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Recent evidence for gamma-ray line emission from Fermi-LAT data: dark matter or artifact?. Meng S u Pappalardo /Einstein Fellow MIT/ CfA Collaborators: Douglas Finkbeiner , Christoph Weniger Fermilab Astrophysics Seminar, March 4 th 2013.

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Recent evidence for gamma-ray line emission from Fermi-LAT data:

  • dark matter or artifact?

Meng Su

Pappalardo/Einstein Fellow


Collaborators: Douglas Finkbeiner, ChristophWeniger

FermilabAstrophysics Seminar, March 4th 2013


Vera Rubin and her colleagues: galaxies rotation curve of nearly galaxies (1970s)

Fritz Zwicky\'s pioneering work in 1933

standard dark matter scenario thermal wimp weakly interacting massive particle
Standard Dark Matter Scenario: Thermal WIMP(Weakly Interacting Massive Particle)
  • Dark matter particles are produced and annihilating in thermal equilibrium after big bang (according to a given particle physics model). Universe is expanding and cooling.
  • Universe becomes too cool to continue producing DM particles. Remaining DM annihilates to standard model particles.
  • Universe’s rate of expansion exceeds rate of particle annihilations, making annihilations rare.
  • “Dark Matter Freezes Out”

Techniques for the Indirect Detection of Dark Matter

•Morphological Differentiation

•Galactic Center Search

•Milky Way Halo

•Dwarf Spheroidal Galaxies

•Anisotropy Power Spectrum

•Gamma-Rays from Galaxy Clusters

•Cosmic Ray Electrons/Gammas from the Sun

•Spectral Differentiation

•Gamma-Ray Line Search

The two-body annihilation of DM into photons produces monochromatic gamma rays

•e+e- ratio and electron + positron Spectrum

(Recall previous talks by Alexander Belikov andLawrence Rudnick)


Gamma-Ray LineSearch

• GOAL: Search for a spectral signal from an annihilation (or decay) of dark matter directly into gammarays.

• Targets: Any annihilating or decaying dark matter particle which produces photons directly, or possibly produces a very hard spectrum of single photons through internal bremsstrahlung

• Benefits: No astrophysical process is expect to produce a gamma-ray line. Resulting constraints can fall far below thermal cross-section

•Difficulties: Direct production of gamma-gamma is not a generic prediction of dark matter annihilation or decay.


Fermi Bubble

Fermi Bubble from three year maps

best limits so far
Best limits so far:

Fermi-LAT Collaboration, arXiv:1205.2739v1

how to define a search region
How to define a search region?
  • Estimate the signal from DM, e.g. Einasto profile squared, projected along line of sight.
  • Estimate background from lower energy (1-20 GeV) photons.
  • Estimate S/N as DM signal/background.
  • Make a cut on estimated S/N.
why the reluctance to call a sharp spectral feature a line
Why the reluctance to call a sharp spectral feature a line?
  • Perhaps because 2 WIMPs -> 2 gammas is usually loop suppressed (in, e.g., the MSSM), so you expect a continuum 100-10,000x brighter. This was not observed, making a line impossible.
  • But there are other theories where a line is OK. Let’s ignore theoretical prejudice and simply ask if there is a line in the Fermi LAT data.
timeline of 130 gev line
Timeline of 130 GeV line:

12 April - Weniger (looks like a line at 130 GeV)

26 April - Profumo & Linden (is it the Fermi bubbles?)

10 May - Tempel et al., (No, it’s not a bubble, could be DM)

21 May - Boyarsky (lots of blobs, probably not DM)

25 May - Acharya, Kane... (It’s a Wino)

29 May - Bergstrom (reviews claims as part of larger review)

30 May - Jim Cline (two lines)

30 May - Buckley & Hooper (theoretical models)

5 June - Geringer-Sameth & Koushiappas (Line search in dwarfs)

7 June - Su & Finkbeiner (Off center 1.5 deg, Einasto, 6.5sigma, usehigh energy-resolution events)

(As of March2013, Wenigerpaper has 120+ citations)

fermi lat maps at 100 180 gev
Fermi-LAT maps at 100–180 GeV

Su & Finkbeiner (arXiv:1206.1616)

null tests
Null tests

Subtraction of other energy maps

Cosmic ray contamination?

  • Make maps in each of 16 energy bins, assume that emission in each bin is a linear combination of template maps, and plot the template coefficients.
  • Coefficients are determined by maximizing the Poisson likelihood of observing the observed counts given the model.
  • Templates choice corresponds to hypothesis to be tested.
not from large scale fermi bubbles
Not from large scale Fermi bubbles

1) χ+χγ+γ

2):χ+χ Z0+γ

E γ=mx-Mz2/4mx

A pair of lines at

110.8±4.4 GeV and 128.8±2.7 GeV

Consistent with single line at

127.3 ± 2.7 GeV

galactic longitude and latitude profile
Galactic longitude and latitude profile

Offset from the GC?

Even though the high-incidence-angle photons (θ > 40◦; right) panels have half the exposure (9.7% vs. 19% for the left panels), they have more than half of the photons, and nearly the same TS due to lower off-line background leaking in.

galactic longitude and latitude profile1
Galactic longitude and latitude profile

Offset from the GC?

Even though the high-incidence-angle photons (θ > 40◦; right) panels have half the exposure (9.7% vs. 19% for the left panels), they have more than half of the photons, and nearly the same TS due to lower off-line background leaking in.

the detection significance of the gamma ray cusp for various models
The detection significance of the gamma-ray cusp for various models

We do the fit in many ways. Off-center Einasto is the best.

assessment of line profile
Assessment of line profile

• The 129 GeV feature shape is strikingly similar to that expected for a line. The 111 GeV feature is

unconvincing, but is also compatible with a line.

• In some cases, fluctuations appear, but are not present in both low and high incidence spectra.

• This test did not have to succeed. The fact that the high-incidence photon sample has sharper spectral features is important.

a modified survey strategy for fermi
  • Thescan strategy of Fermi-LAT could be altered for 1 year to confirm the 130 GeV line!
  • This current strategy is excellent for uniformity of full-sky coverage, but is far from optimal for collecting high-incidence-angle photons from the GC.
  • The exposure time of our (40◦ < θ < 60◦ ) sample exceeds the current strategy (observed 9.7% of the time) by more than a factor of 4. Require GC have an incidence angle of 45◦ < θ < 55◦ .
  • After 1 year of altered observing, we would have a sample of high incidence photons equal to the current sample, and could evaluate their significance directly, in the absence of any trialsfactor!

Vela pulsar The Crab

Atwood et al. 2012

hypothesis 1 the galactic center is bright so instrumental artifacts are more significant there1
Hypothesis 1: The Galactic center is bright, soinstrumental artifacts are more significant there
  • At E > 100 GeV, the Galactic center is only modestly brighter than the surrounding regions, away from the GC or from low/high energy (bright X-ray source?)
  • It is difficult to see how this could happen.
  • Regions with a high gamma ray-to-CR ratio, which are used for calibration purposes: enhance the impact of an instrumental effect, like e.g. energy reconstruction or acceptance anomalies that
hypothesis 2 the galactic center has a hard spectrum making energy mapping errors more significant
Hypothesis 2: The Galactic center has a hard spectrum, making energy mapping errors more significant.
  • The GC spectrum is not much harder than the rest of the Inner Galactic plane, but the latter shows no sign of a feature at 130 GeV.
  • Not enough photon for >300 GeV to mimic the line!
hypothesis 3 the gc observations have a restricted range of incidence angles on the instrument
Hypothesis 3: the GC observations have a restricted rangeof incidence angles on the instrument.
  • Instrumental problems could be projected onto the Galactic center simply for geometric reasons
  • In Dec (Jun) the Sun passes near the Galactic center (anticenter), solar panels orientation determines the GC direction is close to the Sun!
parameterize the shower profile
Parameterize the shower profile
  • Energy leakage requires extensive modeling of showers for energy reconstruction

From TomiYlinen’sthsis


Modeling of the longitudinal and transverse profiles of electromagnetic showers and on the modeling of the development of the showers through the LAT calorimeter.

The shower maximum is not well contained is ∼ 25% for photons at 100 GeV

Philippe Bruel et al. (2012)

shower longitudinal transverse profile parameterization
Shower longitudinal (transverse) profile parameterization

Philippe Bruel et al. (2012)

Simulation of CsIcalorimeter with GEANT4

performance of the shower profile fit bias left resolution center shower containment
Performance of the shower profile fit: bias (left), resolution (center), shower containment

Philippe Bruel et al. (2012)

earth limb photons
“Earth limb” photons

The Fermi-LAT collaboration (2009)

So it looks like there a 3.3 sigma (4.7 sigma?) detection of an artifact in the Fermi data, but only for incidence angles 30-45 degrees

Dip in energy response?

the earth limb line and correlations with the gc signal
The Earth limb line and correlations with the GC signal
  • The majority of high-incidence limb events appear near the orbital pole
But we see no way this can be the explanation. Even if we discard all events with 30 < theta < 45, we get 4 sigma:
Kuhlen et al., arXiv:1208.4844 “Eris” simulation, run with Gasoline n-body codeCosmological zoom-in simulation of a MW-like galaxy.

The region of highest DM density (X) is away from the

dynamical center of the simulation, by 2.5 smoothing

Lengths. This offset is typical over billions of years.


The region of highest DM density (X) is away from the

dynamical center of the simulation, by 2.5 smoothing


This offset is typical over billions of years.

eris conclusions
Eris conclusions:
  • The Kuhlen et al. paper suggests that the distribution of DM in the center of a barred spiral is nontrivial, and the point of maximum density may not (always) be at the center.
  • However, it is not clear at all that this solves our problem with the 130 GeV feature.
background estimation
Background estimation

Su & Finkbeiner(arXiv:1207.7060)

hess ii expected performance in the tens of gev
HESS-II expected performance in the tens of GeV

600 m^2 mirror area and a very high resolution camera.

lower the energy threshold from 100 GeV to about 30 GeV and enhance the HESS sensitivity.

Image Credit: H.E.S.S. Collaboration, Frikkie van Greunen

HESS collaboration, ICRC2009


Line detection sensitivity

Bergstrom et al. (arXiv: 1207.6773)

pass 7 reprocessed data
Pass 7 reprocessed data
  • •  LAT team have reprocessed the data with updated calorimeter response calibration
    • –  Updated light yield calibration in each crystal -> affects energy reconstruction
    • –  Light asymmetry calibration -> affects position resolution, thus improves PSF at high energies
  • •  Impacts and changes– PSF at high energies is improved– Up to 5% shift in the energy scale (time and energy dependent) – Spectrum of background contamination has changed

DAMPE (2015)

The detector will be composed of a telescope (red layers in left figure) and an EM calorimeter.

The EM calorimeter will be composed of 576 BGO crystal bars with dimensions of 2.5 cm×2.5 cm×30cm.

The BGO crystals form 12 layers with an area of about 60cm×60 cm each. The r.l is ~27.

There will be also Silicon detector on the top for charge and position measurement

Total thickness is 34.5 r.l.


Total weight 1.5 tons

J. Chang (DSU 2011)

  • The line signal is not a discovery yet.- need more data (trials factors!)- can change survey strategy to get it fast
  • Want to know:- Is the cusp really off center?- are there two lines (or more)?
  • Doubling the data will address these questions...