---
author:
- 'F. Hofmann'
- 'J.S. Sanders'
- 'K. Nandra'
- 'N. Clerc'
- 'M. Gaspari'
bibliography:
- 'auto.bib'
- 'general.bib'
title: '7.1keV sterile neutrino constraints from X-ray observations of 33clusters of galaxies with *Chandra* ACIS'
---

Introduction {#sec:introduction}
============

@2014ApJ...789...13B and @2014PhRvL.113y1301B recently found indications for a weak unidentified emission line ($\rm{E \sim 3.55~keV}$) in X-ray CCD spectra of the Andromeda galaxy and in deep cluster observations using *Chandra* and **XMM-Newton** data. There is an ongoing discussion on the existence and possible nature of the line with studies using other instruments like the *Suzaku* observatory [@2015MNRAS.451.2447U] and looking at other objects like individual galaxies [e.g. @2015MNRAS.452.3905A]. The line has been proposed as a candidate for a dark matter (DM) decay line and could be explained by decay of sterile neutrinos with a mass of $\rm{m_s=7.1~keV}$. In this model they decay into an X-ray photon with $\rm{E_{\gamma}=m_s/2}$ and an active neutrino $\rm{\nu}$. Sterile neutrinos with masses in the keV range have long been discussed as a possible component of DM [e.g. @1994PhRvL..72...17D; @2001ApJ...562..593A; @2009ARNPS..59..191B], but up until recently only upper limits could be derived [e.g. from observations of the Andromeda galaxy or the Bullet Cluster by @2008MNRAS.387.1361B; @2008ApJ...673..752B].

In 