75 research outputs found
Rapidly rotating neutron star progenitors
Rotating proto-neutron stars can be important sources of gravitational waves
to be searched for by present-day and future interferometric detectors. It was
demonstrated by Imshennik that in extreme cases the rapid rotation of a
collapsing stellar core may lead to fission and formation of a binary
proto-neutron star which subsequently merges due to gravitational wave
emission. In the present paper, we show that such dynamically unstable
collapsing stellar cores may be the product of a former merger process of two
stellar cores in a common envelope. We applied population synthesis
calculations to assess the expected fraction of such rapidly rotating stellar
cores which may lead to fission and formation of a pair of proto-neutron stars.
We have used the BSE population synthesis code supplemented with a new
treatment of stellar core rotation during the evolution via effective
core-envelope coupling, characterized by the coupling time, . The
validity of this approach is checked by direct MESA calculations of the
evolution of a rotating 15 star. From comparison of the calculated
spin distribution of young neutron stars with the observed one, reported by
Popov and Turolla, we infer the value years. We
show that merging of stellar cores in common envelopes can lead to collapses
with dynamically unstable proto-neutron stars, with their formation rate being
of the total core collapses, depending on the common envelope
efficiency.Comment: 10 pages, 4 figures, accepted for publication in MNRA
The second data release from the European Pulsar Timing Array:IV. Implications for massive black holes, dark matter, and the early Universe
The European Pulsar Timing Array (EPTA) and Indian Pulsar Timing Array (InPTA) collaborations have measured a low-frequency common signal in the combination of their second and first data releases, respectively, with the correlation properties of a gravitational wave background (GWB). Such a signal may have its origin in a number of physical processes including a cosmic population of inspiralling supermassive black hole binaries (SMBHBs); inflation, phase transitions, cosmic strings, and tensor mode generation by the non-linear evolution of scalar perturbations in the early Universe; and oscillations of the Galactic potential in the presence of ultra-light dark matter (ULDM). At the current stage of emerging evidence, it is impossible to discriminate among the different origins. Therefore, for this paper, we consider each process separately, and investigated the implications of the signal under the hypothesis that it is generated by that specific process. We find that the signal is consistent with a cosmic population of inspiralling SMBHBs, and its relatively high amplitude can be used to place constraints on binary merger timescales and the SMBH-host galaxy scaling relations. If this origin is confirmed, this would be the first direct evidence that SMBHBs merge in nature, adding an important observational piece to the puzzle of structure formation and galaxy evolution. As for early Universe processes, the measurement would place tight constraints on the cosmic string tension and on the level of turbulence developed by first-order phase transitions. Other processes would require non-standard scenarios, such as a blue-tilted inflationary spectrum or an excess in the primordial spectrum of scalar perturbations at large wavenumbers. Finally, a ULDM origin of the detected signal is disfavoured, which leads to direct constraints on the abundance of ULDM in our Galaxy
The second data release from the European Pulsar Timing Array III. Search for gravitational wave signals
We present the results of the search for an isotropic stochastic gravitational wave background (GWB) at nanohertz frequencies using the second data release of the European Pulsar Timing Array (EPTA) for 25 millisecond pulsars and a combination with the first data release of the Indian Pulsar Timing Array (InPTA). A robust GWB detection is conditioned upon resolving the Hellings-Downs angular pattern in the pairwise cross-correlation of the pulsar timing residuals. Additionally, the GWB is expected to yield the same (common) spectrum of temporal correlations across pulsars, which is used as a null hypothesis in the GWB search. Such a common-spectrum process has already been observed in pulsar timing data. We analysed (i) the full 24.7-year EPTA data set, (ii) its 10.3-year subset based on modern observing systems, (iii) the combination of the full data set with the first data release of the InPTA for ten commonly timed millisecond pulsars, and (iv) the combination of the 10.3-year subset with the InPTA data. These combinations allowed us to probe the contributions of instrumental noise and interstellar propagation effects. With the full data set, we find marginal evidence for a GWB, with a Bayes factor of four and a false alarm probability of 4%. With the 10.3-year subset, we report evidence for a GWB, with a Bayes factor of 60 and a false alarm probability of about 0.1% (≳3σ significance). The addition of the InPTA data yields results that are broadly consistent with the EPTA-only data sets, with the benefit of better noise modelling. Analyses were performed with different data processing pipelines to test the consistency of the results from independent software packages. The latest EPTA data from new generation observing systems show non-negligible evidence for the GWB. At the same time, the inferred spectrum is rather uncertain and in mild tension with the common signal measured in the full data set. However, if the spectral index is fixed at 13/3, the two data sets give a similar amplitude of (2.5 ± 0.7) × 10−15 at a reference frequency of 1 yr−1. Further investigation of these issues is required for reliable astrophysical interpretations of this signal. By continuing our detection efforts as part of the International Pulsar Timing Array (IPTA), we expect to be able to improve the measurement of spatial correlations and better characterise this signal in the coming years
The second data release from the European Pulsar Timing Array IV. Search for continuous gravitational wave signals
We present the results of a search for continuous gravitational wave signals
(CGWs) in the second data release (DR2) of the European Pulsar Timing Array
(EPTA) collaboration. The most significant candidate event from this search has
a gravitational wave frequency of 4-5 nHz. Such a signal could be generated by
a supermassive black hole binary (SMBHB) in the local Universe. We present the
results of a follow-up analysis of this candidate using both Bayesian and
frequentist methods. The Bayesian analysis gives a Bayes factor of 4 in favor
of the presence of the CGW over a common uncorrelated noise process, while the
frequentist analysis estimates the p-value of the candidate to be 1%, also
assuming the presence of common uncorrelated red noise. However, comparing a
model that includes both a CGW and a gravitational wave background (GWB) to a
GWB only, the Bayes factor in favour of the CGW model is only 0.7. Therefore,
we cannot conclusively determine the origin of the observed feature, but we
cannot rule it out as a CGW source. We present results of simulations that
demonstrate that data containing a weak gravitational wave background can be
misinterpreted as data including a CGW and vice versa, providing two plausible
explanations of the EPTA DR2 data. Further investigations combining data from
all PTA collaborations will be needed to reveal the true origin of this
feature.Comment: 12 figures, 15 pages, to be submitte
The second data release from the European Pulsar Timing Array: II. Customised pulsar noise models for spatially correlated gravitational waves
Aims. The nanohertz gravitational wave background (GWB) is expected to be an aggregate signal of an ensemble of gravitational waves emitted predominantly by a large population of coalescing supermassive black hole binaries in the centres of merging galaxies. Pulsar timing arrays (PTAs), which are ensembles of extremely stable pulsars at approximately kiloparsec distances precisely monitored for decades, are the most precise experiments capable of detecting this background. However, the subtle imprints that the GWB induces on pulsar timing data are obscured by many sources of noise that occur on various timescales. These must be carefully modelled and mitigated to increase the sensitivity to the background signal.Methods. In this paper, we present a novel technique to estimate the optimal number of frequency coefficients for modelling achromatic and chromatic noise, while selecting the preferred set of noise models to use for each pulsar. We also incorporated a new model to fit for scattering variations in the Bayesian pulsar timing package temponest. These customised noise models enable a more robust characterisation of single-pulsar noise. We developed a software package based on tempo2 to create realistic simulations of European Pulsar Timing Array (EPTA) datasets that allowed us to test the efficacy of our noise modelling algorithms.Results. Using these techniques, we present an in-depth analysis of the noise properties of 25 millisecond pulsars (MSPs) that form the second data release (DR2) of the EPTA and investigate the effect of incorporating low-frequency data from the Indian Pulsar Timing Array collaboration for a common sample of ten MSPs. We used two packages, enterprise and temponest, to estimate our noise models and compare them with those reported using EPTA DR1. We find that, while in some pulsars we can successfully disentangle chromatic from achromatic noise owing to the wider frequency coverage in DR2, in others the noise models evolve in a much more complicated way. We also find evidence of long-term scattering variations in PSR J1600-3053. Through our simulations, we identify intrinsic biases in our current noise analysis techniques and discuss their effect on GWB searches. The analysis and results discussed in this article directly help to improve the sensitivity to the GWB signal and they are already being used as part of global PTA efforts
The second data release from the European Pulsar Timing Array I. The dataset and timing analysis
Pulsar timing arrays offer a probe of the low-frequency gravitational wave
spectrum (1 - 100 nanohertz), which is intimately connected to a number of
markers that can uniquely trace the formation and evolution of the Universe. We
present the dataset and the results of the timing analysis from the second data
release of the European Pulsar Timing Array (EPTA). The dataset contains
high-precision pulsar timing data from 25 millisecond pulsars collected with
the five largest radio telescopes in Europe, as well as the Large European
Array for Pulsars. The dataset forms the foundation for the search for
gravitational waves by the EPTA, presented in associated papers. We describe
the dataset and present the results of the frequentist and Bayesian pulsar
timing analysis for individual millisecond pulsars that have been observed over
the last ~25 years. We discuss the improvements to the individual pulsar
parameter estimates, as well as new measurements of the physical properties of
these pulsars and their companions. This data release extends the dataset from
EPTA Data Release 1 up to the beginning of 2021, with individual pulsar
datasets with timespans ranging from 14 to 25 years. These lead to improved
constraints on annual parallaxes, secular variation of the orbital period, and
Shapiro delay for a number of sources. Based on these results, we derived
astrophysical parameters that include distances, transverse velocities, binary
pulsar masses, and annual orbital parallaxes.Comment: 29 pages, 9 figures, 13 tables, Astronomy & Astrophysics in pres
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