132 research outputs found
Modelling the Kinked Jet of the Crab Nebula
We investigate the dynamical propagation of the South-East jet from the Crab
pulsar interacting with supernova ejecta by means of three-dimensional
relativistic MHD numerical simulations with the PLUTO code.
The initial jet structure is set up from the inner regions of the Crab
Nebula.
We study the evolution of hot, relativistic hollow outflows initially
carrying a purely azimuthal magnetic field.
Our jet models are characterized by different choices of the outflow
magnetization ( parameter) and the bulk Lorentz factor ().
We show that the jet is heavily affected by the growth of current-driven kink
instabilities causing considerable deflection throughout its propagation
length.
This behavior is partially stabilized by the combined action of larger flow
velocities and/or reduced magnetic field strengths.
We find that our best jet models are characterized by relatively large values
of () and small values of .
Our results are in good agreement with the recent X-ray (\textit{Chandra})
data of the Crab Nebula South-East jet indicating that the jet changes
direction of propagation on a time scale of the order of few years.
The 3D models presented here may have important implications in the
investigation of particle acceleration in relativistic outflows.Comment: 15 pages, 20 figure
The Surprising Crab Nebula
We will present our study of the flux and spectral variability of the Crab
above 100 MeV on different timescales ranging from days to weeks. In addition
to the four main intense and day-long flares detected by AGILE and Fermi-LAT
between Sept. 2007 and Sept. 2012, we find evidence for week-long and less
intense episodes of enhanced gamma-ray emission that we call "waves".
Statistically significant "waves" show timescales of 1-2 weeks, and can occur
by themselves or in association with shorter flares. The Sept. - Oct. 2007
gamma-ray enhancement episode detected by AGILE shows both "wave" and flaring
behavior. We extend our analysis to the publicly available Fermi-LAT dataset
and show that several additional "wave" episodes can be identified. We discuss
the spectral properties of the September 2007 "wave"/flare event and show that
the physical properties of the "waves" are intermediate between steady and
flaring states. Plasma instabilities inducing "waves" appear to involve spatial
distances cm and enhanced magnetic fields }mG. Day-long flares are characterized by smaller distances and larger
local magnetic fields. Typically, the deduced total energy associated with the
"wave" phenomenon (, where is the kinetic
energy of the emitting particles) is comparable with that associated to the
flares, and can reach a few percent of the total available pulsar spindown
energy. Most likely, flares and waves are the product of the same class of
plasma instabilities that we show acting on different timescales and radiation
intensities.Comment: 2012 Fermi Symposium proceedings - eConf C12102
Gamma-ray observations of Cygnus X-1 above 100 MeV in the hard and soft states
We present the results of multi-year gamma-ray observations by the AGILE
satellite of the black hole binary system Cygnus X-1. In a previous
investigation we focused on gamma-ray observations of Cygnus X-1 in the hard
state during the period mid-2007/2009. Here we present the results of the
gamma-ray monitoring of Cygnus X-1 during the period 2010/mid-2012 carried out
for which includes a remarkably prolonged `soft state' phase (June 2010 -- May
2011). Previous 1--10 MeV observations of Cyg X-1 in this state hinted at a
possible existence of a non-thermal particle component with substantial
modifications of the Comptonized emission from the inner accretion disk. Our
AGILE data, averaged over the mid-2010/mid-2011 soft state of Cygnus X-1,
provide a significant upper limit for gamma-ray emission above 100 MeV of
F_soft < 20 x 10^{-8} ph/cm^2/s, excluding the existence of prominent
non-thermal emission above 100 MeV during the soft state of Cygnus X-1. We
discuss theoretical implications of our findings in the context of high-energy
emission models of black hole accretion. We also discuss possible gamma-ray
flares detected by AGILE. In addition to a previously reported episode observed
by AGILE in October 2009 during the hard state, we report a weak but important
candidate for enhanced emission which occurred at the end of June 2010
(2010-06-30 10:00 - 2010-07-02 10:00 UT) exactly in coincidence with a
hard-to-soft state transition and before an anomalous radio flare. An appendix
summarizes all previous high-energy observations and possible detections of
Cygnus X-1 above 1 MeV.Comment: 16 pages, 12 figures, 1 table, accepted for publication in Ap
The extraordinary gamma-ray flare of the blazar 3C 454.3
We present the gamma-ray data of the extraordinary flaring activity above 100
MeV from the flat spectrum radio quasar 3C 454.3 detected by AGILE during the
month of December 2009. 3C 454.3, that has been among the most active blazars
of the FSRQ type since 2007, was detected in the gamma-ray range with a
progressively rising flux since November 10, 2009. The gamma-ray flux reached a
value comparable with that of the Vela pulsar on December 2, 2009. Remarkably,
between December 2 and 3, 2009 the source more than doubled its gamma-ray
emission and became the brightest gamma-ray source in the sky with a peak flux
of F_{\gamma,p} = (2000 \pm 400) x 10^-8 ph cm^-2 s^-1 for a 1-day integration
above 100 MeV. The gamma-ray intensity decreased in the following days with the
source flux remaining at large values near F \simeq (1000 \pm 200) x 10^-8 ph
cm^-2 s^-1 for more than a week. This exceptional gamma-ray flare dissipated
among the largest ever detected intrinsic radiated power in gamma-rays above
100 MeV (L_{\gamma, source, peak} \simeq 3 x 10^46 erg s^-1, for a relativistic
Doppler factor of {\delta} \simeq 30). The total isotropic irradiated energy of
the month-long episode in the range 100 MeV - 3 GeV is E_{\gamma,iso} \simeq
10^56 erg. We report the intensity and spectral evolution of the gamma-ray
emission across the flaring episode. We briefly discuss the important
theoretical implications of our detection.Comment: 17 pages, 3 figures, ApJ accepte
Multiwavelength Variations of 3C 454.3 during the November 2010 to January 2011 Outburst
We present multiwavelength data of the blazar 3C 454.3 obtained during an
extremely bright outburst from November 2010 through January 2011. These
include flux density measurements with the Herschel Space Observatory at five
submillimeter-wave and far-infrared bands, the Fermi Large Area Telescope at
gamma-ray energies, Swift at X-ray, ultraviolet (UV), and optical frequencies,
and the Submillimeter Array at 1.3 mm. From this dataset, we form a series of
52 spectral energy distributions (SEDs) spanning nearly two months that are
unprecedented in time coverage and breadth of frequency. Discrete correlation
anlaysis of the millimeter, far-infrared, and gamma-ray light curves show that
the variations were essentially simultaneous, indicative of co-spatiality of
the emission, at these wavebands. In contrast, differences in short-term
fluctuations at various wavelengths imply the presence of inhomegeneities in
physical conditions across the source. We locate the site of the outburst in
the parsec-scale core, whose flux density as measured on 7 mm Very Long
Baseline Array images increased by 70 percent during the first five weeks of
the outburst. Based on these considerations and guided by the SEDs, we propose
a model in which turbulent plasma crosses a conical standing shock in the
parsec-scale region of the jet. Here, the high-energy emission in the model is
produced by inverse Compton scattering of seed photons supplied by either
nonthermal radiation from a Mach disk, thermal emission from hot dust, or (for
X-rays) synchrotron radiation from plasma that crosses the standing shock. For
the two dates on which we fitted the model SED to the data, the model
corresponds very well to the observations at all bands except at X-ray
energies, where the spectrum is flatter than observed.Comment: Accepted for publication in Astrophysical Journal. 82 pages, 13
figure
Highly magnetized region in pulsar wind nebulae and origin of the Crab gamma-ray flares
The recently discovered gamma-ray flares from the Crab nebula are generally
attributed to the magnetic energy release in a highly magnetized region within
the nebula. I argue that such a region naturally arises in the polar region of
the inner nebula. In pulsar winds, efficient dissipation of the Poynting flux
into the plasma energy occur only in the equatorial belt where the energy is
predominantly transferred by alternating fields. At high latitudes, the pulsar
wind remains highly magnetized therefore the termination shock in the polar
region is weak and the postshock flow remains relativistic. I study the
structure of this flow and show that the flow at first expands and decelerates
and then it converges and accelerates. In the converging part of the flow, the
kink instability triggers the magnetic dissipation. The energy release zone
occurs at the base of the observed jet. A specific turbulence of
relativistically shrinking magnetic loops efficiently accelerates particles so
that the synchrotron emission in the hundreds MeV band, both persistent and
flaring, comes from this site.Comment: Submitted to MNRA
Gamma-ray blazars: the view from AGILE
During the first 3 years of operation the Gamma-Ray Imaging Detector onboard
the AGILE satellite detected several blazars in a high gamma-ray activity: 3C
279, 3C 454.3, PKS 1510-089, S5 0716+714, 3C 273, W Comae, Mrk 421, PKS
0537-441 and 4C +21.35. Thanks to the rapid dissemination of our alerts, we
were able to obtain multiwavelength data from other observatories such as
Spitzer, Swift, RXTE, Suzaku, INTEGRAL, MAGIC, VERITAS, and ARGO as well as
radio-to-optical coverage by means of the GASP Project of the WEBT and the REM
Telescope. This large multifrequency coverage gave us the opportunity to study
the variability correlations between the emission at different frequencies and
to obtain simultaneous spectral energy distributions of these sources from
radio to gamma-ray energy bands, investigating the different mechanisms
responsible for their emission and uncovering in some cases a more complex
behaviour with respect to the standard models. We present a review of the most
interesting AGILE results on these gamma-ray blazars and their multifrequency
data.Comment: 25 pages, 10 figures, accepted for publication on Advances in Space
Research. Talk presented at the 38th COSPAR Scientific Assembly (Bremen,
Germany; July 18-25, 2010
Blazar 3C 454.3 in Outburst and Quiescence During 2005-2007: Two Variable Synchrotron Emission Peaks
We monitored the flaring blazar 3C 454.3 during 2005 June-July with the
Spitzer Infrared Spectrograph (IRS: 15 epochs), Infrared Array Camera (IRAC: 12
epochs) and Multiband Imaging Photometer (MIPS: 2 epochs). We also made Spitzer
IRS, IRAC, and MIPS observations from 2006 December-2007 January when the
source was in a low state, the latter simultaneous with a single Chandra X-ray
observation. In addition, we present optical and sub-mm monitoring data. The
2005-2007 period saw 3 major outbursts. We present evidence that the
radio-optical SED actually consists of two variable synchrotron peaks, the
primary at IR and the secondary at sub-mm wavelengths. The lag between the
optical and sub-mm outbursts may indicate that these two peaks arise from two
distinct regions along the jet separated by a distance of 0.07-5 pc. The flux
at 5-35 microns varied by a factor of 40 and the IR peak varied in frequency
from <1E13 Hz to 4E13 Hz between the highest and lowest states in 2005 and
2006, respectively. Variability was well correlated across the mid-IR band,
with no measurable lag. Flares that doubled in flux occurred on a time scale of
3 days. The IR SED peak moved to higher frequency as a flare brightened, then
returned to lower frequency as it decayed. The fractional variability amplitude
increased with frequency, which we attribute to decreasing synchrotron-self
absorption optical depth. Mid-IR flares may signal the re-energization of a
shock that runs into inhomogeneities along the pre-existing jet or in the
external medium. The synchrotron peak frequencies during each major outburst
may depend upon both the distance from the jet apex and the physical conditions
in the shocks. Variation of the Doppler parameter along a curved or helical jet
is another possibility. Frequency variability of the IR synchrotron peak may
have important consequences for the interpretation of the blazar sequence, and
the presence of a secondary peak may give insight into jet structure.Comment: 38 pages, 15 figures, submitted to ApJS, comments welcom
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