The 50 MeV to 30 TeV γ-ray SED for HESS J1857+026. See Figure 1 below for more details.
Reference: J. Eagle et al. (The Fermi-LAT, VERITAS, and HAWC Collaborations), accepted for publication in ApJ (2026)
Full text version
ArXiv: ArXiV: 2606.10828
Contacts: Yu Chen
We present a new study on the MeV–TeV γ-ray origin of HESS J1857+026 using data collected from the Fermi–LAT, VERITAS, and HAWC observatories. A spatial and spectral study of HESS J1857+026 including radiative modeling of the MeV–TeV spectrum determines the likely dominant γ-ray origin as a 3 pulsar wind nebula (PWN) powered by the energetic pulsar PSR J1856+0245. The MeV–TeV spectrum is further characterized through basic evolutionary radiative modeling assuming a PWN origin to constrain the physical properties of the system such as the magnetic field strength and PWN age. The results of the PWN evolutionary model are consistent with the observational constraints of the system, finding an age of the system between τ = [16, 21] kyr and a magnetic field strength between B = [0.4, 1.6] µG. These estimates support an evolved PWN scenario where the observed γ-ray emission is generated by the relativistic electrons inverse Compton scattering (ICS) off local photon fields, however the low-energy (E < 10 GeV) spectral component could be dominated by hadronic emission originating from a supernova remnant (SNR). For a PWN component above 10 GeV, we measure the conditions for particle diffusion, finding that the local diffusion (D(50 TeV) ∼ 1028 cm−2 s −1 ) is suppressed compared to the interstellar medium (ISM) value, in agreement with similar TeV PWNe. By measuring the radial surface brightness profiles of the γ-ray source across multiple instruments, we demonstrate that the combined MeV–TeV spatial information is a powerful tool to constrain particle diffusion properties.
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Figures from paper (click to get full size image):
Figure 1: The 50MeV to 30TeV γ-ray SED for HESS J1857+026. The blue flux stars are from the Fermi–LAT 4FGL–DR3 catalog (Abdollahi et al. 2022) and the light blue uncertainty flux band is from the Fermi–LAT FGES catalog (Ackermann et al. 2017). The red-filled X-points are from the Fermi–LAT 3FHL catalog (Ajello et al. 2017). The green uncertainty flux band is from the 1–25TeV LHAASO catalog data (Cao et al. 2024) and the 1.3–32TeV cyan uncertainty flux band is from the 3HAWC survey (Albert et al. 2020). The black circles are from the Fermi–LAT between 300MeV and 2TeV (this work, see Section 3), the grey diamonds are from VERITAS (this work, see Section 4), the dark blue hexagons are new HAWC data (this work, see Section 5), the orange points are from HESS (H. E. S. S. Collaboration et al. 2018) and the pink X-points from MAGIC (MAGIC Collaboration et al. 2014). The Fermi–LAT systematics are from Fermi-LAT Collaboration et al. (2025).
Figure 2: The particle column densities estimated from CO (left) and HI (right) emission in the region around HESS J1857+026 in the velocity range 81 km s−1 to 102 km s−1, corresponding to a distance between 5.3 kpc and 6.1 kpc. The larger circle shows the γ-ray size from Fermi-LAT data (see Section 3.1). The smaller circle shows the γ-ray size from the VERITAS data (see Section 4). The white cross sign represents the location of PSR J1856+0245. The 12CO (J = 1−0) data (Dame et al. 2001) are retrieved from the 1.2- m CO Survey Dataverse of the Smithsonian Astrophysical Observatory. The HI data are obtained from the Galactic Archive of the Arecibo L-band Feed Array (GALFA, Peek et al. 2017).
Figure 3: Left: A 3◦ × 3◦ excess counts map of Fermi–LAT data with energy between 300MeV and 2TeV. Unrelated 4FGL sources are in cyan. Additional point sources are labeled in green. 4FGL J1857.7+0246e (white dashed circle) is replaced in the model with the radial Gaussian source (RG) marked as the solid white circle. The 95% positional uncertainty for 2FHL J1856.8+0256 is shown as the smaller white circle, see text for details. Right: A 3 ◦×3 ◦ excess counts map with VERITAS data in the energy range 0.3–10TeV smoothed with a correlation radius of 0.1 ◦. The Gaussian extension of the VERITAS emission associated with HESS J1857+026 in this work is shown as the solid white circle. Unrelated VERITAS emission is seen to the south, corresponding to HESS J1858+020, and is marked with a cyan cross. Both Panels: The HESS J1857+026 extension in the HGPS catalog (H. E. S. S. Collaboration et al. 2018) is indicated as a black circle on the left and a cyan circle on the right. The X-ray position of PSR J1856+0245 is marked with a smaller circle that has the approximate size of the compact PWN (blue color on the left and green color on the right).
Figure 4: Same as Figure 3(a) but comparing the three source models discussed in the main text: the 4FGL, the radial Gaussian model we report in Table 1, and the model comprising three extended sources presented by Guo et al. (2024). The 4FGL source is displayed as the white dashed circle, our best-fit radial Gaussian (“RG”) source as the solid white circle, and the three extended sources reported by Guo et al. (2024) in green. The northern most extended source in Guo et al. (2024) replaces the point source 4FGL J1857.9+0313c, which is shown as the cyan cross. See text for details.
Figure 5: The Fermi–LAT SED for the extended source reported in Section 3 in three energy bands: 1–5GeV (green), 5 GeV–2TeV (red), and 300 MeV–2TeV (black) and compared to the 4FGL–DR3 (blue).
Figure 6: HAWC significance maps in J2000 equatorial degrees of the HESS J1857+026 region in three energy ranges: 1 to 10 TeV (left), 10 TeV to 31.6 TeV (middle) and 31.6 to 316 TeV (right). Emission from HESS J1857+026 cuts off above an energy of 31.6 TeV. The circles shown at the bottom left corner of the maps encompass the 68% containment of the point spread function (PSF) obtained from the sum of individual bin PSF histograms for the corresponding energy range, weighted by the excess2/bkg counts per bin for the given energy range. The 10, 15, 20, 25, and 30 σ significance contours are also shown. Labels mark the positions of source associations in the HESS J1857+026 region.
Figure 7: Left: HAWC residual significance map in equatorial coordinates. The locations and extensions of sources comprising the final source model are displayed. Right: HAWC residual map projected into a 1D histogram. The Gaussian fit values are shown where A is the normalization, μ is the mean value, and σ the variance.
Figure 8: Left: Hadronic scenario. Right: Lepto-hadronic scenario. Both panels: The results of the time-independent NAIMA SED fit. In blue are the Fermi–LAT flux data points for E > 300MeV (this work), in green are VERITAS flux points (this work), and in red are HAWC flux points (this work). We also include TeV data from HESS (H. E. S. S. Collaboration et al. 2018) and MAGIC (MAGIC Collaboration et al. 2014) in gray and orange, respectively. The Fermi–LAT systematics (black) are those of Fermi-LAT Collaboration et al. (2025).
Figure 9: Left: The best-fit SED obtained through the evolutionary model method described in Section 6.2. The colored points (color is proportional to photon energy) represent the values of observed data that the model used as comparison points for fitting: the Fermi–LAT (blue, this work) and HESS and MAGIC (purple, H. E. S. S. Collaboration et al. 2018; MAGIC Collaboration et al. 2014). Right: The best-fit SED obtained through the evolutionary model using E > 10 GeV Fermi–LAT data from Guo et al. (2024).
Figure 10: Radial surface brightness profiles of HESS J1857+026 for Fermi–LAT data in 10 GeV–2 TeV (left), VERITAS data in 0.3–10 TeV (center), and HAWC data for B=1 μG in the 0.67–37 TeV range (right). The profiles are fitted with the diffusion-based surface brightness model given by Equation 16. The theoretical curves are convolved with the instrument PSF during the fitting procedure. The error bars include both statistical uncertainties and the effects of the PSF. For the Fermi–LAT and VERITAS data, the PSF is approximated as a 1D Gaussian with a 1σ containment of 0.1◦. For HAWC, the PSF is modeled as the sum of two Gaussian functions (see Equation 5 in Albert et al. 2024a).
Figure 11: Diffusion coefficient as a function of electron energy in the environment of HESS J1857+026. Left: Assumes a magnetic field of B = 1 μG. Right: Assumes a magnetic field of B = 5.5 μG. The solid line indicates the Galactic average diffusion coefficient following the Kolmogorov (δ = 1/3) regime. The data points are measured from Fermi–LAT, VERITAS, and HAWC radial profiles. The dashed line and the dashed-dotted line are the best-fit diffusion coefficients under the Kolmogorov (δ = 1/3) and Kraichnan (δ = 1/2) diffusion regimes, respectively.
