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Register a new userAn observational review of accretion-driven millisecond X-ray pulsars
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Rudy Wijnands
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I present an observational review of the five currently known accretion-driven millisecond X-ray pulsars. A prominent place in this review is given to SAX J1808.4-3658; it was the first such system discovered and currently four outbursts have been observed from this source. This makes SAX J1808.4-3658 the best studied example of the group. Its most recent outburst in October 2002 is of particular interest because of the discovery of two simultaneous kilohertz quasi-periodic oscillations and nearly coherent oscillations during type-I X-ray bursts. This is the first time that such phenomena are observed in a system for which the neutron star spin frequency is exactly known. The other four systems were discovered within the last two years and only limited results have been published. Since new exiting results are to be expected in the future for all five sources, this review will only represent a snap-shot of the current observational knowledge of accretion-driven millisecond X-ray pulsars. A more extended and fully up-to-date review can be found at http://zon.wins.uva.nl/~rudy/admxp/.
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I present an overview of our current observational knowledge of the six known accretion-driven millisecond X-ray pulsars. A prominent place in this review is given to SAX J1808.4-3658; it was the first such system discovered and currently four outbursts have been observed from this source, three of which have been studied in detail using the Rossi X-ray Timing Explorer satellite. This makes SAX J1808.4-3658 the best studied example of an accretion-driven millisecond pulsar. Its most recent outburst in October 2002 is of particular interest because of the discovery of two simultaneous kilohertz quasi-periodic oscillations and nearly coherent oscillations during type-I X-ray bursts. This is the first (and so far only) time that such phenomena are observed in a system for which the neutron star spin frequency is exactly known. The other five systems were discovered within the last three years (with IGR J00291+5934 only discovered in December 2004) and only limited results have been published.
Neutron Stars are among the most exotic objects in the Universe. A neutron star, with a mass of 1.4-2 Solar masses within a radius of about 10-15 km, is the most compact stable configuration of matter in which degeneracy pressure can still balance gravity, since further compression would lead to gravitational collapse and formation of a black hole. As gravity is extreme, rotation is extreme: neutron stars are the fastest rotating stars known, with periods as short as a millisecond. The presence of a magnetic field not aligned with the rotation axis of the star is the origin of pulsating emission from these sources, which for this reason are dubbed pulsars. The discovery in 1998 of the first Accreting Millisecond X-ray Pulsar, started an exciting season of continuing discoveries. In the last 20 years, thanks to the extraordinary performance of astronomical detectors in the radio, optical, X-ray, and Gamma-ray bands, astrophysicists had the opportunity to thoroughly investigate the so-called Recycling Scenario: the evolutionary path leading to the formation of a Millisecond-spinning Pulsar. In this chapter we review the general properties of Accreting Millisecond X-ray Pulsars, which provide the first evidence that neutron stars are spun up to millisecond periods by accretion of matter and angular momentum from a (low-mass) companion star. We describe the general characteristics of this class of systems with particular attention to their spin and orbital parameters, their short-term and long-term evolution, as well as the information that can be drawn from their X-ray spectra.
Accreting Millisecond X-Ray Pulsars (AMXPs) are astrophysical laboratories without parallel in the study of extreme physics. In this chapter we review the past fifteen years of discoveries in the field. We summarize the observations of the fifteen known AMXPs, with a particular emphasis on the multi-wavelength observations that have been carried out since the discovery of the first AMXP in 1998. We review accretion torque theory, the pulse formation process, and how AMXP observations have changed our view on the interaction of plasma and magnetic fields in strong gravity. We also explain how the AMXPs have deepened our understanding of the thermonuclear burst process, in particular the phenomenon of burst oscillations. We conclude with a discussion of the open problems that remain to be addressed in the future.
Accreting X-ray pulsars are among the most luminous objects in the X-ray sky. In highly magnetized neutron stars (B~10^12 G), the flow of matter is dominated by the strong magnetic field. The general properties of accreting X-ray binaries are presented, focusing on the spectral characteristics of the systems. The use of cyclotron lines as a tool to directly measure a neutron stars magnetic field and to test the theory of accretion are discussed. We conclude with the current and future prospects for accreting X-ray binary studies.
Nuclear-powered X-ray millisecond pulsars are the third type of millisecond pulsars, which are powered by thermonuclear fusion processes. The corresponding brightness oscillations, known as burst oscillations, are observed during some thermonuclear X-ray bursts, when the burning and cooling accreted matter gives rise to an azimuthally asymmetric brightness pattern on the surface of the spinning neutron star. Apart from providing neutron star spin rates, this X-ray timing feature can be a useful tool to probe the fundamental physics of neutron star interior and surface. This chapter presents an overview of the relatively new field of nuclear-powered X-ray millisecond pulsars.
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