Indirect Dark Matter Searches with VERITAS

Indirect Dark Matter
Searches with VERITAS
Matthew Wood (UCLA) for the VERITAS Collaboration
Invisible Universe Conference 2009
July 1st, 2009
Outline
●
Motivation for Indirect Searches
●
IACT Technique and VERITAS Observatory
●
Results of VERITAS observations
●
Interpretation
●
–
Astrophysical Modeling
–
Constraints on WIMP Models
Conclusions
Invisible Universe 2009
Motivation: Indirect Dark Matter Searches with Gamma Rays
●
●
●
●
Weakly interacting massive particles (WIMPs) of mass 50 GeV – 10 TeV are a well­
motivated candidate for dark matter (DM)
GeV­TeV Gamma­rays produced as secondary products of WIMP self­annihilation
Ground­based Imaging Atmospheric Cherenkov Telescopes (IACTs) and space­
based observatories (Fermi) are sensitive in this energy regime
Complementary to accelerator (LHC) and direct searches (CDMS, XENON, etc.)
High Energy (HE)
VERITAS, H.E.S.S., MAGIC
EGRET, Fermi
10 MeV 100 MeV
1 GeV
Very High Energy (VHE)
10 GeV
100 GeV
Invisible Universe 2009
1 TeV
10 TeV
IACT Arrays
●
●
VHE gamma­rays interacting in the atmosphere initiate EM cascades that produce secondary Cherenkov light showers
IACT arrays use stereoscopic images of the Cherenkov light shower to
–
Reconstruct the gamma­ray energy and direction
–
Discriminate between gamma rays and the cosmic­ray background
Invisible Universe 2009
VERITAS
●
●
●
●
Array of four Imaging Atmospheric Telescopes based at Fred Lawrence Whipple Observatory in southern AZ, USA
Fall 2009
Optics: 12 meter diameter reflectors with ~106 m2 mirror area (350 hexagonal mirror facets) and f~1
Camera: 499 PMTs each with 500 MHz FADC for readout
Four telescope operations since April 2007
Invisible Universe 2009
VERITAS Performance
●
●
●
●
Energy Range: 100 GeV­
50 TeV
Sensitivity: ~1% Crab in 50 hours
Energy Resolution: 10­
15%
Angular Resolution: ~0.10 deg at 200 GeV
6 new VHE sources discovered
by VERITAS so far
Invisible Universe 2009
DM Annihilation Signal
Detector Cosmology/Particle Physics Astrophysics
J( , ): Line of sight integral over DM distribution normalized to c2RH
 1%: 1% integral Crab Nebula flux above 100 GeV
3% Crab
1% Crab
0.5% Crab
0.1% Crab
~5 hours
~50 hours
~200 hours
~lifetime of VERITAS
J~104 → 1% Crab flux for <σ v> ~ 10­26 cm3 s­1
J~103 → Detectable during lifetime of VERITAS observatory
Invisible Universe 2009
VERITAS DM Science Program
●
●
Dwarf Galaxies
–
Directly measureable DM mass profile from stellar kinematics
–
Intrinsically dark in VHE gamma rays
Local Group Galaxies/Globular Clusters
–
–
DM cusp formed through baryon­DM interactions could generate extremely large flux enhancement (> 103)
Difficult to use flux limits to constrain WIMP model parameter space
Invisible Universe 2009
Belokurov 2007
Dwarf Galaxy Observations
Significance []
Significance []
Ursa Minor Draco
Exposure 95% C.L. Flux UL [h]
at 1 TeV [m­2 s­1 TeV­1]
19.1
1.0 x 10­9
Ursa Minor 19.4
1.6 x 10­9
Willman I
14.4
2.6 x 10­9
Bootes I
15.4
1.5 x 10­9
Draco
Significance []
Significance []
Willman I Bootes I
Invisible Universe 2009
No Significant Detections
95% C.L. Upper limits on flux > 200 GeV at ~1% of the Crab Nebula
Local Group Galaxies
Exposure [h]
95% C.L. Flux UL at 1 TeV [m­2 s­1 TeV­1]
M32
10.9
1.5 x 10­9
M33
13.8
1.0 x 10­9
Invisible Universe 2009
Significance []
Significance []
M33 M32
Astrophysical Modeling
●
●
To derive constraints on WIMP models we need to know the LOS integral over the DM distribution (J) for each source
We assume that the DM distribution follows an NFW profile as motivated by CDM simulations
NFW
Navarro et al. 2004
Burkert
Invisible Universe 2009
Dwarf Galaxy Modeling: Stellar Kinematics
Constraints on
DM Halo Parameters
Line of Sight Velocities
Assumptions:
Spherical Symmetry
Dynamical Equilibirum
NFW Profile
Martinez et al. 2009
Wilkinson et al. 2004
D [kpc]
rs [kpc]
 s [Msun kpc­3]
J
Draco
80
0.79
4.5 x 107
4
Ursa Minor
66
0.71
3.4 x 107
7
Willman I
38
0.16
4.2 x 108
22
Invisible Universe 2009
Parameterization
from Strigari et al. 2007, 2008
Dwarf Galaxy Modeling: Substructure Boost Factor
●
●
●
DM substructure is a generic prediction of CDM simulations
Posterior Probability Density for Gamma­ray Boost Factor
Can increase the expected annihilation flux from a DM halo by as much as ~100
Uncertainties in Boost Factor Prediction
–
–
Power­law index and cut­off scale of substructure mass spectrum
Mass­Concentration Relation at small DM halo masses
Invisible Universe 2009
Martinez et al. 2009
Local Group Galaxies: M32/M31
●
●
●
●
Compact Elliptical at ~25' separation from M31
VERITAS FoV
Central black hole of mass ~3 x 106 Msun
M32
Stellar power­law cusp with central density > 107 Msun pc­3
Adiabatic compression of DM halo can boost the gamma­
ray luminosity by ~103
M31
NGC 205
30'
Image Credit: SDSS
Invisible Universe 2009
WIMP Particle Candidates
●
Neutralino
–
–
●
Lightest supersymmetric particle in SUSY
Predominantly annihilates to heavy final states due to helicity suppression (bb, WW, ZZ)
Kaluza­Klein Particle
–
Lightest KK mode in UED
–
Predominantly annihilates to charged lepton pairs producing significantly harder spectrum
Invisible Universe 2009
Differential Yield for M = 1 TeV
Neutralino
KK
Internal Brehmsstrahlung
Mχ=907 GeV
Bringmann et al. 2008
BR(WW)=0.54
BR(ZZ)=0.46
Now included in latest
version of DarkSUSY
Spectrum with Brehmsstrahlung Enhancement
Invisible Universe 2009
WIMP Constraints
95% C.L. Upper Limits on <v> vs. M
No Boost
Boost = 100
KK Models
MSSM Models
Invisible Universe 2009
Constraint
from WMAP
WIMP Constraints
95% C.L. Upper Limits on <v> vs. M
KK Models
MSSM Models Constraint from
Future IACT instrument (CTA/AGIS)
Invisible Universe 2009
x 100 Exposure
x 10 Sensitivity
x 5 Reduction E Threshold
AGIS: Advanced Gamma­ray Imaging System
●
●
●
Development of future IACT observatory is currently pursued in both US (AGIS) and Europe (CTA) with the prospect for a joint instrument
With a dedicated dwarf galaxy observing program (1000h) a next­generation IACT could rule out a significant fraction of the MSSM parameter space
AGIS-36 Telescope
Field of View: 8 degrees
AGIS (US contribution) is a 36­
telescope array employing novel Primary Diameter: 11.5m
Camera: 15,000 channels
optical design which combines large FoV (~8°) and good angular Pixel size: 0.06 degrees
Telescope Spacing: 120-150m
resolution (~4')
Invisible Universe 2009
Conclusions
●
●
●
●
VERITAS observations of MW dwarf galaxies can constrain <v> to < 10­22 cm3 s­1 for WIMP masses > 200 GeV with moderate exposure (10­20 h)
Upcoming observational data sets on dwarf galaxies will reduce theoretical uncertainties on mass models and potentially point to better dwarf galaxy candidates
In the future VERITAS can provide follow­up observations of any DM candidates identified by Fermi (i.e. substructures)
A next­generation IACT instrument (CTA/AGIS) could rule out a significant portion of WIMP parameter space Invisible Universe 2009