Searching for Dark Matter in the Galactic Center with ... · x-ray, gamma-ray x-ray, gamma-ray proton proton synchotron radiation inverse Compton scattering bremsstrahlung radiation

Debates on the Nature of Dark MatterSackler 2014

19-22 May 2014

Searching for Dark Matter in the Galactic Center with

Fermi LAT: Challenges

Simona MurgiaUniversity of California, Irvine

Gamma rays from DM Annihilation

arXiv:0908.0195

The Fermi Sky

Fermi LAT data 4 years, E > 1 GeV

Understanding the Gamma-ray Sky

= + +data sources galactic diffuse isotropic

+dark matter??

The diffuse gamma-ray emission from the Milky Way is produced by cosmic rays interacting with the interstellar gas and radiation field and carries important information on the acceleration, distribution, and propagation of cosmic rays.

Galactic Gamma-Ray Interstellar Emission

= + +data sources galactic diffuse isotropic

Inverse Compton Bremsstrahlung π0-decay

x-ray, gamma-rayx-ray, gamma-ray

proton

proton

synchotron radiation inverse Compton scattering

bremsstrahlung radiation

All of these mechanisms create also non γ-ray radiation

Cosmic ray origin, propagation, and properties of the interstellar medium can be constrained by comparing the data to predictions.

Generate models (in agreement with CR data) varying CR source distribution, CR halo size, gas distribution (GALPROP, http://galprop.stanford.edu) and compare with Fermi LAT data

Assume locally measured CR spectra and intensities are representative of the Galaxy

Galactic Gamma-ray Interstellar Emission

http://galprop.stanford.edu




Assume locally measured CR spectra and intensities are representative of the Galaxy

Gamma-ray emissivity of hydrogen gas from within ~ 1kpc from the Sun is in agreement with predictions based on directly measured CR spectra

Gamma Rays from Local HI

arXiv:0908.1171



Example model:CR source distribution: SNRsCR confinement region: 20 kpc radius, 4 kpc height

– 88 –

Fig. 7.— Fractional residual maps, (model ! data)/data, in the energy range 200 MeV –

100 GeV. Shown are residuals for model SSZ4R20T150C5 (top) and model SLZ6R20T"C5

(bottom). The maps have been smoothed with an 0.5! hard edge kernel, see Figure 6.

(data - prediction)/prediction) for example model

All Sky

arXiv:1202.4039 Fermi LAT data 21 months, 200 MeV-100 GeV

On a large scale the agreement between data and prediction overall is good, however some extended excesses and deficits stand out.



Inner galaxy

isotropic

IC

DGE Total

π0-decaybremsstrahlung

sources

Inner Galaxy

arXiv:1202.4039

Fermi LAT data 21 months, 200 MeV-100 GeV




☺ Steep DM profiles predicted by CDM ⇒ Large DM annihilation/decay signal from GC!

Galactic Center Region

☹ Good understanding of the conventional astrophysical background is crucial to extract a potential DM signal from this complex region of the sky:

‣ source confusion: many energetic sources near to or in the line of sight of the GC

‣ diffuse emission modeling: large uncertainties due to the overlap of structures along the line of sight, difficult to model





CR Source Distribution

– 82 –

Fig. 1.— CR source density at z = 0 in arbitrary units as a function of Galactocentric

radius. Solid black curve: SNRs (Case & Bhattacharya 1998). Dashed blue curve: Pulsars

(Lorimer et al. 2006). Dotted red curve: Pulsars (Yusifov & Kucuk 2004). Dash-dotted

green curve: OB stars (Bronfman et al. 2000). While the units are arbitrary, the relative

normalisations of the curves in the figure match those found in the GALPROP models used

in this analysis. The CR flux of the models is normalised to the observed CR flux at the solar

circle after propagation. The normalisation is done at 100 GeV and is therefore una!ected

by modulation.

Cosmic ray source density

SNRs

Pulsars (Lorimer 2006)

Pulsars (Yusifov&Kukuk 2004)

OB stars

arXiv:1202.4039

Fractional residual difference between models generated with two different CR source distributions

Diffuse TeV Emission

The gamma rays of the Galactic center ridge at TeV energies observed by HESS is hard (spectral index~2.3), suggesting spectrum of CRs is harder than measured locally

Aharonian et al 2006

HESS

Aharonian et al 2006





Interstellar Medium

Average gas density

z= 0 pc 100 pc 200 pc

Integrated line intensity of CO (traces H2)





Interstellar Radiation Field

Spatial variation of the radiation field in the Galaxy





‣ Also, unresolved source component likely

Unresolved Sources

Gregoire et al, arXiv:1305.1584

Models based on Story et al 2007, Faucher-Giguere & Loeb 2010

Predicted millisecond pulsar population undetectable by the LAT

Gamma-ray bubbles (Su et al 2010, ApJ 724, 1044):

‣ very extended (~ 50o from plane)

‣ hard spectrum (~E-2, 1-100 GeV)

‣ sharp edges

‣ possible counterparts in other wavelengths (ROSAT, WMAP, and Planck)

Outflow from the center of the Milky Way: jets from the supermassive black hole? starburst?

Su et al 2010, ApJ 724, 1044

Fermi BubblesFermi LAT data, E>10 GeV

Spectrum

A. Franckowiak and D. Malyshev 15

PRELIMINARY

Shift in normalization can be explained by: •  Different

foreground modeling

•  Different definition of the bubble template resulting in different area of the template

Fermi LAT Collaboration, ICRC 2013

Dark Matter Distribution

NFW profile

�0 = 0.3 GeV/cm3

a0 = 20 kpc, r0 = 8.5 kpc

The dark matter annihilation (or decay) signal strongly depends on the dark matter distribution.Cuspier profiles can provide large boost factors

Bertone et al., arXiv:0811.3744

✓ Via Lactea II (Diemand et al 2008) predicts a cuspier profile, ρ(r)∝r-1.2

✓ Aquarius (Springel et al 2008) predicts a shallower than r-1

innermost profile

Navarro, Frenk, and White 1997

17

⇢(r) = ⇢0r0r

(1 + r0/a0)2

(1 + r/a0)2

DATA DATA-MODEL (diffuse)

Fermi’s View of the Inner Galaxy (15ox15o region)

Fermi LAT preliminary results with 32 months of data, E>1 GeV (P7CLEAN_V6, FRONT):

W28

LAT PSR J1809-2332

LAT PSR J1732-3131

2FGL J1745.6-2858

➡ Bright excesses after subtracting diffuse emission model are consistent with known sources.

Galactic diffuse emission model: all sky GALPROP model tuned to the inner galaxy

DATADATA DATA-MODEL (diffuse+sources)

Fermi’s View of the Inner Galaxy (15ox15o region)

Fermi LAT preliminary results with 32 months of data, E>1 GeV (P7CLEAN_V6, FRONT):

➡ Diffuse emission and point sources account for most of the emission observed in the region.

Dark Matter ConstraintsUse LAT data from the inner Galaxy to constrain DM by requiring that the DM contribution doesn’t exceed observed dataConstraints are not competitive unless background is (at least partially) subtracted or for contracted DM profiles

Fermi LAT Collaboration, arXiv:1308.3515

Optimized region for NFWNFW

NFWc

Einasto

Burkert

cc Æ bbPromptPrompt + ICS

5 10 20 50 100 200 500 1000200010-28

10-27

10-26

10-25

10-24

10-23

10-22

m DM @GeVD

<sv>@cm

3 êsD NFW

NFWc

Einasto

Burkert

cc Æ m + m -PromptPrompt + ICS

5 10 20 50 100 200 500 1000200010-28

10-27

10-26

10-25

10-24

10-23

10-22

m DM @GeVD

<sv>@cm

3 êsD

NFW

NFWc

Einasto

Burkert

cc Æ t + t -PromptPrompt + ICS

5 10 20 50 100 200 500 1000200010-28

10-27

10-26

10-25

10-24

10-23

10-22

m DM @GeVD

<sv>@cm

3 êsD

NFW

NFWc

Einasto

Burkert

cc ÆW +W -

W+W-threshold

PromptPrompt + ICS

5 10 20 50 100 200 500 1000200010-28

10-27

10-26

10-25

10-24

10-23

10-22

m DM @GeVD

<sv>@cm

3 êsD

Figure 5: 3� upper limits on the annihilation cross-section of models in which DM annihilates intob¯b, µ+µ� (upper panel), ⌧+⌧� or W+W� (lower panel), for the four DM density profiles discussedin the text. Upper limits set without including the ICS component in the computation are alsogiven as dashed curves (prompt) for comparison. The uncertainty in the diffusion model is shownas the thickness of the solid curves (from top to bottom: MIN, MED, MAX) while the lightershaded regions represent the impact of the different strengths of the Galactic magnetic field withlower(higher) values of the cross-section corresponding to B

0

= 1 µG(B0

= 10 µG). The horizontalline corresponds to the expected value of the thermal cross-section for a generic WIMP candidate.

contribution from prompt gamma rays and the total contribution from prompt plus ICS gammarays.

First, it is worth noting that if the DM density follows an Einasto, NFW or Burkert profile,the upper limits on the annihilation cross section are above the value of the thermal cross-sectionfor any annihilation channel. Nevertheless, the situation is drastically different when we considerthe DM compression due to baryonic infall in the inner region of the Galaxy. Indeed, by adopting

14

3σ

Hooper and Linden, arXiv: 1110.0006

Use LAT data from the inner Galaxy to constrain DM by requiring that the DM contribution doesn’t exceed observed dataConstraints are not competitive unless background is (at least partially) subtracted or for contracted DM profiles

10�22

10�23

10�24

10�25

10�26

h�vi

(cm

3s�

1)

e+e�

Observed Limit

Median Expected

68% Containment

95% Containment

10�22

10�23

10�24

10�25

10�26

h�vi

(cm

3s�

1)

µ+µ�

101 102 103

Mass (GeV/c2)

10�22

10�23

10�24

10�25

10�26

h�vi

(cm

3s�

1)

⌧+⌧�

uu

bb

101 102 103

Mass (GeV/c2)

W+W�

FIG. 5. Constraints on the dark matter annihilation cross section at 95% CL derived from acombined analysis of 15 dwarf spheroidal galaxies assuming an NFW dark matter distribution(solid line). In each panel bands represent the expected sensitivity as calculated by repeatingthe combined analysis on 300 randomly-selected sets of blank fields at high Galactic latitudes inthe LAT data. The dashed line shows the median expected sensitivity while the bands representthe 68% and 95% quantiles. For each set of random locations, nominal J-factors are randomizedin accord with their measurement uncertainties. Thus, the positions and widths of the expectedsensitivity bands reflect the range of statistical fluctuations expected both from the LAT data andfrom the stellar kinematics of the dwarf galaxies. The most significant excess in the observed limitsoccurs for the bb channel between 10 GeV and 25 GeV with TS=8.7 (global p-value of p ⇡ 0.08).

36

Hooper and Linden, arXiv: 1110.0006

Fermi LAT Collaboration, arXiv:1310.0828

Dark Matter Constraints

Sgr A*

Image (width~0.5o) combines a near-infrared view from the Hubble Space Telescope (yellow), an infrared view from the Spitzer Space Telescope (red) and an X-ray view from the Chandra X-ray Observatory blue and violet) into one multi-wavelength picture.

Our knowledge of the conventional astrophysical background is uncertain. This is currently the big limitation for the search of dark matter in the Galactic center with gamma rays.

In addition, better understanding of the dark matter density distribution in the Galactic center is essential in interpreting observations.

Difficult analysis, but the rewards are big!

Conclusions

Thank you!

Documents

Searching for Dark Matter in the Galactic Center with ... · x-ray, gamma-ray x-ray, gamma-ray proton proton synchotron radiation inverse Compton scattering bremsstrahlung radiation