X-ray imaging observations of the high-mass gamma-ray binary HESS J0632+057

TL;DR

This study uses X-ray observations from Chandra and Swift-XRT to analyze the high-mass gamma-ray binary HESS J0632+057, revealing extended emission features, variable absorption, and potential environmental structures around the binary.

Contribution

First detailed X-ray imaging and spectral analysis of HESS J0632+057, identifying extended emission and variable absorption, suggesting complex circumbinary environment and spatial features.

Findings

Detection of asymmetric extended emission near the binary

Identification of a distant extended 'blob' with distinct spectral properties

Evidence of variable absorption ($N_H$) correlated with orbital phase

Abstract

The Chandra X-ray Observatory (CXO) imaged the high-mass gamma-ray binary (HMGB) HESS J0632+057 with the Advanced CCD Imaging Spectrometer (ACIS). We analyzed the CXO data together with 967 ks of archival Swift-XRT observations. On arcsecond scales we find a hint of asymmetric extended emission. On arcminute scales, a region of extended emission ("the blob"), located $\sim$5.5' east of the binary, is seen in both the CXO-ACIS and the Swift-XRT images. The blob does not have a counterpart in the radio, NIR, IR, or optical images. The ACIS spectrum of the blob fits either an absorbed power-law model with photon index $\Gamma\simeq2.6$, or a thermal plasma model with $kT\simeq3$ keV. Since the blob's absorbing column, $N_{\rm H}$, is significantly larger than that of the binary we conclude that the blob and binary are not directly related. The somewhat larger very deep XRT image suggests that the binary may be located within a shell (or cavity). The four ACIS spectra taken within the $\sim$20 day interval near the light curve minimum suggests that the $N_{\rm H}$ is varying on timescales of days, possibly due to the inhomogeneous circumbinary environment. The XRT spectra extracted from the wider orbital phase intervals support significant changes in $N_{\rm H}$ near the light curve maximum/minimum, which may be responsible for the substantial systematic residuals seen near 1 keV, and provide tentative evidence for a Fe line at 6.4 keV. We find no significant periodic signal in the ACIS data up to 0.156 Hz.