arXiv subjects

L. Stawarz

Publications and source records attributed to L. Stawarz.

At least 91 records · Page 5Linked to original sources

arXiv2006

A method of estimation of the dynamical age of FR II-type radio sources from multifrequency data

We propose a new approach in determining ages of FR II type radio sources. We apply the assumed dynamical model of Kaiser et al. (1997) to a number of FR II type radio galaxies observed at different radio frequencies, and fit - for each frequency separately - the model free parameters to the observed sources' quantities. Such a procedure, using enlarged in fact a number of observables, enables us to determine relatively precise ages and other crucial characteristics of the analyzed sources. The resulting age estimates agree very well with those obtained by means of `classical' spectral ageing method for objects not older than 10 Myr, for which good-quality spectral data are available. The presented method is however also applicable in the case of the sources older than this, and/or the ones for which the only available low-resolution radio data do not allow for detailed spectral ageing studies. Our analysis indicates that the main factor precluding precise age determination for FR II type radio galaxies regards the poorly known shape of the initial electron energy distribution injected by the jet terminal shocks to the expanding lobes/cocoons. We briefly consider this issue, and conclude that the broad-band single power-law form assumed here may be accurate enough for the presented estimates, although most likely it does not strictly correspond to some well-defined realistic particle acceleration process. Instead, it should be considered as a simplest model approximation of the initial electron continuum, averaged over a very broad energy range and over the age of the source, with the effective spectral index which may be different for different sources.

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arXiv2006

Absorption of Nuclear Gamma-rays on the Starlight Radiation in FR I Sources: the Case of Centaurus A

Several BL Lac objects are confirmed sources of variable and strongly Doppler-boosted TeV emission produced in the nuclear portions of their relativistic jets. It is more than probable, that also many of the FR I radio galaxies, believed to be the parent population of BL Lacs, are TeV sources, for which Doppler-hidden nuclear gamma-ray radiation may be only too weak to be directly observed. Here we show, however, that about one percent of the total time-averaged TeV radiation produced by the active nuclei of low-power FR I radio sources is inevitably absorbed and re-processed by photon-photon annihilation on the starlight photon field, and the following emission of the created and quickly isotropized electron-positron pairs. In the case of the radio galaxy Centaurus A, we found that the discussed mechanism can give a distinctive observable feature in the form of an isotropic gamma-ray halo. It results from the electron-positron pairs injected to the interstellar medium of the inner parts of the elliptical host by the absorption process, and upscattering starlight radiation via the inverse-Compton process mainly to the GeV-TeV photon energy range. Such a galactic gamma-ray halo is expected to possess a characteristic spectrum peaking at ~0.1 TeV photon energies, and the photon flux strong enough to be detected by modern Cherenkov Telescopes and, in the future, by GLAST. These findings should apply as well to the other nearby FR I sources.

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arXiv2006

Confronting X-ray Emission Models with the Highest-Redshift Kiloparsec-scale Jets: the z=3.89 Jet in Quasar 1745+624

A newly identified kiloparsec-scale X-ray jet in the high-redshift z=3.89 quasar 1745+624 is studied with multi-frequency Very Large Array, Hubble Space Telescope, and Chandra X-ray imaging data. This is only the third large-scale X-ray jet beyond z>3 known and is further distinguished as being the most luminous relativistic jet observed at any redshift, exceeding 10^45 erg/s in both the radio and X-ray bands. Apart from the jet's extreme redshift, luminosity, and high inferred equipartition magnetic field (in comparison to local analogues), its basic properties such as X-ray/radio morphology and radio polarization are similar to lower-redshift examples. Its resolved linear structure and the convex broad-band spectral energy distributions of three distinct knots are also a common feature among known powerful X-ray jets at lower-redshift. Relativistically beamed inverse Compton and `non-standard' synchrotron models have been considered to account for such excess X-ray emission in other jets; both models are applicable to this high-redshift example but with differing requirements for the underlying jet physical properties, such as velocity, energetics, and electron acceleration processes. One potentially very important distinguishing characteristic between the two models is their strongly diverging predictions for the X-ray/radio emission with increasing redshift. This is considered, though with the limited sample of three z>3 jets it is apparent that future studies targeted at very high-redshift jets are required for further elucidation of this issue. Finally, from the broad-band jet emission we estimate the jet kinetic power to be no less than 10^46 erg/s, which is about 10% of the Eddington luminosity corresponding to this galaxy's central [abridged]

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arXiv2006

Dynamics and High Energy Emission of the Flaring HST-1 Knot in the M 87 Jet

Stimulated by recent observations of a radio-to-X-ray synchrotron flare from HST-1, the innermost knot of the M 87 jet, as well as by a detection of a very high energy gamma-ray emission from M 87, we investigated the dynamics and multiwavelength emission of the HST-1 region. We study thermal pressure of the hot interstellar medium in M 87 and argue for a presence of a gaseous condensation in its central parts. Interaction of the jet with such a feature is likely to result in formation of a converging reconfinement shock in the innermost parts of the M 87 jet. We show that for a realistic set of the outflow parameters, a stationary and a flaring part of the HST-1 knot located \~100 pc away from the active center can be associated with the decelerated portion of the jet matter placed immediately downstream of the point where the reconfinement shock reaches the jet axis. We discuss a possible scenario explaining a broad-band brightening of the HST-1 region related to the variable activity of the central core. We show that assuming a previous epoch of the high central black hole activity resulting in ejection of excess particles and photons down along the jet, one may first expect a high-energy flare of HST-1 due to inverse-Comptonisation of the nuclear radiation, followed after a few years by an increase in the synchrotron continuum of this region. If this is the case, then the recently observed increase in the knot luminosity in all spectral bands could be regarded as an unusual echo of the outburst that had happened previously in the active core of the M 87 radio galaxy.

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arXiv2005

Radiative Processes In Extragalactic Large-Scale Jets

About one thousand extragalactic large-scale jets are known, and a few tens of them are confirmed sources of infrared, optical, or X-ray photons. Multiwavelength emission comming directly from these outflows is always non-thermal in origin. This fact constitutes a primary difficulty in extracting unknown parameters of large-scale jets, since the non-thermal featureless continua do not allow to infer undoubtfully (or even at all) bulk velocities and composition of the radiating plasma. In addition, arcsecond spatial resolution, limited sensitivity and narrow energy bands of the best high-frequency telescopes like Spitzer, Hubble and Chandra, preclude precise constraints on the spectral and morphological properties of the discussed objects. Nevertheless, new multiwavelength observations have substantially enriched our knowledge on extragalactic large-scale jets, in many aspects, however, by means of challenging previous predictions and expectations. In this short contribution I will concentrate on the following issue: what can be learned by analyzing broad-band emission of the discussed objects about particle acceleration processes acting thereby and about jet internal parameters.

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arXiv2005

Kiloparsec-Scale Jets in FR I Radio Galaxies and the Gamma-Ray Background

We discuss the contribution of kiloparsec-scale jets in FR I radio galaxies to the diffuse gamma-ray background radiation. The analyzed gamma-ray emission comes from inverse-Compton scattering of starlight photon fields by the ultrarelativistic electrons whose synchrotron radiation is detected from such sources at radio, optical and X-ray energies. We find that these objects, under the minimum-power hypothesis (corresponding to a magnetic field of 300 muG in the brightest knots of these jets), can contribute about one percent to the extragalactic gamma-ray background measured by EGRET. We point out that this result already indicates that the magnetic fields in kpc-scale jets of low-power radio galaxies are not likely to be smaller than 10 muG on average, as otherwise the extragalactic gamma-ray background would be overproduced.

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arXiv2005

On the Magnetic Field in the Kiloparsec-Scale Jet of Radio Galaxy M 87

Several low-power kiloparsec-scale jets in nearby radio galaxies are known for their synchrotron radiation extending up to optical and X-ray photon energies. Here we comment on high-energy gamma-ray emission of one particular object of this kind, i.e. the kiloparsec-scale jet of M 87 radio galaxy, resulting from comptonization of the starlight photon field of the host galaxy by the synchrotron-emitting jet electrons. In the analysis, we include relativistic bulk velocity of the jet, as well as the Klein-Nishina effects. We show, that upper limits to the kiloparsec-scale jet inverse-Compton radiation imposed by HESS and HEGRA Cherenkov Telescopes - which detected a variable source of VHE gamma-ray emission within 0.1 deg (~30 kpc) of the M 87 central region - give us an important constraint on the magnetic field strength in this object, namely that the magnetic field cannot be smaller than the equipartition value (referring solely to the radiating electrons) in the brightest knot of the jet, and most likely, is even stronger. In this context, we point out a need for the amplification of the magnetic energy flux along the M 87 jet from the sub-parsec to kiloparsec scales, suggesting the turbulent dynamo as a plausible process responsible for the aforementioned amplification.

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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 Quasar and FR II Large-Scale Jets

Here we report systematic comparison of the spectral properties of large-scale jets, hotspots and extended lobes in quasars and FR II radio galaxies recently observed with Chandra and ASCA. We argue that if the strong X-ray emission of the jet knots in these objects results from comptonisation of the CMB photons, as usually considered, the powerful large-scale jets are most likely far from the minimum-power condition in the sense that the magnetic field thereby is below equipartition. We also show that the X-ray emission of the hot-spots and lobes in the compiled dataset agrees with the minimum-power condition. In this context, we point out the need for substantial amplification of the magnetic field within the terminal shocks of powerful large-scale jets of quasars and FR II sources.

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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 the Jet Activity in 3C 273

In this paper we comment on the possibility for intermittent jet activity in quasar 3C 273 on different time-scales. We propose, that striking morphology of the large-scale radio jet in this source, as well as the apparent lack of its counterpart on the opposite side of the active center, may be explained in a framework of a restarting jet model. In particular, we propose that 3C 273 radio source is intrinsically two-sided, and represents an analogue of double-double radio galaxies, but only inclined at a small angle to the line of sight. In this case, the apparent one-sideness of the kiloparsec-scale radio structure may be due to combined Doppler and time-travel effects alone, if the 3C 273 large-scale jet itself is relativistic and matter-dominated. We also propose, that knotty morphology of the discussed jet, which is observed now additionally at optical and X-ray frequencies, indicates modulation in the jet kinetic power. This, together with the variability of the jet at small (parsec) scales, indicates that the jet activity in 3C 273, and possibly in other similar sources, is variable/modulated/intermittent over many different time-scales.

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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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arXiv2004

Multifrequency Radiation of Extragalactic Large-Scale Jets

Large-scale extragalactic jets, observed to extend from a few to a few hundred kiloparsecs from active galactic nuclei, are now studied over many decades in frequency of electromagnetic spectrum, from radio until (possibly) TeV gamma rays. For hundreds of known radio jets, only about 25 are observed at optical frequencies. Most of them are relatively short and faint, with only a few exceptions, like 3C 273 or M 87, allowing for detailed spectroscopic and morphological studies. Somewhat surprisingly, the large-scale jets can be very prominent in X-rays. Up to now, about 25 jets were detected within the 1 - 10 keV energy range, although the nature of this emission is still under debate. In general, both optical and X-ray jet observations present serious problems for standard radiation models for the considered objects. Recent TeV observations of M 87 suggest the possibility of generating large photon fluxes at these high energies by its extended jet. In this paper we summarize information about multiwavelength emission of the large-scale jets, and we point out several modifications of the standard jet radiation models (connected with relativistic bulk velocities, jet radial stratification and particle energization all the way along the jet), which can possibly explain some of the mentioned puzzling observations. We also comment on gamma-ray emission of the discussed objects.

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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

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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