1,686 research outputs found
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.Comment: 6 pages, 6 figures included. Proceedings of the Workshop
`Astrophysical Sources of High Energy Particles and Radiation', Torun, Jun.
200
Inverse Compton X-ray Emissions from TeV blazar Mrk421 during a Historical Low-Flux State Observed with NuSTAR
We report on the detection of excess hard X-ray emission from the TeV BL Lac
object Mrk421 during the historical low-flux state of the source in January
2013. NuSTAR observations were conducted four times between MJD56294 and
MJD56312 with a total exposure of 80.9 ksec. The source flux in the 3-40 keV
range was nearly constant except for MJD56307, when the average flux level
increased by a factor of three. Throughout the exposure, the X-ray spectra of
Mrk421 were well represented by a steep power-law model with a photon index of
3.1, although a significant excess was noted above 20 keV in the MJD56302 data
when the source was in its faintest state. Moreover, Mrk421 was detected at
more than the 4-sigma level in the 40-79 keV count maps for both MJD56307 and
MJD56302 but not during the remaining two observations. The detected excess
hard X-ray emissions connect smoothly with the extrapolation of the high-energy
gamma-ray continuum of the blazar constrained by Fermi-LAT during the source
quiescence. These findings indicate that, while the overall X-ray spectrum of
Mrk421 is dominated by the highest-energy tail of the synchrotron continuum,
the variable excess hard X-ray emission above 20 keV (on the timescale of a
week) is related to the inverse Compton emission component. We discuss the
resulting constraints on the variability and spectral properties of the
low-energy segment of the electron energy distribution in the source.Comment: 6 pages, 4 figures, accepted for publication in Ap
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.Comment: 7 pages, 4 figures included. Proceedings of the Workshop
`Extragalactic Jets: Theory and Observation from Radio to Gamma Ray',
Girdwood, May 200
Non-linear shock acceleration and high energy gamma rays from clusters of galaxies
Merger and accretion shocks in clusters of galaxies can accelerate particles
via first order Fermi process. Since this mechanism is believed to be
intrinsically efficient, shocks are expected to be modified by the backreaction
of the accelerated particles. Such a modification might induce appreciable
effects on the non--thermal emission from clusters and a suppression of the
heating of the gas at strong shocks. Here we consider in particular the gamma
ray emission and we discuss the capability of Cherenkov telescopes such as HESS
to detect clusters at TeV energies.Comment: 6 pages, to appear in the proceedings of the Gamma 2004 Symposium on
High Energy Gamma Ray Astronomy, Heidelberg, July 2004 (AIP Proceedings
Series
Blazars studied with the Suzaku X-ray telescope
Blazars, which are radio-loud active galaxies with luminous jets aligned at small angles to the line of sight, form a rather heterogeneous population of extragalactic sources of high energy radiation. Frequently monitored over the last decades with different instruments ranging from low-frequency radio interferometers up to high and very high energy γ-ray telescopes, they enable a unique insight into the innermost parts of active galactic nuclei (AGN) where relativistic jets are launched by supermassive black holes accreting at high rates. The radiative output of blazars is however dominated by a non-thermal emission of high-energy jet particles, and hence extracting the key parameters of these systems requires a good understanding of particle acceleration processes taking place in relativistic collisionless plasma. Here I discuss some selected recent observational results on the topic obtained in the X-ray domain using the Suzaku satellite, which provides detailed broad-band spectra of blazar sources in relatively short (or very short) exposures. Such good-quality spectra are necessary for a precise characterization of the jet variability and energetics. I also comment on the prospects for the X-ray polarimetry of blazar sources with the Soft Gamma-ray Detector onboard the ASTRO-H satellite to be launched in 2015
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