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 approx. 1.0 keV) plus power law (Gamma approx. 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 nu < 221 km/s. We find an equivalent width for the line of 27+2/.3 eV, from which we determine a column density of (7 +/- 1)X10(exp 17) /sq. cm via a curve-of-growth analysis. Using XSTAR simulations, we place a lower limit on the ionization parameter of >103.6 erg cm/s. 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