High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: Gravitational wave observations indicate the existence of merging black holes (BHs) with high spin ($a\gtrsim0.3$), whose formation pathways are still an open question. A possible way to form those binaries is through the tidal spin-up of a Wolf-Rayet (WR) star by its BH companion. In this work, we investigate this scenario by directly calculating the tidal excitation of oscillation modes in WR star models, determining the tidal spin-up rate, and integrating the coupled spin-orbit evolution for WR-BH binaries. We find that for short-period orbits and massive WR stars, the tidal interaction is mostly contributed by standing gravity modes, in contrast to Zahn's model of travelling waves which is frequently assumed in the literature. The standing modes are less efficiently damped than traveling waves, meaning that prior estimates of tidal spin-up may be overestimated. We show that tidal synchronization is rarely reached in WR-BH binaries, and the resulting BH spins have $a \lesssim 0.4$ for all but the shortest period ($P_{\rm orb} \! \lesssim 0.5 \, {\rm d}$) binaries. Tidal spin-up in lower-mass systems is more efficient, providing an anti-correlation between the mass and spin of the BHs, which could be tested in future gravitational wave data. Non-linear damping processes are poorly understood but may allow for more efficient tidal spin-up. We also discuss a new class of gravito-thermal modes that appear in our calculations.

Abstract: Previous studies have identified two emission modes in PSR B1859+07: a normal mode that has three prominent components in the average profile, with the trailing one being the brightest, and an anomalous mode (i.e. the A mode) where emissions seem to be shifted to an earlier phase. Within the normal mode, further analysis has revealed the presence of two sub-modes, i.e. the cW mode and cB mode, where the central component can appear either weak or bright. As for the anomalous mode, a new bright component emerges in the advanced phase while the bright trailing component in the normal mode disappears. New observations of PSR B1859+07 by using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) have revealed the existence of a previously unknown emission mode, dubbed as the Af mode. In this mode, all emission components seen in the normal and anomalous modes are detected. Notably, the mean polarization profiles of both the A and Af modes exhibit an orthogonal polarization angle jump in the bright leading component. The polarization angles for the central component in the original normal mode follow two distinct orthogonal polarization modes in the A and Af modes respectively. The polarization angles for the trailing component show almost the same but a small systematic shift in the A and Af modes, roughly following the values for the cW and cB modes. Those polarization features of this newly detected emission mode imply that the anomalous mode A of PSR B1859+07 is not a result of ``phase shift" or ``swooshes" of normal components, but simply a result of the varying intensities of different profile components. Additionally, subpulse drifting has been detected in the leading component of the Af mode.

Abstract: We compute the structure of a Newtonian, multi-ion radiation-mediated shock for different compositions anticipated in various stellar explosions, including supernovae, gamma-ray bursts, and binary neutron star mergers, using a multi-fluid RMS model that incorporates a self-consistent treatment of electrostatic coupling between the different plasma constituents. We find a significant velocity separation between ions having different charge-to-mass ratios in the immediate shock downstream and demonstrate that in fast enough shocks ion-ion collisions can trigger fusion and fission events at a relatively large rate. Our analysis does not take into account potential kinetic effects, specifically, anomalous coupling through plasma microturbulence, that can significantly reduce the velocity spread downstream, below the activation energy for nuclear reactions. A rough estimate of the scale separation in RMS suggests that for shocks propagating in BNS merger ejecta, the anomalous coupling length may exceed the radiation length, allowing a considerable composition change behind the shock via inelastic collisions of $\alpha$ particles with heavy elements at shock velocities $\beta_u\gtrsim0.2$. Moreover, a sufficient abundance of free neutrons upstream of the shock can also trigger fission through neutron capture reactions downstream. The resultant change in the composition profile may affect the properties of the early kilonova emission. The implications for other exploding systems are also briefly discussed.

Abstract: There are 214 X-ray point-sources ($L_{\rm X}>10^{35} \mathrm{erg/s}$) identified as X-ray binaries (XRBs) in the nearby spiral galaxy M83. Since XRBs are powered by accretion onto a neutron star or a black hole from a companion/donor star these systems are promising progenitors of merging double compact objects (DCOs): black hole - black hole (BH-BH), black hole - neutron star (BH-NS), or neutron star - neutron star (NS-NS) systems. The connection (i.e. XRBs evolving into DCOs) may provide some hints to the yet unanswered question: what is the origin of the LIGO/Virgo/KAGRA mergers? Available observations do not allow to determine what will be the final fate of the XRBs observed in M83. Yet, we can use evolutionary model of isolated binaries to reproduce the population of XRBs in M83 by matching model XRBs numbers/types/luminosities to observations. Knowing the detailed properties of M83 model XRBs (donor/accretor masses, their evolutionary ages and orbits) we follow their evolution to the death of donor stars to check whether any merging DCOs are formed. Although all merging DCOs in our isolated binary evolution model go through the XRB phase (defined as reaching X-ray luminosity from RLOF/wind accretion onto NS/BH above $10^{35}$ erg/s), only very few XRBs evolve to form merging (in Hubble time) DCOs. For M83 with its solar-like metallicity stars and continiuous star-formation we find that only $\sim 1-2\%$ of model XRBs evolve into merging DCOs depending on the adopted evolutionary physics. This is caused by (i) merger of donor star with compact object during common envelope phase, (ii) binary disruption at the supernova explosion of donor star, (iii) formation of a DCO on a wide orbit (merger time longer than Hubble time).

Abstract: The extremely high brightness temperatures of fast radio bursts (FRBs) imply that the radiation process must be coherent, but the radiation mechanism is still unknown. The observed properties of narrow spectra and polarization distributions could be used to constrain the radiation mechanism of FRBs. In this work, we discuss the implications of the spectra and polarizations of FRBs from the perspective of intrinsic radiation mechanisms. We first analyze the observed relative spectral bandwidth of radio bursts from an FRB repeater. Furthermore, we generally discuss the properties of the spectra and polarization of the radiation mechanisms involving the relativistic particle's perpendicular acceleration, which depends on the relation between the particle's deflection angle $\psi$ and the radiation beaming angle $1/\gamma$. We find that: (1) If the narrow spectra of FRBs are attributed to the intrinsic radiation mechanism of a single particle, the condition of $\gamma\psi\ll1$ would be necessary, in which scenario, the observed number fraction between linearly and circularly polarized bursts of some FRB repeaters might be due to the propagation effects; (2) Coherent process by multiple particles with some special distributions can lead to a narrow spectrum even for the scenario with $\gamma\psi\gg1$; (3) If the observed number fraction between linearly and circularly polarized bursts is attributed to the radiation mechanism with $\gamma\psi\gg1$, the cumulative distributions of the linear and circular polarization degrees would mainly depend on the particle's beaming distribution.