We combine simulations with new analyses that overcome previous pitfalls to explicate how nonhelical mean-field dynamos grow and saturate in unstratified, magnetorotationally driven turbulence. Shear of the mean radial magnetic field amplifies the azimuthal component. Radial fields are regenerated by velocity fluctuations that induce shear of radial magnetic fluctuations, followed by Lorentz and Coriolis forces that source a negative off-diagonal component in the turbulent diffusivity tensor. We present a simple schematic to illustrate this dynamo growth. A different part of the Lorentz force forms a third-order correlator in the mean electromotive force that saturates the dynamo.
We are on the cusp of detecting gravitational waves (GWs) from individual massive black hole binaries (MBHBs) with the Laser Interferometer Space Antenna and pulsar-timing arrays. These MBHBs may be surrounded by circumbinary disks and minidisks, the electromagnetic emission from which are essential for localizing the MBHBs on the sky. Here we present the first radiative magnetohydrodynamics (RMHD) minidisk simulation that directly solves the radiative transfer equation on discretized grid rays. The simulation examines one of the minidisks in an equal-mass $2\times10^7\,M_\odot$ MBHB separated by 100 gravitational radii. Minidisks simulated with and without radiative effects resemble each other qualitatively but differ in several key aspects. The RMHD minidisk is denser and geometrically thinner than the magnetohydrodynamics minidisk. Furthermore, the RMHD minidisk, with a nonaxisymmetric photosphere and temperature distribution, produces an anisotropic illumination pattern. As a result, the observed radiative flux of two RMHD minidisks orbiting each other varies at half the binary orbital period, a feature independent of relativistic boosting and lensing effects. Such periodic light curves, if identified in upcoming optical transient surveys, could reveal the existence of MBHBs on the way to merger, particularly if they are in a constant phase relation with detected GWs.
We present panchromatic optical $+$ near-infrared (NIR) $+$ mid-infrared (MIR) observations of the intermediate-luminosity Type Iax supernova (SN Iax) 2024pxl and the extremely low-luminosity SN Iax 2024vjm. JWST observations provide unprecedented MIR spectroscopy of SN Iax, spanning from $+$11 to $+$42 days past maximum light. We detect forbidden emission lines in the MIR at these early times while the optical and NIR are dominated by permitted lines with an absorption component. Panchromatic spectra at early times can thus simultaneously show nebular and photospheric lines, probing both inner and outer layers of the ejecta. We identify spectral lines not seen before in SN Iax, including [Mg II] 4.76 $\mu$m, [Mg II] 9.71 $\mu$m, [Ne II] 12.81 $\mu$m, and isolated O I 2.76 $\mu$m that traces unburned material. Forbidden emission lines of all species are centrally peaked with similar kinematic distributions, indicating that the ejecta are well mixed in both SN 2024pxl and SN 2024vjm, a hallmark of pure deflagration explosion models. Radiative transfer modeling of SN 2024pxl shows good agreement with a weak deflagration of a near-Chandrasekhar-mass white dwarf, but additional IR flux is needed to match the observations, potentially attributable to a surviving remnant. We favor a weak deflagration origin for SN 2024vjm because of its panchromatic spectral similarity to SN 2024pxl, despite the large difference in luminosity. However, our comparison weak deflagration models are all too luminous and rapidly fading compared to SN 2024vjm; future modeling should push to even weaker explosions and include the contribution of a bound remnant. Our observations demonstrate the diagnostic power of panchromatic spectroscopy for unveiling explosion physics in thermonuclear supernovae.
Type-II radio bursts are typically observed below ~ 400 MHz, with narrow-band slowly-drifting fundamental and harmonic structures. Here we report an unusual high-frequency wide-band type-II burst with starting frequency as high as 600 - 700 MHz and the instantaneous bandwidth being as wide as ~ 300 MHz. The estimated average spectral drift is ~ 2.18 MHz/s, its mean duration at each frequency is ~ 3 min, the maximum brightness temperature can exceed 10^11 to 10^12 K. According to the simultaneous EUV and radio imaging data, the radio sources distribute over a relatively broad region that concentrates around a dip of the nose front of the shock-like EUV wave structure. The dip is likely caused by the strong interaction of the eruption with the overlying closed dense loops that are enclosed by the large-scale streamer structure, indicating that the Type-II burst originates from such CME-shock interaction with dense-closed loop structures. The observations suggest that the unusual wide-band high frequency type-II radio burst originates from a dense streamer region in the corona, supported by EUV shock-like structure that get steepened very close to the solar surface ~ 1.23 R0 and type-II radio source coincides with the shock dip. Further the wide-band feature is due to the source stemming from a region with large density variation and not due to the intensity variation across the shock structure.