High resolution X-ray spectroscopy of Supergiant HMXB 4U1700-37 during the compact object eclipse

Martinez-Chicharro, M.; Grinberg, V.; Torrejon, J. M.; Schulz, N.; Oskinova, L.; Nowak, M.; Fürst, F.; Hell, N.; Hainich, R.

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 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 that: 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 Kα emission comes from the bulk of the wind. b) The highly ionised Fe XXV and Fe XXVI Lyα 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 photo ionisation models with low ionisation (log ξ ~ -1) and high ionisation (log ξ ~ 2.4) is required. From their emission measures, the clump-to-interclump density ratio can be estimated to be nc/ni ~ 300. To fit the complex He-like Si XIII profile, the plasma requires a broadening with vbulk ~ 840 km/s. Reproducing the observed r=f line fluxes requires the addition of a third collisionally ionised plasma. d) Emission lines widths appear unresolved at the HETG gratings resolution with exception of Silicon. There is no clear radial segregation between (quasi)neutral and ionised species, consistent with cold wind clumps interspersed in a hot rarefied interclump medium.

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