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A Multiwavelength Interpretation of HESS J1857+026 Emission Using the Fermi-LAT, VERITAS, and HAWC Observatories

This study utilizes multiwavelength data from Fermi-LAT, VERITAS, and HAWC to identify HESS J1857+026 as an evolved pulsar wind nebula powered by PSR J1856+0245, where high-energy gamma rays arise from inverse Compton scattering while lower-energy emission may have a hadronic supernova remnant origin, allowing for the constraint of the system's physical properties and suppressed particle diffusion.

Original authors: Jordan Eagle (on behalf of the Fermi-LAT, VERITAS,,HAWC Collaborations), Yu Chen (on behalf of the Fermi-LAT, VERITAS,,HAWC Collaborations), Ramiro Torres-Escobedo (on behalf of the Fermi-LAT, VERITAS

Published 2026-06-10

📖 5 min read🧠 Deep dive

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Original authors: Jordan Eagle (on behalf of the Fermi-LAT, VERITAS,,HAWC Collaborations), Yu Chen (on behalf of the Fermi-LAT, VERITAS,,HAWC Collaborations), Ramiro Torres-Escobedo (on behalf of the Fermi-LAT, VERITAS,,HAWC Collaborations), Youyou Li (on behalf of the Fermi-LAT, VERITAS,,HAWC Collaborations), Ruo-Yu Shang (on behalf of the Fermi-LAT, VERITAS,,HAWC Collaborations)

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a vast, dark ocean. Most of the time, we can only see the surface waves (visible light). But sometimes, there are massive, invisible underwater currents and storms that release huge amounts of energy. One of these "storms" is a cosmic object called HESS J1857+026.

This paper is like a team of detectives using three different types of high-tech "flashlights" (telescopes) to figure out exactly what is causing this storm and how it works. Here is the story they uncovered, explained simply:

The Cast of Characters

The Storm (HESS J1857+026): A giant, glowing cloud of high-energy particles (gamma rays) floating in space. It's so energetic that it was first spotted by a telescope called HESS.
The Engine (PSR J1856+0245): A rapidly spinning dead star (a pulsar) located right in the middle of the storm. Think of it as a cosmic lighthouse spinning so fast it shoots out a powerful wind.
The Flashlights:
- Fermi-LAT: Looks at lower-energy gamma rays (like seeing the warm glow of a fire).
- VERITAS: Looks at medium-to-high energy rays (seeing the bright flames).
- HAWC: Looks at the highest energy rays (seeing the intense, white-hot core).

The Mystery: What is the Storm?

For a long time, scientists weren't sure if this glowing cloud was a "lemon" (a leftover shell of an exploded star, called a Supernova Remnant) or a "lemonade stand" (a cloud of wind blown by the spinning pulsar, called a Pulsar Wind Nebula).

The "Lemon" Theory (Hadronic): Maybe the explosion of the star smashed into gas clouds, creating a shockwave that accelerated particles like a cosmic pinball machine.
The "Lemonade" Theory (Leptonic/PWN): Maybe the spinning pulsar is blowing a wind of tiny particles (electrons) that are glowing as they fly through space.

The Investigation

The team combined data from all three telescopes to look at the storm from low energy to ultra-high energy. Here is what they found:

1. The Shape Changes with Energy
Imagine looking at a campfire. If you look at the heat (low energy), the glow spreads out far and wide. If you look at the bright white core (high energy), the light is concentrated right in the center.

The Fermi telescope saw the glow spread out over a large area (like the warm heat).
The HAWC telescope saw the high-energy glow concentrated much closer to the pulsar (like the bright core).
The Clue: This "shrinking" shape as energy increases is a classic signature of a Pulsar Wind Nebula. It suggests the particles are being blown out by the pulsar and losing energy as they travel further away.

2. The "Wind" vs. The "Crash"
The team tried to model the physics.

If it were a "crash" (Supernova Remnant), they would expect to see a lot of gas clouds right where the gamma rays are. But when they looked at the gas maps, the gamma rays were actually in a "gas desert"—a place with very little gas. This makes the "crash" theory unlikely.
The "wind" theory fits perfectly. The pulsar is spinning fast enough to power the entire storm. The model suggests the storm is about 16,000 to 21,000 years old. It's an "evolved" storm, meaning it's mature and has cooled down enough that it doesn't glow brightly in X-rays anymore, which explains why we can't see it in X-ray telescopes.

3. The "Traffic Jam" of Particles
One of the most interesting findings is about how the particles move.

In normal space (the "Interstellar Medium"), particles zip around freely, like cars on an empty highway.
Around this pulsar, the particles move much slower. It's like a traffic jam. The magnetic fields around the pulsar are so turbulent that they act like speed bumps, slowing the particles down.
The team calculated that the particles are moving about 100 times slower here than they do in the rest of the galaxy. This "traffic jam" effect is a common feature of these types of cosmic storms.

The Verdict

The paper concludes that HESS J1857+026 is almost certainly a Pulsar Wind Nebula.

The Engine: The spinning pulsar (PSR J1856+0245) is the power source.
The Fuel: It's powered by electrons (leptons) being shot out by the pulsar.
The Mystery Component: There is a tiny hint that the very lowest energy part of the glow might have a different origin (perhaps a leftover shock from the original star explosion), but the main show is definitely the pulsar's wind.

In a nutshell: The universe has a giant, glowing wind-blown cloud powered by a super-fast spinning star. The star is blowing a wind of particles that get stuck in a magnetic traffic jam, creating a beautiful, multi-colored glow that we can now see clearly thanks to our new "flashlights."

Technical Summary: A Multiwavelength Interpretation of HESS J1857+026 Emission

Problem Statement
HESS J1857+026 is an extended, unidentified very-high-energy (VHE) $\gamma$ -ray source coincident with the energetic pulsar PSR J1856+0245. While the source is a candidate for a pulsar wind nebula (PWN), its multiwavelength nature remains complex. Previous observations have revealed a spatial mismatch between the GeV and TeV emission extents and a lack of confirmed X-ray counterparts, raising questions about the dominant emission mechanism. Specifically, it is unclear whether the emission is purely leptonic (inverse Compton scattering from a PWN), purely hadronic (pion decay from a supernova remnant interacting with molecular gas), or a combination of both. Furthermore, the particle diffusion properties in the immediate environment of evolved PWNe remain poorly constrained compared to the average interstellar medium (ISM).

Methodology
This study presents a comprehensive multi-instrument analysis of HESS J1857+026 using 15 years of Fermi-LAT data (300 MeV–2 TeV), VERITAS data (0.3–10 TeV), and new HAWC data (1–316 TeV). The analysis proceeds through several stages:

Data Analysis:
- Fermi-LAT: A binned likelihood analysis was performed on Pass 8 SOURCE class data. The region of interest (ROI) was modeled using the 4FGL-DR3 catalog and diffuse templates. Spatial extension tests compared point-source, radial disk, and radial Gaussian templates.
- VERITAS: A 3D binned maximum-likelihood analysis was conducted on 30 hours of exposure using the Gammapy framework, modeling sources as power laws with symmetric Gaussian spatial shapes.
- HAWC: A forward-folding maximum-likelihood analysis was applied to the Pass 5 dataset (2860 days). The study tested multiple spatial templates (Gaussian vs. diffusion-based) and spectral shapes (power law, log-parabola, power law with exponential cutoff) using Bayesian and Akaike information criteria (BIC/AIC) to select the best-fit models.
Radiative Modeling:
- Time-Independent: Broadband spectral energy distributions (SEDs) were fitted using the NAIMA package to test purely hadronic (SNR) and lepto-hadronic (PWN/SNR) scenarios.
- Time-Dependent: An evolutionary model (similar to Gelfand et al. 2009) was employed to simulate the PWN's expansion and cooling within an SNR shell. A Markov chain Monte Carlo (MCMC) procedure constrained physical parameters such as system age, magnetic field strength, and particle injection spectra.
Diffusion Analysis:
- Radial surface brightness profiles were extracted from all three instruments. These profiles were fitted with an analytical diffusion-based model (Equation 16) to constrain the particle diffusion coefficient, $D(E)$ , and the diffusion suppression relative to the ISM.

Key Results

Spectral and Spatial Characteristics:
- The Fermi-LAT data reveals an extended source best fit by a radial Gaussian with a radius of $0.38^\circ \pm 0.04^\circ$ and a spectral index $\Gamma \approx 2.07$ . A spectral break is observed, with a softer index ( $\Gamma \approx 2.63$ ) below 5 GeV and a harder index ( $\Gamma \approx 1.99$ ) above 5 GeV.
- VERITAS detects the source with a significance of $15.2\sigma$ , measuring a spectral index of $\Gamma = 2.48 \pm 0.06$ and a Gaussian width of $0.236^\circ \pm 0.017^\circ$ .
- HAWC data shows the source extends to $\sim 37$ TeV. The best-fit spatial model favors a diffusion template over a simple Gaussian for a magnetic field of $1 \mu$ G, with a cutoff energy $E_c \approx 14$ TeV.
- The source size decreases with increasing energy (from $0.38^\circ$ at GeV energies to $0.26^\circ$ at HAWC energies), indicating an energy-dependent morphology consistent with an evolved PWN.
Radiative Origin:
- Lepto-hadronic vs. Hadronic: While a purely hadronic model cannot be entirely ruled out for the low-energy ( $<10$ GeV) component, the lepto-hadronic model provides a superior fit to the broadband data. The lack of a detected SNR shell and the low ambient gas density ( $n_0 \lesssim 0.13$ cm $^{-3}$ ) derived from the evolutionary model disfavor a dominant hadronic origin for the TeV emission.
- PWN Evolution: The evolutionary model suggests the system is an evolved PWN with an age of $\tau = [16, 21]$ kyr and a low magnetic field strength of $B = [0.4, 1.6] \mu$ G. These parameters explain the faint X-ray emission (due to synchrotron cooling) and the energy-dependent morphology. The low magnetic field also supports the dominance of inverse Compton scattering (ICS) off local photon fields for the TeV emission.
Particle Diffusion:
- The diffusion coefficient around HESS J1857+026 is found to be suppressed relative to the average Galactic ISM. At 50 TeV, the diffusion coefficient is $D_{PWN} \sim 2.14 \times 10^{28}$ cm $^2$ s $^{-1}$ (for $B=1 \mu$ G), which is approximately two orders of magnitude lower than the ISM value ( $D_{ISM} \sim 1.2 \times 10^{30}$ cm $^2$ s $^{-1}$ ).
- The suppression is primarily in the normalization ( $D_0$ ) rather than the rigidity dependence (spectral index $\delta$ ), which remains consistent with Kolmogorov ( $\delta=1/3$ ) or Kraichnan ( $\delta=1/2$ ) turbulence models. This supports the scenario of locally enhanced magnetic turbulence near the source.

Significance and Claims
The paper claims that the combined multiwavelength data provides a powerful tool for constraining particle diffusion properties in the vicinity of evolved PWNe. The primary conclusion is that HESS J1857+026 is most likely a PWN powered by PSR J1856+0245, where the observed $\gamma$ -ray emission is dominated by relativistic electrons undergoing inverse Compton scattering.

The study highlights that the low-energy spectral component ( $E < 10$ GeV) may still harbor a hadronic contribution from a host or nearby SNR, or alternatively, a distinct population of electrons within the same PWN. However, the high-energy emission is firmly leptonic. The derived diffusion suppression factor is consistent with other TeV PWNe (e.g., Geminga, Monogem), reinforcing the hypothesis that particle transport near these sources is significantly hindered by local magnetic turbulence compared to the general ISM. The authors note that while a neutrino excess has been reported at low significance, the large upper limit on the neutrino emission region compared to the $\gamma$ -ray region suggests a complex or unrelated origin for any potential neutrino signal.

The Cast of Characters

The Mystery: What is the Storm?

The Investigation

The Verdict

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