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Pulsar-Black Hole Binaries In The Galactic Centre


The holy-grail of astrophysics, the pulsar-black hole binary (PSR-BH binary) is a dead-cert to be found, alleges perhaps two of the world’s best living astrophysicists. Abraham Loeb and Claude-André Faucher-Giguere, of Harvard University, announced in the March edition of the Monthly Notices of the Royal Astronomical Society journal that the illusive PSR-BH binary system maybe closer and more attainable than previously thought: simply by focusing attention at the centre of the Galaxy!

The two fabled knights of astrophysics, the pulsating neutron star, “pulsar”, and the black hole, are perhaps the most exotic of all the compact stellar objects. Both formed out of the collapse of massive stars; these objects are numerous in abundance, but virialised (gravitationally bound) together is an altogether rarer sight. (For conciseness, I would suggest sourcing a copy of Shapiro and Tuekolsky’s “Black holes, white dwarfs and neutron stars: The physics of compact objects”, which will discuss in much brevity the exact nature of black holes, pulsars and the wider zoo of compact stellar objects).

Pulsars and black holes are both forged out of the core collapse of massive stars, typically in the 8 to 40 solar mass range, leaving behind a neutron star. Any higher, and we begin to form black holes. The neutron star retains most of its angular momentum, and since it has only a tiny fraction of its progenitor’s radius (and therefore its moment of inertia is sharply reduced), it is formed with very high rotation speed. The “squashing” of the progenitor star will then lead to an intense magnetic field. So from a very simple collapse scenario, we now have the rapidly rotating and higher magnetic neutron star – our pulsar. The pulsation comes from a beam of radiation which is emitted along the magnetic axis of the pulsar, which spins along with the rotation of the neutron star. Observers here on Earth will therefore only momentarily see the beam along the axis of sight, thus we see a pulse. Think of it as a very rapidly spinning lighthouse.

The most common of systems in the Galaxy is the binary system, two stars  orbiting around a common center of mass. Binary systems consisting of a pulsar and a black hole (BH) are the holy grail of astrophysics, both for their significance for stellar evolution and for their potential application as probes of strong gravity. In spite of extensive surveys of our Galaxy and its system of globular clusters, no pulsar-black hole (PSR-BH) binary has been found to date.

But thinking of neutron stars and black holes as astrophysical cousins, both remnants of the core-collapse supernovae of massive progenitor stars, it would be a fair assumption to make that their could exist NS/PSR-BH binary partners. An assumption shared by e.g., Narayan et al. 1991; Phinney 1991; Portegies-Zwart & Yungelson 1998; Bethe & Brown 1999; Sipior & Sigurdsson 2002; Belczynski et al. 2002; Pfahl et al. 2005 to name but a few.

Loeb and Faucher-Giguere theorise that, building on dynamical friction models by Freitag et al. (analogous to those that act to distribute globular clusters), that anywhere up to 25000 stellar sized black holes could reside within the central parsec of our Galaxy, some of which are likely to form binaries with pulsars that may have survived in the central parsec today. They also go on to theorise that similar processes may be happening at the centre of other galaxies, giving added importance to studying the centre of our own Milky Way galaxy.

The significance in such a discovery would lie in using pulsars as tests for general relativity. Pulsars in binary systems provide accurate cosmic clocks that can be used to infer the properties of the binary orbit and its stellar components with great accuracy and precision. In addition to yielding important constraints on stellar evolution, binary pulsars have also been used to test the validity of general relativity and to put stringent constraints on alternative theories of gravity.

The Harvard team conclude that taking into account the much higher retention fraction of neutron stars in the Galactic center relative to globular clusters, as a result of the deeper potential well, several of these systems should survive to the present day and that they may be detectable using existing radio and X-ray telescopes. However, Loeb and Faucher-Giguere go on to express certain caveats on their own simple models.  They state that while their calculations provide predictions for the wheres and hows PSR-BH binaries, that it would be desirable to perform more detailed simulations that stretch to include stellar evolution ad critical binary effects. Nevertheless, the gauntlet has been laid down for the next generation of research astrophysicists to find and locate these cosmic knights, and make theory reality.

Journal References:

  • Loeb, A.; Faucher-Giguere, C.A. (2011)  Pulsar-Black Hole Binaries In The Galactic Centre. Monthly Notices for the Royal Astronomical Society, 414 (1) pp.1012-1025.
  • Deneva, J.S.; et al. (2009) Arecibo Pulsar Survey Using ALFA: Probing Radio Pulsar Intermittency And Transients. The Astrophysical Journal, 703 (1) pp.2259–2274.

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