# EBIT Observation of Ar Dielectronic Recombination Lines Near the Unknown Faint X-Ray Feature Found in the Stacked Spectrum of Galaxy Clusters

## TL;DR

This study used an electron beam ion trap to investigate argon dielectronic recombination lines near the 3.57 keV unidentified X-ray feature in galaxy clusters, finding missing atomic data in current models but concluding it doesn't fully explain the observed line.

## Contribution

The paper provides new EBIT measurements and identifies missing features in atomic databases, improving the understanding of atomic processes near the 3.57 keV line.

## Key findings

- Excellent agreement between NOMAD and EBIT spectra confirms atomic data accuracy.
- Missing atomic features in AtomDB increase flux near 3.57 keV by a factor of 2.
- The additional emission cannot fully account for the unidentified galaxy cluster line.

## Abstract

Motivated by possible atomic origins of the unidentified emission line detected at 3.55 keV to 3.57 keV in a stacked spectrum of galaxy clusters (Bulbul et al. 2014), an electron beam ion trap (EBIT) was used to investigate the resonant dielectronic recombination (DR) process in highly-charged argon ions as a possible contributor to the emission feature. The He-like Ar DR-induced transition 1s$^2$2l - 1s2l3l$^\prime$ was suggested to produce a 3.62 keV photon (Bulbul et al. 2014) near the unidentified line at 3.57 keV and was the starting point of our investigation. The collisional-radiative model NOMAD was used to create synthetic spectra for comparison with both our EBIT measurements and with spectra produced with the AtomDB database/Astrophysical Plasma Emission Code (APEC) used in the Bulbul et al. (2014) work. Excellent agreement was found between the NOMAD and EBIT spectra, providing a high level of confidence in the atomic data used. Comparison of the NOMAD and APEC spectra revealed a number of missing features in the AtomDB database near the unidentified line. At an electron temperature of $T_e$ = 1.72 keV, the inclusion of the missing lines in AtomDB increases the total flux in the 3.5 keV to 3.66 keV energy band by a factor of 2. While important, this extra emission is not enough to explain the unidentified line found in the galaxy cluster spectra.