Abstract
An unidentified 3.5 keV line from X-ray observations of galaxy clusters has been reported recently. Although still under scrutiny, decaying dark matter could be responsible for this signal. We investigate whether an axino with a mass of 7 keV could explain the line, keeping the discussion as model independent as possible. We point out several obstacles, which were overlooked in the literature, and which make the axino an unlikely candidate. The only viable scenario predicts a light metastable neutralino, with a mass between 0.1 and 10 GeV and a lifetime between 10−3 and 104 s.
Highlights
There has been interest recently in an unidentified 3.5 keV line in X-ray observations of galaxy clusters [1,2]
Despite its physical origin being subject to debate [3,4,5,6,7,8], there is still some room to speculate that decaying dark matter is responsible for this signal
As both the axino and the gravitino are in the keV range and below, one might worry about their contribution to the relativistic degrees of freedom at Big Bang Nucleosynthesis (BBN)
Summary
There has been interest recently in an unidentified 3.5 keV line in X-ray observations of galaxy clusters [1,2]. Its lifetime is still longer than the age of the universe, but it can decay into a neutrino and a photon. It is this channel that would produce the 3.5 keV line. The axino could produce an X-ray photon in R-parity conserving SUSY. This is possible if the lightest neutralino is very light or even massless.
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