Searching for the 3.5 keV Line in the Deep Fields with Chandra: The 10 Ms Observations

We report a systematic search for an emission line around 3.5 keV in the spectrum of the cosmic X-ray background using a total of ∼10 Ms Chandra observations toward the COSMOS Legacy and Extended Chandra Deep Field South survey fields. We find marginal evidence of a feature at an energy of ∼3.51 keV with a significance of 2.5-3σ, depending on the choice of statistical treatment. The line intensity is best fit at (8.8 ±2.9) ×10^-7 ph cm^-2 s^-1 when using a simple Δχ ² or ph cm^-2 s^-1 when Markov chain Monte Carlo is used. Based on our knowledge of Chandra and the reported detection of the line by other instruments, an instrumental origin for the line remains unlikely. We cannot, however, rule out a statistical fluctuation, and in that case our results provide a 3σ upper limit at 1.85 ×10^-6 ph cm^-2 s^-1. We discuss the interpretation of this observed line in terms of the iron line background, S xvi charge exchange, as well as potentially being from sterile neutrino decay. We note that our detection is consistent with previous measurements of this line toward the Galactic center and can be modeled as the result of sterile neutrino decay from the Milky Way for the dark matter distribution modeled as a Navarro-Frenk-White profile. For this case, we estimate a mass m _ν ∼ 7.01 keV and a mixing angle sin²(2θ) = (0.83-2.75) ×10^-10. These derived values are in agreement with independent estimates from galaxy clusters, the Galactic center, and M31.