Background modelling in very-high-energy gamma-ray astronomy

Context.Ground based Cherenkov telescope systems measure astrophysical $\gamma$-ray emission against a background of cosmic-ray induced air showers. The subtraction of this background is a major challenge for the extraction of spectra and morphology of $\gamma$-ray sources. Aims.The unprecedented sensitivity of the new generation of ground based very-high-energy $\gamma$-ray experiments such as HESS has lead to the discovery of many previously unknown extended sources. The analysis of such sources requires a range of different background modelling techniques. Here we describe some of the techniques that have been applied to data from the HESS instrument and compare their performance. Methods.Each background model is introduced and discussed in terms of suitability for image generation or spectral analysis and possible caveats are mentioned. Results.We show that there is not a single multi-purpose model, different models are appropriate for different tasks. To keep systematic uncertainties under control it is important to apply several models to the same data set and compare the results

Unexpected Adsorption of Oxygen on TiO₂ Nanotube Arrays: Influence of Crystal Structure

We present kinetics data of O2, n/iso-butane, CO2, and CO adsorbed at ultrahigh vacuum conditions on TiO2 nanotube (TiNTs) arrays produced by electrochemical anodization; amorphous and polycrystalline (anatase and mixed anatase/rutile) TiNTs have been studied addressing structure−activity relationships. Oxygen distinctly interacts with the TiNTs, whereas this process is not observed on fully oxidized TiO2 single crystals. Both molecularly and atomically bonded oxygen have been observed. Variations in the binding energies of alkanes were also detected

Discovery of an X-ray nebula around PSR J1718-3825 and implications for the nature of the α-ray source HESS J1718-385

Combined X-ray synchrotron and inverse-Compton $\gamma$-ray observations of pulsar wind nebulae (PWN) may help to elucidate the processes of acceleration and energy loss in these systems. In particular, such observations provide constraints on the particle injection history and the magnetic field strength in these objects. The newly discovered TeV $\gamma$-ray source HESS J1718-385 has been proposed as the likely PWN of the high spin-down luminosity pulsar PSR J1718-3825. The absence of previous, sensitive X-ray measurements of this pulsar, and the unusual energy spectrum of the TeV source, motivated observations of this region with XMM-Newton. The data obtained reveal a hard-spectrum X-ray source at the position of PSR 1718-3825 and evidence of diffuse emission in the vicinity of the pulsar. We derive limits on the keV emission from the centroid of HESS J1718-385 and discuss the implications of these findings for the PWN nature of this object

Dark matter profiles and annihilation in dwarf spheroidal galaxies: Prospectives for present and futureγ-ray observatories - I. The classical dwarf spheroidal galaxies

Due to their large dynamical mass-to-light ratios, dwarf spheroidal galaxies (dSphs) are promising targets for the indirect detection of dark matter (DM) in γ-rays. We examine their detectability by present and future γ-ray observatories. The key innovative features of our analysis are as follows: (i) we take into account the angular size of the dSphs; while nearby objects have higher γ-ray flux, their larger angular extent can make them less attractive targets for background-dominated instruments; (ii) we derive DM profiles and the astrophysical J-factor (which parametrizes the expected γ-ray flux, independently of the choice of DM particle model) for the classical dSphs directly from photometric and kinematic data. We assume very little about the DM profile, modelling this as a smooth split-power-law distribution, with and without subclumps; (iii) we use a Markov chain Monte Carlo technique to marginalize over unknown parameters and determine the sensitivity of our derived J-factors to both model and measurement uncertainties; and (iv) we use simulated DM profiles to demonstrate that our J-factor determinations recover the correct solution within our quoted uncertainties

Moving Nanoparticles with Raman Scattering

We show how to change optically the distance between two protein-linked gold nanoparticles by Raman-induced motion of the linker protein. Rayleigh scattering spectroscopy of the coupled-particle plasmon allows us to compare the inter-nanoparticle distance of individual protein-linked gold nanoparticle dimers before and after surface-enhanced Raman scattering (SERS). We find that low-intensity (50 μW/μm2) laser light in resonance with the nanoparticle-dimer plasmon provokes a change of the inter-nanoparticle distance on the order of 0.5 nm whenever SERS from the proteins connecting the nanoparticles can be observed

Dark matter annihilation and decay in dwarf spheroidal galaxies: the classical and ultrafaint dSphs

Dwarf spheroidal (dSph) galaxies are prime targets for present and future γ-ray telescopes hunting for indirect signals of particle dark matter. The interpretation of the data requires careful assessment of their dark matter content in order to derive robust constraints on candidate relic particles. Here, we use an optimized spherical Jeans analysis to reconstruct the ‘astrophysical factor’ for both annihilating and decaying dark matter in 21 known dSphs. Improvements with respect to previous works are: (i) the use of more flexible luminosity and anisotropy profiles to minimize biases, (ii) the use of weak priors tailored on extensive sets of contamination-free mock data to improve the confidence intervals, (iii) systematic cross-checks of binned and unbinned analyses on mock and real data, and (iv) the use of mock data including stellar contamination to test the impact on reconstructed signals. Our analysis provides updated values for the dark matter content of 8 ‘classical’ and 13 ‘ultrafaint’ dSphs, with the quoted uncertainties directly linked to the sample size; the more flexible parametrization we use results in changes compared to previous calculations. This translates into our ranking of potentially-brightest and most robust targets – namely Ursa Minor, Draco, Sculptor – and of the more promising, but uncertain targets – namely Ursa Major 2, Coma – for annihilating dark matter. Our analysis of Segue 1 is extremely sensitive to whether we include or exclude a few marginal member stars, making this target one of the most uncertain. Our analysis illustrates challenges that will need to be addressed when inferring the dark matter content of new ‘ultrafaint’ satellites that are beginning to be discovered in southern sky surveys

The radio counterpart of the likely TeV binary HESS J0632+057

The few known γ-ray binary systems are all associated with variable radio and X-ray emission. The TeV source HESS J0632+057, apparently associated with the Be star MWC 148, is plausibly a new member of this class. Following the identification of a variable X-ray counterpart to the TeV source we conducted Giant Metrewave Radio Telescope (GMRT) and Very Large Array (VLA) observations in 2008 June–September to search for the radio counterpart of this object. A point-like radio source at the position of the star is detected in both 1280-MHz GMRT and 5-GHz VLA observations, with an average spectral index, α, of ∼0.6. In the VLA data there is significant flux variability on ∼month time-scales around the mean flux density of ≈0.3 mJy. These radio properties (and the overall spectral energy distribution) are consistent with an interpretation of HESS J0632+057 as a lower power analogue of the established γ-ray binary systems

The GCT camera for the Cherenkov Telescope Array

The Gamma-ray Cherenkov Telescope (GCT) is proposed for the Small-Sized Telescope component of the Cherenkov Telescope Array (CTA). GCT's dual-mirror Schwarzschild-Couder (SC) optical system allows the use of a compact camera with small form-factor photosensors. The GCT camera is ~ 0:4 m in diameter and has 2048 pixels; each pixel has a ~ 0:2° angular size, resulting in a wide field-of-view. The design of the GCT camera is high performance at low cost, with the camera housing 32 front-end electronics modules providing full waveform information for all of the camera's 2048 pixels. The first GCT camera prototype, CHEC-M, was commissioned during 2015, culminating in the first Cherenkov images recorded by a SC telescope and the first light of a CTA prototype. In this contribution we give a detailed description of the GCT camera and present preliminary results from CHEC-M's commissioning

The gamma-ray Cherenkov telescope for the Cherenkov telescope array

The Cherenkov Telescope Array (CTA) is a forthcoming ground-based observatory for very-high-energy gamma rays. CTA will consist of two arrays of imaging atmospheric Cherenkov telescopes in the Northern and Southern hemispheres, and will combine telescopes of different types to achieve unprecedented performance and energy coverage. The Gamma-ray Cherenkov Telescope (GCT) is one of the small-sized telescopes proposed for CTA to explore the energy range from a few TeV to hundreds of TeV with a field of view ≳ 8° and angular resolution of a few arcminutes. The GCT design features dual-mirror Schwarzschild-Couder optics and a compact camera based on densely-pixelated photodetectors as well as custom electronics. In this contribution we provide an overview of the GCT project with focus on prototype development and testing that is currently ongoing. We present results obtained during the first on-telescope campaign in late 2015 at the Observatoire de Paris-Meudon, during which we recorded the first Cherenkov images from atmospheric showers with the GCT multi-anode photomultiplier camera prototype. We also discuss the development of a second GCT camera prototype with silicon photomultipliers as photosensors, and plans toward a contribution to the realisation of CTA

Inauguration and first light of the GCT-M prototype for the Cherenkov telescope array

The Gamma-ray Cherenkov Telescope (GCT) is a candidate for the Small Size Telescopes (SSTs) of the Cherenkov Telescope Array (CTA). Its purpose is to extend the sensitivity of CTA to gamma-ray energies reaching 300 TeV. Its dual-mirror optical design and curved focal plane enables the use of a compact camera of 0.4 m diameter, while achieving a field of view of above 8 degrees. Through the use of the digitising TARGET ASICs, the Cherenkov flash is sampled once per nanosecond contin-uously and then digitised when triggering conditions are met within the analogue outputs of the photosensors. Entire waveforms (typically covering 96 ns) for all 2048 pixels are then stored for analysis, allowing for a broad spectrum of investigations to be performed on the data. Two prototypes of the GCT camera are under development, with differing photosensors: Multi-Anode Photomultipliers (MAPMs) and Silicon Photomultipliers (SiPMs). During November 2015, the GCT MAPM (GCT-M) prototype camera was integrated onto the GCT structure at the Observatoire de Paris-Meudon, where it observed the first Cherenkov light detected by a prototype instrument for CTA