180,707 research outputs found
Binary Pulsar Tests of General Relativity in the Presence of Low-Frequency Noise
The influence of the low-frequency timing noise on the precision of
measurements of the Keplerian and post-Keplerian orbital parameters in binary
pulsars is studied. Fundamental limits on the accuracy of tests of alternative
theories of gravity in the strong-field regime are established. The
gravitational low-frequency timing noise formed by an ensemble of binary stars
is briefly discussed.Comment: 4 pages, contributed paper to the proceedings of the IAU167
colloquium on pulsars, Bonn, August-September 199
Checking the variability of the gravitational constant with binary pulsars
The most precise measurements are done at present by timing of radiopulsars
in binary systems with two neutron stars. The timing measurements of the
Taylor-Hulse pulsar B1913+16 gave the most precise results on testing of
general relativity (GR), finding implicit proof of existence of gravitational
waves. We show that available results of existing measurements, obtained to the
year 1993, in combination with the results of the Mariner 10 in (1992), give
the boundaries for the variation of the gravitational constant
inside the limits year{}.Comment: Submitted to Journa
Bounds on gravitational wave backgrounds from large distance clock comparisons
Our spacetime is filled with gravitational wave backgrounds that constitute a
fluctuating environment created by astrophysical and cosmological sources.
Bounds on these backgrounds are obtained from cosmological and astrophysical
data but also by analysis of ranging and Doppler signals from distant
spacecraft. We propose here a new way to set bounds on those backgrounds by
performing clock comparisons between a ground clock and a remote spacecraft
equipped with an ultra-stable clock, rather than only ranging to an onboard
transponder. This technique can then be optimized as a function of the signal
to be measured and the dominant noise sources, leading to significant
improvements on present bounds in a promising frequency range where different
theoretical models are competing. We illustrate our approach using the SAGAS
project which aims to fly an ultra stable optical clock in the outer solar
system.Comment: 10 pages, 8 figures, minor amendment
Pulsar Timing and its Application for Navigation and Gravitational Wave Detection
Pulsars are natural cosmic clocks. On long timescales they rival the
precision of terrestrial atomic clocks. Using a technique called pulsar timing,
the exact measurement of pulse arrival times allows a number of applications,
ranging from testing theories of gravity to detecting gravitational waves. Also
an external reference system suitable for autonomous space navigation can be
defined by pulsars, using them as natural navigation beacons, not unlike the
use of GPS satellites for navigation on Earth. By comparing pulse arrival times
measured on-board a spacecraft with predicted pulse arrivals at a reference
location (e.g. the solar system barycenter), the spacecraft position can be
determined autonomously and with high accuracy everywhere in the solar system
and beyond. We describe the unique properties of pulsars that suggest that such
a navigation system will certainly have its application in future astronautics.
We also describe the on-going experiments to use the clock-like nature of
pulsars to "construct" a galactic-sized gravitational wave detector for
low-frequency (f_GW ~1E-9 - 1E-7 Hz) gravitational waves. We present the
current status and provide an outlook for the future.Comment: 30 pages, 9 figures. To appear in Vol 63: High Performance Clocks,
Springer Space Science Review
The long-term evolution of the spin, pulse shape, and orbit of the accretion-powered millisecond pulsar SAX J1808.4-3658
We present a 7 yr timing study of the 2.5 ms X-ray pulsar SAX J1808.4-3658,
an X-ray transient with a recurrence time of ~2 yr, using data from the Rossi
X-ray Timing Explorer covering 4 transient outbursts (1998-2005). We verify
that the 401 Hz pulsation traces the spin frequency fundamental and not a
harmonic. Substantial pulse shape variability, both stochastic and systematic,
was observed during each outburst. Analysis of the systematic pulse shape
changes suggests that, as an outburst dims, the X-ray "hot spot" on the pulsar
surface drifts longitudinally and a second hot spot may appear. The overall
pulse shape variability limits the ability to measure spin frequency evolution
within a given X-ray outburst (and calls previous nudot measurements of this
source into question), with typical upper limits of |nudot| < 2.5x10^{-14} Hz/s
(2 sigma). However, combining data from all the outbursts shows with high (6
sigma) significance that the pulsar is undergoing long-term spin down at a rate
nudot = (-5.6+/-2.0)x10^{-16} Hz/s, with most of the spin evolution occurring
during X-ray quiescence. We discuss the possible contributions of magnetic
propeller torques, magnetic dipole radiation, and gravitational radiation to
the measured spin down, setting an upper limit of B < 1.5x10^8 G for the
pulsar's surface dipole magnetic field and and Q/I < 5x10^{-9} for the
fractional mass quadrupole moment. We also measured an orbital period
derivative of Pdot = (3.5+/-0.2)x10^{-12} s/s. This surprising large Pdot is
reminiscent of the large and quasi-cyclic orbital period variation observed in
the so-called "black widow" millisecond radio pulsars, supporting speculation
that SAX J1808.4-3658 may turn on as a radio pulsar during quiescence. In an
appendix we derive an improved (0.15 arcsec) source position from optical data.Comment: 22 pages, 10 figures; accepted for publication in Ap
Testing black hole no-hair theorem with OJ287
We examine the ability to test the black hole no-hair theorem at the 10%
level in this decade using the binary black hole in OJ287. In the test we
constrain the value of the dimensionless parameter q that relates the scaled
quadrupole moment and spin of the primary black hole: q2 = -q 2 . At the
present we can say that q = 1 \pm 0.3 (one), in agreement with General
Relativity and the no-hair theorems. We demonstrate that this result can be
improved if more observational data is found in historical plate archives for
the 1959 and 1971 outbursts. We also show that the predicted 2015 and 2019
outbursts will be crucial in improving the accuracy of the test. Space-based
photometry is required in 2019 July due the proximity of OJ287 to the Sun at
the time of the outburst. The best situation would be to carry out the
photometry far from the Earth, from quite a different vantage point, in order
to avoid the influence of the nearby Sun. We have considered in particular the
STEREO space mission which would be ideal if it has a continuation in 2019 or
LORRI on board the New Horizons mission to Pluto.Comment: 14 pages, 14 figure
Implications of Electronics Constraints for Solid-State Quantum Error Correction and Quantum Circuit Failure Probability
In this paper we present the impact of classical electronics constraints on a
solid-state quantum dot logical qubit architecture. Constraints due to routing
density, bandwidth allocation, signal timing, and thermally aware placement of
classical supporting electronics significantly affect the quantum error
correction circuit's error rate. We analyze one level of a quantum error
correction circuit using nine data qubits in a Bacon-Shor code configured as a
quantum memory. A hypothetical silicon double quantum dot quantum bit (qubit)
is used as the fundamental element. A pessimistic estimate of the error
probability of the quantum circuit is calculated using the total number of
gates and idle time using a provably optimal schedule for the circuit
operations obtained with an integer program methodology. The micro-architecture
analysis provides insight about the different ways the electronics impact the
circuit performance (e.g., extra idle time in the schedule), which can
significantly limit the ultimate performance of any quantum circuit and
therefore is a critical foundation for any future larger scale architecture
analysis.Comment: 10 pages, 7 figures, 3 table
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