J. Aleksić

Cosmology constraints from shear peak statistics in Dark Energy Survey Science Verification data

Shear peak statistics has gained a lot of attention recently as a practical alternative to the two-point statistics for constraining cosmological parameters. We perform a shear peak statistics analysis of the Dark Energy Survey (DES) Science Verification (SV) data, using weak gravitational lensing measurements from a 139 deg² field. We measure the abundance of peaks identified in aperture mass maps, as a function of their signal-to-noise ratio, in the signal-to-noise range 0 4 would require significant corrections, which is why we do not include them in our analysis. We compare our results to the cosmological constraints from the two-point analysis on the SV field and find them to be in good agreement in both the central value and its uncertainty. We discuss prospects for future peak statistics analysis with upcoming DES data.

Magic Constraints on γ-ray Emission from Cygnus X-3

Cygnus X-3 is a microquasar consisting of an accreting compact object orbiting around a Wolf-Rayet star. It has been detected at radio frequencies and up to high-energy gamma rays (above 100 MeV). However, many models also predict a very high energy (VHE) emission (above hundreds of GeV) when the source displays relativistic persistent jets or transient ejections. Therefore, detecting such emission would improve the understanding of the jet physics. The imaging atmospheric Cherenkov telescope MAGIC observed Cygnus X-3 for about 70 hours between 2006 March and 2009 August in different X-ray/radio spectral states and also during a period of enhanced gamma-ray emission. MAGIC found no evidence for a VHE signal from the direction of the microquasar. An upper limit to the integral flux for energies higher than 250 GeV has been set to 2.2 x 10-12 photons cm-2 s-1 (95% confidence level). This is the best limit so far to the VHE emission from this source. The non-detection of a VHE signal during the period of activity in the high-energy band sheds light on the location of the possible VHE radiation favoring the emission from the innermost region of the jets, where absorption is significant. The current and future generations of Cherenkov telescopes may detect a signal under precise spectral conditions.

Optimized analysis method for indirect dark matter searches with imaging air Cherenkov telescopes

We propose a dedicated analysis approach for indirect Dark Matter searches with Imaging Air Cherenkov Telescopes. By using the full likelihood analysis, we take complete advantage of the distinct features expected in the gamma ray spectrum of Dark Matter origin, achieving better sensitivity with respect to the standard analysis chains. We describe the method and characterize its general performance. We also compare its sensitivity with that of the current standards for several Dark Matter annihilation models, obtaining gains of up to factors of order of 10. We compute the improved limits that can be reached using this new approach, taking as an example existing estimates for several benchmark models as well as the recent results from VERITAS on observations of the dwarf spheroidal galaxy Segue 1. Furthermore, we estimate the sensitivity of Cherenkov Telescopes for monochromatic line signals. Predictions are made on improvement that can be achieved for MAGIC and CTA. Lastly, we discuss how this method can be applied in a global, sensitivity-optimized indirect Dark Matter search that combines the results of all Cherenkov observatories of the present generation.

Probing the very high energy γ-ray spectral curvature in the blazar PG 1553+113 with the MAGIC telescopes

PG 1553+113 is a very high energy (VHE, E > 100 GeV) γ-ray emitter classified as a BL Lac object. Its redshift is constrained by intergalactic absorption lines in the range 0.4 < z < 0.58. The MAGIC telescopes have monitored the sources activity since 2005. In early 2012, PG 1553+113 was found in a high state, and later, in April of the same year, the source reached its highest VHE flux state detected so far. Simultaneous observations carried out in X-rays during 2012 April show similar flaring behaviour. In contrast, the γ-ray flux at E < 100 GeV observed by Fermi-LAT is compatible with steady emission. In this paper, a detailed study of the flaring state is presented. The VHE spectrum shows clear curvature, being well fitted either by a power law with an exponential cut-off or by a log-parabola. A simple power-law fit hypothesis for the observed shape of the PG 1553+113 VHE γ-ray spectrum is rejected with a high significance (fit probability P = 2.6 × 10^(−6)). The observed curvature is compatible with the extragalactic background light (EBL) imprint predicted by current generation EBL models assuming a redshift z ∼ 0.4. New constraints on the redshift are derived from the VHE spectrum. These constraints are compatible with previous limits and suggest that the source is most likely located around the optical lower limit, z = 0.4, based on the detection of Lyα absorption. Finally, we find that the synchrotron self-Compton model gives a satisfactory description of the observed multiwavelength spectral energy distribution during the flare.

Weak lensing magnification in the Dark Energy Survey Science Verification data

In this paper, the effect of weak lensing magnification on galaxy number counts is studied by cross-correlating the positions of two galaxy samples, separated by redshift, using the Dark Energy Survey Science Verification data set. This analysis is carried out for galaxies that are selected only by its photometric redshift. An extensive analysis of the systematic effects, using new methods based on simulations is performed, including a Monte Carlo sampling of the selection function of the survey.

Global dark matter limits from a combined analysis of MAGIC and Fermi-LAT data

Gamma-ray instruments like the Fermi-LAT (in space) and the MAGIC telescopes (on the ground) are sensitive to overlapping and complementary ranges of dark matter particle mass, and have dedicated programs to look for dark matter signals coming from the Galactic Center, galaxy clusters, dwarf satellite galaxies and others. The universality of dark matter properties allows the combination of data from different experiments and/or observational targets into a global and sensitiveoptimized search. For a given dark matter particle model, a joint likelihood function can be written as the product of the particular likelihood functions for each of the measurements/instruments – the advantage of such an approach is that the details of each experiment do not need to be combined or averaged. We have implemented this analysis framework and applied it to the MAGIC and Fermi-LAT observations of dwarf satellite galaxies. Here we present the analysis method and the obtained results: the most constraining bounds to dark matter properties for masses between 10 GeV and 100 TeV from dwarf galaxies observations. The approach is completely generic and could be used in the future to merge our results with those from other instruments (H.E.S.S., VERITAS, CTA and/or HAWK), sensitive to the same region of the dark matter parameter space.

Dark Matter Searches in Dwarf Spheroidal Galaxy Segue 1 with MAGIC

This chapter presents the results of this work—the search for signals of dark matter in dSph galaxy Segue 1. The long-term observational campaign was carried out with the MAGIC Telescopes between January 2011 and February 2013. With 157.9 h of good-quality data, this is the deepest survey of any dwarf galaxy by any IACT so far. Here are described the motivation behind the choice of Segue 1 as a suitable dark matter target, details of its observations with MAGIC and the subsequent analysis. Lastly, results from the full likelihood method and their interpretation in the light of various dark matter models are presented.

Dark Matter Paradigm

The notion of dark matter has been present for almost a century, but the question about its nature is still unanswered. Observational evidence and cosmological predictions assure that dark matter represents almost 85 % of the matter content of our Universe, and more than 25 % of its total energy budget. Discovering its essence is one of the most important and exciting tasks of modern science. This chapter is devoted to a brief introduction of the dark matter concept as well as the experimental results and theoretical predictions that support this paradigm.

Dark Matter Searches

The last decades have been marked by ever-growing efforts to discover the true nature of dark matter. Numerous experiments have been devised in attempt to catch a glimpse of the elusive dark matter particle, however, to date, no undeniable results can be claimed. Based on the approach, three main detection techniques can be distinguished: production of dark matter in particle accelerators, direct detection through dark matter scattering off ordinary matter, and indirect detection of primary or secondary SM particles produced in dark matter annihilation or decay. This chapter describes the basic principles behind the first two of these approaches, while indirect detection is described in more detail.

The MAGIC Telescopes

The Florian Goebel Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) Telescopes are two, 17 m diameter telescopes designed to have high sensitivity at low energies and fast response to variable phenomena. This chapter is devoted to the description of technical properties of the MAGIC Telescopes, as well as of the standard observational procedures and of the analysis chain used to process the gathered data.