Discovery of a strong 6.6 keV emission feature from EXO 1745$-$248 after the superburst in 2011 October

TL;DR

This paper reports the discovery of a strong, broad 6.6 keV emission feature in the X-ray spectrum of EXO 1745-248 after a superburst, likely caused by charge exchange interactions near the neutron star, revealing new insights into nuclear burning processes.

Contribution

It introduces the first detection of a broad, strong emission feature at 6.6 keV in this context, suggesting a novel charge exchange mechanism involving metal-enriched burst winds and accretion disks.

Findings

Detected a broad 6.6 keV emission feature with large equivalent width.

The feature's intensity decreased exponentially over about 1 hour.

The emission is consistent with gravitationally redshifted charge exchange from metals.

Abstract

We discover an unidentified strong emission feature in the X-ray spectrum of EXO 1745$-$248 obtained by RXTE at 40 hr after the peak of a superburst. The structure was centered at 6.6 keV and significantly broadened with a large equivalent width of 4.3 keV, corresponding to a line photon flux of 4.7 $\times$ 10$^{-3}$ ph cm$^{-2}$ s$^{-1}$. The 3-20 keV spectrum was reproduced successfully by a power law continuum with narrow and broad (2.7 keV in FWHM) Gaussian emission components. Alternatively, the feature can be described by four narrow Gaussians, centered at 5.5 keV, 6.5 keV, 7.5 keV and 8.6 keV. Considering the strength and shape of the feature, it is unlikely to have originated from reflection of the continuum X-rays by some optically thick materials, such as an accretion disk. Moreover, the intensity of the emission structure decreased significantly with an exponential time scale of 1 hr. The feature was not detected in an INTEGRAL observation performed 10 h before the RXTE observation with a line flux upper limit of 1.5 $\times$ 10$^{-3}$ ph cm$^{-2}$ s$^{-1}$. The observed emission structure is consistent with gravitationally redshifted charge exchange emission from Ti, Cr, Fe, and Co. We suggest that the emission results from a charge exchange interaction between a highly metal-enriched fall back ionized burst wind and an accretion disk, at a distance of $\sim$60 km from the neutron star. If this interpretation is correct, the results provide new information on the understanding of nuclear burning processes during thermonuclear X-ray bursts.