IAC Technique and DM search

1. Indirect Dark Matter Search with the MAGIC Telescope: first results for the Draco dSph observation S. Lombardi(1), A. Biland(2), M. Doro(1), M. Gaug(3), M. Mariotti(1), D. Nieto(4), M. Persic(5), F. Prada(6), M. Rissi(2), M.A. Sanchez-Conde(6), L.S. Stark(2), A. Venturini(1), F. Zandanel(6) for the MAGIC Collaboration* ABSTRACTIn many Dark Matter (DM) frameworks, the annihilation of DM particles can produce a continuum-spectrum gamma radiation that extends up to very high energies. Astrophysical regions supposed to be dynamically dominated by DM, such as Dwarf Spheroidal Galaxies (dSphs), Clusters of Galaxies and Intermediate Mass Black Holes (IMBHs), can be considered as interesting targets for Imaging Atmospheric Cherenkov Telescopes (IACTs). The 17m Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) Telescope, located in the Canary island of La Palma (2200 m a.s.l.), having the lowest energy threshold among current IACTs, is best suited for indirect DM searches. Here, the prospects of the detection of DM with MAGIC and the recent results of the observation of the dSph Draco with MAGIC are reported. The observation of Draco dSph by MAGIC IAC Technique and DM search The nearby dwarf Spheroidal galaxy Draco is a satellite of the Milky Way located at a galactocentric distance of about 82 kpc . The relative small distance from the Earth and the high estimated mass to light ratio (M/L > 200 [5]) make Draco one of the most promising objects for indirect DM search with IACTs among all the possible source- candidates. • The Imaging Atmospheric Cherenkov (IAC) Technique is based on the detection of Cherenkov photons produced by very fast charged particles in the atmosphere created in the atmospheric showers initiated by charged particles and gamma-rays coming from the Universe. • The gamma-rays trace back the place of their origin, being undeflected by cosmic magnetic field, and thus constitute a viable messenger to study the deep Universe and to do fundamental physics research. • A search for Dark Matter particles in the Universe is nowadays constrained by the large uncertainties in the flux prediction which depends on the knowledge of the astrophysical DM density profiles and of the particle physics beyond the Standard Model. These uncertainties put serious hindrances to the estimation of the observability (up to several orders of magnitude). • The second generation IAC Telescopes (IACTs), and among them the MAGIC Telescope, provides low energy threshold, high flux sensitivity and good angular and energy resolution. • MAGIC, thanks to the present largest reflective dish (236 m2), presents the lowest energy threshold (70 GeV at zenith) and for this reason is best suited to search for DM candidates with energy of the neutralino mass (> 45 GeV) or the Kaluza-Klein state (> 400 GeV). • Starting from the end of 2008 a clone of MAGIC I Telescope, dubbed MAGIC II, will begin to take data, allowing a stereoscopic observation of the sky with an improvement of sensitivity of a factor 3, a decrease of energy threshold of about 30% and better energy and angular resolutions. MAGIC I features: High collection area: 236m2 surface, 105m2 effective area Low Energy Threshold (at zenith)50 GeV (trigger), 70-100 GeV (analysis) Energy Resolution 30% (100 GeV), 20% (1 TeV) Angular Resolution 0.1 (Energy Threshold) Flux sensitivity (E > 100 GeV)1.8% Crab-flux/50h(~ 10-11 phcm-2s-1) The DM around the Draco dSph is assumed to be distributed in an extended halo with a radial profile modeled by a power law with an exponential cut-off [3]: The parameter  0 describes the shape of the DM distribution in the crucial innermost region.  = 0 results in a so-called core model with a central flat region, whereas profiles with 0.7 <  < 1.2 denote the so-called cusp profiles. In the plot the <J()> factor is calculated for a core and a cuspy DM profile and an angular resolution of ∆=10-5 sr for the dSph Draco case. With the present angular resolution of the MAGIC telescope (0.1), the two models are indistinguishable due to the limited angular resolution of the telescope which smears the determination of the profiles. Even though the factor <J()> converges for both models to a same value, there is a uncertainty in the distribution of the DM by the existence of a hypothetical central black hole or a clumpy distribution of the DM [6] which could lead to a significant flux enhancement. Gamma-ray flux from DM-annihilating objects A search for a possible DM gamma-ray signal coming from Draco was performed by the MAGIC Telescope during May 2007 [7], for a total of 8 hours of effective observation time: • In the CDM cosmological scenario about 80% of the matter of our Universe is believed to be constituted by cold, neutral, non-baryonic, weak-interacting stable particles (WIMPs) [1]. Among the huge plethora of WIMP candidates, the best motivated ones are related to the Super Symmetrical and Extra Dimensional extensions of the Standard Model of particle physics [2]. • In the widely studied Minimal Supersymmetric extension of the Standard Model (MSSM) the lightest SUSY particle (LSP), the neutralino , is stable (due to the R-parity conservation) and represents an excellent cold DM candidate with a relic density compatible with the WMAP bounds. • Since the neutralinos are Majorana particles, pair of  can annihilate into Standard Model particles, namely quarks, leptons and W bosons. The hadronization of such particles results in a continuum emission of gamma-rays at energies E<M. A direct annihilation in gammas (, Z) with a peculiar sharp line spectrum dependent on the neutralino mass is also possible even if loop-suppressed. • The expected gamma-ray flux from DM-annihilating astrophysical objects can be written as where the particle physics factor fSUSY depends on the features of the neutralino and the factor J() represents the integral of the line-of-sight of the square of the DM density along the direction of observation , taking into account also the beam smearing of the IACT ( B()d) [3]: • Recently it was pointed out that the Internal Bremsstrahlung (IB) process may boost the gamma-ray yield of the neutralino self-annihilation by up to three or four orders of magnitude, even for neutralino masses considerably below the TeV scale [4]. This discovery represents a very important news for the indirect DM search particularly for the IACTs which are sensitive to the energy range most affected by the gamma-ray flux enhancement due to IB process. The plot shows the distribution of the gamma-ray candidates coming from the center of Draco (black marker) and background (grey markers) calculated from the MAGIC data for energies above 140 GeV. No significant gamma-ray excesses within the signal region (below dotted lines) were found. The flux upper limit (calculated from the Rolke method [8] taking into account a systematic error of 30%) for a point-like source and a power law spectrum of index -1.5 was Black marker: signal Grey marker: background Energy > 140 GeV = Due to the high predictive power of the mSUGRA framework, where the SUSY breaking effects are transmitted from the high energy scale to the electroweak scale by the graviton [9], a simulation of several million models (with m0 < 6TeV, m1/2 < 4TeV, -4 TeV  A0  4 TeV,  50, > 0 and no IB corrections) was performed [10]. A comparison between the expected gamma-ray flux and the correspondent upper limit calculated from the data was done for different benchmarks [11]. Although it was not possible to constrain any mSUGRA phase space parameters, very high gamma-ray flux enhancements between O(103) (see benchmark K’) and O(109) (see benchmark E’) were ruled out. Yellow points: mo < 2 TeV (favored by particle physics);Blue points: mo > 2 TeV; Dark blue and dark yellow points: rescaled models with a relic density smaller than the lower WMAP bound The many order of magnitude uncertainties in the flux predictions make impossible to understand exactly the IACTs’ observation time needed for the detection of the gamma-rays coming from DM sources. Nevertheless, it is worth mentioning that many SUSY models predict optimistic gamma-rays fluxes’ upper limits around 10-12 phcm-2s-1. This, together with the new possible DM gamma-ray flux enhancement due to the IB process and the discovery of new promising sources (as the ultra-faint dSph [12]), gives rise to concrete prospects of detection. (1) INFN-University of Padua, Italy (2) ETH Zurich, Switzerland (3) Instituto de Astrofisica de Canarias, Spain (4) Universidad Complutense de Madrid, Spain (5) INAF-Trieste and University of Udine, Italy (6) IAA-CSIC Granada, Spain * for a complete list of MAGIC collaborators and extensive informations on the telescope: http://wwwmagic.mppmu.mpg.de/ References: [1] Spergel, D. N. et al. 2007, ApJS, 170, 377; [2] Bertone, G., Hooper, D., & Silk, J., Phys.Rept.405:279-390,2005; [3] Sanchez-Conde, M. A. et al., Phys.Rev.D76:123509,2007; [4] Bringmann, T., Bergström, L., & Edsjö, J., JHEP 0801:049, 2008; [5] Mayer, L., Kazantzidis, S., Mastropietro, C., & Wadsley, J. 2007, Nature, 445, 738; [6] Strigari, L. E. et al., Phys.Rev.D75:083526,2007; [7] Albert, J. et al., Astrophys.J.679:428-431,2008; [8] Rolke, W. A., Lopez, A. M., & Conrad, J. 2005, Nucl. Instrum. And Meth., A551, 493; [9] Chamseddine, A. H. et al. 1982, Phys. Rev. Lett., 49, 970; [10] Stark, L. S., Hafliger, P., Biland, A., & Pauss, F. 2005, JHEP, 08, 059; [11] Battaglia, M. et al. 2004, Eur. Phys. J., C33, 273; [12] Willman B. et al. 2005, ApJ, 626, L85