High-resolution X-ray spectroscopy of supergiant HMXB 4U 1700-37 during the compact object eclipse

Abstract

We present an analysis of the first observation of the iconic high-mass X-ray binary 4U 1700-37 with the Chandra High-Energy Transmission Gratings (HETGs) during an X-ray eclipse. The goal of the observation was to study the structure/physical conditions in the clumpy stellar wind through high-resolution spectroscopy. We find the following: (a) Emission-line brightness from K-shell transitions, corresponding to near-neutral species, directly correlates with continuum illumination. However, these lines do not greatly diminish during eclipse. This is readily explained if fluorescence K α emission comes from the bulk of the wind. (b) The highly ionized Fe XXV and Fe XXVI Ly α diminish during eclipse. Thus, they must be produced in the vicinity of the compact object where log ξ > 3. (c) To describe the emission-line spectrum, the sum of two self-consistent photoionization models with low ionization (log ξ ∼ -1) and high ionization (log ξ ∼ 2.4) is required. From their emission measures, the clump-to-interclump density ratio can be estimated to be n_c/n_i ∼ 300. To fit the complex He-like Si XIII profile, the plasma requires a broadening with v_bulk ∼ 840 km s^-1. Reproducing the observed r ≈ f line fluxes requires the addition of a third collisionally ionized plasma. (d) Emission-line widths appear unresolved at the HETG resolution with the exception of silicon. There is no clear radial segregation between (quasi-)neutral and ionized species, consistent with cold wind clumps interspersed in a hot rarefied interclump medium.