33 research outputs found
The final phase of inspiral of neutron stars: realistic equations of state
Coalescing compact star binaries are expected to be among the strongest
sources of gravitational radiation to be seen by laser interferometers. We
present calculations of the final phase of inspiral of equal mass irrotational
neutron star binaries and strange quark star binaries. Six types of equations
of state at zero temperature are used - three realistic nuclear equations of
state of various softness and three different MIT bag models of strange quark
matter. We study the precoalescing stage within the Isenberg-Wilson-Mathews
approximation of general relativity using a multidomain spectral method. The
gravitational-radiation driven evolution of the binary system is approximated
by a sequence of quasi-equilibrium configurations at fixed baryon number and
decreasing separation. We find that the innermost stable circular orbit (ISCO)
is given by an orbital instability for binary strange quark stars and by the
mass-shedding limit for neutron star binaries. The gravitational wave frequency
at the ISCO, which marks the end of the inspiral phase, is found to be around
1100-1460 Hz for two 1.35 solar masses irrotational strange stars described by
the MIT bag model and between 800 Hz and 1230 Hz for neutron stars.Comment: 6 pages, 2 figures, Advances in Space Research, in
press,doi:10.1016/j.asr.2006.09.021, available online www.sciencedirect.com,
paper updated according to the reviewer's suggestions (conclusions unchanged
Gravitational Waves: Search Results, Data Analysis and Parameter Estimation. Amaldi 10 Parallel Session C2
The Amaldi 10 Parallel Session C2 on gravitational wave(GW) search results, data analysis and parameter estimation included three lively sessions of lectures by 13 presenters, and 34 posters. The talks and posters covered a huge range of material, including results and analysis techniques for ground-based GW detectors, targeting anticipated signals from different astrophysical sources: compact binary inspiral, merger and ringdown; GW bursts from intermediate mass binary black hole mergers, cosmic string cusps, core-collapse supernovae, and other unmodeled sources; continuous waves from spinning neutron stars; and a stochastic GW background. There was considerable emphasis on Bayesian techniques for estimating the parameters of coalescing compact binary systems from the gravitational waveforms extracted from the data from the advanced detector network. This included methods to distinguish deviations of the signals from what is expected in the context of General Relativity
Dynamic migration of rotating neutron stars due to a phase transition instability
Using numerical simulations based on solving the general relativistic
hydrodynamic equations, we study the dynamics of a phase transition in the
dense core of isolated rotating neutron stars, triggered by the back bending
instability reached via angular momentum loss. In particular, we investigate
the dynamics of a migration from an unstable configuration into a stable one,
which leads to a mini-collapse of the neutron star and excites sizeable
pulsations in its bulk until it acquires a new stable equilibrium state. We
consider equations of state with softening at high densities, a simple analytic
one with a mixed hadron-quark phase in an intermediate pressure interval and
pure quark matter at very high densities, and a microphysical one that has a
first-order phase transition, originating from kaon condensation. Although the
marginally stable initial models are rigidly rotating, we observe that during
the collapse (albeit little) differential rotation is created. We analyze the
emission of gravitational radiation, which in some models is amplified by mode
resonance effects, and assess its prospective detectability by interferometric
detectors. We expect that the most favorable conditions for dynamic migration
exist in very young magnetars. We find that the damping of the post-migration
pulsations strongly depends on the character of the equation of state
softening. The damping of pulsations in the models with the microphysical
equation of state is caused by dissipation associated with matter flowing
through the density jump at the edge of the dense core. If at work, this
mechanism dominates over all other types of dissipation, like bulk viscosity in
the exotic-phase core, gravitational radiation damping, or numerical viscosity.Comment: 23 pages, 18 figures, minor modification
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 transientâs 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
Search for High-energy Neutrinos from Binary Neutron Star Merger GW170817 with ANTARES, IceCube, and the Pierre Auger Observatory
Tensor calculus with open-source software: the SageManifolds project
International audienceThe SageManifolds project aims at extending the mathematics software system Sage towards differential geometry and tensor calculus. Like Sage, SageManifolds is free, open- source and is based on the Python programming language. We discuss here some details of the implementation, which relies on Sage's parent/element framework, and present a concrete example of use
Black Hole Accretion in Gamma Ray Bursts
We study the structure and evolution of the hyperaccreting disks and outflows in the gamma ray bursts central engines. The torus around a stellar mass black hole is composed of free nucleons, Helium, electron-positron pairs, and is cooled by neutrino emission. Accretion of matter powers the relativistic jets, responsible for the gamma ray prompt emission. The significant number density of neutrons in the disk and outflowing material will cause subsequent formation of heavier nuclei. We study the process of nucleosynthesis and its possible observational consequences. We also apply our scenario to the recent observation of the gravitational wave signal, detected on 14 September 2015 by the two Advanced LIGO detectors, and related to an inspiral and merger of a binary black hole system. A gamma ray burst that could possibly be related with the GW150914 event was observed by the Fermi satellite. It had a duration of about 1 s and appeared about 0.4 s after the gravitational-wave signal. We propose that a collapsing massive star and a black hole in a close binary could lead to the event. The gamma ray burst was powered by a weak neutrino flux produced in the star remnantâs matter. Low spin and kick velocity of the merged black hole are reproduced in our simulations. Coincident gravitational-wave emission originates from the merger of the collapsed core and the companion black hole