arXiv subjects

Dieter Horns

Publications and source records attributed to Dieter Horns.

At least 55 records · Page 3Linked to original sources

arXiv2013

WISPers from the Dark Side: Radio Probes of Axions and Hidden Photons

Measurements in the radio regime embrace a number of effective approaches for WISP searches, often covering unique or highly complementary ranges of the parameter space compared to those explored in other research domains. These measurements can be used to search for electromagnetic tracers of the hidden photon and axion oscillations, extending down to ~10^-19 eV the range of the hidden photon mass probed, and closing the last gaps in the strongly favoured 1-5 micro-eV range for axion dark matter. This provides a strong impetus for several new initiatives in the field, including the WISP Dark Matter eXperiment (WISPDMX) and novel conceptual approaches for broad-band WISP searches in the 0.1-1000 micro-eV range.

physics.ins-det↗

arXiv2013

Pair-production opacity at high and very-high gamma-ray energies

The propagation of high energy (HE, $E_γ>100$ MeV) and very high-energy gamma-rays (VHE, $E_γ>100$ GeV) in the extra-galactic photon field leads to pair-production and consequently energy- and distance-dependent attenuation of the primary intensity. The spectroscopy of an increasing number of extra-galactic objects at HE and VHE energies has demonstrated indeed the presence of such an attenuation which in turn has been used to constrain the photon density in the medium. At large optical depth ($τ\gtrsim 2$) potential modifications of pair-production due to competing but rare processes (as, e.g., the presence of sub-neV axion-like particle) may be found. Indications for a pair-production anomaly have previously been found with VHE-spectra. Here, we present further indications (at the level of $3.68 σ$) for a reduced optical depth at high energies from an analysis of Fermi-\textit{LAT} data.

astro-ph.HE↗

arXiv2013

First lower limits on the photon-axion-like particle coupling from very high energy gamma-ray observation

The intrinsic flux of very high energy (VHE, Energy > 100 GeV) gamma-rays from extragalactic sources is attenuated due to pair production in the interaction with photons of the extragalactic background light (EBL). Depending on the distance of the source, the Universe should be opaque to VHE photons above a certain energy. However, indications exist that the Universe is more transparent than previously thought. A recent statistical analysis of a large sample of VHE spectra shows that the correction for absorption with current EBL models is too strong for the data points with the highest attenuation. An explanation might be the oscillation of VHE photons into hypothetical axion-like particles (ALPs) in ambient magnetic fields. This mechanism would decrease the opacity as ALPs propagate unimpeded over cosmological distances. Here, a large sample of VHE gamma-ray spectra obtained with imaging air Cherenkov telescopes is used to set, for the first time, lower limits on the photon-ALP coupling constant over a large range of ALP masses. The conversion in different magnetic field configurations, including intra-cluster and intergalactic magnetic fields together with the magnetic field of the Milky Way, is investigated taking into account the energy dependence of the oscillations. For optimistic scenarios of the intervening magnetic fields, a lower limit on the photon-ALP coupling of the order of $10^{-12}\mathrm{GeV}^{-1}$ is obtained whereas more conservative model assumptions result in $2\times10^{-11}\mathrm{GeV}^{-1}$. The latter value is within reach of future dedicated ALP searches.

astro-ph.HE↗

arXiv2013

Any Light Particle Search II -- Technical Design Report

This document constitutes an excerpt of the Technical Design Report for the second stage of the "Any Light Particle Search" (ALPS-II) at DESY as submitted to the DESY PRC in August 2012 and reviewed in November 2012. ALPS-II is a "Light Shining through a Wall" experiment which searches for photon oscillations into weakly interacting sub-eV particles. These are often predicted by extensions of the Standard Model and motivated by astrophysical phenomena. The first phases of the ALPS-II project were approved by the DESY management on February 21st, 2013.

physics.ins-det↗

arXiv2013

Event reconstruction techniques for the wide-angle air Cherenkov detector HiSCORE

Wide-angle, non-imaging air Cherenkov detectors provide a way to observe cosmic gamma-rays which is complementary to observations by imaging Cherenkov telescopes. Their particular strength lies in the multi-TeV to ultra high energy range (E > 30 TeV), where large effective areas, yet small light sensitive areas per detector station are needed. To exploit this potential to full extent, a large station spacing is required to achieve a large effective area at a reasonable effort. In such a detector, the low number of signals per event, the absence of imaging information, and the poor signal to noise ratio of Cherenkov light to night sky brightness pose considerable challenges for the event reconstruction, especially the gamma hadron separation. The event reconstruction presented in this paper has been developed for the wide-angle detector HiSCORE, but the concepts may be applied more generically. It is tested on simulated data in the 10 TeV to 5 PeV energy range. For the tests, a regular grid of 22 x 22 detector stations with a spacing of 150 m is assumed, covering an area of 10 km^2. The angular resolution of individual events is found to be about 0.3 degree near the energy threshold, improving to below 0.1 degree at higher energies. The relative energy resolution is 20% at the threshold and improves to 10% at higher energies. Several parameters for gamma hadron separation are described. With a combination of these parameters, 80% to 90% of the hadronic background can be suppressed, while about 60% of the gamma-ray events are retained. The point source sensitivity to gamma-ray sources is estimated, with conservative assumptions, to be about 8 x 10^(-13) erg / s / cm^2 at 100 TeV gamma-ray for a 10 km^2 array. With more optimistic assumptions, and a 100 km^2 array, a sensitivity of about 1 x 10^(-13) erg / s / cm^2 can be achieved (at 100 TeV).

astro-ph.IM↗

arXiv2013

Revisiting the Indication for a low opacity Universe for very high energy gamma-rays

Very high energy (VHE, energy >~ 100 GeV) γ-rays undergo pair production with photons of the extragalactic background light (EBL). Thus, the intrinsic γ-ray flux of cosmological sources is attenuated and the Universe should be opaque to γ-rays above a redshift dependent energy. Recently, an indication has been found that the Universe is more transparent than predicted by a lower-limit EBL model. Here, this indication is confronted with additional VHE γ-ray spectra and different EBL models. Depending on the model for the opacity, the indication persist between a ~2.6 σ and ~4.3 σ confidence level.

astro-ph.HE↗

arXiv2013

Chandra, Keck and VLA Observations of the Crab Nebula during the 2011-April Gamma-ray Flare

We present results from our analysis of Chandra X-ray Observatory, W. M. Keck Observatory, and Karl G. Jansky Very Large Array (VLA) images of the Crab Nebula that were contemporaneous with the gamma-ray flare of 2011 April. Despite hints in the X-ray data, we find no evidence for statistically significant variations that pinpoint the specific location of the flares within the Nebula. The Keck observations extend this conclusion to the "inner knot", i.e., the feature within an arcsecond of the pulsar. The VLA observations support this conclusion. We also discuss theoretical implications of the gamma-ray flares and suggest that the most dramatic gamma-ray flares are due to radiation-reaction-limited synchrotron emission associated with sudden, dissipative changes in the current system sustained by the central pulsar.

astro-ph.HE↗

arXiv2012

Searching for WISPy Cold Dark Matter with a Dish Antenna

The cold dark matter of the Universe may be comprised of very light and very weakly interacting particles, so-called WISPs. Two prominent examples are hidden photons and axion-like particles. In this note we propose a new technique to sensitively search for this type of dark matter with dish antennas. The technique is broadband and allows to explore a whole range of masses in a single measurement.

hep-ph↗

arXiv2012

The effect of photon-axion-like particle conversions in galaxy clusters on very high energy γ-ray spectra

Very high energy (VHE, energy >~ 100 GeV) γ-rays originating from extragalactic sources interact with low energy photons of background radiation fields and produce electron-positron pairs. Alternatively, in the presence of ambient magnetic fields, they can convert into hypothetical axion-like particles (ALPs), pseudo-scalar spin-0 bosons, predicted by extensions of the standard model. These particles propagate unimpeded over cosmological distances. Here, the effect of photon-ALP oscillations in magnetic fields of galaxy clusters and the Milky Way on VHE γ-ray spectra is studied. It is shown that this mechanism can lead to a substantial enhancement of the VHE flux and a spectral hardening, thus effectively reducing the opacity of the Universe to VHE γ-rays.

astro-ph.HE↗

arXiv2012

Hardening of TeV gamma spectrum of AGNs in galaxy clusters by conversions of photons into axion-like particles

A fraction of AGN producing VHE gamma-rays are located in galaxy clusters. The magnetic field present in the intra-cluster medium would lead to conversions of VHE photons into axion-like particles (ALPs), which are a generic prediction of several extensions of the Standard Model. ALPs produced in this way would traverse cosmological distances unaffected by the extragalactic background light at variance with VHE photons which undergo a substantial absorption. Eventually, a nontrivial fraction of ALPs would re-convert into VHE photons in the magnetic field of the Milky Way. This mechanism produces a significant hardening of the VHE spectrum of AGN in galaxy clusters. As a specific example we consider the energy spectra of two observed VHE gamma-ray sources located in galaxy clusters, namely 1ES 0414+009 at redshift z=0.287 and Mkn 501 at z=0.034. We find that the hardening in the observed spectra becomes relevant at E > 1 TeV. The detection of this signature would allow to indirectly probe the existence of ultra-light ALPs with mass m_a < 10^{-8} eV and photon-ALP coupling g_{a gamma} < 10^{-10} GeV^{-1} with the presently operating Imaging Atmospheric Cherenkov Telescopes like H.E.S.S., MAGIC, VERITAS and CANGAROO-III and even more likely with the planned detectors like CTA, HAWC and HiSCORE. An independent laboratory check of ultra-light ALPs invoked in this mechanism can be performed with the planned upgrade of the photon regeneration experiment ALPS at DESY and with the next generation solar axion detector IAXO.

astro-ph.HE↗

arXiv2012

Suzaku Observations of the Non-thermal Supernova Remnant HESS J1731-347

A detailed analysis of the nonthermal X-ray emission from the North-Western and Southern parts of the supernova remnant (SNR) HESS J1731$ - $347 with {\it Suzaku} is presented. The shell portions covered by the observations emit hard and line-less X-rays. The spectrum can be reproduced by a simple absorbed power-law model with a photon index $Γ$ of 1.8-2.7 and an absorption column density $N_{\rm H}$ of (1.0-2.1)$\times 10^{22}$ cm$^{-2}$. These quantities change significantly from region to region; the North-Western part of the SNR has the hardest and most absorbed spectrum. The Western part of the X-ray shell has a smaller curvature than North-Western and Southern shell segments. A comparison of the X-ray morphology to the Very High Energy (VHE) gamma-ray and radio images was performed. The efficiency of electron acceleration and emission mechanism in each portion of the shell are discussed. Thermal X-ray emission from the SNR was searched for but could not be detected at a significant level.

astro-ph.HE↗

arXiv2012

Limits on the extragalactic background light in the Fermi era

Very high energy (VHE, energy $E \gtrsim 100$\,GeV) γ-rays from cosmological sources are attenuated due to the interaction with photons of the extragalactic background light (EBL) in the ultraviolet to infrared wavelength band. The EBL, thus, leaves an imprint on the observed energy spectra of these objects. In the last four years, the number of extragalactic VHE sources discovered with imaging atmospheric Cherenkov telescopes (IACTs), such as MAGIC, H.E.S.S., and VERITAS, has doubled. Furthermore, the measurements of the \emph{Fermi} satellite brought new insights into the intrinsic spectra of the sources at GeV energies. In this paper, upper limits on the EBL intensity are derived by considering the most extensive VHE source sample ever used in this context. This is accomplished by constructing a large number of generic EBL shapes and combining spectral informations from \emph{Fermi} and IACTs together with minimal assumptions about the source physics at high and very high γ-ray energies. The evolution of the EBL with redshift is accounted for and the possibility of the formation of an electromagnetic cascade and the implications on the upper limits are explored. The EBL density at $z=0$ is constrained over a broad wavelength range between 0.4 and 100\,μm. At optical wavelengths, the EBL density is constrained below 24\,nW\,m$^{-2}$\,sr$^{-1}$ and below 5\,nW\,m$^{-2}$\,sr$^{-1}$ between 8\,μm and 31\,μm.

astro-ph.CO↗

arXiv2012

AstroFit: An Interface Program for Exploring Complementarity in Dark Matter Research

AstroFit is an interface adding astrophysical components to programs for fitting physics beyond the Standard Model (BSM) to experimental data from collider searches. The project aims at combining a wide range of experimental results from indirect, direct and collider serarches for Dark Matter (DM) and confronting it with theoretical expectations in various DM models. Here, we introduce AstroFit and discuss first results.

astro-ph.HE↗

arXiv2012

Indications for a pair-production anomaly from the propagation of VHE gamma-rays

In the recent years, the number of detected very high energy (VHE: E > 100 GeV) gamma-ray sources has increased rapidly. The sources have been observed at redshifts up to z = 0.536 without strong indications for the presence of absorption features in the energy spectra. Absorption is however expected due to pair-production processes of the propagating photons with the photon bath in intergalactic space. Even though this photon density is not well known, lower limits can be firmly set by the resolved emission from galaxy counts. Using this guaranteed background light, we investigate the behaviour of the energy spectra in the transition region from the optically thin to the optically thick regime. Among the sample of 50 energy spectra, 7 spectra cover the the range from optical depth $τ< 1$ to $τ> 2$. For these sources, the transition to $τ> 2$ takes place at widely different energies ranging from 0.4 TeV to 21 TeV. Consistently, in all of these sources, an upturn of the absorption-corrected spectrum is visible at this transition with a combined significance of 4.2 standard deviations. Given the broad range of energies and redshifts covered by the sample, source-intrinsic features are unlikely to explain the observed effect. Systematic effects related to observations have been investigated and found to be not sufficient to account for the observed effect. The pair-production process seems to be suppressed in a similar way as expected in the extension of the standard model by a light (<neV) pseudoscalar (axion-like) particle.

astro-ph.CO↗

arXiv2011

MAGIC measurement of the Crab Nebula spectrum over three decades in energy

The Crab Pulsar Wind Nebula is the best studied source of $γ$-ray astrophysics. The contribution of the various soft radiation fields to the Inverse Compton component of its high energy emission, the strenght of the internal magnetic field and the maximum energies reached by primary electrons are however still matter of study. The MAGIC stereoscopic system recorded almost 50 hours of Crab Nebula data in the last two years, between October 2009 and April 2011. Analysis of this data sample using the latest improvements in the MAGIC stereo software provided an unprecedented differential energy spectrum spanning three decades in energy, from 50 GeV up to 45 TeV. At low energies, the MAGIC results, combined with the Fermi/LAT data, yield a precise measurement of the Inverse Compton peak. In addition, we present light curves of the Crab Nebula at different time scales, including a measurement simultaneous to one of the Crab Nebula flares recently detected by both Fermi/LAT and AGILE. Using the MAGIC spectrum together with multiwavelength data, we discuss the implications for the modeling of the Crab Nebula.

astro-ph.HE↗

arXiv2011

The ground-based large-area wide-angle gamma-ray and cosmic-ray experiment HiSCORE

The question of the origin of cosmic rays and other questions of astroparticle and particle physics can be addressed with indirect air-shower observations above 10 TeV primary energy. We propose to explore the cosmic ray and gamma-ray sky (accelerator sky) in the energy range from 10 TeV to 1 EeV with the new ground-based large-area wide angle (~0.85 sterad) air-shower detector HiSCORE (Hundred*i Square-km Cosmic ORigin Explorer). The HiSCORE detector is based on non-imaging air-shower Cherenkov light-front sampling using an array of light-collecting stations. A full detector simulation and basic reconstruction algorithms have been used to assess the performance of HiSCORE. First prototype studies for different hardware components of the detector array have been carried out. The resulting sensitivity of HiSCORE to gamma-rays will be comparable to CTA at 50 TeV and will extend the sensitive energy range for gamma-rays up to the PeV regime. HiSCORE will also be sensitive to charged cosmic rays between 100 TeV and 1 EeV.

astro-ph.IM↗

arXiv2011

Long-term optical variability of PKS 2155-304

Aims: The optical variability of the blazar PKS 2155-304 is investigated to characterise the red noise behaviour at largely different time scales from 20 days to O(>10 yrs). Methods: The long-term optical light curve of PKS 2155-304 is assembled from archival data as well as from so-far unpublished observations mostly carried out with the ROTSE-III and the ASAS robotic telescopes. A forward folding technique is used to determine the best-fit parameters for a model of a power law with a break in the power spectral density function (PSD). The best-fit parameters are estimated using a maximum-likelihood method with simulated light curves in conjunction with the Lomb Scargle Periodogram (LSP) and the first-order Structure Function (SF). In addition, a new approach based upon the so-called Multiple Fragments Variance Function (MFVF) is introduced and compared to the other methods. Simulated light curves have been used to confirm the reliability of these methods as well as to estimate the uncertainties of the best-fit parameters. Results: The light curve is consistent with the assumed broken power-law PSD. All three methods agree within the estimated uncertainties with the MFVF providing the most accurate results. The red-noise behaviour of the PSD in frequency f follows a power law with f^-β, β=1.8 +0.1/-0.2 and a break towards f^0 at frequencies lower than f_min=(2.7 +2.2/-1.6 yrs)^-1.

astro-ph.HE↗

arXiv2011

Measurement of night sky brightness in southern Australia

Night sky brightness is a major source of noise both for Cherenkov telescopes as well as for wide-angle Cherenkov detectors. Therefore, it is important to know the level of night sky brightness at potential sites for future experiments. The measurements of night sky brightness presented here were carried out at Fowler's Gap, a research station in New South Wales, Australia, which is a potential site for the proposed TenTen Cherenkov telescope system and the planned wide-angle Cherenkov detector system HiSCORE. A portable instrument was developed and measurements of the night sky brightness were taken in February and August 2010. Brightness levels were measured for a range of different sky regions and in various spectral bands. The night sky brightness in the relevant wavelength regime for photomultipliers was found to be at the same level as measured in similar campaigns at the established Cherenkov telescope sites of Khomas, Namibia, and at La Palma. The brightness of dark regions in the sky is about 2 x 10^12 photons/(s sr m^2) between 300 nm and 650 nm, and up to four times brighter in bright regions of the sky towards the galactic plane. The brightness in V band is 21.6 magnitudes per arcsec^2 in the dark regions. All brightness levels are averaged over the field of view of the instrument of about 1.3 x 10^(-3) sr. The spectrum of the night sky brightness was found to be dominated by longer wavelengths, which allows to apply filters to separate the night sky brightness from the blue Cherenkov light. The possible gain in the signal to noise ratio was found to be up to 1.2, assuming an ideal low-pass filter.

astro-ph.IM↗