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

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 -1s2l 3l 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 (Ralchenko & Maron 2001) 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) (Foster et al. 2012; Smith et al. 2001) 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.

Decaying dark matter in dwarf spheroidal galaxies: Prospects for x-ray and gamma-ray telescopes

“…This claim triggered multiple follow-up papers, intense debates, reinvestigating the excess at the 3.5 keV, or also trying to explain it with different phenomena (e.g., [10][11][12][13][14][15][16][17][18][19]). A recent study [20] found some missing features in the originally used database near the unidentified line. The inclusion of these features raises the total flux value in the region around 3.5 keV but is still not enough to explain the line found in the stacked galaxy cluster spectra.…”

A Critical Compilation of Experimental Data on the 1s2$l$2$l^{\prime}$ Core-Excited States of Li-like Ions from Carbon to Uranium

“…When reading the literature on the core-excited levels of Li-like ions, one faces a difficulty with heterogeneous spectroscopic notation. In the early papers [7,8] dealing with moderately charged ions, a peculiar kind of LS -coupling scheme was used to designate the 1s2s 2 , 1s2p 2 , and 1s2s2p energy levels, which we will call Gabriel's scheme. For the 1s2p 2 configuration having two equivalent p-electrons it is impossible to distinguish between them, so the two p-electrons are combined first to produce an intermediate singlet ( 1 S or 1 D) or triplet ( 3 P) term, which is then combined with the 2 S term of the 1s shell to produce the final LS term.…”

“…Dielectronic satellite spectra of highly charged ions are an important source of spectroscopic data widely used for analyzing astrophysical plasmas as well as spectra from electron beam ion traps, tokamaks and laserproduced plasmas (see, e.g., [1][2][3][4]). Such spectra provide crucial information on the electron temperature and density, the ionization state distribution, and other characteristics of hot plasmas.…”

Probing the Milky Way's Dark Matter Halo for the 3.5 keV Line

“…Various non-dark matter, astrophysical explanations for the line have been discussed. Among these interpretations is contamination due to nearby K and Ar dielectric emission, both of which have been evaluated and subsequently ruled out using electron beam ion trap (EBIT) experiments (Bulbul et al 2019;Gall et al 2019;Weller et al 2019). A current leading interpretation is charge exchange (CX) between bare Sulfur ions and neutral Hydrogen, described by Bulbul et al (2014a), Gu et al (2015), Shah et al (2016), and others referenced therein.…”