An Fe Xxvi Absorption Line in the Persistent Spectrum of the Dipping Low Mass X-ray Binary 1a 1744−361

We report on Chandra X-ray Observatory (CXO) High-Energy Transmission Grating (HETG) spectra of the dipping Low Mass X-ray Binary (LMXB) 1A 1744−361 during its July 2008 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 at 7.6 keV. 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. We find an equivalent width for the line of 27 −3 eV, from which we determine a column density of 7±1×1017 cm via a curve-of-growth analysis. Using XSTAR simulations, we place a lower limit on the ionization parameter of > 10 erg cm s. The properties of this line are consistent with those observed in other dipping LMXBs. Using Rossi X-ray Timing Explorer (RXTE) data accumulated during this latest outburst we present an updated color-color diagram which clearly shows that 1A 1744−361 is an “atoll” source. Finally, using additional dips found in the RXTE and CXO data we provide an updated orbital period estimate of 52±5 minutes. Subject headings: line: identification — binaries: general — stars: individual(1A 1744−361) — stars: neutron — X-rays: binaries — X-rays: stars