Publications
Published Papers:
 Benyamin, D. , Piran, T. & Shaviv, N. J. “ElectronCapture
Isotopes could Constrain CosmicRay Propagation Models”, To be appear in The
Astrophysical Journal.
ABSTRACT
Electron
capture (EC) isotopes are known to provide constraints on the low
energy behavior of cosmic rays (CRs), such as reacceleration. Here we
study the EC isotopes within the framework of the dynamic spiral arms
CR propagation model in which most of the CR sources reside in the
galactic spiral arms. The model was previously used to explain the B/C
and subFe/Fe ratios (Benyamin et al. 2014, 2016). We show that the
known inconsistency between the 49Ti/49V and 51V/51Cr ratios remains
also in the spiralarms model. On the other hand, unlike the general
wisdom in which the isotope ratios depend primarily on reacceleration,
we find here that the ratio also depends on the halo size (Zh) and in
spiralarms models also on the time since the last spiral arm passage
(τarm). Namely, EC isotopes can in principle provide interesting
constraints on the diffusion geometry. However, with the present
uncertainties in the lab measurements of both the electron attachment
rate and the fragmentation crosssections, no meaningful constraint can
be placed.
preprint can be found here
 Nava, L. , Benyamin, D. , Piran, T. & Shaviv, N. J. “Reconciling
the diffuse Galactic γray and the cosmic ray spectra” Monthly
Notices of the Royal Astronomical Society, Volume 466, Issue 3, 21
April 2017, Pages 3674–3681.
ABSTRACT
Most of the diffuse
Galactic GeV γray emission is produced via collisions of cosmic ray
(CR) protons with ISM protons. As such the observed spectra of the
γrays and the CRs should be strongly linked. Recent observations of
FermiLAT exhibit a hardening of the γray spectrum at around a hundred
GeV, between the Sagittarius and Carina tangents, and a further harden
ing at a few degrees above and below the Galactic plane. However,
standard CR propagation models that assume a time independent source
distribution and a location independent diffu sion cannot give rise to
a spatially dependent CR (and hence γray) spectral slopes. Here we
consider a dynamic spiral arm model in which the distribution of CR
sources is concentrated in the (dynamic) spiral arms, and we study the
effects of this model on the π0decay produced γray spectra. Within
this model, near the Galactic arms the observed γray spectral slope is
not trivially related to the CR injection spectrum and energy
dependence of the diffusion co efficient. We find unique signatures
that agree with the FermiLAT observations. This model also provides a
physical explanation for the difference between the local CR spectral
slope and the CR slope inferred from the average γray spectrum.
preprint can be found here
 Benyamin, D. , Nakar, E. , Piran, T. & Shaviv, N. J. “The B/C and
SubIron/Iron Cosmic Ray RatiosFurther Evidence in Favor of the
SpiralArm Diffusion Model” The
Astrophysical Journal, 826, 47, 2016
July 19.
ABSTRACT
The boron to carbon (B/C)
and subFe/Fe ratios provide an important clue on cosmic ray (CR)
propagation within the Galaxy. These ratios estimate the grammage that
the CRs traverse as they propagate from their sources to Earth.
Attempts to explain these ratios within the standard CR propagation
models require ad hoc modifications and even with those these models
necessitate inconsistent grammages to explain both ratios. As an
alternative, physically motivated model, we have proposed that CRs
originate preferably within the galactic spiral arms. CR propagation
from dynamic spiral arms has important imprints on various secondary to
primary ratios, such as the B/C ratio and the positron fraction. We use
our spiralarm diffusion model with the spallation network extended up
to nickel to calculate the subFe/Fe ratio. We show that without any
additional parameters the spiralarm model consistently explains both
ratios with the same grammage, providing further evidence in favor of
this model.
preprint can be found here
 Benyamin, D. , Nakar, E. , Piran, T. & Shaviv, N. J. “Recovering
the observed B/C ratio in a dynamic spiralarmed cosmic ray model” The
Astrophysical Journal, 782, 34, 2014 January 23.
ABSTRACT
We develop a fully
threedimensional numerical code describing the diffusion of cosmic
rays (CRs) in the Milky Way. It includes the nuclear spallation chain
up to oxygen, and allows the study of various CR properties, such as
the CR age, grammage traversed, and the ratio between secondary and
primary particles. This code enables us to explore a model in which a
large fraction of the CR acceleration takes place in the vicinity of
galactic spiral arms that are dynamic. We show that the effect of
having dynamic spiral arms is to limit the age of CRs at low energies.
This is because at low energies the time since the last spiral arm
passage governs the CR age, and not diffusion. Using the model, the
observed spectral dependence of the secondary to primary ratio is
recovered without requiring any further assumptions such as a galactic
wind, reacceleration or various assumptions on the diffusivity. In
particular, we obtain a secondary to primary ratio which increases with
energy below about 1 GeV.
preprint can be found here
Submitetd Papers:
 Shaviv, N. J. , Benyamin, D. , Murase, K. & Piran, T.
“Implications of Smaller Cosmic Ray Halo and Diffusion Coefficient to
understanding the Knees and Observed Anisotropy”, Submitted to The
Astrophysical Journal Letters.
ABSTRACT
Cosmic Ray diffusion
models lacking nearby sources require a smaller halo and smaller
diffusion coefficient in order to fit the average grammage inferred
from ratios between secondary to primary cosmic rays as well as the
10Be/9Be which “dates” the cosmic rays. We show here that models with
such modified parameters lead to a notably smaller anisotropy,
releasing the tension between standard predictions and observations.
For the lower diffusion coefficient, the “knee” at a few PeV is then
unavoidably explained as the energy for which the Larmor radius is
equal to the mean free path, because of which the ISM diffusion
coefficient will increase fast above this energy (for protons). This
would imply that the knee is a propagation effect and the main cosmic
ray acceleration site (presumably supernova remnants) should in
principle be able to accelerate up to higher energies. The smaller
scales also imply that fewer sources contribute to the cosmic ray
density at any given energy, giving rise to large fluctuations which
can explain the anisotropy behavior above 10 TeV. Although the energy
dependent composition is similar to the behavior expected in standard
models, here one predicts a larger anisotropy for lighter elements at a
given energy above the knee.
preprint can be found here
 Benyamin, D. & Shaviv, N. J. “Lower Limits on the
Nucleosynthezis of 44Ti and 60Fe in the Dynamic SpiralArms Model”,
Submitted to The
Astrophysical Journal Letters.
ABSTRACT
We have previously
focused on studying the electroncapture isotopes within the dynamic
spiralarms model and empirically derived the energy dependence of the
electron attachment rate using the observation of 49Ti/49V and 51V/51Cr
ratios (Benyamin et al. 2017). We have also shown how this relation
recovers the energy dependence seen in the lab measurements (Letaw et
al. 1985). In this work we use this relation to construct the 44Ca/44Ti
ratio and place a lower limit on the amount of 44Ti that is required to
be nucleosynthesized at the source. The results also imply that the
acceleration process of the radioisotopes cannot be much longer than a
century time scale (or else the required nucleosynthesized amount has
to be correspondingly larger). We also provide a similar lower limit on
the source 60Fe by comparing to the recently observed 60Fe/56Fe (Binns
et al.
2016).
preprint can be found here
*This work has been done in collaboration with Professor Michael Paul.


