Chandra explores interstellar medium of a bright low-mass X-ray binary

Using NASA’s Chandra X-ray space telescope, astronomers have performed high-resolution X-ray spectroscopic observations of a bright low-mass X-ray binary known as GX 340+0. Results of the observational campaign, published April 3 on the arXiv pre-print server, shed more light on the composition of interstellar medium (ISM) in this system.

Classification and characteristics

X-ray binaries are composed of a normal star or a white dwarf transferring mass onto a compact neutron star or a black hole. Based on the mass of the companion star, astronomers divide them into low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBs).

GX 340+0 is a persistent, bright LMXB at a distance of some 33,000 light years, containing a weakly magnetized neutron star. The object was first identified in 1969 and is classified as a Z source.

In general, the so-called Z sources represent a particular class of neutron star LMXBs characterized by a wide Z-like track in their hard color-soft color (or hardness-intensity) diagrams, with three branches: the horizontal (HB), the normal (NB), and the flaring branch (FB).

Deep spectroscopy with Chandra

A team of astronomers led by Daniele Rogantini of the University of Chicago, decided to take a closer look at GX 340+0 with Chandra’s High-Energy Transmission Grating (HETG), hoping to get more insights into the properties of this system. HETG enables high-resolution X-ray spectroscopy, which is a powerful probe of dust chemistry and grain physics in the ISM of X-ray sources.

“We present a study of the gas and dust along the line of sight to the bright low-mass X-ray binary GX 340+0, which samples higher-density gases in the ISM. Using deep Chandra/HETG spectra, we characterize X-ray absorption fine structure from dust, gas absorption lines, and the optical depths of the silicon, sulfur, and iron K-edges,” the researchers explain.

Insights into ISM dust composition and atomic data

The study found that amorphous olivine is the dominant component of dust in the ISM of GX 340+0. It accounts for more than 60% of the total dust column density in all statistically selected models. This result is consistent with previous findings that in general estimate 70–90% of olivine contribution.

The collected data indicate that the iron budget is dominated by iron incorporated into silicates (about 74%), with an additional contribution from iron-bearing sulfides and metallic iron (about 8% and 18%, respectively). Moreover, the models used by the authors of the paper favor sulfur-bearing dust in the form of iron sulfides (pyrrhotite/troilite), and they infer a sulfur depletion of approximately 35% into the condensed phase.

The study also detected a tentative absorption structure associated with low-abundance elements present in the ISM of GX 340+0—argon and calcium. This structure appears offset with respect to some commonly adopted cross sections. According to the authors of the paper, this finding highlights the importance of accurate atomic photoabsorption data.

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