A VERITAS view of HESS J1857+026 within a multi-wavelength analysis

HESS J1857+026 remains a mysterious gamma-ray emitter since its discovery in 2008. Despite the disclosure of a nearby pulsar and multiple studies in the high-energy (HE, E > 100 MeV) and very-high-energy (VHE, E > 100 GeV) regimes, there have been no confirmed counterparts (e.g., an SNR shell or other extended structure) in X-ray or other wavelengths. We present the result of our study of the VHE emission of HESS~J1857+026 with VERITAS as part of a multi-wavelength investigation to uncover its emission mechanisms. Our result confirms the extended nature of the source and we characterize its spectral and morphological features in the VHE band. Using the morphology of the source revealed in our analysis, we also explore the underlying transport process of a possible electron population in a leptonic PWN scenario for the gamma-ray emission.

💡 Research Summary

This paper presents a detailed VERITAS study of the very‑high‑energy (VHE) gamma‑ray source HESS J1857+026 within a broader multi‑wavelength effort. The source, first discovered by HESS in 2008, lies near the energetic pulsar PSR J1856+0245 (spin‑down power 4.6 × 10³⁶ erg s⁻¹, characteristic age 21 kyr). Previous observations with Fermi‑LAT, MAGIC, and radio surveys have suggested an extended GeV counterpart, a possible two‑component TeV morphology (a southern PWN‑like component and a northern component associated with a CO cavity/H II region), and even a surrounding HI superbubble.

VERITAS observed the region from 2008 to 2016, accumulating ≈30 h of quality‑selected data (mean elevation 58°, mean offset 1.21°). Event reconstruction and gamma‑hadron separation were performed with EventDisplay, and the data were converted to DL3 format using V2DL3. Background templates were generated with Gammapy‑tools, and a 3‑D stacked analysis employing a field‑of‑view background method was carried out. The source model consists of a power‑law spectrum (E₀ = 1 TeV) and a symmetric Gaussian spatial distribution; exclusion regions were set around the pulsar, HESS J1858+020, and the supernova remnant W44.

The maximum‑likelihood fit yields a detection significance of 15.3 σ. The best‑fit centroid is at RA 284.37°, Dec 2.72° (J2000) with a Gaussian extension of 0.24° ± 0.015°. The spectrum is described by Γ = 2.46 ± 0.06 and a normalization N₀ = (5.5 ± 0.43) × 10⁻¹² TeV⁻¹ s⁻¹ cm⁻² in the 0.3–10 TeV band; an upper limit is set above 10 TeV. A secondary, harder component appears above 1 TeV toward the north, hinting at either faster diffusion of higher‑energy electrons or a distinct source.

To probe particle transport, the authors constructed a radial surface‑brightness profile and fitted it with a diffusion‑motivated function f(θ)=f₀ θ/(θ+0.06θ_d) exp(−θ²/θ_d²). The characteristic diffusion angle is θ_d = 1.025° ± 0.317°, which at the assumed distance of 5.5 kpc corresponds to a diffusion length d ≈ 98 ± 30 pc. Using d = 2√(D t) with t ≈ 21 kyr (the pulsar age) gives a diffusion coefficient D ≈ (3.5 ± 2.2) × 10²⁸ cm² s⁻¹, roughly an order of magnitude below the canonical Galactic value (~10²⁹ cm² s⁻¹) and comparable to the suppressed diffusion inferred around Geminga and other middle‑aged pulsars. Assuming a magnetic field of ~1 µG, the electron cooling time is of order tens of kiloyears, comparable to the pulsar age, supporting a leptonic PWN origin for the VHE emission.

The study concludes that VERITAS has firmly detected and characterized HESS J1857+026, confirming its extended morphology and providing the first quantitative estimate of the diffusion coefficient in this region. The suppressed diffusion strengthens the emerging picture that many middle‑aged pulsars reside within low‑diffusion “halos.” The northern excess warrants deeper observations and a full multi‑wavelength synthesis (including forthcoming Fermi‑LAT, HAWC, radio, and X‑ray data) to disentangle possible additional sources and to refine the transport modeling.