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

Kavli Affiliate: Eric Miller

| First 5 Authors: Nico Cappelluti, Esra Bulbul, Adam Foster, Priyamvada Natarajan, Megan C. Urry

| Summary:

In this paper 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 $sim$10 Ms
Chandra observations towards the COSMOS Legacy and CDFS survey fields. We find
a marginal evidence of a feature at an energy of $sim$3.51 keV with a
significance of 2.5-3 $sigma$, depending on the choice of the statistical
treatment. The line intensity is best fit at $8.8 pm {2.9}times10^{-7}$ ph
cm$^{-2}$s$^{-1}$ when using a simple $Deltachi^2$ or $10.2 ^{+0.2}_{-0.4}
times10^{-7}$ ph cm$^{-2}$s$^{-1}$ when MCMC 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 though rule out a
statistical fluctuation and in that case our results provide a 3$sigma$ upper
limit at 1.85$times$10$^{-6}$ ph cm$^{-2}$s$^{-1}$. We discuss the
interpretation of this observed line in terms of the iron line background; S
{sc XVI} charge exchange as well as potentially 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
an NFW profile. For this case, we estimate a mass m$_{nu}sim$7.01 keV and a
mixing angle sin$^2$(2$theta$)= 0.83–2.75 $times 10^{-10}$. These derived
values are in agreement with independent estimates from galaxy clusters; the
Galactic center and M31.

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