16,004 research outputs found
Prediction of biopore- and matrix-dominated flow from X-ray CT-derived macropore network characteristics
Peer reviewedPublisher PD
The (Double) White Dwarf Binary SDSS 1257+5428
SDSS 1257+5428 is a white dwarf in a close orbit with a companion that has
been suggested to be a neutron star. If so, it hosts the closest known neutron
star, and its existence implies a great abundance of similar systems and a rate
of white-dwarf neutron-star mergers similar to that of the type Ia supernova
rate. Here, we present high signal-to-noise spectra of SDSS 1257+5428, which
confirm an independent finding that the system is in fact composed of two white
dwarfs, one relatively cool and with low mass, and the other hotter and more
massive. With this, the demographics and merger rate are no longer puzzling
(various factors combine to lower the latter by more than two orders of
magnitude). We show that the spectra are fit well with a combination of two
hydrogen model atmospheres, as long as the lines of the higher-gravity
component are broadened significantly relative to what is expected from just
pressure broadening. Interpreting this additional broadening as due to
rotation, the inferred spin period is short, about 1 minute. Similarly rapid
rotation is only seen in accreting white dwarfs that are magnetic; empirically,
it appears that in non-magnetized white dwarfs, accreted angular momentum is
lost by nova explosions before it can be transferred to the white dwarf. This
suggests that the massive white dwarf in SDSS 1257+5428 is magnetic as well,
with B~10^5 G. Alternatively, the broadening seen in the spectral lines could
be due to a stronger magnetic field, of ~10^6 G. The two models could be
distinguished by further observations.Comment: 9 pages, 3 figures, submitted to Ap
PTF 11kx: A Type Ia Supernova with a Symbiotic Nova Progenitor
There is a consensus that type Ia supernovae (SNe Ia) arise from the thermonuclear explosion of white dwarf stars that accrete matter from a binary companion. However, direct observation of SN Ia progenitors is lacking, and the precise nature of the binary companion remains uncertain. A temporal series of high-resolution optical spectra of the SN Ia PTF 11kx reveals a complex circumstellar environment that provides an unprecedentedly detailed view of the progenitor system. Multiple shells of circumstellar material are detected, and the SN ejecta are seen to interact with circumstellar material starting 59 days after the explosion. These features are best described by a symbiotic nova progenitor, similar to RS Ophiuchi
SN1998bw: The Case for a Relativistic Shock
SN1998bw shot to fame by claims of association with GRB980425. Independent of
its presumed association with a GRB, this SN is unusual in its radio
properties. A simple interpretation of the unusually bright radio emission
leads us to the conclusion that there are two shocks in this SN: a slow moving
shock containing most of the ejecta and a relativistic shock (Gamma=2) which is
responsible for the radio emission. This is the first evidence for the
existence of relativistic shocks in supernovae. It is quite plausible that this
shock may produce high energy emission (at early times and by inverse Compton
scattering). As with other supernovae, we expect radio emission at much later
times powered primarily by the slow moving ejecta. This expectation has
motivated us to continue monitoring this unusual SN.Comment: A&A (in press), Rome GRB Symposium, Nov. 199
Permanent spin currents in cavity-qubit systems
In a recent remarkable experiment [P. Roushan et al., Nature Physics 13, 146
(2017)], a spin current in an architecture of three superconducting qubits was
produced during a few microseconds by creating synthetic magnetic fields. The
life-time of the current was set by the typical dissipative mechanisms that
occur in those systems. We propose a scheme for the generation of permanent
currents, even in the presence of such imperfections, and scalable to larger
system sizes. It relies on striking a subtle balance between multiple
nonequilibrium drives and the dissipation mechanisms, in order to engineer and
stimulate chiral excited states which can carry current.Comment: 4 pages, 3 figure
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