A Pulsed Discovery in Globular Cluster Omega Centauri | WPN World People News

What Lies At the Core

Located just 17,000 light-years away, Omega Centauri is an intriguing object of study. Though we know of more than 200 globular clusters — compact spheres of old stars — that lie in the outer regions of the Milky Way, Omega Centauri is the most massive and the most luminous. Its properties have led scientists to speculate that this cluster was once a dwarf galaxy that was captured by the Milky Way and had its outer stars stripped away.

The globular cluster Omega Centauri contains as many as ten million stars. A new study has found five millisecond pulsars at its core.

Omega Centauri’s large mass and unusual formation history open two interesting possibilities:

1. The cluster might contain massive black holes.
Theory predicts that the conditions at the center of massive stellar clusters are ripe for collisions that drive the growth of intermediate-mass black holes with perhaps hundreds to tens of thousands of solar masses.

Image of two globular clusters (can you spot them? Look carefully!) in the Milky Way. Omega Centauri is brighter and more massive than either of these — or any other Milky Way globular cluster.

2. The cluster core might show evidence of dark matter annihilation.
If Omega Centauri was once a dark-matter-dominated dwarf galaxy, then its relatively close distance makes it an excellent target to search for dark matter annihilating at its center.

Artist’s illustration of a pulsar, a fast-spinning, magnetized neutron star.

Pulsars as Probes

How can we explore these possibilities? Our best bet would be to study the motions and signatures of radio pulsars — rapidly rotating, magnetized neutron stars — in the cluster’s core. The motions of these dense objects would provide information about the core’s dynamics, potentially revealing the gravitational influence of lurking massive black holes. The signatures of the pulsars’ emission could also tell us about the interstellar medium of the cluster, constraining particle dark matter annihilation models.

The pulse profiles of the five newly discovered millisecond radio pulsars in Omega Centauri’s core.

But though radio pulsars are common in the cores of other globular clusters, they’ve remained elusive in Omega Centauri. A tantalizing hint came in 2010 with the discovery of a gamma-ray source in the cluster’s core — but years of searching for pulsed radio emission from this location turned up nothing.

A Population Found

A new study led by Shi Dai (CSIRO Australia Telescope National Facility) has now used the high sensitivity of the Ultra-Wideband Low receiver on the Parkes radio telescope in Australia to up the search intensity. The result? The team found five faint millisecond pulsars hiding in Omega Centauri’s core.

These newly discovered pulsars have spin periods that range from 4.1 to 6.8 milliseconds. While four of them are isolated objects, the fifth lies in an eclipsing binary system with a very low-mass star, orbiting once every 2.1 hours.

These pulsars may just be the tip of the iceberg: Dai and collaborators found additional compact sources in deep radio continuum images of Omega Centauri’s center, suggesting there may be more pulsars awaiting discovery. In the meantime, tracking the five pulsars now known will give us an excellent opportunity to probe the properties of this massive, bright cluster and learn more about its secrets.

Source: American Astronomical Society