The Astrophysical Journal
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The Astrophysical Journal, Volume 883; doi:10.3847/1538-4357/ab304c
Abstract:Coronal mass ejections (CMEs) often exhibit the classic three-part structure in a coronagraph, i.e., the bright front, dark cavity, and bright core, which are traditionally considered as the manifestations of coronal plasma pileup, magnetic flux rope (MFR), and filament, respectively. However, a recent survey based on 42 CMEs all possessing the three-part structure found that a large majority (69%) do not contain an eruptive filament at the Sun. Therefore, a challenging opinion is proposed and claims that the bright core can also correspond to the MFR, which is supported by the CME simulation. Then what is the nature of the CME core? In this paper, we address this issue through a CME associated with the eruption of a filament-hosting MFR on 2013 September 29. This CME exhibits the three-part morphology in multiple white-light coronagraphs from different perspectives. The new finding is that the bright core contains both a sharp and a fuzzy component. Through tracking the filament motion continuously from its source region to the outer corona, we conclude that the sharp component corresponds to the filament. The fuzzy component is suggested to result from the MFR that supports the filament against the gravity in the corona. Our study can shed more light on the nature of CME cores, and explain the core whether or not the filament is involved with a uniform scenario. The nature of the CME cavity is also discussed.
The Astrophysical Journal, Volume 883; doi:10.3847/1538-4357/ab357e
Abstract:The Low-Frequency Array radio telescope discovered the 707 Hz binary millisecond pulsar (MSP) J0952−0607 in a targeted radio pulsation search of an unidentified Fermi gamma-ray source. This source shows a weak energy flux of F γ = 2.6 × 10−12 erg cm−2 s−1 in the energy range between 100 MeV and 100 GeV. Here we report the detection of pulsed gamma-ray emission from PSR J0952−0607 in a very sensitive gamma-ray pulsation search. The pulsar's rotational, binary, and astrometric properties are measured over 7 years of Fermi-Large Area Telescope data. For this we take into account the uncertainty on the shape of the gamma-ray pulse profile. We present an updated radio-timing solution now spanning more than 2 years and show results from optical modeling of the black-widow-type companion based on new multiband photometric data taken with HiPERCAM on the Gran Telescopio Canarias on La Palma and ULTRACAM on the New Technology Telescope at ESO La Silla (based on observations collected at the European Southern Observatory, Chile; programme 0101.D-0925, PI: Clark, C. J.). PSR J0952−0607 is now the fastest-spinning pulsar for which the intrinsic spin-down rate has been reliably constrained (). The inferred surface magnetic field strength of is among the 10 lowest of all known pulsars. This discovery is another example of an extremely fast spinning black-widow pulsar hiding within an unidentified Fermi gamma-ray source. In the future such systems might help to pin down the maximum spin frequency and the minimum surface magnetic field strength of MSPs.
The Astrophysical Journal, Volume 883; doi:10.3847/1538-4357/ab3850
Abstract:During star formation, the accretion disk drives fast MHD winds, which usually contain two components, a collimated jet and a radially distributed wide-angle wind. These winds entrain the surrounding ambient gas producing molecular outflows. We report a recent observation of 12CO (2–1) emission of the HH 46/47 molecular outflow by the Atacama Large Millimeter/submillimeter Array, in which we identify multiple wide-angle outflowing shell structures in both the blueshifted and redshifted outflow lobes. These shells are highly coherent in position–position–velocity space, extending to 40–50 km s−1 in velocity and 104 au in space, with well-defined morphology and kinematics. We suggest these outflowing shells are the result of the entrainment of ambient gas by a series of outbursts from an intermittent wide-angle wind. Episodic outbursts in collimated jets are commonly observed, yet detection of a similar behavior in wide-angle winds has been elusive. Here we show clear evidence that the wide-angle component of the HH 46/47 protostellar outflows experiences variability similar to that seen in the collimated component.
The Astrophysical Journal, Volume 882; doi:10.3847/1538-4357/ab3d47
The Astrophysical Journal, Volume 882; doi:10.3847/1538-4357/ab36b7
Abstract:We report on the successive occurrence of 05 wide photospheric vortices with strong transverse shear flows at the edge of a sunspot light bridge (LB), and the subsequent ejection of chromospheric surges observed using a Visible Inteferometry Spectrograph, a broadband TiO filter, and a Near InfRared Imaging Spectrograph of the Goode Solar Telescope operating at Big Bear Solar Observatory. The Hα surges ejected at the location of the vortices often appeared in a hollow cylindrical structure. We also observed quasi-periodic vortex-associated bright Hα plasma blobs moving upward with a speed of up to 4 km s−1. In view of the strong shear flow at the edge of the LB, it is likely that the vortices form under the Kelvin–Helmholtz instability. The surges may result from either the magnetic tension generated after magnetic reconnection or an acoustic impulse of a fast photospheric transverse flow. Otherwise, the surges could also be associated with Alfvénic waves, in which case their origin could be torsional magnetic fields generated in the process of the vortex formation.
The Astrophysical Journal, Volume 882; doi:10.3847/1538-4357/ab298a
Abstract:The most important factors determining solar coronal activity are believed to be the availability of magnetic free energy and the constraint of magnetic helicity conservation. Direct measurements of the helicity and magnetic free energy in the coronal volume are difficult, but their values may be estimated from measurements of the helicity and free energy transport rates through the photosphere. We examine these transport rates for a topologically open system such as the corona, in which the magnetic fields have a nonzero normal component at the boundaries, and derive a new formula for the helicity transport rate at the boundaries. In addition, we derive new expressions for helicity transport due to flux emergence/submergence versus photospheric horizontal motions. The key feature of our formulas is that they are manifestly gauge invariant. Our results are somewhat counterintuitive in that only the lamellar electric field produced by the surface potential transports helicity across boundaries, and the solenoidal electric field produced by a surface stream function does not contribute to the helicity transport. We discuss the physical interpretation of this result. Furthermore, we derive an expression for the free energy transport rate and show that a necessary condition for free energy transport across a boundary is the presence of a closed magnetic field at the surface, indicating that there are current systems within the volume. We discuss the implications of these results for using photospheric vector magnetic and velocity field measurements to derive the solar coronal helicity and magnetic free energy, which can then be used to constrain and drive models for coronal activity.
The Astrophysical Journal, Volume 882; doi:10.3847/1538-4357/ab3348
Abstract:The t hermal force (TF) is an exchange force mediated by Coulomb collisions between electrons and ions in a heat-conducting astrophysical plasma, is one of three, non-inertial, balancing terms in the parallel component of the generalized Ohm's law, and is magnetic field aligned with a size that scales with and is parallel to the dimensionless heat flux. The TF (i) increases the size of E ∥ above that implied by the electron pressure divergence; (ii) deepens the electrostatic trap for electrons about the Sun; (iii) strengthens the electron kurtosis and skewness, further levitating ions out of their gravitational well, (iv) constrains the heat flow in a plasma where parallel currents are preempted; and (v) is shown to be directly measurable using the full electron velocity distribution function above and below thermal energies. (vi) The usually ignored TF modifies all species internal energy equations; it enhances the rate of conduction cooling by the electrons, increases the ion entropy, and forestalls adiabatic behavior. Using estimates at 1 au this effect is especially strong in the higher speed wind U > 400 km s−1 regime. (vii) On rather general grounds any physical heat transport is accompanied by an underlying TF; in almost all known cases of modeling astrophysical plasmas this dependence is ignored or demonstrably incorrect. It follows that attempts to predict species specific pressures without inclusion of the TF is futile.
The Astrophysical Journal, Volume 882; doi:10.3847/1538-4357/ab3650
Abstract:We present the first realistic 3D simulations of flame front instabilities during type I X-ray bursts. The unperturbed front is characterized by the balance between the pressure gradient and the Coriolis force of a spinning neutron star (ν = 450 Hz in our case). This balance leads to a fast horizontal velocity field parallel to the flame front. This flow is strongly sheared in the vertical direction. When we perturb the front an instability quickly corrugates the front. We identify this instability as the baroclinic instability. Most importantly, the flame is not disrupted by the instability and there are two major consequences: the overall flame propagation speed is ~10 times faster than in the unperturbed case and distinct flame vortices appear. The speedup is due to the corrugation of the front and the dynamics of the vortices. These vortices may also be linked to the oscillations observed in the light curves of the bursts.
The Astrophysical Journal, Volume 882; doi:10.3847/2041-8213/ab3d41
Abstract:The 180 day Space Telescope and Optical Reverberation Mapping campaign on NGC 5548 discovered an anomalous period, the broad-line region (BLR) holiday, in which the emission lines decorrelated from the continuum variations. This is important since the correlation between the continuum-flux variations and the emission-line response is the basic assumption for black hole (BH) mass determinations through reverberation mapping. During the BLR holiday the high-ionization intrinsic absorption lines also decorrelated from the continuum as a result of the variable covering factor of the line-of-sight (LOS) obscurer. The emission lines are not confined to the LOS, so this does not explain the BLR holiday. If the LOS obscurer is a disk wind, its streamlines must extend down to the plane of the disk and the base of the wind would lie between the BH and the BLR, forming an equatorial obscurer. This obscurer can be transparent to ionizing radiation, or can be translucent, blocking only parts of the spectral energy distribution, depending on its density. An emission-line holiday is produced if the wind density increases only slightly above its transparent state. Both obscurers are parts of the same wind, so they can have associated behavior in a way that explains both holidays. A very dense wind would block nearly all ionizing radiation, producing a Seyfert 2 and possibly providing a contributor to the changing-look active galactic nucleus phenomenon. Disk winds are very common and we propose that the equatorial obscurers are too, but mostly in a transparent state.
The Astrophysical Journal, Volume 882; doi:10.3847/1538-4357/ab364a
Abstract:Interchange reconnection is thought to play an important role in driving the dynamics of the slow solar wind. To understand the details of this process, it is important to catalog the various magnetic structures that are present at the boundary between open and closed magnetic flux. To this end we have developed a numerical method for partitioning the coronal volume into individual flux domains using volume segmentation along layers of high magnetic squashing degree (Q). Our publicly available implementation of this method is able to identify the different magnetic structures within a coronal magnetic field model that define the open-closed boundary and comprise the so-called Separatrix-Web (S-Web). With this we test previous predictions of how different configurations of high-Q arcs within the S-Web are related to coronal magnetic field structures. Here we present our findings from a survey of 11 different potential field source surface models, spanning from 2008 to 2017, which offer a representative sample of the coronal magnetic field across nearly a complete solar cycle. Two key findings of our analysis are that (i) "vertex" structures—where arcs of the S-Web meet away from the heliospheric current sheet—are associated with underlying magnetic dome structures, and (ii) that any given arc of the S-Web is almost equally as likely to be formed by a narrow corridor of open flux (corresponding to a hyperbolic flux tube) as by the separatrix surface of a magnetic null. Together, these findings highlight the importance of a variety of topological configurations for future studies of interchange reconnection and the acceleration of the solar wind.