357 research outputs found
Practical Methods for Continuous Gravitational Wave Detection using Pulsar Timing Data
Gravitational Waves (GWs) are tiny ripples in the fabric of space-time
predicted by Einstein's General Relativity. Pulsar timing arrays (PTAs) are
well poised to detect low frequency ( -- Hz) GWs in the near
future. There has been a significant amount of research into the detection of a
stochastic background of GWs from supermassive black hole binaries (SMBHBs).
Recent work has shown that single continuous sources standing out above the
background may be detectable by PTAs operating at a sensitivity sufficient to
detect the stochastic background. The most likely sources of continuous GWs in
the pulsar timing frequency band are extremely massive and/or nearby SMBHBs. In
this paper we present detection strategies including various forms of matched
filtering and power spectral summing. We determine the efficacy and
computational cost of such strategies. It is shown that it is computationally
infeasible to use an optimal matched filter including the poorly constrained
pulsar distances with a grid based method. We show that an Earth-term-matched
filter constructed using only the correlated signal terms is both
computationally viable and highly sensitive to GW signals. This technique is
only a factor of two less sensitive than the computationally unrealizable
optimal matched filter and a factor of two more sensitive than a power spectral
summing technique. We further show that a pairwise matched filter, taking the
pulsar distances into account is comparable to the optimal matched filter for
the single template case and comparable to the Earth-term-matched filter for
many search templates. Finally, using simulated data optimal quality, we place
a theoretical minimum detectable strain amplitude of from
continuous GWs at frequencies on the order .Comment: submitted to Ap
Detection of variable frequency signals using a fast chirp transform
The detection of signals with varying frequency is important in many areas of
physics and astrophysics. The current work was motivated by a desire to detect
gravitational waves from the binary inspiral of neutron stars and black holes,
a topic of significant interest for the new generation of interferometric
gravitational wave detectors such as LIGO. However, this work has significant
generality beyond gravitational wave signal detection.
We define a Fast Chirp Transform (FCT) analogous to the Fast Fourier
Transform (FFT). Use of the FCT provides a simple and powerful formalism for
detection of signals with variable frequency just as Fourier transform
techniques provide a formalism for the detection of signals of constant
frequency. In particular, use of the FCT can alleviate the requirement of
generating complicated families of filter functions typically required in the
conventional matched filtering process. We briefly discuss the application of
the FCT to several signal detection problems of current interest
Constraining the coalescence rate of supermassive black-hole binaries using pulsar timing
Pulsar timing observations are used to place constraints on the rate of
coalescence of supermassive black-hole (SMBH) binaries as a function of mass
and redshift. In contrast to the indirect constraints obtained from other
techniques, pulsar timing observations provide a direct constraint on the
black-hole merger rate. This is possible since pulsar timing is sensitive to
the gravitational waves (GWs) emitted by these sources in the final stages of
their evolution. We find that upper bounds calculated from the recently
published Parkes Pulsar Timing Array data are just above theoretical
predictions for redshifts below 10. In the future, with improved timing
precision and longer data spans, we show that a non-detection of GWs will rule
out some of the available parameter space in a particular class of SMBH binary
merger models. We also show that if we can time a set of pulsars to 10ns timing
accuracy, for example, using the proposed Square Kilometre Array, it should be
possible to detect one or more individual SMBH binary systems
A coherent method for the detection and estimation of continuous gravitational wave signals using a pulsar timing array
The use of a high precision pulsar timing array is a promising approach to
detecting gravitational waves in the very low frequency regime ( Hz) that is complementary to the ground-based efforts (e.g., LIGO,
Virgo) at high frequencies ( Hz) and space-based ones (e.g.,
LISA) at low frequencies ( Hz). One of the target sources for
pulsar timing arrays are individual supermassive black hole binaries that are
expected to form in galactic mergers. In this paper, a likelihood based method
for detection and estimation is presented for a monochromatic continuous
gravitational wave signal emitted by such a source. The so-called pulsar terms
in the signal that arise due to the breakdown of the long-wavelength
approximation are explicitly taken into account in this method. In addition,
the method accounts for equality and inequality constraints involved in the
semi-analytical maximization of the likelihood over a subset of the parameters.
The remaining parameters are maximized over numerically using Particle Swarm
Optimization. Thus, the method presented here solves the monochromatic
continuous wave detection and estimation problem without invoking some of the
approximations that have been used in earlier studies.Comment: 33 pages, 10 figures, submitted to Ap
Coherent network analysis for continuous gravitational wave signals in a pulsar timing array: Pulsar phases as extrinsic parameters
Supermassive black hole binaries are one of the primary targets for
gravitational wave searches using pulsar timing arrays. Gravitational wave
signals from such systems are well represented by parametrized models, allowing
the standard Generalized Likelihood Ratio Test (GLRT) to be used for their
detection and estimation. However, there is a dichotomy in how the GLRT can be
implemented for pulsar timing arrays: there are two possible ways in which one
can split the set of signal parameters for semi-analytical and numerical
extremization. The straightforward extension of the method used for continuous
signals in ground-based gravitational wave searches, where the so-called pulsar
phase parameters are maximized numerically, was addressed in an earlier paper
(Wang et al. 2014). In this paper, we report the first study of the performance
of the second approach where the pulsar phases are maximized semi-analytically.
This approach is scalable since the number of parameters left over for
numerical optimization does not depend on the size of the pulsar timing array.
Our results show that, for the same array size (9 pulsars), the new method
performs somewhat worse in parameter estimation, but not in detection, than the
previous method where the pulsar phases were maximized numerically. The origin
of the performance discrepancy is likely to be in the ill-posedness that is
intrinsic to any network analysis method. However, scalability of the new
method allows the ill-posedness to be mitigated by simply adding more pulsars
to the array. This is shown explicitly by taking a larger array of pulsars.Comment: 30 pages, 11 figures, revised version, published in Ap
The Arecibo Observatory as an Instrument for Investigating Orbital Debris: Legacy and Next Generation Performance
In this paper, we investigate the ability of the Arecibo Observatory to characterize the orbital debris environment and compare it to the primary instrument used by NASA\u27s Orbital Debris Program Office, the Haystack Ultra-Wideband Satellite Imaging Radar (HUSIR). Arecibo\u27s location (183 N) increases the percentage of observable orbits (relative to HUSIR) by 27%, which gives Arecibo access to a much larger and previously unmeasured portion of the environment. Due to the recent collapse of the Arecibo dish, in addition to exploring historic capabilities of the Legacy Arecibo Telescope, estimates of the performance of the proposed Next Generation Arecibo Telescope (NGAT) are explored. We show that the current NGAT design could have a sensitivity comparable to the Goldstone Orbital Debris Radar, currently NASA\u27s most sensitive orbital debris radar. Additionally, design suggestions are presented that would significantly improve the capabilities of the NGAT for orbital debris investigations. We show that, with appropriate hardware upgrades, it would be possible to achieve a minimum-detectable debris size as small as 1 mm. These capabilities would allow data from Arecibo to significantly improve short-term debris environment models, which are used to inform spacecraft design and operations, particularly for orbital debris smaller than 3 mm, which pose the highest penetration risk to most spacecraft
Radio Pulse Properties of the Millisecond Pulsar PSR J0437-4715. I. Observations at 20cm
We present a total of 48 minutes of observations of the nearby, bright
millisecond pulsar PSR J0437-4715 taken at the Parkes radio observatory in
Australia. The data were obtained at a central radio frequency of 1380 MHz
using a high-speed tape recorder that permitted coherent Nyquist sampling of 50
MHz of bandwidth in each of two polarizations. Using the high time resolution
available from this voltage recording technique, we have studied a variety of
single-pulse properties, most for the first time in a millisecond pulsar. We
find no evidence for "diffractive" quantization effects in the individual pulse
arrival times or amplitudes as have been reported for this pulsar at lower
radio frequency using coarser time resolution (Ables et al. 1997). Overall, we
find that the single pulse properties of PSR J0437-4715 are similar to those of
the common slow-rotating pulsars, even though this pulsar's magnetosphere and
surface magnetic field are several orders of magnitude smaller than those of
the general population. The pulsar radio emission mechanism must therefore be
insensitive to these fundamental neutron star properties.Comment: 24 Postscript pages, 11 eps figures. Accepted for publication in the
Astrophysical Journal. Abbreviated abstract follow
The intrinsic intensity modulation of PSR B1937+21 at 1410 MHz
The single-pulse properties of the millisecond radio pulsar PSR B1937+21 are
studied in the 1410 MHz radio band. Aside from occasional ``giant pulses''
occurring in restricted regions of pulse phase, the emission appears to be
remarkably stable, showing no pulse-to-pulse fluctuations other then those
induced by propagation through the interstellar medium. This type of behavior
has not been seen in any other pulsar although it was seen in previous 430 MHz
observations of this source.
The stability of PSR B1937+21 can be understood in the context of the
sparking gap model of radio pulsar emission. Given the emission properties of
this source at 430 MHz, this model predicts that the emission at all higher
frequencies will be just as stable. Since the stability depends on the outflow
velocity of the emitting plasma, an upper bound may be placed on its Lorentz
factor.Comment: submitted to ApJ
Detecting gravitational wave memory with pulsar timing
We compare the detectability of gravitational bursts passing through the solar system with those passing near each millisecond pulsar in an N-pulsar timing array. The sensitivity to Earth-passing bursts can exploit the correlation expected in pulse arrival times while pulsar-passing bursts, though uncorrelated between objects, provide an N-fold increase in overall time baseline that can compensate for the lower sensitivity. Bursts with memory from mergers of supermassive black holes produce step functions in apparent spin frequency that are the easiest to detect in pulsar timing. We show that the burst rate and amplitude distribution, while strongly dependent on inadequately known cosmological evolution, may favor detection in the pulsar terms rather than the Earth timing perturbations. Any contamination of timing data by red spin noise makes burst detection more difficult because both signals grow with the length of the time data span T. Furthermore, the different bursts that could appear in one or more data sets of length T 10yr also affect the detectability of the gravitational wave stochastic background that, like spin noise, has a red power spectrum. A burst with memory is a worthwhile target in the timing of multiple pulsars in a globular cluster because it should produce a correlated signal with a time delay of less than about 10years in some cases. © 2012. The American Astronomical Society. All rights reserved.
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