Simultaneous NICER and NuSTAR Observations of the Ultracompact X-Ray Binary 4U 0614+091
David MoutardR. M. LudlamJavier A. GarcíaD. AltamiranoD J K BuissonEdward M. CackettJ. ChenevezN. DegenaarA. C. FabianJ. HomanAmruta JaodandSean N. PikeA. W. ShawTod E. StrohmayerJohn A. TomsickBenjamin M. Coughenour
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Abstract We present the first joint NuSTAR and NICER observations of the ultracompact X-ray binary 4U 0614+091. This source shows quasiperiodic flux variations on the timescale of ∼days. We use reflection modeling techniques to study various components of the accretion system as the flux varies. We find that the flux of the reflected emission and the thermal components representing the disk and the compact object trend closely with the overall flux. However, the flux of the power-law component representing the illuminating X-ray corona scales in the opposite direction, increasing as the total flux decreases. During the lowest flux observation, we see evidence of accretion disk truncation from roughly 6 gravitational radii to 11.5 gravitational radii. This is potentially analogous to the truncation seen in black hole low-mass X-ray binaries, which tends to occur during the low/hard state at sufficiently low Eddington ratios.Keywords:
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With H. A. Bethe, G. E. Brown worked on the merger rate of neutron star binaries for the gravitational wave detection. Their prediction has to be modified significantly due to the observations of [Formula: see text] neutron stars and the detection of gravitational waves. There still, however, remains a possibility that neutron star-low mass black hole binaries are significant sources of gravitational waves for the ground-based detectors. In this paper, I review the evolution of neutron star binaries with super-Eddington accretion and discuss the future prospect.
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Equations of State of Neutron Star Matter.- Computing Initial Conditions.- Methods of Simulations.- Diagnostics for Numerical Simulations.- The Merger of Nonspinning Black Hole-Neutron Star Binaries.- The Merger of Spinning Black Hole-Neutron Star Binaries.
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The recent direct detection of gravitational waves (GWs) from binary black hole mergers (2016, Phys. Rev. Lett. 116, no. 6, 061102; no. 24, 241103) opens up an entirely new non-electromagnetic window into the Universe making it possible to probe physics that has been hidden or dark to electromagnetic observations. In addition to cataclysmic events involving black holes, GWs can be triggered by physical processes and systems involving neutron stars. Properties of neutron stars are largely determined by the equation of state (EOS) of neutron-rich matter, which is the major ingredient in calculating the stellar structure and properties of related phenomena, such as gravitational wave emission from elliptically deformed pulsars and neutron star binaries. Although the EOS of neutron-rich matter is still rather uncertain mainly due to the poorly known density dependence of nuclear symmetry energy at high densities, significant progress has been made recently in constraining the symmetry energy using data from terrestrial nuclear laboratories. These constraints could provide useful information on the limits of GWs expected from neutron stars. Here after briefly reviewing our previous work on constraining gravitational radiation from elliptically deformed pulsars with terrestrial nuclear laboratory data in light of the recent gravitational wave detection, we estimate the maximum gravitational wave strain amplitude, using an optimistic value for the breaking strain of the neutron star crust, for 15 pulsars at distances 0.16 kpc to 0.91 kpc from Earth, and find it to be in the range of $\sim[0.2-31.1]\times 10^{-24}$, depending on the details of the EOS used to compute the neutron star properties. Implications are discussed.
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Abstract I review the current global status of research on gravitational waves emitted from mergers of binary neutron star systems, focusing on general-relativistic simulations and their use to interpret data from the gravitational-wave detectors, especially in relation to the equation of state of compact stars.
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The X-ray transient, 4U 1730-22, has not been detected in outburst since 1972, when a single outburst was detected by the Uhuru satellite. This neutron star or black hole X-ray binary is presumably in quiescence now, and here, we report on X-ray and optical observations of the 4U 1730-22 field designed to identify the system's quiescent counterpart. Using Chandra, we have found a very likely counterpart. The candidate counterpart is close to the center of the Uhuru error region and has a thermal spectrum. The 0.3-8 keV spectrum is well-described by a neutron star atmosphere model with an effective temperature of 131+/-21 eV. For a neutron star with a 10 km radius, the implied source distance is 10(+12)(-4) kpc, and the X-ray luminosity is 1.9E33 ergs/s assuming a distance of 10 kpc. Accretion from a companion star is likely required to maintain the temperature of this neutron star, which would imply that it is an X-ray binary and therefore, almost certainly the 4U 1730-22 counterpart. We do not detect an optical source at the position of the Chandra source down to R > 22.1, and this is consistent with the system being a Low-Mass X-ray Binary at a distance greater than a few kpc. If our identification is correct, 4U 1730-22 is one of the 5 most luminous of the 20 neutron star transients that have quiescent X-ray luminosity measurements.
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Abstract I discuss the nature of the compact X-ray source inside the supernova remnant RCW 103. Several models, based on the accretion of matter onto a compact object such as a neutron star or a black hole (isolated or binary), are analysed. I show that it is more likely that the X-ray source is an accreting neutron star than an accreting black hole. I also argue that models of a binary system with an old accreting neutron star are most favoured.
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The discovery of GW170817 with gravitational waves (GWs) and electromagnetic (EM) radiation is prompting new questions in strong-gravity astrophysics. Importantly, it remains unknown whether the progenitor of the merger comprised two neutron stars (NSs), or a NS and a black hole (BH). Using new numerical-relativity simulations and incorporating modeling uncertainties we produce novel GW and EM observables for NS-BH mergers with similar masses. A joint analysis of GW and EM measurements reveals that if GW170817 is a NS-BH merger, <40% of the GW parameters are compatible with EM observations.
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The stages that follow the merging of two neutron stars are discussed. It is shown that if a rapidly rotating grav- itationally bound object is formed after the merging (a spinar or a massive neutron star), then the characteristic time of its evolution is determined by a fundamental value tspin =3 k m p e 2 h 1 = 2 m 3 c 5 = 2 G 1 = 2 10 3 s ; where the dimensionless value = 100 1000 depends on the exact equation of state of nuclear matter. The hypothesis is discussed as to whether the residual optical emission of the gamma-rayburstsispulsar-likeanditsevolutiondrivenbymag- netodipole energy losses. It is shown that binary neutron star mergings can be accompanied by two gravitational wave burst separatedeitherbythetimeofspinar'scollapsetspin orneutron star cooling time ( 10 s), depending on the masses of neutron stars.
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A neutron star low-mass binary is a binary stellar system with a neutron star and a low-mass companion star rotating around each other. In this system the neutron star accretes mass from the companion, and as this matter falls into the deep potential well of the neutron star, the gravitational potential energy is released primarily in the wavelengths. Such a source was first discovered in X-rays in 1962, and this discovery formally gave birth to the X-ray astronomy. In the subsequent decades, our knowledge of these sources has increased enormously by the observations with several space missions. Here we give a brief overview of our current understanding of the observational aspects of these systems.
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The stages that follow the merging of two neutron stars are discussed. It is shown that if a rapidly rotating gravitationally bound object is formed after the merging (a spinar or a massive neutron star), then the characteristic time of its evolution is determined by a fundamental value t_{spin} = 3 k (m_p e^2 \hbar^{1/2}) / (m_e^3 c^{5/2} G^{1/2}) \approx 7e+5 s k, where the dimensionless value k depends on the exact equation of state of nuclear matter. The hypothesis is discussed as to whether the residual optical emission of the gamma-ray burst GRB 970228 is pulsar-like and its evolution driven by magnetodipole energy losses. It is shown that binary neutron star mergings can be accompanied by two gravitational wave burst separated either by the time interval t_{spin} or neutron star cooling time (~10 s), depending on the masses of neutron stars.
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