Predictions for the Detection by CELESTE (E>30GeV) of
Neutralino Annihilation Photons from DRACO.
Javier Bussóns Gordo, Edmond Giraud, A. Falvard, A. Jachołkowska, J.
Lavalle, E. Nuss, F. Piron, M. Sapiński
Groupe d’Astroparticules de Montpellier
August 12, 2002
Abstract. By assuming that the halo of the dwarf spheroidal galaxy Draco is made of neutralinos and invoking various halo profiles, we estimate the fluxes of E>1GeV photons arising
from neutralino annihilation for five supersymmetric (SUSY) dark matter benchmark scenarios (B, C, G, I, L). We also calculate the expected number of γ -photons for 30 hours of
ON-source observation by CELESTE, a former solar plant in the French Pyrenees, converted
into a γ -ray detector with energy threshold of 30 GeV and aperture of 8×10−5 sr. We expect
a photon rate between 0.01 and 1 times that of the Crab Nebula —standard source measured at
6 photons/min. Part of the supersymmetry parameter space above the I benchmark point may
thus be sampled.
Keywords: dwarf spheroidals, supersymmetry, dark matter
1. Introduction
Within the Minimal Supersymmetric Standard Model, if R-parity is conserved, a stable weakly interactive massive particle exists: the neutralino. A
relic population of such particles from the early universe could contribute to
the dark matter and could be indirectly detected on Earth via high energy
γ -ray photons —being Majorana particles, neutralino pairs can annihilate to
particle-antiparticle pairs which may generate such photons.
The choice of Draco in the indirect search for dark matter is justified by
its huge mass to luminosity ratio (∼ 200, to be compared to ∼10 in bright
spiral galaxies) and its flat velocity profile, suggestive of a mass distribution
which does not follow the luminosity and of an extended dark matter halo.
7
At a distance of 82 ± 7 kpc and with a mass of 8 +3
−2 × 10 M at 3 core radii,
i.e. nearby and dark-matter dominated, Draco is one of the most massive substructures orbiting around our Galaxy and could well be one of the numerous
large clumps predicted by Cold Dark Matter (CDM) simulations which would
not be disrupted by the galactic tidal field —the data indicate that Draco has
not been tidally truncated within ∼ 1 kpc (Kleyna et al., 2001). Moreover,
simulations of the growth of CDM halos have revealed that a significant
fraction of the mass lies in substructures orbiting in virialized dark matter
halos and that such substructures appear down to the smallest mass scale,
well below that of dwarf spheroidals.
© 2003 Kluwer Academic Publishers. Printed in the Netherlands.
CELESTE Predictions for Neutralino Annihilation Photons from DRACO
7
High-energy γ -rays traversing the atmosphere generate showers of charged
particles and Cherenkov light. CELESTE (CErenkov Low Energy Sampling
and Timing Experiment) (Paré, 2002), an atmospheric Cherenkov telescope
which makes use of a decommissioned solar electric plant (Thémis) in the
French Pyrénées, samples the time of arrival and intensity of the Cherenkov
wavefront. The combination of a large (2900 m 2 ) mirror surface area from
53 independent heliostats, a secondary optics where each heliostat is seen
by an independent photomultiplier and an analog-sum trigger system with 1
GHz signal digitisation (flash ADCs) makes possible the detection of cosmic
γ -rays down to 30 GeV.
2. Halo Mass Models
Firstly, we have fitted the Draco radial velocity profile with various singlecomponent mass density profiles consistent with N-body simulations, namely
Navarro-Frenk-White (NFW) (Navarro et al., 1996) and Moore (Moore et al.,
1999), given, respectively, by:
r −2
r −1.5
rs
rs
1+
, ρ(r) = ρ0 ( )1.5 1 +
(1)
r
rs
r
rs
where the scale parameter r s and the density ρ0 are derived from the velocity profile. But since the observed profile is more compact, we have also
considered double-component models.
As with M31, the Draco profile is reminiscent of a two-component distribution with different scales. One may think of two CDM sub-halos, e.g.
a compact component and a larger halo, both originally with similar CDM
profiles, which have merged and are presently seen at an epoch when the
centers of mass are roughly at the same position but do not yet form a single
NFW halo. Another possibility is the presence of a central black hole.
We fit the outer component with an NFW of ∼ 1 kpc scale, the value
expected by making this model evolve from z = 4 to a present concentration
parameter (virial-to-scale radius ratio) ∼ 10. For the inner component, we
consider two cases: i) a CDM sub-halo (fitted with another NFW profile,
Fig 1, left) would produce, if the first velocity point is taken at face value, ∼
9×105 M within 60 pc, which is enormous; ii) a massive black hole (MBH),
around which various CDM distributions have been tried. If the surrounding
material is dense and has low velocity dispersion, an incipient MBH makes a
strong spike and increases adiabatically (Gondolo & Silk, 1999). A number
of criticisms were made on this scenario, mainly on its ability to survive over
gigayears (Merritt et al., 2002). Nevertheless, in a stable environment, the
MBH will continuously reconstruct a spike in a short dynamical scale as we
have numerically tested. The best fit with a central MBH and an NFW outer
ρ(r) = ρ0
8
Javier Bussòns Gordo
Figure 1. Radial velocity profiles considered in our simulations.
halo with scale radius r s = 1.3 kpc yields a central mass of 6×10 5 M (Fig 1,
right).
3. Gamma-Ray Flux Prediction
Using the SUSPECT code (Kneur, 2002), we deal with the neutralino in the
minimal supergravity scheme (mSUGRA) at a Great Unification Theory scale
which evolves, via renormalisation couplings and masses, to the electroweak
scale. Then, with the DarkSUSY program (Gondolo et al., 2001), we determine the relevant masses, couplings and cross sections; check for consistency
with both a non-charged lightest SUSY particle, the quark masses and the
experimental limits; calculate the relic density of neutralinos in the universe
and the γ -ray fluxes expected on Earth. It is at this last stage that the halo
density profiles need to be introduced. We have merged both programs into
DarkSUSPECT (Nuss, Kneur, Moultaka, 2002).
Some SUSY benchmark models (Ellis et al., 2002) have been proposed to
provide a common way of comparing the discovery potential of future accelerators. These scenarios correspond to 13 configurations of the five mSUGRA
parameters, with the trilinear coupling parameter set to zero.
By assuming isotropic neutralino velocities within the halo, we can decouple the γ -flux into its astrophysical and particle physics parts:
d 2 Nγ
hNγ σ vi
=
dd E
4πm 2χ
Z
l.o.s.
ρχ2 dl(ψ),
(2)
where hNγ σ vi averages the product of the cross section and the relative particle velocity, and ρχ is the density profile of halo neutralinos (determined from
CELESTE Predictions for Neutralino Annihilation Photons from DRACO
9
Integrated Flux (cm
-2
-1
s )
astrophysical data), integrated along the line of sight. Figure 2 shows the γ ray fluxes from Draco for the 5 selected benchmarks (filled dots: CGBLI in
order of increasing flux). These fluxes are given in Table I (see Fig 3) for
different halo profiles along with the number of γ -rays expected to be seen
by CELESTE (8×10−5 sr aperture) in 30 hours of ON-source observation
for the brightest neutralino models and for the brightest benchmark model
(I). The bulk of the integrated photon fluxes lies between 10 −15 and 10−13 :
this determines the sensitivity which next-generation detectors must attain in
order to probe a reasonable fraction of the parameter space.
10
10
10
10
10
-11
renormalization not needed
initial
renormalized
benchmark
-12
-13
-14
0.9 < gaugino fraction < 1
0.1 < gaugino fraction < 0.9
-15
0
50
100
150
200
250
300
Neutralino Mass (GeV)
Figure 2. γ -ray fluxes from Draco for the 5 selected benchmarks (see text for details and
Table I presented in Figure 3).
Regarding the detectability by CELESTE (or a similar instrument), dual
halo mass models with inner sub-haloes of classical NFW or Moore types
yield too few photons (less than 100 in 30 hours even for the brightest neutralino models) and therefore have no chances of detection. However, the
massive-black-hole / NFW combinations (BH+NFW in Table I), where various CDM distributions are placed around the central MBH, are more likely
to be detected. In particular, BH+NFW1, a combination with r s = 0.1pc and
a CDM-to-MBH mass fraction f =10%, is the most optimistic among those
tested in this work: 1741 photons (in 30 hours) for the I neutralino benchmark
and over 10000 photons for the brightest neutralino model. Models with less
dense spikes around the MBH (like BH+NFW2, where f =1% or BH+NFW4,
10
Javier Bussòns Gordo
Figure 3.
where rs = 0.34pc) yield more modest expectations. We have not taken into
account the clumpiness factor, which should be of order 10 in the inner region and of order 100 at large radii (Calcáneo-Roldán & Moore, 2000) . It is
worth reminding that all neutralino models have equal weight and that those
producing the highest fluxes correspond to the largest annihilation sections
compatible with cosmology.
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