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Investigation of the high-energy emission from the TeV gamma-ray binary HESS J0632+057

Presentation #116.21 in the session Stellar/Compact Objects.

Published onJul 01, 2023
Investigation of the high-energy emission from the TeV gamma-ray binary HESS J0632+057

Existence of highly energetic particles in TeV gamma-ray binaries (TGBs) is well established by detection of very high-energy gamma rays from them. It is thought that the particles are accelerated in the intrabinary shock produced by wind-wind interaction (intrabinary shock scenario) or in the bipolar jets of a black hole (microquasar scenario). These scenarios have been applied to a handful of TGBs discovered thus far and provided insights into the nature of TGBs and particle acceleration mechanisms in them. HESS J0632+057 (J0632), in a 317-day orbit with a Be companion, exhibits X-ray and TeV modulations on the orbital period, and thus is a well-identified TGB. The X-ray and TeV orbital light curves (LCs) show two broad bumps at orbital phases ϕ ∼ 0.3 and ∼0.7, and a narrow spike at ϕ ∼ 0.35. The phase-resolved spectral energy distributions (SEDs) of J0632 show a hint for a TeV excess which was seen also in the other TGB PSR J2032+4127. These LC and SED features can provide important clues to the mechanisms for particle acceleration in TGBs. Although the orbit of J0632 has not been reliably determined and it was unclear whether the high-energy emission in the source is produced in an IBS or a blackhole jet, Tokayer et al. (2021) demonstrated that an intrabinary shock (IBS) scenario explains the X-ray data, and derived the system orbit based on the X-ray modeling. Additional support to the IBS scenario was made by a discovery of extended X-ray emission which was interpreted to be a signature of the wind-wind interaction. In light of these findings, we developed an IBS model and applied it to the multi-band data of J0632 to cross-check the X-ray-inferred orbit and to further understand the nature of particles and their emission in the source.

For the X-ray-inferred orbit, our IBS model reproduces general features of the measured broadband (phase averaged and resolved) SEDs and multi-band LCs. We found that the particles are energized at the shock to >TeV energies, and their bulk motion is accelerated to highly relativistic speed with a Lorentz factor of ~6. More intriguingly, we found that the TeV bump in the SED could not be reproduced by the IBS particles with a featureless power-law distribution. The SED bump instead demands particles with a narrow distribution with the average Lorentz factor of 106, possibly the preshock particles of the pulsar wind. If so, the TeV SED bumps in TGBs provide opportunities to probe the energy conversion processes in pulsar wind: from magnetic to particle energies.

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