390 research outputs found
The dynamics of internal working surfaces in MHD jets
The dynamical effects of magnetic fields in models of radiative, Herbig-Haro
(HH) jets have been studied in a number of papers. For example, magnetized,
radiative jets from variable sources have been studied with axisymmetric and 3D
numerical simulations. In this paper, we present an analytic model describing
the effect of a toroidal magnetic field on the internal working surfaces that
result from a variability in the ejection velocity. We find that for parameters
appropriate for HH jets the forces associated with the magnetic field dominate
over the gas pressure force within the working surfaces. Depending on the ram
pressure radial cross section of the jet, the magnetic field can produce a
strong axial pinch, or, alternatively, a broadening of the internal working
surfaces. We check the validity of the analytic model with axisymmetric
numerical simulations of variable, magnetized jets.Comment: 14 pages, 4 figures. ApJ in pres
Simulation of dark lanes in post--flare supra--arcades
Using two simulations of 1.5D, for the first time, in Costa et al. (2009) and
Shulz et al. (2010) we numerically reproduce the observational dark inflows
described in Verwichte et al. (2005). We show that the dark tracks can be
explained as hot plasma vacuums generated upstream of a slow magnetoacoustic
shock wave produced by a localized deposition of energy. In this work, we show
that the overall 2D results are in agreement with the observational behaviour.
However they show a slight shift in the characteristic parameter with respect
to those found previously. Also, we confirm qualitatively the behaviour found
in the previous papers, i.e. for a given numerical domain the period of the
kink--like structure is a function of the magnetic field intensity: larger
periods are associated with lower magnetic field intensities. Contrary to the
1D result -where the sunward dynamic is independent of the magnetic field
intensity due to its exclusively waveguide role- in the 2D simulation the
sunward speed is larger for larger values of the magnetic field. This can be
interpreted as the capability of the low coronal plasma to collimate the
deposition of energy into the magnetic field direction. The moving features
consistent of low--density and high--temperature plasma cavities have larger
inside values of the structuring parameter beta than the neighboring media.
Thus, the voids seem to be the emergence structures of a whole nonlinear
interacting plasma context of shocks and waves more than voided plasma loops
magnetically structured.Comment: 5 pages, 5 figure
Single-bubble and multi-bubble cavitation in water triggered by laser-driven focusing shock waves
In this study a single laser pulse spatially shaped into a ring is focused
into a thin water layer, creating an annular cavitation bubble and cylindrical
shock waves: an outer shock that diverges away from the excitation laser ring
and an inner shock that focuses towards the center. A few nanoseconds after the
converging shock reaches the focus and diverges away from the center, a single
bubble nucleates at the center. The inner diverging shock then reaches the
surface of the annular laser-induced bubble and reflects at the boundary,
initiating nucleation of a tertiary bubble cloud. In the present experiments,
we have performed time-resolved imaging of shock propagation and bubble wall
motion. Our experimental observations of single-bubble cavitation and collapse
and appearance of ring-shaped bubble clouds are consistent with our numerical
simulations that solve a one dimensional Euler equation in cylindrical
coordinates. The numerical results agree qualitatively with the experimental
observations of the appearance and growth of bubble clouds at the smallest
laser excitation rings. Our technique of shock-driven bubble cavitation opens
novel perspectives for the investigation of shock-induced single-bubble or
multi-bubble cavitation phenomena in thin liquids
The broad-line type Ic SN 2020bvc: signatures of an off-axis gamma-ray burst afterglow
Long-duration gamma-ray bursts (GRBs) are almost unequivocally associated
with very energetic, broad-lined supernovae (SNe) of Type Ic-BL. While the
gamma-ray emission is emitted in narrow jets, the SN emits radiation
isotropically. Therefore, some SN Ic-BL not associated with GRBs have been
hypothesized to arise from events with inner engines such as off-axis GRBs or
choked jets. Here we present observations of the nearby ( Mpc) SN
2020bvc (ASAS-SN 20bs) which support this scenario. \textit{Swift} UVOT
observations reveal an early decline (up to two days after explosion) while
optical spectra classify it as a SN Ic-BL with very high expansion velocities
( 70,000 km/s), similar to that found for the jet-cocoon emission in
SN 2017iuk associated with GRB 171205A. Moreover, \textit{Swift} X-Ray
Telescope and \textit{CXO} X-ray Observatory detected X-ray emission only three
days after the SN and decaying onwards, which can be ascribed to an afterglow
component. Cocoon and X-ray emission are both signatures of jet-powered GRBs.
In the case of SN 2020bvc, we find that the jet is off axis (by 23
degrees), as also indicated by the lack of early ( day) X-ray
emission which explains why no coincident GRB was detected promptly or in
archival data. These observations suggest that SN 2020bvc is the first orphan
GRB detected through its associated SN emission.Comment: 9 pages, 6 figures, 5 tables. Accepted for publication in A&
Description of Atypical Bursts Seen Slightly Off-axis
The detection of gravitational waves together with their electromagnetic counterpart, in the gamma-ray burst GRB 170817A, marked a new era o multi-messenger astronomy. Several theoretical models have been propose to explain the atypical behaviour of this event. Recently, it was show that the multi-wavelength afterglow of GRB 170817A was consistent with synchrotron forward-shock model when the outflow was viewed off-axis decelerated in a uniform medium and parametrized through a power-la energy emission, and the stratified medium in the close vicinity of binary neutron star merger proposed to explain the gamma-ray flux in th short GRB 150101B, we extend the mechanism proposed to explain GR 170817A to a more general scenario deriving the synchrotron self-Compto (SSC) and synchrotron forward-shock model when the off-axis outflow i decelerated in a uniform and stratified circumburst density. A particular cases, we show that the delayed and long-lasting afterglo emission observed in GRB 080503, GRB140903A, GRB 150101B and GRB 160821 could be interpreted by a similar scenario to the one used to describ GRB 170817A. In addition, we show that the proposed scenario agrees wit the MAGIC, Fermi-LAT and H.E.S.S upper limits on gamma-ray emission fro GRB 160821B and GRB 170817A
Radiation-magnetohydrodynamic simulations of the photoionisation of magnetised globules
We present the first three-dimensional radiation-magnetohydrodynamic
simulations of the photoionisation of a dense, magnetised molecular globule by
an external source of ultraviolet radiation. We find that, for the case of a
strong ionising field, significant deviations from the non-magnetic evolution
are seen when the initial magnetic field threading the globule has an
associated magnetic pressure that is greater than one hundred times the gas
pressure. In such a strong-field case, the photoevaporating globule will adopt
a flattened or "curled up" shape, depending on the initial field orientation,
and magnetic confinement of the ionised photoevaporation flow can lead to
recombination and subsequent fragmentation during advanced stages of the
globule evolution. We find suggestive evidence that such magnetic effects may
be important in the formation of bright, bar-like emission features in H II
regions. We include simple but realistic fits to heating and cooling rates in
the neutral and molecular gas in the vicinity of a high-mass star cluster and
show that the frequently used isothermal approximation can lead to an
overestimate of the importance of gravitational instability in the radiatively
imploded globule. For globules within 2 parsecs of a high-mass star cluster, we
find that heating by stellar x rays prevents the molecular gas from cooling
below 50 K.Comment: 19 pages, 16 figures, many in full color. Accepted by MNRAS. Updated
to reflect the accepted version. Significantly expanded and improved with
respect to the first version. Well worth downloading agai
- …