3,130 research outputs found
Breaking degeneracy in jet dynamics: multi-epoch joint modelling of the BL Lac PKS 2155-304
Supermassive black holes can launch powerful jets which can be some of the
most luminous multi-wavelength sources; decades after their discovery their
physics and energetics are still poorly understood. The past decade has seen a
dramatic improvement in the quality of available data, but despite this
improvement the semi-analytical modelling of jets has advanced slowly: simple
one-zone models are still the most commonly employed method of interpreting
data, in particular for AGN jets. These models can roughly constrain the
properties of jets but they cannot unambiguously couple their emission to the
launching regions and internal dynamics, which can be probed with simulations.
However, simulations are not easily comparable to observations because they
cannot yet self-consistently predict spectra. We present an advanced
semi-analytical model which accounts for the dynamics of the whole jet,
starting from a simplified parametrization of Relativistic Magnetohydrodynamics
in which the magnetic flux is converted into bulk kinetic energy. To benchmark
the model we fit six quasisimultaneous, multi-wavelength spectral energy
distributions of the BL Lac PKS 2155-304 obtained by the TANAMI program, and we
address the degeneracies inherent to such a complex model by employing a
state-of-the-art exploration of parameter space, which so far has been mostly
neglected in the study of AGN jets. We find that this new approach is much more
effective than a single-epoch fit in providing meaningful constraints on model
parameters.Comment: Accepted for publication on MNRA
The jet-disk symbiosis without maximal jets: 1-D hydrodynamical jets revisited
In this work we discuss the recent criticism by Zdziarski of the maximal jet
model derived in Falcke & Biermann (1995). We agree with Zdziarski that in
general a jet's internal energy is not bounded by its rest-mass energy density.
We describe the effects of the mistake on conclusions that have been made using
the maximal jet model and show when a maximal jet is an appropriate assumption.
The maximal jet model was used to derive a 1-D hydrodynamical model of jets in
agnjet, a model that does multiwavelength fitting of quiescent/hard state X-ray
binaries and low-luminosity active galactic nuclei. We correct algebraic
mistakes made in the derivation of the 1-D Euler equation and relax the maximal
jet assumption. We show that the corrections cause minor differences as long as
the jet has a small opening angle and a small terminal Lorentz factor. We find
that the major conclusion from the maximal jet model, the jet-disk symbiosis,
can be generally applied to astrophysical jets. We also show that isothermal
jets are required to match the flat radio spectra seen in low-luminosity X-ray
binaries and active galactic nuclei, in agreement with other works.Comment: 7 pages, accepted by A&
A study of omega bands and Ps6 pulsations on the ground, at low altitude and at geostationary orbit
We investigate the electrodynamic coupling between auroral omega bands and the inner magnetosphere. The goal of this study is to determine the features to which omega bands map in the magnetosphere. To establish the auroral-magnetosphere connection, we appeal to the case study analysis of the data rich event of September 26, 1989. At 6 magnetic local time (MLT), two trains of Ps6 pulsations (ground magnetic signatures of omega bands) were observed to drift over the Canadian Auroral Network For the OPEN Program Unified Study (CANOPUS) chain. At the same time periodic ionospheric flow patterns moved through the collocated Bistatic Auroral Radar System (BARS) field of view. Similar coincident magnetic variations were observed by GOES 6, GOES 7 and SCATHA, all of which had magnetic foot points near the CANOPUS/BARS stations. SCATHA, which was located at 6 MLT, 0.5 RE earthward of GOES 7 observed the 10 min period pulsations, whereas GOES 7 did not. In addition, DMSP F6 and F8 were over-flying the region and observed characteristic precipitation and flow signatures. From this fortunate constellation of ground and space observations, we conclude that auroral omega bands are the electrodynamic signature of a corrugated current sheet (or some similar spatially localized magnetic structure) in the near-Earth geostationary magnetosphere
Magnetic flux transfer in the 5 April 2010 Galaxy 15 substorm: an unprecedented observation
At approximately 08:25 UT on 5 April 2010, a CME-driven shock compressed
Earth's magnetosphere and applied about 15 nT of southward IMF for nearly an
hour. A substorm growth phase and localized dipolarization at 08:47 UT were
followed by large dipolarizations at 09:03 UT and 09:08 UT, observed by GOES
West (11) in the midnight sector, and by three THEMIS spacecraft near X=−11,
Y=−2 RE. A large electric field at the THEMIS spacecraft indicates so
much flux transfer to the inner magnetosphere that "overdipolarization" took
place at GOES 11. This transfer is consistent with the ground and space
magnetic signature of the substorm current wedge. Significant particle
injections were also observed. The ensemble of extreme geophysical
conditions, never previously observed, is consistent with the Near-Earth
Neutral Line interpretation of substorms, and subjected the Galaxy 15
geosynchronous satellite to space weather conditions which appear to have
induced a major operational anomaly
Multi-wavelength astronomical searches for primordial black holes
If primordial black holes of O(1–100) M_⊙ constitute a significant portion of the dark matter in the Universe, they should be very abundant in our Galaxy. We present here a detailed analysis of the radio and X-ray emission that these objects are expected to produce due to the accretion of gas from the interstellar medium. With respect to previous studies, we relax the assumption of a monochromatic mass function, and introduce an improved treatment of the physics of gas accretion onto isolated, moving compact objects, based on a set of state-of-the-art numerical simulations. By comparing our predictions with known radio and X-ray sources in the Galactic center region, we show that the maximum relic density of primordial black holes in the mass range of interest is ~ 10^(−3) smaller than that of dark matter. The new upper bound is two orders of magnitude stronger with respect to previous results, based on a conservative phenomenological treatment of the accretion physics. We also provide a comprehensive critical discussion on the reliability of this bound, and on possible future developments in the field. We argue in particular that future multi-wavelength searches will soon start to probe the galactic population of astrophysical black holes
Multi-wavelength astronomical searches for primordial black holes
If primordial black holes of constitute a
significant portion of the dark matter in the Universe, they should be very
abundant in our Galaxy. We present here a detailed analysis of the radio and
X-ray emission that these objects are expected to produce due to the accretion
of gas from the interstellar medium. With respect to previous studies, we relax
the assumption of a monochromatic mass function, and introduce an improved
treatment of the physics of gas accretion onto isolated, moving compact
objects, based on a set of state-of-the-art numerical simulations. By comparing
our predictions with known radio and X-ray sources in the Galactic center
region, we show that the maximum relic density of primordial black holes in the
mass range of interest is smaller than that of dark matter. The
new upper bound is two orders of magnitude stronger with respect to previous
results, based on a conservative phenomenological treatment of the accretion
physics. We also provide a comprehensive critical discussion on the reliability
of this bound, and on possible future developments in the field. We argue in
particular that future multi-wavelength searches will soon start to probe the
galactic population of astrophysical black holes.Comment: 21 pages, 6 figures. Prepared for submission to JCA
The Locations of Gamma-Ray Bursts Measured by COMPTEL
The COMPTEL instrument on the Compton Gamma Ray Observatory is used to
measure the locations of gamma-ray bursts through direct imaging of MeV
photons. In a comprehensive search, we have detected and localized 29 bursts
observed between 1991 April 19 and 1995 May 31. The average location accuracy
of these events is 1.25\arcdeg (1), including a systematic error of
\sim0.5\arcdeg, which is verified through comparison with Interplanetary
Network (IPN) timing annuli. The combination of COMPTEL and IPN measurements
results in locations for 26 of the bursts with an average ``error box'' area of
only 0.3 deg (1). We find that the angular distribution of
COMPTEL burst locations is consistent with large-scale isotropy and that there
is no statistically significant evidence of small-angle auto-correlations. We
conclude that there is no compelling evidence for burst repetition since no
more than two of the events (or 7% of the 29 bursts) could possibly have
come from the same source. We also find that there is no significant
correlation between the burst locations and either Abell clusters of galaxies
or radio-quiet quasars. Agreement between individual COMPTEL locations and IPN
annuli places a lower limit of 100~AU (95% confidence) on the distance to
the stronger bursts.Comment: Accepted for publication in the Astrophysical Journal, 1998 Jan. 1,
Vol. 492. 33 pages, 9 figures, 5 table
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