Monday, May 5, 2014

Possible Exotic Object around a Pulsar

Pulsars are highly magnetised, spinning compact stars that emit intense bipolar beams of electromagnetic radiation. A pulse is observed when such a beam happens to sweep pass the Earth, hence the term ‘pulsars’. These objects are the superdense and compact remnant cores of massive stars that have gone supernova. A typical compact star packs as much mass as the Sun within an object just several kilometres in size.

Figure 1: Schematic view of a pulsar. The sphere in the middle represents the compact star; the curves indicate the magnetic field lines and the protruding bipolar cones represent the beam emission zones. Credit: Roy Smits.

A paper by M. Bailes et al. (2011) reported on the detection of a Jupiter-mass object in a very close-in orbit around the pulsar PSR J1719-1438. This Jupiter-mass object is so close to the pulsar that its density has to be greater than 23 grams per cubic centimetre for it to not get torn apart by the pulsar’s titanic gravity. Such a high density is not typical for Jupiter-mass objects. In comparison, Jupiter has a mean density of only 1.3 grams per cubic centimetre. It is theorised that the Jupiter-mass object around PSR J1719-1438 is the dense remnant core of a carbon white dwarf that had its outer layers stripped away and accreted by the pulsar. An object like this would easily have the high density necessary to avoid being tidally torn apart by the pulsar’s gravity.

Figure 2: Artist’s impression of the orbit of the Jupiter-mass object around PSR J1719-1438.

Nevertheless, a paper by J.E. Horvath (2012) suggests that the Jupiter-mass object around PSR J1719-1438 might not be the dense remnant core of a white dwarf. Instead, it could be something more exotic. The proposed solution is that the object is an exotic Jupiter-mass clump of superdense strange quark matter. An object like this is so compact; it would measure a mere ~1 km in radius.

There are a number of mechanisms that can explain the origin of a Jupiter-mass quark matter object. In one such process, the pulsar itself is already comprised of quark matter (i.e. the pulsar is a quark star) and its formation involved the conversion of neutrons to quarks in an explosive and turbulent manner. The turbulence can eject sufficient quantities of quark matter to form a Jupiter-mass quark matter object in a very close-in orbit around the pulsar. Another process involves the merger of two low-mass quark stars. A. Bauswein et al. (2009) show that a merger event like this can easily eject enough quark matter to form a Jupiter-mass object.

Future observations might reveal whether or not the Jupiter-mass object around PSR J1719-1438 is indeed an exotic object. For example, a Jupiter-mass quark matter object measuring ~1 km in radius would be too small to generate a detectable photometric signal (i.e. reflected light from the pulsar). Moreover, an exotic quark matter object is not expected to produce any detectable evaporation signature since there would be no “normal matter” for the pulsar’s energetic radiation to strip off into space. For this reason, the non-detection of circumstellar material around PSR J1719-1438 can be evidence that is consistent with the pulsar’s companion being an exotic quark matter object.

- M. Bailes et al. (2011), “Transformation of a Star into a Planet in a Millisecond Pulsar Binary”, arXiv:1108.5201 [astro-ph.SR]
- J.E. Horvath (2012), “The nature of the companion of PSR J1719-1438: a white dwarf or an exotic object?”, arXiv:1205.1410 [astro-ph.HE]
- A. Bauswein et al. (2009), “Mass ejection by strange star mergers and observational implications”, arXiv:0812.4248 [astro-ph]