1. The Detection of an Unexplained Emission Line at 3.56 keV Esra Bulbul Harvard-Smithsonian Center for Astrophysics Maxim Markevitch (NASA/GSFC), Adam Foster (CfA), Randall Smith (CfA), Mike Loewenstein (NASA/GSFC), and Scott Randall (CfA)

2. Line Searches in X-ray Observations of Clusters  Largest gravitationally bound aggregations of hot ICM and DM  Gas is enriched mainly via SN explosions by metals produced by stars in galaxies  Weak emission lines are now being discovered through X-ray spectroscopy Werner et al. (2006) 2A 0335+096

3. Why Stacking?  73 galaxy clusters at their rest frame  Increased S/N  0.01 < z < 0.35  Smeared non-source features, e.g. instrumental, background XMM-Newton Stamp Images of 73 Galaxy Clusters

4. Background Before Stacking

5. Background After Stacking

6. Detection of An Unidentified Emission Line Flux: 4.0 ± 0.8 x 10 -6 phts cm -2 s -1 Energy: XMM - MOS Full Sample 6 Ms XMM - MOS Full Sample 6 Ms

7. with an Additional Gaussian Model… XMM - MOS Full Sample 6 Ms XMM - MOS Full Sample 6 Ms

8. Detected in Independent PN Observations XMM - PN Full Sample 2 Ms XMM - PN Full Sample 2 Ms

9. Detected in Fainter 69 Clusters XMM - PN Full Sample 1.8 Ms XMM - PN Full Sample 1.8 Ms

10. Chandra ACIS-S 883 ks Detected in Chandra Observations of the Perseus cluster

11. XMM-Newton MOS 317 ks Detected in XMM Observations of the Perseus cluster

12. Confirmation Boyarsky et al. (2014a, b) detected the line  independent XMM OFFSET observations of the Perseus cluster  M31  Milky Way Energy Normalized Count Rate cnts/keV/s

13. Unlikely Astrophysical Scenarios  K XVIII line at 3.52 keV Atomic Database Estimates of K XVIII line at 3.52 keV and Ar XVII DR line at 3.62 keV

14. Unlikely Astrophysical Scenarios  K XVIII line at 3.52 keV  Ar XVII DR line at 3.62 keV Atomic Database Estimates of K XVIII line at 3.52 keV and Ar XVII DR line at 3.62 keV

15. Unlikely Astrophysical Scenarios  K XVIII line at 3.52 keV  Ar XVII DR line at 3.62 keV Atomic Database Estimates Ar XVII DR line at 3.62 keV Based on Ar XVII triplet at 3.12 keV

16. Unlikely Astrophysical Scenarios  K XVIII line at 3.52 keV  Ar XVII DR line at 3.62 keV  Emission lines of strong hydrogen- and helium-like ions Atomic Database Estimates of K XVIII line at 3.52 keV and Ar XVII DR line at 3.62 keV

17. Unlikely Astrophysical Scenarios  K XVIII line at 3.52 keV  Ar XVII DR line at 3.62 keV  Emission lines of strong hydrogen- and helium-like ions  Charge exchange Atomic Database Estimates of K XVIII line at 3.52 keV and Ar XVII DR line at 3.62 keV

18. Unlikely Astrophysical Scenarios  K XVIII line at 3.52 keV  Ar XVII DR line at 3.62 keV  Emission lines of strong hydrogen- and helium-like ions  Charge exchange  Radiative recombination continuum (RRC) feature Atomic Database Estimates of K XVIII line at 3.52 keV and Ar XVII DR line at 3.62 keV

19. Unlikely Astrophysical Scenarios  K XVIII line at 3.52 keV  Ar XVII DR line at 3.62 keV  Emission lines of strong hydrogen- and helium-like ions  Charge exchange  Radiative recombination continuum (RRC) feature  Any Other Astrophysical Explanations? Atomic Database Estimates of K XVIII line at 3.52 keV and Ar XVII DR line at 3.62 keV

20. What is the origin?  Could this be a sterile neutrino decay signature?  Warm dark matter candidate sterile neutrinos decay into an active neutrino and emission line  Neutrino Properties Mass = 2E γ Mixing angle Mass (keV) The diffuse X-ray background (Boyarsky et al. 2006), cluster X-ray (Boyarsky et al. 2006b), BMW (Boyarsky et al. (2007), M31 (Watson et al. 2006), the Tremaine-Gunn bound (Bode et al. 2001), and Fornax dwarf galaxy (Strigari et al. 2006)

21. Sterile Neutrino vs Astrophysical Line? Astro-H Perseus 1 Ms ν bar = 300 km s -1 ν DM = 1300 km s -1

22. Sterile Neutrino Abundance  Sterile neutrinos constitutes ∼ 19% of the dark matter (Shi & Fuller 1999)  may also be produced by means that do not involve oscillations, such as inflaton or Higgs decay (Kusenko 2006; Petraki & Kusenko 2008). This detection is consistent with 100% of dark matter composed of sterile neutrinos produced by these mechanisms.  If generated via split seesaw mechanism, this detection indicates that 100% of dark matter composed of sterile neutrinos (Kusenko, Takahashi, & Yanagida 2010).

23. If you do not want to wait for Astro-H Alternatives for the Confirmation  Stacked Suzaku and Chandra Observations of large number of galaxy clusters – NASA/ADAP grant (PI: Bulbul)  Deep observations of dwarf spheroidal galaxies  Stacked observations of more Local Group galaxies (cannot nail it if it is a detector artifact)  Deep Chandra observations of the Galactic Center

24. Limits from Our Detections 10 -11 Mixing angle Mass (keV)

25. Bright Clusters  MOS Observations of the Bright Clusters (Coma, Perseus, Ophiuchus, Centaurus) Flux: phts cm -2 s -1  MOS Observations of the Perseus cluster phts cm -2 s -1  MOS Observations of the Perseus cluster (core cut) phts cm -2 s -1  not due to a dominant source in the sample, not a background or effective area feature Δχ 2 = 17.1 (1 dof) Δχ 2 = 15.7 (1 dof)