12,966 research outputs found
No periodicity revealed for an "eclipsing" ultraluminous supersoft X-ray source in M81
Luminous supersoft X-ray sources found in the Milky Way and Magellanic Clouds
are likely white dwarfs that steadily or cyclically burn accreted matter on
their surface, which are promising type Ia supernova progenitors. Observations
of distant galaxies with Chandra and XMM-Newton have revealed supersoft sources
that are generally hotter and more luminous, including some ultraluminous
supersoft sources (ULSs) that are possibly intermediate mass black holes of a
few thousand solar masses. In this paper we report our X-ray spectral and
timing analysis for M81-ULS1, an ultraluminous supersoft source in the nearby
spiral galaxy M81. M81-ULS1 has been persistently supersoft in 17 Chandra ACIS
observations spanning six years, and its spectrum can be described by either a
eV blackbody for a white dwarf, or a
eV multicolor accretion disk for a
intermediate mass black hole. In two observations, the light curves exhibited
dramatic flux drop/rise on time scales of seconds, reminiscent of
eclipse ingress/egress in eclipsing X-ray binaries. However, the exhaustive
search for periodicity in the reasonable range of 50 ksec to 50 days failed to
reveal an orbital period. The failure to reveal any periodicity is consistent
with the long period ( yrs) predicted for this system given the optical
identification of the secondary with an asymptotic giant star. Also, the
eclipse-like dramatic flux changes in hours are hard to explain under the white
dwarf model, but can in principle be explained by disk temperature changes
induced by accretion rate variations under the intermediate mass black hole
model.Comment: 19 pages, 7 figures, 1 table, to appear in ApJ
Variable Ly alpha sheds light on the environment surrounding GRB 090426
Long duration gamma-ray bursts are commonly associated with the deaths of
massive stars. Spectroscopic studies using the afterglow as a light source
provide a unique opportunity to unveil the medium surrounding it, probing the
densest region of their galaxies. This material is usually in a low ionisation
state and at large distances from the burst site, hence representing the normal
interstellar medium in the galaxy. Here we present the case of GRB 090426 at
z=2.609, whose optical spectrum indicates an almost fully ionised medium
together with a low column density of neutral hydrogen. For the first time, we
also observe variations in the Ly alpha absorption line. Photoionisation
modeling shows that we are probing material from the vicinity of the burst (~80
pc). The host galaxy is a complex of two luminous interacting galaxies, which
might suggest that this burst could have occurred in an isolated star-forming
region outside its host galaxy created in the interaction of the two galaxies.Comment: 11 pages, 9 figures, submitted to MNRA
Genealogies of rapidly adapting populations
The genetic diversity of a species is shaped by its recent evolutionary
history and can be used to infer demographic events or selective sweeps. Most
inference methods are based on the null hypothesis that natural selection is a
weak or infrequent evolutionary force. However, many species, particularly
pathogens, are under continuous pressure to adapt in response to changing
environments. A statistical framework for inference from diversity data of such
populations is currently lacking. Toward this goal, we explore the properties
of genealogies in a model of continual adaptation in asexual populations. We
show that lineages trace back to a small pool of highly fit ancestors, in which
almost simultaneous coalescence of more than two lineages frequently occurs.
While such multiple mergers are unlikely under the neutral coalescent, they
create a unique genetic footprint in adapting populations. The site frequency
spectrum of derived neutral alleles, for example, is non-monotonic and has a
peak at high frequencies, whereas Tajima's D becomes more and more negative
with increasing sample size. Since multiple merger coalescents emerge in many
models of rapid adaptation, we argue that they should be considered as a
null-model for adapting populations.Comment: to appear in PNA
To Be Captured
To Be Captured, a 3D animated short, utilizes some toys as the film’s main characters to increase the sense of artificiality, while trying to animate their movements as closely to the forces of the real world as possible. It tells a story of a group of stuffed animals enclosed in a doll claw machine waiting to be captured. The main character, one of the smarter toys, is a seahorse who plays tricks to be captured when a little girl starts to play the game. Once out of the box, he despairs realizing that the girl is a fanatic who only cares about catching the biggest toy in the machine, which is the bear. While playing, she becomes furious when she fails to capture her prize. The bear in the box trembles with fear when he witnesses her ill-treatment of her previous prizes, the carrots and the seahorse, and he begs for help. The animals work together to protect the bear and then have to rely only on themselves to face the challenges of a truly free life. This thesis short shows that the use of fictional narrative, artificial stage setting, and anthropomorphic dolls in an animated film featuring toy characters can still produce a strong sense of realism if viewed from Brecht’s A-Effect perspective and balanced with the realistic representational approaches in animation, such as facial expression personification, varied camera angles, detailed movement analysis, expressive area lighting design, and texture parameter adjustment. This thesis presents a journey of exploration of how to create this fantasy short film and animate it in a realistic manner
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