A white dwarf (WD) approaching the Chandrasekhar mass may in several circumstances undergo accretion–induced collapse (AIC) to a neutron star (NS) before a thermonuclear explosion ensues. It has ...generally been assumed that such an AIC does not produce a detectable supernova (SN). If, however, the progenitor WD is rapidly rotating (as may be expected due to its prior accretion), a centrifugally supported disc forms around the NS upon collapse. We calculate the subsequent evolution of this accretion disc and its nuclear composition using time–dependent height–integrated simulations with initial conditions taken from the AIC calculations of Dessart and collaborators. Soon after its formation, the disc is cooled by neutrinos and its composition is driven neutron rich (electron fraction Ye∼ 0.1) by electron captures. However, as the disc viscously spreads, it is irradiated by neutrinos from the central proto–NS, which dramatically alters its neutron–to–proton ratio. We find that electron neutrino captures increase Ye to ∼0.5 by the time that weak interactions in the disc freeze out. Because the disc becomes radiatively inefficient and begins forming α–particles soon after freeze out, powerful winds blow away most of the disc's remaining mass. These Ye∼ 0.5 outflows synthesize up to a few times 10−2 M⊙ in 56Ni. As a result, AIC may be accompanied by a radioactively powered SN–like transient that peaks on a time–scale of ∼1 d. Since few intermediate mass elements are likely synthesized, these nickel–rich explosions should be spectroscopically distinct from other SNe. The time–scale, velocity and composition of the AIC transient can be modified if the disc wind sweeps up a ∼0.1 M⊙ remnant disc created by a WD–WD merger; such an ‘enshrouded’ AIC may account for sub–luminous, sub–Chandrasekhar Type I SNe. Optical transient surveys such as the Panoramic Survey Telescope and Rapid Response System and the Palomar Transient Factory should detect a few AIC transients per year if their true rate is ∼10−2 of the type Ia rate, and Large Synoptic Survey Telescope should detect several hundred per year. High cadence observations (≲1 d) are optimal for the detection and followup of AIC.
We propose that pressure anisotropy causes weakly collisional turbulent plasmas to self-organize so as to resist changes in magnetic-field strength. We term this effect ‘magneto-immutability’ by ...analogy with incompressibility (resistance to changes in pressure). The effect is important when the pressure anisotropy becomes comparable to the magnetic pressure, suggesting that in collisionless, weakly magnetized (high-
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) plasmas its dynamical relevance is similar to that of incompressibility. Simulations of magnetized turbulence using the weakly collisional Braginskii model show that magneto-immutable turbulence is surprisingly similar, in most statistical measures, to critically balanced magnetohydrodynamic turbulence. However, in order to minimize magnetic-field variation, the flow direction becomes more constrained than in magnetohydrodynamics, and the turbulence is more strongly dominated by magnetic energy (a non-zero ‘residual energy’). These effects represent key differences between pressure-anisotropic and fluid turbulence, and should be observable in the
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turbulent solar wind.
We study the linear stability of weakly magnetized differentially rotating plasmas in both collisionless kinetic theory and Braginskii's theory of collisional, magnetized plasmas. We focus on the ...very weakly magnetized limit in which ..., where beta is the ratio of thermal to magnetic energy and ... is the ratio of the cyclotron frequency to rotation frequency. This regime is important for understanding how astrophysical magnetic fields originate and are amplified at high redshift. We show that the single instability of fluid theory -- the magnetorotational instability mediated by magnetic tension -- is replaced by two distinct instabilities, one associated with ions and one with electrons. Each of these has a different way of tapping into the free energy of differential rotation. The ion instability is driven by viscous transport of momentum across magnetic field lines due to a finite ion cyclotron frequency (gyroviscosity); the fastest growing modes have wavelengths significantly longer than magnetohydrodynamic (MHD) and Hall MHD predictions. The electron instability is a whistler mode driven unstable by the temperature anisotropy generated by differential rotation; the growth time can be orders of magnitude shorter than the rotation period. The electron instability is an example of a broader class of instabilities that tap into the free energy of differential rotation or shear via the temperature anisotropy they generate. We briefly discuss the application of our results to the stability of planar shear flows and show that such flows are linearly overstable in the presence of fluid gyroviscosity. We also briefly describe the implications of our results for magnetic field amplification in the virialized haloes of high-redshift galaxies. (ProQuest: ... denotes formulae/symbols omitted.)
I calculate the linear stability of a stratified low-collisionality plasma in the presence of a weak magnetic field. Heat is assumed to flow only along magnetic field lines. In the absence of a heat ...flux in the background plasma, Balbus demonstrated that plasmas in which the temperature increases in the direction of gravity are buoyantly unstable to convective-like motions (the "magnetothermal instability"). I show that in the presence of a background heat flux, an analogous instability is present when the temperature decreases in the direction of gravity. The instability is driven by the background heat flux, and the fastest growing mode has a growth time of order the local dynamical time. Thus, independent of the sign of the temperature gradient, weakly magnetized low-collisionality plasmas are unstable on a dynamical time to magnetically mediated buoyancy instabilities. The instability described in this paper is predicted to be present in clusters of galaxies at radii similar to 0.1-100 kpc, where the observed temperature increases outward. Possible consequences for the origin of cluster magnetic fields, "cooling flows," and the thermodynamics of the intercluster medium are briefly discussed.
Pressure anisotropy can strongly influence the dynamics of weakly collisional, high-beta plasmas, but its effects are missed by standard magnetohydrodynamics (MHD). Small changes to the ...magnetic-field strength generate large pressure-anisotropy forces, heating the plasma, driving instabilities and rearranging flows, even on scales far above the particles’ gyroscales where kinetic effects are traditionally considered most important. Here, we study the influence of pressure anisotropy on turbulent plasmas threaded by a mean magnetic field (Alfvénic turbulence). Extending previous results that were concerned with Braginskii MHD, we consider a wide range of regimes and parameters using a simplified fluid model based on drift kinetics with heat fluxes calculated using a Landau-fluid closure. We show that viscous (pressure-anisotropy) heating dissipates between a quarter (in collisionless regimes) and half (in collisional regimes) of the turbulent-cascade power injected at large scales; this does not depend strongly on either plasma beta or the ion-to-electron temperature ratio. This will in turn influence the plasma's thermodynamics by regulating energy partition between different dissipation channels (e.g. electron and ion heat). Due to the pressure anisotropy's rapid dynamic feedback onto the flows that create it – an effect we term ‘magneto-immutability’ – the viscous heating is confined to a narrow range of scales near the forcing scale, supporting a nearly conservative, MHD-like inertial-range cascade, via which the rest of the energy is transferred to small scales. Despite the simplified model, our results – including the viscous heating rate, distributions and turbulent spectra – compare favourably with recent hybrid-kinetic simulations. This is promising for the more general use of extended-fluid (or even MHD) approaches to model weakly collisional plasmas such as the intracluster medium, hot accretion flows and the solar wind.
We report on observations of GRB 080503, a short gamma-ray burst (GRB) with very bright extended emission (about 30 times the gamma-ray fluence of the initial spike) in conjunction with a thorough ...comparison to other short Swift events. In spite of the prompt-emission brightness, however, the optical counterpart is extraordinarily faint, never exceeding 25 mag in deep observations starting at ~1 hr after the Burst Alert Telescope (BAT) trigger. The optical brightness peaks at ~1 day and then falls sharply in a manner similar to the predictions of Li & Paczynski (1998) for supernova-like emission following compact binary mergers. However, a shallow spectral index and similar evolution in X-rays inferred from Chandra observations are more consistent with an afterglow interpretation. The extreme faintness of this probable afterglow relative to the bright gamma-ray emission argues for a very low density medium surrounding the burst (a 'naked' GRB), consistent with the lack of a coincident host galaxy down to 28.5 mag in deep Hubble Space Telescope imaging. The late optical and X-ray peak could be explained by a slightly off-axis jet or by a refreshed shock. Our observations reinforce the notion that short GRBs generally occur outside regions of active star formation, but demonstrate that in some cases the luminosity of the extended prompt emission can greatly exceed that of the short spike, which may constrain theoretical interpretation of this class of events. This extended emission is not the onset of an afterglow, and its relative brightness is probably either a viewing-angle effect or intrinsic to the central engine itself. Because most previous BAT short bursts without observed extended emission are too faint for this signature to have been detectable even if it were present at typical level, conclusions based solely on the observed presence or absence of extended emission in the existing Swift sample are premature.
This paper reports measurements of Sgr A* made with NACO in L' band (3.80 is a subset of m), Ks band (2.12 is a subset of m), and H band (1.66 is a subset of m), and with VISIR in N band (11.88 is a ...subset of m) at the ESO VLT, as well as with XMM-Newton at X-ray (2-10 keV) wavelengths. On 2007 April 4, a very bright flare was observed from Sgr A* simultaneously at L' band and X-ray wavelengths. No emission was detected using VISIR. The resulting spectral energy distribution has a blue slope ( beta >0 for Delta L Delta Delta beta , consistent with Delta L Delta Delta 0.4) between 12 is a subset of m and 3.8 is a subset of m. For the first time, our high-quality data allow a detailed comparison of infrared (IR) and X-ray light curves with a resolution of a few minutes. The IR and X-ray flares are simultaneous to within 3 minutes. However, the IR flare lasts significantly longer than the X-ray flare (both before and after the X-ray peak), and prominent substructures in the 3.8 is a subset of m light curve are clearly not seen in the X-ray data. From the shortest timescale variations in the L'-band light curve, we find that the flaring region must be no more than 1.2RS in size. The high X-ray to IR flux ratio, blue Delta L Delta slope MIR to L' band, and the soft Delta L Delta spectral index of the X-ray flare together place strong constraints on possible flare emission mechanisms. We find that it is quantitatively difficult to explain this bright X-ray flare with inverse Compton processes. A synchrotron emission scenario from an electron distribution with a cooling break is a more viable scenario.
We explore the effects of r-process nucleosynthesis on fallback accretion in neutron star (NS)–NS and black hole–NS mergers, and the resulting implications for short-duration gamma-ray bursts (GRBs). ...Though dynamically important, the energy released during the r-process is not yet taken into account in merger simulations. We use a nuclear reaction network to calculate the heating (due to β decays and nuclear fission) experienced by material on the marginally bound orbits nominally responsible for late-time fallback. Since matter with longer orbital periods torb experiences lower densities, for longer periods of time, the total r-process heating rises rapidly with torb, such that material with torb≳ 1 s can become completely unbound. Thus, r-process heating fundamentally changes the canonical prediction of an uninterrupted power-law decline in the fallback rate at late times. When the time-scale for r-process to complete is ≳1 s, the heating produces a complete cut-off in fallback accretion after ∼1 s; if robust, this would imply that fallback accretion cannot explain the late-time X-ray flaring observed following some short GRBs. However, for a narrow, but physically plausible, range of parameters, fallback accretion can resume after ∼10 s, despite having been strongly suppressed for ∼1–10 s after the merger. This suggests the intriguing possibility that the gap observed between the prompt and extended emission in short GRBs is a manifestation of r-process heating.
We model the interaction between the wind from a newly formed rapidly rotating magnetar and the surrounding supernova shock and host star. The dynamics is modelled using the two-dimensional, ...axisymmetric thin-shell equations. In the first ∼10–100 s after core-collapse the magnetar inflates a bubble of plasma and magnetic fields behind the supernova shock. The bubble expands asymmetrically because of the pinching effect of the toroidal magnetic field, even if the host star is spherically symmetric, just as in the analogous problem of the evolution of pulsar wind nebulae. The degree of asymmetry depends on Emag/Etot, the ratio of the magnetic energy to the total energy in the bubble. The correct value of Emag/Etot is uncertain because of uncertainties in the conversion of magnetic energy into kinetic energy at large radii in relativistic winds; we argue, however, that bubbles inflated by newly formed magnetars are likely to be significantly more magnetized than their pulsar counterparts. We show that for a ratio of magnetic to total power supplied by the central magnetar the bubble expands relatively spherically. For , however, most of the pressure in the bubble is exerted close to the rotation axis, driving a collimated outflow out through the host star. This can account for the collimation inferred from observations of long-duration gamma-ray bursts (GRBs). Outflows from magnetars become increasingly magnetically dominated at late times, due to the decrease in neutrino-driven mass loss as the young neutron star cools. We thus suggest that the magnetar-driven bubble initially expands relatively spherically, enhancing the energy of the associated supernova, while at late times it becomes progressively more collimated, producing the GRB. The same physical processes may operate in more modestly rotating neutron stars to produce asymmetric supernovae and lower energy transients such as X-ray flashes.
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