1,013 research outputs found
On the production of heavy axion-like particles in the accretion disks of gamma-ray bursts
Heavy axion-like particles have been introduced in several scenarios beyond
the Standard Model and their production in some astrophysical systems should be
possible. In this work, we re-examine the possibility that these type of
particles can be generated in the accretion disks of gamma-ray bursts (GRB),
the most powerful events in the universe. If the produced axions decay into
photons or pairs at the correct distances, a fireball is generated. We
calculate the structure transient accretion disks in GRBs (density, temperature
and thickness profiles) taking into account the effect of heavy axion emission
as well as the rest of the relevant standard cooling processes. This allows us
to obtain the values of the coupling constant g_{aN} in order for the axions
not to become trapped, and we can also compute the emitted heavy axion
luminosity from the entire disk. We find that for the couplings within the
ranges found, then the mechanism for powering GRBs based on heavy axion
production and decay becomes an alternative to the standard picture based upon
magnetohydrodynamic processes and neutrino-antineutrino annihilation.
Otherwise, if heavy axions are produced in the disk but their decay to takes
place further away, the mechanism fails. Still, the decay products (gamma rays
or electrons and positrons) should leave observable signatures which are not
observed for different ranges of values of the coupling constants, depending on
the mass of the heavy axionComment: 14 pages, 7 figures. Updated version that matches the one to be
published on Physics Letters
A two-zone approach to neutrino production in gamma-ray bursts
Gamma-ray bursts (GRB) are the most powerful events in the universe. They are
capable of accelerating particles to very high energies, so are strong
candidates as sources of detectable astrophysical neutrinos. We study the
effects of particle acceleration and escape by implementing a two-zone model in
order to assess the production of high-energy neutrinos in GRBs associated with
their prompt emission. Both primary relativistic electrons and protons are
injected in a zone where an acceleration mechanism operates and dominates over
the losses. The escaping particles are re-injected in a cooling zone that
propagates downstream. The synchrotron photons emitted by the accelerated
electrons are taken as targets for interactions, which generate pions
along with the collisions with cold protons in the flow. The distribution
of these secondary pions and the decaying muons are also computed in both
zones, from which the neutrino output is obtained. We find that for escape
rates lower than the acceleration rate, the synchrotron emission from electrons
in the acceleration zone can account for the GRB emission, and the production
of neutrinos via interactions in this zone becomes dominant for
GeV. For illustration, we compute the corresponding diffuse
neutrino flux under different assumptions and show that it can reach the level
of the signal recently detected by IceCube.Comment: 11 pages, 7 figures, to appear in A&
Propagation of high-energy neutrinos in a background of ultralight scalar dark matter
If high-energy neutrinos propagate in a background of ultralight scalar field
particles of dark matter (eV), neutrino-dark matter
interactions can play a role and affect the neutrino flux. In this work we
analyse this effect using transport equations that account for the neutrino
regeneration as well as absorption, and we consider the neutrino flux
propagation in the extragalactic medium and also through the galactic halo of
dark matter. We show the results for the final flux to arrive on Earth for
different cases of point and diffuse neutrino fluxes. {We conclude that this
type of neutrino interactions with ultralight scalar particles as dark matter
can yield very different results in the neutrino flux and in the flavor ratios
that can be measured in neutrino detectors such as IceCube.Comment: 11 pages, 11 figures, accepted for publication in Astroparticle
Physic
Gamma-ray absorption in the microquasar SS433
We discuss the gamma-ray absorption in the inner region of the microquasar
SS433. Our investigation includes several contributions to the opacity of this
system. They result from the ambient fields generated by the primary star,
possibly an A-type supergiant, and a very extended disk around the black hole.
Besides the sharp and dramatic absorption effect that occurs every time the
star crosses the emission zone, we find in the UV photon field from the
extended disk an important source of absorption for very high energy
gamma-rays. This results in periodic gamma-ray observational signatures.Comment: 8 pages, 9 figures, to appear in Astropart.Phy
On the Radio Polarization Signature of Efficient and Inefficient Particle Acceleration in Supernova Remnant SN 1006
We present a radio polarization study of SN 1006, based on combined VLA and
ATCA observations at 20 cm that resulted in sensitive images with an angular
resolution of 10 arcsec. The fractional polarization in the two bright radio
and X-ray lobes of the SNR is measured to be 0.17, while in the southeastern
sector, where the radio and non-thermal X-ray emission are much weaker, the
polarization fraction reaches a value of 0.6 +- 0.2, close to the theoretical
limit of 0.7. We interpret this result as evidence of a disordered, turbulent
magnetic field in the lobes, where particle acceleration is believed to be
efficient, and a highly ordered field in the southeast, where the acceleration
efficiency has been shown to be very low. Utilizing the frequency coverage of
our observations, an average rotation measure of ~12 rad/m2 is determined from
the combined data set, which is then used to obtain the intrinsic direction of
the magnetic field vectors. While the orientation of magnetic field vectors
across the SNR shell appear radial, a large fraction of the magnetic vectors
lie parallel to the Galactic Plane. Along the highly polarized southeastern
rim, the field is aligned tangent to the shock, and therefore also nearly
parallel to the Galactic Plane. These results strongly suggest that the ambient
field surrounding SN 1006 is aligned with this direction (i.e., from northeast
to southwest) and that the bright lobes are due to a polar cap geometry. Our
study establishes that the most efficient particle acceleration and generation
of magnetic turbulence in SN 1006 is attained for shocks in which the magnetic
field direction and shock normal are quasi-parallel, while inefficient
acceleration and little to no generation of magnetic turbulence obtains for the
quasi-perpendicular case.Comment: Accepted for publication in Astronomical Journa
An MHD study of SN 1006 and determination of the ambient magnetic field direction
In this work we employ an MHD numerical code to reproduce the morphology
observed for SN 1006 in radio synchrotron and thermal X-ray emission. We
introduce a density discontinuity, in the form of a flat cloud parallel to the
Galactic Plane, in order to explain the NW filament observed in optical
wavelengths and in thermal X-rays. We compare our models with observations. We
also perform a test that contrasts the radio emitting bright limbs of the SNR
against the central region, finding additional support to our results. Our main
conclusion is that the most probable direction of the ambient magnetic field is
on average perpendicular to the Galactic Plane.Comment: 7 pages, 5 figures, accepted by MNRA
3D MHD simulation of polarized emission in SN 1006
We use three dimensional magnetohydrodynamic (MHD) simulations to model the
supernova remnant SN 1006. From our numerical results, we have carried out a
polarization study, obtaining synthetic maps of the polarized intensity, the
Stokes parameter , and the polar-referenced angle, which can be compared
with observational results. Synthetic maps were computed considering two
possible particle acceleration mechanisms: quasi-parallel and
quasi-perpendicular. The comparison of synthetic maps of the Stokes parameter
maps with observations proves to be a valuable tool to discern
unambiguously which mechanism is taking place in the remnant of SN 1006, giving
strong support to the quasi-parallel model.Comment: 6 pages, 4 figures, accepted by MNRA
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