Dark Matter Search Using Chandra Observations of Willman 1, and a Spectral Feature Consistent with a Decay Line of a 5 keV Sterile Neutrino
📝 Abstract
We report the results of a search for an emission line from radiatively decaying dark matter in the Chandra X-ray Observatory spectrum of the ultra-faint dwarf spheroidal galaxy Willman 1. 99% confidence line flux upper limits over the 0.4-7 keV Chandra bandpass are derived and mapped to an allowed region in the sterile neutrino mass-mixing angle plane that is consistent with recent constraints from Suzaku X-ray Observatory and Chandra observations of the Ursa Minor and Draco dwarf spheroidals. A significant excess to the continuum, detected by fitting the particle-background-subtracted source spectrum, indicates the presence of a narrow emission feature with energy 2.51 +/- 0.07 (0.11) keV and flux [3.53 +/- 1.95 (2.77)] X 10^(-6) photons/cm^2/s at 68% (90%) confidence. Interpreting this as an emission line from sterile neutrino radiative decay, we derive the corresponding allowed range of sterile neutrino mass and mixing angle using two approaches. The first assumes that dark matter is solely composed of sterile neutrinos, and the second relaxes that requirement. The feature is consistent with the sterile neutrino mass of 5.0 +/- 0.2 keV and a mixing angle in a narrow range for which neutrino oscillations can produce all of the dark matter and for which sterile neutrino emission from the cooling neutron stars can explain pulsar kicks, thus bolstering both the statistical and physical significance of our measurement.
💡 Analysis
We report the results of a search for an emission line from radiatively decaying dark matter in the Chandra X-ray Observatory spectrum of the ultra-faint dwarf spheroidal galaxy Willman 1. 99% confidence line flux upper limits over the 0.4-7 keV Chandra bandpass are derived and mapped to an allowed region in the sterile neutrino mass-mixing angle plane that is consistent with recent constraints from Suzaku X-ray Observatory and Chandra observations of the Ursa Minor and Draco dwarf spheroidals. A significant excess to the continuum, detected by fitting the particle-background-subtracted source spectrum, indicates the presence of a narrow emission feature with energy 2.51 +/- 0.07 (0.11) keV and flux [3.53 +/- 1.95 (2.77)] X 10^(-6) photons/cm^2/s at 68% (90%) confidence. Interpreting this as an emission line from sterile neutrino radiative decay, we derive the corresponding allowed range of sterile neutrino mass and mixing angle using two approaches. The first assumes that dark matter is solely composed of sterile neutrinos, and the second relaxes that requirement. The feature is consistent with the sterile neutrino mass of 5.0 +/- 0.2 keV and a mixing angle in a narrow range for which neutrino oscillations can produce all of the dark matter and for which sterile neutrino emission from the cooling neutron stars can explain pulsar kicks, thus bolstering both the statistical and physical significance of our measurement.
📄 Content
arXiv:0912.0552v3 [astro-ph.HE] 12 May 2010 UCLA/09/TEP/57 Dark Matter Search Using Chandra Observations of Willman 1, and a Spectral Feature Consistent with a Decay Line of a 5 keV Sterile Neutrino Michael Loewenstein1,2, Alexander Kusenko3,4 ABSTRACT We report the results of a search for an emission line from radiatively decay- ing dark matter in the Chandra X-ray Observatory spectrum of the ultra-faint dwarf spheroidal galaxy Willman 1. 99% confidence line flux upper limits over the 0.4-7 keV Chandra bandpass are derived and mapped to an allowed region in the sterile neutrino mass-mixing angle plane that is consistent with recent con- straints from Suzaku X-ray Observatory and Chandra observations of the Ursa Minor and Draco dwarf spheroidals. A significant excess to the continuum, de- tected by fitting the particle-background-subtracted source spectrum, indicates the presence of a narrow emission feature with energy 2.51 ± 0.07(0.11) keV and flux [3.53 ± 1.95(2.77)] × 10−6 photons cm−2 s−1 at 68% (90%) confidence. Inter- preting this as an emission line from sterile neutrino radiative decay, we derive the corresponding allowed range of sterile neutrino mass and mixing angle using two approaches. The first assumes that dark matter is solely composed of sterile neutrinos, and the second relaxes that requirement. The feature is consistent with the sterile neutrino mass of 5.0 ± 0.2 keV and a mixing angle in a nar- row range for which neutrino oscillations can produce all of the dark matter and for which sterile neutrino emission from the cooling neutron stars can explain pulsar kicks, thus bolstering both the statistical and physical significance of our measurement. 1Department of Astronomy, University of Maryland, College Park, MD. 2CRESST and X-ray Astrophysics Laboratory NASA/GSFC, Greenbelt, MD. 3Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA 4Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa, Chiba 277- 8568, Japan – 2 – 1. Introduction 1.1. Context None of the Standard Model particles can account for the dark matter that makes up most of the mass in the universe. We report the latest results of a search for dark matter in the form of relic sterile neutrinos (see Kusenko 2009 for an up-to-date review). The motiva- tion for considering the sterile neutrino, one of a number of feasible dark-matter candidates, is two-fold. First, the discovery of ordinary neutrino masses is most easily accommodated by means of the so-called seesaw mechanism, which calls for some new gauge-singlet fermions. If all of these fermions have very large Majorana masses, there are no additional degrees of freedom (particles) at the low energy scale. However, if one of these mass parameters lies in the 1–30 keV range, the corresponding sterile neutrino can be the long sought-after dark matter particle (Dodelson & Widrow 1994). Second, the physics of supernovae, that often assists in ruling out hypothetical low-mass particles, provides some intriguing clues in favor of the existence of sterile neutrinos with the same parameters that are required to explain dark matter. Such particles would be anisotropically emitted from a cooling newly born neutron star, inducing a sufficient recoil momentum in the neutron star to explain the observed velocities of pulsars (Kusenko & Segr`e 1997; Fuller et al. 2003). X-ray obser- vations offer the best probe of this well-motivated dark-matter candidate (Kusenko 2009). In the early universe, sterile neutrinos may be produced through non-resonant oscillations (Dodelson & Widrow 1994), resonant oscillations (Shi & Fuller 1999) and via various other channels (Kusenko 2006; Shaposhnikov & Tkachev 2006; Petraki & Kusenko 2008). The ki- netic properties of dark matter relevant for structure formation on small scales (i.e., how “warm” or “cold” the dark matter is) depend on the production scenario, as well as the particle mass (Kusenko 2006; Petraki 2008; Boyanovsky 2008b). The generic prediction is that relic sterile neutrinos must decay into a lighter neutrino and a photon (Pal & Wolfenstein 1982). Since this two-body decay of a 1–30 keV mass particle produces a narrow line with energy Eγ = mstc2/2, X-ray astronomy provides a unique opportunity to discover these relic particles. Archival X-ray data has been used to set limits on relic sterile neutrinos (see the review in Kusenko 2009). In addition, a dedicated search for sterile neutrinos using the Suzaku X-ray telescope has recently been conducted by Loewenstein, Kusenko, & Biermann (2009) (hereafter, LKB). Here we will present new results from the search using the Chandra X-ray Observatory. Our analysis of data on the Willman 1 dwarf spheroidal galaxy yields evidence of a 2.5 keV line, consistent with the decay of a relic sterile neutrino with mass 5 keV; and, with an inferred line strength consistent with that expected if all of the dark matter is composed of sterile neutri
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