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[20.13] Isolated magnetar spin-down and soft X-ray emission: RX J1856.5-3754

K. Mori (Columbia Astrophysics Laboratory), M.A. Ruderman (Columbia University, Physics Department)

The spin-down rate ( \dot \Omega ) of an isolated magnetar, canonically described by I \dot \Omega = - K\Omegaⁿ, gives the time after birth for achieving \Omega as \tau_sd ~\Omega /(n-1) \dot{\Omega} only for n > 1. When pulsar wind carries off stellar spin angular momentum, n ~3 and \Omega/(n-1)\dot \Omega ~ the time since birth to a currently observed spin angular frequency \Omega. However, when a magnetar with magnetic dipole field B_d ~10¹⁵ G moves at high velocity (v > 10⁷ cm\,s^-1) through the ISM, a transition to ``propeller effect'' spin-down with n <= 1 may occur in less than 10⁶ years. Then a measured \Omega / (1-n) \dot \Omega becomes the time remaining before the stellar spin is quenched. We consider RX J1856.5-3754 as such a magnetar. This is consistent with observed thermal X-ray emission (featureless and nearly blackbody for one X-ray polarization mode and almost no emission in the orthogonal mode). We propose that this magnetar's spin period has already grown to >> 10⁴ sec and compare this conjecture to predictions from a large number of different propeller effect spin-down models.

Bulletin of the American Astronomical Society, 35#2
© 2003. The American Astronomical Soceity.