Veritas explores the nature of a mysterious gamma-ray emitter

Astronomers have employed the Very Energetic Radiation Imaging Telescope Array System (VERITAS) to observe a mysterious gamma-ray emitting source designated HESS J1857+026. Results of the observational campaign, published December 19 on the pre-print server arXiv, shed more light on the nature of this source.

Sources emitting gamma radiation with photon energies between 100 GeV and 100 TeV are called very high energy (VHE) gamma-ray sources. Observations show that these sources are often blazars or binary star systems containing a compact object. However, the nature of many VHE gamma-ray sources is still not well understood.

The nature of HESS J1857+026 perplexes astronomers

Discovered in 2008 with the High Energy Stereoscopic System (HESS), HESS J1857+026 is one of such VHE gamma-ray sources. It has been the target of multiple observations in high-energy and very-high energy bands, however its true nature still remains a mystery.

Despite the detection of a nearby pulsar, designated PSR J1856+0245, there have been no confirmed counterparts, like a supernova remnant (SNR) shell or other extended structure, in X-ray or other wavelengths.

That is why a team of astronomers led by Yu Chen of the University of California, Los Angeles (UCLA) decided to take a closer look at HESS J1857+026 with VERITAS, which is an array of four imaging atmospheric Cherenkov telescopes located at the Fred Lawrence Whipple Observatory in Arizona. VERITAS is sensitive to gamma rays in the energy range of 100 GeV to above 30 TeV and has an angular resolution of below 0.1 degrees at 1 TeV.

"VERITAS has observed the region of HESS J1857+026 from 2008 to 2016, including serendipitous observation of other targets, e.g., the supernova remnant W44, in the FOV [field-of-view]. After quality selection requiring good weather and a stable trigger rate, about 30 hours of data are used in this analysis," the researchers explain.

Where do these gamma-rays come from?

According to the paper, the significance map of the region around HESS J1857+026 in 0.3–1 TeV and in 1–10 TeV shows that the pulsar PSR J1856+0245 is clearly displaced from the VHE emission center. This supports previous suggestions that the gamma-ray emission seen from HESS J1857+026 is potentially produced by a pulsar wind nebula (PWN) powered by PSR J1856+0245.

Furthermore, the observations identified a northern component, which shows up at energies above 1 TeV. This additional structure could indicate a separate source or it could originate from the expanse of the source itself due to faster diffusion of electrons with higher energies. Therefore, additional observations are required to make any further conclusions.

Moreover, based on the collected data, the astronomers calculate that HESS J1857+026 has a diffusion length of about 321 light years. They estimate that the cooling time for the electron population responsible for the gamma rays are on the order of tens of thousand years, thus larger or comparable to the age of the pulsar. These calculations point to a diffusion an order of magnitude lower than the galactic average.

In concluding remarks, the authors of the paper underline that HESS J1857+026 has an extended nature and its morphology seems to indicate an expansion of the source region or an unrelated source with increased energy.

© 2025 Science X Network