Array (VLA). Nevertheless, no significant radio emission was detected in this region.…”