21 research outputs found
A repeating fast radio burst associated with a persistent radio source
The dispersive sweep of fast radio bursts (FRBs) has been used to probe the ionized baryon content of the intergalactic medium1, which is assumed to dominate the total extragalactic dispersion. Although the host-galaxy contributions to the dispersion measure appear to be small for most FRBs2, in at least one case there is evidence for an extreme magneto-ionic local environment3,4 and a compact persistent radio source5. Here we report the detection and localization of the repeating FRB 20190520B, which is co-located with a compact, persistent radio source and associated with a dwarf host galaxy of high specific-star-formation rate at a redshift of 0.241 ± 0.001. The estimated host-galaxy dispersion measure of approximately 903−111+72 parsecs per cubic centimetre, which is nearly an order of magnitude higher than the average of FRB host galaxies2,6, far exceeds the dispersion-measure contribution of the intergalactic medium. Caution is thus warranted in inferring redshifts for FRBs without accurate host-galaxy identifications
The NANOGrav 15-year Data Set: Search for Anisotropy in the Gravitational-Wave Background
The North American Nanohertz Observatory for Gravitational Waves (NANOGrav)
has reported evidence for the presence of an isotropic nanohertz gravitational
wave background (GWB) in its 15 yr dataset. However, if the GWB is produced by
a population of inspiraling supermassive black hole binary (SMBHB) systems,
then the background is predicted to be anisotropic, depending on the
distribution of these systems in the local Universe and the statistical
properties of the SMBHB population. In this work, we search for anisotropy in
the GWB using multiple methods and bases to describe the distribution of the
GWB power on the sky. We do not find significant evidence of anisotropy, and
place a Bayesian upper limit on the level of broadband anisotropy such
that . We also derive conservative estimates on the
anisotropy expected from a random distribution of SMBHB systems using
astrophysical simulations conditioned on the isotropic GWB inferred in the
15-yr dataset, and show that this dataset has sufficient sensitivity to probe a
large fraction of the predicted level of anisotropy. We end by highlighting the
opportunities and challenges in searching for anisotropy in pulsar timing array
data.Comment: 19 pages, 11 figures; submitted to Astrophysical Journal Letters as
part of Focus on NANOGrav's 15-year Data Set and the Gravitational Wave
Background. For questions or comments, please email [email protected]
The NANOGrav 12.5 yr Data Set: A Computationally Efficient Eccentric Binary Search Pipeline and Constraints on an Eccentric Supermassive Binary Candidate in 3C 66B
The radio galaxy 3C 66B has been hypothesized to host a supermassive black hole binary (SMBHB) at its center based on electromagnetic observations. Its apparent 1.05 yr period and low redshift (∼0.02) make it an interesting testbed to search for low-frequency gravitational waves (GWs) using pulsar timing array (PTA) experiments. This source has been subjected to multiple searches for continuous GWs from a circular SMBHB, resulting in progressively more stringent constraints on its GW amplitude and chirp mass. In this paper, we develop a pipeline for performing Bayesian targeted searches for eccentric SMBHBs in PTA data sets, and test its efficacy by applying it to simulated data sets with varying injected signal strengths. We also search for a realistic eccentric SMBHB source in 3C 66B using the NANOGrav 12.5 yr data set employing PTA signal models containing Earth term-only as well as Earth+pulsar term contributions using this pipeline. Due to limitations in our PTA signal model, we get meaningful results only when the initial eccentricity e 0 < 0.5 and the symmetric mass ratio η > 0.1. We find no evidence for an eccentric SMBHB signal in our data, and therefore place 95% upper limits on the PTA signal amplitude of 88.1 ± 3.7 ns for the Earth term-only and 81.74 ± 0.86 ns for the Earth+pulsar term searches for e 0 < 0.5 and η > 0.1. Similar 95% upper limits on the chirp mass are (1.98 ± 0.05) × 109 and (1.81 ± 0.01) × 109 M ☉. These upper limits, while less stringent than those calculated from a circular binary search in the NANOGrav 12.5 yr data set, are consistent with the SMBHB model of 3C 66B developed from electromagnetic observations
The NANOGrav 15-Year Data Set: Detector Characterization and Noise Budget
Pulsar timing arrays (PTAs) are galactic-scale gravitational wave detectors.
Each individual arm, composed of a millisecond pulsar, a radio telescope, and a
kiloparsecs-long path, differs in its properties but, in aggregate, can be used
to extract low-frequency gravitational wave (GW) signals. We present a noise
and sensitivity analysis to accompany the NANOGrav 15-year data release and
associated papers, along with an in-depth introduction to PTA noise models. As
a first step in our analysis, we characterize each individual pulsar data set
with three types of white noise parameters and two red noise parameters. These
parameters, along with the timing model and, particularly, a piecewise-constant
model for the time-variable dispersion measure, determine the sensitivity curve
over the low-frequency GW band we are searching. We tabulate information for
all of the pulsars in this data release and present some representative
sensitivity curves. We then combine the individual pulsar sensitivities using a
signal-to-noise-ratio statistic to calculate the global sensitivity of the PTA
to a stochastic background of GWs, obtaining a minimum noise characteristic
strain of at 5 nHz. A power law-integrated analysis shows
rough agreement with the amplitudes recovered in NANOGrav's 15-year GW
background analysis. While our phenomenological noise model does not model all
known physical effects explicitly, it provides an accurate characterization of
the noise in the data while preserving sensitivity to multiple classes of GW
signals.Comment: 67 pages, 73 figures, 3 tables; published in Astrophysical Journal
Letters as part of Focus on NANOGrav's 15-year Data Set and the Gravitational
Wave Background. For questions or comments, please email
[email protected]
The NANOGrav 12.5 yr Data Set: Search for Gravitational Wave Memory
We present the results of a Bayesian search for gravitational wave (GW) memory in the NANOGrav 12.5 yr data set. We find no convincing evidence for any gravitational wave memory signals in this data set. We find a Bayes factor of 2.8 in favor of a model that includes a memory signal and common spatially uncorrelated red noise (CURN) compared to a model including only a CURN. However, further investigation shows that a disproportionate amount of support for the memory signal comes from three dubious pulsars. Using a more flexible red-noise model in these pulsars reduces the Bayes factor to 1.3. Having found no compelling evidence, we go on to place upper limits on the strain amplitude of GW memory events as a function of sky location and event epoch. These upper limits are computed using a signal model that assumes the existence of a common, spatially uncorrelated red noise in addition to a GW memory signal. The median strain upper limit as a function of sky position is approximately 3.3 × 10−14. We also find that there are some differences in the upper limits as a function of sky position centered around PSR J0613−0200. This suggests that this pulsar has some excess noise that can be confounded with GW memory. Finally, the upper limits as a function of burst epoch continue to improve at later epochs. This improvement is attributable to the continued growth of the pulsar timing array
The NANOGrav 15-year Data Set: Observations and Timing of 68 Millisecond Pulsars
We present observations and timing analyses of 68 millisecond pulsars (MSPs)
comprising the 15-year data set of the North American Nanohertz Observatory for
Gravitational Waves (NANOGrav). NANOGrav is a pulsar timing array (PTA)
experiment that is sensitive to low-frequency gravitational waves. This is
NANOGrav's fifth public data release, including both "narrowband" and
"wideband" time-of-arrival (TOA) measurements and corresponding pulsar timing
models. We have added 21 MSPs and extended our timing baselines by three years,
now spanning nearly 16 years for some of our sources. The data were collected
using the Arecibo Observatory, the Green Bank Telescope, and the Very Large
Array between frequencies of 327 MHz and 3 GHz, with most sources observed
approximately monthly. A number of notable methodological and procedural
changes were made compared to our previous data sets. These improve the overall
quality of the TOA data set and are part of the transition to new pulsar timing
and PTA analysis software packages. For the first time, our data products are
accompanied by a full suite of software to reproduce data reduction, analysis,
and results. Our timing models include a variety of newly detected astrometric
and binary pulsar parameters, including several significant improvements to
pulsar mass constraints. We find that the time series of 23 pulsars contain
detectable levels of red noise, 10 of which are new measurements. In this data
set, we find evidence for a stochastic gravitational-wave background.Comment: 90 pages, 74 figures, 6 tables; published in Astrophysical Journal
Letters as part of Focus on NANOGrav's 15-year Data Set and the Gravitational
Wave Background. For questions or comments, please email
[email protected]
How to Detect an Astrophysical Nanohertz Gravitational-Wave Background
Analysis of pulsar timing data have provided evidence for a stochastic
gravitational wave background in the nHz frequency band. The most plausible
source of such a background is the superposition of signals from millions of
supermassive black hole binaries. The standard statistical techniques used to
search for such a 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 datasets. The dataset
length, observing schedule, and noise levels were chosen to emulate the
NANOGrav 15-year dataset. 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 datasets, despite their fundamental
assumptions not being 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.Comment: 14 pages, 8 figure
The NANOGrav 15 yr Data Set: Search for Transverse Polarization Modes in the Gravitational-wave Background
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
The NANOGrav 15-year data set: Search for Transverse Polarization Modes in the Gravitational-Wave Background
Recently we found compelling evidence for a gravitational wave background
with Hellings and Downs (HD) correlations in our 15-year 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-year
data set for evidence of a gravitational wave background with quadrupolar
Hellings and Downs (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 when comparing HD to ST correlations,
and 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.Comment: 11 pages, 5 figure
The NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background
We report multiple lines of evidence for a stochastic signal that is
correlated among 67 pulsars from the 15-year pulsar-timing data set collected
by the North American Nanohertz Observatory for Gravitational Waves. The
correlations follow the Hellings-Downs pattern expected for a stochastic
gravitational-wave background. The presence of such a gravitational-wave
background with a power-law-spectrum is favored over a model with only
independent pulsar noises with a Bayes factor in excess of , and this
same model is favored over an uncorrelated common power-law-spectrum model with
Bayes factors of 200-1000, depending on spectral modeling choices. We have
built a statistical background distribution for these latter Bayes factors
using a method that removes inter-pulsar correlations from our data set,
finding (approx. ) for the observed Bayes factors in the
null no-correlation scenario. A frequentist test statistic built directly as a
weighted sum of inter-pulsar correlations yields (approx. ). Assuming a fiducial
characteristic-strain spectrum, as appropriate for an ensemble of binary
supermassive black-hole inspirals, the strain amplitude is (median + 90% credible interval) at a reference frequency of
1/(1 yr). The inferred gravitational-wave background amplitude and spectrum are
consistent with astrophysical expectations for a signal from a population of
supermassive black-hole binaries, although more exotic cosmological and
astrophysical sources cannot be excluded. The observation of Hellings-Downs
correlations points to the gravitational-wave origin of this signal.Comment: 30 pages, 18 figures. Published in Astrophysical Journal Letters as
part of Focus on NANOGrav's 15-year Data Set and the Gravitational Wave
Background. For questions or comments, please email [email protected]