In the cosmological paradigm, cold Dark Matter (DM) dominates the mass content of the Universe and is present at every scale. Candidates for DM include many extensions of the standard model, with a weakly interacting massive particle (WIMP) in the mass range from 50 GeV to greater than 10 TeV. The self-annihilation of WIMPs in astrophysical regions of high DM density can produce secondary particles, including very high energy (VHE) gamma rays, with energies up to the dark matter particle mass. The VERITAS array of Cherenkov telescopes, designed for the detection of VHE gamma rays in the 100 GeV-10 TeV energy range, is an appropriate instrument for the detection of DM and is complementary to Fermi-LAT. Dwarf spheroidal galaxies (dSphs) of the Local Group are potentially the best targets to search for the annihilation signature of DM due to their proximity and large DM content. We report on the latest VERITAS observations of dSphs and discuss the results in the framework of WIMP models.
The compelling evidence for the presence of DM in the different structures of the Universe [1] has motivated numerous efforts to search for DM by means of astrophysical observations. If DM is made of WIMPs annihilating into standard model particles, indirect searches for DM annihilations with VHE γ-rays provide a very promising way to constrain the nature of the DM particle: γ-rays are free of any propagation effects on short distances (≤ 1 Mpc) and DM particle annihilation is predicted to give a unique γ-ray spectrum. Such searches are often conducted using pointed observations toward nearby DM overdensities, because the annihilation rate is proportional to the squared DM density. Popular targets include the Galactic Center [2][3][4][5], satellite galaxies of the Milky-Way (MW) [6][7][8][9][10][11][12], globular clusters [13,14] and clusters of galaxies [15]. The dSphs of the Local Group best meet the criteria for a clear and unambiguous detection of DM. They are gravitationally bound objects and contain up to O(10 3 ) more mass in DM than in visible matter. As opposed to the Galactic Center, they are environments with a favorably low astrophysical γ-ray background. Neither astrophysical γ-ray sources (supernova remnants, pulsar wind nebulae, etc) nor gas acting as target material for cosmic rays have been observed in these systems [16]. Furthermore, their relative proximity and high galactic latitude make them the best astrophysical targets for high signal-to-noise detection. This paper gives a status of the dSph observational program carried out by the ground-based VHE γ-ray observatory VERITAS. Section II presents the instrument and summarizes the VERITAS dSph observations and data analysis. Section III interprets the re-sults in terms of constraints on DM models, focusing on models which include a Sommerfeld enhancement. Finally, section IV is devoted to the conclusion.
VERITAS is an array of four 12-meter imaging atmospheric Cherenkov telescopes (IACTs) located at the base camp of the F. L. Whipple Observatory in southern Arizona. Each VERITAS telescope consists of a large optical reflector which focuses the Cherenkov light emitted by particle air showers onto a camera of 499 photomultiplier tubes. The large effective area (∼ 10 5 m 2 ), in conjunction with the stereoscopic imaging of air showers, enables VERITAS to be sensitive over a wide range of energies (from 100 GeV to 30 TeV) with an energy and angular resolution of 15-20% and 0. I: The VERITAS dSph observations and corresponding analysis results. The significance is calculated according to the method of Li & Ma [19]. The 95% CL ULs on the number of γ-rays in the ON-source region is derived using the Rolke prescription [20]. The ULs on the integrated flux have been computed assuming a power law of index -2.6.
thus reducing the systematic uncertainties in the background determination. Data reduction follows the methods described in [18]. After calibration of the data, the γ-rays are selected by calculating the Hillas parameters of the recorded camera images. The selection cuts were optimized for the detection of a 5% Crab Nebula-like source. After the γ-ray selection, the residual hadronic background in a circular region of 0.12 deg radius centered on the target position (called the ON-source region) is estimated using the reflected background model. The analysis of the data did not reveal any significant excess over the estimated background in any of these observations. Table I displays the measured γ-ray excesses for each of the dSph dataset, along with the corresponding significances, the resulting upper limits (ULs) on the number of γrays in the ON source region and on the integrated flux above 300 GeV. The 95% confidence level (CL) integral flux ULs lie in the range 0.4-1.2% of the Crab Nebula flux.
The absence of signal in any of these observations can be used to derive constraints on various dark matter models. The γ-ray flux from the annihilations of DM particles, of mass m DM , in a spherical DM halo is given by a particle physics term times an astrophysics term:
The particle physics term contains all the information about the DM particle: its mass m DM , its total velocity-weighted annihilation cross-section σv and the differential γ-ray spectrum from all final states weighted by their corresponding branching ratios, dN γ /dE. The astrophysical factor J(∆Ω) is the square of the DM density integrated along the line of sight (s) and over the solid angle ∆Ω:
The solid angle is given here by the size of the signal search region defined previously in the analysis, i.e. θ ≤ 0.12 deg. The estimate of the astrophysical factor requires a modeling of the dSph DM distribution. Each dSph DM distribution has been modeled with a Navarro, Frenk & White (NFW) profile [21], except Segue 1 for which the DM distribution has been modeled with an Einasto profile [22]. The parameters of a dSph profile are constrained using its star kinematics and have
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