239,020 research outputs found
Viewing angle of binary neutron star mergers
The joint detection of the gravitational wave (GW) GW170817 and its
electromagnetic (EM) counterparts GRB170817A and kilonova AT 2017gfo has
triggered extensive study of the EM emission of binary neutron star mergers. A
parameter which is common to and plays a key role in both the GW and the EM
analyses is the viewing angle of the binary's orbit. If a binary is viewed from
different angles, the amount of GW energy changes (implying that orientation
and distance are correlated) and the EM signatures can vary, depending on the
structure of the emission. Information about the viewing angle of the binary
orbital plane is therefore crucial to the interpretation of both the GW and the
EM data, and can potentially be extracted from either side.
In the first part of this study, we present a systematic analysis of how well
the viewing angle of binary neutron stars can be measured from the GW data. We
show that if the sky position and the redshift of the binary can be identified
via the EM counterpart and an associated host galaxy, then for 50 of the
systems the viewing angle can be constrained to uncertainty
from the GW data, independent of electromagnetic emission models. On the other
hand, if no redshift measurement is available, the measurement of the viewing
angle with GW alone is not informative, unless the true viewing angle is close
to . This holds true even if the sky position is measured
independently.
Then, we consider the case where some constraints on the viewing angle can be
placed from the EM data itself. We show that the EM measurements can then be
used in the analysis of GW data to improve the precision of the luminosity
distance, and hence of the Hubble constant, by a factor of 2 to 3.Comment: Accepted by Physical Review
VLBI observations of seven BL Lac objects from RGB sample
We present EVN observations of seven BL Lac objects selected from the RGB
sample. To investigate the intrinsic radiation property of BL Lac objects, we
estimated the Doppler factor with the VLA or MERLIN core and the total 408 MHz
luminosity for a sample of 170 BL Lac objects. The intrinsic (comoving)
synchrotron peak frequency was then calculated by using the estimated Doppler
factor. Assuming a Lorentz factor of 5, the viewing angle of jets was
constrained. The high-resolution VLBI images of seven sources all show a
core-jet structure. We estimated the proper motions of three sources with the
VLBI archive data, and find that the apparent speed increases with the distance
of components to the core for all of them. In our BL Lacs sample, the Doppler
factor of LBLs is systematically larger than that of IBLs and HBLs. We find a
significant anti-correlation between the total 408 MHz luminosity and the
intrinsic synchrotron peak frequency. However, the scatter is much larger than
for the blazar sequence. Moreover, we find a significant positive correlation
between the viewing angle and the intrinsic synchrotron peak frequency. The BL
Lac objects show a continuous distribution on the viewing angle. While LBLs
have a smaller viewing angle than that of IBLs and HBLs, IBLs are comparable to
HBLs. We conclude that the intrinsic synchrotron peak frequency is not only
related to the intrinsic radio power (though with a large scatter), but also to
the viewing angle for the present sample.Comment: 22 pages,15figures, published by A&
Is There a Disc in the Superluminal Quasars?
We present a new evidence of the accretion disc in active galactic nuclei, by
examining the properties of the Ha emission line versus viewing angle, in a
sample of superluminal (SL) quasars. Both line velocity width (FWHM) and rest
equivalent width (EW) correlate with viewing angle. These correlations are
quantitatively consistent with a disc geometry for both the line and the
underlying continuum source.Comment: 18 pages with 6 figures, accepted for publication in MNRA
New constraints on a triaxial model of the Galaxy
We determine the values of parameters of an N-body model for the Galaxy
developed by Fux via comparison with an unbiased, homogeneous sample of OH/IR
stars. Via Monte-Carlo simulation, we find the plausibilities of the
best-fitting models, as well as their errors. The parameters that are
constrained best by these projected data are the total mass of the model and
the viewing angle of the central Bar, although the distribution of the latter
has multiple maxima. The best model has a viewing angle of 44 degrees,
semi-major axis of 2.5 kpc, a bar mass of 1.7E10 solar masses and a tangential
velocity of the local standard of rest of 171 km/s . We argue that the lower
values that are commonly found from stellar data for the viewing angle (around
25 degrees) arise when too few coordinates are available, when the longitude
range is too narrow or when low latitudes are excluded from the fit. The new
constraints on the viewing angle of the galactic Bar from stellar line-of-sight
velocities decrease further the ability of the Bar's distribution to account
for the observed micro-lensing optical depth toward Baade's window : our model
reproduces only half the observed value. The signal of triaxiality diminishes
quickly with increasing latitude, fading within approximately one scaleheight.
This suggests that Baade's window is not a very appropriate region to sample
Bar properties.Comment: 10 pages, 8 figures, TeX, accepted for publication in MNRA
Gamma-ray burst beaming: a universal configuration with a standard energy reservoir?
We consider a gamma-ray burst (GRB) model based on an anisotropic fireball
with an axisymmetric power-law distribution of the energy per solid angle with
index -k, and allow for the observer's viewing direction being at an arbitrary
angle with respect to the jet axis. This model can reproduce the key features
expected from the conventional on-axis uniform jet models, with the novelty
that the achromatic break time in the broadband afterglow lightcurves
corresponds to the epoch when the relativistic beaming angle is equal to the
viewing angle rather than to the jet half opening angle. If all the GRB
fireballs have such a similar energy distribution form with 1.5 < k < (or \sim)
2, GRBs may be modeled by a quasi-universal beaming configuration, and an
approximately standard energy reservoir. The conclusion also holds for some
other forms of angular energy distributions, such as the Gaussian function.Comment: Slightly expanded version accepted for publication in Ap
Resonant Scattering of Emission Lines in Coronal Loops: Effects on Image Morphology and Line Ratios
We have investigated the effects of resonant scattering of emission lines on
the image morphology and intensity from coronal loop structures. It has
previously been shown that line of sight effects in optically thin line
emission can yield loop images that appear uniformly bright at one viewing
angle, but show ``looptop sources'' at other viewing angles. For optically
thick loops where multiple resonant scattering is important, we use a 3D Monte
Carlo radiation transfer code. Our simulations show that the intensity
variation across the image is more uniform than the optically thin simulation
and, depending on viewing angle, the intensity may be lower or higher than that
predicted from optically thin simulations due to scattering out of or into the
line of sight.Comment: Accepted for publication in Ap
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