Search for Dark Matter
Galactic Satellites with
Fermi-LAT
Ping Wang
KIPAC-SLAC, Stanford University
Representing the Fermi LAT
Collaboration
1
Outline
• Gamma-rays produced from Weakly
Interacting Massive Particle (WIMP)
annihilation and Final State Radiation
(FSR)
• Fermi LAT sensitivity to Dark Matter (DM)
satellites
• Criteria for DM satellite candidate
selection
• Preliminary results for 3-month LAT data
2
Gamma-rays from WIMP Annihilation
and FSR
• Secondary photons mainly • Photons directly radiated from
through p0 decay; dominant
the external legs; dominant if
if mainly annihilation into
mainly annihilating into
heavy quarks or gauge
bosons
charged leptons
– Non-power-law; a soft cutoff at
DM particle’s mass
– Very hard spectrum (~1/E); a
sharp cutoff at DM particle’s
mass
Birkedal et al., 2005
Photon spectrum ~ 1/E
3
Fermi LAT Sensitivity to DM Satellites
~3 with >5sigma for 1 year
~10 with >5sigma for 5 years
• Simulation of Milky Way dark matter satellites with truncated NFW profile
from Taylor & Babul, 2004, 2005
• Background estimate using Galprop diffuse and isotropic power law
extragalactic diffuse (Sreekumar et al.)
• Generic WIMP model: WIMP mass 100GeV, cross-section 2.3e-26 cm^3/s,
bb-bar only
– This model can give the required relic density
• Compare the signal, background flux inside the tidal radius
• No. of sigma = signal / sqrt (bkgd + signal)
4
Fermi LAT Sensitivity to DM Satellites
DM satellites with >5 sigma for 1yr LAT observation
24 realizations
Galprop conv.
•
Average of radial
Simulation of Milky Way dark matter satellites
with truncated
NFW
extension
(observed
by profile
from Taylor & Babul, 2004, 2005
LAT) ~ 1 deg
• Background estimate using Galprop diffuse and isotropic power law
extragalactic diffuse (Sreekumar et al.)
• Generic WIMP model: WIMP mass 100GeV, cross-section 2.3e-26 cm^3/s,
bb-bar only
• Compare the signal, background flux inside the tidal radius
• No. of sigma = signal / sqrt (bkgd + signal)
5
Radial extension (deg)
Criteria for DM Satellite Candidate
Selection
• Source has spatial extension (can be
resolved by the LAT)
• Source energy spectrum is non-power-law
(if WIMP annihilation) or 1/E power-law (if
FSR)
• Source is not variable
• Source has no counterparts at other
wavelengths
6
Analysis Method to Search for DM
Satellites using LAT Data
• Blind search strategy
– Optimize the analysis method using 3-month LAT
data
– Fix the analysis method and analyze 1-year LAT data
• Analysis method
– Search for 5 sigma detections with |b|>10deg, which
are not identified in Fermi LAT catalog
– Test source extension
• Hypothesis testing: NFW model VS. point source model
– Test source energy spectrum
• Hypothesis testing: WIMP annihilation spectrum VS. power
law spectrum
– Test source variability
• Light curve 1 week bins
7
Preliminary Results for 3-month
LAT Data
• 3-month data
– Aug 8th – Nov 7th, 2008
– 200 MeV – 60 GeV
– “Diffuse” class
• One interesting example
– Possibly extended source
– Possibly non-power-law
– Not variable based on light curve with time interval 1
week
– No molecular cloud counterpart
– No dSph counterpart
8
TS Map and Residual TS Map for
200 MeV – 60 GeV: 3-month data
•Pixel size = 0.125 deg
•Grid size = 2 deg x 1 deg
TS Map: point source model
Residual TS Map
9
Residual TS>30 (>4sigma); like another source.
TS Map and Residual TS Map for
200 MeV – 300 GeV: 10-month data
•Pixel size = 0.125 deg
•Grid size = 2 deg x 1 deg
TS Map: point source model
Residual TS Map
10
Residual TS>40 (>5sigma); like another source.
Smoothed Counts Map for 200
MeV – 300 GeV: 10-month data
•Pixel size = 0.125 deg
•Grid size = 2 deg x 1 deg
Smoothed Counts Map
11
Also seems another source in this counts map
Preliminary Results for 3-month
LAT Data
• One interesting example
– Possibly extended source
– Possibly non-power-law
– Not variable based on light curve with time
interval 1 week
– No molecular cloud counterpart
– No dSph counterpart
– Two close sources
• Confirmed by 10-month data
12
Summary
• The Fermi LAT offers a unique opportunity to
discover DM satellites by the gamma rays
produced in WIMP annihilations
–
–
–
–
Energy Spectrum
Spatial extent
Steady source
No counterparts at other wavelengths
• No DM satellite found in 3-month LAT data, and
this result is consistent with generic WIMP
model and N-body simulations.
13
Back up slides
14
TS Map and Residual TS Map for
1.6GeV – 3.2 GeV: 10-month data
•Pixel size = 0.125 deg
•Grid size = 2 deg x 1 deg
TS Map: point source model
Residual TS Map
15
Residual TS~25 (~5sigma); like another source.
Light Curve
•Light curve with time interval 1 week
•Within 2 deg radius region around the source
16
Test Statistic
• TS = 2 * (ln(L1) – ln(L0))
• TS approximately follows a chi-square
distribution if comparing two hierarchically
nested models.
– L1: bkgd and a source
– L0: bkgd only
• Otherwise, need to define a threshold of TS to
accept (or reject) hypothesis 0.
– Gtlike: DM spectrum VS. power law spectrum
• L1: bkgd and a DM point source (DMFit model for bb-bar
channel only)
• L0: bkgd and a pwl point source model (pwl spectrum model)
– Sourcelike: NFW source VS. point source
• L1: bkgd and a NFW source
• L0: bkgd and a point source
17
DM Spectrum VS. Pwl
Data 0:
Data 0: flux No. of
Significance deltaTS
pwl index /m^2/s
samples level
(10MeV600GeV)
2.0
1.0e-3
10,000 0.001
2.95
2.0
6.0e-4
10,000
0.001
6.35
1.5
7.0e-5
10,000
0.001
4.52
1.5
4.0e-5
10,000
0.001
3.48
2.5
1.0e-2
10,000
0.001
-0.06
2.5
5.0e-3
10,000
0.001
4.3118
NFW VS. Point source
Data 0:
Data 0: flux No. of
Significance deltaTS
pwl index /m^2/s
samples level
(10MeV600GeV)
2.0
1.0e-3
1,000
0.001
5.21
2.0
6.0e-4
1,000
0.001
6.82
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Simulation of Gamma-ray Flux from
Neutralino Annihilation in the Milky Way
•Gamma-ray flux from WIMP annihilation
d
dN
1   annv 
E , l, b  

Bf 
ddE
4p
2
f dE
f

 2 (r ( , l , b))
LOS
Milky Way Halo simulated by Taylor & Babul (2005)
2
m
d
All-sky map of DM gamma ray emission (Baltz 2006)
20