Gravitational waves from binary neutron stars

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.

The Effect of Post-Newtonian Spin Precessions on the Evolution of Exomoons’ Obliquity

Putative natural massive satellites (exomoons) have gained increasing attention when they orbit Jupiter-like planets within the habitable zone of their host main-sequence star S. An exomoon s is expected to move within the equatorial plane of its host planet p, with its spin S s aligned with its orbital angular momentum L , which, in turn, is parallel to the planetary spin S p. If, in particular, the common tilt ε of such angular momenta to the plane of the satellite–planet motion about the star, assumed fixed, has certain values, the stellar latitudinal irradiation experienced on the exomoon may allow it to sustain life as we know it, at least for certain orbital configurations. An Earth analog (similar in mass, radius, oblateness, and obliquity) is considered, which orbits within 5–10 planetary radii R p from its Jupiter-like host planet. The de Sitter and Lense–Thirring spin precessions due to the general relativistic post-Newtonian (pN) field of the host planet have an impact on an exomoon’s habitability for a variety of different initial spin–orbit configurations. Here I show it by identifying long-term variations in the satellite’s obliquity ε s, where variations can be ≲10°–100°, depending on the initial spin–orbit configuration, with a timescale of ≃0.1–1 million years. Also the satellite’s quadrupole mass moment J 2 s induces obliquity variations that are faster than the pN ones but do not cancel them. Tidal dissipations, which may potentially have a relevant impact on the outlined pattern, are not included in the present analysis.

Transonic accretion and winds around Pseudo-Kerr black holes andcomparison with general relativistic solutions

Spectral and timing properties of accretion flows on a black hole depend on their density and temperature distributions, which, in turn come from the underlying dynamics. Thus, an accurate description of the flow which includes hydrodynamics and radiative transfer is a must to interpret the observational results. In the case of non-rotating black holes, Pseudo- Newtonian description of surrounding space-time enables one to make a significant progress in predicting spectral and timing properties. This formalism is lacking for the spinning black holes. In this paper, we show that there exists an exact form of ‘natural’ potential derivable from the general relativistic (GR) radial momentum equation written in the local corotating frame. Use of this potential in an otherwise Newtonian set of equations, allows us to describe transonic flows very accurately as is evidenced by comparing with solutions obtained from the full GR framework. We study the properties of the sonic points and the centrifugal pressure supported shocks in the parameter space spanned by the specific energy and the angular momentum, and compare with the results of GR hydrodynamics. We show that this potential can safely be used for the entire range of Kerr parameter −1 < a < 1 for modeling of observational results around spinning black holes. We assume the flow to be inviscid. Thus, it is non-dissipative with constant energy and angular momentum. These assumptions are valid very close to the black hole horizon as the infall time scale is much shorter as compared to the viscous time scale.

Editorial for the Special Issue “Relativistic Astrophysics”

Relativistic Astrophysics is the branch of astrophysics that studies astronomical phenomena and celestial bodies, for which classical mechanics and Newton’s law of gravitation are inapplicable to creation of suitable models and we have to generalize these approaches following general relativistic prescriptions [...]

The Response of Black Hole Spark Gaps to External Changes: A Production Mechanism of Rapid TeV Flares?

We study the response of a starved Kerr black hole magnetosphere to abrupt changes in the intensity of disk emission and in the global magnetospheric current, by means of one-dimensional general relativistic particle-in-cell simulations. Such changes likely arise from the intermittency of the accretion process. We find that in cases where the pair-production opacity contributed by the soft disk photons is modest, as in, e.g., M87, such changes can give rise to delayed, strong teraelectronvolt (TeV) flares, dominated by curvature emission of particles accelerated in the gap. The flare rise time, and the delay between the external variation and the onset of the flare emitted from the outer gap boundary, are of the order of the light-crossing time of the gap. The rapid, large-amplitude TeV flares observed in M87, and perhaps, other active galactic nuclei may be produced by such a mechanism.

Brightness Asymmetry of Black Hole Images as a Probe of Observer Inclination

The Event Horizon Telescope recently captured images of the supermassive black hole in the center of the M87 galaxy, which shows a ring-like emission structure with the south side only slightly brighter than the north side. This relatively weak asymmetry in the brightness profile along the ring has been interpreted as a consequence of the low inclination of the observer (around 17° for M87), which suppresses the Doppler beaming and boosting effects that might otherwise be expected due to the nearly relativistic velocities of the orbiting plasma. In this work, we use a large suite of general relativistic magnetohydrodynamic simulations to reassess the validity of this argument. By constructing explicit counterexamples, we show that low inclination is a sufficient but not necessary condition for images to have low brightness asymmetry. Accretion flow models with high accumulated magnetic flux close to the black hole horizon (the so-called magnetically arrested disks) and low black hole spins have angular velocities that are substantially smaller than the orbital velocities of test particles at the same location. As a result, such models can produce images with low brightness asymmetry even when viewed edge on.

Black Hole Flares: Ejection of Accreted Magnetic Flux through 3D Plasmoid-mediated Reconnection

Magnetic reconnection can power bright, rapid flares originating from the inner magnetosphere of accreting black holes. We conduct extremely high-resolution (5376 × 2304 × 2304 cells) general-relativistic magnetohydrodynamics simulations, capturing plasmoid-mediated reconnection in a 3D magnetically arrested disk for the first time. We show that an equatorial, plasmoid-unstable current sheet forms in a transient, nonaxisymmetric, low-density magnetosphere within the inner few Schwarzschild radii. Magnetic flux bundles escape from the event horizon through reconnection at the universal plasmoid-mediated rate in this current sheet. The reconnection feeds on the highly magnetized plasma in the jets and heats the plasma that ends up trapped in flux bundles to temperatures proportional to the jet’s magnetization. The escaped flux bundles can complete a full orbit as low-density hot spots, consistent with Sgr A* observations by the GRAVITY interferometer. Reconnection near the horizon produces sufficiently energetic plasma to explain flares from accreting black holes, such as the TeV emission observed from M87. The drop in the mass accretion rate during the flare and the resulting low-density magnetosphere make it easier for very-high-energy photons produced by reconnection-accelerated particles to escape. The extreme-resolution results in a converged plasmoid-mediated reconnection rate that directly determines the timescales and properties of the flare.

Curvature Tensor of the Stationary Accelerated Frame in Gravity Field

We define an accelerated frame that moves along rˆ -axis in the general relativistic curved space-time. We then calculate the curvature tensor of this accelerated frame in the stationary gravity field. The curvature tensor is divided into two parts: the curvature tensor as observed by the observer and the curvature tensor of the observer’s own planet in the gravity field.

On the dynamics of the torus around the kicked black hole

There is a special interest to understand the dynamical properties of the accretion disk created around the newly formed black hole due to the supermassive black hole binaries which merge inside the gaseous disk. The newly formed black hole would have a kick velocity up to thousands of km/s that drives a perturbation on a newly accreted torus around the black hole. Some of the observed supermassive black holes at the center of the Active Galactic Nucleus (AGN) move with a certain velocity relative to its broader accretion disk. In this paper, the effects of the kicked black holes onto the infinitesimally thin accreted torus are studied by using the general relativistic hydrodynamical code, focusing on changing the dynamics of the accretion disk during the accretion disk–black hole interaction. We have found that the non-axisymmetric global mode [Formula: see text] inhomogeneity, which causes a spiral-wave-structure, is excited on the torus due to kicked black hole. The higher the perturbation velocity produced by the kicked black hole, the longer the time the torus takes to reach the saturation point. The created spiral density waves which rapidly evolve into the spiral shocks are also observed from the numerical simulations. The spiral shock is responsible for accreting matter toward the black hole. First, the spiral-wave-structure is developed and the accretion through the spiral arms is stopped around the black hole. At the later time of simulation, the formed spiral shocks partly cause the angular momentum loss across the torus.