arXiv subjects

Lukasz Stawarz

Publications and source records attributed to Lukasz Stawarz.

At least 37 records · Page 2Linked to original sources

arXiv2014

Discovery of Dramatic Optical Variability in SDSS J1100+4421: A Peculiar Radio-Loud Narrow-Line Seyfert 1 Galaxy?

We present our discovery of dramatic variability in SDSS J1100+4421 by the high-cadence transient survey Kiso Supernova Survey (KISS). The source brightened in the optical by at least a factor of three within about half a day. Spectroscopic observations suggest that this object is likely a narrow-line Seyfert 1 galaxy (NLS1) at z=0.840, however with unusually strong narrow emission lines. The estimated black hole mass of ~ 10^7 Msun implies bolometric nuclear luminosity close to the Eddington limit. SDSS J1100+4421 is also extremely radio-loud, with a radio loudness parameter of R ~ 4 x 10^2 - 3 x 10^3, which implies the presence of relativistic jets. Rapid and large-amplitude optical variability of the target, reminiscent of that found in a few radio- and gamma-ray loud NLS1s, is therefore produced most likely in a blazar-like core. The 1.4 GHz radio image of the source shows an extended structure with a linear size of about 100 kpc. If SDSS J1100+4421 is a genuine NLS1, as suggested here, this radio structure would then be the largest ever discovered in this type of active galaxies.

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arXiv2014

Jet contributions to the broad-band spectrum of Cyg X-1 in the hard state

We apply the jet model developed in the preceding paper of Zdziarski et al. to the hard-state emission spectra of Cyg X-1. We augment the model for the analytical treatment of the particle evolution beyond the energy dissipation region, and allow for various forms of the acceleration rate. We calculate the resulting electron and emission spectra as functions of the jet height, along with the emission spectra integrated over the outflow. The model accounts well for the observed radio, infrared, and GeV fluxes of the source, although the available data do not provide unique constraints on the model free parameters. The contribution of the jet emission in the UV--to--X-ray range turns out to be in all the cases negligible compared to the radiative output of the accretion component. Nevertheless, we find out that it is possible to account for the observed flux of Cyg X-1 at MeV energies by synchrotron jet emission, in accord with the recent claims of the detection of strong linear polarization of the source in that range. However, this is possible only assuming a very efficient particle acceleration leading to the formation of flat electron spectra, and jet magnetic fields much above the equipartition level.

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arXiv2014

Jet models for black-hole binaries in the hard spectral state

This is part one of our study of models of jets with distributed electron acceleration. We present here our assumptions, basic equations, and their solutions for the steady-state electron distribution. We assume the shape of the rate of electron acceleration and the dependencies of its normalization and the magnetic field strength on the height along the jet. Our focus is on the hard spectral state of black-hole binaries, for which we take into account that their typical radio spectra are flat. This appears to require a constant dissipation rate per unit logarithmic length and conservation of the magnetic energy flux. Our electron kinetic equation includes adiabatic and radiative losses and advection, and our photon radiative transfer equation includes synchrotron absorption and emission and Compton emission. Apart from the self-Compton process, we take into account Compton scattering of stellar and accretion photons and absorption of very-high energy gamma-rays by pair production on soft photons. We present a general solution of the kinetic equation with advection and radiative and adiabatic losses and an analytic solution in the case of dominant synchrotron losses in conical jets. In the following paper, we present detailed spectra resulting from our equations as applied to Cyg X-1.

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arXiv2014

Low-frequency High-resolution Radio Observations of the TeV-emitting Blazar SHBLJ001355.9-185406

In the framework of the unification scheme of radio-loud active galactic nuclei, BL Lac objects and quasars are the beamed end-on counterparts of low-power (FRI) and high-power (FRII) radio galaxies, respectively. Some BL Lacs have been found to possess the FRII-type large-scale radio morphology, suggesting that the parent population of BL Lacs is a mixture of low- and high-power radio galaxies. This seems to apply only to `low frequency-peaked' BL Lacs, since all the `high frequency-peaked' BL Lacs studied so far were shown to host exclusively the FRI-type radio jets. While analyzing the NVSS survey maps of the TeV detected BL Lacs, we have however discovered that the high frequency-peaked object SHBL J001355.9-185406 is associated uniquely with the one-sided, arcmin-scale, and edge-brightened jet/lobe-like feature extending to the south-west from the blazar core. In order to investigate in detail the large-scale morphology of SHBL J001355.9-185406, we have performed low-frequency and high-resolution observations of the source at 156, 259 and 629 MHz using the Giant Metrewave Radio Telescope. Our analysis indicates that no diffuse arcmin-scale emission is present around the unresolved blazar core, and that the arcmin-scale structure seen on the NVSS map breaks into three distinct features unrelated to the blazar, but instead associated with background AGN. The upper limits for the extended radio halo around the TeV-emitting BL Lac object SHBL J001355.9-185406 read as < 10% - 1% at $156-629$ MHz. The fact that the integrated radio spectrum of the unresolved blazar core is flat down to 156 MHz indicates that a self-similar character of the jet in the source holds up to relatively large distances from the jet base.

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arXiv2013

Jet Emission in Young Radio Sources: a Fermi-LAT Gamma-ray View

We investigate the contribution of the beamed jet component to the high energy emission in young and compact extragalactic radio sources, focusing for the first time on the gamma-ray band. We derive predictions on the gamma-ray luminosities associated with the relativistic jet assuming a leptonic radiative model. The high energy emission is produced via Compton scattering by the relativistic electrons in a spherical region at the considered scales ($\lesssim$10 kpc). Simulations show a wide range of gamma-ray luminosities, with intensities up to $\sim10^{46}-10^{48}$ erg s$^{-1}$ depending on the assumed jet parameters. We find a highly linear relation between the simulated X-ray and gamma-ray luminosities that can be used to select candidates for a gamma-ray detection. We compare the simulated luminosity distributions in the radio, X-ray and gamma-ray regimes with observations for the largest sample of X-ray detected young radio quasars. Our analysis of $\sim$4-year Fermi Large Area Telescope (LAT) data does not give any statistically significant detection. However, the majority of the model-predicted gamma-ray fluxes for the sample are near or below the current Fermi-LAT flux threshold and compatible with the derived upper limits. Our study gives constraints on the minimum jet power ($L_{jet,kin}/L_{disk}>0.01$), on a potential jet contribution to the X-ray emission in the most compact sources ($\lesssim1$ kpc) and on the particles to magnetic field energy density ratio in broad agreement with equipartition assumption.

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arXiv2013

Discovery of GeV Emission from the Circinus galaxy with the Fermi-LAT

We report the discovery of gamma-ray emission from the Circinus galaxy using the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. Circinus is a nearby (~4 Mpc) starburst with a heavily obscured Seyfert-type active nucleus, bipolar radio lobes perpendicular to the spiral disk, and kpc-scale jet-like structures. Our analysis of 0.1-100 GeV events collected during 4 years of LAT observations reveals a significant (~ 7.3 sigma) excess above the background. We find no indications of variability or spatial extension beyond the LAT point-spread function. A power-law model used to describe the 0.1-100 GeV gamma-ray spectrum yields a flux of (18.8+/-5.8)x10^{-9} ph cm^{-2} s^{-1} and photon index 2.19+/-0.12, corresponding to an isotropic gamma-ray luminosity of 3 x 10^{40} erg s^{-1}. This observed gamma-ray luminosity exceeds the luminosity expected from cosmic-ray interactions in the interstellar medium and inverse Compton radiation from the radio lobes. Thus the origin of the GeV excess requires further investigation.

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arXiv2013

Polarimetry and the High-Energy Emission Mechanisms in Quasar Jets. The Case of PKS 1136-135

Since the discovery of kiloparsec-scale X-ray emission from quasar jets, the physical processes responsible for their high-energy emission have been poorly defined. A number of mechanisms are under active debate, including synchrotron radiation, inverse-Comptonized CMB (IC/CMB) emission, and other Comptonization processes. In a number of cases, the optical and X-ray emission of jet regions are inked by a single spectral component, and in those, high- resolution multi-band imaging and polarimetry can be combined to yield a powerful diagnostic of jet emission processes. Here we report on deep imaging photometry of the jet of PKS 1136$-$135 obtained with the {\it Hubble Space Telescope.} We find that several knots are highly polarized in the optical, with fractional polarization $Π>30%$. When combined with the broadband spectral shape observed in these regions, this is very difficult to explain via IC/CMB models, unless the scattering particles are at the lowest-energy tip of the electron energy distribution, with Lorentz factor $γ\sim 1$, and the jet is also very highly beamed ($δ\geq 20$) and viewed within a few degrees of the line of sight. We discuss both the IC/CMB and synchrotron interpretation of the X-ray emission in the light of this new evidence, presenting new models of the spectral energy distribution and also the matter content of this jet. The high polarizations do not completely rule out the possibility of IC/CMB optical-to-X-ray emission in this jet, but they do strongly disfavor the model. We discuss the implications of this finding, and also the prospects for future work.

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arXiv2012

Multiwavelength Astronomy and CTA: X-rays

We discuss how future X-ray instruments which are under development can contribute to our understanding of the non-thermal Universe. Much progress has been made in the field of X-ray Astronomy recently, thanks to the operation of modern X-ray telescopes such as Chandra, XMM-Newton, Suzaku, and Swift, but more in-depth investigation awaits future missions. These future missions include ASTROSAT, NuStar, e-ROSITA, ASTRO-H and GEMS, which will be realized in the next decade, and also much larger projects such as Athena and LOFT, which have been proposed for the 2020's. All of those are expected to bring a variety of novel observational results regarding astrophysical sources of high-energy particles and radiation, i.e. supernova remnants, neutron stars, stellar-mass black holes, active galaxies, and clusters of galaxies among others. The operation of the future X-ray instruments will proceed in parallel with the operation of Fermi-LAT and the Cherenkov Telescope Array. We emphasize that the synergy between the X-ray and gamma-ray observations is particularly important, and that the planned X-ray missions, when in conjunction with the modern gamma-ray observatories, will indeed provide a qualitatively better insight into the high-energy Universe.

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arXiv2012

Probing the origin of VHE emission from M 87 with MWL observations in 2010

The large majority of extragalactic very high energy (VHE; E>100 GeV) sources belongs to the class of active galactic nuclei (AGN), in particular the BL Lac sub-class. AGNs are characterized by an extremely bright and compact emission region, powered by a super-massive black hole (SMBH) and an accretion disk, and relativistic outflows (jets) detected all across the electro-magnetic spectrum. In BL Lac sources the jet axis is oriented close to the line of sight, giving rise to a relativistic boosting of the emission. In radio galaxies, on the other hand, the jet makes a larger angle to the line of sight allowing to resolve the central core and the jet in great details. The giant radio galaxy M 87 with its proximity (1 6Mpc) and its very massive black hole ((3-6) x 10^9 M_solar) provides a unique laboratory to investigate VHE emission in such objects and thereby probe particle acceleration to relativistic energies near SMBH and in jets. M 87 has been established as a VHE emitter since 2005. The VHE emission displays strong variability on time-scales as short as a day. It has been subject of a large joint VHE and multi-wavelength (MWL) monitoring campaign in 2008, where a rise in the 43 GHz VLBA radio emission of the innermost region (core) was found to coincide with a flaring activity at VHE. This had been interpreted as a strong indication that the VHE emission is produced in the direct vicinity of the SMBH black hole. In 2010 again a flare at VHE was detected triggering further MWL observations with the VLBA, Chandra, and other instruments. At the same time M 87 was also observed with the Fermi-LAT telescope at GeV energies and the European VLBI Network (EVN). In this contribution preliminary results from the campaign will be presented.

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arXiv2011

Optical Polarization and Spectral Variability in the M87 Jet

During the last decade, M87's jet has been the site of an extraordinary variability event, with one knot (HST-1) increasing by over a factor 100 in brightness. Variability was also seen on timescales of months in the nuclear flux. Here we discuss the optical-UV polarization and spectral variability of these components, which show vastly different behavior. HST-1 shows a highly significant correlation between flux and polarization, with P increasing from $\sim 20%$ at minimum to >40% at maximum, while the orientation of its electric vector stayed constant. HST-1's optical-UV spectrum is very hard ($α_{UV-O}\sim0.5$, $F_ν\proptoν^{-α}$), and displays "hard lags" during epochs 2004.9-2005.5, including the peak of the flare, with soft lags at later epochs. We interpret the behavior of HST-1 as enhanced particle acceleration in a shock, with cooling from both particle aging and the relaxation of the compression. We set 2$σ$ upper limits of $0.5 δ$ parsecs and 1.02$c$ on the size and advance speed of the flaring region. The slight deviation of the electric vector orientation from the jet PA, makes it likely that on smaller scales the flaring region has either a double or twisted structure. By contrast, the nucleus displays much more rapid variability, with a highly variable electric vector orientation and 'looping' in the $(I,P)$ plane. The nucleus has a much steeper spectrum ($α_{UV-O} \sim 1.5$) but does not show UV-optical spectral variability. Its behavior can be interpreted as either a helical distortion to a steady jet or a shock propagating through a helical jet.

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arXiv2011

Unraveling the Nature of Unidentified High Galactic Latitude Fermi/LAT Gamma-ray Sources with Suzaku

We report on the results of deep X-ray follow-up observations of four unidentified Fermi/LAT gamma-ray sources at high Galactic latitudes using Suzaku. The studied objects were detected with high significance during the first 3 months of Fermi/LAT operation, and subsequently better localized in the Fermi/LAT 1 year catalog (1FGL). Possible associations with pulsars and active galaxies have subsequently been discussed, and our observations provide an important contribution to this debate. In particular, an X-ray point source was found within the 95% confidence error circle of 1FGL J1231.1-1410. X-ray spectrum is well-fitted by a blackbody with an additional power-law. This supports the recently claimed identification of this source with a millisecond pulsar (MSP) PSR J1231-1411. Concerning 1FGL J1311.7-3429, two X-ray sources were found within the LAT error circle. Even though the X-ray spectral and variability properties were accessed, their nature and relationship with the gamma-ray source remain uncertain. We found several weak X-ray sources in the field of 1FGL J1333.2+5056, one coinciding with CLASS J1333+5057. We argue the available data are consistent with the association between these two objects. Finally, we have detected an X-ray source in the vicinity of 1FGL J2017.3+0603. This object was recently suggested to be associated with a newly discovered MSP PSR J2017+0603, because of the spatial-coincidence and the gamma-ray pulse detection. We have only detected the X-ray counterpart of the CLASS J2017+0603, while we determined an X-ray flux upper limit at the pulsar position. All in all, our studies indicate while a significant fraction of unidentified high Galactic latitude gamma-ray sources is related to the pulsar and blazar phenomena, associations with other classes of astrophysical objects are still valid options.

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arXiv2010

Intermittent Activity of Jets in AGN

Large scale X-ray jets that extend to >100 kpc distances from the host galaxy indicate the importance of jets interactions with the environment on many different physical scales. Morphology of X-ray clusters indicate that the radio-jet activity of a cD galaxy is intermittent. This intermittency might be a result of a feedback and/or interactions between galaxies within the cluster. Here we consider the radiation pressure instability operating on short timescales (<10^5 years) as the origin of the intermittent behaviour. We test whether this instability can be responsible for short ages (< 10^4 years) of Compact Symmetric Objects measured by hot spots propagation velocities in VLBI observations. We model the accretion disk evolution and constrain model parameters that may explain the observed compact radio structures and over-abundance of GPS sources. We also describe effects of consequent outbursts.

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arXiv2009

On the Energy Spectra of GeV/TeV Cosmic Ray Leptons

Recent observations of cosmic ray electrons from several instruments have revealed various degrees of deviation in the measured electron energy distribution from a simple power-law, in a form of an excess around TeV energies. An even more prominent deviation has been observed in the fraction of cosmic ray positrons around 100 GeV energies. In this paper we show that the observed excesses in the electron spectrum may be easily re-produced without invoking any unusual sources other than the general diffuse Galactic components of cosmic rays. The primary physical effect involved is the Klein-Nishina suppression of the electron cooling rate around TeV energies. With a very reasonable choice of the model parameters characterizing the local interstellar medium, we can reproduce the most recent observations by Fermi and HESS experiments. We also find that high positron fraction increasing with energy, as claimed by the PAMELA experiment, cannot be explained in our model with the conservative set of the model parameters. We are able, however, to reproduce the PAMELA results assuming high values of the starlight and interstellar gas densities, which would be more appropriate for vicinities of supernova remnants. A possible solution to this problem may be that cosmic rays undergo most of their interactions near their sources due to the efficient trapping in the far upstream of supernova shocks by self-generated, cosmic ray-driven turbulence.

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arXiv2009

Constraining Emission Models of Luminous Blazar Sources

Many luminous blazars which are associated with quasar-type active galactic nuclei display broad-band spectra characterized by a large luminosity ratio of their high-energy (gamma-ray) and low-energy (synchrotron) spectral components. This large ratio, reaching values up to 100, challenges the standard synchrotron self-Compton models by means of substantial departures from the minimum power condition. Luminous blazars have also typically very hard X-ray spectra, and those in turn seem to challenge hadronic scenarios for the high energy blazar emission. As shown in this paper, no such problems are faced by the models which involve Comptonization of radiation provided by a broad line-region, or dusty molecular torus. The lack or weakness of bulk Compton and Klein-Nishina features indicated by the presently available data favors production of gamma-rays via up-scattering of infrared photons from hot dust. This implies that the blazar emission zone is located at parsec-scale distances from the nucleus, and as such is possibly associated with the extended, quasi-stationary reconfinement shocks formed in relativistic outflows. This scenario predicts characteristic timescales for flux changes in luminous blazars to be days/weeks, consistent with the variability patterns observed in such systems at infrared, optical and gamma-ray frequencies. We also propose that the parsec-scale blazar activity can be occasionally accompanied by dissipative events taking place at sub-parsec distances and powered by internal shocks and/or reconnection of magnetic fields. These could account for the multiwavelength intra-day flares occasionally observed in powerful blazars sources.

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arXiv2009

Variability Timescales in the M87 Jet: Signatures of E-Squared Losses, Discovery of a Quasi-period in HST-1, and the Site of TeV Flaring

We investigate the variability timescales in the jet of M87 with two goals. The first is to use the rise times and decay times in the radio, ultraviolet and X-ray lightcurves of HST-1 to constrain the source size and the energy loss mechanisms affecting the relativistic electron distributions. HST-1 is the first jet knot clearly resolved from the nuclear emission by Chandra and is the site of the huge flare of 2005. We find clear evidence for a frequency-dependent decrease in the synchrotron flux being consistent with E-squared energy losses. Assuming that this behavior is predominantly caused by synchrotron cooling, we estimate a value of 0.6 mG for the average magnetic field strength of the HST-1 emission region, a value consistent with previous estimates of the equipartition field. In the process of analyzing the first derivative of the X-ray light curve of HST-1, we discovered a quasi-periodic oscillation which was most obvious in 2003 and 2004 prior to the major flare in 2005. The four cycles observed have a period of order 6 months. The second goal is to search for evidence of differences between the X-ray variability timescales of HST-1 and the unresolved nuclear region (diameter <0.6"). These features, separated by more than 60 pc, are the two chief contenders for the origin of the TeV variable emissions observed by HESS in 2005 and by MAGIC and VERITAS in 2008. The X-ray variability of the nucleus appears to be at least twice as rapid as that of the HST-1 knot. However, the shortest nuclear variability timescale we can measure from the Chandra data (<= 20 days) is still significantly longer than the shortest TeV variability of M87 reported by the HESS and MAGIC telescopes (1-2 days).

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arXiv2009

Accretion Disk Model of Short-Timescale Intermittent Activity in Young Radio Sources

We associate the existence of short-lived compact radio sources with the intermittent activity of the central engine caused by a radiation pressure instability within an accretion disk. Such objects may constitute a numerous sub-class of Giga-Hertz Peaked Spectrum sources, in accordance with the population studies of radio-loud active galaxies, as well as detailed investigations of their radio morphologies. We perform the model computations assuming the viscosity parametrization as proportional to a geometrical mean of the total and gas pressure. The implied timescales are consistent with the observed ages of the sources. The duration of an active phase for a moderate accretion rate is short enough (< 10^3-10^4 years) that the ejecta are confined within the host galaxy and thus these sources cannot evolve into large size radio galaxies unless they are close to the Eddington limit.

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arXiv2008

On the Possible Association of Ultra High Energy Cosmic Rays with Nearby Active Galaxies

(Abridged) Data collected by the Pierre Auger Observatory (Auger) provide evidence for anisotropy in the arrival directions of cosmic rays (CRs) with energies >57 EeV that suggests a correlation with the positions of AGN located within ~75 Mpc. A detailed study of the sample of AGN whose positions are located within 3.2 degrees of the CR events (and extending our analysis out to ~150 Mpc) shows that most of them are classified as Seyfert 2 and low-ionization nuclear emission-line region (LINER) galaxies whose properties do not differ substantially from other local AGN of the same types. Therefore, if the production of the highest energy CRs is persistent in nature, i.e., operates in a single object on long (>Myr) timescales, the claimed correlation between the CR events observed by Auger and local active galaxies should be considered as resulting from a chance coincidence. Additionally, most of the selected sources do not show significant jet activity, and hence, in most conservative scenarios, there are no reasons for expecting them to accelerate CRs up to the highest energies, ~10^20 eV. A future analysis has to take into account AGN morphology and may yield a correlation with a larger deflection angle and/or more distant sources. We further argue that the nearby radio galaxy NGC 5128 (Cen A) alone could be associated with at least 4 events due to its large radio extent, and PKS 1343-60 (Cen B), another nearby radio galaxy, can be associated with more than 1 event due to its proximity to the Galactic plane and, correspondingly, the stronger Galactic magnetic field the UHECRs encounter during propagation to the Earth. Future gamma-ray observations (by, e.g., Fermi Gamma-ray Space Telescope, and HESS) may provide additional clues to the nature of the accelerators of the UHECRs in the local Universe.

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arXiv2007

The Case of the 300 kpc Long X-ray Jet in PKS 1127-145 at z=1.18

The complex X-ray morphology of the 300 kpc long X-ray jet in PKS1127-145 (z=1.18 quasar) is clearly discerned in a ~100 ksec Chandra observation. The jet X-ray surface brightness gradually decreases by an order of magnitude going out from the core. The X-ray spectrum of the inner jet is relatively flat with alpha_X=0.66+/-0.15 and steep in the outer jet with alpha_X=1.0+/-0.2. The X-ray and radio jet intensity profiles are strikingly different, with the radio emission peaking strongly at the two outer knots while the X-ray emission is strongest in the inner jet region. We discuss the constraints implied by these data on the X-ray emission models and conclude that ``one-zone'' models fail and that at least a two-component model is needed to explain the jet's broadband emission. We propose that the X-ray emission originates in the jet proper while the bulk of the radio emission comes from a surrounding jet sheath. We also consider intermittent jet activity as a possible cause of the observed jet morphology.

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