Research Note

The local Galactic magnetic field in the direction of Geminga

INAF - Osservatorio Astrofisico di Arcetri Largo Enrico Fermi 5, 50125 Firenze, Italy e-mail: salvati@arcetri.astro.it

Received: 5 October 2009
Accepted: 25 January 2010

Abstract

Context. The Milagro hot spot A, close to the Galactic anticenter direction, has been tentatively attributed to cosmic rays from a local reservoir (at a distance ≈100 pc), freely streaming along diverging and smooth magnetic field lines. This is at variance with the geometry of the ≈kpc scale Galactic magnetic field, which is known to be aligned with the spiral arms.

Aims. We investigate the information available on the geometry of the magnetic field on the scales (≈100 pc) of relevance here.

Methods. The magnetic field immediately upstream of the heliosphere has been investigated by previous authors by modeling the interaction of this field with the solar wind. At larger distances, we use the dispersion measure and the rotation measure of nearby pulsars (especially towards the third Galactic quadrant). Additional information about the local field towards the North Polar Spur is taken from previous studies of the diffuse radio emission and the polarization of starlight.

Results. The asymmetry of the heliosphere with respect to the incoming interstellar medium implies a magnetic field almost orthogonal to the local spiral arm, in the general direction of hot spot A, but more to the south. This is in good agreement with the nearby pulsar data on the one side, and the North Polar Spur data on the other.

Conclusions. The local magnetic field on scales of ≈100 parsecs around the Sun seems to be oriented so as to provide a direct connection between the Solar System and a possible site of the Geminga supernova; the residual angular difference and the shape and orientation of the Milagro hot spot can be attributed to the field trailing in the wake of the heliosphere.