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

TL;DR

This paper proposes a model attributing the observed cosmic ray spectrum bump to reaccelerated particles from stellar bow shocks, specifically from stars like Epsilon Indi or Epsilon Eridani, with no ad hoc parameters needed.

Contribution

The model explains the cosmic ray bump using only three physical parameters and predicts the specific stellar sources responsible, unlike previous models requiring multiple adjustable parameters.

Findings

The model fits the cosmic ray data with approximately 0.08% accuracy.

It constrains the shock Mach number and distance, linking them to the observed bump.

Epsilon Indi and Epsilon Eridani are identified as likely sources.

Abstract

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, $\zeta_{\text{obs}}$. 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$$0.08\%$ accuracy. The fit critically requires the I-K spectrum predicted by the model and rules out the alternatives. These fit's attributes make an accidental agreement highly unlikely. In turn, $R_{0}$ and $M$ derived from the fit impose the distance-size %($\zeta_{{\rm obs}}$$-$$l_{\perp}$) relation on the shock: $\zeta_{{\rm obs}}$(pc)$\sim$$10^{2}\sqrt{l_{\perp}(\text{pc})}$. For sufficiently large bow shocks, $l_{\perp}$$=$$10^{-3}$$-$$10^{-2}$ pc, we find the distance of $\zeta_{{\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.