dark matter searches with balloons and -ray telescopes ullrich schwanke (humboldt university,...
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Dark Matter Searches with Balloons and -ray Telescopes
Ullrich Schwanke (Humboldt University, Berlin)
Workshop on Exotic Physics with Neutrino Telescopes, Uppsala, 2006
Indirect Dark Matter Searches Search for positron and antiproton
signals• The HEAT balloon experiment
Gamma-ray Astronomy• 511 keV annihilation line (Integral)
• Diffuse galactic gamma-ray emission (EGRET)
• {Extragalactic gamma-ray background (EGRET)}
• Gamma-rays from the Galactic centre (H.E.S.S.)
Summary and Outlook
Overview
Indirect Searches
Extraterrestrial sources. Detection in orbit/atmosphere. Potentially large amount of DM (~entire Universe). Competition from less exotic production mechanisms. Modelling of Milky Way required.
Antiprotons
• Propagation effects
• Expect energy spectrum with cut-off at mass of DM particle
Positrons
• lower range than antiprotons
Gammas
• Directional information can be correlated with (dark) matter density in the Milky Way
• Gamma-line(s) would be unique signature.GLAST Simulation
Search for Antiprotons and Positrons
HEAT
1987
Historic claims for a sizable fraction of positrons/antiprotons in the cosmic radiation Experimental challenge: small fraction of e+/p-, wealth of background with opposite charge Good particle ID required
BESS, CAPRICE, High-Energy Antimatter Telescope, ...
BESS
HEAT Positron Fraction
1987
Measured by two different detectors (HEAT-e and HEAT-pbar) Near solar maximum (1994 and 1995) and solar minimum (2000) Different vertical geomagnetic cutoffs: ~1 GeV (1995) and ~4 GeV
(1994, 2000) Statistical significance ? Interpretation ?
HEAT-pbar
Interpretation of the Positron Fraction
D. Hooper, hep-ph/0409272
Neutralino DM
• inefficient generation of positrons
• increase annihilation rate by clumping
Kaluza-Klein Dark Matter
• viable positron source for mass range 300..400 GeV
(Annihilation rate normalized to data)
e+ diffusion parameters
Antiproton Fraction and Flux
1987
Some claimed excesses in the past Now, measurements seem to be
consistent with purely secondary production of antiprotons
Expect BESS 2004 data (factor ~10 longer flights)
Primary antiproton flux from annihilation of a 964 GeV MSSM neutralino (P. Ullio, astro-ph/9904086 (1999))
Gamma-Ray Telescopes
Soft -rays: < 1 MeV
IntegralHigh energy -rays: 10 MeV – 100 GeVEGRET, GLAST
Very high energy -rays:
> 100 GeVAir-CherenkovTelescopesH.E.S.S.Whipple/VeritasMAGICCANGAROO
Galactic 511 keV Annihilation Line
Instrument Year Flux
(10-3 cm-2 s-1)
Centroid (keV)
Width (keV)
HEAO-3 79-80 1.130.13 510.920.23 1.6+0.9-1.6
GRIS 88 and
92
0.880.07 2.50.4
HEXAGONE 89 1.000.24 511.330.41 2.90+1.10-1.01
TGRS 95-97 1.070.05 510.980.10 1.810.54
• Accurate tracer of galactic positrons.
• Thermalization of positrons required.
• Various detections since initial discovery in 1973.
• Agreement on absolut flux, no time dependence
• Morphology less clear (halo + galactic disk component)
e+e-
Latest Data: Integral and SPI
launched in Oct 02
SPectromètre Integral 16° FoV (FWHM) 20 keV – 10 MeV 2 keV energy
resolution (at 1 MeV) 2° angular resolution
Observations of the Galactic Centre
Measurement relies on accurate subtraction of instrumental annihilation line
Flux and intrinsic line width compatible with earlier measurements
20
Energy (keV)
Exposure
Source Morphology
0.00 photons/cm2/s/sr 0.04
DIRBE
Gaussian with FWHM=9°
15 kpc
Interpretations (I)
Conventional Interpretations Supernovae Wolf-Rayet Stars Neutron stars, pulsars Cosmic rays ...and (of course) Black
holes Dark Matter Interpretation
C. Boehm et al., astro-ph/0309686
F. Beacom et al., astro-ph/0409403
Contributionfrom disk expected, i.e. smaller bulge-to-disk ratio (Integral: B/D > 0.3..0.5)
Interpretations (II) DM annihilation occurs
„invisibly“ Light (1-100 MeV) scalar
DM particles Exchange particle could be
fermion (with suppressed Z couplings) or new gauge boson („U boson“)
Correct DM relic density is obtained
Caveat: COMPTEL and EGRET data require m<20 MeV when internal bremsstrahlung is taken into account
Cannot be excluded....
Flu
x()
/Flu
x(0)
r
1
Soft -rays: < 1 MeV
IntegralHigh energy -rays: 10 MeV – 100 GeVEGRET, GLAST
Very high energy -rays:
> 100 GeVAir-CherenkovTelescopesH.E.S.S.Whipple/VeritasMAGICCANGAROO
Gamma-Ray Telescopes
Diffuse Gamma-Ray Emission
CGRO (1991-2000)
EGRET 20 MeV – 30 GeV energy resolution 20% angular resolution:
1.3° at 1 GeV 0.4° at 10 GeV
EGRET Gamma-Ray Data
Subtraction of 271 EGRET point sources Diffuse galactic gamma-ray emission remains Excess observed from all directions Right now, EGRET data (and more) can be described by scenarios with and without DM
S. D. Hunter et al.
Astrophys. J. 481, 205 (1997)
1) Solution without DM: Strong, Moskalenko & Reimer, Astrophys. J. 613, 962 (2004)
2) Solution with DM: W. de Boer et al., A&A 444 (2005) 51.
GALPROP: Cosmic Ray Propagation• radiation field
• nuclear reaction networks
• spatial distribution of sources
• energy spectra at injection
• solar modulation
Model
Geometry Diffusion
1) Solution without Dark Matter
(30.5°<l<179.5°, 180.5°<l<330.5°)0 decay
Inverse Compton
1.0-2.0 GeV
Bremsstrahlung
Extragalactic Gamma-RayBackground
GALPROP: Numeric evaluation of Diffusion-Loss-Equations. Obtains (anti)proton and electron/positron spectra, too. -ray data can be described fairly well, albeit at the expense of a slightly worse matching of the local electron and proton spectra
2) Solution with Dark Matter
Explains EGRET data with a photon component from neutralino annihilation Gets background shape from GALPROP, signal shape from SUSY generators Determines halo structure, needs two rings of stars around Milky Way Locates WIMP mass in 50-100 GeV range DM signal compatible with supersymmetry for boost factors of ~20
(-30°<l<+30°) E>0.5 GeV
Neutralino annihilation
Backgrounds
Soft -rays: < 1 MeV
IntegralHigh energy -rays: 10 MeV – 100 GeVEGRET, GLAST
Very high energy -rays:
> 100 GeVAir-CherenkovTelescopesH.E.S.S.Whipple/VeritasMAGICCANGAROO
Gamma-Ray Telescopes
Performance
Trigger threshold: 40 – 100 GeV
Angular resolution is a few arcminutes (~0.1°, stereo)
Collection area: 50000 m2
Relative energy resolution ~20%
Factor 102 improved sensitivity
Duty cycle: 1000 h per year
H.E.S.S.EGRET
The Crab Nebula
30 sec
1 night
1 yearCas A2002
Crab1989
H.E.S.S. 2004
H.E.S.S.Field of View (5°)
Observations of the Galactic Centre
The Dynamical Centre:Sgr A*
3 106 solar mass black hole Very low luminosity Highly variable non-thermal
emission in IR and X-ray Surrounded by supernova-
remnant Sgr A East and H II region Sgr A West
MPE / R. Genzel et al.
Sgr A East
Sgr A*
3‘
H.E.S.S. Result (2003) 17 hours of data Taken with 2
telescopes during construction of the array
160 GeV threshold 11 signal from close
to Sgr A* Point-like source See A&A 425, L13-16
(2004)
G0.9 ist SNR mit Pulsarwind-Nebel
Starkes Signal vom Galaktischen Zentrum
G0.9+0.1
HESS J1745-290
H.E.S.S. Result (2004)
Position
Chandra GC surveyNASA/UMass/D.Wang et al.
CANGAROO (80%)
Whipple
(95%)
H.E.S.S.
Contours from Hooper et al. 2004
Chandra GC surveyNASA/UMass/D.Wang et al.
CANGAROO (80%)
Whipple
(95%)
H.E.S.S. (95%)
Position: Compatible with Sgr-A*
H.E.S.S.
ChandraF. Banagoff et al.
95%
68%
Energy Spectrum
HESS:dN/dE E-2.2
Flux > 160 GeV:
5 % of Crab flux
CANGAROO:
dN/dE E-4.6
Flux > 160 GeV:
~ 1 Crab
H.E.S.S. 2004 Data Two years of data: 40 h with full 4 telescope array
Significance of HESS J1745-290 is 35
Position, flux and spectrum (~2.3) compatible
No Variability on scales of• Years• Months• Days• Minutes
Interpretations of the TeV Signal from the Galatic Centre
1) Particle Acceleration near the Black Hole Sgr A*: F. Aharonian & A. Neronov, astro-ph/0408303 (2004); Atoyan & Dermer, astro-ph/0401243 (2004).
2) Particle Acceleration in the supernova remnant Sgr A East: Crocker et al. astro-ph/0408183 (2004)
3) Dark Matter Annihilation: D. Horns, astro-ph/0408192; Bergström et al., astro-ph/0410359
1) Particle Acceleration close to Sgr A*
Low luminosity of Sgr A* ~10 TeV photons can escape without e+e- conversion
There is evidence that Sgr A* is spinning at a good fraction of the maximum possible speed.
Rotation in a magnetic field produces a strong electro-motoric force
Acceleration of protons to 1018 eV (?)
• VHE gamma-rays via curvature radiation or hadronic interactions
Acceleration of electrons (?)
• TeV Gamma-rays via Inverse Compton Scattering
• More efficient than proton acceleration Or acceleration at shocks in the accretion disk
• TeV radiation via: p + p +/-,
VHE -rays from Sgr A* ?Aharonian et al. 2004
Log E (eV) Data can be explained as radiation of accelerated protons… or
electrons close (<10 Rg) to Sgr A* Absence of variability does not support BH origin of -rays
2) Particle Acceleration in Sgr A East
Spectral index measured by H.E.S.S. close to expectation from Fermi acceleration
Sgr A East is a powerful SNR• 10,000 years old
• Compact (~3 arcmins)
• Energy: 4 x 1052 erg
Crocker et al. explain overabundance of cosmic rays from the GC around 1018 eV• Flux normalization from H.E.S.S. (or a nearby EGRET source)
under the assumption of pp induced decay
• Explains particle acceleration up to the ankle (3 1018 eV)
• Note: SUGAR/AGASA CR anisotropies are constrained by AUGER data
Association with CR Anisotropy?
Crocker et al 2004, astro-ph/0408183
H.E.S.S.
EGRET
AGASA (1018 eV)
Log (E/eV)
Log (
dF/
dE /
cm
-2 s
-2 e
V-1)
Fit
n+Xp+p 0+X
No indication for CR anisotropies in AUGER data, but plausible explanation for H.E.S.S. data
3) DM Interpretation: Spectrum
Wimp annihilationspectra have a cutoff at ~(0.2…0.3) M
• CANGAROO Spectrum consistent with a 1.1 TeV neutralino-type WIMP
• HESS Spectrum requires a mass > 12 TeV
• Most models favour a < 2 TeV WIMP
• Requires high DM density and/or cross section
• Kaluza-Klein DM requires large boost factors (>103)
• DM interpretation is stretched further by H.E.S.S. 2004 data
Summary and Outlook
There is no WIMP that can explain more than one measurement….
For antiprotons and positrons, future space-borne experiments (AMS02, Pamela) will do a lot better than balloon experiments.
511 keV line: Interpretation? Galactic Centre region
• Multi-wavelength approach
• Continue identifying and subtracting conventional sources
GLAST (5/2007) and low-threshold IACTs will provide improved sensitivity below 100 GeV
• Search for gamma-lines and continuum.
• Observation of other DM candidates (e.g. dwarf galaxies orbiting the Milky Way)
GLAST
PAMELA
Extragalactic -ray background (EGB)
Various contributions: Seyfert galaxies, quasars, type 1a supernovae... Re-analysis of complete EGRET data set found that galactic background (from GALPROP) was underestimated (i.e. EGB overestimated)
1 GeV
1998
EGB and Dark Matter
Re-analysis of complete EGRET data set, GALPROP for foreground subtraction D. Elsässer and K. Mannheim, PRL 94, 171302 (2005) Annihilation of a MSSM neutralino in NFW-type DM halos Integration from z=20 to present, factor 106 enhance due to structure formation
Gaugino-line WIMP with mass 515+110-75 GeV
Caveats S. Ando, PRL 94, 171303 (2005) Assume universality of halo density profile, same WIMP mass and cross-section Use EGRET, H.E.S.S. and CANGAROO galactic centre data as upper limit on the neutralino flux and predict EGB If DM component in EGB is real, it would imply a much higher flux from the Galactic Centre 515 GeV WIMP is at odds with DM interpretation of galactic EGRET data