University of Maryland
4:30 PM Monday, March 1, 2021
Talk Recording

Mikhail Malkov
University of San Diego

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 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. Ad hoc fitting of the bump shape requires six adjustable parameters. Our model requires none, merely depending on 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 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: the 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.