126 research outputs found
Integration of the atmospheric fluctuations in a dual-field optical interferometer: the short exposure regime
Spatial phase-referencing in dual-field optical interferometry is
reconsidered. Our analysis is based on the 2-sample variance of the
differential phase between target and reference star. We show that averaging
over time of the atmospheric effects depends on this 2-sample phase variance
(Allan variance) rather than on the true variance. The proper expression for
fringe smearing beyond the isoplanatic angle is derived. With simulations of
atmospheric effects, based on a Paranal turbulence model, we show how the
performances of a dual-field optical interferometer can be evaluated in a
diagram 'separation angle' versus 'magnitude of faint object'. In this diagram,
a domain with short exposure is found to be most useful for interferometry,
with about the same magnitude limits in the H and K bands. With star counts
from a Galaxy model, we evaluate the sky coverage for differential astrometry
and detection of exoplanets, i.e. likelihood of faint reference stars in the
vicinity of a bright target. With the 2mass survey, we evaluate sky coverage
for phase-referencing, i.e. avaibility of a bright enough star for main delay
tracking in the vicinity of any target direction.Comment: 9 pages, 8 figures, accepted for publication in A&
Stationary equatorial MHD flows in general relativity
We derive a new formulation of the fully general relativistic equations describing a stationary equatorial MHD outflow from a rotating central object. The wind solution appears as a level contour of a `Bernoulli' function fixed by the requirements that it must pass through the slow and fast critical points. This approach is the general relativistic extension to the classical treatment of Sakurai (1985). We discuss in details how the efficiency of the magnetic to kinetic energy conversion depends mainly on the geometry of the flux tubes and show that the magnetic acceleration can work very well under some conditions. We show how this tool can be used for the study of several astrophysical phenomena, among which gamma-ray bursts
Prompt high-energy emission from gamma-ray bursts in the internal shock model
The prompt GRB emission is thought to arise from electrons accelerated in
internal shocks propagating within a highly relativistic outflow. The launch of
Fermi offers the prospect of observations with unprecedented sensitivity in
high-energy (>100 MeV) gamma-rays. The aim is to explore the predictions for HE
emission from internal shocks, taking into account both dynamical and radiative
aspects, and to deduce how HE observations constrain the properties of the
relativistic outflow. The emission is modeled by combining a time-dependent
radiative code with a dynamical code giving the evolution of the physical
conditions in the shocked regions.Synthetic lightcurves and spectra are
compared to observations. The HE emission deviates significantly from
analytical estimates, which tend to overpredict the IC component, when the time
dependence and full cross-sections are included. The exploration of the
parameter space favors the case where the dominant process in the BATSE range
is synchrotron emission. The HE component becomes stronger for weaker magnetic
fields. The HE lightcurve can display a prolonged pulse duration due to IC
emission, or even a delayed peak compared to the BATSE range.Alternatively,
having dominant IC emission in the BATSE range requires most electrons to be
accelerated into a steep power-law distribution and implies strong 2nd order IC
scattering. In this case, the BATSE and HE lightcurves are very similar. The
combined dynamical and radiative approach allows a firm appraisal of GRB HE
prompt emission. A diagnostic procedure is presented to identify from
observations the dominant emission process and derive constrains on the bulk
Lorentz factor, particle density and magnetic field of the outflow.Comment: 28 pages, 20 figures, accepted for publication in A&
Reconciling observed GRB prompt spectra with synchrotron radiation ?
(abridged)Prompt GRB emission is often interpreted as synchrotron radiation
from high-energy electrons accelerated in internal shocks. Fast synchrotron
cooling predicts that the photon index below the spectral peak is alpha=-3/2.
This differs significantly from the observed median value alpha \approx -1. We
quantify the influence of inverse Compton and adiabatic cooling on alpha to
understand whether these processes can reconcile the observations with a
synchrotron origin. We use a time-dependent code that follows both the shock
dynamics and electron energy losses. We investigate the dependence of alpha on
the parameters of the model. Slopes between -3/2 and -1 are reached when
electrons suffer IC losses in the Klein-Nishina regime. This does not
necessarily imply a strong IC component in the Fermi/LAT range because
scatterings are only moderately efficient. Steep slopes require that a large
fraction (10-30%) of the dissipated energy is given to a small fraction (<~1%)
of the electrons and that the magnetic energy density fraction remains low (<~
0.1%). Values of alpha up to -2/3 can be obtained with relatively high
radiative efficiencies (>50%) when adiabatic cooling is comparable with
radiative cooling (marginally fast cooling). This requires collisions at small
radii and/or with low magnetic fields. Amending the standard fast cooling
scenario to account for IC cooling naturally leads to alpha up to -1.
Marginally fast cooling may also account for alpha up to -2/3, although the
conditions required are more difficult to reach. About 20% of GRBs show spectra
with slopes alpha>-2/3. Other effects, not investigated here, such as a thermal
component in the electron distribution or pair production by HE photons may
further affect alpha. Still, the majority of observed GRB prompt spectra can be
reconciled with a synchrotron origin, constraining the microphysics of mildly
relativistic internal shocks.Comment: 14 pages, 10 figures, accepted for publication in A&A (10/10/2010
Large scale magnetic fields and their dissipation in GRB fireballs
We consider possible geometries of magnetic fields in GRB outflows, and their
evolution with distance from the source. For magnetically driven outflows, with
an assumed ratio of magnetic to kinetic energy density of order unity, the
field strengths are sufficient for efficient production of gamma-rays by
synchrotron emission in the standard internal shock scenario, without the need
for local generation of small scale fields. In these conditions, the MHD
approximation is valid to large distances (>10^19cm). In outflows driven by
nonaxisymmetric magnetic fields, changes of direction of the field cause
dissipation of magnetic energy by reconnection. This dissipation takes place
outside the photosphere of the outflow, and can convert a significant fraction
of the magnetic energy flux into radiation.Comment: 12 pages including 3 figures, submitted to A&
Evidence for a Photospheric Component in the Prompt Emission of the Short GRB120323A and its Effects on the GRB Hardness-Luminosity Relation
The short GRB 120323A had the highest flux ever detected with the Fermi/GBM.
Here we study its remarkable spectral properties and their evolution using two
spectral models: (i) a single emission component scenario, where the spectrum
is modeled by the empirical Band function, and (ii) a two component scenario,
where thermal (Planck-like) emission is observed simultaneously with a
non-thermal component (a Band function). We find that the latter model fits the
integrated burst spectrum significantly better than the former, and that their
respective spectral parameters are dramatically different: when fit with a Band
function only, the Epeak of the event is unusually soft for a short GRB, while
adding a thermal component leads to more typical short GRB values. Our
time-resolved spectral analysis produces similar results. We argue here that
the two-component model is the preferred interpretation for GRB 120323A, based
on: (i) the values and evolution of the Band function parameters of the two
component scenario, which are more typical for a short GRB, and (ii) the
appearance in the data of a significant hardness-intensity correlation,
commonly found in GRBs, when we employee two-component model fits; the
correlation is non-existent in the Band-only fits. GRB 110721A, a long burst
with an intense photospheric emission, exhibits the exact same behavior. We
conclude that GRB 120323A has a strong photospheric emission contribution,
first time observed in a short GRB. Magnetic dissipation models are difficult
to reconcile with these results, which instead favor photospheric thermal
emission and fast cooling synchrotron radiation from internal shocks. Finally,
we derive a possibly universal hardness-luminosity relation in the source frame
using a larger set of GRBs L,i=(1.59+/-0.84).10^50 (Epeak,i)^(1.33+/-0.07)
erg/s), which could be used as a possible redshift estimator for cosmology.Comment: 27 pages, 13 figures, Accepted by ApJ (April, 7th 2013
M2000 : an astrometric catalog in the Bordeaux Carte du Ciel zone +11 degrees < {delta} < +18 degrees
During four years, systematic observations have been conducted in drift scan
mode with the Bordeaux automated meridian circle in the declination band [+11 ;
+18]. The resulting astrometric catalog includes about 2 300 000 stars down to
the magnitude limit V_M=16.3. Nearly all stars (96%) have been observed at
least 6 times, the catalog being complete down to V_M=15.4. The median internal
standard error in position is about 35 mas in the V_M magnitude range [11 ;
15], which degrades to about 50 mas when the faintest stars are considered.
M2000 provides also one band photometry with a median internal standard error
of 0.04 mag. Comparisons with the Hipparcos and bright part of Tycho-2 catalogs
have enabled to estimate external errors in position to be lower than 40 mas.
In this zone and at epoch 1998, the faint part of Tycho-2 is found to have an
accuracy of 116 mas in alpha instead of 82 mas deduced from the model-based
standard errors given in the catalog.Comment: The catalogue can be fetched directly from:
ftp://cdsarc.u-strasbg.fr/cats/I/272 or queried from:
http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I/272 More information at :
http://www.observ.u-bordeaux.fr/~soubiran/m2000.ht
Gravitational radiation from precessing accretion disks in gamma-ray bursts
We study the precession of accretion disks in the context of gamma-ray burst
inner engines. Our aim is to quantitatively estimate the characteristics of
gravitational waves produced by the precession of the transient accretion disk
in gamma-ray bursts. We evaluate the possible periods of disk precession caused
by the Lense-Thirring effect using an accretion disk model that allows for
neutrino cooling. Assuming jet ejection perpendicular to the disk plane and a
typical intrinsic time-dependence for the burst, we find gamma-ray light curves
that have a temporal microstructure similar to that observed in some reported
events. The parameters obtained for the precession are then used to evaluate
the production of gravitational waves. We find that the precession of accretion
disks of outer radius smaller than cm and accretion rates above 1 solar
mass per second could be detected by Advanced LIGO if they occur at distances
of less than 100 Mpc. We conclude that the precession of a neutrino-cooled
accretion disk in long gamma-ray bursts can be probed by gravitational wave
astronomy. Precession of the disks in short gamma-ray events is undetectable
with the current technology.Comment: 5 pages, 5 figures, accepted for publication in A&
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