The hot interstellar medium towards 4U 1820-30: a Bayesian analysis

Abstract

Context. High-ionisation lines in the soft X-ray band are generally associated to either interstellar hot gas along the line of sight or to photoionised gas intrinsic to the source. In the low-mass X-ray binary 4U 1820-30, the nature of these lines is not well understood.
Aims: We aim to characterise the ionised gas present along the line of sight towards 4U 1820-30 producing the X-ray absorption lines of Mg XI, Ne IX, Fe XVII, O VII, and O VIII.
Methods: We analysed all the observations available for this source in the XMM-Newton and Chandra archives that were taken with the RGS, HETG, and LETG spectrometers. We accurately examined the high-resolution grating spectra using a standard X-ray analysis procedure based on the C-statistic and using Bayesian parameter inference. We tested two physical models which describe a plasma in either collisional ionisation or photoionisation equilibrium. We adopted the Bayesian model comparison to statistically compare the different combinations of models used for the analysis.
Results: We find that the lines are consistent with hot gas in the interstellar medium rather than the intrinsic gas of the X-ray binary. Our best-fit model reveals the presence of a collisionally ionised plasma with a temperature of T = (1.98 ± 0.05) × 10⁶ K. The photoionisation model fails to fit the Fe XVII line (which is detected with a significance of 6.5σ) because of the low column density predicted by the model. Moreover, the low inclination of the binary system is likely the reason for the non-detection of ionised gas intrinsic to the source.