Abstract
We report the discovery of peculiar X-ray spectral variability in the binary radio millisecond pulsar PSR J0024--7204W in the globular cluster 47 Tucanae. The observed emission consists of a dominant non-thermal component, which is eclipsed for a portion of the orbit, and a thermal component, which appears to be persistent. We propose that the non-thermal X-rays originate in a relativistic intrabinary shock, formed due to interaction between the relativistic particle wind from the pulsar and matter from the main-sequence companion star, while the thermal photons are from the heated magnetic polar caps of the millisecond pulsar. At optical wavelengths, the emission exhibits large-amplitude variations at the orbital period, which can be attributed to heating of one side of the tidally-locked secondary star by the pulsar wind. The observed X-ray and optical properties of PSR J0024--7204W are remarkably similar to those of the low mass X-ray binary and X-ray millisecond pulsar SAX J1808.4--3658 in quiescence. This supports the conjecture that the non-thermal X-ray emission and optical modulations seen in the SAX J1808.4--3658 system in a quiescent state are due to interaction between the wind from a reactivated rotation-powered pulsar and matter from the companion star. The striking similarities between the two systems provide support for the long-sought connection between millisecond radio pulsars and accreting neutron star systems.
Highlights
Radio millisecond pulsars (MSPs) form a separate class of rotation-powered pulsars, characterized by small spin periods, P P 25 ms, and spin-down rates, P 1⁄4 dP/dt $ 10À19 to 10À21, implying surface magnetic field strengths of B / (PP )1=2 $ 108 109 G and characteristic ages of P/2Pk109 yr
We propose that the nonthermal X-rays originate in a relativistic intrabinary shock, formed due to interaction between the relativistic particle wind from the pulsar and matter from the main-sequence companion star, while the thermal photons are from the heated magnetic polar caps of the millisecond pulsar
The observed X-ray and optical properties of PSR J0024À7204W are remarkably similar to those of the low-mass X-ray binary and X-ray millisecond pulsar SAX J1808.4À3658 in quiescence. This supports the conjecture that the nonthermal X-ray emission and optical modulations seen in the SAX J1808.4À3658 system in a quiescent state are due to interaction between the wind from a reactivated rotationpowered pulsar and matter from the companion star
Summary
Radio millisecond pulsars (MSPs) form a separate class of rotation-powered pulsars, characterized by small spin periods, P P 25 ms, and spin-down rates, P 1⁄4 dP/dt $ 10À19 to 10À21, implying surface magnetic field strengths of B / (PP )1=2 $ 108 109 G and characteristic ages of P/2Pk109 yr. These neutron stars (NSs) are widely believed to be the end-products of a more exotic channel of binary evolution, involving an extended period of accretion of matter and angular momentum from a close stellar companion in X-ray binary systems (Alpar et al 1982; Bhattacharya & van den Heuvel 1991). Vol 630 atypical of the MSP sample in 47 Tuc, but similar to PSR J1740À5340 in the globular cluster NGC 6397 (D’Amico et al 2001; Grindlay et al 2002), the only other known MSP with a nondegenerate stellar companion
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