Particle/high-energy
astrophysics, Galactic cosmic-ray production and
propagation,
Cosmic-ray and neutrino observations, Ultra-high energy
cosmic-ray,
Particle interactions with fields and matter, X- rays and
γ-rays,
Nuclear astrophysics, Galactic structure, Astrophysical
plasma kinetics.
Current Projects:
I am presently working on analyzing the new AMS-02
observations data with the spiral-arms model framework, in 3
different projects:
The latest data exhibits a break in the primary cosmic
rays spectra, at
around 300 GeV, which is different than hitherto
assumed. Propagation
models should consider this with respect to their
injection spectra.
The new lepton data exhibits in more details the rise of
the positron
fraction at about 10 GeV (also known as the Pamela
anomaly). Shaviv
et al.(2009) have shown using a 2D model that a
spiral-arms model should easily recover this break. This
is now studied in a full 3D dynamic spiral-arms model
which includes the lepton cross-sections and cooling
effects.
I am presently working on including the antiprotons
cross-sections into the code and comparing the model
predictions with the AMS-02 data.
My Research Summary:
I developed the first fully 3D model describing the
diffusion of cosmic rays (CRs) in the Milky Way while
considering that
a large
fraction of the CR acceleration takes place in the
vicinity of the
dynamic galactic spiral arms. It presently
includes the nuclear
spallation chain up to Nickel, and allows the study of
various CR
properties, such as their age, grammage traversed, and
various
secondary and primary particle ratios. With the code, it
was possible to show that
many poorly understood CR characteristics can be naturally
explained. I
intend to continue developing and applying the code to
understand
further cosmic ray behavior in the Galaxy and their
relation to other phenomena in high energy astrophysics.
Figure 1: A simulation of the cosmic
ray density in the Milky Way plane, at different energies.
The black
dot represents the solar system which revolves once every
300 Myr or so. The color scale denotes the log of the
density at different
energies:
Research Highlights:
• In Benyamin
et
al.(2014) we presented the first results describing
the
first fully three dimensional
model of the diffusion of cosmic rays in the Milky Way,
which
considers that most cosmic ray acceleration takes place in
the
galactic spiral arms and that these spiral arms are dynamic.
We use the
model to
study different cosmic ray properties, such as their age,
grammage
traversed, and the ratio between secondary cosmic rays
produced along
the way and primary cosmic rays accelerated at the sources.
Figure 2: Relative CR density distribution in the Milky Way
at two energies,
normalized to the density at the location of the solar
system. Left: At
1 GeV. Right: At 100 GeV. The small circle denotes the
location of the
solar system. At low energies,
the diffusion timescale and the timescale to escape the
galaxy are
comparable. As a consequence, it is possible to see the
advection of
the disk relative to the spiral arms. At higher energies,
the escape is
much faster and the advection cannot be seen:
• We showed that the effect of having dynamic spiral arms is
to limit
the
age of cosmic rays at low energies. This is because this age
is
determined by the time elapsed since the last spiral arm
passage and
not by their diffusion time. In different projects, we show
that the
model recovers the different observations of cosmic rays.
Some of these
observations did not have any natural explanation.
Figure 3: We recover the observed spectral dependence of the
secondary to primary ratio without requiring any further
assumptions
such as having a galactic wind, re-acceleration or ad hoc
assumptions
on the diffusivity. In particular, we obtain a secondary to
primary
ratio (B/C) which increases with energy below about 1 GeV.
Moreover, we
recover all the spectra in the range of Beryllium to Oxygen:
• In Benyamin
et
al. (2016), we explain the previously obtained
“discrepancy”
between the sub-Fe/Fe ratio and the B/C, which manifests
itself as an
apparent paucity in short paths in standard models.
Figure 4: Contour plot of χ2 for the disk-like
model. The red contours correspond
to the B/C fit. The blue contours correspond to the
sub-Fe/Fe fit. The
dashed lines correspond to the same contours as obtained
when replacing
the cross-sections of Silberberg & Tsao (1990) with
those compiled
by Webber et al. (2003). Note that the discrepancy between
the B/C and
the sub-Fe/Fe remains in the “disk-like” model (left) but
not in the
spiral-arms model (right).
• In Benyamin et
al. (2017),
we continue by studying the electron
capture
isotopes. These isotopes are not radioactive if their
K-shell
electrons are stripped, as is the case in all but the lowest
energy
cosmic rays.
Figure 5: We show that we recover the standard results for
electron capture isotopes, but also
that they can also be used to constrain diffusion model
parameters.
However, given the large uncertainties in the spallation
cross-sections, no meaningful constraints can yet be placed.
Figure 6: In an ongoing
project, collaborating with Prof. Michael Paul, we
continue our research on the radioactive
nuclei using the results from Benyamin et al. (2017) to
place a lower
limits on the nucleosynthesis of 44Ti and 60Fe.
Figure 7: In Nava
et
al. (2016) we couple the cosmic ray diffusion model to
a
γ-ray production code and construct the γ-ray sky observed
by
satellites . We have shown that the model predicts
changes in the γ-ray spectrum which cannot arise in the
standard model,
and that these variations are apparent in the FERMI data.
Figure 8: In Shaviv
et
al.(2017) we have shown that with its nominal
parameters, the model correctly predicts the small observed
anisotropy at TeV energies. It also gives a non-standard
interpretation to the cosmic-ray “knee”. Instead of it being
the
highest energy accelerated by the galactic sources, it
naturally
becomes a propagation effect.
