Chandra Finds Long-Sought Link to Origin of Millisecond Pulsars

Jul 21, 2005
Chandra Finds Long-Sought Link to Origin of Millisecond Pulsars

The peculiar cosmic object known as 47 Tuc W (denoted by arrow in the X-ray image) is a double star system consisting of a normal star and a neutron star that makes a complete rotation every 2.35 milliseconds. Blink your eye and a superdense star the size of Manhattan Island will have rotated 25 or more times!

Image credit: X-ray: NASA/CXC/Northwestern U./C.Heinke et al.; Optical: ESO/Danish 1.54-m/W.Keel et al.

New Chandra observations give the best information yet on why such neutron stars, called millisecond pulsars, are rotating so fast. The key, as in real estate, is location, location, location - in this case the crowded confines of the globular star cluster 47 Tucanae, where stars are less than a tenth of a light year apart. Almost two dozen millisecond pulsars are located there. This large sample is a bonanza for astronomers seeking to test theories for the origin of millisecond pulsars, and increases the chances that they will find a critical transitional object such 47 Tuc W.

47 Tuc W stands out from the crowd because it produces more high-energy X-rays than the others. This anomaly points to a different origin of the X-rays, namely a shock wave due to a collision between matter flowing from a companion star and particles racing away from the pulsar at near the speed of light. Regular variations in the optical and X-ray light corresponding to the 3.2-hour orbital period of the stars support this interpretation.

A team of astronomers from the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA pointed out that the X-ray signature and variability of the light from 47 Tuc W are nearly identical to those observed from an X-ray binary source known as J1808. They suggest that these similarities between a known millisecond pulsar and a known X-ray binary provide the long-sought link between these types of objects.

In theory, the first step toward producing a millisecond pulsar is the formation of a neutron star when a massive star goes supernova. If the neutron star is in a globular cluster, it will perform an erratic dance around the center of the cluster, picking up a companion star which it may later swap for another.

As on a crowded dance floor, the congestion in a globular cluster can cause the neutron star to move closer to its companion, or to swap partners to form an even tighter pair. When the pairing becomes close enough, the neutron star begins to pull matter away from its partner. As matter falls onto the neutron star, it gives off X-rays. An X-ray binary system has been formed, and the neutron star has made the crucial second step toward becoming a millisecond pulsar.

The matter falling onto the neutron star slowly spins it up, in the same way that a child's carousel can be spun up by pushing it every time it comes around. After 10 to 100 million years of pushing, the neutron star is rotating once every few milliseconds. Finally, due to the rapid rotation of the neutron star, or the evolution of the companion, the infall of matter stops, the X-ray emission declines, and the neutron star emerges as a radio-emitting millisecond pulsar.

Illustration of Shock Wave Around Millisecond Pulsar

Illustration of Shock Wave Around Millisecond Pulsar
In binary star systems such as 47 Tuc W, which contains a normal star and an extremely rapidly rotating neutron star called a millisecond pulsar (MSP), matter is pulled from the normal star by the gravitational tug of the more massive neutron star. In contrast to X-ray binary systems, this matter (yellow streamer in illustration) does not form a hot disk around the neutron star. Instead, it is pushed back by the pressure of a wind of fast-moving particles (blue) produced by the pulsar. The resulting shock wave (white) is a source of high-energy X-rays. (Illustration: NASA/CfA/S.Bogdanov)

It is likely that the companion star in 47 Tuc W - a normal star with a mass greater than about an eighth that of the Sun - is a new partner, rather than the companion that spun up the pulsar. The new partner, acquired fairly recently in an exchange that ejected the previous companion, is trying to dump on the already spun-up pulsar, creating the observed shock wave. In contrast, the X-ray binary J1808 is not in a globular cluster, and is very likely making do with its original companion, which has been depleted to a brown dwarf size with a mass less than 5% that of the Sun.

Most astronomers accept the binary spin-up scenario for creating millisecond pulsars because they have observed neutron stars speeding up in X-ray binary systems, and almost all radio millisecond pulsars are observed to be in binary systems. Until now, definitive proof has been lacking, because very little is known about transitional objects between the second and final steps.

That is why 47 Tuc W is hot. It links a millisecond pulsar with many of the properties of an X-ray binary, to J1808, an X-ray binary that behaves in many ways like a millisecond pulsar, thus providing a strong chain of evidence to support the theory.

Source: Chandra X-ray Observatory

Explore further: Image: Hubble serves a slice of stars

add to favorites email to friend print save as pdf

Related Stories

Fermi finds a 'transformer' pulsar

Jul 22, 2014

( —In late June 2013, an exceptional binary containing a rapidly spinning neutron star underwent a dramatic change in behavior never before observed. The pulsar's radio beacon vanished, while at ...

Upgraded telescope opens window to universe

Jul 18, 2014

An international team of astrophysicists including University of Adelaide researchers have announced the successful detection of pulsed gamma rays from the neutron star, the Vela pulsar, using their newly upgraded telescope ...

A 3-D model of stellar core collapse

May 02, 2014

( —What happens when massive stars collapse? One potential result is a core-collapse supernova. Astronomers can make observations of such events that tell us what is happening on the surface of ...

Recommended for you

SHERLOC to micro-map Mars minerals and carbon rings

42 minutes ago

( —An ultraviolet-light instrument on the robotic arm of NASA's Mars 2020 rover will use two types of ultraviolet-light spectroscopy, plus a versatile camera, to help meet the mission's ambitious ...

NuSTAR celebrates two years of science in space

1 hour ago

( —NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, a premier black-hole hunter among other talents, has finished up its two-year prime mission, and will be moving onto its next phase, ...

Video: ATV-5 separation from Ariane 5

1 hour ago

These images were taken by cameras on the Ariane 5 launcher that rocketed skywards on 29 July 2014 with Europe's last cargo vessel to visit the International Space Station, ATV-5. The video shows the separation of ATV Georges ...

User comments : 0