Propagation of magnetized neutron stars through the interstellar medium

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WIND ACCRETION TO MAGNETIZED STARS

Propagation of magnetized neutron stars through the interstellar medium

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We investigate the propagation of magnetized, isolated old neutron stars through the interstellar medium. We performed axisymmetric non relativistic magnetohydrodynamic (MHD) simulations of the propagation of a non-rotating star with dipole magnetic field aligned with its velocity through the interstellar medium (ISM). We observed, that:

1. The magnetized star acts an obstacle for the flow of the ISM, and a conical shock wave forms as in the hydrodynamic case.

2. Long magnetotails form behind the star, and reconnection is observed in the tail.

3. In the R_A ~ R_acc regime, some matter accumulates around the star, but most of the matter is deflected by the magnetic field of the star and flies away. The accretion rate to the star is much smaller than that to a non-magnetized star.

4. In the magnetic plow regime, R_A >> R_acc and at high Mach numbers M ~ 10 - 50, no matter accumulation is observed around the star. The density of matter in the tail is very low. Some matter accretes from the upwind pole. Accretion rate is larger than Bondi-Hoyle accretion rate, but much smaller than the incoming matter flux.

5. When R_A > R_acc, the magnetic energy-density predominates in the magnetotail.

Part of this energy may radiate owing to reconnection processes. The power is however small ( ~ 10²¹ erg/s for typical parameters for evolved pulsars and ~ 10²⁴ erg/s for magnetars), so that only the closest magnetars may be possibly observed. For tail magnetic fields B ~ 10^-4 - 10^-6 G, the tail flares or substorms may give emission in the radio band.

6. Similar power to that released by field reconnection is released in the bow shock, which gives radiation in the band from optical to X-ray.

7. Magnetic tails are expected to also form in case of propagation of pulsars through the ISM. In this case particles accelerated by the pulsar will propagate preferentially along the tail to give an elongated structure.

8. The presented simulations and estimations can also be applied to other magnetized stars propagating through the ISM, such as magnetized white dwarfs, Ap stars, and young stellar objects.

9. Propagation of magnetized stars can lead to the appearance of ordered magnetized structures in the ISM. Also, these stars may give a contribution to the magnetic flux of the Galaxy.

INTERACTION OF EVOLVED PULSARS AND MAGNETARS WITH THE ISM

Proceedings of Relativistic Astrophysics: 20-th Texas Symposium, 2001
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