XMM-Newton X-ray spectroscopy of the high-mass X-ray binary 4U 1700-37 at low flux

We present results of a monitoring campaign of the high-mass X-ray binary system <ASTROBJ>4U 1700-37</ASTROBJ>/<ASTROBJ>HD 153919</ASTROBJ>, carried out with XMM-Newton in February 2001. The system was observed at four orbital phase intervals, covering 37% of one 3.41-day orbit. The lightcurve includes strong flares, commonly observed in this source. We focus on three epochs in which the data are not affected by photon pile up: the eclipse, the eclipse egress and a low-flux interval in the lightcurve around orbital phase phi ~ 0.25. The high-energy part of the continuum is modelled as a direct plus a scattered component, each represented by a power law with identical photon index (alpha ~ 1.4), but with different absorption columns. We show that during the low-flux interval the continuum is strongly reduced, probably due to a reduction of the accretion rate onto the compact object. A soft excess is detected in all spectra, consistent with either another continuum component originating in the outskirts of the system or a blend of emission lines. Many fluorescence emission lines from near-neutral species and discrete recombination lines from He- and H-like species are detected during eclipse and egress. The fluorescence Fe Kalpha line at 6.4 keV is very prominent; a second Kalpha line is detected at slightly higher energies (up to 6.7 keV) and a Kbeta line at 7.1 keV. In the low-flux interval the Fe Kalpha line at 6.4 keV is strongly (factor ~ 30) reduced in strength. In eclipse, the Fe Kbeta/Kalpha ratio is consistent with a value of 0.13. In egress we initially measure a higher ratio, which can be explained by a shift in energy of the Fe K-edge to ~ 7.15 keV, which is consistent with moderately ionised iron, rather than neutral iron, as expected for the stellar wind medium. The detection of recombination lines during eclipse indicates the presence of an extended ionised region surrounding the compact object. The observed increase in strength of some emission lines corresponding to higher values of the ionisation parameter xi further substantiates this conclusion.