59,362 research outputs found
Reverse Shock Emission in Gamma-ray Bursts Revisited
A generic synchrotron external shock model is the widely preferred paradigm
used to interpret the broad-band afterglow data of gamma-ray bursts (GRBs),
including predicted observable signatures from a reverse shock which have been
confirmed by observations. Investigations of the nature of the reverse shock
emission can provide valuable insights into the intrinsic properties of the GRB
ejecta. Here we briefly review the standard and the extended models of the
reverse shock emission, discussing the connection between the theory and
observations, including the implications of the latest observational advances.Comment: Invited review, to be published in special issue on "GRB in Swift and
Fermi Era" in Journal of Advances in Astronom
Fast Radio Burst/Gamma-Ray Burst Cosmography
Recently, both theoretical arguments and observational evidence suggested
that a small fraction of fast radio bursts (FRBs) could be associated with
gamma-ray bursts (GRBs). If such FRB/GRB association systems are commonly
detected in the future, the combination of dispersion measure (DM) derived from
FRBs and redshifts derived from GRBs makes these systems a plausible tool to
conduct cosmography. We quantify uncertainties in deriving the
redshift-dependent DM_{IGM} as a function of z, and test how well dark energy
models can be constrained with Monte Carlo simulations. We show that with
potentially several 10s of FRB/GRB systems detected in a decade or so, one may
reach reasonable constraints on wCDM models. When combined with SN Ia data,
unprecedented constraints on dark energy equation of state may be achieved,
thanks to the prospects of detecting FRB/GRB systems at relatively high
redshifts. The ratio between the mean value and luminosity
distance (D_{L} (z)) is insensitive to dark energy models. This gives the
prospects of applying SN Ia data to calibrate using a
relatively small sample of FRB/GRB systems, allowing a reliable constraint on
the baryon inhomogeneity distribution as a function of redshift. The
methodology developed in this paper can also be applied, if the FRB redshifts
can be measured by other means. Some caveats of putting this method into
practice are also discussed.Comment: Accepted for publication in Ap
Possible High-Energy Neutrino and Photon Signals from Gravitational Wave Bursts due to Double Neutron Star Mergers
As the technology of gravitational-wave and neutrino detectors becomes
increasingly mature, a multi-messenger era of astronomy is ushered in. Advanced
gravitational wave detectors are close to making a ground-breaking discovery of
gravitational wave bursts (GWBs) associated with mergers of double neutron
stars (NS-NS). It is essential to study the possible electromagnetic (EM) and
neutrino emission counterparts of these GWBs. Recent observations and numerical
simulations suggest that at least a fraction of NS-NS mergers may leave behind
a massive millisecond magnetar as the merger product. Here we show that protons
accelerated in the forward shock powered by a magnetar wind pushing the ejecta
launched during the merger process would interact with photons generated in the
dissipating magnetar wind and emit high energy neutrinos and photons. We
estimate the typical energy and fluence of the neutrinos from such a scenario.
We find that PeV neutrinos could be emitted from the shock front as long
as the ejecta could be accelerated to a relativistic speed. The diffuse
neutrino flux from these events, even under the most optimistic scenarios, is
too low to account for the two events announced by the IceCube Collaboration,
but it is only slightly lower than the diffuse flux of GRBs, making it an
important candidate for the diffuse background of PeV neutrinos. The
neutron-pion decay of these events make them a moderate contributor to the
sub-TeV gamma-ray diffuse background.Comment: Accepted for publication in PRD, minor revisio
Labelled tree graphs, Feynman diagrams and disk integrals
In this note, we introduce and study a new class of "half integrands" in
Cachazo-He-Yuan (CHY) formula, which naturally generalize the so-called
Parke-Taylor factors; these are dubbed Cayley functions as each of them
corresponds to a labelled tree graph. The CHY formula with a Cayley function
squared gives a sum of Feynman diagrams, and we represent it by a combinatoric
polytope whose vertices correspond to Feynman diagrams. We provide a simple
graphic rule to derive the polytope from a labelled tree graph, and classify
such polytopes ranging from the associahedron to the permutohedron.
Furthermore, we study the linear space of such half integrands and find (1) a
nice formula reducing any Cayley function to a sum of Parke-Taylor factors in
the Kleiss-Kuijf basis (2) a set of Cayley functions as a new basis of the
space; each element has the remarkable property that its CHY formula with a
given Parke-Taylor factor gives either a single Feynman diagram or zero. We
also briefly discuss applications of Cayley functions and the new basis in
certain disk integrals of superstring theory.Comment: 30+8 pages, many figures;typos fixe
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