Circinus X-1: survivor of a highly asymmetric supernova

We have analyzed the kinematical parameters of Cir X-1 to constrain the nature of its companion star, the eccentricity of the binary and the pre-supernova parameter space. We argue that the companion is most likely to be a low-mass (< 2.0 M_sun) unevolved star and that the eccentricity of the orbit is 0.94 +/- 0.04. We have evaluated the dynamical effects of the supernova explosion and we find it must have been asymmetric. On average, we find that a kick of 740 km/s is needed to account for the recently measured radial velocity of +430 km/s (Johnston, Fender & Wu) for this extreme system. The corresponding minimum kick velocity is 500 km/s. This is the largest kick needed to explain the motion of any observed binary system. If Cir X-1 is associated with the supernova remnant G321.9-0.3 then we find a limiting minimum age of this remnant of 60000 yr. Furthermore, we predict that the companion star has lost 10% of its mass as a result of stripping and ablation from the impact of the supernova shell shortly after the explosion

Formation of the black-hole binary M33 X-7 via mass-exchange in a tight massive system

M33 X-7 is among the most massive X-Ray binary stellar systems known, hosting a rapidly spinning 15.65 Msun black hole orbiting an underluminous 70 Msun Main Sequence companion in a slightly eccentric 3.45 day orbit. Although post-main-sequence mass transfer explains the masses and tight orbit, it leaves unexplained the observed X-Ray luminosity, star's underluminosity, black hole's spin, and eccentricity. A common envelope phase, or rotational mixing, could explain the orbit, but the former would lead to a merger and the latter to an overluminous companion. A merger would also ensue if mass transfer to the black hole were invoked for its spin-up. Here we report that, if M33 X-7 started as a primary of 85-99 Msun and a secondary of 28-32 Msun, in a 2.8-3.1 day orbit, its observed properties can be consistently explained. In this model, the Main Sequence primary transferred part of its envelope to the secondary and lost the rest in a wind; it ended its life as a ~16 Msun He star with a Fe-Ni core which collapsed to a black hole (with or without an accompanying supernova). The release of binding energy and, possibly, collapse asymmetries "kicked" the nascent black hole into an eccentric orbit. Wind accretion explains the X-Ray luminosity, while the black hole spin can be natal.Comment: Manuscript: 18 pages, 2 tables, 2 figure. Supplementary Information: 34 pages, 6 figures. Advance Online Publication (AOP) on http://www.nature.com/nature on October 20, 2010. To Appear in Nature on November 4, 201

A 15.65 solar mass black hole in an eclipsing binary in the nearby spiral galaxy Messier 33

Stellar-mass black holes are discovered in X-ray emitting binary systems, where their mass can be determined from the dynamics of their companion stars. Models of stellar evolution have difficulty producing black holes in close binaries with masses >10 solar masses, which is consistent with the fact that the most massive stellar black holes known so all have masses within 1 sigma of 10 solar masses. Here we report a mass of 15.65 +/- 1.45 solar masses for the black hole in the recently discovered system M33 X-7, which is located in the nearby galaxy Messier 33 (M33) and is the only known black hole that is in an eclipsing binary. In order to produce such a massive black hole, the progenitor star must have retained much of its outer envelope until after helium fusion in the core was completed. On the other hand, in order for the black hole to be in its present 3.45 day orbit about its 70.0 +/- 6.9 solar mass companion, there must have been a ``common envelope'' phase of evolution in which a significant amount of mass was lost from the system. We find the common envelope phase could not have occured in M33 X-7 unless the amount of mass lost from the progenitor during its evolution was an order of magnitude less than what is usually assumed in evolutionary models of massive stars.Comment: To appear in Nature October 18, 2007. Four figures (one color figure degraded). Differs slightly from published version. Supplementary Information follows in a separate postin

The effects of supernovae on the dynamical evolution of binary stars and star clusters

In this chapter I review the effects of supernovae explosions on the dynamical evolution of (1) binary stars and (2) star clusters. (1) Supernovae in binaries can drastically alter the orbit of the system, sometimes disrupting it entirely, and are thought to be partially responsible for `runaway' massive stars - stars in the Galaxy with large peculiar velocities. The ejection of the lower-mass secondary component of a binary occurs often in the event of the more massive primary star exploding as a supernova. The orbital properties of binaries that contain massive stars mean that the observed velocities of runaway stars (10s - 100s km s$^{-1}$) are consistent with this scenario. (2) Star formation is an inherently inefficient process, and much of the potential in young star clusters remains in the form of gas. Supernovae can in principle expel this gas, which would drastically alter the dynamics of the cluster by unbinding the stars from the potential. However, recent numerical simulations, and observational evidence that gas-free clusters are observed to be bound, suggest that the effects of supernova explosions on the dynamics of star clusters are likely to be minimal.Comment: 16 pages, to appear in the 'Handbook of Supernovae', eds. Paul Murdin and Athem Alsabti. This version replaces an earlier version that contained several typo

Transient radio bursts from rotating neutron stars

The `radio sky' is relatively unexplored for transient signals, although the potential of radio-transient searches is high, as demonstrated recently by the discovery of a previously unknown type of source which varies on timescales of minutes to hours. Here we report a new large-scale search for radio sources varying on much shorter timescales. This has revealed 11 objects characterized by single, dispersed bursts having durations between 2 and 30 ms. The average time intervals between bursts range from 4 minutes to 3 hours, with radio emission typically detectable for < 1 s per day. From an analysis of the burst arrival times, we have identified periodicities in the range 0.4 - 7 s for ten of the 11 sources, suggesting a rotating neutron star origin. Despite the small number of sources presently detected, their ephemeral nature implies a total Galactic population which significantly exceeds that of the regularly pulsing radio pulsars. Five of the ten sources have periods greater than 4 s, and period derivatives have been measured for three of the sources, with one having a very high inferred magnetic field of 5e13 G, suggesting that this new population is related to other classes of isolated neutron stars observed at X-ray and gamma-ray wavelengths.Comment: 10 pages, 4 figures. Accepted by Natur

Accreting Millisecond X-Ray Pulsars

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.Comment: Review to appear in "Timing neutron stars: pulsations, oscillations and explosions", T. Belloni, M. Mendez, C.M. Zhang Eds., ASSL, Springer; [revision with literature updated, several typos removed, 1 new AMXP added

Binary and Millisecond Pulsars at the New Millennium

We review the properties and applications of binary and millisecond pulsars. Our knowledge of these exciting objects has greatly increased in recent years, mainly due to successful surveys which have brought the known pulsar population to over 1300. There are now 56 binary and millisecond pulsars in the Galactic disk and a further 47 in globular clusters. This review is concerned primarily with the results and spin-offs from these surveys which are of particular interest to the relativity community.Comment: 59 pages, 26 figures, 5 tables. Accepted for publication in Living Reviews in Relativity (http://www.livingreviews.org

The Evolution of Compact Binary Star Systems

We review the formation and evolution of compact binary stars consisting of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and BHs are thought to be the primary astrophysical sources of gravitational waves (GWs) within the frequency band of ground-based detectors, while compact binaries of WDs are important sources of GWs at lower frequencies to be covered by space interferometers (LISA). Major uncertainties in the current understanding of properties of NSs and BHs most relevant to the GW studies are discussed, including the treatment of the natal kicks which compact stellar remnants acquire during the core collapse of massive stars and the common envelope phase of binary evolution. We discuss the coalescence rates of binary NSs and BHs and prospects for their detections, the formation and evolution of binary WDs and their observational manifestations. Special attention is given to AM CVn-stars -- compact binaries in which the Roche lobe is filled by another WD or a low-mass partially degenerate helium-star, as these stars are thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure

High-frequency variability in neutron-star low-mass X-ray binaries

Binary systems with a neutron-star primary accreting from a companion star display variability in the X-ray band on time scales ranging from years to milliseconds. With frequencies of up to ~1300 Hz, the kilohertz quasi-periodic oscillations (kHz QPOs) represent the fastest variability observed from any astronomical object. The sub-millisecond time scale of this variability implies that the kHz QPOs are produced in the accretion flow very close to the surface of the neutron star, providing a unique view of the dynamics of matter under the influence of some of the strongest gravitational fields in the Universe. This offers the possibility to probe some of the most extreme predictions of General Relativity, such as dragging of inertial frames and periastron precession at rates that are sixteen orders of magnitude faster than those observed in the solar system and, ultimately, the existence of a minimum distance at which a stable orbit around a compact object is possible. Here we review the last twenty years of research on kHz QPOs, and we discuss the prospects for future developments in this field.Comment: 66 pages, 37 figures, 190 references. Review to appear in T. Belloni, M. Mendez, C. Zhang, editors, "Timing Neutron Stars: Pulsations, Oscillations and Explosions", ASSL, Springe

Binary and Millisecond Pulsars

We review the main properties, demographics and applications of binary and millisecond radio pulsars. Our knowledge of these exciting objects has greatly increased in recent years, mainly due to successful surveys which have brought the known pulsar population to over 1700. There are now 80 binary and millisecond pulsars associated with the disk of our Galaxy, and a further 103 pulsars in 24 of the Galactic globular clusters. Recent highlights have been the discovery of the first ever double pulsar system and a recent flurry of discoveries in globular clusters, in particular Terzan 5.Comment: 77 pages, 30 figures, available on-line at http://www.livingreviews.org/lrr-2005-