1,032 research outputs found
Impact of basic angle variations on the parallax zero point for a scanning astrometric satellite
Determination of absolute parallaxes by means of a scanning astrometric
satellite such as Hipparcos or Gaia relies on the short-term stability of the
so-called basic angle between the two viewing directions. Uncalibrated
variations of the basic angle may produce systematic errors in the computed
parallaxes. We examine the coupling between a global parallax shift and
specific variations of the basic angle, namely those related to the satellite
attitude with respect to the Sun. The changes in observables produced by small
perturbations of the basic angle, attitude, and parallaxes are calculated
analytically. We then look for a combination of perturbations that has no net
effect on the observables. In the approximation of infinitely small fields of
view, it is shown that certain perturbations of the basic angle are
observationally indistinguishable from a global shift of the parallaxes. If
such perturbations exist, they cannot be calibrated from the astrometric
observations but will produce a global parallax bias. Numerical simulations of
the astrometric solution, using both direct and iterative methods, confirm this
theoretical result. For a given amplitude of the basic angle perturbation, the
parallax bias is smaller for a larger basic angle and a larger solar aspect
angle. In both these respects Gaia has a more favourable geometry than
Hipparcos. In the case of Gaia, internal metrology is used to monitor basic
angle variations. Additionally, Gaia has the advantage of detecting numerous
quasars, which can be used to verify the parallax zero point.Comment: 8 pages, 2 figures; Accepted for publication in Astronomy &
Astrophysic
Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
We derive the luminosity-temperature relation for the super-critically
accreting black holes (BHs) and compare it to the data on ultraluminous X-ray
sources (ULXs). At super-Eddington accretion rates, an outflow forms within the
spherization radius. We construct the accretion disc model accounting for the
advection and the outflow, and compute characteristic disc temperatures. The
bolometric luminosity exceeds the Eddington luminosity L_Edd by a logarithmic
factor 1+0.6 ln mdot (where mdot is the accretion rate in Eddington units) and
the wind kinetic luminosity is close to L_Edd. The apparent luminosity for the
face-on observer is 2-7 times higher because of geometrical beaming. Such an
observer has a direct view of the inner hot accretion disc, which has a peak
temperature T_max of a few keV in stellar-mass BHs. The emitted spectrum
extends as a power-law F_E ~ E**{-1} down to the temperature at the
spherization radius T_sp ~ mdot**(-1/2) keV. We associate T_max with a few keV
spectral components and T_sp with the soft, 0.1-0.2 keV components observed in
ULXs. An edge-on observer sees only the soft emission from the extended
envelope, with the photosphere radius exceeding the spherization radius by
orders of magnitude. The dependence of the photosphere temperature on
luminosity is consistent with that observed in the super-Eddington accreting
BHs SS 433 and V4641 Sgr. Strong outflows combined with the large intrinsic
X-ray luminosity of the central BH explain naturally the presence of the
photoionized nebulae around ULXs. An excellent agreement between the model and
the observational data strongly argues in favour of ULXs being super-critically
accreting, stellar-mass BHs similar to SS 433, but viewed close to the symmetry
axis.Comment: 8 pages, 5 figures; heavily revised version; accepted to MNRA
Charged Current Neutrino Cross Section and Tau Energy Loss at Ultra-High Energies
We evaluate both the tau lepton energy loss produced by photonuclear
interactions and the neutrino charged current cross section at ultra-high
energies, relevant to neutrino bounds with Earth-skimming tau neutrinos, using
different theoretical and phenomenological models for nucleon and nucleus
structure functions. The theoretical uncertainty is estimated by taking
different extrapolations of the structure function F2 to very low values of x,
in the low and moderate Q2 range for the tau lepton interaction and at high Q2
for the neutrino-nucleus inelastic cross section. It is at these extremely low
values of x where nuclear shadowing and parton saturation effects are unknown
and could be stronger than usually considered. For tau and neutrino energies
E=10^9 GeV we find uncertainties of a factor 4 for the tau energy loss and of a
factor 2 for the charged current neutrino-nucleus cross section.Comment: 20 pages and 11 figure
Transverse Enhancement Model and MiniBooNE Charge Current Quasi-Elastic Neutrino Scattering Data
Recently proposed Transverse Enhancement Model of nuclear effects in Charge
Current Quasi-Elastic neutrino scattering [A. Bodek, H. S. Budd, and M. E.
Christy, Eur. Phys. J. C{\bf 71} (2011) 1726] is confronted with the MiniBooNE
high statistics experimental data. It is shown that the {\it effective} large
axial mass model leads to better agreement with the data.Comment: 4 pages, 6 figure
Neutrino Interactions at Ultrahigh Energies
We report new calculations of the cross sections for deeply inelastic
neutrino-nucleon scattering at neutrino energies between 10^{9}\ev and
10^{21}\ev. We compare with results in the literature and assess the
reliability of our predictions. For completeness, we briefly review the cross
sections for neutrino interactions with atomic electrons, emphasizing the role
of the -boson resonance in interactions for neutrino
energies in the neighborhood of 6.3\pev. Adopting model predictions for
extraterrestrial neutrino fluxes from active galactic nuclei, gamma-ray
bursters, and the collapse of topological defects, we estimate event rates in
large-volume water \v{C}erenkov detectors and large-area ground arrays.Comment: 32 pages, 11 figures, uses RevTeX and boxedep
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