Unveiling the environment surrounding low-mass X-ray binary SAX J1808.4-3658

Abstract

Context. Low-mass X-ray binaries (LMXBs) are a natural workbench to study accretion disk phenomena and optimal background sources to measure elemental abundances in the interstellar medium (ISM). In high-resolution XMM-Newton spectra, the LMXB SAX J1808.4-3658 in the past showed a neon column density significantly higher than expected given its small distance, presumably due to additional absorption from a neon-rich circumstellar medium (CSM). Aims. It is possible to detect intrinsic absorption from the CSM by evidence of Keplerian motions or outflows. For this purpose, we analyze a recent, deep (100 ks long), high-resolution Chandra/LETGS spectrum of SAX J1808.4-3658 in combination with archival data. Methods. We estimated the column densities of the different absorbers through the study of their absorption lines. We used both empirical and physical models involving photo- and collisional-ionization to determine the nature of the absorbers. Results. The abundances of the cold interstellar gas match the solar values as expected given the proximity of the X-ray source. For the first time in this source, we detected neon and oxygen blueshifted absorption lines, which can be well modeled with outflowing photoionized gas. The wind is neon rich (Ne/O ≠3) and may originate from processed, ionized gas near the accretion disk or its corona. The kinematics (v = 500-1000 km s-1) are indeed similar to those seen in other accretion disks. We also discovered a system of emission lines with very high Doppler velocities (v ∼ 24 000 km s-1) originating presumably closer to the compact object. Additional observations and UV coverage are needed to accurately determine the abundances and the ionization structure of the wind. © ESO, 2014.