X-ray and optical monitoring of the peculiar source 4U 1700+24/V934 Her

We report on ASCA and BeppoSAX X-ray broad band observations of the galactic low-luminosity X-ray source 4U 1700+24 performed on 1995 and 1998, respectively, and on (quasi-)simultaneous ground observations of its optical counterpart, V934 Her, from the Loiano 1.5-meter telescope. In order to better understand the nature of the source we also analyze public archival ROSAT and RXTE data as well as the RXTE ASM light curve of 4U 1700+24; we also re-analyze a 1985 EXOSAT pointing. The optical spectra are typical of a M2 III star; this allows us to determine a revised distance to the object of ~ 400 pc. While these spectra do not show either any spectral change between the two epochs or any peculiar feature apart from those observed in normal red giants, the spectroscopic measurements carried out in X-rays reveal a complex and long-term variable spectrum, with a clear soft excess. The X-ray spectral properties of the source are best described by a thermal Comptonization spectrum plus a soft energy (<1 keV) excess, which can be modeled in the form of a blackbody emission with kT_BB ~ 1 keV; the latter component is not detected at the lowest source flux levels. The ratio between the two components varies substantially with the source flux. The X-ray emission from the object appears to become harder as its luminosity increases: indeed, the RXTE data acquired during an outburst occurred in October-November 1997 display a hard tail, detected up to 100 keV and modeled with a comptonizing cloud which is hotter and less opaque than that seen in the low intensity state. Apart from erratic shot-noise variability on timescales of tens to thousands of seconds, no significant properties (such as pulsations or QPOs) are found from the timing analysis of the X-ray light curves extracted from the observations presented here. With the new distance determination, the 2-10 keV X-ray luminosity range spanned in the considered observations lies between ~ 2x1032 and ~ 1x1034 erg s-1. All this information, combined with the findings by other authors, allows us to suggest that the scenario which best describes the object consists of a wide binary system in which a neutron star accretes matter from the wind of a M-type giant star. Implications of such a model are discussed. Partly based on observations collected at the Bologna Astronomical Observatory in Loiano, Italy.