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