VHE Observations of the Binary Candidate HESS J0632+057 with H.E.S.S. and VERITAS

HESS J0632+057 is an unidentified gamma-ray source located in the Monoceros region, probably associated with the massive Be star MWC 148. H.E.S.S. and VERITAS observations in the very high energy (VHE) range combined with Swift X-ray data indicate that this object is a new member of the elusive gamma-ray binary class. We present here results of VHE gamma-ray observations from VERITAS and HESS at energies above 100 GeV taken over more than six years. The observations confirm HESS J0632+057 as a point-like VHE source with a significance of more than 12 standard deviations. The VHE gamma-ray results are discussed in the context of contemporaneous X-ray observations with Swift XRT.Comment: Presented at the 32nd ICRC, Beijing, 201

Search for Galactic dark matter substructures with Cherenkov telescopes

Weakly interacting massive dark matter (DM) particles are expected to self-annihilate or decay, generating high-energy photons in these processes. This establishes the possibility for indirect detection of DM by \gamma-ray telescopes. For probing the secondary products of DM, accurate knowledge about the DM density distribution in potential astrophysical targets is crucial. In this contribution, the prospects for the detection of subhalos in the Galactic DM halo with present and future imaging atmospheric Cherenkov telescopes (IACT) are investigated. The source count distribution and angular power spectra for \gamma-rays originating from annihilating DM in subhalos are calculated from N-body simulation results. To study the systematic uncertainties coming from the modeling of the DM density distribution, parameters describing the \gamma-ray yield from subhalos are varied in 16 benchmark models. We conclude that Galactic subhalos of annihilating DM are probably too faint to be a promising target for IACT observations, even with the prospective Cherenkov Telescope Array (CTA).Comment: 8 pages, 5 figures. Included in Proceedings of the 34th International Cosmic Ray Conference, The Hague, The Netherlands (July 30 - August 6, 2015

Optimal strategies for observation of active galactic nuclei variability with Imaging Atmospheric Cherenkov Telescopes

Variable emission is one of the defining characteristic of active galactic nuclei (AGN). While providing precious information on the nature and physics of the sources, variability is often challenging to observe with time- and field-of-view-limited astronomical observatories such as Imaging Atmospheric Cherenkov Telescopes (IACTs). In this work, we address two questions relevant for the observation of sources characterized by AGN-like variability: what is the most time-efficient way to detect such sources, and what is the observational bias that can be introduced by the choice of the observing strategy when conducting blind surveys of the sky. Different observing strategies are evaluated using simulated light curves and realistic instrument response functions of the Cherenkov Telescope Array (CTA), a future gamma-ray observatory. We show that strategies that makes use of very small observing windows, spread over large periods of time, allows for a faster detection of the source, and are less influenced by the variability properties of the sources, as compared to strategies that concentrate the observing time in a small number of large observing windows. Although derived using CTA as an example, our conclusions are conceptually valid for any IACTs facility, and in general, to all observatories with small field of view and limited duty cycle.Comment: 14 pages, 11 figure

Improved γ\gamma/hadron separation for the detection of faint gamma-ray sources using boosted decision trees

Imaging atmospheric Cherenkov telescopes record an enormous number of cosmic-ray background events. Suppressing these background events while retaining $\gamma$-rays is key to achieving good sensitivity to faint $\gamma$-ray sources. The differentiation between signal and background events can be accomplished using machine learning algorithms, which are already used in various fields of physics. Multivariate analyses combine several variables into a single variable that indicates the degree to which an event is $\gamma$-ray-like or cosmic-ray-like. In this paper we will focus on the use of boosted decision trees for $\gamma$/hadron separation. We apply the method to data from the Very Energetic Radiation Imaging Telescope Array System (VERITAS), and demonstrate an improved sensitivity compared to the VERITAS standard analysis.Comment: accepted for publication in Astroparticle Physic

Dark matter substructure modelling and sensitivity of the Cherenkov Telescope Array to Galactic dark halos

Hierarchical structure formation leads to a clumpy distribution of dark matter in the Milky Way. These clumps are possible targets to search for dark matter annihilation with present and future $\gamma$-ray instruments. Many uncertainties exist on the clump distribution, leading to disputed conclusions about the expected number of detectable clumps and the ensuing limits that can be obtained from non-detection. In this paper, we use the CLUMPY code to simulate thousands of skymaps for several clump distributions. This allows us to statistically assess the typical properties (mass, distance, angular size, luminosity) of the detectable clumps. Varying parameters of the clump distributions allows us to identify the key quantities to which the number of detectable clumps is the most sensitive. Focusing our analysis on two extreme clump configurations, yet consistent with results from numerical simulations, we revisit and compare various calculations made for the Fermi-LAT instrument, in terms of number of dark clumps expected and the angular power spectrum for the Galactic signal. We then focus on the prospects of detecting dark clumps with the future CTA instrument, for which we make a detailed sensitivity analysis using open-source CTA software. Based on a realistic scenario for the foreseen CTA extragalactic survey, and accounting for a post-trial sensitivity in the survey, we show that we obtain competitive and complementary limits to those based on long observation of a single bright dwarf spheroidal galaxy.Comment: 29 pages + appendix, 15 figures. V2: Sects. 3.3, 4, and 5.3 extended, results unchanged (matching accepted JCAP version