THE TeV BINARY HESS J0632+057 IN THE LOW AND HIGH X-RAY STATE

We report on a 40 ks Chandra observation of the TeV emitting high-mass X-ray binary HESS J0632+057 performed in 2011 February during a high state of X-ray and TeV activity. We have used the ACIS-S camera in continuous 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 47 ks XMM-Newton observation on 2007 September. We did not find any periodic or quasi-periodic signal in any of the two observations. We derived an average pulsed fraction 3 sigma upper limit for the presence of a periodic signal of less than or similar to 35% and 25% during the low and high emission states, 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 N-H similar or equal to (2.1-4.3) x 10(21) cm(-2) and Gamma similar or equal to 1.18-1.61. In contrast to what has been observed in other TeV binaries, it seems that in this source the higher the flux, the softer the X-ray spectrum.