61 research outputs found
Testing the Universal Structured Jet Models of Gamma-Ray Bursts by BATSE Observations
Assuming that the observed gamma-ray burst (GRB) rate as a function of
redshift is proportional to a corrected star formation rate, we derive the
empirical distribution of the viewing angles of long BATSE GRBs, , and the distribution of these bursts in the plane of
against redshift, , by using a tight correlation between
) and ). Our results show that
is well fitted by a log-normal distribution centering at with a width of . We test different
universal structured jet models by comparing model predictions with our
empirical results. To make the comparisons reasonable, an "effective"
threshold, which corresponds to the sample selection criteria of the long GRB
sample, is used. We find that the predictions of a two-Gaussian jet model are
roughly consistent with our empirical results. A brief discussion shows that
cosmological effect on the relation does not
significantly affect our results, but sample selection effects on this
relationship might significantly influence our results.Comment: 5 pages, 6 figures, accepted for publication in A
Identification of Two categories of optically bright gamma-ray bursts
We present the results of a systematical analysis of the intrinsic optical
afterglow light curves for a complete sample of gamma-ray bursts (GRBs)
observed in the period from Feb. 1997 to Aug. 2005. These light curves are
generally well-sampled, with at least four detections in the band. The
redshifts of all the bursts in the sample are available. We derive the
intrinsic band afterglow lightcurves (luminosity versus time within the
cosmic proper rest frame) for these GRBs, and discover a fact that they
essentially follow two universal tracks after 2 hours since the GRB triggers.
The optical luminosities at 1 day show a clear bimodal distribution, peaking at
1.4*10^{46} ergs~s^{-1} for the luminous group and 5.3*10^{44} ergs~s^{-1} for
the dim group. About 75% of the GRBs are in the luminous group, and the other
25% belong to the dim group. While the luminous group has a wide range of
redshift distribution, the bursts in the dim group all appear at a redshift
lower than 1.1.Comment: 10 pages, 2 figures, emulateapj style, accepted for publication by
ApJ Letter
Plant geographic phenotypic variation drives diversification in its associated community of a phytophagous insect and its parasitoids
International audienceBackground: While the communities constituted by phytophageous insects and their parasites may represent half of all terrestrial animal species, understanding their diversification remains a major challenge. A neglected idea is that geographic phenotypic variation in a host plant may lead to heterogeneous evolutionary responses of the different members of the associated communities. This could result in diversification on a host plant by ecological speciation in some species, leading to geographic variation in community composition. In this study we investigated geographic variation of inflorescence receptacle size in a plant, Ficus hirta, and how the hymenopteran community feeding in the inflorescences has responded. Our predictions were:1) Inflorescence size variation affects wasp species differently depending on how they access oviposition sites.2) In some affected lineages of wasps, we may observe vicariant, parapatric species adapted to different inflorescence sizes.Results: We show that fig (the enclosed inflorescence of Ficus) wall thickness varies geographically. The fig-entering pollinating wasp was not affected, while the parasites ovipositing through the fig wall were. Two parapatric species of Philotrypesis, exhibiting strikingly different ovipositor lengths, were recorded. One species of Sycoscapter was also present, and it was restricted, like the shorter-ovipositor Philotrypesis, to the geographic zone where fig walls were thinner.Conclusions: Previous work on fig wasps suggested that parapatric geographic ranges among congenerics were due to adaptation to variation in abiotic factors, complemented by interspecific competition. Our results show that parapatric ranges may also result from adaptation to variation in biotic factors. Within an insect community, differences among species in their response to geographic phenotypic variation of their host plant may result in geographically heterogeneous community structure. Such heterogeneity leads to heterogeneous interaction networks among sites. Our results support the hypothesis that plant geographic phenotypic variation can be a driver of diversification in associated insect communities, and can complement other diversification processes
Influences of tilted thin accretion disks on the optical appearance of hairy black holes in Horndeski gravity
Research on the optical appearance of black holes, both in general relativity
and modified gravity, has been in full swing since the Event Horizon Telescope
Collaboration announced photos of M87 and Sagittarius A.
Nevertheless, limited attention has been given to the impact of tilted
accretion disks on black hole images. This paper investigates the GHz
images of non-rotating hairy black holes illuminated by tilted, thin accretion
disks in Horndeski gravity with the aid of a ray tracing method. The results
indicate that reducing the scalar hair parameter effectively diminishes image
luminosity and extends both the critical curve and the inner shadow. This trend
facilitates the differentiation between hairy black holes and Schwarzschild
black holes. Furthermore, we observe that the inclination of the tilted
accretion disk can mimic the observation angle, consequently affecting image
brightness and the morphology of the inner shadow. In specific parameter
spaces, the disk inclination has the ability to shift the position of the light
spot in the images of hairy black holes. This finding may provide potential
theoretical evidence for the formation of three flares at different positions
in the Sagittarius A image. Additionally, by examining the images of
hairy black holes surrounded by two thin accretion disks, we report the
obscuring effect of the accretion environment on the inner shadow of the black
hole.Comment: 26 pages, 14 figure
Density Functional Theory Calculations on the Interstellar Formation of Biomolecules
The density functional theory (DFT) is the most versatile electronic
structure method used in quantum chemical calculations, and is increasingly
applied in astrochemical research. This mini-review provides an overview of the
applications of DFT calculations in understanding the chemistry that occurs in
star-forming regions. We survey investigations into the formation of
biologically-relevant compounds such as nucleobases in the interstellar medium,
and also covers the formation of both achiral and chiral amino acids, as well
as biologically-relevant molecules such as sugars, and nitrogen-containing
polycyclic aromatic hydrocarbons. Additionally, DFT calculations are used to
estimate the potential barriers for chemical reactions in astronomical
environments. We conclude by noting several areas that require more research,
such as the formation pathways of chiral amino acids, complex sugars and other
biologically-important molecules, and the role of environmental factors in the
formation of interstellar biomolecules
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