4U1700-37

1 Introduction

First detected by the Uhuru satellite (Jones et al. 1973) the eclipsing X-ray source 4U 1700-37 was quickly associated with the luminous O6.5 Iaf⁺ star HD 153919, confirming 4U 1700-37 as a high-mass X-ray binary (henceforth HMXB). HMXBs are systems composed of an OB star and a compact companion (neutron star or black hole), with the X-ray emission resulting from the accretion of material by the compact companion. In the subclass of supergiant HMXB systems material is accreted either via Roche lobe overflow or directly from the powerful stellar wind of the OB primary. Given that HD 153919 slightly underfills its Roche Lobe (e.g. Conti 1978) mass transfer proceeds via the latter mechanism.

Although an orbital period of $\sim$3.412 days (Jones et al. 1973) was quickly identified for 4U 1700-37, extensive searches (e.g. Rubin et al. 1996 and references therein) have failed to identify any other X-ray periodicities within the system that might correspond to the pulse period for a possible neutron star (although Konig & Maisack 1997 claim the presence of a 13.81 day period in CGRO BATSE & RXTE ASM datasets). Given the absence of any X-ray pulsations and the unusually hard nature of the spectrum various authors (e.g. Brown et al. 1996) have suggested that the compact companion could be a low mass black hole rather than a neutron star. However, Reynolds et al. (1999) point out that the 2-200 keV spectrum of 4U 1700-37 differs from those commonly observed for black hole candidates such as Cygnus X-1. Given that the X-ray spectrum of 4U 1700-37 is qualitatively similar to those of accreting neutron stars they suggest that the compact object is also a neutron star, and explain the lack of pulsations as due to either a weak magnetic field or an alignment of the magnetic field with the spin axis.

With a spectral type of O6.5 Iaf⁺, HD 153919 is the hottest and potentially most massive mass donor of any of the the HMXB systems. As such, determination of its fundamental parameters (radius, temperature, mass and chemical composition) is of importance given that these will potentially provide valuable insights into the evolution of very massive stars and their ultimate fate. In particular by determining the masses of the components of HMXB systems, limits to the progenitor masses for neutron stars and black holes in such systems can be found while the chemical composition of the mass donor can shed light on the pre-supernova (SN) mass transfer mechanisms.

The paper is ordered as follows. Section 2 describes the determination of the physical properties of HD 153919; the complete dataset and the non-Local Thermal Equilibrium (non-LTE) code used to analyse it. Section 3 describes the Monte-Carlo technique used to determine the masses of both components of the binary system. In Sects. 4 and 5 we discuss the implications of our results for the evolution of hot massive stars, limits for the progenitor masses of compact objects and the equation of state for nuclear matter. Finally in Sect. 6 we summarize the main results of the paper.

4U1700-37