191 research outputs found
On the magnetization of BL Lac jets
The current paradigm foresees that relativistic jets are launched as
magnetically dominated flows, whose magnetic power is progressively converted
to kinetic power of of the matter of the jet, until equipartition is reached.
Therefore, at the end of the acceleration phase, the jet should still carry a
substantial fraction ( half) of its power in the form of a Poynting
flux. It has been also argued that, in these conditions, the best candidate
particle acceleration mechanism is efficient reconnection of magnetic field
lines, for which it is predicted that magnetic field and accelerated
relativistic electron energy densities are in equipartition.Through the
modeling of the jet non--thermal emission, we explore if equipartition is
indeed possible in BL Lac objects, i.e. low-power blazars with weak or absent
broad emission lines. We find that one-zone models (for which only one region
is involved in the production of the radiation we observe) the particle energy
density is largely dominating (by 1-2 orders of magnitude) over the magnetic
one. As a consequence, the jet kinetic power largely exceeds the magnetic
power. Instead, if the jet is structured (i.e. made by a fast spine surrounded
by a slower layer), the amplification of the IC emission due to the radiative
interplay between the two components allows us to reproduce the emission in
equipartition conditions.Comment: 9 pages, 7 figures, improved version accepted by MNRAS for
publicatio
Charge-starved, relativistic jets and blazar variability
High energy emission from blazars is thought to arise in a relativistic jet
launched by a supermassive black hole. The emission site must be far from the
hole and the jet relativistic, in order to avoid absorption of the photons. In
extreme cases, rapid variability of the emission suggests that structures of
length-scale smaller than the gravitational radius of the central black hole
are imprinted on the jet as it is launched, and modulate the radiation released
after it has been accelerated to high Lorentz factor. We propose a mechanism
which can account for the acceleration of the jet, and for the rapid
variability of the radiation, based on the propagation characteristics of
large-amplitude waves in charge-starved, polar jets. Using a two-fluid
(electron-positron) description, we find the outflows exhibit a delayed
acceleration phase, that starts at roughly 1pc, where the inertia associated
with the wave currents becomes important. The time-structure imprinted on the
jet at launch modulates photons produced by the accelerating jet provided the
pair multiplicity in the black-hole magnetosphere is sufficiently small,
suggesting that very rapid variability is confined to sources in which the
electromagnetic cascade in the black-hole magnetosphere is not prolific.Comment: 8 pages, 1 figure. Momentum equation corrected. Conclusions
unchanged. Erratum submitted to Ap
First NuSTAR Observations of Mrk 501 within a Radio to TeV Multi-Instrument Campaign
We report on simultaneous broadband observations of the TeV-emitting blazar Markarian 501 between 1 April and 10 August 2013, including the first detailed characterization of the synchrotron peak with Swift and NuSTAR. During the campaign, the nearby BL Lac object was observed in both a quiescent and an elevated state. The broadband campaign includes observations with NuSTAR, MAGIC, VERITAS, the Fermi Large Area Telescope (LAT), Swift X-ray Telescope and UV Optical Telescope, various ground-based optical instruments, including the GASP-WEBT program, as well as radio observations by OVRO, MetsÀhovi and the F-Gamma consortium. Some of the MAGIC observations were affected by a sand layer from the Saharan desert, and had to be corrected using event-by-event corrections derived with a LIDAR (LIght Detection And Ranging) facility. This is the first time that LIDAR information is used to produce a physics result with Cherenkov Telescope data taken during adverse atmospheric conditions, and hence sets a precedent for the current and future ground-based gamma-ray instruments. The NuSTAR instrument provides unprecedented sensitivity in hard X-rays, showing the source to display a spectral energy distribution between 3 and 79 keV consistent with a log-parabolic spectrum and hard X-ray variability on hour timescales. None (of the four extended NuSTAR observations) shows evidence of the onset of inverse-Compton emission at hard X-ray energies. We apply a single-zone equilibrium synchrotron self-Compton model to five simultaneous broadband spectral energy distributions. We find that the synchrotron self-Compton model can reproduce the observed broadband states through a decrease in the magnetic field strength coinciding with an increase in the luminosity and hardness of the relativistic leptons responsible for the high-energy emission
The COSPIX mission: focusing on the energetic and obscured Universe
Tracing the formation and evolution of all supermassive black holes,
including the obscured ones, understanding how black holes influence their
surroundings and how matter behaves under extreme conditions, are recognized as
key science objectives to be addressed by the next generation of instruments.
These are the main goals of the COSPIX proposal, made to ESA in December 2010
in the context of its call for selection of the M3 mission. In addition,
COSPIX, will also provide key measurements on the non thermal Universe,
particularly in relation to the question of the acceleration of particles, as
well as on many other fundamental questions as for example the energetic
particle content of clusters of galaxies. COSPIX is proposed as an observatory
operating from 0.3 to more than 100 keV. The payload features a single long
focal length focusing telescope offering an effective area close to ten times
larger than any scheduled focusing mission at 30 keV, an angular resolution
better than 20 arcseconds in hard X-rays, and polarimetric capabilities within
the same focal plane instrumentation. In this paper, we describe the science
objectives of the mission, its baseline design, and its performances, as
proposed to ESA.Comment: 7 pages, accepted for publication in Proceedings of Science, for the
25th Texas Symposium on Relativistic Astrophysics (eds. F. Rieger & C.
van Eldik), PoS(Texas 2010)25
The characterization of the distant blazar GB6 J1239+0443 from flaring and low activity periods
In 2008 AGILE and Fermi detected gamma-ray flaring activity from the
unidentified EGRET source 3EG J1236+0457, recently associated with a flat
spectrum radio quasar GB6 J1239+0443 at z=1.762. The optical counterpart of the
gamma-ray source underwent a flux enhancement of a factor 15-30 in 6 years, and
of ~10 in six months. We interpret this flare-up in terms of a transition from
an accretion-disk dominated emission to a synchrotron-jet dominated one. We
analysed a Sloan Digital Sky Survey (SDSS) archival optical spectrum taken
during a period of low radio and optical activity of the source. We estimated
the mass of the central black hole using the width of the CIV emission line. In
our work, we have also investigated SDSS archival optical photometric data and
UV GALEX observations to estimate the thermal-disk emission contribution of GB6
J1239+0443. Our analysis of the gamma-ray data taken during the flaring
episodes indicates a flat gamma-ray spectrum, with an extension of up to 15
GeV, with no statistically-relevant sign of absorption from the broad line
region, suggesting that the blazar-zone is located beyond the broad line
region. This result is confirmed by the modeling of the broad-band spectral
energy distribution (well constrained by the available multiwavelength data) of
the flaring activity periods and by the accretion disk luminosity and black
hole mass estimated by us using archival data.Comment: 30 pages, 7 figures, 4 tables MNRAS Accepted on 2012 June 1
The structure and emission model of the relativistic jet in the quasar 3C 279 inferred from radio to high-energy gamma-ray observations in 2008-2010
We present time-resolved broad-band observations of the quasar 3C 279
obtained from multi-wavelength campaigns conducted during the first two years
of the Fermi Gamma-ray Space Telescope mission. While investigating the
previously reported gamma-ray/optical flare accompanied by a change in optical
polarization, we found that the optical emission appears delayed with respect
to the gamma-ray emission by about 10 days. X-ray observations reveal a pair of
`isolated' flares separated by ~90 days, with only weak gamma-ray/optical
counterparts. The spectral structure measured by Spitzer reveals a synchrotron
component peaking in the mid-infrared band with a sharp break at the
far-infrared band during the gamma-ray flare, while the peak appears in the
mm/sub-mm band in the low state. Selected spectral energy distributions are
fitted with leptonic models including Comptonization of external radiation
produced in a dusty torus or the broad-line region. Adopting the interpretation
of the polarization swing involving propagation of the emitting region along a
curved trajectory, we can explain the evolution of the broad-band spectra
during the gamma-ray flaring event by a shift of its location from ~ 1 pc to ~
4 pc from the central black hole. On the other hand, if the gamma-ray flare is
generated instead at sub-pc distance from the central black hole, the
far-infrared break can be explained by synchrotron self-absorption. We also
model the low spectral state, dominated by the mm/sub-mm peaking synchrotron
component, and suggest that the corresponding inverse-Compton component
explains the steady X-ray emission.Comment: 23 pages, 18 figures 5 tables, Accepted for publication in The
Astrophysical Journa
Magnetoluminescence
Pulsar Wind Nebulae, Blazars, Gamma Ray Bursts and Magnetars all contain
regions where the electromagnetic energy density greatly exceeds the plasma
energy density. These sources exhibit dramatic flaring activity where the
electromagnetic energy distributed over large volumes, appears to be converted
efficiently into high energy particles and gamma-rays. We call this general
process magnetoluminescence. Global requirements on the underlying, extreme
particle acceleration processes are described and the likely importance of
relativistic beaming in enhancing the observed radiation from a flare is
emphasized. Recent research on fluid descriptions of unstable electromagnetic
configurations are summarized and progress on the associated kinetic
simulations that are needed to account for the acceleration and radiation is
discussed. Future observational, simulation and experimental opportunities are
briefly summarized.Comment: To appear in "Jets and Winds in Pulsar Wind Nebulae, Gamma-ray Bursts
and Blazars: Physics of Extreme Energy Release" of the Space Science Reviews
serie
Fermi Gamma-ray Space Telescope Observations of Recent Gamma-ray Outbursts from 3C 454.3
The flat spectrum radio quasar 3C~454.3 underwent an extraordinary outburst
in December 2009 when it became the brightest gamma-ray source in the sky for
over one week. Its daily flux measured with the Fermi Large Area Telescope at
photon energies E>100 MeV reached F = 22+/-1 x 10^-6 ph cm^-2 s^-1,
representing the highest daily flux of any blazar ever recorded in high-energy
gamma-rays. It again became the brightest source in the sky in 2010 April,
triggering a pointed-mode observation by Fermi. The correlated gamma-ray
temporal and spectral properties during these exceptional events are presented
and discussed. The main results show flux variability over time scales less
than 3 h and very mild spectral variability with an indication of gradual
hardening preceding major flares. No consistent loop pattern emerged in the
gamma-ray spectral index vs flux plane. A minimum Doppler factor of ~ 15 is
derived, and the maximum energy of a photon from 3C 454.3 is ~ 20 GeV. The
spectral break at a few GeV is inconsistent with Klein-Nishina softening from
power-law electrons scattering Ly_alpha line radiation, and a break in the
underlying electron spectrum in blazar leptonic models is implied.Comment: submitted to the Astrophysical Journa
A change in the optical polarization associated with a gamma-ray flare in the blazar 3C 279
It is widely accepted that strong and variable radiation detected over all
accessible energy bands in a number of active galaxies arises from a
relativistic, Doppler-boosted jet pointing close to our line of sight. The size
of the emitting zone and the location of this region relative to the central
supermassive black hole are, however, poorly known, with estimates ranging from
light-hours to a light-year or more. Here we report the coincidence of a
gamma-ray flare with a dramatic change of optical polarization angle. This
provides evidence for co-spatiality of optical and gamma-ray emission regions
and indicates a highly ordered jet magnetic field. The results also require a
non-axisymmetric structure of the emission zone, implying a curved trajectory
for the emitting material within the jet, with the dissipation region located
at a considerable distance from the black hole, at about 10^5 gravitational
radii.Comment: Published in Nature issued on 18 February 2010. Corresponding
authors: Masaaki Hayashida and Greg Madejsk
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