Reducing the number of templates for aligned-spin compact binary coalescence gravitational wave searches using metric-agnostic template nudging

Efficient multi-dimensional template placement is crucial in computationally intensive matched-filtering searches for Gravitational Waves (GWs). Here, we implement the Neighboring Cell Algorithm (NCA) to improve the detection volume of an existing Compact Binary Coalescence (CBC) template bank. This algorithm has already been successfully applied for a binary millisecond pulsar search in data from the Fermi satellite. It repositions templates from over-dense regions to under-dense regions and reduces the number of templates that would have been required by a stochastic method to achieve the same detection volume. Our method is readily generalizable to other CBC parameter spaces. Here we apply this method to the aligned--single-spin neutron-star--black-hole binary coalescence inspiral-merger-ringdown gravitational wave parameter space. We show that the template nudging algorithm can attain the equivalent effectualness of the stochastic method with 12% fewer templates

A Spectral Approach to the Relativistic Inverse Stellar Structure Problem

A new method for solving the relativistic inverse stellar structure problem is presented. This method determines a spectral representation of the unknown high density portion of the stellar equation of state from a knowledge of the total masses M and radii R of the stars. Spectral representations of the equation of state are very efficient, generally requiring only a few spectral parameters to achieve good accuracy. This new method is able, therefore, to determine the high density equation of state quite accurately from only a few accurately measured [M,R] data points. This method is tested here by determining the equations of state from mock [M,R] data computed from tabulated "realistic" neutron-star equations of state. The spectral equations of state obtained from these mock data are shown to agree on average with the originals to within a few percent (over the entire high density range of the neutron-star interior) using only two [M,R] data points. Higher accuracies are achieved when more data are used. The accuracies of the equations of state determined in these examples are shown to be nearly optimal, in the sense that their errors are comparable to the errors of the best-fit spectral representations of these realistic equations of state.Comment: 12 pages; 1 table; v2 minor changes to version accepted in Phys. Rev.

Spectral approach to the relativistic inverse stellar structure problem II

The inverse stellar structure problem determines the equation of state of the matter in stars from a knowledge of their macroscopic observables (e.g. their masses and radii). This problem was solved in a previous paper by constructing a spectral representation of the equation of state whose stellar models match a prescribed set of macroscopic observables. This paper improves and extends that work in two significant ways. (i) The method is made more robust by accounting for an unexpected feature of the enthalpy-based representations of the equations of state used in this work. After making the appropriate modifications, accurate initial guesses for the spectral parameters are no longer needed, so Monte Carlo techniques can now be used to ensure the best fit to the observables. (ii) The method is extended here to use masses and tidal deformabilities (which will be measured by gravitational wave observations of neutron-star mergers) as the macroscopic observables instead of masses and radii. The accuracy and reliability of this extended and more robust spectral method is evaluated in this paper using mock data for observables from stars based on 34 different theoretical models of the high-density neutron-star equation of state. In qualitative agreement with earlier work, these tests suggest the high-density part of the neutron-star equation of state could be determined at the few-percent accuracy level using high-quality measurements of the masses and radii (or masses and tidal deformabilities) of just two or three neutron stars

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

Optimal template placement for searches of gravitational waves from precessing compact binary coalescences

A new field of astronomy was opened up on Monday September 14th 2015, when the first detection of a Gravitational Wave was observed, GW150914. So far there have been no observations of gravitational waves produced from neutron-star--black-hole mergers. The observation of a neutron-star--black-hole merger would be significant because it would provide another way to study how compact binary coalescence gravitational wave signals relate to companion electromagnetic and neutrino emission signals. The difficulty in detecting precessing neutron-star--black-hole binaries (systems that allow for the full range of physical spin configurations) is that template banks for these precessing compact binary coalescence systems require many more templates than compact binary coalescence template banks that have been implemented previously. The purpose of the work presented in this thesis is to both improve the construction and efficiency of compact binary coalescence template banks. Template bank construction in the high dimensional precessing compact binary coalescence parameter spaces is complicated by the absence of a known analytic expression of mismatch inspiral-merger-ringdown metric. Previously, these restraints only permitted the use of the inefficient and slow to converge stochastic template placement algorithms. In this thesis, I constructed a template bank, the face-on-precessing template bank, over a subspace of this precessing neutron-star--black-hole template bank parameter space. I accomplished this by implementing a new algorithm for speeding up the convergence of the stochastic placement of these templates. I found that this subspace required 53 times more templates than the aligned-spin bank. Additionally, I developed an alternative template placement algorithm, the template nudging algorithm, to reposition compact binary coalescence templates into more effectual configurations in order to eliminate the effect of gridlines, artificially dense regions of the bank, that are characteristic of the hybrid template bank construction methods previously implemented in LIGO-Virgo compact binary coalescence searches. Finally, I developed a technique for constructing flat coordinates for compact binary coalescence template placements in high dimensional parameter spaces to ease in the construction of a fully precessing compact binary coalescence template bank

A Spectral Approach to the Relativistic Inverse Stellar Structure Problem

We present a new method for solving the inverse stellar structure problem, which determines an expression for the high density range of the neutron star equation of state (EOS) based on observations of total masses, M, and radii, R of these stars. This approach determines spectral representations of the EOS that are very accurate and require only a few spectral parameters to converge. This method can determine the underlying high density EOS from just a few mass-radii observations {Mi, Ri}. While accurate mass-radii data are not available yet, we tested the accuracy of this method to determine the EOS from a set of {Mi, Ri} values computed from two tabulated theoretical EOS. When applied to the PAL6 [ 6] and MS 1 [7] tabulated EOS, this method converged to the original EOS to within a few percent using {Mi, Ri} data from only two stellar models

Search for intermediate mass black hole binaries in the first observing run of Advanced LIGO

International audienceDuring their first observational run, the two Advanced LIGO detectors attained an unprecedented sensitivity, resulting in the first direct detections of gravitational-wave signals produced by stellar-mass binary black hole systems. This paper reports on an all-sky search for gravitational waves (GWs) from merging intermediate mass black hole binaries (IMBHBs). The combined results from two independent search techniques were used in this study: the first employs a matched-filter algorithm that uses a bank of filters covering the GW signal parameter space, while the second is a generic search for GW transients (bursts). No GWs from IMBHBs were detected; therefore, we constrain the rate of several classes of IMBHB mergers. The most stringent limit is obtained for black holes of individual mass 100  M⊙, with spins aligned with the binary orbital angular momentum. For such systems, the merger rate is constrained to be less than 0.93  Gpc−3 yr−1 in comoving units at the 90% confidence level, an improvement of nearly 2 orders of magnitude over previous upper limits