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