1,307 research outputs found
Early Afterglows of Gamma-Ray Bursts in a Stratified Medium with a Power-Law Density Distribution
A long-duration gamma-ray burst (GRB) has been widely thought to arise from
the collapse of a massive star, and it has been suggested that its ambient
medium is a homogenous interstellar medium (ISM) or a stellar wind. There are
two shocks when an ultra-relativistic fireball that has been ejected during the
prompt gamma-ray emission phase sweeps up the circumburst medium: a reverse
shock that propagates into the fireball, and a forward shock that propagates
into the ambient medium. In this paper, we investigate the temporal evolution
of the dynamics and emission of these two shocks in an environment with a
general density distribution of (where is the radius) by
considering thick-shell and thin-shell cases. A GRB afterglow with one smooth
onset peak at early times is understood to result from such external shocks.
Thus, we can determine the medium density distribution by fitting the onset
peak appearing in the light curve of an early optical afterglow. We apply our
model to 19 GRBs, and find that their values are in the range of 0.4 - 1.4,
with a typical value of , implying that this environment is neither a
homogenous interstellar medium with nor a typical stellar wind with
. This shows that the progenitors of these GRBs might have undergone a new
mass-loss evolution.Comment: 32 pages, 5 figures, 1 table, published in Ap
Primordial Black Holes from Sound Speed Resonance during Inflation
We report on a novel phenomenon of the resonance effect of primordial density
perturbations arisen from a sound speed parameter with an oscillatory behavior,
which can generically lead to the formation of primordial black holes in the
early Universe. For a general inflaton field, it can seed primordial density
fluctuations and their propagation is governed by a parameter of sound speed
square. Once if this parameter achieves an oscillatory feature for a while
during inflation, a significant non-perturbative resonance effect on the
inflaton field fluctuations takes place around a critical length scale, which
results in significant peaks in the primordial power spectrum. By virtue of
this robust mechanism, primordial black holes with specific mass function can
be produced with a sufficient abundance for dark matter in sizable parameter
ranges.Comment: 6 pages, 4 figures; v2: figures replotted with corrections, analysis
extended, version accepted by Phys.Rev.Let
N′-[1-(2-Pyridyl)ethylidene]nicotinohydrazide
In the the title compound, C13H12N4O, the dihedral angle between the aromatic ring planes is 21.7 (3)°. In the crystal structure, intermolecular N—H⋯O hydrogen bonds lead to C(4) chains
Bis(2-aminopyrimidine-κN 1)diaquadinitrato-κO;κ2 O,O′-cadmium(II) monohydrate
In the title compound, [Cd(NO3)2(C4H5N3)2(H2O)2]·H2O, the Cd atom is seven-coordinated by two 2-aminopyrimidine molecules, two water molecules, one bidentate nitrate anion and one monodentate nitrate anion. A network of N—H⋯O, N—H⋯N and O—H⋯O hydrogen bonds helps to consolidate the crystal structure
Pentaaquabis[4-(2-hydroxybenzylideneamino)benzenesulfonato]lead(II)
In the structure of the title compound, [Pb(C13H10NO4S)2(H2O)5], two S—O bonds and one C—N bond have lengths of 1.421 (9), 1.425 (8) and 1.268 (11) Å, respectively, which suggests they are double bonds. Molecules form a two-dimensional layered structure via O—H⋯O and O—H⋯N interactions. The Pb atom adopts distorted cubo-octahedral coordination
1-Hydroxymethyl-3,12-dioxa-14-azatetracyclo[9.2.1.04,14.05,10]tetradecane
In the title fused-ring compound, C12H13NO3, the two five-membered C3NO rings both approximate to envelope conformations with C atoms in the flap positions. The OH group of the pendant CH2OH unit is disordered over two positions in a 0.528 (5):0.472 (5) ratio. One of the OH groups participates in an O—H⋯N hydrogen bond, generating centrosymmetric dimers in the crystal structure
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