184 research outputs found
Extending the WMAP Bound on the Size of the Universe
Clues to the shape of our Universe can be found by searching the CMB for
matching circles of temperature patterns. A full sky search of the CMB, mapped
extremely accurately by NASA's WMAP satellite, returned no detection of such
matching circles and placed a lower bound on the size of the Universe at 24
Gpc. This lower bound can be extended by optimally filtering the WMAP power
spectrum. More stringent bounds can be placed on specific candidate topologies
by using a a combination statistic. We use optimal filtering and the
combination statistic to rule out the infamous "soccer ball universe'' model.Comment: 9 pages, 16 figure
Data Analysis Challenges for the Einstein Telescope
The Einstein Telescope is a proposed third generation gravitational wave
detector that will operate in the region of 1 Hz to a few kHz. As well as the
inspiral of compact binaries composed of neutron stars or black holes, the
lower frequency cut-off of the detector will open the window to a number of new
sources. These will include the end stage of inspirals, plus merger and
ringdown of intermediate mass black holes, where the masses of the component
bodies are on the order of a few hundred solar masses. There is also the
possibility of observing intermediate mass ratio inspirals, where a stellar
mass compact object inspirals into a black hole which is a few hundred to a few
thousand times more massive. In this article, we investigate some of the data
analysis challenges for the Einstein Telescope such as the effects of increased
source number, the need for more accurate waveform models and the some of the
computational issues that a data analysis strategy might face.Comment: 18 pages, Invited review for Einstein Telescope special edition of
GR
The Mock LISA Data Challenges: from Challenge 3 to Challenge 4
The Mock LISA Data Challenges are a program to demonstrate LISA data-analysis
capabilities and to encourage their development. Each round of challenges
consists of one or more datasets containing simulated instrument noise and
gravitational waves from sources of undisclosed parameters. Participants
analyze the datasets and report best-fit solutions for the source parameters.
Here we present the results of the third challenge, issued in Apr 2008, which
demonstrated the positive recovery of signals from chirping Galactic binaries,
from spinning supermassive--black-hole binaries (with optimal SNRs between ~ 10
and 2000), from simultaneous extreme-mass-ratio inspirals (SNRs of 10-50), from
cosmic-string-cusp bursts (SNRs of 10-100), and from a relatively loud
isotropic background with Omega_gw(f) ~ 10^-11, slightly below the LISA
instrument noise.Comment: 12 pages, 2 figures, proceedings of the 8th Edoardo Amaldi Conference
on Gravitational Waves, New York, June 21-26, 200
First narrow-band search for continuous gravitational waves from known pulsars in advanced detector data
Spinning neutron stars asymmetric with respect to their rotation axis are potential sources of
continuous gravitational waves for ground-based interferometric detectors. In the case of known pulsars a
fully coherent search, based on matched filtering, which uses the position and rotational parameters
obtained from electromagnetic observations, can be carried out. Matched filtering maximizes the signalto-
noise (SNR) ratio, but a large sensitivity loss is expected in case of even a very small mismatch
between the assumed and the true signal parameters. For this reason, narrow-band analysis methods have
been developed, allowing a fully coherent search for gravitational waves from known pulsars over a
fraction of a hertz and several spin-down values. In this paper we describe a narrow-band search of
11 pulsars using data from Advanced LIGO’s first observing run. Although we have found several initial
outliers, further studies show no significant evidence for the presence of a gravitational wave signal.
Finally, we have placed upper limits on the signal strain amplitude lower than the spin-down limit for 5 of
the 11 targets over the bands searched; in the case of J1813-1749 the spin-down limit has been beaten for
the first time. For an additional 3 targets, the median upper limit across the search bands is below the
spin-down limit. This is the most sensitive narrow-band search for continuous gravitational waves carried
out so far
Searching for continuous Gravitational Waves in the second data release of the International Pulsar Timing Array
The International Pulsar Timing Array 2nd data release is the combination of
datasets from worldwide collaborations. In this study, we search for continuous
waves: gravitational wave signals produced by individual supermassive black
hole binaries in the local universe. We consider binaries on circular orbits
and neglect the evolution of orbital frequency over the observational span. We
find no evidence for such signals and set sky averaged 95% upper limits on
their amplitude h 95 . The most sensitive frequency is 10nHz with h 95 = 9.1
10-15 . We achieved the best upper limit to date at low and high frequencies of
the PTA band thanks to improved effective cadence of observations. In our
analysis, we have taken into account the recently discovered common red noise
process, which has an impact at low frequencies. We also find that the peculiar
noise features present in some pulsars data must be taken into account to
reduce the false alarm. We show that using custom noise models is essential in
searching for continuous gravitational wave signals and setting the upper
limit
Comparing Recent Pulsar Timing Array Results on the Nanohertz Stochastic Gravitational-wave Background
The Australian, Chinese, European, Indian, and North American pulsar timing array (PTA) collaborations recently reported, at varying levels, evidence for the presence of a nanohertz gravitational-wave background (GWB). Given that each PTA made different choices in modeling their data, we perform a comparison of the GWB and individual pulsar noise parameters across the results reported from the PTAs that constitute the International Pulsar Timing Array (IPTA). We show that despite making different modeling choices, there is no significant difference in the GWB parameters that are measured by the different PTAs, agreeing within 1σ. The pulsar noise parameters are also consistent between different PTAs for the majority of the pulsars included in these analyses. We bridge the differences in modeling choices by adopting a standardized noise model for all pulsars and PTAs, finding that under this model there is a reduction in the tension in the pulsar noise parameters. As part of this reanalysis, we "extended" each PTA's data set by adding extra pulsars that were not timed by that PTA. Under these extensions, we find better constraints on the GWB amplitude and a higher signal-to-noise ratio for the Hellings–Downs correlations. These extensions serve as a prelude to the benefits offered by a full combination of data across all pulsars in the IPTA, i.e., the IPTA's Data Release 3, which will involve not just adding in additional pulsars but also including data from all three PTAs where any given pulsar is timed by more than a single PTA
Comparing recent PTA results on the nanohertz stochastic gravitational wave background
The Australian, Chinese, European, Indian, and North American pulsar timing
array (PTA) collaborations recently reported, at varying levels, evidence for
the presence of a nanohertz gravitational wave background (GWB). Given that
each PTA made different choices in modeling their data, we perform a comparison
of the GWB and individual pulsar noise parameters across the results reported
from the PTAs that constitute the International Pulsar Timing Array (IPTA). We
show that despite making different modeling choices, there is no significant
difference in the GWB parameters that are measured by the different PTAs,
agreeing within . The pulsar noise parameters are also consistent
between different PTAs for the majority of the pulsars included in these
analyses. We bridge the differences in modeling choices by adopting a
standardized noise model for all pulsars and PTAs, finding that under this
model there is a reduction in the tension in the pulsar noise parameters. As
part of this reanalysis, we "extended" each PTA's data set by adding extra
pulsars that were not timed by that PTA. Under these extensions, we find better
constraints on the GWB amplitude and a higher signal-to-noise ratio for the
Hellings and Downs correlations. These extensions serve as a prelude to the
benefits offered by a full combination of data across all pulsars in the IPTA,
i.e., the IPTA's Data Release 3, which will involve not just adding in
additional pulsars, but also including data from all three PTAs where any given
pulsar is timed by more than as single PTA.Comment: 21 pages, 9 figures, submitted to Ap
The NANOGrav 15 yr Data Set: Search for Transverse Polarization Modes in the Gravitational-wave Background
\ua9 2024. The Author(s). Published by the American Astronomical Society.Recently we found compelling evidence for a gravitational-wave background with Hellings and Downs (HD) correlations in our 15 yr data set. These correlations describe gravitational waves as predicted by general relativity, which has two transverse polarization modes. However, more general metric theories of gravity can have additional polarization modes, which produce different interpulsar correlations. In this work, we search the NANOGrav 15 yr data set for evidence of a gravitational-wave background with quadrupolar HD and scalar-transverse (ST) correlations. We find that HD correlations are the best fit to the data and no significant evidence in favor of ST correlations. While Bayes factors show strong evidence for a correlated signal, the data does not strongly prefer either correlation signature, with Bayes factors ∼2 when comparing HD to ST correlations, and ∼1 for HD plus ST correlations to HD correlations alone. However, when modeled alongside HD correlations, the amplitude and spectral index posteriors for ST correlations are uninformative, with the HD process accounting for the vast majority of the total signal. Using the optimal statistic, a frequentist technique that focuses on the pulsar-pair cross-correlations, we find median signal-to-noise ratios of 5.0 for HD and 4.6 for ST correlations when fit for separately, and median signal-to-noise ratios of 3.5 for HD and 3.0 for ST correlations when fit for simultaneously. While the signal-to-noise ratios for each of the correlations are comparable, the estimated amplitude and spectral index for HD are a significantly better fit to the total signal, in agreement with our Bayesian analysis
How to Detect an Astrophysical Nanohertz Gravitational Wave Background
\ua9 2023. The Author(s). Published by the American Astronomical Society.Analyses of pulsar timing data have provided evidence for a stochastic gravitational wave background in the nanohertz frequency band. The most plausible source of this background is the superposition of signals from millions of supermassive black hole binaries. The standard statistical techniques used to search for this background and assess its significance make several simplifying assumptions, namely (i) Gaussianity, (ii) isotropy, and most often, (iii) a power-law spectrum. However, a stochastic background from a finite collection of binaries does not exactly satisfy any of these assumptions. To understand the effect of these assumptions, we test standard analysis techniques on a large collection of realistic simulated data sets. The data-set length, observing schedule, and noise levels were chosen to emulate the NANOGrav 15 yr data set. Simulated signals from millions of binaries drawn from models based on the Illustris cosmological hydrodynamical simulation were added to the data. We find that the standard statistical methods perform remarkably well on these simulated data sets, even though their fundamental assumptions are not strictly met. They are able to achieve a confident detection of the background. However, even for a fixed set of astrophysical parameters, different realizations of the universe result in a large variance in the significance and recovered parameters of the background. We also find that the presence of loud individual binaries can bias the spectral recovery of the background if we do not account for them
Effects of Data Quality Vetoes on a Search for Compact Binary Coalescences in Advanced LIGO's First Observing Run
The first observing run of Advanced LIGO spanned 4 months, from September 12,
2015 to January 19, 2016, during which gravitational waves were directly
detected from two binary black hole systems, namely GW150914 and GW151226.
Confident detection of gravitational waves requires an understanding of
instrumental transients and artifacts that can reduce the sensitivity of a
search. Studies of the quality of the detector data yield insights into the
cause of instrumental artifacts and data quality vetoes specific to a search
are produced to mitigate the effects of problematic data. In this paper, the
systematic removal of noisy data from analysis time is shown to improve the
sensitivity of searches for compact binary coalescences. The output of the
PyCBC pipeline, which is a python-based code package used to search for
gravitational wave signals from compact binary coalescences, is used as a
metric for improvement. GW150914 was a loud enough signal that removing noisy
data did not improve its significance. However, the removal of data with excess
noise decreased the false alarm rate of GW151226 by more than two orders of
magnitude, from 1 in 770 years to less than 1 in 186000 years.Comment: 27 pages, 13 figures, published versio
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