A search for iron emission lines in the Chandra X-ray spectra of neutron star low-mass X-ray binaries

TL;DR

This study investigates the presence of iron emission lines in neutron star low-mass X-ray binaries using simultaneous Chandra and RXTE observations, finding significant lines in two sources and suggesting ionization effects influence line strength.

Contribution

First systematic search for iron lines in neutron star LMXBs with simultaneous high-resolution and broadband data, revealing variability and ionization effects.

Findings

Significant iron lines detected in GX 17+2 and GX 349+2.

Line equivalent width in GX 349+2 increases as flux decreases.

No clear correlation between line presence and spectral state.

Abstract

While iron emission lines are well studied in black hole systems, both in X-ray binaries and Active Galactic Nuclei, there has been less of a focus on these lines in neutron star low-mass X-ray binaries (LMXBs). However, recent observations with Suzaku and XMM-Newton have revealed broad asymmetric iron line profiles in 4 neutron star LMXBs, confirming an inner disk origin for these lines in neutron star systems. Here, we present a search for iron lines in 6 neutron star LMXBs. For each object we have simultaneous Chandra and RXTE observations at 2 separate epochs, allowing for both a high resolution spectrum, as well as broadband spectral coverage. Out of the six objects in the survey, we only find significant iron lines in two of the objects, GX 17+2 and GX 349+2. However, we cannot rule out that there are weak, broad lines present in the other sources. The equivalent width of the line in GX 17+2 is consistent between the 2 epochs, while in GX 349+2 the line equivalent width increases by a factor of ~3 between epochs as the source flux decreases by a factor of 1.3. This suggests that the disk is highly ionized, and the line is dominated by recombination emission. We find that there appears to be no specific locations in the long-term hardness-intensity diagrams where iron emission lines are formed, though more sources and further observations are required.