Recent observations of short gamma-ray bursts (SGRBs) suggest that binary neutron star (NS) mergers can create highly magnetized, millisecond NSs. Sharp cut-offs in X-ray afterglow plateaus of some ...SGRBs hint at the gravitational collapse of these remnant NSs to black holes. The collapse of such ‘supramassive’ NSs also describes the blitzar model, a leading candidate for the progenitors of fast radio bursts (FRBs). The observation of an FRB associated with an SGRB would provide compelling evidence for the blitzar model and the binary NS merger scenario of SGRBs, and lead to interesting constraints on the NS equation of state. We predict the collapse times of supramassive NSs created in binary NS mergers, finding that such stars collapse ∼10–4.4 × 104 s (95 per cent confidence) after the merger. This directly impacts observations targeting NS remnants of binary NS mergers, providing the optimal window for high time resolution radio and X-ray follow-up of SGRBs and gravitational wave bursts.
We present timing models for 20 millisecond pulsars in the Parkes Pulsar Timing Array. The precision of the parameter measurements in these models has been improved over earlier results by using ...longer data sets and modelling the non-stationary noise. We describe a new noise modelling procedure and demonstrate its effectiveness using simulated data. Our methodology includes the addition of annual dispersion measure (DM) variations to the timing models of some pulsars. We present the first significant parallax measurements for PSRs J1024−0719, J1045−4509, J1600−3053, J1603−7202, and J1730−2304, as well as the first significant measurements of some post-Keplerian orbital parameters in six binary pulsars, caused by kinematic effects. Improved Shapiro delay measurements have resulted in much improved pulsar mass measurements, particularly for PSRs J0437−4715 and J1909−3744 with M
p = 1.44 ± 0.07 and 1.47 ± 0.03 M⊙, respectively. The improved orbital period-derivative measurement for PSR J0437−4715 results in a derived distance measurement at the 0.16 per cent level of precision, D = 156.79 ± 0.25 pc, one of the most fractionally precise distance measurements of any star to date.
Neutron stars spin down over time due to a number of energy-loss processes. We provide tantalizing population-based evidence that millisecond pulsars (MSPs) have a minimum ellipticity of ϵ 10−9 ...around their spin axis and that, consequently, some spin down mostly through gravitational-wave emission. We discuss the implications of such a minimum ellipticity in terms of the internal magnetic field strengths and nuclear matter composition of neutron stars and show it would result in the Advanced LIGO and Virgo gravitational-wave detectors, or their upgrades, detecting gravitational waves from some known MSPs in the near future.
Gravitational waves are expected to be radiated by supermassive black hole binaries formed during galaxy mergers. A stochastic superposition of gravitational waves from all such binary systems would ...modulate the arrival times of pulses from radio pulsars. Using observations of millisecond pulsars obtained with the Parkes radio telescope, we constrained the characteristic amplitude of this background, Ac,yr, to be <1.0 × 10–15 with 95% confidence. This limit excludes predicted ranges for Ac,yr from current models with 91 to 99.7% probability. We conclude that binary evolution is either stalled or dramatically accelerated by galactic-center environments and that higher-cadence and shorter-wavelength observations would be more sensitive to gravitational waves.
Rapidly rotating neutron stars (NSs) in low-mass X-ray binaries (LMXBs) are thought to be interesting sources of gravitational waves (GWs) for current and next generation ground-based detectors, such ...as Advanced LIGO and the Einstein Telescope. The main reason is that many of the NSs in these systems appear to be spinning well below their Keplerian break-up frequency, and it has been suggested that torques associated with GW emission may be setting the observed spin period. This assumption has been used extensively in the literature to assess the strength of the likely GW signal. There is now, however, a significant amount of theoretical and observation work that suggests that this may not be the case, and that GW emission is unlikely to be setting the spin equilibrium period in many systems. In this paper we take a different starting point and predict the GW signal strength for two physical mechanisms that are likely to be at work in LMXBs: crustal mountains due to thermal asymmetries and magnetically confined mountains. We find that thermal crustal mountains in transient LMXBs are unlikely to lead to detectable GW emission, while persistent systems are good candidates for detection by Advanced LIGO and by the Einstein Telescope. Detection prospects are pessimistic for the magnetic mountain case, unless the NS has a buried magnetic field of B ≈ 1012 G, well above the typically inferred exterior dipole fields of these objects. Nevertheless, if a system were to be detected by a GW observatory, cyclotron resonant scattering features in the X-ray emission could be used to distinguish between the two different scenarios.
Certain multiwavelength observations of neutron stars, such as intermittent radio emissions from rotation-powered pulsars beyond the pair-cascade death line, the pulse profile of the magnetar SGR ...1900+14 after its 1998 August 27 giant flare and X-ray spectral features of PSR J0821−4300 and SGR 0418+5729, suggest that the magnetic fields of non-accreting neutron stars are not purely dipolar and may contain higher order multipoles. Here, we calculate the ellipticity of a non-barotropic neutron star with (i) a quadrupole poloidal-toroidal field, and (ii) a purely poloidal field containing arbitrary multipoles, deriving the relation between the ellipticity and the multipole amplitudes. We present, as a worked example, a purely poloidal field comprising dipole, quadrupole and octupole components. We show the correlation between field energy and ellipticity for each multipole, that the l = 4 multipole has the lowest energy, and that l = 5 has the lowest ellipticity. We show how a mixed multipolar field creates an observationally testable mismatch between the principal axes of inertia (to be inferred from gravitational wave data) and the magnetic inclination angle. Strong quadrupole and octupole components (with amplitudes ∼102 times higher than the dipole) in SGR 0418+5729 still yield ellipticity ∼10−8, consistent with current gravitational wave upper limits. The existence of higher multipoles in fast-rotating objects (e.g. newborn magnetars) has interesting implications for the braking law and hence phase tracking during coherent gravitational wave searches.
We present the results of a cross-correlation search for gravitational waves from SNR 1987A using the second year of LIGO science run 5 data. The frequency band 75–450 Hz is searched. No evidence of ...gravitational waves is found. A 90% confidence upper limit of h0≤3.8×10−25 is placed on the gravitational-wave strain at the most sensitive frequency near 150 Hz. This corresponds to an ellipticity of ε≤8.2×10−4 and improves on previously published strain upper limits by a factor of approximately 4. We perform a comprehensive suite of validations of the search algorithm and identify several computational savings that marginally sacrifice sensitivity in order to streamline the parameter space being searched. We estimate detection thresholds and sensitivities through Monte Carlo simulations.
ABSTRACT
Gravitational waves provide a unique tool for observational astronomy. While the first LIGO–Virgo catalogue of gravitational-wave transients (GWTC-1) contains 11 signals from black hole and ...neutron star binaries, the number of observations is increasing rapidly as detector sensitivity improves. To extract information from the observed signals, it is imperative to have fast, flexible, and scalable inference techniques. In a previous paper, we introduced bilby: a modular and user-friendly Bayesian inference library adapted to address the needs of gravitational-wave inference. In this work, we demonstrate that bilby produces reliable results for simulated gravitational-wave signals from compact binary mergers, and verify that it accurately reproduces results reported for the 11 GWTC-1 signals. Additionally, we provide configuration and output files for all analyses to allow for easy reproduction, modification, and future use. This work establishes that bilby is primed and ready to analyse the rapidly growing population of compact binary coalescence gravitational-wave signals.
ABSTRACT
Gravitational waves from binary neutron star post-merger remnants have the potential to uncover the physics of the hot nuclear equation of state. These gravitational-wave signals are high ...frequency (∼kHz) and short-lived ($\mathcal {O}(10\, \mathrm{ms})$), which introduces potential problems for data analysis algorithms due to the presence of non-stationary and non-Gaussian noise artefacts in gravitational-wave observatories. We quantify the degree to which these noise features in LIGO data may affect our confidence in identifying post-merger gravitational-wave signals. We show that the combination of vetoing data with non-stationary glitches and the application of the Allen χ2 veto (usually reserved for long-lived lower frequency gravitational-wave signals), allows one to confidently detect post-merger signals with signal-to-noise ratio ρ ≳ 8. We discuss the need to incorporate the data quality checks and vetoes into realistic post-merger gravitational-wave searches, and describe their relevance to calculating realistic false-alarm and false-dismissal rates.
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