David Benyamin

Position: Post-doc
Office: Kaplun 100
Phone: (972-54) 5330991
Email: david.benyamin@mail.huji.ac.il

Research Interests:

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.

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