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

## TL;DR

This study searches for a 3.5 keV emission line in deep Chandra X-ray observations, finding marginal evidence that could suggest sterile neutrino decay, but remains inconclusive due to statistical limitations.

## Contribution

First systematic search for the 3.5 keV line in deep Chandra data, providing constraints on sterile neutrino dark matter and discussing potential astrophysical origins.

## Key findings

- Marginal 3.5 keV line detection at 2.5-3 sigma significance.
- Line flux consistent with previous detections in Galactic center.
- Results support sterile neutrino decay as a possible origin.

## Abstract

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 $\Delta\chi^2$ 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.