arXiv subjects

Markus Boettcher

Publications and source records attributed to Markus Boettcher.

At least 73 records · Page 4Linked to original sources

arXiv2012

Synchrotron Emission from Pair Cascades in AGN Environments

Recent detections of very-high-energy (VHE, E > 100 GeV) gamma-ray blazars which do not belong to the high frequency peaked BL Lac (HBL) class, suggest that gamma-gamma absorption and pair cascading might occur in those objects. In the presence of even weak magnetic fields, these Compton-supported pair cascades will be deflected and contribute to the Fermi gamma-ray flux of radio galaxies. We demonstrate that, in this scenario, the magnetic field can not be determined from a fit of the cascade emission to the gamma-ray spectrum alone, and the degeneracy can only be lifted if the synchrotron emission from the cascades is observed as well. We illustrate this fact with the example of NGC 1275. We point out that the cascade synchrotron emission may produce spectral features reminiscent of the big blue bump observed in the spectral energy distributions of several blazars, and illustrate this idea for 3C 279.

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arXiv2012

Time-dependent simulations of emission from FSRQ PKS1510-089: multiwavelength variability of external Compton and SSC models

[abridged] We present results of modeling the SED and multiwavelength variability of the bright FSRQ PKS1510-089 with our time-dependent multizone Monte Carlo/Fokker-Planck code (Chen et al. 2001). As primary source of seed photons for inverse Compton scattering, we consider radiation from the broad line region (BLR), from the molecular torus, and the local synchrotron radiation (SSC). Different scenarios are assessed by comparing simulated light curves and SEDs with one of the best flares by PKS1510-089, in March 2009. The time-dependence of our code and its correct handling of light travel time effects allow us to fully take into account the effect of the finite size of the active region, and in turn to fully exploit the information carried by time resolved observed SEDs, increasingly available since the launch of Fermi. We confirm that the spectrum adopted for the external radiation has an important impact on the modeling of the SED, in particular for the lower energy end of the Compton component, observed in the X-ray band, which in turn is one of the most critical bands to assess the differences between EC and SSC emission. In the context of the scenario presented here, where the flaring is caused by the increase of the number of relativistic electrons ascribed to the effect of the interaction of a portion of the jet (blob) with a shock, we can not firmly discriminate the three main scenarios for gamma-ray emission. However, results show clearly the differences produced by a more realistic treatment of the emitting source in the shape of SEDs and their time variability over relevant, observable time-scales, and demonstrate the crucial importance of time-dependent multi-zone models to advance our understanding of the physics of these sources, by taking full advantage of the wealth of information offered by the high quality data of current multiwavelength campaigns.

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arXiv2012

Modeling the Spectral Energy Distributions and Variability of Blazars

In this review, recent progress in theoretical models for the broadband (radio through gamma-ray) emission from blazars are summarized. The salient features of both leptonic and hadronic models are reviewed. I present sample modeling results of spectral energy distributions (SEDs) of different types of Fermi-detected blazars along the traditional blazar sequence, using both types of models. In many cases, the SEDs of high-frequency peaked blazars (HBLs) have been found to be well represented by simple synchrotron + synchrotron self-Compton (SSC) models. However, a few HBLs recently discovered as very-high-energy (VHE) gamma-ray emitters by VERITAS are actually better represented by either external-Compton or hadronic models. Often, spectral modeling with time-independent single-zone models alone is not sufficient to constrain models, as both leptonic and lepto-hadronic models are able to provide acceptable fits to the overall SED. This degeneracy can be lifted by considering further constraints from spectral variability. Recent developments of time-dependent and inhomogeneous blazar models will be discussed, including detailed numerical simulations as well as a semi-analytical approach to the time-dependent radiation signatures of shock-in-jet models.

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arXiv2012

Synchrotron Emission from VHE Gamma-Ray Induced Pair Cascades in AGN Environments

The discovery of very-high-energy (VHE, E > 100 GeV) gamma-ray emission from intermediate- and low-frequency peaked blazars suggests that gamma-gamma absorption and pair cascading might occur in those objects. In previous papers, we investigated the Compton emission from VHE gamma-ray induced pair cascades, deflected by moderate magnetic fields, in a largely model-independent way, and demonstrated that this emission can explain the Fermi fluxes and spectra of the radio galaxies Cen A and NGC 1275. In this paper, we describe a generalization of our Monte-Carlo cascade code to include the angle-dependent synchrotron output from the cascades, allowing for the application to situations with non-negligible magnetic fields, leading to potentially observable synchrotron signatures, but still not dominating the radiative energy loss of cascade particles. We confirm that the synchrotron radiation from the cascades in NGC 1275 and Cen A are negligible for the parameters used in our previous works. We demonstrate that the magnetic field can not be determined from a fit of the cascade emission to the gamma-ray spectrum alone, and the degeneracy can only be lifted if the synchrotron emission from the cascades is observed as well. We illustrate this fact with the example of NGC 1275. We point out that the cascade synchrotron emission may produce spectral features in the same energy range in which the big blue bump is observed in the spectral energy distributions of several blazars, and may make a non-negligible contribution to this feature. We illustrate this idea with the example of 3C 279.

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arXiv2012

Simultaneous B'V'R' Monitoring of BL Lacertae Object S5~0716+714 and Detection of Inter-Band Time Delay

We present the results of our optical monitoring of the BL Lac object S5 0716+714 on seven nights in 2006 December. The monitoring was carried out simultaneously at three optical wavelengths with a novel photometric system. The object did not show large-amplitude internight variations during this period. Intranight variations were observed on four nights and probably on one more. Strong bluer-when-brighter chromatism was detected on both intranight and internight timescales. The intranight variation amplitude decreases in the wavelength sequence of B', R', and V'. Cross correlation analyses revealed that the variability at the $B'$ and $V'$ bands lead that at the $R'$ band by about 30 minutes on one night.

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arXiv2011

Multiwavelength Spectral Studies Of Fermi-LAT Blazars

We present multiwavelength spectral analyses of two Fermi-LAT blazars, OJ 287 and 3C 279, that are part of the Boston University multiwaveband polarization program. The data have been compiled from observations with Fermi, RXTE, the VLBA, and various ground-based optical and radio telescopes. We simulate the dynamic spectral energy distributions (SEDs) within the framework of a multi-slice, time-dependent leptonic jet model for blazars, with radiation feedback, in the internal shock scenario. We use the physical jet parameters obtained from the VLBA monitoring to guide our modeling efforts. We discuss the role of intrinsic parameters and the interplay between synchrotron and inverse Compton radiation processes responsible for producing the resultant SEDs.

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arXiv2011

Time dependent simulations of multiwavelength variability of the blazar Mrk 421 with a Monte Carlo multi-zone code

(abridged) We present a new time-dependent multi-zone radiative transfer code and its application to study the SSC emission of Mrk 421. The code couples Fokker-Planck and Monte Carlo methods, in a 2D geometry. For the first time all the light travel time effects (LCTE) are fully considered, along with a proper treatment of Compton cooling, which depends on them. We study a set of simple scenarios where the variability is produced by injection of relativistic electrons as a `shock front' crosses the emission region. We consider emission from two components, with the second one either being pre-existing and co-spatial and participating in the evolution of the active region, or spatially separated and independent, only diluting the observed variability. Temporal and spectral results of the simulation are compared to the multiwavelength observations of Mrk 421 in March 2001. We find parameters that can adequately fit the observed SEDs and multiwavelength light curves and correlations. There remain however a few open issues, most notably: i) systematic soft intra-band X-ray lags. ii) The quadratic correlation between the TeV and X-ray flux during the flare decay has not been reproduced. These features are among those affected by the spatial extent and geometry of the source. The difficulty of producing hard X-ray lags is exacerbated by a bias towards soft lags caused by the combination of energy dependent radiative cooling time-scales and LCTE. About the second emission component, our results strongly favor the scenario where it is co-spatial and it participates in the flare evolution, suggesting that different phases of activity may occur in the same region. The cases presented in this paper represent only an initial study, and despite their limited scope they make a strong case for the need of true time-dependent and multi-zone modeling.

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arXiv2010

VHE Gamma-Ray Induced Pair Cascades in the Radiation Fields of Dust Tori of AGN: Application to Cen A

The growing number of extragalactic high-energy (HE, E > 100 MeV) and very-high-energy (VHE, E > 100 GeV) gamma-ray sources that do not belong to the blazar class suggests that VHE gamma-ray production may be a common property of most radio-loud Active Galactic Nuclei (AGN). In a previous paper, we have investigated the signatures of Compton-supported pair cascades initiated by VHE gamma-ray absorption in monochromatic radiation fields, dominated by Ly-alpha line emission from the Broad Line Region. In this paper, we investigate the interaction of nuclear VHE gamma-rays with the thermal infrared radiation field from a circumnuclear dust torus. Our code follows the spatial development of the cascade in full 3-dimensional geometry. We provide a model fit to the broadband SED of the dust-rich, gamma-ray loud radio galaxy Cen A and show that typical blazar-like jet parameters may be used to model the broadband SED, if one allows for an additional cascade contribution to the Fermi gamma-ray emission.

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arXiv2010

Time-dependent Radiation Transfer in the Internal Shock Model Scenario for Blazar Jets

We describe the time-dependent radiation transfer in blazar jets, within the internal shock model. We assume that the central engine, which consists of a black hole and an accretion disk, spews out relativistic shells of plasma with different velocity, mass, and energy. We consider a single inelastic collision between a faster (inner) and a slower (outer) moving shell. We study the dynamics of the collision and evaluate the subsequent emission of radiation via the synchrotron and synchrotron self Compton (SSC) processes after the interaction between the two shells has begun. The collision results in the formation of a forward shock (FS) and a reverse shock (RS) that convert the ordered bulk kinetic energy of the shells into magnetic field energy and accelerate the particles, which then radiate. We assume a cylindrical geometry for the emission region of the jet. We treat the self-consistent radiative transfer by taking into account the inhomogeneity in the photon density throughout the region. In this paper, we focus on understanding the effects of varying relevant input parameters on the simulated spectral energy distribution (SED) and spectral variability patterns.

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arXiv2010

Models for the Spectral Energy Distributions and Variability of Blazars

In this review, recent progress in theoretical models for blazar emission will be summarized. The salient features of both leptonic and lepto-hadronic approaches to modeling blazar spectral energy distributions will be reviewed. I will present sample modeling results of spectral energy distributions (SEDs) of different types of blazars along the blazar sequence, including Fermi high-energy gamma-ray data, using both types of models. Special emphasis will be placed on the implications of the recent very-high-energy (VHE) gamma-ray detections of non-traditional VHE gamma-ray blazars, including intermediate and low-frequency-peaked BL Lac objects and even flat-spectrum radio quasars. Due to the featureless optical spectra of BL Lac objects, the redshifts of several BL Lacs remain unknown. I will briefly discuss possible constraints on their redshift using spectral modeling of their SED including Fermi + ground-based VHE gamma-ray data. It will be shown that in some cases, spectral modeling with time-independent single-zone models alone is not sufficient to constrain models, as both leptonic and lepto-hadronic models are able to provide acceptable fits to the overall SED. Subsequently, recent developments of time-dependent and inhomogeneous blazar models will be discussed, including detailed numerical simulations as well as a semi-analytical approach to the time-dependent radiation signatures of shock-in-jet models.

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arXiv2010

VHE Gamma-Ray Induced Pair Cascades in Blazars and Radio Galaxies: Application to NGC 1275

Recent blazar detections by HESS, MAGIC, and VERITAS suggest that very-high-energy (VHE, E > 100 GeV) gamma-rays may be produced in most, if not all, types of blazars, including those that possess intense circumnuclear radiation fields. In this paper, we investigate the interaction of nuclear VHE gamma-rays with the circumnuclear radiation fields through gamma-gamma absorption and pair production, and the subsequent Compton-supported pair cascades. We have developed a Monte-Carlo code to follow the spatial development of the cascade in full 3-dimensional geometry, and calculate the radiative output due to the cascade as a function of viewing angle with respect to the primary VHE gamma-ray beam (presumably the jet axis of the blazar). We show that even for relatively weak magnetic fields, the cascades can be efficiently isotropized, leading to substantial off-axis cascade emission peaking in the Fermi energy range at detectable levels for nearby radio galaxies. We demonstrate that this scenario can explain the Fermi flux and spectrum of the radio galaxy NGC 1275.

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arXiv2010

Timing Signatures of the Internal-Shock Model for Blazars

We investigate the spectral and timing signatures of the internal-shock model for blazars. For this purpose, we develop a semi-analytical model for the time-dependent radiative output from internal shocks arising from colliding relativistic shells in a blazar jet. The emission through synchrotron and synchrotron-self Compton (SSC) radiation as well as Comptonization of an isotropic external radiation field are taken into account. We evaluate the discrete correlation function (DCF) of the model light curves in order to evaluate features of photon-energy dependent time lags and the quality of the correlation, represented by the peak value of the DCF. The almost completely analytic nature of our approach allows us to study in detail the influence of various model parameters on the resulting spectral and timing features. This paper focuses on a range of parameters in which the gamma-ray production is dominated by Comptonization of external radiation, most likely appropriate for gamma-ray bright flat-spectrum radio quasars (FSRQs) or low-frequency peaked BL Lac objects (LBLs). In most cases relevant for FSRQs and LBLs, the variability of the optical emission is highly correlated with the X-ray and high-energy (HE: > 100 MeV) gamma-ray emission. Our baseline model predicts a lead of the optical variability with respect to the higher-energy bands by 1 - 2 hours and of the HE gamma-rays before the X-rays by about 1 hour. We show that variations of certain parameters may lead to changing signs of inter-band time lags, potentially explaining the lack of persistent trends of time lags in most blazars.

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arXiv2009

Implications of the VHE Gamma-Ray Detection of the Quasar 3C279

The MAGIC collaboration recently reported the detection of the quasar 3C279 at > 100 GeV gamma-ray energies. Here we present simultaneous optical (BVRI) and X-ray (RXTE PCA) data from the day of the VHE detection and discuss the implications of the snap-shot spectral energy distribution for jet models of blazars. A one-zone synchrotron-self-Compton origin of the entire SED, including the VHE gamma-ray emission can be ruled out. The VHE emission could, in principle, be interpreted as Compton upscattering of external radiation (e.g., from the broad-line regions). However, such an interpretation would require either an unusually low magnetic field of B ~ 0.03 G or an unrealistically high Doppler factor of Gamma ~ 140. In addition, such a model fails to reproduce the observed X-ray flux. This as well as the lack of correlated variability in the optical with the VHE gamma-ray emission and the substantial gamma-gamma opacity of the BLR radiation field to VHE gamma-rays suggests a multi-zone model. In particular, an SSC model with an emission region far outside the BLR reproduces the simultaneous X-ray -- VHE gamma-ray spectrum of 3C279. Alternatively, a hadronic model is capable of reproducing the observed SED of 3C279 reasonably well. However, the hadronic model requires a rather extreme jet power of L_j ~ 10^{49} erg s^{-1}, compared to a requirement of L_j ~ 2 X 10^{47} erg s^{-1} for a multi-zone leptonic model.

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arXiv2008

Gamma-Ray Studies of Blazars: Synchro-Compton Analysis of Flat Spectrum Radio Quasars

We extend a method for modeling synchrotron and synchrotron self-Compton radiations in blazar jets to include external Compton processes. The basic model assumption is that the blazar radio through soft X-ray flux is nonthermal synchrotron radiation emitted by isotropically-distributed electrons in the randomly directed magnetic field of outflowing relativistic blazar jet plasma. Thus the electron distribution is given by the synchrotron spectrum, depending only on the Doppler factor $δ_{\rm D}$ and mean magnetic field $B$, given that the comoving emission region size scale $R_b^\prime \lesssim c \dD t_v/(1+z)$, where $t_v$ is variability time and $z$ is source redshift. Generalizing the approach of Georganopoulos, Kirk, and Mastichiadis (2001) to arbitrary anisotropic target radiation fields, we use the electron spectrum implied by the synchrotron component to derive accurate Compton-scattered $γ$-ray spectra throughout the Thomson and Klein-Nishina regimes for external Compton scattering processes. We derive and calculate accurate $γ$-ray spectra produced by relativistic electrons that Compton-scatter (i) a point source of radiation located radially behind the jet, (ii) photons from a thermal Shakura-Sunyaev accretion disk and (iii) target photons from the central source scattered by a spherically-symmetric shell of broad line region (BLR) gas. Calculations of broadband spectral energy distributions from the radio through $γ$-ray regimes are presented, which include self-consistent $γγ$ absorption on the same radiation fields that provide target photons for Compton scattering. Application of this baseline flat spectrum radio/$γ$-ray quasar model is considered in view of data from $γ$-ray telescopes and contemporaneous multi-wavelength campaigns.

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arXiv2008

Synchrotron Self-Compton Analysis of TeV X-ray Selected BL Lacertae Objects

We introduce a methodology for analysis of multiwavelength data from X-ray selected BL Lac (XBL) objects detected in the TeV regime. By assuming that the radio--through--X-ray flux from XBLs is nonthermal synchrotron radiation emitted by isotropically-distributed electrons in the randomly oriented magnetic field of a relativistic blazar jet, we obtain the electron spectrum. This spectrum is then used to deduce the synchrotron self-Compton (SSC) spectrum as a function of the Doppler factor, magnetic field, and variability timescale. The variability timescale is used to infer the comoving blob radius from light travel-time arguments, leaving only two parameters. With this approach, we accurately simulate the synchrotron and SSC spectrum of flaring XBLs in the Thomson through Klein-Nishina regimes. Photoabsorption by interactions with internal jet radiation and the intergalactic background light (IBL) is included. Doppler factors, magnetic fields, and absolute jet powers are obtained by fitting the {\em HESS} and {\em Swift} data of the recent giant TeV flare observed from \object{PKS 2155--304}. For the contemporaneous {\em Swift} and {\em HESS} data from 28 and 30 July 2006, respectively, Doppler factors $\gtrsim 60$ and absolute jet powers $\gtrsim 10^{46}$ ergs s$^{-1}$ are required for a synchrotron/SSC model to give a good fit to the data, for a low intensity of the IBL and a ratio of 10 times more energy in hadrons than nonthermal electrons. Fits are also made to a TeV flare observed in 2001 from Mkn 421 which require Doppler factors $\gtrsim 30$ and jet powers $\gtrsim 10^{45}$ erg s$^{-1}$.

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arXiv2008

The Hard VHE Gamma-ray Emission in High-Redshift TeV Blazars: Comptonization of Cosmic Microwave Background Radiation in an Extended Jet?

Observations of very-high-energy (VHE, E > 250 GeV) gamma-ray emission from several blazars at z > 0.1 have placed stringent constraints on the elusive spectrum and intensity of the intergalactic infrared background radiation (IIBR). Correcting their observed VHE spectrum for gamma-gamma absorption even by the lowest plausible level of the IIBR provided evidence for a very hard (photon spectral index Gamma_{ph} < 2) intrinsic source spectrum out to TeV energies. Such a hard VHE gamma-ray spectrum poses a serious challenge to the conventional synchrotron-self-Compton interpretation of the VHE emission of TeV blazars and suggests the emergence of a separate emission component beyond a few hundred GeV. Here we propose that such a very hard, slowly variable VHE emission component in TeV blazars may be produced via Compton upscattering of Cosmic Microwave Background (CMB) photons by shock-accelerated electrons in an extended jet. For the case of 1ES 1101-232, this component could dominate the bolometric luminosity of the extended jet if the magnetic fields are of the order of typical intergalactic magnetic fields B ~ 10 micro-Gauss and electrons are still being accelerated out to TeV energies gamma > 4 X 10^6) on kiloparsec scales along the jet.

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arXiv2008

Nonthermal Synchrotron and Synchrotron Self-Compton Emission from GRBs: Predictions for {\em Swift} and {\em GLAST}

Results of a leptonic jet model for the prompt emission and early afterglows of GRBs are presented. The synchrotron component is modeled with the canonical Band spectrum and the synchrotron self-Compton component is calculated from the implied synchrotron-emitting electron spectrum in a relativistic plasma blob. In the comoving frame the magnetic field is assumed to be tangled and the electron and photon distributions are assumed to be isotropic. The Compton-scattered spectrum is calculated using the full Compton cross-section in the Thomson through Klein-Nishina using the Jones formula. Pair production photoabsorption, both from ambient radiation in the jet and from the extragalactic background light (EBL), is taken into account. Results are presented as a function of a small set of parameters: the Doppler factor, the observed variability timescale, the comoving magnetic field, the peak synchrotron flux, and the redshift of the burst. Model predictions will be tested by multiwavelength observations, including the {\em Swift} and {\em GLAST} satellites, which will provide unprecedented coverage of GRBs.

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arXiv2007

X-ray Spectral Signatures of the Photon Bubble Model for Ultraluminous X-ray Sources

The nature of ultraluminous X-ray sources in nearby galaxies is one of the major open questions in modern X-ray astrophysics. One possible explanation for these objects is an inhomogeneous, radiation dominated accretion disk around a $\sim 10 M_{\odot}$ black hole -- the so-called ``photon bubble'' model. While previous studies of this model have focused primarily on its radiation-hydrodynamics aspects, in this paper, we provide an analysis of its X-ray spectral (continuum and possible edge and line) characteristics. Compton reflection between high and low density regions in the disk may provide the key to distinguishing this model from others, such as accretion onto an intermediate mass black hole. We couple a Monte Carlo/Fokker-Planck radiation transport code with the XSTAR code for reflection to simulate the photon spectra produced in a photon bubble model for ULXs. We find that reflection components tend to be very weak and in most cases not observable, and make predictions for the shape of the high-energy Comptonizing spectra. In many cases the Comptonization dominates the spectra even down to $\sim$ a few keV. In one simulation, a $\sim 9 \kev$ feature was found, which may be considered a signature of photon bubbles in ULXs; furthermore, we make predictions of high energy power-laws which may be observed by future instruments.

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