Investigating the Nature of Absorption Lines in the Chandra X‐Ray Spectra of the Neutron Star Binary 4U 1820−30

Abstract

We use four Chandra gratings spectra of the neutron star low-mass X-raybinary 4U 1820-30 to better understand the nature of certain X-rayabsorption lines in X-ray binaries, including the Ne II, Ne III, Ne IX,O VII, and O VIII lines. The equivalent widths of the lines aregenerally consistent between the observations, as expected if theselines originate in the hot interstellar medium. No evidence was foundthat the lines were blueshifted, again supporting the interstellarmedium origin, although this may be due to poor statistics. There isapparent variability in the O VIII Ly{α} line equivalent widthproviding some evidence that at least some of the O VIII absorptionarises within the system. However, the significance is marginal (2.4{σ}), and the lack of variation in the other lines casts some doubton the reality of the variability. From calculating the equivalenthydrogen column densities for a range of Doppler parameters, we findthey are consistent with the interstellar origin of the lines. Inaddition, we fit the spectra with photoionization models for locallyabsorbing material, and find that they can reproduce the spectrum well,but only when there is an extremely low filling factor. We conclude thatboth the ISM and local absorption remain possible for the origin of thelines, but that more sensitive observations are needed to search forlow-level variability.