176 research outputs found
A ground track control algorithm for the Topographic Mapping Laser Altimeter (TMLA)
The results of an analysis of an algorithm that will provide autonomous onboard orbit control using orbits determined with Global Positioning System (GPS) data. The algorithm uses the GPS data to (1) compute the ground track error relative to a fixed longitude grid, and (2) determine the altitude adjustment required to correct the longitude error. A program was written on a personal computer (PC) to test the concept for numerous altitudes and values of solar flux using a simplified orbit model including only the J sub 2 zonal harmonic and simple orbit decay computations. The algorithm was then implemented in a precision orbit propagation program having a full range of perturbations. The analysis showed that, even with all perturbations (including actual time histories of solar flux variation), the algorithm could effectively control the spacecraft ground track and yield more than 99 percent Earth coverage in the time required to complete one coverage cycle on the fixed grid (220 to 230 days depending on altitude and overlap allowance)
Epicyclic oscillations of non-slender fluid tori around Kerr black holes
Considering epicyclic oscillations of pressure-supported perfect fluid tori
orbiting Kerr black holes we examine non-geodesic (pressure) effects on the
epicyclic modes properties. Using a perturbation method we derive fully general
relativistic formulas for eigenfunctions and eigenfrequencies of the radial and
vertical epicyclic modes of a slightly non-slender, constant specific angular
momentum torus up to second-order accuracy with respect to the torus thickness.
The behaviour of the axisymmetric and lowest-order () non-axisymmetric
epicyclic modes is investigated. For an arbitrary black hole spin we find that,
in comparison with the (axisymmetric) epicyclic frequencies of free test
particles, non-slender tori receive negative pressure corrections and exhibit
thus lower frequencies. Our findings are in qualitative agreement with the
results of a recent pseudo-Newtonian study of analogous problem defined within
the Paczy{\'n}ski-Wiita potential. Implications of our results on the
high-frequency QPO models dealing with epicyclic oscillations are addressed.Comment: 24 pages, 8 figure
Line emission from optically thick relativistic accretion tori
We calculate line emission from relativistic accretion tori around Kerr black
holes and investigate how the line profiles depend on the viewing inclination,
spin of the central black hole, parameters describing the shape of the tori,
and spatial distribution of line emissivity on the torus surface. We also
compare the lines with those from thin accretion disks. Our calculations show
that lines from tori and lines from thin disks share several common features.
In particular, at low and moderate viewing inclination angles they both have
asymmetric double-peaked profiles with a tall, sharp blue peak and a shorter
red peak which has an extensive red wing. At high viewing inclination angles
they both have very broad, asymmetric lines which can be roughly considered
single-peaked. Torus and disk lines may show very different red and blue line
wings, but the differences are due to the models for relativistic tori and
disks having differing inner boundary radii. Self-eclipse and lensing play some
role in shaping the torus lines, but they are effective only at high
inclination angles. If inner and outer radii of an accretion torus are the same
as those of an accretion disk, their line profiles show substantial differences
only when inclination angles are close to 90 degrees, and those differences
manifest mostly at the central regions of the lines instead of the wings.Comment: 14 pages, 17 figures. Accepted to A&
Period distribution of old accreting isolated neutron stars
In this paper we present calculations of period distribution for old
accreting isolated neutron stars (INSs). After few billion years of evolution
low velocity INSs come to the stage of accretion. At this stage INS's period
evolution is governed by magnetic braking and angular momentum accreted. Since
the interstellar medium is turbulized accreted momentum can either accelerate
or decelerate spin of an INS, therefore the evolution of period has chaotic
character.
Our calculations show that in the case of constant magnetic field accreting
INSs have relatively long spin periods (some hours and more, depending on INS's
spatial velocity, its magnetic field and density of the surrounding medium).
Such periods are much longer than the values measured by {\emph ROSAT} for 3
radio-silent isolated neutron stars. Due to long periods INSs should have high
spin up/down rates, , which should fluctuate on a time scale of yr.Comment: 7 pages, 3 figure
The proper motion and energy distribution of the isolated neutron star RX J0720.4-3125
ESO 4m class telescope and VLT deep imaging of the isolated neutron star RX
J0720.4-3125 reveals a proper motion of mu = 97 +/-12 mas/yr and a blue U-B
color index. We show that a neutron star atmosphere model modified to account
for a limited amount of hydrogen on the star's surface can well represent both
the optical and X-ray data without invoking any additional components. The
large proper motion almost completely excludes the possibility that accretion
from interstellar medium is the powering mechanism of the X-ray emission. It
also implies that the proposed spin down is entirely due to magnetic dipole
losses. RX J0720.4-3125 is thus a very likely middle aged cooling neutron star.
Its overall properties are quite similar to some of the long period radio
pulsars recently discovered, giving further support to the idea that RX
J0720.4-3125 may be a pulsar whose narrow radio beam does not cross the Earth.Comment: Submitted to Astronomy and Astrophysics, 8 pages 5 figure
Discovery of 5.16s pulsations from the isolated neutron star RBS 1223
The isolated neutron star candidate RBS 1223 was observed with the Advanced
CCD Imaging Spectrometer aboard the Chandra X-ray observatory on 2000 June 24.
A timing analysis of the data yielded a periodic modulation with a period
P=5.1571696^(+1.57*10^(-4) -1.36*10^(-4)s. Using ROSAT HRI archived
observations we detected a period P=5.1561274 \pm 4.4*10^(-4)s and determined
period derivative dP/dt=(0.7 - 2.0)*10^(-11) s*s^(-1). The detection of this
period and dP/dt indicates that RBS 12223 has a ``characteristic'' age of
6000-12000 years and huge magnetic field at the surface (B(dipole)~(1.7-
3.2)*10^(+14) G) typical for anomalous X-ray pulsars (AXPs).Comment: 7 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic
Three-Dimensional Hydrodynamic Simulations of Accretion Tori in Kerr Spacetimes
This paper presents results of three-dimensional simulations of global
hydrodynamic instabilities in black hole tori, extending earlier work by Hawley
to Kerr spacetimes. This study probes a three-dimensional parameter space of
torus angular momentum, torus size, and black hole angular momentum. We have
observed the growth of the Papaloizou-Pringle instability for a range of torus
configurations and the resultant formation of m=1 planets. We have also
observed the quenching of this instability in the presence of early accretion
flows; however, in one simulation both early accretion and planet formation
occurred. Though most of the conclusions reached in Hawley's earlier work on
Schwarzschild black holes carry over to Kerr spacetime, the presence of frame
dragging in the Kerr geometry adds an element of complexity to the simulations;
we have seen especially clear examples of this phenomenon in the accretion
flows that arise from retrograde tori.Comment: Accepted for publication in ApJ. 23 pages, 11 figures, animations
available at http://www.astro.virginia.edu/~jd5v/h91redux/h91redux.htm
Stability of general-relativistic accretion disks
Self-gravitating relativistic disks around black holes can form as transient
structures in a number of astrophysical scenarios such as binary neutron star
and black hole-neutron star coalescences, as well as the core-collapse of
massive stars. We explore the stability of such disks against runaway and
non-axisymmetric instabilities using three-dimensional hydrodynamics
simulations in full general relativity using the THOR code. We model the disk
matter using the ideal fluid approximation with a -law equation of
state with . We explore three disk models around non-rotating black
holes with disk-to-black hole mass ratios of 0.24, 0.17 and 0.11. Due to metric
blending in our initial data, all of our initial models contain an initial
axisymmetric perturbation which induces radial disk oscillations. Despite these
oscillations, our models do not develop the runaway instability during the
first several orbital periods. Instead, all of the models develop unstable
non-axisymmetric modes on a dynamical timescale. We observe two distinct types
of instabilities: the Papaloizou-Pringle and the so-called intermediate type
instabilities. The development of the non-axisymmetric mode with azimuthal
number m = 1 is accompanied by an outspiraling motion of the black hole, which
significantly amplifies the growth rate of the m = 1 mode in some cases.
Overall, our simulations show that the properties of the unstable
non-axisymmetric modes in our disk models are qualitatively similar to those in
Newtonian theory.Comment: 30 pages, 21 figure
The proper motion of the isolated neutron star RX J1605.3+3249
We obtained deep optical imaging of the thermally emitting X-ray bright and
radio-quiet isolated neutron star RX J1605.3+3249 with the Subaru telescope in
1999 and 2003. Together with archival HST images acquired in 2001 these data
reveal a proper motion of mu = 144.5 +/- 13.2 mas/yr. This implies a relatively
high spatial velocity and indicates that the star is unlikely to be re-heated
by accretion of matter from the interstellar medium. Assuming that RX
J1605.3+3249 is a young (10^5-10^6 yr) cooling neutron star, its apparent
trajectory is consistent with a birth in the nearby Sco OB2 OB association at a
location close to that derived for RX J1856.5-3754 and perhaps also to that of
RX J0720.4-3125. This suggests that the X-ray bright part of ROSAT-discovered
isolated neutron stars is dominated by the production of the Sco OB2 complex
which is the closest OB association and a part of the Gould belt. The B and R
magnitudes of the faint optical counterpart did not vary from 1999 to 2003 at B
= 27.22 +/- 0.10. Its B-R colour index of +0.32 +/- 0.17 is significantly
redder than that of other isolated neutron stars and the optical flux lies a
factor 11.5 above the extrapolation of the X-ray blackbody-like spectrum. The
red optical colour reveals the presence of an additional emitting component in
the optical regime over the main neutron star thermal emission. We also
discovered a small elongated Halpha nebula approximately centered on the
neutron star and aligned with the direction of motion. The width of the nebula
is unresolved and smaller than ~ 0.4" for a length of about 1". The shape of
the Balmer emitting nebula is very different from those seen close to other
neutron stars and should be confirmed by follow-up observations. We shortly
discuss the possible mechanisms which could give rise to such a geometry.Comment: 9 pages, 6 figures, resolution of some figures reduced to meet
astro-ph file size restriction, accepted for publication in A&
Accretion Discs in Blazars
The characteristic properties of blazars (rapid variability, strong
polarization, high brightness) are widely attributed to a powerful relativistic
jet oriented close to our line of sight. Despite the spectral energy
distributions (SEDs) being strongly jet-dominated, a "big blue bump" has been
recently detected in sources known as flat spectrum radio quasars (FSRQs).
These new data provide a unique opportunity to observationally test coupled
jet-disc accretion models in these extreme sources. In particular, as energy
and angular momentum can be extracted by a jet magnetically coupled to the
accretion disc, the thermal disc emission spectrum may be modified from that
predicted by the standard model for disc accretion. We compare the
theoretically predicted jet-modified accretion disc spectra against the new
observations of the "big blue bump" in FSRQs. We find mass accretion rates that
are higher, typically by a factor of two, than predicted by standard accretion
disc theory. Furthermore, our results predict that the high redshift blazars
PKS 0836+710, PKS 2149-307, B2 0743+25 and PKS 0537-286 may be predominantly
powered by a low or moderate spin (a < 0.6) black hole with high mass accretion
rates mdot_a ~ 50 - 200 msol/yr, while 3C 273 harbours a rapidly spinning black
hole (a = 0.97) with mdot_a ~ 20 msol/yr. We also find that the black hole
masses in these high redshift sources must be > 5 * 10^9 msol.Comment: Accepted for publication (17 August 2009) in MNRA
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