arXiv subjects

F. Schüssler

Publications and source records attributed to F. Schüssler.

32 records · Page 2Linked to original sources

arXiv2017

Target of Opportunity Observations of Blazars with H.E.S.S

The very-high-energy (VHE, E>100 GeV) extragalactic sky is dominated by blazars, a class of active galactic nuclei which show rapid variability at all wavelengths. Target of Opportunity (ToO) observations triggered by flaring activity detected at longer wavelengths are, thus, an important part of the blazar observing strategy of H.E.S.S., an array of five imaging atmospheric Cherenkov telescopes sensitive to VHE photons. In this contribution we detail the H.E.S.S. extragalactic ToO program, describing the specific procedures currently in place to follow up on multi-wavelength alerts. The program is illustrated by discussing a few recent noteworthy targets observed with the H.E.S.S. phase II array over the last two years of blazar ToO observations.

astro-ph.HE↗

arXiv2017

GRB Observations with H.E.S.S. II

The High Energy Stereoscopic System (H.E.S.S.) has been searching for counterparts of Gamma Ray Bursts (GRBs) for many years. In 2012 the system was upgraded with a fifth $28$ m diameter telescope (CT5) which is equipped with faster motors for rapid repointing, marking the start of the second phase of H.E.S.S. operation (H.E.S.S. II). CT5s large light collection area of $600\,{\rm m}^{2}$ improves the sensitivity to low-energy gamma-rays and even extends the energy range below $100$ GeV. The search for counterparts continues now in the energy range of tens of GeV to tens of TeV. A detection in this energy range would open a new window to the part of the spectrum of these highly energetic explosions which Fermi-LAT has only successfully detected in a reduced subset of events, with rather limited statistics. In the past years, H.E.S.S. has performed followup observations based on GRB detections by Swift-BAT and Fermi-GBM/-LAT. This Target of Opportunity observation program was carried out with a generalised Target of Opportunity Alert system. This contribution will highlight key features of the Target of Opportunity Alert system, present follow-up statistics of GRBs as well as detailed results of promising follow-up observations.

astro-ph.HE↗

arXiv2017

Searching for VHE gamma-ray emission associated with IceCube astrophysical neutrinos using FACT, H.E.S.S., MAGIC, and VERITAS

The detection of an astrophysical flux of high-energy neutrinos by IceCube is a major step forward in the search for the origin of cosmic rays, as this emission is expected to originate in hadronic interactions taking place in or near cosmic-ray accelerators. No neutrino point sources, or a significant correlation with known astrophysical objects, have been identified in the IceCube data so far that could reveal the location of the neutrino emission sites. The hadronic interactions responsible for the neutrino emission should also lead to the production of high-energy gamma rays from neutral pion decays. The search for neutrino sources can therefore be performed by studying the spatial and temporal correlations between neutrino events and very-high-energy (VHE, E > 100 GeV) gamma rays. We report on the search for VHE gamma-ray emission at the reconstructed position of muon neutrino events detected by IceCube using the FACT, H.E.S.S., MAGIC, and VERITAS imaging atmospheric Cherenkov telescopes (IACTs). No significant steady gamma-ray counterparts have been identified for the neutrino events observed so far. Finally, we outline recent programs to perform prompt IACT observations of realtime IceCube neutrino event positions.

astro-ph.HE↗

arXiv2017

Gravitational wave alert follow-up strategy in the H.E.S.S. multi-messenger framework

The H.E.S.S. high-energy gamma-ray observatory is member of the Virgo/LIGO electromagnetic follow-up effort since early 2014. Its capability for transient follow-up studies benefits from its large field of view, rapid response time and high sensitivity. Drawing from the experience gained from other science cases like gamma-ray bursts and high-energy neutrino follow-ups we demonstrate the high perspectives for new types of analyses like the search for gravitational wave counterparts and the study of multi-messenger signals from binary neutron star mergers. This contribution aims to present the potential pointing strategy that the H.E.S.S. observatory would carry out following an alert from gravitational wave observatories. We will discuss several key points like the use of information from a galaxy catalogue, the time-dependent visibility of sky regions and the automatic handling of gravitational wave uncertainty maps, that will enable an optimized choice of the pointing directions. Finally, based on simulated binary neutron star mergers, the performance of the outlined gravitational wave-alert observations will be presented.

astro-ph.IM↗

arXiv2017

H.E.S.S. multi-messenger observations

The H.E.S.S. Imaging Air Cherenkov Telescope system is very well suited to perform follow-up observations of detections at other wavelengths or messengers due to its fast reaction and its comparably low energy threshold. These advantages are fully exploited via a largely automatized system reacting to alerts from various partner observatories covering all wavelengths and astrophysical messengers. In this contribution we provide an overview and recent results from H.E.S.S. programs to follow up on multi-wavelength and multi-messenger alerts. We present ToO observations searching for high-energy gamma-ray emission in coincidence with electromagnetic transients Fast Radio Bursts as well as multi-messenger transients related to high-energy neutrinos and Gravitational Waves.

astro-ph.HE↗

arXiv2016

Observations of Binary Systems with the H.E.S.S. Telescopes

Observations of binary systems obtained recently with the High Energy Stereoscopic System (H.E.S.S) of Cherenkov telescopes are reported. The outcomes of a detailed observation campaign on PSR B1259-63 during its periastron passage in 2014 will be presented. This system was observed for the first time with H.E.S.S. II, providing spectra and light curves down to 200 GeV, which will be compared with observations conducted during previous periastron passages and with results from an analysis of contemporaneously taken Fermi-LAT data. Also long-term observations of LS 5039 with H.E.S.S in phase I and phase II are reported. This source was monitored at very high energies (VHEs) in a period of time spanning more than ten years. Its spectral energy distribution measured with H.E.S.S. II extends down to 120 GeV. Spectral results from the Fermi-LAT observations are shown as well, and the compatibility with H.E.S.S. results in the overlapping energy range is discussed. The identification of the new gamma-ray binary candidate HESS J1832-093 will also be presented. Furthermore, the search for VHE emission from the microquasars GRS 1915+105, Circinus X-1 and V4641 Sgr based on data from H.E.S.S. observations conducted contemporaneously with the RXTE satellite experiment will be reported. These data provide constraints on the integral gamma-ray flux at different X-ray states of the three sources.

astro-ph.HE↗

arXiv2016

A search for Secluded Dark Matter in the Sun with the ANTARES neutrino telescope

A search for Secluded Dark Matter annihilation in the Sun using 2007-2012 data of the ANTARES neutrino telescope is presented. Three different cases are considered: a) detection of dimuons that result from the decay of the mediator, or neutrino detection from: b) mediator that decays into a dimuon and, in turn, into neutrinos, and c) mediator that decays directly into neutrinos. As no significant excess over background is observed, constraints are derived on the dark matter mass and the lifetime of the mediator.

hep-ex↗

arXiv2015

The H.E.S.S. multi-messenger program

Based on fundamental particle physics processes like the production and subsequent decay of pions in interactions of high-energy particles, close connections exist between the acceleration sites of high-energy cosmic rays and the emission of high-energy gamma rays and high-energy neutrinos. In most cases these connections provide both spatial and temporal correlations of the different emitted particles. The combination of the complementary information provided by these messengers allows to lift ambiguities in the interpretation of the data and enables novel and highly sensitive analyses. In this contribution the H.E.S.S. multi-messenger program is introduced and described. The current core of this newly installed program is the combination of high-energy neutrinos and high-energy gamma rays. The search for gamma-ray emission following gravitational wave triggers is also discussed. Furthermore, the existing program for following triggers in the electromagnetic regime was extended by the search for gamma-ray emission from Fast Radio Bursts (FRBs). An overview over current and planned analyses is given and recent results are presented.

astro-ph.HE↗

arXiv2015

Simultaneous H.E.S.S. and RXTE observations of the microquasars GRS 1915+105, Circinus X-1 and V4641 Sgr

Microquasars, Galactic binary systems showing extended and variable radio emission, are potential gamma-ray emitters. Indications of gamma-ray transient episodes have been reported in at least two systems, Cyg X-1 and Cyg X-3. The identification of additional gamma-ray emitting microquasars is key for a better understanding of these systems. Very-high-energy gamma-ray emission from microquasars has been predicted to happen during periods of transient outbursts potentially connected with the formation of a jet-like outflow. Contemporaneous observations using the H.E.S.S. telescope array and the RXTE satellite were obtained on three microquasars: GRS 1915+105, Circinus X-1 and V4641 Sgr with the aim of detecting a broadband flaring event in the very-high-energy gamma-ray and X-ray bands. We report here on the analysis of these data for each system, including a detailed X-ray analysis assessing the location of the sources in a hardness-intensity diagram during the observations. Finally we discuss the derived upper limits on their very-high-energy gamma-ray flux.

astro-ph.HE↗

arXiv2013

Multiwavelength study of the region around the ANTARES neutrino excess

The ANTARES collaboration reported the results of a search for point-like neutrino sources using data taken in the period 2007-2010. An unbinned maximum likelihood based all-sky search yielded a cluster of 9 (5) events within a cone of 3 (1) degrees around (R.A., Dec) = (-46.5deg, -65.0deg). The trial factor corrected p-value of 2.6% (2.2 sigma) is not significant enough to claim the observation of an astrophysical point source. However, it currently constitutes the most significant localized neutrino excess observed by ANTARES. Here we present a multi-wavelength analysis including optical to X-ray archival data and a dedicated analysis of gamma-ray data from Fermi-LAT. In order to cover the TeV domain, dedicated observations with the H.E.S.S. telescope array were carried out. We present these data and discuss implications of the results in terms of signatures for a cosmic-ray acceleration site.

astro-ph.HE↗

arXiv2008

Third Level Trigger for the Fluorescence Telescopes of the Pierre Auger Observatory

The trigger system for the Auger fluorescence telescopes is implemented in hard- and software for an efficient selection of fluorescence light tracks induced by high-energy extensive air showers. The algorithm of the third stage uses the multiplicity signal of the hardware for fast rejection of lightning events with above 99% efficiency. In a second step direct muon hits in the camera and random triggers are rejected by analyzing the space-time correlation of the pixels. The trigger algorithm was tested with measured and simulated showers and implemented in the electronics of the fluorescence telescopes. A comparison to a prototype trigger without multiplicity shows the superiority of this approach, e.g. the false rejection rate is a factor 10 lower.

astro-ph↗

arXiv2008

Reconstruction of Longitudinal Profiles of Ultra-High Energy Cosmic Ray Showers from Fluorescence and Cherenkov Light Measurements

We present a new method for the reconstruction of the longitudinal profile of extensive air showers induced by ultra-high energy cosmic rays. In contrast to the typically considered shower size profile, this method employs directly the ionization energy deposit of the shower particles in the atmosphere. Due to universality of the energy spectra of electrons and positrons, both fluorescence and Cherenkov light can be used simultaneously as signal to infer the shower profile from the detected light. The method is based on an analytic least-square solution for the estimation of the shower profile from the observed light signal. Furthermore, the extrapolation of the observed part of the profile with a Gaisser-Hillas function is discussed and the total statistical uncertainty of shower parameters like total energy and shower maximum is calculated.

astro-ph↗

arXiv2007

On the relation between the proton-air cross section and fluctuations of the shower longitudinal profile

The current status and prospects of deducing the proton-air cross section from fluorescence telescope measurements of extensive air showers are discussed. As it is not possible to observe the point of first interaction, $X_ {\rm 1}$, directly, other observables closely linked to $X_{\rm 1}$ must be inferred from the measured longitudinal profiles. This introduces a dependence on the models used to describe the shower development. Systematic uncertainties arising from this model dependence, from the reconstruction method itself and from a possible non-proton contamination of the selected shower sample are discussed.

astro-ph↗

arXiv2007

Longitudinal Shower Profile Reconstruction from Fluorescence and Cherenkov Light

Traditionally, longitudinal shower profiles are reconstructed in fluorescence light experiments by treating the Cherenkov light contribution as background. Here we will argue that, due to universality of the energy spectra of electrons and positrons, both fluorescence and Cherenkov light can be used simultaneously as signal to infer the longitudinal shower development. We present a new profile reconstruction method that is based on the analytic least-square solution for the estimation of the shower profile from the observed light signal and discuss the extrapolation of the profile with a Gaisser-Hillas function.

astro-ph↗