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The binary system AR Scorpii hosts an M-type main sequence cool star orbiting around a magnetic white dwarf in the Milky Way Galaxy. The broadband non-thermal emission over radio, optical and X-ray wavebands observed from AR Scorpii indicates strong modulations on the spin frequency of the white dwarf as well as the spin-orbit beat frequency of the system. Therefore, AR Scorpii is also referred to as a white dwarf pulsar wherein a fast spinning white dwarf star plays very crucial role in the broadband non-thermal emission. Several interpretations for the observed features of AR Scorpii appear in the literature without firm conclusions. In this paper, we investigate connection between some of the important physical properties like spin-down power, surface magnetic field, equation of state, temperature and gravity associated with the white dwarf in the binary system AR Scorpii and its observational characteristics. We explore the plausible effects of white dwarf surface magnetic field on the absence of substantial accretion in this binary system and also discuss the gravitational wave emission due to magnetic deformation mechanism.

We show that a common evolutionary history can produce the black hole binaries in the Galaxy in which the black holes have masses of ~ 5 - 10M_⊙. In the black hole binaries with low-mass, ≲ 2.5M_⊙ ZAMS (zero age main sequence) companions, the latter remain in main sequence during the active stage of soft X-ray transients (SXT's), most of them being of K or M classification. In two intermediate cases, IL Lupi and Nova Scorpii with ZAMS ~ 2.5M_⊙ companions the orbits are greatly widened because of large mass loss in the explosion forming the black hole, and whereas these companions are in late main sequence evolution, they are close to evolving. Binaries with companion ZAMS masses ≳ 3M_⊙ are initially "silent" until the companion begins evolving across the Herzsprung gap.

We provide evidence that the narrower, shorter period binaries, with companions now in main sequence, are fossil remnants of gamma ray bursters (GRB's). We also show that the GRB is generally accompanied by a hypernova explosion (a very energetic supernova explosion). We further show that the binaries with evolved companions are good models for some of the ultraluminous X-ray sources (ULX's) recently seen by Chandra in other galaxies.

The great regularity in our evolutionary history, especially the fact that most of the companions of ZAMS mass ≲ 2.5M_⊙ remain in main sequences as K or M stars can be explained by the mass loss in common envelope evolution to be Case C; i.e. to occur only after core He burning has finished. Since our argument for Case C mass transfer is not generally understood in the community, we add an appendix, showing that with certain assumptions which we outline we can reproduce the regularities in the evolution of black hole binaries by Case C mass transfer.

In this paper we use ΔP = -1.772341 ± 13.153788 s between the phenomenologically determined orbital period P_b of the PSR J0737-3039A/B double pulsar system and the purely Keplerian period calculated with the system's parameters, determined independently of the third Kepler law itself, in order to put constraints on some models of modified gravity (f(R), Yukawa-like fifth force, MOND). The major source of error affecting ΔP is not the one in the phenomenologically measured period (δP_b = 4×10^-6s), but the systematic uncertainty δP⁽⁰⁾ in the computed Keplerian one due to the relative semimajor axis a mainly caused, in turn, by the errors in the ratio of the pulsars' masses and in sin i. We get |κ| ≤ 0.8 × 10^-26m^-2 for the parameter that in the f(R) framework is a measure of the nonlinearity of the theory, |α| ≤ 5.5 × 10^-4 for the fifth-force strength parameter (for λ ≈ a = 0.006 AU). The effects predicted by the strong-acceleration regime of MOND are far too small to be constrained with some effectiveness today and in the future as well. In view of the continuous timing of such an important system, it might happen that in the near future it will be possible to obtain somewhat tighter constraints.

We present the state-of-the-art of the numerical simulations of the supernova (SN) explosion of a carbon-oxygen core $({\mathrm{\text{CO}}}_{\mathrm{\text{core}}})$ that forms a compact binary with a neutron star (NS) companion, following the induced gravitational collapse (IGC) scenario of long gamma-ray bursts (GRBs) associated with type Ic supernovae (SNe). We focus on the consequences of the hypercritical accretion of the SN ejecta onto the NS companion which either becomes a more massive NS or gravitationally collapses forming a black hole (BH). We summarize the series of results on this topic starting from the first analytic estimates in 2012 all the way up to the most recent three-dimensional (3D) smoothed-particle-hydrodynamics (SPH) numerical simulations in 2018. We present a new SN ejecta morphology, highly asymmetric, acquired by binary interaction and leading to well-defined, observable signatures in the gamma- and X-rays emission of long GRBs.

We report on the results from the observations in very high energy band (VHE, E_γ ≥ 100 GeV) of the γ-ray binary LS I +61 303 and the black hole X-ray binary (BHXB) Cygnus X-1. LS I +61 303 was recently discovered at VHE by MAGIC¹ and here we present the preliminary results from an extensive observation campaign, comprising 112 observation hours covering 4 orbital cycles, aiming at determining the time-dependent features of the VHE emission. Cygnus X-1 was observed for a total of 40 hours during 26 nights, spanning the period between June and November 2006. We report on the results of the searches for steady and variable γ-ray signals from Cygnus X-1, including the first experimental evidence for an intense flare, of duration between 1.5 and 24 hours.

The hydrodynamics of the interaction of pulsar and stellar winds in binary systems harboring a pulsar and its impact on the nonthermal radiation of the binary pulsar PSR B1259-63/SS2883 is discussed. The collision of an ultrarelativistic pulsar wind with a nonrelativistic stellar outflow results in significant bulk acceleration of the shocked material from the pulsar wind. Already at distances comparable to the size of the binary system, the Lorentz factor of the shocked flow can be as large as γ ~ 4. This results in significant anisotropy of the inverse Compton radiation of accelerated electrons. Because of the Doppler boosting of the produced radiation, one should expect a variable gamma-ray signal from the system. In particular, this effect may naturally explain the reported tendency of a decrease of TeV gamma-ray flux close to the periastron. The modeling of the interaction of pulsar and stellar winds allows self-consistent calculations of adiabatic losses. Our results show that adiabatic losses dominate over the radiative losses. These results have direct impact on the orbital variability of radio, X-ray and gamma-ray signals detected from the binary pulsar PSR B1259-63/SS2883.

Gamma-ray-loud binary systems are a newly identified class of X-ray binaries detected up to TeV energies. Three such systems — PSR B1259–63, LS 5039 and LSI +61 303 — have been firmly detected as persistent or regularly variable TeV γ-ray emitters. The origin of the high-energy activity of these sources is not clear. In this paper we review the multiwavelength properties of these systems and discuss their similarities and peculiarities.

Our concept of induced gravitational collapse (IGC paradigm) starting from a supernova occurring with a companion neutron star, has unlocked the understanding of seven different families of gamma ray bursts (GRBs), indicating a path for the formation of black holes in the universe. An authentic laboratory of relativistic astrophysics has been unveiled in which new paradigms have been introduced in order to advance knowledge of the most energetic, distant and complex systems in our universe. A novel cosmic matrix paradigm has been introduced at a relativistic cosmic level, which parallels the concept of an S-matrix introduced by Feynmann, Wheeler and Heisenberg in the quantum world of microphysics. Here the “in” states are represented by a neutron star and a supernova, while the “out” states, generated within less than a second, are a new neutron star and a black hole. This novel field of research needs very powerful technological observations in all wavelengths ranging from radio through optical, X-ray and gamma ray radiation all the way up to ultra-high-energy cosmic rays.

Pulsars (PSRs) are some of the most accurate clocks found in nature, while black holes (BHs) offer a unique arena for the study of quantum gravity. As such, PSR–BH binaries provide ideal astrophysical systems for detecting effects of quantum gravity. With the success of aLIGO and the advent of instruments like the Square Kilometer Array (SKA) and Evolved Laser Interferometer Space Antenna (eLISA), the prospects for discovery of such PSR–BH binaries are very promising. We argue that PSR–BH binaries can serve as ready-made testing grounds for proposed resolutions to the BH information paradox. We propose using timing signals from a PSR beam passing through the region near a BH event horizon as a probe of quantum gravitational effects. In particular, we demonstrate that fluctuations of the geometry outside a BH lead to an increase in the measured root-mean-square deviation of arrival times of PSR pulsar traveling near the horizon.

We present the first results obtained in the elaboration of a complete model of a microquasar where the donor star is from Population III. These stars do not produce stellar winds so we consider that the mass loss is due exclusively to matter overflowing the Roche lobe towards the compact object, a maximally rotating black hole. The rate of accretion is extremely super-Eddington, with an intense mass loss from the system in the form of winds and jets. We calculate the relativistic particle content of the jet and the corresponding spectral energy distribution (SED) considering a lepto-hadronic model. Prospects for the cosmological implications of these objects are briefly discussed.

We argue that the Black Hole-Neutron Star (BH-NS) binaries are the natural astrophysical probes of quantum gravity in the context of the new era of multi-messenger astronomy. In particular, we discuss the observable effect of enhanced BH mass loss in a BH-NS binary, due to the presence of an additional length scale tied to the intrinsic noncommutativity of quantum spacetime in quantum gravity.

Binary stars are important for understanding stellar structure and evolution. Binary systems with an evolved component give us an important constraint about the role played by convection on the characteristic time for tidal synchronization and circularization. On this study, we discuss about the role of convection in binary stars with evolved components. Base on a stellar sample composed by 260 binary stars with surface convective mass determined from evolutionary models computed with the Toulouse-Geneva Evolution Code (TGEC) as in do Nascimento et al. (2009). The stars with different convective deepening are represented in the Hertzsprung-Russell diagram (HR diagram). We are focused on the important question of how convection influence the evolution of the tidal synchronization and circularization of binary systems with an evolved component.

We take the data of twin kilohertz quasi-periodic oscillations (kHz QPOs) in neutron stars (NSs) published before 2011 as the samples, which include 15 Atoll sources and 8 Z sources. Then we test the correlation between the twin peak kHz QPOs, and compare with the samples. We find the power law relation between the lower-frequency (ν₁) and the upper-frequency (ν₂) of kHz QPOs can fit the data much better than the other models. For the Atoll (Z) sources, the best fitting coefficient and index of power law are a = 687 (720) and b =1.50 (1.83) respectively. The Chi-square per degree of freedom (χ²/d.o.f.) are 1.93 and 1.49 respectively.

Observations of the binary pulsar PSR B1259-63/LS2883 in the high energy and very high energy domains have revealed a few quite unusual features. One of the most puzzling phenomena is the bright GeV flare detected with Fermi/LAT in 2011 January, approximately one month after periastron passage. Since the maximum luminosity in the high energy band during the flare nearly achieved the level of the pulsar spin-down energy losses, it is likely that the particles, responsible for this emission component, had a strongly anisotropic distribution, which resulted in the emission enhancement. One of the most prolific scenarios for such an emission enhancement is the Doppler boosting, which is realized in sources with relativistic motions. Interestingly, a number of hydrodynamical simulations have predicted a formation of highly relativistic outflows in binary pulsar systems, therefore scenarios, involving relativistic boosting, are very natural for these systems. However a more detailed analysis of such a possibility, presented in this study, reveals certain limitations which put strict constraints on the maximum luminosity achievable in this scenario. These constraints render the "Doppler boosting" scenario to be less feasible, especially for the synchrotron models.

New observations of the binary pulsar B1913 + 16 are presented. Since 1978 the leading component of the pulse profile has weakened dramatically by about 40%. For the first time, a decrease in component separation is observed, consistent with expectations of geodetic precession. Assuming the correctness of general relativity and a circular hollow-cone–like beam, a fully consistent model for the system geometry is developed. The misalignment angle between pulsar spin and orbital momentum is determined, giving direct evidence for an asymmetric kick during the second supernova explosion. It is argued that the orbital inclination angle is 132°.8 (rather than 47°.2). A prediction of this model is that PSR B1913 + 16 will no longer be observable after the year 2025.

Our concept of induced gravitational collapse (IGC paradigm) starting from a supernova occurring with a companion neutron star, has unlocked the understanding of seven different families of gamma ray bursts (GRBs), indicating a path for the formation of black holes in the universe. An authentic laboratory of relativistic astrophysics has been unveiled in which new paradigms have been introduced in order to advance knowledge of the most energetic, distant and complex systems in our universe. A novel cosmic matrix paradigm has been introduced at a relativistic cosmic level, which parallels the concept of an S-matrix introduced by Feynmann, Wheeler and Heisenberg in the quantum world of microphysics. Here the “in” states are represented by a neutron star and a supernova, while the “out” states, generated within less than a second, are a new neutron star and a black hole. This novel field of research needs very powerful technological observations in all wavelengths ranging from radio through optical, X-ray and gamma ray radiation all the way up to ultra-high-energy cosmic rays.

The evolution of close-orbit progenitor binaries of double neutron star (DNS) systems leads to supernova (SN) explosions of ultra-stripped stars. The amount of SN ejecta mass is very limited from such, more or less, naked metal cores with envelope masses of only 0.01 - 0.2 M_⊙. The combination of little SN ejecta mass and the associated possibility of small NS kicks is quite important for the characteristics of the resulting DNS systems left behind. Here, we discuss theoretical predictions for DNS systems, based on Case BB Roche-lobe overflow prior to ultra-stripped SNe, and briefly compare with observations.

The explosion of ultra-stripped stars in close binaries may explain new discoveries of weak and fast optical transients. We have demonstrated that helium star companions to neutron stars (NSs) may evolve into naked metal cores as low as ∼ 1.5 M_⊙, barely above the Chandrasekhar mass limit, by the time they explode. Here we present a new systematic investigation of the progenitor evolution leading to such ultra-stripped supernovae (SNe), in some cases yielding pre-SN envelopes of less than 0.01 M_⊙. We discuss the nature of these SNe (electron-capture vs iron core-collapse) and their observational light-curve properties. Ultra-stripped SNe are highly relevant for binary pulsars, as well as gravitational wave detection of merging NSs by LIGO/VIRGO, since these events are expected to produce mainly low-kick NSs in the mass range 1.10 − 1.80 M_⊙.

Motivated by detections of hypervelocity stars that may originate from the Galactic Center, we revist the problem of a binary disruption by a passage near a much more massive point mass. The six order of magnitude mass ratio between the Galactic Center black hole and the binary stars allows us to formulate the problem in the restricted parabolic three-body approximation. In this framework, results can be simply rescaled in terms of binary masses, its initial separation and binary-to-black hole mass ratio. Consequently, an advantage over the full three-body calculation is that a much smaller set of simulations is needed to explore the relevant parameter space. Contrary to previous claims, we show that, upon binary disruption, the lighter star does not remain preferentially bound to the black hole. In fact, it is ejected exactly in 50% of the cases. Nonetheless, lighter objects have higher ejection velocities, since the energy distribution is independent of mass. Focusing on the planar case, we provide the probability distributions for disruption of circular binaries and for the ejection energy. We show that even binaries that penetrate deeply into the tidal sphere of the black hole are not doomed to disruption, but survive in 20% of the cases. Nor do these deep encounters produce the highest ejection energies, which are instead obtained for binaries arriving to 0:1 – 0:5 of the tidal radius in a prograde orbit. Interestingly, such deep-reaching binaries separate widely after penetrating the tidal radius, but always approach each other again on their way out from the black hole. Finally, our analytic method allows us to account for a finite size of the stars and recast the ejection energy in terms of a minimal possible separation. We find that, for a given minimal separation, the ejection energy is relatively insensitive to the initial binary separation (see Sari, Kobayashi and Rossi 2010, ApJ 708, 605 for the full discussion).

The Chandrasekhar limit is of key importance for the evolution of white dwarfs in binary systems and for the formation of neutron stars and black holes in binaries. Mass transfer can drive a white dwarf in a binary over the Chandrasekhar limit, which may lead to a Type Ia supernova (in case of a CO white dwarf) or an Accretion-Induced Collapse (AIC, in the case of an O-Ne-Mg white dwarf; and possibly also in some CO white dwarfs) which produces a neutron star. The direct formation of neutron stars or black holes out of degenerate stellar cores that exceed the Chandrasekhar limit, occurs in binaries with components that started out with masses ≥ 8 M_⊙.

This paper first discusses possible models for Type Ia supernovae, and then focusses on the formation of neutron stars in binary systems, by direct core collapse and by the AIC of O-Ne-Mg white dwarfs in binaries. Observational evidence is reviewed for the existence of two different direct neutron-star formation mechanisms in binaries: (i) by electron-capture collapse of the degenerate O-Ne-Mg core in stars with initial masses in the range of 8 to about 12 M_⊙, and (ii) by iron-core collapse in stars with inital masses above this range. Observations of neutron stars in binaries are consistent with a picture in which neutron stars produced by e-capture collapse have relatively low masses, ˜1.25M_⊙, and received hardly any velocity kick at birth, whereas neutron stars produced by iron-core collapses are more massive and received large velocity kicks at birth. Many of the globular cluster neutron stars and also some of the neutron stars in low-mass binaries in the Galactic disk are likely to have been produced by AIC of O-Ne-Mg white dwarfs in binaries. AIC is expected to produce normal strongly magnetized neutron stars, which in binaries can evolve into millisecond pulsars through the usual recycling scenario.