Multiband Nonthermal Radiative Properties of HESS J1813-178

The source HESS J1813-178 was detected in the survey of the inner Galaxy in TeV gamma-rays, and a SNR G12.8-0.0 was identified in the radio band to be associated with it. The PWN embedded in the SNR is powered by an energetic pulsar PSR J1813-1749, which was recently discovered. Whether the TeV gamma-rays originate from the SNR shell or the PWN is uncertain now. We investigate theoretically the multiwavelength nonthermal radiation from the composite SNR G12.8-0.0. The emission from the particles accelerated in the SNR shell is calculated based on a semianalytical method to the nonlinear diffusive shock acceleration mechanism. In the model, the magnetic field is self-generated via resonant streaming instability, and the dynamical reaction of the field on the shock is taken into account. Based on a model which couples the dynamical and radiative evolution of a PWN in a non-radiative SNR, the dynamics and the multi-band emission of the PWN are investigated. The particles are injected with a spectrum of a relativistic Maxwellian plus a power law high-energy tail with an index of -2.5. Our results indicate that the radio emission from the shell can be well reproduced as synchrotron radiation of the electrons accelerated by the SNR shock; with an ISM number density of 1.4 cm^{-3} for the remnant, the gamma-ray emission from the SNR shell is insignificant, and the observed X-rays and VHE gamma-rays from the source are consistent with the emission produced by electrons/positrons injected in the PWN via synchrotron radiation and IC scattering, respectively; the resulting gamma-ray flux for the shell is comparable to the detected one only with a relatively larger density of about 2.8 cm^{-3}. The VHE gamma-rays of HESS J1813-178 can be naturally explained to mainly originate from the nebula although the contribution of the SNR shell becomes significant with a denser ambient medium.

💡 Research Summary

The paper presents a comprehensive theoretical study of the multi‑wavelength non‑thermal emission from the composite supernova remnant (SNR) G12.8‑0.0 and its embedded pulsar wind nebula (PWN), which are associated with the very‑high‑energy (VHE) gamma‑ray source HESS J1813‑178. The authors adopt two complementary frameworks: (i) a semi‑analytical, non‑linear diffusive shock acceleration (NL‑DSA) model for particles accelerated at the SNR forward shock, and (ii) a dynamical‑radiative model for the evolution of a PWN inside a non‑radiative SNR shell.

In the NL‑DSA part, the steady‑state transport equation for protons is solved in one dimension, including a free‑escape boundary upstream of the shock to mimic the loss of the highest‑energy particles. The diffusion coefficient follows Bohm scaling, D = pc²/(3eB). Magnetic field amplification is treated self‑consistently via resonant streaming instability, with the amplified magnetic pressure P_w(x) expressed in terms of the local flow speed. The model incorporates the dynamical feedback of the amplified field on the shock structure, yielding a total compression ratio R_tot that depends on the sonic Mach number, the background magnetic field (B₀ = 5 µG), and the upstream density (n₀). Particle injection follows the thermal‑leakage prescription, characterized by ξ ≈ 3.8, which sets the injection momentum p_inj = ξ p_th. Electrons share the same spectral shape as protons up to a maximum energy E_max,e, determined by equating the acceleration time with the synchrotron loss time; the resulting cutoff lies at a few tens of TeV for the adopted shock speed (u₀ ≈ 10⁸ cm s⁻¹). The electron distribution is then written as f_e(p) = K_ep p f(p) exp