K. Katarzynski

Hard TeV spectra of blazars and the constraints to the infrared intergalactic background

Recent gamma-ray observations of the blazar 1ES 1101−232 (redshift z = 0.186) reveal that the unabsorbed TeV spectrum is hard, with spectral index α 0.5 [F(ν) ∝ ν −α ]. We show that simple one-zone synchrotron self-Compton model can explain such hard spectra if we assume a power law energy distribution of the emitting electrons with a relatively high minimum energy. In this case the intrinsic TeV spectrum can be as hard as F(ν) ∝ ν 1/3 , while the predicted X-ray spectrum can still be much softer. The observations of 1ES 1101−232 can therefore be reconciled with relatively high intensities of the infrared background, even if some extreme background levels can indeed be excluded. We show that the other TeV sources (Mrk 421, Mrk 501 and PKS 2155−304) can be interpreted in the same framework, with a somewhat larger minimum energy.

Stochastic particle acceleration and synchrotron self-Compton radiation in TeV blazars

Aims. We analyse the influence of the stochastic particle accelera tion for the evolution of the electron spectrum. We assume that all investigated spectra are generated inside a spherical, homogeneous source and also analyse the synchrotron and inverse Compton emission generated by such an object. Methods. The stochastic acceleration is treated as the diffusion of the particle momentum and is described by the momentum‐diffusion equation. We investigate the stationary and time dependent solutions of the equation for several different evolutionary scenarios. The scenarios are divided into two general classes. First, we analyse a few cases without injection or escape of the particles during th e evolution. Then we investigate the scenarios where we assume continuous injection and simultaneous escape of the particles. Results. In the case of no injection and escape the acceleration process, competing with the radiative cooling, only modifies the i nitial particle spectrum. The competition leads to a thermal or quasi‐thermal distribution of the particle energy. In the case of the inj ection and simultaneous escape the resulting spectra depend mostly on the energy distribution of the injected particles. In the simplest case, w here the particles are injected at the lowest possible energies, the competition b etween the acceleration and the escape forms a power‐law energy distribution. We apply our modeling to the high energy activity of the blazar Mrk 501 observed in April 1997. Calculating the evolution of the electron spectrum self‐consistently we can reproduce the observed spectra well with a number of free parameters that is comparable to or less than in the “classic stationary” one‐zone synchrotron self‐Compton scenario.

Correlation between the TeV and X-ray emission in high-energy peaked BL Lac objects

We discuss the correlation between the evolution of the TeV emission and X-ray radiation observed in high-energy peaked BL Lac objects. We describe such a correlation by a simple power law FTeV (t) ∝ F x−ray (t). In the first part of this work we present correlations obtained for the activity of Mrk 501 observed in 1997 April and for the activity of Mrk 421 observed in 2000 February. Our results obtained for Mrk 501 show that the index of the correlation (x) may strongly depend on the width and position of the spectral bands used for the comparison. The result of the correlation which we have obtained for Mrk 421 is not informative. However, we discuss results of similar correlation obtained for this source by other authors. They report an almost quadratic (x ∼ 2) correlations observed between the evolution of the TeV and X-ray emission. In the second part of this paper we present a phenomenological model which describes the evolution of the synchrotron and inverse Compton emission of a simple spherical homogeneous source. Neglecting the radiative cooling of the particles we derive analytical expressions that describe the evolution. Then we use a numerical code to investigate the impact of radiative cooling on the evolution. We show that different forms of correlations can be obtained depending on the assumed evolution scenario and the spectral bands used for the calculation. However, the quadratic correlation observed during the decay phase of the flare observed in Mrk 421 on 2001 March 19 appears problematic for this basic modeling. The quadratic correlation can be explained only for specific choices of the spectral bands used for the calculation. Therefore, looking for more robust solutions, we investigate the evolution of the emission generated by a cylindrical source. However this model does not provide robust solutions for the problem of a quadratic correlation. In principle the problem could be solved by the TeV emission generated by the self Compton scattering in the Thomson limit. However, we show that such a process requires unacceptably large values of the Doppler factor. Finally we briefly discuss the possible influence of the light travel time effect on our results.

A synchrotron self-Compton scenario for the very high energy γ-ray emission of the radiogalaxy M 87 - Unifying the TeV emission of blazars and other AGNs?

M87 is the first extragalactic source detected in the TeV range that is not a blazar. With the increasing performances of ground-based Cherenkov telescopes, we can now probe the variability in the gamma-ray flux at small timescales, thus putting strong constraints on the size of the emitting zone. A modification of standard emission models of TeV blazars appears necessary to account for the gamma-ray observations despite this misalignment. We explain TeV gamma-ray spectra and fast variability of M87 by invoking an emission zone close to the central supermassive black hole, which is filled with several plasma blobs moving in the large opening angle of the jet formation zone. We develop a new multi-blob synchrotron self-Compton (SSC) model with emitting blobs beyond the Alfven surface in the jet, at a distance of about 100 r_g from the central engine. This model is explicitly adapted to deal with large viewing angles and moderate values of the Lorentz factor inferred from (general relativistic) magnetohydrodynamic models of jet formation. This scenario can account for the recent gamma-ray observations of M87 made by the High Energy Stereoscopic System (H.E.S.S.) telescope array. We find individual blob radii of about 10^{14} cm, which is compatible with the variability on timescales of days recently reported by the H.E.S.S. collaboration and is of the order of the black hole gravitational radius. Predictions of the very high energy emission for three other sources with extended optical or X-ray jet which could be misaligned blazars still with moderate beaming are presented for one Seyfert 2 radiogalaxy, namely Cen A, one peculiar BL Lac, PKS 0521-36, and one quasar, 3C 273.

Active Galactic Nuclei under the scrutiny of CTA

Abstract Active Galactic Nuclei (hereafter AGN) produce powerful outflows which offer excellent conditions for efficient particle acceleration in internal and external shocks, turbulence, and magnetic reconnection events. The jets as well as particle accelerating regions close to the supermassive black holes (hereafter SMBH) at the intersection of plasma inflows and outflows, can produce readily detectable very high energy gamma-ray emission. As of now, more than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the present ground-based gamma-ray telescopes, which represents more than one third of the cosmic sources detected so far in the VHE gamma-ray regime. The future Cherenkov Telescope Array (CTA) should boost the sample of AGN detected in the VHE range by about one order of magnitude, shedding new light on AGN population studies, and AGN classification and unification schemes. CTA will be a unique tool to scrutinize the extreme high-energy tail of accelerated particles in SMBH environments, to revisit the central engines and their associated relativistic jets, and to study the particle acceleration and emission mechanisms, particularly exploring the missing link between accretion physics, SMBH magnetospheres and jet formation. Monitoring of distant AGN will be an extremely rewarding observing program which will inform us about the inner workings and evolution of AGN. Furthermore these AGN are bright beacons of gamma-rays which will allow us to constrain the extragalactic infrared and optical backgrounds as well as the intergalactic magnetic field, and will enable tests of quantum gravity and other “exotic” phenomena.

Modelling rapid TeV variability of PKS 2155−304

We present theoretical modelling for the very rapid TeV variability of PKS 2155–304 observed recently by the H.E.S.S. experiment. To explain the light-curve, where at least five flaring events were well observed, we assume five independent components of a jet that are characterized by slightly different physical parameters. An additional, significantly larger component is used to explain the emission of the source at long time scales. This component dominates the emission in the X-ray range, whereas the other components are dominant in the TeV range. The model used for our simulation describes precisely the evolution of the particle energy spectrum inside each component and takes into account light travel time effects. We show that a relatively simple synchrotron self-Compton scenario may explain this very rapid variability. Moreover, we find that absorption of the TeV emission inside the components due to the pair creation process is negligible.

On the correlation between the X-ray and gamma-ray emission in TeV blazars

Aims. The observations of TeV blazars published recently show an unexpected quadratic or even cubic correlation between the X-ray and gamma-ray emission. A standard model of the synchrotron self-Compton emission of a compact source inside a jet is not able to explain such a correlation. Therefore, we propose an alternative scenario where the emission of at least two independent compact components is observed at the same time. Methods. We compare two different models. The first model assumes the injection of relativistic particles into a downstream region of a shock wave inside a jet that creates the emitting source. The model precisely describes the evolution of the particle energy spectrum inside the source and takes into account a light-crossing time effect for the produced radiation. The second model assumes an intrinsically constant emission of a homogeneous source that travels inside the jet along a curved trajectory, where the activity is produced simply by different values of the sources Doppler factor. To verify the two models we use recently published observations of Mrk 421. Results. Our simulations show that simultaneous radiation of at least two independent sources, where the first source dominates the emission in the X-ray range and the second source radiates strongly in the gamma-ray range, can explain the observed correlations. However, the injection model provides inadequate results because it gives different values for the correlation of the rise and decay of a flare. This problem is negligible in the scenario that uses the Doppler boosting effect. Therefore, this approach yields much better results.

The cyclo-synchrotron process and particle heating through the absorption of photons

Aims. We propose a new approximation for the cyclo-synchrotron emissivity of a single electron. In the second part of this work, we discuss a simple application for our approximation, and investigate the heating of electrons through the self-absorption process. Finally, we investigate the self-absorbed part of the spectrum produced by a power-law population of electrons. Methods. In comparison to earlier approximations, our formula provides a few significant advantages. Integration of the emissivity over the whole frequency range, starting from the proper minimal emitting frequency, gives the correct cooling rate for any energy particle. Further, the spectrum of the emission is well-approximated over the whole frequency range, even for relatively low particle energies (

Physical properties and astrometry of radio-emitting brown dwarf TVLM 513-46546 revisited

\beta \ll 0.1

Hard MeV–GeV spectra of blazars

), where most of the power is emitted in the first harmonic. In order to test our continuous approximation, we compare it with a recently derived approximation of the first ten harmonics. Finally, our formula connects relatively smooth to the synchrotron emission at