arXiv subjects

M. Sikora

Publications and source records attributed to M. Sikora.

At least 37 records · Page 2Linked to original sources

arXiv2007

Radio Loudness of AGNs: Host Galaxy Morphology and the Spin Paradigm

We investigate how the total radio luminosity of AGN-powered radio sources depends on their accretion luminosity and the central black hole mass. We find that AGNs form two distinct and well separated sequences on the radio-loudness - Eddington-ratio plane. We argue that these sequences mark the real upper bounds of radio-loudness of two distinct populations of AGNs: those hosted respectively by elliptical and disk galaxies. Both sequences show the same dependence of the radio-loudness on the Eddington ratio (an increase with decreasing Eddington ratio), which suggests that another parameter in addition to the accretion rate must play a role in determining the jet production efficiency in active galactic nuclei, and that this parameter is related to properties of the host galaxy. The revealed host-related radio dichotomy breaks down at high accretion rates where the dominant fraction of luminous quasars hosted by elliptical galaxies is radio quiet. We argue that the huge difference between the radio-loudness reachable by AGNs in disc and elliptical galaxies can be explained by the scenario according to which the spin of a black hole determines the outflow's power, and central black holes can reach large spins only in early type galaxies (following major mergers), and not (in a statistical sense) in spiral galaxies.

astro-ph↗

arXiv2006

Radio-Loudness of Active Galactic Nuclei: Observational Facts and Theoretical Implications

We investigate how the total radio luminosity of AGN-powered radio sources depends on their accretion luminosity and the central black hole mass. Our studies cover about seven orders of magnitude in accretion luminosity and the full range of AGN black hole masses. We find that AGNs form two distinct and well separated sequences on the radio-loudness -- Eddington-ratio plane. The `upper' sequence is formed by radio selected AGNs, the `lower' sequence contains mainly optically selected objects. Whereas an apparent `gap' between the two sequences may be an artifact of selection effects, the sequences themselves mark the real upper bounds of radio-loudness of two distinct populations of AGNs: those hosted respectively by elliptical and disk galaxies. Both sequences show the same dependence of the radio-loudness on the Eddington ratio (an increase with decreasing Eddington ratio), which suggests that the normalization of this dependence is determined by the black hole spin. This implies that central black holes in giant elliptical galaxies have (on average) much larger spins than black holes in spiral/disc galaxies. This galaxy-morphology related radio-dichotomy breaks down at high accretion rates where the dominant fraction of luminous quasars hosted by elliptical galaxies is radio quiet. This led to speculations in the literature that formation of powerful jets at high accretion rates is intermittent and related to switches between two disk accretion modes, as directly observed in some BH X-ray binaries. We argue that such intermittency can be reconciled with the spin paradigm, provided that successful formation of relativistic jets by rotating black holes requires collimation by MHD outflows from accretion disks.

astro-ph↗

arXiv2006

On magnetic fields in broad-line blazars

High energy spectra of broad-line blazars can be reproduced by both synchrotron-self-Compton (SSC) models and external-Compton (EC) models. However, as is known from numerical modeling, SSC scenarios require much weaker magnetic field than EC ones. In this paper we quantify these results analytically.

astro-ph↗

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.

astro-ph↗

arXiv2005

Direct evidence of the orbital contribution to the magnetic moment in AA'FeReO6 double perovskites

Spin and orbital magnetic moments of Re in AA'FeReO6 double perovskites (A,A' = Ba, Sr, Ca) have been directly probed employing XMCD spectroscopy at the Re L2,3-edges. A considerable orbital magnetic moment is observed in all the studied compounds despite octahedral coordination. Relative orbital to spin contribution per Re atom rises with lattice distortion from mL/mS = -0.28 to -0.34 for AA'=Ba2FeReO6 and Ca2FeReO6, respectively. A preliminary XMCD measurements at the Fe L2,3-edges reveals also a significant orbital moment of iron in Ca2FeReO6. The relation of the results to the magnetic properties of the compounds is discussed.

cond-mat.str-el↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

arXiv2004

Evidence for Strange Quark Contributions to the Nucleon's Form Factors at $Q^2$ = 0.108 (GeV/c)$^2$

We report on a measurement of the parity violating asymmetry in the elastic scattering of polarized electrons off unpolarized protons with the A4 apparatus at MAMI in Mainz at a four momentum transfer value of $Q^2$ = \Qsquare (GeV/c)$^2$ and at a forward electron scattering angle of 30$^\circ < θ_e < 40^\circ$. The measured asymmetry is $A_{LR}(\vec{e}p)$ = (\Aphys $\pm$ \Deltastat$_{stat}$ $\pm$ \Deltasyst$_{syst}$) $\times$ 10$^{-6}$. The expectation from the Standard Model assuming no strangeness contribution to the vector current is A$_0$ = (\Azero $\pm$ \DeltaAzero) $\times$ 10$^{-6}$. We have improved the statistical accuracy by a factor of 3 as compared to our previous measurements at a higher $Q^2$. We have extracted the strangeness contribution to the electromagnetic form factors from our data to be $G_E^s$ + \FakGMs $G_M^s$ = \GEsGMs $\pm $ \DeltaGEsGMs at $Q^2$ = \Qsquare (GeV/c)$^2$. As in our previous measurement at higher momentum transfer for $G_E^s$ + 0.230 $G_M^s$, we again find the value for $G_E^s$ + \FakGMs $G_M^s$ to be positive, this time at an improved significance level of 2 $σ$.

nucl-ex↗

arXiv2004

Soft X-ray precursors of the non-thermal flares in blazars - theoretical predictions

Popular internal shock models, developed to explain production of high energy flares in blazar jets, involve collisions between local overdensities of matter being ejected by a central engine and moving along the jet with different velocities. Prior to such collisions, the matter is relatively cold and therefore does not produce intrinsic non-thermal radiation. However, due to Comptonization of external radiation by cold electrons, the presence of such matter should be apparent by prominent precursor soft X-ray flares, visible prior to non-thermal $γ$-ray flares. In this paper we discuss the predicted properties of such precursors and study the dependence of their properties (luminosities and light curves) on kinematic parameters of relativistic ejecta and on an angle of view. We demonstrate that the lack of evidence for luminous soft X-ray precursors can be reconciled with our predictions for their properties if acceleration and collimation of a jet takes about three distance decades. We briefly discuss the severe constraints on the internal shock models that would be imposed by a non-detection of such precursors.

astro-ph↗

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.

astro-ph↗

arXiv2003

Gamma-ray bursts from alternating-current jets

We propose a gamma-ray burst scenario involving relativistic jets dominated by Poynting flux with alternating toroidal magnetic fields. Such a structure may arise naturally if the jet is formed and powered by the accretion flow in the core of a collapsar. We conjecture that the polarity of the toroidal magnetic field changes randomly due to hydromagnetic turbulence driven by the magneto-rotational instability (MRI), with the typical reversal time determined by the time scale for amplifying magnetic fields up to a dynamically important level. Poynting flux-dominated jets with reversing B-fields provide a natural and efficient way to dissipate energy via the reconnection process. Gamma-rays are produced at the spatially separated reconnection sites. In this scenario, the emergent synchrotron radiation can be highly polarized and can form both smooth and spiky light curves. We note the possibility that cold and dense filaments can form in the reconnection zones as the result of thermal instability. One could then explain the production of very hard X-ray spectra, as due to bound-free absorption of the synchrotron radiation.

astro-ph↗

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.

astro-ph↗

arXiv2003

BeppoSAX and multiwavelength observations of BL Lacertae in 2000

We present two BeppoSAX observations of BL Lac (2200+420) as part of a multiwavelength campaign performed in 2000. The source was in different states of activity: in June, the X-ray spectrum was faint and hard (alpha sim 0.8), with positive residuals towards low energies. In October we detected the highest [2-10] keV flux ever measured for the source. During this observation, the spectrum was soft (alpha sim 1.56) up to 10 keV, while above this energy a hard component was dominating. The BeppoSAX data are confirmed by simultaneous RXTE short observations. During the first observation the soft X-ray flux was variable on timescales of a few hours, while the hard X-ray flux was almost constant. During the second observation, instead, the soft spectrum displayed an erratic behaviour with large variations (up to factors 3-4) on timescales smaller than 2 hrs. The analysis of the multiwavelength SED of October evidenced an intriguing feature: the optical and X-ray sections of the SED are misaligned, while in the prevailing standard picture, they are both thought to be produced via synchrotron emission. We suggested four scenarios to account for this discrepancy: a higher than galactic dust-to-gas ratio towards the source, the first detection of bulk Compton emission in the X-ray band, the presence of two synchrotron emitting regions located at different distances from the nucleus, the detection of a Klein-Nishina effect on the synchrotron spectrum. We evidenced the favorable and critical points of each scenario, but, at present, we cannot discriminate between them.

astro-ph↗

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.

astro-ph↗

arXiv2003

Numerical simulations of radiation from blazar jets

We present a description of our numerical code BLAZAR. This code calculates spectra and light curves of blazars during outbursts. The code is based on a model in which the non-thermal flares in blazars are produced in thin shells propagating down a conical jet with relativistic velocities. Such shells may represent layers of a shocked plasma, enclosed between the forward and reverse fronts of an internal shock. In the model adopted by us, the production of non-thermal radiation is assumed to be dominated by electrons and positrons which are accelerated directly, rather then injected by pair cascades. The code includes synchrotron emission and inverse-Compton process as the radiation mechanisms. Both synchrotron photons and external photons are included as the seed photons for Comptonization. At the present stage, the code is limited to treat the inverse Compton process only within the Thomson limit and is specialized to model radiation production in the flat spectrum radio quasars. As an example, we present the results of modeling an outburst in 3C 279 - the most extensively monitored gamma-ray - bright quasar.

astro-ph↗

arXiv2002

Unifying MeV-blazars with GeV-blazars

We demonstrate that the spectral differences between Flat Spectrum Radio Quasars (FSRQ) with steep gamma-ray spectra (MeV-blazars) and FSRQ with flat gamma-ray spectra (GeV-blazars) can be explained by assuming that in the MeV-blazars, the production of gamma-rays is dominated by Comptonization of infrared radiation of hot dust, whereas in the GeV-blazars -- by Comptonization of broad emission lines. Additional ingredient, required to reach satisfactory unification, is an assumption that the radiating electrons are accelerated via a two step process, in the lower energy range -- following instabilities driven by shock-reflected ions, and in the higher energy range -- via resonant scatterings off Alfven waves. Our model predicts that on average, the MeV-blazars should vary on longer time scales than GeV-blazars.

astro-ph↗

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.

astro-ph↗