arXiv subjects

Ullrich Schwanke

Publications and source records attributed to Ullrich Schwanke.

18 recordsLinked to original sources

arXiv2026

Coulomb crystallization of xenon highly charged ions in a laser-cooled Ca+ matrix

We report on the sympathetic cooling and Coulomb crystallization of xenon highly charged ions (HCIs) with laser-cooled Ca$^+$ ions. The HCIs are produced in a compact electron beam ion trap, then charge selected, decelerated, and finally injected into a cryogenic linear Paul trap. There, they are captured into $^{40}$Ca$^+$ Coulomb crystals, and co-crystallized within them, causing dark voids in their fluorescence images. Fine control over the number of trapped ions and HCIs allows us to realize mixed-species crystals with arbitrary ordering patterns. By investigating Xe$^{q+}$--Ca$^+$ strings, we confirm the HCI charge states, measure their lifetime and characterize the mixed-species motional modes. Our system effectively combines the established quantum control toolbox for Ca$^+$ with the rich set of atomic properties of Xe highly charged ions, providing a resourceful platform for optical frequency metrology, searches for signatures of new physics, and quantum information science.

physics.atom-ph↗

arXiv2021

Probing the hadronic nature of the gamma-ray emission associated with Westerlund 2

Star-forming regions have been proposed as potential Galactic cosmic-ray accelerators for decades. Cosmic ray acceleration can be probed through observations of gamma-rays produced in inelastic proton-proton collisions, at GeV and TeV energies. We analyze more than 11 years of Fermi-LAT data from the direction of Westerlund 2, one of the most massive and best-studied star-forming regions in our Galaxy. The spectral and morphological characteristics of the LAT source agree with the ones in the TeV regime (HESS J1023-575), allowing the description of the gamma-ray source from a few hundreds of MeV to a few tens of TeVs. We will present the results and discuss the implications of the identification with the stellar cluster and the radiation mechanism involved.

astro-ph.HE↗

arXiv2021

CTbend: A Bayesian open-source framework to model pointing corrections for Cherenkov telescopes

The pointing of Cherenkov telescopes is subject to imperfections which are, for example, related to the bending of the telescopes mechanical structure. These imperfections must be measured, modeled, and finally corrected to achieve an optimal telescope pointing precision. The measurement of pointing deviations is often performed while the telescope points to different stars and a CCD camera monitors the offsets of the star images to the center of the focal plane. Outliers in these measurements can propagate into the pointing model and lead to imprecise model predictions. CTbend is a simple and standalone open-source framework that uses a Bayesian analysis with an outlier resilient likelihood function to model the pointing of Cherenkov telescopes with parametric standard models like TPoint. The framework is described in the following.

astro-ph.IM↗

arXiv2021

Monte Carlo Simulations and Validation of NectarCAM, a Medium Sized Telescope Camera for CTA

The upcoming Cherenkov Telescope Array (CTA) ground-based gamma-ray observatory will open up our view of the very high energy Universe, offering an improvement in sensitivity of 5-10 times that of previous experiments. NectarCAM is one of the proposed cameras for the Medium-Sized Telescopes (MST) which have been designed to cover the core energy range of CTA, from 100 GeV to 10 TeV. The final camera will be capable of GHz sampling and provide a field of view of 8 degrees with its 265 modules of 7 photomultiplier each (for a total of 1855 pixels). In order to validate the performance of NectarCAM, a partially-equipped prototype has been constructed consisting of only the inner 61-modules. It has so far undergone testing at the integration test-bench facility in CEA Paris-Saclay (France) and on a prototype of the MST structure in Adlershof (Germany). To characterize the performance of the prototype, Monte Carlo simulations were conducted using a detailed model of the 61 module camera in the CORSIKA/sim_telarray framework. This contribution provides an overview of this work including the comparison of trigger and readout performance on test-bench data and trigger and image parameterization performance during on-sky measurements.

astro-ph.IM↗

arXiv2021

Probing the hadronic nature of the gamma-ray emission associated with Westerlund 2

Star-forming regions have been proposed as potential Galactic cosmic-ray accelerators for decades. Cosmic-ray acceleration can be probed through observations of gamma-rays produced in inelastic proton-proton collisions, at GeV and TeV energies. In this paper, we analyze more than 11 years of Fermi-LAT data from the direction of Westerlund 2, one of the most massive and best-studied star-forming regions in our Galaxy. In particular, we investigate the characteristics of the bright pulsar PSR J1023-5746 that dominates the gamma-ray emission below a few GeV at the position of Westerlund 2, and the underlying extended source FGES J1023.3-5747. The analysis results in a clear identification of FGES J1023.3-5747 as the GeV counterpart of the TeV source HESS J1023-575, through its morphological and spectral properties. This identification provides new clues about the origin of the HESS J1023-575 gamma-ray emission, favouring a hadronic origin of the emission, powered by Westerlund 2, rather than a leptonic origin related to either the pulsar wind nebula associated with PSR J1023-5746 or the cluster itself. This result indirectly supports the hypothesis that star-forming regions can contribute to the cosmic-ray sea observed in our Galaxy

astro-ph.HE↗

arXiv2019

A Condition Monitoring Concept Studied at the MST Prototype for the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is a future ground-based gamma-ray observatory that will provide unprecedented sensitivity and angular resolution for the detection of gamma rays with energies above a few tens of GeV. In comparison to existing instruments (like H.E.S.S., MAGIC, and VERITAS) the sensitivity will be improved by installing two extended arrays of telescopes in the northern and southern hemisphere, respectively. A large number of planned telescopes (>100 in total) motivates the application of predictive maintenance techniques to the individual telescopes. A constant and automatic condition monitoring of the mechanical telescope structure and of the drive system (motors, gears) is considered for this purpose. The condition monitoring system aims at detecting degradations well before critical errors occur; it should help to ensure long-term operation and to reduce the maintenance efforts of the observatory. We present approaches for the condition monitoring of the structure and the drive system of Medium-Sized Telescopes (MSTs), respectively. The overall concept has been developed and tested at the MST prototype for CTA in Berlin. The sensors used, the joint data acquisition system, possible analysis methods (like Operational Modal Analysis, OMA, and Experimental Modal Analysis, EMA) and first performance results are discussed.

astro-ph.IM↗

arXiv2018

Application of Deep Learning methods to analysis of Imaging Atmospheric Cherenkov Telescopes data

Ground based gamma-ray observations with Imaging Atmospheric Cherenkov Telescopes (IACTs) play a significant role in the discovery of very high energy (E > 100 GeV) gamma-ray emitters. The analysis of IACT data demands a highly efficient background rejection technique, as well as methods to accurately determine the energy of the recorded gamma-ray and the position of its source in the sky. We present results for background rejection and signal direction reconstruction from first studies of a novel data analysis scheme for IACT measurements. The new analysis is based on a set of Convolutional Neural Networks (CNNs) applied to images from the four H.E.S.S. phase-I telescopes. As the H.E.S.S. cameras pixels are arranged in a hexagonal array, we demonstrate two ways to use such image data to train CNNs: by resampling the images to a square grid and by applying modified convolution kernels that conserve the hexagonal grid properties. The networks were trained on sets of Monte-Carlo simulated events and tested on both simulations and measured data from the H.E.S.S. array. A comparison between the CNN analysis to current state-of-the-art algorithms reveals a clear improvement in background rejection performance. When applied to H.E.S.S. observation data, the CNN direction reconstruction performs at a similar level as traditional methods. These results serve as a proof-of-concept for the application of CNNs to the analysis of events recorded by IACTs.

astro-ph.IM↗

arXiv2017

Probing Convolutional Neural Networks for Event Reconstruction in γ-Ray Astronomy with Cherenkov Telescopes

A dramatic progress in the field of computer vision has been made in recent years by applying deep learning techniques. State-of-the-art performance in image recognition is thereby reached with Convolutional Neural Networks (CNNs). CNNs are a powerful class of artificial neural networks, characterized by requiring fewer connections and free parameters than traditional neural networks and exploiting spatial symmetries in the input data. Moreover, CNNs have the ability to automatically extract general characteristic features from data sets and create abstract data representations which can perform very robust predictions. This suggests that experiments using Cherenkov telescopes could harness these powerful machine learning algorithms to improve the analysis of particle-induced air-showers, where the properties of primary shower particles are reconstructed from shower images recorded by the telescopes. In this work, we present initial results of a CNN-based analysis for background rejection and shower reconstruction, utilizing simulation data from the H.E.S.S. experiment. We concentrate on supervised training methods and outline the influence of image sampling on the performance of the CNN-model predictions.

astro-ph.IM↗

arXiv2017

A pointing solution for the medium size telescopes for the Cherenkov Telescope Array

The pointing capability of a telescope in the Cherenkov Telescope Array (CTA) is a crucial aspect in the calibration of the instrument. It describes how a position in the sky is transformed to the focal plane of the telescope and allows precise directional reconstructions of atmospheric particle showers. The favoured approach for pointing calibrations of the Medium Size Telescopes (MST) is the utilisation of an CCD-camera installed in the centre of the dish, which images the night sky and the focal plane simultaneously. The technical implementation of this solution and test results taken over a period of one year at the MST prototype in Berlin/Adlershof are presented. Investigations of pointing calibration precision with simulated data and real data taken during test runs of the prototype telescope will also be shown.

astro-ph.IM↗

arXiv2017

Dark matter line searches towards dwarf galaxies with H.E.S.S

High energy $γ$-rays are powerful probes in the search for annihilations of dark matter (DM) par- ticles in dense environments. In several DM particle models their annihilation produces characteristic features such as lines, bumps or cut-offs in their energy spectrum. The High Energy Stereoscopic System (H.E.S.S.) of imaging atmospheric Cherenkov telescopes is perfectly suited to search for such features from multi-TeV mass DM particles. The Dwarf Spheroidal Galaxies (dSphs) of the Local Group are the most common satellites of the Milky Way and assumed to be gravitationally bound dominantly by DM, with up to O(10 3 ) times more mass in DM than in visible matter. Over the past decade, several observational campaigns on dwarf satellite galaxies were launched by H.E.S.S. amounting to more than 140 hours of exposure in total. The observations are reviewed here. In the absence of clear signals, the expected spectral and spatial morphologies of signal and background are used to derive constraints on the DM particle annihilation cross- section for particle models producing line-like signals. The combination of the data of all the dwarf galaxies allows a significant improvement in the HESS sensitivity.

astro-ph.HE↗

arXiv2017

Radio Observations of the Region around the Pulsar Wind Nebula HESS J1303-631 with ATCA

Radio observations of the region surrounding PSR J1301-6305 at 5.5 GHz and 7.5 GHz were conducted with ATCA on September 5th, 2013. They were dedicated to the search of the radio counterpart of the evolved pulsar wind nebula HESS J1303-631, detected in X-rays and GeV-TeV gamma-rays. The collected data do not reveal any significant extended emission associated with PSR J1301-6305. In addition, archival 1.384 GHz and 2.368 GHz data do not show any evidence for a radio counterpart of HESS J1303-631. Archival 1.384 GHz observations reveal a detection of an extended structure centred at an angular distance of 190 from the pulsar. This extended structure might be a Supernova remnant (SNR) and a potential birth place of PSR J1301-6305. The implications of the lack of radio counterpart of HESS J1303-631 on the understanding of the nature of the PWN are discussed.

astro-ph.HE↗

arXiv2015

Status of the Medium-Sized Telescope for the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA), is an international project for the next generation ground- based observatory for gamma-ray astronomy in the energy range from 20 GeV to 300 TeV. The sensitivity in the core energy range will be dominated by up to 40 Medium-Sized Telescopes (MSTs). The MSTs, of Davies-Cotton type with a 12 m diameter reflector are currently in the prototype phase. A full-size mechanical telescope structure has been assembled in Berlin. The telescope is partially equipped with different mirror prototypes, which are currently being tested and evaluated for performances characteristics. A report concentrating on the details of the tele- scope structure, the drive assemblies and the optics of the MST prototype will be given.

astro-ph.IM↗

arXiv2015

Radio Observations of the Pulsar Wind Nebula HESS J1303-631 Field of View with ATCA

Based on its energy-dependent morphology the initially unidentified very high energy (VHE; E>100 GeV) gamma-ray source HESS J1303-631 was recently associated with the pulsar PSR J1301-6305. Subsequent detection of X-ray and GeV counterparts also supports the identification of the H.E.S.S. source as evolved pulsar wind nebula (PWN). We report here on recent radio observations of the PSR J1301-6305 field of view (FOV) with ATCA dedicated to search for the radio counterpart of this evolved PWN. Observations at 5.5 GHz and 7.5 GHz do not reveal any extended emission associated with the pulsar. The analysis of the archival 1.384 GHz and 2.368 GHz data also does not show any significant emission. The 1.384 GHz data reveal a hint of an extended shell-like emission in the PSR J1301-6305 FOV which might be a supernova remnant. We discuss the implications of the non-detection at radio wavelengths on the nature and evolution of the PWN as well as the possibility of the SNR candidate being the birth place of PSR J1301-6305.

astro-ph.HE↗

arXiv2013

Pointing Calibration for the Cherenkov Telescope Array Medium Size Telescope Prototype

Pointing calibration is an offline correction applied in order to obtain the true pointing direction of a telescope. The Cherenkov Telescope Array (CTA) aims to have the precision to determine the position of point-like as well as slightly extended sources, with the goal of systematic errors less than 7 arc seconds in space angle. This poster describes the pointing calibration concept being developed for the CTA Medium Size Telescope (MST) prototype at Berlin-Adlershof, showing test results and preliminary measurements. The MST pointing calibration method uses two CCD cameras, mounted on the telescope dish, to determine the true pointing of the telescope. The "Lid CCD" is aligned to the optical axis of the telescope, calibrated with LEDs on the dummy gamma-camera lid; the "Sky CCD" is pre-aligned to the Lid CCD and the transformation between the Sky and Lid CCD camera fields of view is precisely modelled with images from special pointing runs which are also used to determine the pointing model. During source tracking, the CCD cameras record images which are analysed offline using software tools including Astrometry.net to determine the true pointing coordinates.

astro-ph.IM↗

arXiv2013

Impact of the Cherenkov Telescope Array (CTA) altitude on Dark Matter searches in the Milky Way Halo

Observations of dwarf galaxies and of the Milky Way halo with current ground-based Cherenkov telescopes have resulted in interesting limits on the cross-section for dark matter (DM) self- annihilation for WIMP masses above some 100 GeV. The future Cherenkov Telescope Array (CTA) is expected to further explore the parameter space of dark matter candidates that are predicted in extensions of the standard model of particle physics. Due to its low energy threshold (of order of few tens of GeV) and high sensitivity, CTA will also probe lower WIMP masses than current experiments, but the actual performance in this regime will be influenced by the altitude of the observatory above sea level. Using the response of possible CTA candidate arrays to simulated photons and hadrons, we estimate how searches for a WIMP annihilation signal from the Milky Way halo will be influenced by altitude of different possible CTA sites.

astro-ph.HE↗

arXiv2013

Strategies for the Detection of Gamma Rays from Dark Matter Annihilation Towards the Galactic Center Region with the High Energy Stereoscopic System

The central region of the dark matter halo of the Milky Way is a promising target for a search for a particle dark matter annihilation signal. The H.E.S.S. Collaboration has published a search for a photon flux originating from dark matter particles annihilating in the galactic center region. No significant excess was observed and upper limits on the velocity averaged dark matter self annihilation cross section were derived. The limits exclude the self annihilation of dark matter particles in the $\sim 1$ TeV to $\sim 4$ TeV mass range with a velocity averaged annihilation cross section larger than $\sim 3\cdot 10^{-25}\:\mathrm{cm^3/s}$, i.e. about one order of magnitude above the prediction for a thermal relic dark matter particle. A detailed and realistic Monte Carlo study of new strategies for the search for a particle dark matter annihilation signal from the Milky Way dark matter halo with the High Energy Stereoscopic System is presented and the sensitivity of different experimental approaches is compared.

astro-ph.HE↗

arXiv2012

Trigger and data rates expected for the CTA Observatory

The Cherenkov Telescope Array (CTA) is an initiative to build a next-generation observatory for very-high energy $γ$-rays. Its expected large effective area ($\mathcal{O}(10^{7} \mathrm{m}^2)$) and energy threshold as low as 25 GeV imply a challenge for triggering and data acquisition systems. The analysis of the official CTA Monte Carlo production-1 simulations leads to array trigger rates of $\mathcal{O}$(10 kHz) and data rates ranging from $\mathcal{O}$(100 MB/s) to $\mathcal{O}$(1000 MB/s), depending on the read-out scenario.

astro-ph.IM↗

arXiv2012

H.E.S.S. deeper observations on SNR RX J0852.0-4622

Supernova Remnants (SNRs) are believed to be acceleration sites of Galactic cosmic rays. Therefore, deep studies of these objects are instrumental for an understanding of the high energy processes in our Galaxy. RX J0852.0-4622, also known as Vela Junior, is one of the few (4) shell-type SNRs resolved at Very High Energies (VHE; E > 100 GeV). It is one of the largest known VHE sources (~ 1.0 deg radius) and its flux level is comparable to the flux level of the Crab Nebula in the same energy band. These characteristics allow for a detailed analysis, shedding further light on the high-energy processes taking place in the remnant. In this document we present further details on the spatial and spectral morphology derived with an extended data set. The analysis of the spectral morphology of the remnant is compatible with a constant power-law photon index of 2.11 +/- 0.05_stat +/- 0.20_syst from the whole SNR in the energy range from 0.5 TeV to 7 TeV. The analysis of the spatial morphology shows an enhanced emission towards the direction of the pulsar PSR J0855-4644, however as the pulsar is lying on the rim of the SNR, it is difficult to disentangle both contributions. Therefore, assuming a point source, the upper limit on the flux of the pulsar wind nebula (PWN) between 1 TeV and 10 TeV, is estimated to be ~ 2% of the Crab Nebula flux in the same energy range.

astro-ph.HE↗