arXiv subjects

Markus Böttcher

Publications and source records attributed to Markus Böttcher.

At least 37 records · Page 2Linked to original sources

arXiv2024

Revisiting the TeV flare of PKS 2155-304 in 2006

Blazars, a subclass of active galactic nuclei (AGN), are known to be bright $γ$-ray sources, frequently exhibiting active (flaring) periods. The blazar PKS~2155-304 is a high synchrotron-peaked BL Lac object located at redshift $z=0.116$. On 2006 July 28, an extremely remarkable outburst of VHE $γ$-ray emission from this blazar was reported by the H.E.S.S. experiment, with an average flux more than 10 times the low-state level. The variability timescale of this extraordinary flare was as short as approximately 200~s. In order to guarantee the transparency of the emission region for TeV photons, the fast variability demands an extremely high Doppler factor $δ_{\rm D}>50$ of the jet within the classical one-zone model, leading to the so-called "Doppler factor crisis". Here we demonstrate that the stochastic dissipation model, which is a multi-blob scenario for blazars, can self-consistently explain the giant TeV flares of PKS~2155-304 and the low-state emission before and after the flares, in terms of both multi-wavelength spectral and variability characteristics. The required Doppler factor in this model can be as low as 20, which is a reasonable and typical value for blazar jets. The obtained model parameters may shed some light on the physical properties of the relativistic jet.

astro-ph.HE↗

arXiv2023

Shaken, not stirred: Test particles in binary black hole mergers

Since 2015 the advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) has detected a large number of gravitational wave events, originating from both binary neutron stars and binary black hole (BBH) mergers. In light of these detections, we simulate the dynamics of ambient test particles in the gravitational potential well of a BBH system close to its inspiral phase with the goal of simulating the associated electromagnetic radiation and resulting spectral energy distribution of such a BBH system. This could shed light on possible detection ranges of electromagnetic counterparts to BBH mergers. The potentials are numerically calculated using finite difference methods, under the assumption of non-rotating black holes with the post-Newtonian Paczynski-Wiita potential approximation in tandem with retarded time concepts analogous to electrodynamics. We find that the frequencies of potential electromagnetic radiation produced by these systems (possibly reaching Earth), range between a few $\text{kHz}$ to a few $100 \text{MHz}$.

astro-ph.HE↗

arXiv2023

Investigating the Lorentz Invariance Violation effect using different cosmological backgrounds

Familiar concepts in physics, such as Lorentz symmetry, are expected to be broken at energies approaching the Planck energy scale as predicted by several quantum-gravity theories. However, such very large energies are unreachable by current experiments on Earth. Current and future Cherenkov telescope facilities may have the capability to measure the accumulated deformation from Lorentz symmetry for photons traveling over large distances via energy-dependent time delays. One of the best natural laboratories to test Lorentz Invariance Violation~(LIV) signatures are Gamma-ray bursts~(GRBs). The calculation of time delays due to the LIV effect depends on the cosmic expansion history. In most of the previous works calculating time lags due to the LIV effect, the standard $Λ$CDM (or concordance) cosmological model is assumed. In this paper, we investigate whether the LIV signature is significantly different when assuming alternatives to the $Λ$CDM cosmological model. Specifically, we consider cosmological models with a non-trivial dark-energy equation of state ($w \neq -1$), such as the standard Chevallier-Polarski-Linder~(CPL) parameterization, the quadratic parameterization of the dark-energy equation of state, and the Pade parameterizations. We find that the relative difference in the predicted time lags is small, of the order of at most a few percent, and thus likely smaller than the systematic errors of possible measurements currently or in the near future.

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arXiv2023

Inverse Compton Emission and Cooling of Relativistic Particles Accelerated at Shear Boundary Layers in Relativistic Jets

Both observational evidence and theoretical considerations from MHD simulations of jets suggest that the relativistic jets of active galactic nuclei (AGN) are radially stratified, with a fast inner spine surrounded by a slower-moving outer sheath. The resulting relativistic shear layers are a prime candidate for the site of relativistic particle acceleration in the jets of AGN and gamma-ray bursts (GRBs). In this article, we present outcomes of particle-in-cell simulations of magnetic-field generation and particle acceleration in the relativistic shear boundary layers (SBLs) of jets in AGN and GRBs. We investigate the effects of inverse Compton cooling on relativistic particles that are accelerated in the SBLs of relativistic jets including the self-consistent calculation of the radiation spectrum produced by inverse Compton scattering of relativistic electrons in an isotropic external soft photon field. We find that the Compton cooling can be substantial, depending on the characteristic energy (blackbody temperature and energy density) of the external radiation field. The produced Compton emission is highly anisotropic and more strongly beamed along the jet direction than the characteristic $1/Γ$ pattern expected from intrinsically isotropic emission in the comoving frame of an emission region moving along the jet with bulk Lorentz factor $Γ$. We suggest that this may resolve the long-standing problem of the Doppler Factor Crisis.

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arXiv2023

The High Energy X-ray Probe (HEX-P): the most powerful jets through the lens of a superb X-ray eye

A fraction of the active supermassive black holes at the centers of galaxies in our Universe are capable of launching extreme kiloparsec-long relativistic jets. These jets are known multiband (radio to $γ$-ray) and multimessenger (neutrino) emitters, and some of them have been monitored over several decades at all accessible wavelengths. However, many open questions remain unanswered about the processes powering these highly energetic phenomena. These jets intrinsically produce soft-to-hard X-ray emission that extends from $E\sim0.1\,\rm keV$ up to $E>100\,\rm keV$. Simultaneous broadband X-ray coverage, combined with excellent timing and imaging capabilities, is required to uncover the physics of jets. Indeed, truly simultaneous soft-to-hard X-ray coverage, in synergy with current and upcoming high-energy facilities (such as IXPE, COSI, CTAO, etc.) and neutrino detectors (e.g., IceCube), would enable us to disentangle the particle population responsible for the high-energy radiation from these jets. A sensitive hard X-ray survey ($F_{8-24\,\rm keV}<10^{-15}\,\rm erg~cm^{-2}~s^{-1}$) could unveil the bulk of their population in the early Universe. Acceleration and radiative processes responsible for the majority of their X-ray emission would be pinned down by microsecond timing capabilities at both soft and hard X-rays. Furthermore, imaging jet structures for the first time in the hard X-ray regime could unravel the origin of their high-energy emission. The proposed Probe-class mission concept High Energy X-ray Probe (HEX-P) combines all these required capabilities, making it the crucial next-generation X-ray telescope in the multi-messenger, time-domain era. HEX-P will be the ideal mission to unravel the science behind the most powerful accelerators in the universe.

astro-ph.HE↗

arXiv2023

A multi-zone view on the multi-wavelength emission of blazars

In this work, a time-dependent modeling is developed to study the emission properties of blazars in the low state. Motivated by various observations, we speculate and assume that numerous discrete radiation zones throughout the jet of a blazar contribute to the broadband emission. We model the temporal evolution of the electron spectrum in each emission zone taking into account the injection, cooling and escape of relativistic electrons. By doing so, we are able to calculate the multi-wavelength emission of each radiation zone. The observed emission of a blazar is then the superposition of the emission from all discrete radiation zones. We revisit the multi-wavelength spectral energy distributions, light curves and polarisation under the model, and discuss its potential to reproduce the flat radio spectra, the core-shift phenomena, the minute-scale gamma-ray variability, and the large polarisation-angle swings, which are difficult to explain under the conventional one-zone models simultaneously.

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arXiv2023

The Compton Spectrometer and Imager

The Compton Spectrometer and Imager (COSI) is a NASA Small Explorer (SMEX) satellite mission in development with a planned launch in 2027. COSI is a wide-field gamma-ray telescope designed to survey the entire sky at 0.2-5 MeV. It provides imaging, spectroscopy, and polarimetry of astrophysical sources, and its germanium detectors provide excellent energy resolution for emission line measurements. Science goals for COSI include studies of 0.511 MeV emission from antimatter annihilation in the Galaxy, mapping radioactive elements from nucleosynthesis, determining emission mechanisms and source geometries with polarization measurements, and detecting and localizing multimessenger sources. The instantaneous field of view for the germanium detectors is >25% of the sky, and they are surrounded on the sides and bottom by active shields, providing background rejection as well as allowing for detection of gamma-ray bursts and other gamma-ray flares over most of the sky. In the following, we provide an overview of the COSI mission, including the science, the technical design, and the project status.

astro-ph.HE↗

arXiv2023

The cosipy library: COSI's high-level analysis software

The Compton Spectrometer and Imager (COSI) is a selected Small Explorer (SMEX) mission launching in 2027. It consists of a large field-of-view Compton telescope that will probe with increased sensitivity the under-explored MeV gamma-ray sky (0.2-5 MeV). We will present the current status of cosipy, a Python library that will perform spectral and polarization fits, image deconvolution, and all high-level analysis tasks required by COSI's broad science goals: uncovering the origin of the Galactic positrons, mapping the sites of Galactic nucleosynthesis, improving our models of the jet and emission mechanism of gamma-ray bursts (GRBs) and active galactic nuclei (AGNs), and detecting and localizing gravitational wave and neutrino sources. The cosipy library builds on the experience gained during the COSI balloon campaigns and will bring the analysis of data in the Compton regime to a modern open-source likelihood-based code, capable of performing coherent joint fits with other instruments using the Multi-Mission Maximum Likelihood framework (3ML). In this contribution, we will also discuss our plans to receive feedback from the community by having yearly software releases accompanied by publicly-available data challenges.

astro-ph.HE↗

arXiv2023

Paarl Africa Underground Laboratory (PAUL)

Establishing a deep underground physics laboratory to study, amongst others, double beta decay, geoneutrinos, reactor neutrinos and dark matter has been discussed for more than a decade within the austral African physicists' community. PAUL, the Paarl Africa Underground Laboratory, is an initiative foreseeing an open international laboratory devoted to the development of competitive science in the austral region. It has the advantage that the location, the Huguenot tunnel, exists already and the geology and the environment of the site is appropriate for an experimental facility. The paper describes the PAUL initiative, presents the physics prospects and discusses the capacity for building the future experimental facility.

hep-ex↗

arXiv2021

Modeling the Spectral Energy Distributions and Spectropolarimetry of Blazars -- Application to 4C+01.02 in 2016-2017

The optical radiation emitted by blazars contains contributions from synchrotron radiation by relativistic electrons in the jets, as well as thermal radiation emitted mainly by the Accretion Disk (AD), the Broad Line Region (BLR) and the host galaxy. The unpolarized radiation components from the AD, BLR and host galaxy present themselves by decreasing the total polarization in the optical/ultraviolet(UV) spectrum. A combined model for the Spectral Energy Distribution (SED) and degree of optical/UV polarization is constructed, enabling the disentanglement of the synchrotron and AD components. Our model is applied to the multi-wavelength SED and spectropolarimetry observations of the Flat Spectrum Radio Quasar 4C+01.02 ($z = 2.1$) in its 2016 July-August flaring state and July-August 2017 quiescent state, using data from the Fermi Large Area Telescope, the Southern African Large Telescope and the Las Cumbres Observatory network of telescopes. By constraining the AD component, the mass of the super massive black hole is obtained as $3 \times 10^9 \rm M_{\odot}$. Furthermore, the model retrieves the characteristics of the relativistic electron distribution in the jet and the degree of ordering of the magnetic field. Our results highlight the potential of spectropolarimetry observations for disentangling thermal from non-thermal (jet) emission components and thus revealing the physics of particle acceleration and high-energy emission in active galactic nuclei jets.

astro-ph.HE↗

arXiv2021

Simulations of Stochastic Long-Term Variability in Leptonic Models for External-Compton and Synchrotron Self-Compton Dominated Blazars

In this work we investigate the nature of multi-wavelength variability of blazars from a purely numerical approach. We use a time-dependent one-zone leptonic blazar emission model to simulate multi-wavelength variability by introducing stochastic parameter variations in the emission region. These stochastic parameter variations are generated by Monte Carlo methods and have a characteristic power law index of $α=-2$ in their power spectral densities. We include representative blazar test cases for a flat spectrum radio quasar and a high synchrotron peaked BL Lacertae object for which the high energy component of the Spectral Energy Distribution is dominated by external Compton and synchrotron self-Compton emission, respectively. The simulated variability is analyzed in order to characterise the distinctions between the two blazar cases and the physical parameters driving the variability. We show that the variability's power spectrum is closely related to underlying stochastic parameter variations for both cases. Distinct differences between the different progenitor variations are present in the multi-wavelength cross-correlation functions.

astro-ph.HE↗

arXiv2021

Radio and $γ$-ray activity in the jet of the blazar S5 0716$+$714

We explore the connection between the $γ$-ray and radio emission in the jet of the blazar 0716$+$714 by using 15, 37, and 230 GHz radio and 0.1$-$200 GeV $γ$-ray light curves spanning 10.5 years (2008$-$2019). We find significant positive and negative correlations between radio and $γ$-ray fluxes in different time ranges. The time delays between radio and $γ$-ray emission suggest that the observed $γ$-ray flares originated from multiple regions upstream of the radio core, within a few parsecs from the central engine. Using time-resolved 43 GHz VLBA maps we identified 14 jet components moving downstream along the jet. Their apparent speeds range from 6 to 26 $c$, showing notable variations in their position angles upstream the stationary component ($\sim$0.53 mas from the core). The brightness temperature declines as function of distance from the core according to a power-law which becomes shallower at the location of the stationary component. We also find that the periods at which significant correlations between radio and $γ$-ray emission occur overlap with the times when the jet was oriented to the north. Our results indicate that the passage of a propagating disturbance (or shock) through the radio core and the orientation of the jet might be responsible for the observed correlation between the radio and $γ$-ray variability. We present a scenario that connects the positive correlation and the unusual anti-correlation by combining the production of a flare and a dip at $γ$-rays by a strong moving shock at different distances from the jet apex.

astro-ph.HE↗

arXiv2021

Radiation and Polarization Signatures from Magnetic Reconnection in Relativistic Jets--II. Connection with $γ$-rays

It is commonly believed that blazar jets are relativistic magnetized plasma outflows from supermassive black holes. One key question is how the jets dissipate magnetic energy to accelerate particles and drive powerful multi-wavelength flares. Relativistic magnetic reconnection has been proposed as the primary plasma physical process in the blazar emission region. Recent numerical simulations have shown strong acceleration of nonthermal particles that may lead to multi-wavelength flares. Nevertheless, previous works have not directly evaluated $γ$-ray signatures from first-principle simulations. In this paper, we employ combined particle-in-cell and polarized radiation transfer simulations to study multi-wavelength radiation and optical polarization signatures under the leptonic scenario from relativistic magnetic reconnection. We find harder-when-brighter trends in optical and {\it Fermi-LAT} $γ$-ray bands as well as closely correlated optical and $γ$-ray flares. The optical polarization angle swings are also accompanied by $γ$-ray flares with trivial time delays. Intriguingly, we find highly variable synchrotron self Compton signatures due to inhomogeneous particle distributions during plasmoid mergers. This feature may result in fast $γ$-ray flares or orphan $γ$-ray flares under the leptonic scenario, complementary to the frequently considered mini-jet scenario. It may also infer neutrino emission with low secondary synchrotron flux under the hadronic scenario, if plasmoid mergers can accelerate protons to very high energy.

astro-ph.HE↗

arXiv2021

A tidal disruption event coincident with a high-energy neutrino

Cosmic neutrinos provide a unique window into the otherwise-hidden mechanism of particle acceleration in astrophysical objects. A flux of high-energy neutrinos was discovered in 2013, and the IceCube Collaboration recently reported the likely association of one high-energy neutrino with a flare from the relativistic jet of an active galaxy pointed towards the Earth. However a combined analysis of many similar active galaxies revealed no excess from the broader population, leaving the vast majority of the cosmic neutrino flux unexplained. Here we present the likely association of a radio-emitting tidal disruption event, AT2019dsg, with a second high-energy neutrino. AT2019dsg was identified as part of our systematic search for optical counterparts to high-energy neutrinos with the Zwicky Transient Facility. The probability of finding any coincident radio-emitting tidal disruption event by chance is 0.5%, while the probability of finding one as bright in bolometric energy flux as AT2019dsg is 0.2%. Our electromagnetic observations can be explained through a multi-zone model, with radio analysis revealing a central engine, embedded in a UV photosphere, that powers an extended synchrotron-emitting outflow. This provides an ideal site for PeV neutrino production. Assuming that the association is genuine, our observations suggest that tidal disruption events with mildly-relativistic outflows contribute to the cosmic neutrino flux.

astro-ph.HE↗

arXiv2021

The Blazar Hadronic Code Comparison Project

Blazar hadronic models have been developed in the past decades as an alternative to leptonic ones. In hadronic models the gamma-ray emission is associated with synchrotron emission by protons, and/or secondary leptons produced in proton-photon interactions. Together with photons, hadronic emission models predict the emission of neutrinos that are therefore the smoking gun for acceleration of relativistic hadrons in blazar jets. The simulation of proton-photon interactions and all associated radiative processes is a complex numerical task, and different approaches to the problem have been adopted in the literature. So far, no systematic comparison between the different codes has been performed, preventing a clear understanding of the underlying uncertainties in the numerical simulations. To fill this gap, we have undertaken the first comprehensive comparison of blazar hadronic codes, and the results from this effort will be presented in this contribution.

astro-ph.HE↗

arXiv2021

Monte-Carlo Applications for Partially Polarised Inverse External-Compton Scattering (MAPPIES) -- I. Description of the code and First Results

The radiation mechanisms responsible for the multiwavelength emission from relativistic jet sources are poorly understood. The modelling of the spectral energy distributions (SEDs) and light curves alone is not adequate to distinguish between existing models. Polarisation in the $X$-ray and $γ$-ray regime of these sources may provide new and unique information about the jet physics and radiation mechanisms. Several upcoming projects will be able to deliver polarimetric measurements of the brightest $X$-ray sources, including active galactic nuclei (AGN) jets and $γ$-ray bursts (GRBs). This article describes the development of a new Monte-Carlo code -- MAPPIES (Monte-Carlo Applications for Partially Polarised Inverse External-Compton Scattering) -- for polarisation-dependent Compton scattering in relativistic jet sources. Generic results for Compton polarisation in the Thomson and Klein-Nishina regimes are presented.

astro-ph.HE↗

arXiv2021

Monte-Carlo Applications for Partially Polarized Inverse External-Compton Scattering (MAPPIES) II -- Application to the UV/Soft X-ray Excess in Blazar Spectra

The spectral energy distributions (SEDs) of some blazars exhibit an ultraviolet (UV) and/or soft X-ray excess, which can be modelled with different radiation mechanisms. Polarization measurements of the UV/X-ray emission from blazars may provide new and unique information about the astrophysical environment of blazar jets, and could thus help to distinguish between different emission scenarios. In this paper, a new Monte-Carlo code -- MAPPIES (Monte-Carlo Applications for Partially Polarized Inverse External-Compton Scattering) -- for polarization-dependent Compton scattering is used to simulate the polarization signatures in a model where the UV/soft X-ray excess arises from the bulk Compton process. Predictions of the expected polarization signatures of Compton emission from the soft X-ray excess in the SED of AO 0235+164, and the UV excess in the SED of 3C 279 are made for upcoming and proposed polarimetry missions.

astro-ph.HE↗

arXiv2020

High-energy and Very High-energy Constraints from Log-parabolic Spectral Models in Narrow-line Seyfert 1 Galaxies

Narrow-line Seyfert 1 galaxies (NLSy1s) are a well-established class of $γ$-ray sources, showing the presence of a jet like the more common flat-spectrum radio quasars. The evidence of $γ$-ray emission poses the issue of the location of the $γ$-ray emitting zone and of the contribution of the $γ$-$γ$ absorption within the broad-line region (BLR), since such objects have been detected by Fermi-LAT in the MeV-GeV energy range but not by imaging atmospheric Cherenkov telescopes beyond 100 GeV. We discuss how the spectral properties of three NLSy1s (SBS 0846+513, PMN J0948+0022, and PKS 1502+036) derived from the Fermi Large Area Telescope Fourth Source Catalog (4FGL) compare with theoretical models based on the observed properties of the BLR. In particular, we focus on the question on how simple power-law spectral models and log-parabolic ones could be disentangled in $γ$-ray narrow-line Seyfert 1 galaxies by means of current Fermi-LAT or future imaging atmospheric Cherenkov telescopes data. We find that the only possibility for a log-parabolic model to mimic a power-law model in the energy band above $E \sim 100$ GeV is to have a very small value of the curvature parameter $ β\sim 0.05$.

astro-ph.HE↗