The polarization signatures of the blazar emissions are known to be highly variable. In addition to small fluctuations of the polarization angle around a mean value, sometimes large (> 180^o) polarization angle swings are observed. We suggest that such p henomena can be interpreted as arising from light-travel-time effects within an underlying axisymmetric emission region. We present the first simultaneous fitting of the multi-wavelength spectrum, variability and time-dependent polarization features of a correlated optical and gamma-ray flaring event of the prominent blazar 3C279, which was accompanied by a drastic change of its polarization signatures. This unprecedented combination of spectral, variability, and polarization information in a coherent physical model allows us to place stringent constraints on the particle acceleration and magnetic-field topology in the relativistic jet of a blazar, strongly favoring a scenario in which magnetic energy dissipation is the primary driver of the flare event.

Haocheng Zhang Xuhui Chen Markus Boettcher Fan Guo Hui Li

In this study, we use a one-zone leptonic and a lepto-hadronic model to investigate the multi-wavelength emission and the prominent flare of the flat spectrum radio quasar 3C 454.3 in Nov 2010. We perform a parameter study with both models to obtain broadband fits to the spectral energy distribution of 3C 454.3. Starting with the baseline parameters obtained from the fits, we then investigate different flaring scenarios for both models to explain an extreme outburst and spectral hardening of 3C 454.3 that occurred in Nov 2010. We find that the one zone lepto-hadronic model can successfully explain both the broadband multi-wavelength spectral energy distribution and light curves in the optical R, Swift XRT and Fermi gamma-ray bandpasses for 3C 454.3 during quiescence and the peak of the Nov. 2010 flare. We also find that the one-zone leptonic model produces poor fits to the broadband spectra in the X-ray and high energy gamma-ray band passes for the Nov. 2010 flare.

Christopher S. Diltz Markus Boettcher

The unexpectedly hard very-high-energy (VHE; $E > 100$ GeV) $\gamma$-ray spectra of a few distant blazars have been interpreted as evidence for a reduction of the $\gamma\gamma$ opacity of the Universe due to the interaction of VHE $\gamma$-rays with the extragalactic background light (EBL) compared to the expectation from our current knowledge of the density and cosmological evolution of the EBL. One of the suggested solutions to this problem consisted of the inhomogeneity of the EBL. In this paper, we study the effects of such inhomogeneities on the energy density of the EBL (which then also becomes anisotropic) and the resulting $\gamma\gamma$ opacity. Specifically, we investigate the effects of cosmic voids along the line of sight to a distant blazar. We find that the effect of such voids on the $\gamma\gamma$ opacity, for any realistic void size, is only of the order of $\lesssim 1$ % and much smaller than expected from a simple linear scaling of the $\gamma\gamma$ opacity with the line-of-sight galaxy under-density due to a cosmic void.

Hassan Abdalla Markus Boettcher

We report on recent results from a target-of-opportunity program to obtain spectropolarimetry observations with the Southern African Large Telescope (SALT) on flaring gamma-ray blazars. SALT spectropolarimetry and contemporaneous multi-wavelength spectral energy distribution (SED) data are being modelled self-consistently with a leptonic single-zone model. Such modeling provides an accurate estimate of the degree of order of the magnetic field in the emission region and the thermal contributions (from the host galaxy and the accretion disk) to the SED, thus putting strong constraints on the physical parameters of the gamma-ray emitting region. For the specific case of the $\gamma$-ray blazar 4C+01.02, we demonstrate that the combined SED and spectropolarimetry modeling constrains the mass of the central black hole in this blazar to $M_{\rm BH} \sim 10^9 \, M_{\odot}$.

Markus Boettcher Brian van Soelen Richard J. Britto David A. H. Buckley Johannes P. Marais Hester Schutte

This review provides an overview of recent advances in multi-wavelength and multi-messenger observations of blazars, the current status of theoretical models for blazar emission, and prospects for future facilities. The discussion of observational results will focus on advances made possible through the Fermi Gamma-Ray Space Telescope and ground-based gamma-ray observatories (H.E.S.S., MAGIC, VERITAS) as well as the recent first evidence for a blazar being a source of IceCube neutrinos. The main focus of this review will be the discussion of our current theoretical understanding of blazar multi-wavelength and multi-messenger emission, in the spectral, time, and polarization domains. Future progress will be expected in particular through the development of the first X-ray polarimeter, IXPE, and the installation of the Cherenkov Telescope Array (CTA), both expected to become operational in the early to mid 2020s.

Markus Boettcher

Mildly relativistic, oblique shocks are frequently invoked as possible sites of relativistic particle acceleration and production of strongly variable, polarized multi-wavelength emission from relativistic jet sources such as blazars, via diffusive shock acceleration (DSA). In recent work, we had self-consistently coupled DSA and radiation transfer simulations in blazar jets. These one-zone models determined that the observed spectral energy distributions (SEDs) of blazars strongly constrain the nature of the hydromagnetic turbulence responsible for pitch-angle scattering. In this paper, we expand our previous work by including full time dependence and treating two emission zones, one being the site of acceleration. This modeling is applied to a multiwavelength flare of the flat spectrum radio quasar 3C~279, fitting snap-shot SEDs and light curves. We predict spectral hysteresis patterns in various energy bands as well as cross-band time lags with optical and GeV gamma-rays as well as radio and X-rays tracing each other closely with zero time lag, but radio and X-rays lagging behind the optical and gamma-ray variability by several hours.

Markus Boettcher Matthew G. Baring

This contribution reviews recent advances in the possible identification of blazars as potential sources of at least some of the very-high-energy neutrinos detected by the IceCube neutrino detector at the South Pole. The basic physical requirements for neutrino production and physics constraints that may be drawn from neutrino - blazar associations are reviewed. Several individual cases of possible associations will be discussed in more detail. It is emphasized that due to $\gamma\gamma$ opacity constraints in efficiently neutrino-producing blazars, an association between X-ray -- soft $\gamma$-ray activity and very-high-energy neutrino production is more naturally expected than a connection between neutrino and high-energy/very-high-energy $\gamma$-ray activity.

Markus Boettcher Matthew Fu Timothy Govenor Quentin King Parisa Roustazadeh

A discrete jet component (blob) ejection and its subsequent deceleration was observed in the 2019/2020 outburst in the low-mass X-ray binary MAXI J1348-630. A first kinematic analysis of the deceleration due to an abrupt transition from an evacuated cavity to the interstellar medium suggested a kinetic energy exceeding 1046 erg, surpassing estimates of the available total ejection energy. However, incorporating a transition layer with exponential density growth between the cavity and interstellar medium recently enabled a kinematic analysis with much more realistic energy requirements of approximately $10^{44}$ erg. Here, we study the expected radiative signatures of electrons accelerated within the decelerating blob by introducing a model akin to the relativistic blast wave model for gamma-ray bursts, considering radiative energy losses and radiation drag, to simulate the deceleration of a relativistically moving plasmoid. This model yields snap-shot spectral energy distributions and multi-wavelength light curves from synchrotron and Synchrotron-Self-Compton (SSC) emission. Notably, the synchrotron emission peaks in the X-rays, and the predicted Radio and X-ray light curves closely resemble the observed ones during the jet decleration phase following the outburst in 2019/2020.

Aishwarya Sarath Markus Boettcher

The contribution of the pair annihilation process in relativistic electron-positron jets to the gamma-ray emission of blazars is calculated. Under the same assumptions as for the calculation of the yield of inverse Compton scattered accretion disk radiation (Dermer and Schlickeiser 1993) we calculate the emerging pair annihilation radiation taking into account all spectral broadening effects due to the energy spectra of the annihilating particles and the bulk motion of the jet. It is shown that the time-integrated pair annihilation spectrum appears almost like the well-known gamma-ray spectrum from decaying $\pi^o$-mesons at rest, yielding a broad bumpy feature located between 50 and 100 MeV. We also demonstrate that for pair densities $> 10^9$ cm$^{-3}$ in the jet the annihilation radiation will dominate the inverse Compton radiation, and indeed may explain reported spectral bumps at MeV energies. The refined treatment of the inverse Compton radiation leads to spectral breaks of the inverse Compton emission in the MeV energy range with a change in spectral index $\Delta \alpha $ larger than 0.5 as detected in PKS 0528+134 and 3C273.

M. Boettcher R. Schlickeiser

The evolution of the particle distribution functions inside a relativistic jet containing an e^-e^+ pair plasma and of the resulting $\gamma$-ray and X-ray spectra is investigated. The first phase of this evolution is governed by heavy radiative energy losses. For this phase, approximative expressions for the energy-loss rates due to inverse-Compton scattering, using the full Klein-Nishina cross section, are given as one-dimensional integrals for both cooling by inverse-Compton scattering of synchrotron photons (SSC) and of accretion disk photons (EIC). We calculate instantaneous and time-integrated $\gamma$-ray spectra emitted by such a jet for various sets of parameters, deducing some general implications on the observable broadband radiation. Finally, we present model fits to the broadband spectrum of Mrk 421. We find that the most plausible way to explain both the quiescent and the flaring state of Mrk 421 consists of a model where EIC and SSC dominate the observed spectrum in different frequency bands. In our model the flaring state is primarily related to an increase of the maximum Lorentz factor of the injected pairs.

M. Boettcher H. Mause R. Schlickeiser