9 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
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
Flavor composition of neutrinos from choked gamma-ray bursts
Choked gamma-ray bursts (CGRBs) are possible neutrino sources that have been
proposed as capable of generating the flux detected by IceCube, since no
accompanying gamma-ray signal is expected, as required by observations. We
focus on obtaining the neutrino flux and flavor composition corresponding to
CGRBs under different assumptions for the target photon density and the
magnetic field of the emission region. We consider the injection of both
electrons and protons into the internal shocks of CGRBs, and using a
steady-state transport equation, we account for all the relevant cooling
processes. In particular, we include the usually adopted background of soft
photons, which is the thermalized emission originated at the shocked jet head.
Additionally, we consider the synchrotron photons emitted by the electrons
co-accelerated with the protons at the internal shocks in the jet. We also
obtain the distributions of pions, kaons, and muons using the transport
equation to account for the cooling effects due not only to synchrotron
emission but also interactions with the soft photons in the ambient. We
integrate the total diffuse flux of neutrinos of different flavors and compute
the flavor ratios to be observed on Earth. As a consequence of the losses
suffered mainly by pions and muons, we find these ratios to depend on the
energy: for energies above ~(10^5-10^6) GeV (depending on the magnetic field,
proton-to-electron ratio, and jet power), we find that the electron flavor
ratio decreases and the muon flavor ratio increases, while the tau flavor ratio
increases only moderately. Our results are sensitive to the mentioned key
physical parameters of the emitting region of CGRBs. Hence, the obtained flavor
ratios are to be contrasted with cumulative data from ongoing and future
neutrino instruments in order to assess the contribution of these sources to
the diffuse flux of astrophysical neutrinos.Comment: 23 pages, 18 figures. Accepted for publication in A&
Precession of neutrino-cooled accretion disks in gamma-ray burst engines
We study the precession of accretion disks in the context of gamma-ray burst
inner engines. With an accretion disk model that allows for neutrino cooling,
we evaluate the possible periods of disk precession and nutation due to the
Lense-Thirring effect. Assuming jet ejection perpendicular to the disk midplane
and a typical intrinsic time dependence for the burst, we find possible
gamma-ray light curves with temporal microstructure similar to what is observed
in some subsamples. We conclude that the precession and nutation of a
neutrino-cooled accretion disk in the burst engine might be responsible for
some events, especially those with a slow rise and a fast decay.Comment: 7 pages, 8 figures, accepted for publication in A&
Simulated X-ray emission for a runaway model of Kepler's supernova remnant
We present two-dimensional numerical simulations of a model for Kepler's supernova remnant (SNR) carried out with the YGUAZU-A code. Following previous studies, we have assumed that the peculiar shape of this young remnant arises as a consequence of the interaction of the SNR blast wave with the bow shock formed by the wind of its high velocity progenitor. Furthermore, from our numerical results we have obtained synthetic X-ray emission maps, which can be directly compared with recent and previous observations of this SNR. Our models show that a nice fit with respect to the X-ray morphology and luminosity is obtained for a SN progenitor with mass-loss rate of 5 x 10(-5) M-circle dot yr(-1), an ambient medium density of 10(-2) cm(-3), an initial explosion energy of 8 x 10(50) ergs, and a total ejected mass within 1.4-2.5 M-circle dot. In our simulations, parameters typical of a young population progenitor have not been considered. This model also predicts a similar to 0.3% yearly decrease in the total X-ray luminosity, which is consistent with observed values. The parameters employed in our runs correspond to a Type Ia supernova. Based on our simulations, we find that the expansion rate increases after the SNR blast wave overruns the bow shock, and we discuss whether this can explain the observed difference between the expansion rates measured from sequences of radio and X-ray images