AN Fe XXIV ABSORPTION LINE IN THE PERSISTENT SPECTRUM OF THE DIPPING LOW-MASS X-RAY BINARY 1A 1744–361

摘要

We report on Chandra X-ray Observatory (Chandra) High Energy Transmission Grating spectra of the dipping low-mass X-ray binary 1A 1744–361 during its 2008 July outburst. We find that its persistent emission is well modeled by a blackbody (kT ~ 1.0?keV) plus power law (Γ ~ 1.7) with an absorption edge. In the residuals of the combined spectrum, we find a significant absorption line at 6.961 ± 0.002?keV, consistent with the Fe XXVI (hydrogen-like Fe) 2-1 transition. We place an upper limit on the velocity of a redshifted flow of v 221?km?s–1. We find an equivalent width for the line of 27+2 – 3?eV, from which we determine a column density of (7 ± 1) × 1017?cm–2 via a curve-of-growth analysis. Using XSTAR simulations, we place a lower limit on the ionization parameter of 103.6?erg?cm?s–1. We discuss what implications the feature has on the system and its geometry. We also present Rossi X-ray Timing Explorer data accumulated during this latest outburst and, via an updated color-color diagram, clearly show that 1A 1744–361 is an "atoll" source.