Multi-messenger astronomy combining Gravitational Wave (GW) and Electromagnetic Wave (EM) observation brings huge impact on physics, astrophysics and cosmology. However, the majority of sources to be detected with currently running ground-based GW ob
servatories are binary black hole (BBH) mergers, which are expected disappointedly to have no EM counterparts. In this letter, we propose that if the BBH merger happens in a gaseous disk around a supermassive black hole, the merger can be accompanied by a transient radio flare alike a fast radio burst (FRB). We argue that the total mass and the effective spin derived from GW detection can be used to distinguish such a source from other channels of BBH mergers. If the prediction is confirmed with future observation, multi-messenger astronomy can be brought to a distance which is one order of magnitude farther than present. The mystery of the origin of FRBs can also be revealed partially.
We study linear polarization of optical emission from white dwarf (WD) binary system AR~Scorpii. The optical emission from this binary is modulating with the beat frequency of the system, and it is highly polarized, with the degree of the polarizatio
n reaching $sim 40$%. The angle of the polarization monotonically increases with the spin phase, and the total swing angle can reach $360^{circ}$ over one spin phase. It is also observed that the morphology of the pulse profile and the degree of linear polarization evolve with the orbital phase. These polarization properties can constrain the scenario for nonthermal emission from AR Scorpii. In this paper, we study the polarization properties predicted by the emission model, in which (i) the pulsed optical emission is produced by the synchrotron emission from relativistic electrons trapped by magnetic field lines of the WD and (ii) the emission is mainly produced at magnetic mirror points of the electron motion. We find that this model can reproduce the large swing of the polarization angle, provided that the distribution of the initial pitch angle of the electrons that are leaving the M-type star is biased to a smaller angle rather than a uniform distribution. The observed direction of the swing suggests that the Earth viewing angle is less than $90^{circ}$ measured from the WD spin axis. The current model prefers an Earth viewing angle of $50^{circ}-60^{circ}$ and a magnetic inclination angle of $50^{circ}-60^{circ}$ (or $120^{circ}-130^{circ}$). We discuss that the different contribution of the emission from M-type star to total emission causes a large variation in the pulsed fraction and the degree of the linear polarization along the orbital phase.
Among the four black hole binary merger events detected by LIGO, six progenitor black holes have masses greater than 20,$M_odot$. The existence of such massive BHs calls for extreme metal-poor stars as the progenitors. An alternative possibility that
a pair of stellar mass black holes each with mass $sim7,M_odot$ increases to $>20,M_odot$ via accretion from a disk surrounding a super massive black hole in an active galactic nucleus is considered. The growth of mass of the binary and the transfer of orbital angular momentum to the disk accelerates the merger. Based on the recent numerical work of Tang et al. (2017), it is found that, in the disk of a low mass AGN with mass $sim10^6,M_odot$ and Eddington ratio $>0.01$, the mass of an individual BH in the binary can grow to $>20,M_odot$ before coalescence provided that accretion takes place at a rate more than 10 times the Eddington value. The mechanism predicts a new class of gravitational wave sources involving the merger of two extreme Kerr black holes associated with active galactic nuclei and a possible electromagnetic wave counterpart.
AR~Scorpii is an intermediate polar system composed of a magnetic white dwarf (WD) and an M-type star, and shows non-thermal, pulsed, and highly linearly polarized emission. The radio/optical emission modulates with the WDs spin and show the double p
eak structure in the light curves. In this paper, we discuss a possible scenario for the radiation mechanism of AR~Scorpii. The magnetic interaction on the surface of the companion star produces an outflow from the companion star, the heating of the companion star surface, and the acceleration of electrons to a relativistic energy. The accelerated electrons, whose typical Lorentz factor is $sim 50-100$, from the companion star move along the magnetic field lines toward the WD surface. The electrons injected with the pitch angle of $sintheta_{p,0}>0.05$ are subject to the magnetic mirror effect and are trapped in the closed magnetic field line region.We find that the emission from the first magnetic mirror points mainly contributes to the observed pulsed emission and the formation of the double-peak structure in the light curve. For the inclined rotator, the pulse peak in the calculated light curve shifts the position in the spin phase, and a Fourier analysis exhibits a beat frequency feature, which are consistent with the optical/UV observations. The pulse profile also evolves with the orbital phase owing to the effect of the viewing geometry. The model also interprets the global features of the observed spectral energy distribution in radio to X-ray energy bands. We also discuss the curvature radiation and the inverse-Compton scattering process in the outer gap accelerator of the WD in AR Scorpii and discuss the possibility of the detection by future high-energy missions.
PSR B1259-63/LS2883 is a binary system composed of a pulsar and a Be star. The Be star has an equatorial circumstellar disk (CD). The {it Fermi} satellite discovered unexpected gamma-ray flares around 30 days after the last two periastron passages. T
he origin of the flares remain puzzling. In this work, we explore the possibility that, the GeV flares are consequences of inverse Compton-scattering of soft photons by the pulsar wind. The soft photons are from an accretion disk around the pulsar, which is composed by the matter from CD captured by the pulsars gravity at disk-crossing before the periastron. At the other disk-crossing after the periastron, the density of the CD is not high enough so that accretion is prevented by the pulsar wind shock. This model can reproduce the observed SEDs and light curves satisfactorily.
We discuss X-ray and gamma-ray emissions from Crab-like pulsars, PSRs~J0537-6910 and~J0540-6919, in Large Magellanic Cloud. Fermi-LAT observations have resolved the gamma-ray emissions from these two pulsars and found the pulsed emissions from PSR~J0
540-6919. The total pulsed radiation in the X-ray/gamma-ray energy bands of PSR~J0540-6919 is observed with the efficiency $eta_{J0540}sim 0.06$ (in 4$pi$ sr), which is about a factor of ten larger than $eta_{Crab}sim 0.006$ of the Crab pulsar. Although PSR~J0537-6910 has the highest spin-down power among currently known pulsars, the efficiency of the observed X-ray emissions is about two orders of magnitude smaller than that of PSR~J0540-6919. This paper mainly discusses what causes the difference in the radiation efficiencies of these three energetic Crab-like pulsars. We discuss electron/positron acceleration and high-energy emission processes within the outer gap model. By solving the outer gap structure with the dipole magnetic field, we show that the radiation efficiency decreases as the inclination angle between the magnetic axis and the rotation axis increases. To explain the difference in the pulse profile and in the radiation efficiency, our model suggests that PSR~J0540-6919 has an inclination angle much smaller than the that of Crab pulsar (here we assume the inclination angles of both pulsars are $alpha<90^{circ}$). On the other hand, we speculate that the difference in the radiation efficiencies between PSRs~J0537-6910 and J0549-6919 is mainly caused by the difference in the Earth viewing angle, and that we see PSR~J0537-6910 with an Earth viewing angle $zeta>>90^{circ}$ (or $<<90^{circ}$) measured from the spin axis, while we see PSR~J0540-6919 with $zetasim 90^{circ}$.
We study a possible connection between processes of gamma-ray emission and hydrogen ionization in a few pc of central region around Sgr A*. Previous investigations showed there is a discrepancy between interpretation of gamma-ray and ionization data
if gamma-rays are generated by proton-proton collisions. Here we provided analysis of processes of ionization and emission basing on analytical and numerical calculations of kinetic equations which describe processes of particle propagation and their energy losses. The origin of gamma rays could be either due to collisions of relativistic protons with the dense gas of the surrounding circumnuclear disk (CND) or bremsstrahlung and inverse Compton scattering of relativistic electrons. The hydrogen ionization in this case is produced by a low energy component of the CR spectrum. We found that if ionization is produced by protons the expected ionization rate of hydrogen in the CND is of the same order as derived from IR observations. So we do not see any discrepancy between the gamma-ray and ionization data for the hadronic model. In the case of ionization by electrons we obtained the ionization rate one order of magnitude higher than follows from the IR data. In principle, a selection between the leptonic and hadronic interpretations can be performed basing on measurements of radio and X-ray fluxes from this region because the leptonic and hadronic models give different values of the fluxes from there. We do not exclude that gamma-ray production and hydrogen ionization in the CND are due to a past activity of Sgr A* which occurred about 100 year ago. Then we hypothesize that there may be connection between a past proton eruption and a flux of hard X-rays emitted by Sgr A* hundred years ago as follows from the observed time variability of the iron line seen in the direction of GC molecular clouds.
Fermi bubbles are giant gamma-ray structures extended north and south of the Galactic center with characteristic sizes of order of 10 kpc recently discovered by Fermi Large Area Telescope. Good correlation between radio and gamma-ray emission in the
region covered by Fermi bubbles implies the presence of high-energy electrons in this region. Since it is relatively difficult for relativistic electrons of this energy to travel all the way from the Galactic sources toward Fermi bubbles one can assume that they accelerated in-situ. The corresponding acceleration mechanism should also affect the distribution of the relativistic protons in the Galaxy. Since protons have much larger lifetimes the effect may even be observed near the Earth. In our model we suggest that Fermi bubbles are created by acceleration of electrons on series of shocks born due to periodic star accretions by supermassive black hole Sgr A*. We propose that hadronic CR within the knee of the observed CR spectrum are produced by Galactic supernova remnants distributed in the Galactic disk. Reacceleration of these particles in the Fermi Bubble produces CRs beyond the knee. This model provides a natural explanation of the observed CR flux, spectral indexes, and matching of spectra at the knee.
The discovery of multiband afterglows definitely shows that most $gamma$-ray bursts are of cosmological origin. $gamma$-ray bursts are found to be one of the most violent explosive phenomena in the Universe, in which astonishing ultra-relativistic mo
tions are involved. In this article, the multiband observational characteristics of $gamma$-ray bursts and their afterglows are briefly reviewed. The standard model of $gamma$-ray bursts, i.e. the fireball model, is described. Emphasis is then put on the importance of the nonrelativistic phase of afterglows. The concept of deep Newtonian phase is elaborated. A generic dynamical model that is applicable in both the relativistic and nonrelativistic phases is introduced. Based on these elaborations, the overall afterglow behaviors, from the very early stages to the very late stages, can be conveniently calculated.
Non-thermal TeV $gamma$-ray emission within a multiparsec has been observed from the center region of our Galaxy. We argue that these $gamma$-rays are the result of transient activity of the massive black hole Sgr A$^*$ that resides at the Galactic C
enter. Several thousand years ago, the black hole may have experienced an active phase by capturing a red giant star and forming an accretion disk, temporarily behaving like an active galactic nucleus. A powerful jet, which contains plenty of high speed protons, was launched during the process. These runaway protons interact with the dense ambient medium, producing TeV $gamma$-ray emission through the $pi^0$-decay process. We show that the total energy deposited in this way is large enough to account for observations. The diffusion length of protons is also consistent with the observed size of the TeV source.
