The accretion disk corona and disk atmosphere of 4U 1624-490 as viewed by the Chandra-HETGS

We present a detailed spectral study (photoionization modelling and variability) of the “Big Dipper” 4U 1624-490 based on a \chandra-High Energy Transmission Gratings Spectrometer (HETGS) observation over the $\sim 76$ ks binary orbit of 4U 1624-490. While the continuum spectrum can be modeled using a blackbody plus power-law, a slightly better fit is obtained using a single $\mathrm{\Gamma }=2.25$ power-law partially (71%) covered by a local absorber of column density $N_{\mathrm{H},\mathrm{L}\mathrm{o}\mathrm{c}\mathrm{a}\mathrm{l}}=8.1_{-0.6}^{+0.7}\times {10}^{22}\mathrm{c}{\mathrm{m}}^{-2}$. The data show a possible quasi-sinusoidal modulation with period $43_{-9}^{+13}$ ks that might be due to changes in local obscuration. Photoionization modeling with the {\sc xstar} code and variability studies of the observed strong \ion{Fe}{25} and \ion{Fe}{26} absorption lines point to a two-temperature plasma for their origin: a highly ionized component of ionization parameter ${\xi }_{\mathrm{h}\mathrm{o}\mathrm{t}}\approx {10}^{4.3}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{s}\mathrm{c}\mathrm{m}{\mathrm{s}}^{-1}$ ($T\sim 3.0\times {10}^6$ K) associated with an extended accretion disk corona of radius $R \sim3\times10^{10}$cm,andalessionizedmorevariablecomponentof$\xi \approx 10^{3.4} {\rm ergs cm s^{-1}}$($T\sim 1.0\times 10^{6}$K)and$\rm \xi \approx 10^{3.1} ergs cm s^{-1}$($T\sim 0.9\times 10^{6}$ K) coincident with the accretion disk rim. We use this, with the observed \ion{Fe}{25} and \ion{Fe}{26} absorption line variations (in wavelength, strength, and width) to construct a viewing geometry that is mapped to changes in plasma conditions over the 4U 1624-490 orbital period.