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High-energy positrons and bright {\gamma}-ray sources are unique both for fundamental research and practical applications. However, GeV electron-positron pair jets and {\gamma}-ray flashes are still hardly produced in laboratories. Here we demonstrate that, by irradiating two 10 PW-scale laser pulses onto a near-critical density plasma channel, highly-directional GeV electron-positron pairs and bright {\gamma}-ray beams can be efficiently generated. Three-dimensional particle-in-cell simulations show that GeV positron jets show high density (8*10^21/cm^3), attosecond duration (400 as) and a divergence angle of 14{\deg}. Additionally, ultrabright (2*10^25 photons/s/mm2/mrad2/0.1%BW) collimated attosecond (370 as) {\gamma}-ray flashes with a laser energy conversion efficiency of 5.6% are emitted. Once realized in experiment, it may open up new possibilities for a wide variety of applications.

The gravitational-wave background from supermassive binary black holes (SMBBHs) has yet to be detected. This has led to speculations as to whether current pulsar timing array limits are in tension with theoretical predictions. In this paper, we use electromagnetic observations to constrain the SMBBH background from above and below. To derive the {\em maximum} amplitude of the background, we suppose that equal-mass SMBBH mergers fully account for the local black hole number density. This yields a maximum characteristic signal amplitude at a period of one year $A_{\rm{yr}}<2.4\times 10^{-15}$, which is comparable to the pulsar timing limits. To derive the {\em minimum} amplitude requires an electromagnetic observation of an SMBBH. While a number of candidates have been put forward, there are no universally-accepted electromagnetic detections in the nanohertz band. We show the candidate 3C 66B implies a lower limit, which is inconsistent with limits from pulsar timing, casting doubt on its binary nature. Alternatively, if the parameters of OJ 287 are known accurately, then $A_{\rm{yr}}> 6.1\times 10^{-17}$ at 95\% confidence level. If one of the current candidates can be established as a bona fide SMBBH, it will immediately imply an astrophysically interesting lower limit., Comment: Accepted for publication in MNRAS; 9 pages, 7 figures

Although several laser-plasma-based methods have been proposed for generating energetic electrons, positrons and {\gamma}-photons, manipulation of their microstructures is still challenging, and their angular momentum control has not yet been achieved. Here, we present and numerically demonstrate an all-optical scheme to generate bright GeV {\gamma}-photon and positron beams with controllable angular momentum by use of two counter-propagating circularly-polarized lasers in a near-critical-density plasma. The plasma acts as a "switching medium", where the trapped electrons first obtain angular momentum from the drive laser pulse and then transfer it to the {\gamma}-photons via nonlinear Compton scattering. Further through the multiphoton Breit-Wheeler process, dense energetic positron beams are efficiently generated, whose angular momentum can be well controlled by laser-plasma interactions. This opens up a promising and feasible way to produce ultra-bright GeV {\gamma}-photons and positron beams with desirable angular momentum for a wide range of scientific research and applications.

By means of systematic simulations we demonstrate generation of a variety of ring-shaped optical vortices (OVs) from a two-dimensional input with embedded vorticity, in a dissipative medium modeled by the cubic-quintic complex Ginzburg-Landau equation with an inhomogeneous effective diffusion (spatial-filtering) term, which is anisotropic in the transverse plane and periodically modulated in the longitudinal direction. We show the generation of stable square- and gear-shaped OVs, as well as tilted oval-shaped vortex rings, and string-shaped bound states built of a central fundamental soliton and two vortex satellites, or of three fundamental solitons. Their shape can be adjusted by tuning the strength and modulation period of the inhomogeneous diffusion. Stability domains of the generated OVs are identified by varying the vorticity of the input and parameters of the inhomogeneous diffusion. The results suggest a method to generate new types of ring-shaped OVs with applications to the work with structured light., Comment: 24 pages, 5 figures; Nonlinear Dynamics, in press

We investigate Airy-soliton interactions in self-defocusing media with PT potentials in one transverse dimension. We discuss different potentials in which the interacting beams with different phases are launched into the media at different separation distances. During interactions, there exist a primary collision region and a relaxation region accompanied by continuous interaction with the dispersed Airy tail. In the relaxation region, the beams exist soliton-like and breathers-like propagation. The beam width and mean power are influenced by initial separation, phase shift and modulation depth of PT potentials. Especially, the collision distance decreases with the spatial beam separation and the mean power possesses sinusoidal dependence on the phase shift., Comment: 16 pages, 5 figures

The recent LIGO-Virgo detection of gravitational waves from a binary neutron star inspiral event GW170817 and the discovery of its accompanying electromagnetic signals mark a new era for multimessenger astronomy. In the coming years, advanced gravitational-wave detectors are likely to detect tens to hundreds of similar events. Neutron stars in binaries can possess significant spin, which is imprinted on the gravitational waveform via the effective spin parameter $\chi_\text{eff}$. We explore the astrophysical inferences made possible by gravitational-wave measurements of $\chi_\text{eff}$. First, using a fiducial model informed by radio observations, we estimate that $\approx15-30\%$ of binary neutron stars should have spins measurable at $\gtrsim 90\%$ confidence level by advanced detectors assuming the spin axis of the recycled neutron star aligns with the total orbital angular momentum of the binary. Second, using Bayesian inference, we show that it is possible to tell whether or not the spin axis of the recycled neutron star tends to be aligned with the binary orbit using $\gtrsim 30$ detections. Finally, interesting constraints can be placed on neutron star magnetic field decay after $\gtrsim 300$ detections, if the spin periods and magnetic field strengths of Galactic binary neutron stars are representative of the merging population., Comment: Accepted for publication in Phys. Rev. D; 14 pages, 8 figures, 4 tables

We have investigated mixed-gap vector solitons involving incoherently coupled fundamental and dipole components in a parity-time (PT) symmetric lattice with saturable nonlinearity. For the focusing case, vector solitons emerge from the semi-infinite and the first finite gaps, while for the defocusing case, vector solitons emerge from the first finite and the second finite gaps. For both cases, we find that stronger saturable nonlinearity is relative to sharper increase/decrease of soliton power with propagation constant and to narrower existence domain of vector solitons. This finding is helpful for realizing high-power solitons with limited range of propagation constant. Additionally, our numerical calculations show that increasing the weight of dipole component results in destabilization of vector solitons, while stronger saturable nonlinearity to certain extent suppresses the instability of vector solitons., Comment: 16 pages, 7 figures