Probing the Properties of the Pulsar Wind in the Gamma-Ray Binary HESS J0632+057 with NuSTAR and VERITAS Observations

TL;DR

This study uses simultaneous NuSTAR and VERITAS observations of HESS J0632+057 to analyze its pulsar wind properties, revealing spectral variations and supporting a leptonic emission model with a pulsar origin.

Contribution

It provides the first simultaneous hard X-ray and TeV gamma-ray observations of HESS J0632+057, constraining the pulsar wind magnetization and supporting a leptonic emission scenario.

Findings

Spectral index varies from 1.77 to 1.56 in X-ray data.

Multi-wavelength SED fits support a pulsar wind shock model.

Constraints on pulsar wind magnetization are consistent with other systems.

Abstract

HESS J0632+057 is a gamma-ray binary composed of a compact object orbiting a Be star with a period of about $315$ days. Extensive X-ray and TeV gamma-ray observations have revealed a peculiar light curve containing two peaks, separated by a dip. We present the results of simultaneous observations in hard X-rays with NuSTAR and in TeV gamma-rays with VERITAS, performed in November and December 2017. These observations correspond to the orbital phases $\phi\approx0.22$ and $0.3$, where the fluxes are rising towards the first light-curve peak. A significant variation of the spectral index from 1.77$\pm$0.05 to 1.56$\pm$0.05 is observed in the X-ray data. The multi-wavelength spectral energy distributions (SED) derived from the observations are interpreted in terms of a leptonic model, in which the compact object is assumed to be a pulsar and non-thermal radiation is emitted by high-energy electrons accelerated at the shock formed by the collision between the stellar and pulsar wind. The results of the SED fitting show that our data can be consistently described within this scenario, and allow us to estimate the magnetization of the pulsar wind at the location of the shock formation. The constraints on the pulsar-wind magnetization provided by our results are shown to be consistent with those obtained from other systems.