ABSTRACT
Wind-fed models offer a unique way to form predictive models of the accretion flow surrounding Sagittarius A*. We present 3D wind-fed magnetohydrodynamic (MHD) and general relativistic ...magnetohydrodynamic (GRMHD) simulations spanning the entire dynamic range of accretion from parsec scales to the event horizon. We expand on previous work by including non-zero black hole spin and dynamically evolved electron thermodynamics. Initial conditions for these simulations are generated from simulations of the observed Wolf–Rayet stellar winds in the Galactic Centre. The resulting flow tends to be highly magnetized (β ≈ 2) with an ∼r−1 density profile independent of the strength of magnetic fields in the winds. Our simulations reach the magnetically arrested disc (MAD) state for some, but not all cases. In tilted flows, standard and normal evolution (SANE) jets tend to align with the angular momentum of the gas at large scales, even if that direction is perpendicular to the black hole spin axis. Conversely, MAD jets tend to align with the black hole spin axis. The gas angular momentum shows similar behaviour: SANE flows tend to only partially align while MAD flows tend to fully align. With a limited number of dynamical free parameters, our models can produce accretion rates, 230 GHz flux, and unresolved linear polarization fractions roughly consistent with observations for several choices of electron heating fraction. Absent another source of large-scale magnetic field, winds with a higher degree of magnetization (e.g. where the magnetic pressure is 1/100 of the ram pressure in the winds) may be required to get a sufficiently large rotation measure with consistent sign.
ABSTRACT
Neutron star mergers are unique laboratories of accretion, ejection, and r-process nucleosynthesis. We used 3D general relativistic magnetohydrodynamic simulations to study the role of the ...post-merger magnetic geometry in the evolution of merger remnant discs around stationary Kerr black holes. Our simulations fully capture mass accretion, ejection, and jet production, owing to their exceptionally long duration exceeding 4 s. Poloidal post-merger magnetic field configurations produce jets with energies Ejet ∼ (4–30) × 1050 erg, isotropic equivalent energies Eiso ∼ (4–20) × 1052 erg, opening angles θjet ∼ 6–13°, and durations tj ≲ 1 s. Accompanying the production of jets is the ejection of $f_\mathrm{ej}\sim 30\!-\!40{{\ \rm per\ cent}}$ of the post-merger disc mass, continuing out to times >1 s. We discover that a more natural, purely toroidal post-merger magnetic field geometry generates large-scale poloidal magnetic flux of alternating polarity and striped jets. The first stripe, of $E_\mathrm{jet}\simeq 2\times 10^{48}\, \mathrm{erg}$, Eiso ∼ 1051 erg, θjet ∼ 3.5–5°, and tj ∼ 0.1 s, is followed by ≳4 s of striped jet activity with $f_\mathrm{ej}\simeq 27{{\ \rm per\ cent}}$. The dissipation of such stripes could power the short-duration gamma-ray burst (sGRB) prompt emission. Our simulated jet energies and durations span the range of sGRBs. We find that although the blue kilonova component is initially hidden from view by the red component, it expands faster, outruns the red component, and becomes visible to off-axis observers. In comparison to GW 170817/GRB 170817A, our simulations underpredict the mass of the blue relative to red component by a factor of few. Including the dynamical ejecta and neutrino absorption may reduce this tension.
The most promising astrophysical sources of kHz gravitational waves (GWs) are the inspiral and merger of binary neutron star(NS)/black hole systems. Maximizing the scientific return of a GW detection ...will require identifying a coincident electromagnetic (EM) counterpart. One of the most likely sources of isotropic EM emission from compact object mergers is a supernova-like transient powered by the radioactive decay of heavy elements synthesized in ejecta from the merger. We present the first calculations of the optical transients from compact object mergers that self-consistently determine the radioactive heating by means of a nuclear reaction network; using this heating rate, we model the light curve with a one-dimensional Monte Carlo radiation transfer calculation. For an ejecta mass ∼10−2 M⊙ (10−3 M⊙) the resulting light-curve peaks on a time-scale ∼1 d at a V-band luminosity νLν∼ 3 × 1041 (1041) erg s−1MV=−15(−14); this corresponds to an effective ‘f’ parameter ∼3 × 10−6 in the Li–Paczynski toy model. We argue that these results are relatively insensitive to uncertainties in the relevant nuclear physics and to the precise early-time dynamics and ejecta composition. Since NS merger transients peak at a luminosity that is a factor of ∼103 higher than a typical nova, we propose naming these events ‘kilo-novae’. Because of the rapid evolution and low luminosity of NS merger transients, EM counterpart searches triggered by GW detections will require close collaboration between the GW and astronomical communities. NS merger transients may also be detectable following a short-duration gamma-ray burst or ‘blindly’ with present or upcoming optical transient surveys. Because the emission produced by NS merger ejecta is powered by the formation of rare r-process elements, current optical transient surveys can directly constrain the unknown origin of the heaviest elements in the Universe.
ABSTRACT It is shown that low-collisionality plasmas cannot support linearly polarized shear-Alfvén fluctuations above a critical amplitude , where β is the ratio of thermal to magnetic pressure. ...Above this cutoff, a developing fluctuation will generate a pressure anisotropy that is sufficient to destabilize itself through the parallel firehose instability. This causes the wave frequency to approach zero, interrupting the fluctuation before any oscillation. The magnetic field lines rapidly relax into a sequence of angular zig-zag structures. Such a restrictive bound on shear-Alfvén-wave amplitudes has far-reaching implications for the physics of magnetized turbulence in the high-β conditions prevalent in many astrophysical plasmas, as well as for the solar wind at ∼1 au where β 1.
ABSTRACT
We present the implementation and the first results of cosmic ray (CR) feedback in the Feedback In Realistic Environments (FIRE) simulations. We investigate CR feedback in non-cosmological ...simulations of dwarf, sub-L⋆ starburst, and L⋆ galaxies with different propagation models, including advection, isotropic, and anisotropic diffusion, and streaming along field lines with different transport coefficients. We simulate CR diffusion and streaming simultaneously in galaxies with high resolution, using a two-moment method. We forward-model and compare to observations of γ-ray emission from nearby and starburst galaxies. We reproduce the γ-ray observations of dwarf and L⋆ galaxies with constant isotropic diffusion coefficient $\kappa \sim 3\times 10^{29}\, {\rm cm^{2}\, s^{-1}}$. Advection-only and streaming-only models produce order of magnitude too large γ-ray luminosities in dwarf and L⋆ galaxies. We show that in models that match the γ-ray observations, most CRs escape low-gas-density galaxies (e.g. dwarfs) before significant collisional losses, while starburst galaxies are CR proton calorimeters. While adiabatic losses can be significant, they occur only after CRs escape galaxies, so they are only of secondary importance for γ-ray emissivities. Models where CRs are ‘trapped’ in the star-forming disc have lower star formation efficiency, but these models are ruled out by γ-ray observations. For models with constant κ that match the γ-ray observations, CRs form extended haloes with scale heights of several kpc to several tens of kpc.
The proton temperature anisotropy in the solar wind is known to be constrained by the theoretical thresholds for pressure-anisotropy-driven instabilities. Here, we use approximately 1x10;{6} ...independent measurements of gyroscale magnetic fluctuations in the solar wind to show for the first time that these fluctuations are enhanced along the temperature anisotropy thresholds of the mirror, proton oblique firehose, and ion cyclotron instabilities. In addition, the measured magnetic compressibility is enhanced at high plasma beta (beta_{ parallel} greater, similar1) along the mirror instability threshold but small elsewhere, consistent with expectations of the mirror mode. We also show that the short wavelength magnetic fluctuation power is a strong function of collisionality, which relaxes the temperature anisotropy away from the instability conditions and reduces correspondingly the fluctuation power.
ABSTRACT
We present 3D general relativistic magnetohydrodynamic simulations of zero angular momentum accretion around a rapidly rotating black hole, modified by the presence of initially uniform ...magnetic fields. We consider several angles between the magnetic field direction and the black hole spin. In the resulting flows, the mid-plane dynamics are governed by magnetic reconnection-driven turbulence in a magnetically arrested (or a nearly arrested) state. Electromagnetic jets with outflow efficiencies ∼10–200 per cent occupy the polar regions, reaching several hundred gravitational radii before they dissipate due to the kink instability. The jet directions fluctuate in time and can be tilted by as much as ∼30○ with respect to black hole spin, but this tilt does not depend strongly on the tilt of the initial magnetic field. A jet forms even when there is no initial net vertical magnetic flux since turbulent, horizon-scale fluctuations can generate a net vertical field locally. Peak jet power is obtained for an initial magnetic field tilted by 40○–80○ with respect to the black hole spin because this maximizes the amount of magnetic flux that can reach the black hole. These simulations may be a reasonable model for low luminosity black hole accretion flows such as Sgr A* or M87.
A three-dimensional, nonlinear gyrokinetic simulation of plasma turbulence resolving scales from the ion to electron gyroradius with a realistic mass ratio is presented, where all damping is provided ...by resolved physical mechanisms. The resulting energy spectra are quantitatively consistent with a magnetic power spectrum scaling of k(-2.8) as observed in in situ spacecraft measurements of the "dissipation range" of solar wind turbulence. Despite the strongly nonlinear nature of the turbulence, the linear kinetic Alfvén wave mode quantitatively describes the polarization of the turbulent fluctuations. The collisional ion heating is measured at subion-Larmor radius scales, which provides evidence of the ion entropy cascade in an electromagnetic turbulence simulation.
Approximately 1/4-1/2 of short duration gamma-ray bursts (GRBs) are followed by variable X-ray emission lasting ∼100 s with a fluence comparable or exceeding that of the initial burst itself. The ...long duration and significant energy of this 'extended emission' (EE) poses a major challenge to the standard binary neutron star (NS) merger model. Metzger et al. recently proposed that the EE is powered by the spin-down of a strongly magnetized neutron star (a millisecond protomagnetar), which either survives the NS-NS merger or is created by the accretion-induced collapse (AIC) of a white dwarf. However, the effects of surrounding material on the magnetar outflow have not yet been considered. Here we present time-dependent axisymmetric relativistic magnetohydrodynamic simulations of the interaction of the relativistic protomagnetar wind with a surrounding 10−1-10−3 M⊙ envelope, which represents material ejected during the merger, in the supernova following AIC, or via outflows from the initial accretion disc. The collision between the relativistic magnetar wind and the expanding ejecta produces a termination shock and a magnetized nebula inside the ejecta. A strong toroidal magnetic field builds up in the nebula, which drives a bipolar jet out through the ejecta, similar to the magnetar model developed in the case of long-duration GRBs. We quantify the 'breakout' time and opening angle of the jet θj as a function of the wind energy flux
and ejecta mass M
ej. We show that
and θj are inversely correlated, such that the beaming-corrected (isotropic) luminosity of the jet (and hence the observed EE) is primarily a function of M
ej. Both variability arguments, and the lower limit on the power of magnetar outflows capable of producing bright emission, suggest that the true opening angle of the magnetar jet must be relatively large. The model thus predicts a class of events for which the EE is observable with no associated short GRB. These may appear as long-duration GRBs or X-ray flashes unaccompanied by a bright supernova and not solely associated with massive star formation, which may be detected by future all-sky X-ray survey missions.
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