Suzaku search for evidence of sterile neutrinos in the X-ray spectra of dwarf spheroidal galaxies Michael Loewenstein, Alexander Kusenko (UCLA), Peter Biermann (MPIfr, UAla)

Meet the Sterile Neutrino Right-handed (sterile) neutrinos may be introduced in extensions of the Standard Model that account for neutrino masses.  mix Sterile ’s may be thermally produced via neutrino oscillations in the early universe at a rate that depends on the active-sterile mixing angle,  mix. m ster- Dodelson & Widrow (1994) showed that sterile ’s could be produced in this manner in sufficient numbers to account for dark matter if m ster- ~1 keV.  mix. There are alternative production scenarios that decouple or alter the dependence of the rate on  mix.

Astrophysical Context (Warm) dark matter in the form of the keV sterile neutrino could resolve discrepancies between CDM and small scale structure. Sterile ’s can explain pulsar kicks: magnetic field  polarization- dependent scattering  asymmetric momentum distribution  They may also facilitate primordial star formation. They must not wipe out small scale structure seen in the Ly  forest.

Motivation for this project  Sterile neutrinos decay into an active neutrino and an X-ray photon (e=m/2) -- producing an emission line amenable to X-ray spectroscopic investigation.  Detection and measurement of such a feature would point the way to physics beyond the standard model, and enable one to map out the distribution of dark matter using X-ray spectroscopy.  There’s an interesting regime in which sterile neutrino radiative decay may be detectable by X-ray instruments presently in orbit.

Motivation for this project

Where to look, What to look with Target selection: maximize dark matter surface density and minimize other X-ray emitting components. Local group dwarf spheroidals are ideal: X-ray dark, (no hot gas, AGN, X-ray binaries) nearby concentrations of dark matter with just enough stars to measure their distance and total mass.

Dwarf spheroidals are… old and metal-poor... …and DM-dominated

Where to look, What to look with Need imaging (in case there are other sources in the field) X-ray spectroscopy with the best possible spectral resolution and lowest background to optimize sensitivity to weak emission lines. Because Suzaku XIS has the lowest background (orbit, design), and the sharpest spectral resolution among X-ray CCD detectors (dispersive spectrometers optimized for point sources) currently in orbit, it is the observatory of choice to search for weak lines from extended sources. Suzaku has three operational co-aligned detector/telescope pairs…

Where to look, What to look with Of the LG dSphs, the Ursa Minor and Draco systems are optimal based on their distances and large dark matter densities derived from stellar kinematics. In our pilot Suzaku Cycle 2 program, we observe the Ursa Minor and Draco dwarf spheroidal galaxies for ~ 67 ksec each (200 ksec of data). M/L L  

Ursa Minor (DSS)

How to look -- spectral analysis Extract spectra from cleaned photon event file in source-free region of all three active chips Generate spectral response matrices, efficiency curves

How to look -- spectral analysis Subtract NXB derived from night-earth data

How to look -- spectral analysis Fit to GXB+CXB with constraints from RASS  Derive upper limits to line fluxes when an extra emission line component is added to the spectral model, using Monte Carlo simulations to identify confidence levels

What we’ve found

Limits and what to do with them Using the Ursa Minor mass profile (Strigari et al.), convert flux limit to decay rate limit at each X-ray energy (=m ster- /2)… Using the Ursa Minor mass profile (Strigari et al.), convert flux limit to decay rate limit at each X-ray energy (=m ster- /2)… … which maps onto an excluded region in m ster- -  (mixing angle) plane -- that is more restrictive than previous limits… … which maps onto an excluded region in m ster- -  (mixing angle) plane -- that is more restrictive than previous limits…

Improved constraints The solid excluded region applies even if sterile neutrinos constitute a fraction of dark matter. The preliminary Suzaku Ursa Minor constraints shift this limit to the solid thick red line, thus eliminating a substantial portion of the (hatched) pulsar kick region. The thin dashed line shows the pre-Suzaku X-ray bound assuming that sterile neutrinos make up all the dark matter; this shifts to the broken red line.

What’s New, What’s NeXT Draco: a different approach (using maximum likelihood on the unsubtracted, unbinned data) yields preliminary results comparable to those for Ursa Minor (out to higher energies) … Draco: a different approach (using maximum likelihood on the unsubtracted, unbinned data) yields preliminary results comparable to those for Ursa Minor (out to higher energies) …

What’s New, What’s NeXT The Sloan Digital Sky Survey (SDSS) has doubled the number of know dwarf spheroidal galaxies by discovering ultra faint dwarfs, and implies that there may be ~1000 within the Milky Way halo (Tollerud et al., Bovill and Ricotti). The Sloan Digital Sky Survey (SDSS) has doubled the number of know dwarf spheroidal galaxies by discovering ultra faint dwarfs, and implies that there may be ~1000 within the Milky Way halo (Tollerud et al., Bovill and Ricotti). These are promising future targets. These are promising future targets.

What’s New, What’s NeXT

Summary  Our preliminary results for Ursa Minor (and Draco) demonstrate that, with Suzaku, we can extend the explored region of sterile neutrino parameter space into a new regime that will test some theories for pulsar kicks.  We are in the final analysis stage, and applying new statistical techniques. We expect to rule out some production mechanisms under the assumption that sterile neutrinos are the dominant constituent of dark matter (or discover evidence for their existence!).

Coda   Future observations with X-ray calorimeters (e.g., SXS on NeXT) will either detect emission lines from sterile neutrino radiative decay, or definitively imply a low abundance and/or mixing angle.   Sterile neutrinos are a well-motivated dark matter candidate, but one with mass and relic abundance poorly determined a priori. As long as dark matter remains unidentified, we intend to continue searching the X-ray sky…