arXiv subjects

Andreas Kopp

Publications and source records attributed to Andreas Kopp.

7 recordsLinked to original sources

arXiv2026

On the Expected Orbitally-modulated TeV Signatures of Spider Binaries: The Effect of Intrabinary Shock Geometry

'Spider' binary systems - black widow and redback compact binaries differentiated by their companion's mass and nature - are an important type of pulsar system exhibiting a rich empirical phenomenology, including radio eclipses, optical light curves from a heated companion, as well as non-thermal X-ray and GeV orbital light curves and spectra. Multi-wavelength observations have now resulted in the detection of >~50 of these systems in which a millisecond pulsar heats and ablates its low-mass companion via its intense pulsar wind. Broadband observations have established the presence of relativistic leptons that have been accelerated in the pulsar magnetosphere and near the intrabinary shock, as well as a hot companion, presenting an ideal environment for the creation of orbitally-modulated inverse Compton fluxes that should be within reach of current and future Cherenkov telescopes. We have included an updated synchrotron kernel, different parametric injection spectral shapes, and several intrabinary shock geometries in our emission code to improve our predictions of the expected TeV signatures from spider binaries. Our updated phase-dependent spectral and energy-dependent light curve outputs may aid in constraining particle energetics, wind properties, shock geometry, and system inclination of several spider binaries.

astro-ph.HE↗

arXiv2020

Assessing Uncertainties in the Predicted Very-High-Energy Flux of Globular Clusters in the Cherenkov Telescope Array Era

Terzan 5 is the only Galactic globular cluster that has plausibly been detected in the very-high-energy range. Stacking upper limits by H.E.S.S. on the integral $γ$-ray flux of a population of other globular clusters are very constraining for leptonic cluster emission models. We demonstrate that uncertainty in model parameters leads to a large spread in the predicted flux, and there are indeed regions in parameter space for which the stringent stacking upper limits are satisfied. We conduct two more case studies: we study the uncertainties in differential TeV flux for M15, showing that our model can satisfy the stringent MAGIC upper limits for this cluster, for typical cluster parameters. We also calculate the differential flux at TeV energies for $ω$ Cen, from which five pulsars have recently been detected at radio energies. It is thus important to increase measurement accuracy on key model parameters in order to improve predictions of cluster fluxes so as to better guide the observational strategy of the Cherenkov Telescope Array.

astro-ph.HE↗

arXiv2019

Probing the pulsar population of Terzan 5 via spectral modeling

Terzan 5 is the only Galactic globular cluster that has plausibly been detected at very-high energies by the High Energy Stereoscopic System. It has an unexpectedly asymmetric very-high-energy morphology that is offset from the cluster center, in addition to a large-scale, offset radio structure and compact diffuse X-ray emission associated with this cluster. We present new data from the Fermi Large Area Telescope on this source. We model the updated broadband spectral energy distribution, attributing this to cumulative pulsed emission from a population of embedded millisecond pulsars as well as unpulsed emission from the interaction of their leptonic winds with the ambient magnetic and soft-photon fields. In particular, our model invokes unpulsed synchrotron and inverse Compton components to model the radio and TeV data, cumulative pulsed curvature radiation to fit the Fermi data, and explains the hard Chandra X-ray spectrum via a "new" cumulative synchrotron component from electron-positron pairs within the pulsar magnetospheres that has not been implemented before. We find reasonable spectral fits for plausible model parameters. We also derive constraints on the millisecond pulsar luminosity function using the diffuse X-ray data and the Chandra sensitivity. Future higher-quality spectral and spatial data will help discriminate between competing scenarios (such as dark matter annihilation, white dwarf winds or hadronic interactions) proposed for the broadband emission as well as constrain degenerate model parameters.

astro-ph.HE↗

arXiv2015

The millisecond pulsar contribution to the rising positron fraction

Pair cascades from millisecond pulsars (MSPs) may be a primary source of Galactic electrons and positrons that contribute to the increase in positron flux above 10 GeV as observed by PAMELA and AMS-02. The Fermi Large Area Telescope (LAT) has increased the number of detected gamma-ray MSPs tremendously. Light curve modelling furthermore favours abundant pair production in MSP magnetospheres, so that models of primary cosmic-ray positrons from pulsars should include the contribution from the larger numbers of MSPs and their potentially higher positron output per source. We model the contribution of Galactic MSPs to the terrestrial cosmic-ray electron / positron flux by using a population synthesis code to predict the source properties of present-day MSPs. We simulate pair spectra assuming an offset-dipole magnetic field which boosts pair creation rates. We also consider positrons and electrons that have additionally been accelerated to very high energies in the strong intrabinary shocks in black widow (BW) and redback (RB) binary systems. We transport these particles to Earth by calculating their diffusion and the radiative energy losses they suffer in the Galaxy using a model. Our model particle flux increases for non-zero offsets of the magnetic polar caps. We find that pair cascades from MSP magnetospheres contribute only modestly around a few tens of GeV to the measured fluxes. BW and RB fluxes may reach a few tens of percent of the observed flux up to a few TeV. Future observations should constrain the source properties in this case.

astro-ph.HE↗

arXiv2014

Cosmic Ray Transport in Heliospheric Magnetic Structures: I. Modeling Background Solar Wind Using the CRONOS MHD Code

The transport of energetic particles such as Cosmic Rays is governed by the properties of the plasma being traversed. While these properties are rather poorly known for galactic and interstellar plasmas due to the lack of in situ measurements, the heliospheric plasma environment has been probed by spacecraft for decades and provides a unique opportunity for testing transport theories. Of particular interest for the 3D heliospheric transport of energetic particles are structures such as corotating interaction regions (CIRs), which, due to strongly enhanced magnetic field strengths, turbulence, and associated shocks, can act as diffusion barriers on the one hand, but also as accelerators of low energy CRs on the other hand as well. In a two-fold series of papers we investigate these effects by modeling inner-heliospheric solar wind conditions with a numerical magnetohydrodynamic (MHD) setup (this paper), which will serve as an input to a transport code employing a stochastic differential equation (SDE) approach (second paper). In this first paper we present results from 3D MHD simulations with our code CRONOS: for validation purposes we use analytic boundary conditions and compare with similar work by Pizzo. For a more realistic modeling of solar wind conditions, boundary conditions derived from synoptic magnetograms via the Wang-Sheeley-Arge (WSA) model are utilized, where the potential field modeling is performed with a finite-difference approach (FDIPS) in contrast to the traditional spherical harmonics expansion often utilized in the WSA model. Our results are validated by comparing with multi-spacecraft data for ecliptical (STEREO-A/B) and out-of-ecliptic (Ulysses) regions.

astro-ph.SR↗

arXiv2012

A novel code for numerical 3-D MHD studies of CME expansion

A recent third-order, essentially non-oscillatory central scheme to advance the equations of single-fluid magnetohydrodynamics (MHD) in time has been implemented into a new numerical code. This code operates on a 3-D Cartesian, non-staggered grid, and is able to handle shock-like gradients without producing spurious oscillations. To demonstrate the suitability of our code for the simulation of coronal mass ejections (CMEs) and similar heliospheric transients, we present selected results from test cases and perform studies of the solar wind expansion during phases of minimum solar activity. We can demonstrate convergence of the system into a stable Parker-like steady state for both hydrodynamic and MHD winds. The model is subsequently applied to expansion studies of CME-like plasma bubbles, and their evolution is monitored until a stationary state similar to the initial one is achieved. In spite of the model's (current) simplicity, we can confirm the CME's nearly self-similar evolution close to the Sun, thus highlighting the importance of detailed modelling especially at small heliospheric radii. Additionally, alternative methods to implement boundary conditions at the coronal base, as well as strategies to ensure a solenoidal magnetic field, are discussed and evaluated.

astro-ph.SR↗

arXiv2009

Plans for PANDA Online Computing

The PANDA experiment will not use any hardware trigger, i.e. all raw data are streaming in the data acquisition with a bandwidth of ~280 GB/s. The PANDA Online System is designed to perform data reduction by a factor of ~800 by reconstruction algorithms programmed in VHDL (Very High Speed Integrated Circuit Hardware Description Language) on FPGAs (Field Programmable Gate Arrays).

physics.ins-det↗