Investigation of the Broadband Emission of the Gamma-Ray Binary HESS J0632+057 Using an Intrabinary Shock Model

Abstract

We investigated a wealth of X-ray and gamma-ray spectral energy distribution (SED) and multiband light-curve (LC) data of the gamma-ray binary HESS J0632+057 using a phenomenological intrabinary shock (IBS) model. Our baseline model assumes that the IBS is formed by colliding winds from a putative pulsar and its Be companion and that particles accelerated in the IBS emit broadband radiation via synchrotron (SY) and inverse Compton upscattering (ICS) processes. Adopting the latest orbital solution and system geometry, we reproduced the global X-ray and TeV LC features, two broad bumps at ϕ ∼ 0.3 and ∼0.7, with the SY and ICS model components. We found that these TeV LC peaks originate from ICS emission caused by the enhanced seed photon density near periastron and superior conjunction or Doppler-beamed emission of bulk-accelerated particles in the IBS at inferior conjunction. While our IBS model successfully explained most of the observed SED and LC data, we found that phase-resolved SED data in the TeV band require an additional component associated with ICS emission from preshock particles (produced by the pulsar wind). This finding indicates a possibility of delineating the IBS emission components and determining the bulk Lorentz factors of the pulsar wind at certain orbital phases.

Modeling X-Ray and Gamma-Ray Emission from Redback Pulsar Binaries

“…Alternatively, the IBS model constructed based on Scenario 2 could easily accommodate the broadband SEDs and multiband LCs of the three RBs (Figure 3). However, we should note that the parameter values reported in Table 4 are not unique due to parameter degeneracies (see Kim et al 2022), and so the MeV and TeV flux predictions here should be taken as one potential realization among a landscape of possibilities. Some of the degeneracy can be broken by high-quality optical, X-ray, MeV, and TeV data or limits.…”

“…i and η s values can be inferred from optical LC modeling (e.g., Breton et al 2013) and LAT measurements of the pulsar flux, respectively. The X-ray data help constrain v IBS (by the widths and amplitudes of the LC peaks; Kim et al 2022), B s within the IBS (Equation ( 5)), and s,max g (synchrotron cutoff). Current constraints on these parameters are not stringent because the quality of the X-ray LC measurements is poor and the SEDs do not fully cover the energy range of the synchrotron spectrum.…”

“…In this section, we describe the emission zones and computational methods used in our numerical model. See Kim et al (2022) for a comprehensive understanding of the model components, parameters, and their covariance.…”

The high energy X-ray probe (HEX-P): Galactic PeVatrons, star clusters, superbubbles, microquasar jets, and gamma-ray binaries

“…Our results, shown in the Figures 12C, suggest that HEX-P will be capable of detecting an exponential cut-off at E c ≤ 70 keV with 50-ks exposure, and even a milder cut-off (e.g., a smaller exponential index) can be detectable with HEX-P. Moreover, the more accurate spectral measurements and identification of spectral features at different orbital phases can contribute to distinguishing between various emission components in TGBs, such as the IBS vs. preshock emission (e.g., Kim et al, 2022).…”

Constraining the orbital solution of the gamma-ray binary HESS J0632+057

“…The newer solution presented by M18 suggests the peaks in the light curves occur at points near the circumstellar disc crossing. In addition to the radial velocity solutions by C12 and M18, a number of different solutions have been suggested by modelling the non-thermal emission [12][13][14][15].…”