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<jats:title>Abstract</jats:title> <jats:p>The calling card of solar magnetism is the sunspot cycle, during which sunspots regularly reverse their polarity sense every 11 yr. However, a number of more complicated time-dependent behaviors have also been identified. In particular, there are temporal modulations associated with active longitudes and hemispheric asymmetry, when sunspots appear at certain solar longitudes or else in one hemisphere preferentially. So far, a direct link between this asymmetric temporal behavior and the underlying solar dynamo has remained elusive. In this work, we present results from global 3D magnetohydrodynamic simulations, which display both behavior reminiscent of the sunspot cycle (regular polarity reversals and equatorward migration of internal magnetic field) and asymmetric, irregular behavior which we interpret as active longitudes and hemispheric asymmetry in the simulations. The simulations are thus bistable, in that the turbulent convection can stably support two distinct flavors of magnetism at different times, in superposition or with smooth transitions from one state to the other. We discuss this new family of dynamo models in the context of the extensive observations of the Sun’s surface magnetic field with the <jats:italic>Solar and Heliospheric Observatory</jats:italic> and the <jats:italic>Solar Dynamics Observatory</jats:italic>, as well as earlier observations of sunspot number and synoptic maps. We suggest that the solar dynamo itself may be bistable in nature, exhibiting two types of temporal behavior in the magnetic field.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Standing shocks are believed to be responsible for stationary Type II solar radio bursts, whereas drifting Type II bursts are excited by moving shocks often related to coronal mass ejections (CMEs). Observations of either stationary or drifting Type II bursts are common, but a transition between the two states has not yet been reported. Here, we present a Type II burst which shows a clear, continuous transition from a stationary to a drifting state, the first observation of its kind. Moreover, band splitting is observed in the stationary parts of the burst, as well as intriguing negative and positive frequency-drift fine structures within the stationary emissions. The relation of the radio emissions to an observed jet and a narrow CME were investigated across multiple wavelengths, and the mechanisms leading to the transitioning Type II burst were determined. We find that a jet eruption generates a streamer-puff CME and that the interplay between the CME-driven shock and the streamer is likely to be responsible for the observed radio emissions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>There is no complete description of the emission physics during the prompt phase in gamma-ray bursts. Spectral analyses, however, indicate that many spectra are narrower than what is expected for nonthermal emission models. Here, we reanalyze the sample of 37 bursts in Yu et al. by fitting the narrowest time-resolved spectrum in each burst. We perform a model comparison between photospheric and synchrotron emission models based on Bayesian evidence. We compare the shapes of the narrowest expected spectra: emission from the photosphere in a non-dissipative flow and slow cooled synchrotron emission from a narrow electron distribution. We find that the photospheric spectral shape is preferred by 54% ± 8% of the spectra (20/37), while 38% ± 8% of the spectra (14/37) prefer the synchrotron spectral shape; three spectra are inconclusive. We hence conclude that GRB spectra are indeed very narrow and that more than half of the bursts have a photospheric emission episode. We also find that a third of all analyzed spectra, not only prefer, but are also compatible with a non-dissipative photosphere, confirming previous similar findings. Furthermore, we notice that the spectra that prefer the photospheric model all have low-energy power-law indices <jats:italic>α</jats:italic> ≳ −0.5. This means that <jats:italic>α</jats:italic> is a good estimator for which model is preferred by the data. Finally, we argue that the spectra that statistically prefer the synchrotron model could equally as well be caused by subphotospheric dissipation. If that is the case, photospheric emission during the early, prompt phase would be even more dominant.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The magnetic field structure of a star-forming Bok globule BHR 71 was determined based on near-infrared polarimetric observations of background stars. The magnetic field in BHR 71 was mapped from 25 stars. By using a simple 2D parabolic function, the plane-of-sky magnetic axis of the core was found to be <jats:italic>θ</jats:italic> <jats:sub>mag</jats:sub> = 125° ± 11°. The plane-of-sky mean magnetic field strength of BHR 71 was found to be <jats:italic>B</jats:italic> <jats:sub>pos</jats:sub> = 8.8–15.0 <jats:italic>μ</jats:italic>G, indicating that the BHR 71 core is magnetically supercritical with <jats:italic>λ</jats:italic> = 1.44–2.43. Taking into account the effect of thermal/turbulent pressure and the plane-of-sky magnetic field component, the critical mass of BHR 71 was <jats:italic>M</jats:italic> <jats:sub>cr</jats:sub> = 14.5–18.7 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, which is consistent with the observed core mass of <jats:italic>M</jats:italic> <jats:sub>core</jats:sub> ≈ 14.7 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. We conclude that BHR 71 is in a condition close to a kinematically critical state, and the magnetic field direction lies close to the plane of sky. Since BHR 71 is a star-forming core, a significantly subcritical condition (i.e., the magnetic field direction deviating from the plane of sky) is unlikely, and collapsed from a condition close to a kinematically critical state. There are two possible scenarios to explain the curved magnetic fields of BHR 71, one is an hourglass-like field structure due to mass accumulation and the other is the Inoue &amp; Fukui mechanism, which proposes the interaction of the core with a shock wave to create curved magnetic fields wrapping around the core.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Although their existence is not yet confirmed observationally, intermediate-mass black holes (IMBHs) may play a key role in the dynamics of galactic nuclei. In this paper, we neglect the effect of the nuclear star cluster itself and investigate only how a small reservoir of IMBHs influences the secular dynamics of stellar-mass black hole binaries, using <jats:italic>N</jats:italic>-body simulations. We show that our simplifications are valid and that the IMBHs significantly enhance binary evaporation by pushing the binaries into the Hill-unstable region of parameter space, where they are separated by the supermassive black hole’s tidal field. For binaries in the <jats:italic>S</jats:italic>-cluster region of the Milky Way, IMBHs drive the binaries to merge in up to 1%–6% of cases, assuming five IMBHs within 5 pc of mass 10<jats:sup>4</jats:sup> <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab7921ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> each. Observations of binaries in the Galactic center may strongly constrain the population of IMBHs therein.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study the ensemble of linear polarization measurements in the optical afterglows of long-duration gamma-ray bursts. We assume a non-sideways-expanding top-hat jet geometry and use the relatively large number of measurements under the assumption that they represent a statistically unbiased sample. This allows us to constrain the ratio between the maximum predicted polarization and the measured one, which is an indicator of the geometry of the magnetic field in the downstream region of the external shock. We find that the measured polarization is substantially suppressed with respect to the maximum possible for either a completely ordered magnetic field parallel to the shock normal or to a field that is entirely contained in the shock plane. The measured polarization is limited, on average, to between 25% and 30% of the maximum theoretically possible value. This reduction requires the perpendicular component of the magnetic field to be dominant in energy with respect to the component parallel to the shock front, as expected for a shock-generated and/or shock-compressed field. We find, however, that the data only marginally support the assumption of a simple top-hat jet, pointing toward a more complex geometry for the outflow.</jats:p>