arXiv subjects

R. Moderski

Publications and source records attributed to R. Moderski.

At least 73 records · Page 4Linked to original sources

arXiv2010

Discovery and follow-up studies of the extended, off-plane, VHE gamma-ray source HESS J1507-622

Context: The detection of gamma-rays in the very-high-energy (VHE) range (100 GeV-100 TeV) offers the possibility of studying the parent population of ultrarelativistic particles found in astrophysical sources, so it is useful for understanding the underlying astrophysical processes in nonthermal sources. Aim: The discovery of the VHE gamma-ray source HESS J1507-622 is reported and possibilities regarding its nature are investigated. Methods: The H.E.S.S. array of imaging atmospheric Cherenkov telescopes (IACTs) has a high sensitivity compared with previous instruments (~1% of the Crab flux in 25 hours observation time for a 5 sigma point-source detection) and has a large field of view (~5 deg in diameter). HESS J1507-622 was discovered within the ongoing H.E.S.S. survey of the inner Galaxy, and the source was also studied by means of dedicated multiwavelength observations. Results: A Galactic gamma-ray source, HESS J1507-622, located ~3.5 deg from the Galactic plane was detected with a statistical significance > 9 sigma. Its energy spectrum is well fitted by a power law with spectral index Γ= 2.24 +/- 0.16_{stat} +/- 0.20_{sys} and a flux above 1 TeV of (1.5 +/- 0.4_{stat} +/- 0.3_{sys}) X 10^{-12} cm^{-2} s^{-1}. Possible interpretations (considering both hadronic and leptonic models) of the VHE gamma-ray emission are discussed in the absence of an obvious counterpart.

astro-ph.HE↗

arXiv2010

Discovery of VHE gamma-rays from the BL Lac object PKS 0548-322

PKS 0548-322 (z=0.069) is a ``high-frequency-peaked'' BL Lac object and a candidate very high energy (VHE, E>100 GeV) gamma-ray emitter, due to its high X-ray and radio flux. Observations at the VHE band provide insights into the origin of very energetic particles present in this source and the radiation processes at work. We report observations made between October 2004 and January 2008 with the H.E.S.S. array, a four imaging atmospheric-Cherenkov telescopes. Contemporaneous UV and X-ray observations with the Swift satellite in November 2006 are also reported. PKS 0548-322 is detected for the first time in the VHE band with H.E.S.S. We measure an excess of 216 gamma-rays corresponding to a significance of 5.6 standard deviations. The photon spectrum of the source is described by a power-law, with a photon index of Gamma=2.86 +/- 0.34 (stat) +/- 0.10 (sys). The integral flux above 200 GeV is 1.3 % of the flux of the Crab Nebula, and is consistent with being constant in time. Contemporaneous Swift/XRT observations reveal an X-ray flux between 2 and 10 keV of F_{2-10 keV}=2.3 +/- 0.2 x 10^{-11} erg.cm^{-2}. s^{-1}, an intermediate intensity state with respect to previous observations. The spectral energy distribution can be reproduced using a simple one-zone synchrotron self Compton model, with parameters similar those observed for other sources of this type.

astro-ph.HE↗

arXiv2010

X-ray Emitting GHz-Peaked Spectrum Galaxies: Testing a Dynamical-Radiative Model with Broad-Band Spectra

In a dynamical-radiative model we recently developed to describe the physics of compact, GHz-Peaked-Spectrum (GPS) sources, the relativistic jets propagate across the inner, kpc-sized region of the host galaxy, while the electron population of the expanding lobes evolves and emits synchrotron and inverse-Compton (IC) radiation. Interstellar-medium gas clouds engulfed by the expanding lobes, and photoionized by the active nucleus, are responsible for the radio spectral turnover through free-free absorption (FFA) of the synchrotron photons. The model provides a description of the evolution of the spectral energy distribution (SED) of GPS sources with their expansion, predicting significant and complex high-energy emission, from the X-ray to the gamma-ray frequency domain. Here, we test this model with the broad-band SEDs of a sample of eleven X-ray emitting GPS galaxies with Compact-Symmetric-Object (CSO) morphology, and show that: (i) the shape of the radio continuum at frequencies lower than the spectral turnover is indeed well accounted for by the FFA mechanism; (ii) the observed X-ray spectra can be interpreted as non-thermal radiation produced via IC scattering of the local radiation fields off the lobe particles, providing a viable alternative to the thermal, accretion-disk dominated scenario. We also show that the relation between the hydrogen column densities derived from the X-ray (N_H) and radio (N_HI) data of the sources is suggestive of a positive correlation, which, if confirmed by future observations, would provide further support to our scenario of high-energy emitting lobes.

astro-ph.CO↗

arXiv2008

Modelling the broad-band spectra of X-ray emitting GPS galaxies

The study of the broad-band emission of GHz-Peaked-Spectrum (GPS) radio galaxies is a powerful tool to investigate the physical processes taking place in the central, kpc-sized region of their active hosts, where the jets propagate and the lobes expand, interacting with the surrounding interstellar medium (ISM). We recently developed a new dynamical-radiative model to describe the evolution of the GPS phenomenon (Stawarz et al. 2008): as the relativistic jets propagate through the ISM, gradually engulfing narrow-line emitting gas clouds along their way, the electron population of the expanding lobes evolves, emitting synchrotron light, as well as inverse-Compton radiation via up-scattering of the photon fields from the host galaxy and its active nucleus. The model, which successfully reproduces the key features of the GPS radio sources as a class, provides a description of the evolution of their spectral energy distribution (SED) with the lobes' expansion, predicting significant and complex X-ray to gamma-ray emission. We apply here the model to the broad-band SED's of a sample of known, X-ray emitting GPS galaxies, and show that: (i) the free-free absorption mechanism enables us to reproduce the radio continuum at frequencies below the turnover; (ii) the lobes' non-thermal, inverse-Compton emission can account for the observed X-ray spectra, providing a viable alternative to the thermal, accretion-dominated scenario. We also show that, in our sample, the relationship between the X-ray and radio hydrogen column densitities, N_H and N_HI, is suggestive of a positive correlation, which, if confirmed, would support the scenario of high-energy emitting lobes.

astro-ph↗

arXiv2008

On the Evolution of and High-Energy Emission from GHz-Peaked-Spectrum Sources

Here we discuss evolution and broad-band emission of compact (< kpc) lobes in young radio sources. We propose a simple dynamical description for these objects, consisting of a relativistic jet propagating into a uniform gaseous medium in the central parts of an elliptical host. In the framework of the proposed model, we follow the evolution of ultrarelativistic electrons injected from a terminal hotspot of a jet to expanding lobes, taking into account their adiabatic energy losses as well as radiative cooling. This allows us to discuss the broad-band lobe emission of young radio sources. In particular, we argue that the observed spectral turnover in the radio synchrotron spectra of these objects cannot originate from the synchrotron self-absorption process but is most likely due to free-free absorption effects connected with neutral clouds of interstellar medium engulfed by the expanding lobes and photoionized by active centers. We also find a relatively strong and complex high-energy emission component produced by inverse-Compton up-scattering of various surrounding photon fields by the lobes' electrons. We argue that such high energy radiation is strong enough to account for several observed properties of GHz-peaked-spectrum (GPS) radio galaxies at UV and X-ray frequencies. In addition, this emission is expected to extend up to GeV (or possibly even TeV) photon energies and can thus be probed by several modern gamma-ray instruments. In particular, we suggest that GPS radio galaxies should constitute a relatively numerous class of extragalactic sources detected by GLAST.

astro-ph↗

arXiv2007

3C454.3 reveals the structure and physics of its 'blazar zone'

Recent multi-wavelength observations of 3C454.3, in particular during its giant outburst in 2005, put severe constraints on the location of the 'blazar zone', its dissipative nature, and high energy radiation mechanisms. As the optical, X-ray, and millimeter light-curves indicate, significant fraction of the jet energy must be released in the vicinity of the millimeter-photosphere, i.e. at distances where, due to the lateral expansion, the jet becomes transparent at millimeter wavelengths. We conclude that this region is located at ~10 parsecs, the distance coinciding with the location of the hot dust region. This location is consistent with the high amplitude variations observed on ~10 day time scale, provided the Lorentz factor of a jet is ~20. We argue that dissipation is driven by reconfinement shock and demonstrate that X-rays and gamma-rays are likely to be produced via inverse Compton scattering of near/mid IR photons emitted by the hot dust. We also infer that the largest gamma-to-synchrotron luminosity ratio ever recorded in this object - having taken place during its lowest luminosity states - can be simply due to weaker magnetic fields carried by a less powerful jet.

astro-ph↗

arXiv2007

Multiwavelength Observations of the Powerful Gamma-ray Quasar PKS 1510-089: Clues on the Jet Composition

We present the results from a multiwavelength campaign of the powerful Gamma-ray quasar PKS 1510-089. This campaign commenced with a deep Suzaku observation lasting three days for a total exposure time of 120 ks, and continued with Swift monitoring over 18 days. Besides Swift observations, the campaign included ground-based optical and radio data, and yielded a quasi-simultaneous broad-band spectral energy distribution from 10^9 Hz to 10^{19} Hz. The Suzaku observation provided a high S/N X-ray spectrum, which is well represented by an extremely hard power-law with photon index Gamma ~ 1.2, augmented by a soft component apparent below 1 keV, which is well described by a black-body model with temperature kT ~ 0.2 keV. Monitoring by Suzaku revealed temporal variability which is different between the low and high energy bands, again suggesting the presence of a second, variable component in addition to the primary power-law emission.We model the broadband spectrum of PKS 1510-089 assuming that the high energy spectral component results from Comptonization of infrared radiation produced by hot dust located in the surrounding molecular torus. In the adopted internal shock scenario, the derived model parameters imply that the power of the jet is dominated by protons but with a number of electrons/positrons exceeding a number of protons by a factor ~10. We also find that inhomogeneities responsible for the shock formation, prior to the collision may produce bulk-Compton radiation which can explain the observed soft X-ray excess and possible excess at ~18 keV. We note, however, that the bulk-Compton interpretation is not unique, and the observed soft excess could arise as well via some other processes discussed briefly in the text.

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

Gravitational lensing as a probe of compact object population in the Galaxy

The population of solitary compact objects in the Galaxy is very diffcult to investigate. In this paper we analyze the possibility of using microlensing searches to detect and to analyze the properties of the solitary black holes and neutron stars. Evolution of single and binary stars is considered using the StarTrack population synthesis code. We investigate the properties of the Galactic population of compact objects numerically. We find that the compact object lensing events are concentrated in a region with the radius of $\approx 5$ degrees around the Galactic center. The distribution of masses of the lenses for the models we consider differs but only slightly from the underlying massdistribution. The expected detection rates are of the order of a few per year.

astro-ph↗

arXiv2006

Thick Domain Walls in AdS Black Hole Spacetimes

Equations of motion for a real self-gravitating scalar field in the background of a black hole with negative cosmological constant were solved numerically. We obtain a sequence of static axisymmetric solutions representing thick domain wall cosmological black hole systems, depending on the mass of black hole, cosmological parameter and the parameter binding black hole mass with the width of the domain wall. For the case of extremal cosmological black hole the expulsion of scalar field from the black hole strongly depends on it.

gr-qc↗

arXiv2006

On the origin of bimodal duration distribution of Gamma Ray Bursts

The modified version of a bullet model for gamma ray bursts is studied. The central engine of the source produces multiple sub-jets that are contained within a cone. The emission of photons in the source frame of a sub-jet either takes part in an infinitesimally thin shell, or during its expansion for a finite time. The analysis of the observed profiles of GRBs taken by BATSE leads us to the conclusion that the latter possibility is much more favored. We also study the statistical distribution of GRBs, in the context of their bimodality of durations, taking into account the detector's capability of observing the signal above a certain flux limit. The model with shells emitting for a finite time is able to reproduce only one class of bursts, short or long, depending on the adopted physical parameters. Therefore we suggest that the GRB bimodality is intrinsically connected with two separate classes of sources.

astro-ph↗

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

Reissner-Nordstrom Black Holes and Thick Domain Walls

We solve numerically equations of motion for real self-interacting scalar fields in the background of Reissner-Nordstrom black hole and obtained a sequence of static axisymmetric solutions representing thick domain walls charged black hole systems. In the case of extremal Reissner-Nordstrom black hole solution we find that there is a parameter depending on the black hole mass and the width of the domain wall which constitutes the upper limit for the expulsion to occur.

gr-qc↗

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

Thick Domain Walls and Charged Dilaton Black Holes

We study a black hole domain wall system in dilaton gravity which is the low-energy limit of the superstring theory. We solve numerically equations of motion for real self-interacting scalar field and justify the existence of static axisymmetric field configuration representing the thick domain wall in the background of a charged dilaton black hole. It was also confirmed that the extreme dilaton black hole always expelled the domain wall.

hep-th↗

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

Late-time evolution of a self-interacting scalar field in the spacetime of dilaton black hole

We investigate the late-time tails of self-interacting (massive) scalar fields in the spacetime of dilaton black hole. Following the no hair theorem we examine the mechanism by which self-interacting scalar hair decay. We revealed that the intermediate asymptotic behavior of the considered field perturbations is dominated by an oscillatory inverse power-law decaying tail. The numerical simulations showed that at the very late-time massive self-interacting scalar hair decayed slower than any power law.

gr-qc↗

arXiv2001

Modeling the production of flares in $γ$-ray quasars

Theories of high energy radiation production in quasar jets can be verified by studies of both time-averaged spectra and variability patterns. While the former has been explored extensively, the latter is in its infancy. In this paper, we study the production of short-term flares in the shock-in-jet model. We examine how the flares' profiles depend on such parameters as shock/dissipation lifetime, electron-injection time profile, adiabaticity, and half-opening angle of the jet. In particular, we demonstrate the large difference between flare profiles produced in the radiative and adiabatic regimes. We apply our model to the $\sim$day timescale flares observed in optically violently variable (OVV) quasars, checking whether the external-radiation-Compton (ERC) model for $γ$-ray flares at energies $> 30$ MeV (EGRET range) can be reconciled with the flares observed at lower energies. Specifically, we show that the strict correlation between X-ray and $γ$-ray flares strongly supports the dominance of the synchrotron self-Compton mechanism in the X-ray band. We also derive conditions that must be satisfied by the ERC model in order to explain a lag of the $γ$-ray peak behind the optical one, as claimed to be observed in PKS 1406-076. Finally, we predict that in ERC models where the MeV peak is related to the break in electron distribution due to inefficient cooling of electrons below a certain energy, the flares should decay significantly more slowly in the soft $γ$-ray band than at energies greater than 30 MeV.

astro-ph↗