arXiv subjects

M. Ostrowski

Publications and source records attributed to M. Ostrowski.

At least 73 records · Page 4Linked to original sources

arXiv2005

On Multiwavelength Emission and Morphology of Large-Scale Quasar Jets

We propose that the knots of the large-scale jets in powerful radio sources represent moving and separate portions of the jet matter, with the excess kinetic power. This can explain many morphological properties of quasar jets - like high knot-to-interknot brightness contrasts, frequency-independent knot profiles and almost universal extents of the knot regions - independently of the exact emission mechanisms responsible for producing broad-band jet emission. We briefly discuss a possible connection between this scenario and the idea of highly modulated/intermittent activity of the jet engine.

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arXiv2005

On the Magnetic Field in Kpc-Scale Jets of FR I Radio Galaxies

The energy content of large-scale jets in FR I radio galaxies is still an open issue. Here we show that upper limits on the high-energy and very high-energy gamma-ray emission of the kpc-scale jet in M 87 radio galaxy imposed by EGRET, Whipple, and - most importantly - HEGRA and HESS observations, provide important constraints on the magnetic field strength in this object. In particular, a non-detection of gamma-ray radiation from the brightest part of this jet (knot A), expected from the inverse-Compton scattering of the starlight photons by the synchrotron-emitting jet electrons, implies that the magnetic field cannot be smaller than the equipartition value (referring solely to the radiating ultrarelativistic electrons), and most likely, is even stronger. In this context, we point out several consequences of the obtained result for the large-scale jet structures in FR I radio galaxies and the M 87 jet in particular. For example, we discuss a potential need for amplification of the magnetic field energy flux along these jets (from sub-pc to kpc scales), suggesting a turbulent dynamo as a plausible process responsible for the aforementioned amplification.

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arXiv2004

Discovery of hard X-ray features around hotspots of Cygnus A

We present results of analysis of a Chandra observation of Cygnus A in which the X-ray hotspots at the ends of the jets are mapped in detail. A hardness map reveals previously unknown structure in the form of outer and inner hard arcs around the hotspots, with hardness significantly enhanced compared with the hotspot central regions. The outer hard arcs may constitute the first detection of the bow shock; the inner hard arcs may reveal where the jets impact on the hotspots. We argue that these features cannot result from electrons radiating by the synchrotron self-Compton process. Instead we consider two possible sources of the hard emission: the outer arcs may be due to thermal radiation of hot intracluster gas compressed at the bow shock. Alternatively, both outer and inner arcs may be due to synchrotron radiation of electrons accelerated in turbulent regions highly perturbed by shocks and shear flows. Comparison of measured hardness ratios with simulations of the hardness ratios resulting from these processes show that it is more diffcult to explain the observations with a thermal model. Although we cannot rule out a thermal model, we argue in favour of the non-thermal explanation. The hard regions in the secondary hotspots suggest that jet activity is still powering these hotspots.

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arXiv2004

On Multiwavelength Emission of Large-Scale Quasar Jets

We discuss morphological properties of the large-scale jets in powerful radio sources, which are now observed at radio, optical and X-ray frequencies, in order to determine the origin of their X-ray radiation and the nature of the particle acceleration responsible for their multiwavelength emission. We show that modeling knots of the these objects as stationary regions of energy dissipation within the uniform and continuous jet flow leads to several problems. Such problems are especially pronounced if the observed X-ray emission is due to inverse-Compton scattering of the CMB radiation. We explore another possibility, namely that the knots represent moving and separate portions of the jet matter, with excess kinetic power. We suggest a possible connection between this scenario and the idea of intermittent/highly modulated jet activity. The proposed model can explain some morphological properties of quasar jets -- like high knot-to-interknot brightness contrasts, frequency-independent knot profiles and almost universal extents of the knot regions -- independently of the exact emission mechanism responsible for producing the X-rays. In this context, we consider different possibilities for the production of such X-ray radiation, and discuss the related issue of particle acceleration. We conclude that the appropriate process cannot be designated for certain yet. We suggest, however, that X-ray observations of the jet in 3C 120 (in which intermittent jet activity is quite distinct) already seem to support the synchrotron origin of the X-ray emission, at least in this object.

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arXiv2003

High Energy gamma-rays From FR I Jets

Thanks to Hubble and Chandra telescopes, some of the large scale jets in extragalactic radio sources are now being observed at optical and X-ray frequencies. For the FR I objects the synchrotron nature of this emission is surely established, although a lot of uncertainties - connected for example with the particle acceleration processes involved - remain. In this paper we study production of high energy gamma-rays in FR I kiloparsec-scale jets by inverse-Compton emission of the synchrotron-emitting electrons. We consider different origin of seed photons contributing to the inverse-Compton scattering, including nuclear jet radiation as well as ambient, stellar and circumstellar emission of the host galaxies. We discuss how future detections or non-detections of the evaluated gamma-ray fluxes can provide constraints on the unknown large scale jet parameters, i.e. the magnetic field intensity and the jet Doppler factor. For the nearby sources Centaurus A and M 87, we find measurable fluxes of TeV photons resulting from synchrotron self-Compton process and from comptonisation of the galactic photon fields, respectively. In the case of Centaurus A, we also find a relatively strong emission component due to comptonisation of the nuclear blazar photons, which could be easily observed by GLAST at energy ~10 GeV, providing important test for the unification of FR I sources with BL Lac objects.

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arXiv2003

VHE gamma-rays From Extragalactic Large Scale Jets

We discuss production of very high energy (VHE) gamma-rays in large scale extragalactic jets of FR I sources. Based on recent optical and X-ray observations of these objects we evaluate their VHE emission in a framework of one electron population model. We consider possibility for detection of the predicted radiation, which could provide important constraints on the jet structure and some hardly known large scale jet parameters.

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arXiv2003

A Role of the Boundary Shear Layer in Modeling of Large Scale Jets

We discuss a role of the boundary shear layer of large scale jets for their observed multiwavelength emission. We consider a simple mechanism of a turbulent particle acceleration acting within such regions as an alternative mechanism with respect to the standard approaches involving internal shocks in the jet. We show, that under simple assumptions the boundary layer acceleration can provide very high energy electrons possibly generating the X-ray emission observed by Chandra from some large scale jets in powerful radiogalaxies.

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arXiv2002

Comment on the first-order Fermi acceleration at ultra-relativistic shocks

The first-order Fermi acceleration process at an ultra-relativistic shock wave is expected to create a particle spectrum with the unique asymptotic spectral index sigma_{gamma >> 1} approximately 2.2. Below, we discuss this result and differences in its various derivations, which -- explicitly or implicitly -- always require highly turbulent conditions downstream of the shock. In the presence of medium amplitude turbulence the generated particle spectrum can be much steeper than the above asymptotic one. We also note problems with application of the pitch angle diffusion model for particle transport near the ultra-relativistic shocks.

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arXiv2002

Mechanisms and sites of ultra high energy cosmic ray origin

We shortly discuss several astrophysical scenarios leading to cosmic ray acceleration up to extremely high energies reaching the scale of 10^{20} eV. The processes suggested in the literature include acceleration at relativistic jet terminal shocks and shear boundary layers, shocks in large scale accretion flows onto supergalactic cosmic structures, particle reflections from ultra-relativistic shocks postulated to exist in sources of gamma ray bursts, the processes involving the neutron star rotating magnetospheres and dormant quasars. Some of these objects can explain cosmic rays with highest energies if one tunes the model parameters to limits enabling the highest acceleration efficiency. We also note that some of the considered processes allow for acceleration efficiency in the Hillas diagram, beta, to be much larger than unity. The present paper is based on a review talk presented during the European Cosmic Ray Symposium in Lodz (2000).

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arXiv2002

On particle acceleration in turbulent shear boundary layers of relativistic jets

We discuss processes accelerating cosmic ray electrons at boundaries of relativistic jets. The resulting spectrum is expected to posses two characteristic components: a power-law distribution at lower energies and a harder high energy component preceding the high energy cut-off. An example of high energy spectrum from such electron distribution is presented, including the synchrotron and the inverse-Compton components. A characteristic feature of the synchrotron spectrum of such electrons is its highest frequency part which occurs above the power-law fitted to the low frequency spectral range.

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arXiv2002

On relativistic jet/counterjet asymmetry in the presence of radiating jet boundary layer

We consider radiation of relativistic electrons accelerated within the jet extended boundary layer. Due to velocity shear across the boundary the observed jet/counterjet brightness ratio is diminished as compared to the one derived for the jet spine. Thus the jet Lorentz factor evaluated from the observed jet asymmetry can be an underestimated value influenced by observation of the slower boundary layer. We briefly discuss several consequences of the radiating boundary layer model in the context of recent Chandra and XMM observations of the large scale jets.

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arXiv2002

Radiation from the Relativistic Jet: a Role of the Shear Boundary Layer

Recent radio and optical large scale jets' observations suggest a two-component jet morphology, consisting of a fast central spine surrounded with a boundary layer with a velocity shear. We study radiation of electrons accelerated at such boundary layers as an option for standard approaches involving internal shocks in jets. The acceleration process in the boundary layer yields in a natural way a two component electron distribution: a power-law continuum with a bump at the energy, where energy gains equal radiation losses, followed by a cut-off. For such distributions we derive the observed spectra of synchrotron and inverse-Compton radiation, including comptonization of synchrotron and CMB photons. Under simple assumptions of energy equipartition between the relativistic particles and the magnetic field, the relativistic jet velocity at large scales and a turbulent character of the shear layer, the considered radiation can substantially contribute to the jet radiative output. In the considered conditions the synchrotron emission is characterized by a spectral index of the radio-to-optical continuum being approximately constant along the jet. A characteristic feature of the obtained broad-band synchrotron spectrum is an excess at X-ray frequencies, similar to the one observed in some objects by Chandra. As compared to the uniform jet models, the velocity shear across the radiating boundary region leads to decrease and frequency dependence of the observed jet-counterjet radio brightness asymmetry. We conclude that a careful investigation of the observational data looking for the derived effects can allow to evaluate the role of the boundary layer acceleration processes and/or impose constraints for the physical parameters of such layers in large scale jets.

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arXiv2002

Cosmic ray acceleration at supergalactic accretion shocks: a new upper energy limit due to a finite shock extension

Accretion flows onto supergalactic-scale structures are accompanied with large spatial scale shock waves. These shocks were postulated as possible sources of ultra-high energy cosmic rays. The highest particle energies were expected for perpendicular shock configuration in the so-called "Jokipii diffusion limit", involving weakly turbulent conditions in the large-scale magnetic field imbedded in the accreting plasma. For such configuration we discuss the process limiting the highest energy that particles can obtain in the first-order Fermi acceleration process due to finite shock extensions to the sides, along and across the mean magnetic field. Cosmic ray outflow along the shock structure can substantially lower (below ~10^18 eV for protons) the upper particle energy limit for conditions considered for supergalactic shocks.

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arXiv2001

Cosmic ray propagation and acceleration

This report covers the studies of energetic particle propagation and acceleration presented during the 27th ICRC in sessions OG 1.3 and 1.4, plus a few selected papers from neighbouring OG sections. We list problems discussed within these sections and we shortly describe selected interesting achievements and/or novel approaches.

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arXiv2001

High Energy Radiation Generated at Boundary Shear Layers of Relativistic Jets

A simple model of cosmic ray electron acceleration at the jet boundary (Ostrowski 2000) yields a power-law particle energy distribution of ultrarelativistic electrons with an energy cut-off growing with time and finally, a growing particle bump at the energy, where energy gains equal radiation losses. For such electron distribution, in tens-of-kpc scale jets, we derived the observed time varying spectra of synchrotron and inverse-Compton radiation, including comptonization of synchrotron and cosmic microwave background photons. Slowly varying spectral index along the jet in the `low-frequency' spectral range is a natural consequence of boundary layer acceleration. Variations of the high energy bump of the electron distribution can give rise to anomalous behaviour in X-ray band in comparison to the lower frequencies.

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arXiv2001

Numerical Simulations of Relativistic Shock Acceleration

We review the present status of the cosmic ray acceleration theory in mildly relativistic shock waves. Due to the involved substantial particle anisotropies analytical methods can tackle only simple situations involving weakly turbulent conditions near the shock. The numerical Monte Carlo methods are used to study the acceleration process in more general conditions. Contrary to non-relativistic shocks, the cosmic ray spectra at relativistic shock waves depend substantially on the magnetic field configurations and turbulence spectra. Depending on these conditions both very flat and very steep spectra can be created, with the characteristic acceleration time scales changing non-monotonously with the turbulence amplitude. Thus, the discussed theory is not able to uniquely explain the observed synchrotron spectra of relativistic shocks. We also mention an interesting possibility of particle acceleration at incompressible flow discontinuities (shear layers) at boundaries of relativistic jets.

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arXiv2001

Pressure imbalance of FRII radio source lobes: a role of energetic proton population

Recently Hardcastle & Worrall (MNRAS, 319, 562) analyzed 63 FRII radio galaxies imbedded in the X-ray radiating gas in galaxy clusters and concluded, that pressures inside its lobes seem to be a factor of a few lower than in the surrounding gas. One of explanations of the existing `blown up' radio lobes is the existence of invisible internal pressure component due to energetic cosmic ray nuclei (protons). Here we discuss a possible mechanism providing these particles in the acceleration processes acting at side boundaries of relativistic jets. The process can accelerate particles to ultra high energies with possibly a very hard spectrum. Its action provides also an additional viscous jet breaking mechanism. The work is still in progress.

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arXiv2000

An attempt to identify the extended synchrotron structure associated with the micro-quasar GRS 1915+105

The energy ejected from the galaxy micro-quasar GRS1915+105 in the form of jets is expected to lead to formation of an extended double lobe/hot-spot structure with the energy content comparable to an average supernova remnant. We used the Effelsberg 100m telescope at 10.45 GHz in attempt to identify such structures. For this distant galactic plane source any definite identification was not possible due to high confusion by numerous background sources, however, a few suspect structures were pointed out.

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