arXiv subjects

Mikhail A. Malkov

Publications and source records attributed to Mikhail A. Malkov.

3 recordsLinked to original sources

arXiv2022

On the origin of observed cosmic ray spectrum below 100 TV

Recent precise measurements of primary and secondary cosmic rays (CRs) in the TV rigidity domain have unveiled a bump in their spectra, located between 0.5-50 TV. We argue that a local shock may generate such a bump by increasing the rigidity of the preexisting CRs below 50 TV by a mere factor of ~1.5. Reaccelerated particles below ~0.5 TV are convected with the interstellar medium (ISM) flow and do not reach the Sun, thus creating the bump. This single universal process is responsible for the observed spectra of all CR species in the rigidity range below 100 TV. We propose that one viable shock candidate is the Epsilon Eridani star at 3.2 pc from the Sun, which is well-aligned with the direction of the local magnetic field. Other shocks, such as old supernova shells, may produce a similar effect. We provide a simple formula, Eq. (9), that reproduces the spectra of all CR species with only two nonadjustable shock parameters, uniquely derived from the proton data. We show how our formalism predicts helium and carbon spectra and the B/C ratio.

astro-ph.HE↗

arXiv2021

The TeV Cosmic Ray Bump: a Message from Epsilon Indi or Epsilon Eridani Star?

A recently observed bump in the cosmic ray (CR) spectrum from 0.3--30 TV is likely caused by a stellar bow shock that reaccelerates \emph{preexisting} CRs, which further propagate to the Sun along the magnetic field lines. Along their way, these particles generate an Iroshnikov-Kraichnan (I-K) turbulence that controls their propagation and sustains the bump. {\it Ad hoc} fitting of the bump shape requires six adjustable parameters. Our model requires none, merely depending on \emph{three physical unknowns that we constrain using the fit.} These are the shock Mach number, $M$, its size, $l_{\perp}$, and the distance to it, $ζ_{\text{obs}}$. Altogether, they define the bump rigidity $R_{0}$. With $M$$\approx$1.5--1.6 and $R_{0}$$\approx$4.4 TV, the model fits the data with $\approx$$0.08\%$ accuracy. The fit critically requires the I-K spectrum predicted by the model and rules out the alternatives. These fit's attributes make an accidental agreement highly unlikely. In turn, $R_{0}$ and $M$ derived from the fit impose the distance-size %($ζ_{\rm obs}$$-$$l_{\perp}$) relation on the shock: $ζ_{\rm obs}$(pc)$\sim$$10^{2}\sqrt{l_{\perp}(\text{pc})}$. For sufficiently large bow shocks, $l_{\perp}$$=$$10^{-3}$$-$$10^{-2}$ pc, we find the distance of $ζ_{\rm obs}$$=$3--10 pc. Three promising stars in this range are: Scholz's Star at 6.8 pc, Epsilon Indi at 3.6 pc, and Epsilon Eridani at 3.2 pc. Based on their current positions and velocities, we propose that Epsilon Indi and Epsilon Eridani can produce the observed spectral bump. Moreover, Epsilon Eridani's position is only $\sim$$6.7^{\circ}$ off of the magnetic field direction in the solar neighborhood, which also changes the CR arrival direction distribution. Given the proximity of these stars, the bump appearance may change in a relatively short time.

astro-ph.HE↗

arXiv2020

Powerlaw spectra from stochastic acceleration

Numerical simulations of particle acceleration in magnetized turbulence have recently observed powerlaw spectra where pile-up distributions are rather expected. We interpret this as evidence for particle segregation based on acceleration rate, which is likely related to a non-trivial dependence of the efficacy of acceleration on phase space variables other than the momentum. We describe the corresponding transport in momentum space using continuous-time random walks, in which the time between two consecutive momentum jumps becomes a random variable. We show that powerlaws indeed emerge when the experimental (simulation) timescale does not encompass the full extent of the distribution of waiting times. We provide analytical solutions, which reproduce dedicated numerical Monte Carlo realizations of the stochastic process, as well as analytical approximations. Our results can be readily extrapolated for applications to astrophysical phenomenology.

astro-ph.HE↗