28,556 research outputs found
Signatures of photon and axion-like particle mixing in the gamma-ray burst jet
Photons couple to Axion-Like Particles (ALPs) or more generally to any pseudo
Nambu-Goldstone boson in the presence of an external electromagnetic field.
Mixing between photons and ALPs in the strong magnetic field of a Gamma-Ray
Burst (GRB) jet during the prompt emission phase can leave observable imprints
on the gamma-ray polarization and spectrum. Mixing in the intergalactic medium
is not expected to modify these signatures for ALP mass > 10^(-14) eV and/or
for < nG magnetic field. We show that the depletion of photons due to
conversion to ALPs changes the linear degree of polarization from the values
predicted by the synchrotron model of gamma ray emission. We also show that
when the magnetic field orientation in the propagation region is perpendicular
to the field orientation in the production region, the observed synchrotron
spectrum becomes steeper than the theoretical prediction and as detected in a
sizable fraction of GRB sample. Detection of the correlated polarization and
spectral signatures from these steep-spectrum GRBs by gamma-ray polarimeters
can be a very powerful probe to discover ALPs. Measurement of gamma-ray
polarization from GRBs in general, with high statistics, can also be useful to
search for ALPs.Comment: 17 pages, 3 figures. Accepted for publication in JCAP with minor
change
Linear and Second-order Optical Response of the III-V Mono-layer Superlattices
We report the first fully self-consistent calculations of the nonlinear
optical properties of superlattices. The materials investigated are mono-layer
superlattices with GaP grown on the the top of InP, AlP and GaAs (110)
substrates. We use the full-potential linearized augmented plane wave method
within the generalized gradient approximation to obtain the frequency dependent
dielectric tensor and the second-harmonic-generation susceptibility. The effect
of lattice relaxations on the linear optical properties are studied. Our
calculations show that the major anisotropy in the optical properties is the
result of strain in GaP. This anisotropy is maximum for the superlattice with
maximum lattice mismatch between the constituent materials. In order to
differentiate the superlattice features from the bulk-like transitions an
improvement over the existing effective medium model is proposed. The
superlattice features are found to be more pronounced for the second-order than
the linear optical response indicating the need for full supercell calculations
in determining the correct second-order response.Comment: 9 pages, 4 figures, submitted to Phy. Rev.
Revisiting the SN1987A gamma-ray limit on ultralight axion-like particles
We revise the bound from the supernova SN1987A on the coupling of ultralight
axion-like particles (ALPs) to photons. In a core-collapse supernova, ALPs
would be emitted via the Primakoff process, and eventually convert into gamma
rays in the magnetic field of the Milky Way. The lack of a gamma-ray signal in
the GRS instrument of the SMM satellite in coincidence with the observation of
the neutrinos emitted from SN1987A therefore provides a strong bound on their
coupling to photons. Due to the large uncertainty associated with the current
bound, we revise this argument, based on state-of-the-art physical inputs both
for the supernova models and for the Milky-Way magnetic field. Furthermore, we
provide major amendments, such as the consistent treatment of
nucleon-degeneracy effects and of the reduction of the nuclear masses in the
hot and dense nuclear medium of the supernova. With these improvements, we
obtain a new upper limit on the photon-ALP coupling: g_{a\gamma} < 5.3 x
10^{-12} GeV^{-1}, for m_a < 4.4 x 10^{-10} eV, and we also give its dependence
at larger ALP masses. Moreover, we discuss how much the Fermi-LAT satellite
experiment could improve this bound, should a close-enough supernova explode in
the near future.Comment: Accepted for publication in JCAP (December 22nd, 2014
Late-time magnetogenesis driven by ALP dark matter and dark photon
We propose a mechanism generating primordial magnetic fields after the
annihilations. Our mechanism involves an ultra-light axion-like
particle (ALP) which constitutes the dark matter, and a dark gauge
boson introduced to bypass the obstacle placed by the conductivity of cosmic
plasma. In our scheme, a coherently oscillating ALP amplifies the dark photon
field, and part of the amplified dark photon field is concurrently converted to
the ordinary magnetic field through the ALP-induced magnetic mixing. For the
relevant ALP mass range , our mechanism can generate with a coherent length kpc, which is large enough to provide a seed of the galactic
magnetic fields. The mechanism also predicts a dark electromagnetic
field , which
can result in interesting astrophysical/cosmological phenomena by inducing the
mixings between the ALP, ordinary photon, and dark photon states.Comment: 6 pages, 2 figures; discussions rearranged, minor numerical errors
fixed, conclusion unchanged; discussion improved, accepted for publication in
PR
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