Post on 25-Feb-2016
description
Gamma-Ray Results from FermiIndirect Detection of Dark Matter
Robert P. JohnsonU.C. Santa Cruz
Department of Physics andSanta Cruz Institute for Particle Physics
Representing the Fermi-LAT Collaboration
Large Area Telescope (LAT)D.o.E.
Fermi Observatory
• Successful collaboration of particle physicists and astrophysicists.
• 3 ton particle detector “telescope”.• million amplifiers; 5 computers.
GBM-ray Burst
Monitor
ApJ 697, 1071, 2009
STScI 2011
R.P. Johnson 3STScI 2011
2-Year All-Sky Map, E>1 GeV
The full Fermi-LAT photon data set is public, so contributions to DM searches have come from both within and outside the LAT collaboration.
Counts Map
The entire sky is viewed by the LAT every three hours.
R.P. Johnson 4STScI 2011
1451 Sources (>4 significance)
630
First 11 months of data
New classes not associated (confidently) with -ray sources in 3rd EGRET catalog.
2FGL is coming soon: >1800 sources
Ap.J. Supp. 188 (2010) 405
R.P. Johnson 5
Searches for Dark Matter Annihilation (or Decay)
• Exclusive final states: or Z– Unambiguous line signature.– But very low expected rates.
• Inclusive production (primarily 0 decays and inverse Compton scattering).– Much higher rates, but the signal spectrum is not so easy to
differentiate from diffuse and point-source backgrounds.– Localized sources:
• Galactic center.• Dwarf spheroidal galaxies.• Dark satellites.• Galaxy clusters.
– Diffuse sources:• Galactic halo.• Isotropic extragalactic diffuse emission.
STScI 2011
?
R.P. Johnson 6
Gamma-Ray Line Search, Year 1Example fit, at 40 GeV (the fit with the largest line “signal”)
Fit to a power-law background plus a line at 40 GeV.
The signal fraction and the power-law index float freely in the fit.
STScI 2011
11 Months of data
Almost all sky:
Galactic plane removed, except for Galactic center.
Sources removed by 0.2 cut.
PRL 104, 091302 (2010)
R.P. Johnson 7
Gamma-Ray Line Search DM LimitsCross Section Upper Limits, for annihilation to
0 50 100 150 200
1.4E-28
1.4E-27
1.4E-26
NFWEinastoIsothermal
Energy (GeV)
95%
C.L
. upp
er li
mit
on
v (c
m3/
s)
(i.e. annihilation cross section times B.R. to )
PRL 104, 091302 (2010)
STScI 2011
Can rule out some esoteric models such as G. Kane et al., PAMELA Satellite Data as a Signal of Non-Thermal Wino LSP Dark Matter, Phys. Lett. B681:151, 2009.
R.P. Johnson 8
Line Analysis of 2 Years of Data
STScI 2011
Work extending the Fermi-LAT line analysis to 2 years of data will be presented next week at the Fermi Symposium.• Improved (not yet public) energy
estimator based on shower profiles.• Studies of systematic effects in the
energy using control samples (e.g. limb photons).
Energy (GeV)
R.P. Johnson 9
WIMP Mass (GeV)
Line Analysis of 2 Years of Data
STScI 2011
Non-LAT-collaboration analysis,Gilles Vertongen, Christoph Weniger, arXiv:1101.2610v1.
This extends to much lower energy than the LAT results. The LAT team has been investigating systematic effects there, including a 4 significance “signal” at 6.5 GeV caused by systematic errors in the energy estimator.
Work extending the Fermi-LAT line analysis to 2 years of data will be presented next week at the Fermi Symposium.• Improved (not yet published)
energy estimator based on shower profiles.
• Studies of systematic effects in the energy using control samples (e.g. limb photons).
Preliminary
R.P. Johnson 10
Galactic Center
STScI 2011 10
>32 months counts map with 1FGL sources plotted, LAT front section only for E>1 GeV
+ Pulsarso Other Sources
Two LAT sources closest to the GC:• 1FGL J1745.6-2900c, 0.08 (HESS J1745-290?)• 1FGL J1746.4-2849c, 0.2 (PWN)
PWNHESS? PWN?
10 square
b
R.P. Johnson 11
Galactic Center
STScI 2011
>32 months counts map with 1FGL sources plotted, LAT front section only for E>1 GeV
+ Pulsarso Other Sources
Two LAT sources closest to the GC:• 1FGL J1745.6-2900c, 0.08 (HESS J1745-290?)• 1FGL J1746.4-2849c, 0.2 (PWN)
PWNHESS? PWN?
10 square
b
LAT front PSF at 1 GeV is about 0.5.
A 0.5 cone at the GC covers 150 parsec diameter!
ChandraX-rayimage
52 pc
40 pc
R.P. Johnson 12
DM Detection in the GC?• Thin converters (front section)
only, so the 68% containment angle at 1 GeV is 0.5.
• The disk model is fit to data along the Galactic ridge near the GC.
• The bulge model is spherically symmetric and fits the data well outside of 2 degrees.
STScI 2011
Hooper, Goodenough, Phys. Lett. B697, 412-428, 2011
Dashed: disk modelDotted: bulge model
Excess point-like contribution
R.P. Johnson 13
DM Detection in the GC?
STScI 2011
Hooper, Goodenough, Phys. Lett. B697, 412-428, 2011
Dashed: disk modelDotted: bulge model
Spherical point-like
excess
Extrapolation from TeV HESS source
• Thin converters (front section) only, so the 68% containment angle at 1 GeV is 0.5.
• The disk model is fit to data along the Galactic ridge near the GC.
• The bulge model is spherically symmetric and fits the data well outside of 2 degrees.
• The point-like excess cuts off above about 8 GeV and is not consistent with extrapolation from the HESS TeV source HESS J1745-290.
• Inclusion of a cusped DM profile improves the fit, and the data are consistent with annihilation of a 7 to 10 GeV WIMP of <v>31026 cm3/s.
• Other analyses, e.g. Boyarski et al., ArXiV:1012.5839v1, show consistency with diffuse emission and a GC point source.
MSP explanation: Kevork N. Abazajian, arXiv:1011.4275v3
R.P. Johnson 14
LAT Galactic Center Region Spectrum
STScI 2011
The Fermi-LAT collaboration has yet to publish on the GC region, other than the published catalog sources.This plot is for next week’s Fermi Symposium, showing an example spectral fit in a 5 square region about the GC, including numerous sources plus a particular GALROP model for the diffuse.Some residuals are apparent around a few GeV but are at no more than the 5% level.
Sources (solid)
Dashed Lines:Galactic Diffuse ModelIsotropic Diffuse
Solid Lines: Sources
Frac
tiona
l Re
sidu
als
Preliminary
Preliminary
R.P. Johnson 15
LAT Galactic Center Spatial Residuals
STScI 2011
1<E<3 GeV35% +35%
3<E<10 GeV55% +90%
10<E<30 GeV60% +250%
5 degree square region about the GC
0.1 degree square pixels
(counts model)/model
Preliminary Preliminary Preliminary
R.P. Johnson 16
Galactic Halo
STScI 2011
• 100 GeV WIMP annihilating to b quarks with v=31026 cm3/s.
• Via-Lactea II Galaxy model, including boost for unresolved substructure.
GALPROP
|b|>10
The DM signal is a small bump on a very large diffuse background.Fermi will have the statistical power to see it, but Systematic uncertainties in the background models! Systematic uncertainties in the instrumental effective area!
Via Lactea II: Diemand et al 2008, Nature 454, 735.
GALPROP: Strong, Moskalenko, Reimer 2000, ApJ. 437, 763.
R.P. Johnson 17
High Latitude Diffuse Emission
• The first Fermi-LAT publication on the galactic diffuse spectrum strongly disagreed with the EGRET spectrum.
• In particular, there is no huge “GeV excess” with respect to standard models of the diffuse production from cosmic rays.
STScI 2011
PRL 103, 251101, 2009
|b|>10, so most of the diffuse emission is local (especially for 0)
R.P. Johnson 18
Local Diffuse Emission• View toward the 2nd Galactic quadrant, including the
Galactic plane. – Dominated by the Gould belt and local arm.– Good kinematic separation of radio signatures
• View toward the 3rd Galactic quadrant, but large Galactic latitude. Region with no large molecular clouds and with most of the atomic hydrogen within 1 kpc.
STScI 2011
The gamma-ray data are well modeled by the local cosmic-ray spectra, but the HI emissivity in the 2nd quadrant must be increased relative to prior estimates.
Abdo et al. 2009, ApJ 703, 1249
15<b<30
|b|>22
Abdo et al. 2009, ApJ 710, 133
R.P. Johnson 19
High Latitude Diffuse Emission
STScI 2011
Source Dist., Halo h, Halo R, Spin Temp, Dust
XCO is fit to gamma rays
From Dec. 2010 talk by Gudlaugur Johannesson in Paris (“Dark Matter All Around”)
• Extensive work under way to model the diffuse emission.
• Right: one of a grid of 128 GALPROP models.
• All models respect constraints from local cosmic ray measurements (e.g. B/C ratio).
• No obvious “best model”, and small but significant residuals persist.
• This high latitude excess is similar to what some analyses see in the GC region.
• Spatial residuals are under 10% except in lobes, Loop-1, and outer galaxy.
R.P. Johnson 20
Halo Dark Matter Limits
STScI 2011
Two groups within the LAT collaboration have been working for some time on deriving DM limits from the diffuse analysis.
– No models give high quality fits over the full sky (|b|>10).• Large spatial and spectral residuals, compared with the small LAT
statistical errors!• (The official LAT diffuse model (ring model) has far too many ad-hoc
parameters to be used in searches for new physics.)– Not surprising, given the residuals that we have seen above, the fits
often give a positive value for the DM abundance, but• No confidence in ruling out systematic errors in the diffuse background
model and in the LAT effective area as the source of the residuals!– Very difficult to quantify with any rigor the effects of uncertainty in the
diffuse background model and the effective area.– Nevertheless, we expect these analyses to converge on publishable
limits this year
R.P. Johnson 21
DM Limits from High Latitude Diffuse• 21 months of data from 800 MeV to 100 GeV• ROI: 5|b|15; 75 75• Blue curves are very conservative upper limits derived assuming that all
diffuse photons are from dark matter.• The shaded region is excluded by a fit that includes a Galactic diffuse model
as well as dark matter and the isotropic diffuse.• Normalizations, in several Galactocentric rings, of GALPROP cosmic-ray
interactions with gas maps (HI and H2) and ISRF, are allowed to vary freely in the fit, as is the isotropic diffuse.
STScI 2011
R.P. Johnson 22
Dwarf Spheroidal Galaxies
Large satellite galaxies
Well-known dSphs
dSphs discovered by SDSS
Belokurov, V., et al. 2007, ApJ, 654, 897
STScI 2011
R.P. Johnson 23
Dwarf Spheroidal Galaxies, DM Search
Belokurov, V., et al. 2007, ApJ, 654, 897
Select 10 dSphs away from the galactic plane and not too distant.Require good stellar kinematic data and high mass/light of 10 to over 1000.
STScI 2011
No gamma ray signal is seen yet from any of these sources.
R.P. Johnson 24
Dwarf Spheroidal Galaxies DM Limits• Stellar data from Keck (Bullock, Kaplinghat, Martinez) were used to evaluate the
DM content of each of 8 dwarfs, to translate the flux limits into annihilation cross section limits. No substructure boost assumed.
• Red points are MSSM models with a cosmological WIMP thermal relic density compatible with WMAP data.
STScI 2011
ApJ 712 (2010) 147.
31026 cm3/s
Published results based on the first 11 months of data.
v
(10
26 cm
3 /s)
WIMP Mass (GeV)
R.P. Johnson 25
Dwarf “Stacking” Analysis• 24 months of data• Combined fit to 10 dwarfs, with common DM v free parameter
– Carina and Segue-1 added to the analysis• Analysis takes into account uncertainties of the astrophysics “J factors”:
STScI 2011
l.o.s.
2 )())(()( dllJ
The J factors are the integral of the following function over a 0.5 radius cone about the dwarf location, assuming an NFW profile:
v
No substructure boost assumed.
R.P. Johnson 26
Dwarf “Stacking” Analysis Upper Limits
STScI 2011
33 GeV
Thermal WIMP
Limits for other DM channels will be shown at the Fermi Symposium in Rome next week.
• 24 months of data• Combined fit to 10 dwarfs, with common DM v free parameter
– Carina and Segue-1 added to the analysis• Analysis takes into account uncertainties of the astrophysics “J factors”:
10 GeV WIMP apparently
ruled out
R.P. Johnson 27
Dark Satellites; Expectations• Via Lactea-2 simulation of the DM galaxy (Nature 454, 735)
– Including a boost for unresolved substructure– Sample 10 viewing points 8 kpc from the Galactic center
• WIMP annihilation to b,b-bar using Dark-SUSY (JCAP 0407, 008)– Nominal expected thermal WIMP cross section: 3×1026 cm3/s
• MC simulation of the Fermi-LAT instrument response• 10 year observation time
STScI 2011
B. Anderson et al., Ap. J. 718 (2010) 899.
Expected number of DM halo objects visible at 3 std. dev. significance.
Expected number of DM halo objects visible at 5 std. dev. significance.
Simulation only—no data
R.P. Johnson 28
Dark Satellites; Searches• One year of data.• Test unidentified sources (|b|>20) for
– Non-power-law spectrum,– Detectable source extension (non point-like). This test is essential to
remove contamination from high latitude gamma-ray pulsars.• No unidentified sources satisfy both selection criteria.
STScI 2011
One source passed both tests but appeared to be a conjunction of two sources, one of which was subsequently discovered to be a millisecond pulsar.
• A paper is in preparation to interpret this null result in terms of DM limits, based on the Via Lactea II and Aquarius galaxy simulations. Aquarius: V. Springel et al., ArXiv:0809.0898
R.P. Johnson 29
Galaxy Clusters• No observation thus far of gamma rays from galaxy clusters (whether
originating from DM or CR), besides clusters hosting AGN.• Fermi-LAT publication based on 11 months of data:
STScI 2011
JCAP, doi:10.1088/1475-7516/2010/05/025
MSSM
bb
Effect of substructure
Limits for just 2 of the clusters
R.P. Johnson 30
Galaxy Cluster “Stacking” Analysis• 24 months of data• 200 MeV to 100 GeV• 5 “nearby” clusters:
– AWM7– Fornax– Centaurus– Coma– M49
STScI 2011
R.P. Johnson 31
Preliminary
Galaxy Cluster “Stacking” Analysis
STScI 2011
• 24 months of data• 200 MeV to 100 GeV• 5 “nearby” clusters:
– AWM7– Fornax– Centaurus– Coma– M49
• DM limits from combined likelihood fit.
– Smooth NFW profile assumed (no substructure)
– Up to a factor of 2 improvement by use of the combined fit
(J-Factor uncertainties not considered)
R.P. Johnson 32
Isotropic (Extragalactic) Diffuse Emission
A published Fermi-LAT analysis has extracted the isotropic flux of gamma rays (believed to be primarily extragalactic) by reducing and understanding the residual CR background.
– Based on Fermi measurements of the blazar luminosity function (Ap.J. 720, 435, 2010), unresolved AGN can account for up to 30% of this diffuse (blue shaded).
– Star forming galaxies account for much of the rest. See the estimates above for two different assumptions on the spectrum (red and green shaded)
STScI 2011
PRL 104, 101101, 2010
=2.41 0.05
|b|>10
Models:
Star forming galaxies
Preliminary
R.P. Johnson 33
Cosmological Dark Matter
STScI 2011
The isotropic extragalactic contribution have been interpreted in terms of limits on cosmological dark matter annihilation:
Assuming a power-law model for astrophysical background
DM could supply all the photons in a given bin
DM structure evolution scenarios Models of absorption by EBL
JCAP04 (2010) 014
Large dependence on
model of DM structure
R.P. Johnson 34
Conclusions• No unambiguous signature of dark matter annihilation in the gamma-ray sky,
despite some intriguing residuals.• Some of the most interesting limits are from the dwarf satellites
– Low background– Solid interpretation, based on DM content derived from stellar velocity
measurements• Besides expected statistical improvements by up to a factor of 2, the DM
sensitivity from dwarfs may improve to 100 GeV or more due to– More and better stellar velocity measurements– Discovery of more dwarfs, especially in the southern hemisphere using
new survey telescopes.• Better sensitivity to DM annihilation in the diffuse Galactic halo or the
Galactic center requires improved understanding of the diffuse background caused by cosmic-ray propagation and interaction.
• In general this exercise would be tremendously invigorated and aided by the discovery of WIMP candidates at the LHC!
STScI 2011
R.P. Johnson 35
EXTRA SLIDES
STScI 2011
R.P. Johnson 36
High Latitude Diffuse Emission
STScI 2011
From Dec. 2010 talk by Gudlaugur Johannesson in Paris (“Dark Matter All Around”)
• Extensive work under way to model the diffuse emission.
• Right: one of a grid of 128 GALPROP models.
• All models respect constraints from local cosmic ray measurements (e.g. B/C ratio).
• No obvious “best model”, and small but significant residuals persist.
• High latitude excess is very similar to what we saw in the GC region.
• Spatial residuals are under 10% except in lobes, Loop-1, and outer galaxy.
• Still, many models explain the data reasonably well.
R.P. Johnson 37
GC MSP Interpretation
STScI 2011
• Hooper & Goodenough discredit an MSP explanation:
a. Spectrum is different from the average of known gamma-ray MSPs.
b. Would require a dense population of MSPs near the GC.
Kevork N. Abazajian, arXiv:1011.4275v3• Others disagree. For example, Abazajian argues that
a. The H&G extracted GC spectrum is consistent with gamma-ray spectra from four globular clusters. Although as noted by H&G, the fitted spectral indices do have large statistical errors.
b. The central star cluster of the GC fits within the LAT PSF and is 1000 times more massive than the largest globular cluster (Omega Cen.).
c. The analysis has not fully considered the systematic effects of the subtracted background model on the spectrum.
In any case, mundane astrophysics explanations of the GC source cannot be ruled out!
R.P. Johnson 38
DM Upper Limits from the Full |b|>10 SkyA profile likelihood fit to the DM content (for a given DM model and WIMP mass) encompasses variations over many systematic parameters:
– GALPROP CR propagation parameters (constrained by a 2 fit to local CR spectra and isotope ratios):
• CR source distribution• Halo height , diffusion constant, and Alfven velocity• Electron and hadron injection indices• Electron normalization (allowed to vary well outside of the local measurement)
– Normalization of maps of CR targets, in several Galactic rings:• 0 production of gammas:
– Atomic hydrogen (large uncertainties in spin temperature)– Ionized hydrogen (minor contribution)– Molecular hydrogen (uncertainties and variations in XCO)
• Inter-Stellar Radiation Field (inverse-Compton production of gamma rays)
– Energy-dependent uncertainties in the LAT effective area– Normalization of the isotropic diffuse, including instrumental
backgrounds
STScI 2011
R.P. Johnson 39
Methodology to Account for Diffuse BackgroundFor practical reasons, GALPROP parameters cannot be varied continuously in a fit. We must make a discrete sampling of the likelihood space.
STScI 2011
log of fit to the -ray data
½
2 of fi
t to
loca
l CR
data
contours of constant global
Each point represents the best DM fit when using a single GALPROP model to describe the diffuse background.
Course sampling of GALPROP parameters
Finer sampling near the minimum
Work in progress: dark matter upper limits will be derived from the ensemble of likelihood profiles after • finalizing the ranges of variations of
parameters and • filling in a denser sampling near the
minimum.
Likelihood profile for each GALPROP model
Amount of Dark Matter
log