The TeV binary HESS J0632+057 in the low and high X-ray state

TL;DR

This study analyzes X-ray observations of the TeV binary HESS J0632+057 during high and low emission states, finding spectral changes but no evidence of X-ray pulsations, and noting an unusual flux-spectrum relationship.

Contribution

First detailed comparison of X-ray spectral properties of HESS J0632+057 during different emission states, with constraints on pulsations and spectral variability.

Findings

No periodic or quasi-periodic X-ray signals detected.

Spectral parameters vary significantly between high and low states.

Higher flux correlates with softer X-ray spectrum, contrary to other TeV binaries.

Abstract

We report on a 40ks Chandra observation of the TeV emitting high mass X-ray binary HESS J0632+057 performed in February 2011 during a high-state of X-ray and TeV activity. We have used the ACIS-S camera in Continuos Clocking mode to search for a possible X-ray pulsar in this system. Furthermore, we compare the emission of the source during this high state, with its X-ray properties during a low state of emission, caught by a 47ks XMM-Newton observation on September 2007. We did not find any periodic or quasi-periodic signal in any of the two observations. We derived an average pulsed fraction 3sigma upper limit for the presence of a periodic signal of ~35% and 25% during the low and high emission state, respectively (although this limit is strongly dependent on the frequency and the energy band). Using the best X-ray spectra derived to date for HESS J0632+057, we found evidence for a significant spectral change between the low and high X-ray emission states, with the absorption value and the photon index varying between Nh ~ 2.1-4.3x10^{21} cm^{-2} and Gamma ~ 1.18-1.61. At variance with what observed in other TeV binaries, it seems that in this source the higher the flux the softer the X-ray spectrum.