From AAS NOVA: “A Pulsed Discovery In Omega Centauri”

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From AAS NOVA

12 February 2020
Susanna Kohler

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The globular cluster Omega Centauri contains as many as ten million stars. A new study has found five millisecond pulsars at its core. [ESO/La Silla Observatory]

ESO/Cerro LaSilla, 600 km north of Santiago de Chile at an altitude of 2400 metres.

The globular cluster Omega Centauri makes for an impressive sight — millions of stars gravitationally bound into a beautiful sphere, its core alight from the glow of densely packed bodies. A recent study has unveiled a new discovery at the heart of this cluster: five long-anticipated pulsars.

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.

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

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.
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.

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

Women in STEM – Dame Susan Jocelyn Bell Burnell

Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.

Dame Susan Jocelyn Bell Burnell at work on first plusar chart 1967 pictured working at the Four Acre Array in 1967. Image courtesy of Mullard Radio Astronomy Observatory.

Dame Susan Jocelyn Bell Burnell 2009

Dame Susan Jocelyn Bell Burnell (1943 – ), still working from http://www. famousirishscientists.weebly.com

Biography

British astrophysicist, scholar and trailblazer Jocelyn Bell Burnell discovered the space-based phenomena known as pulsars, going on to establish herself as an esteemed leader in her field.Who Is Jocelyn Bell Burnell?
Jocelyn Bell Burnell is a British astrophysicist and astronomer. As a research assistant, she helped build a large radio telescope and discovered pulsars, providing the first direct evidence for the existence of rapidly spinning neutron stars. In addition to her affiliation with Open University, she has served as dean of science at the University of Bath and president of the Royal Astronomical Society. Bell Burnell has also earned countless awards and honors during her distinguished academic career.

Early Life

Jocelyn Bell Burnell was born Susan Jocelyn Bell on July 15, 1943, in Belfast, Northern Ireland. Her parents were educated Quakers who encouraged their daughter’s early interest in science with books and trips to a nearby observatory. Despite her appetite for learning, however, Bell Burnell had difficulty in grade school and failed an exam intended to measure her readiness for higher education.

Undeterred, her parents sent her to England to study at a Quaker boarding school, where she quickly distinguished herself in her science classes. Having proven her aptitude for higher learning, Bell Burnell attended the University of Glasgow, where she earned a bachelor’s degree in physics in 1965.

Little Green Men

In 1965, Bell Burnell began her graduate studies in radio astronomy at Cambridge University. One of several research assistants and students working under astronomers Anthony Hewish, her thesis advisor, and Martin Ryle, over the next two years she helped construct a massive radio telescope designed to monitor quasars. By 1967, it was operational and Bell Burnell was tasked with analyzing the data it produced. After spending endless hours pouring over the charts, she noticed some anomalies that did not fit with the patterns produced by quasars and called them to Hewish’s attention.

Over the ensuing months, the team systematically eliminated all possible sources of the radio pulses—which they affectionately labeled Little Green Men, in reference to their potentially artificial origins—until they were able to deduce that they were made by neutron stars, fast-spinning collapsed stars too small to form black holes.

Pulsars and Nobel Prize Controversy

Their findings were published in the February 1968 issue of Nature and caused an immediate sensation. Intrigued as much by the novelty of a woman scientist as by the astronomical significance of the team’s discovery, which was labeled pulsars—for pulsating radio stars—the press picked up the story and showered Bell Burnell with attention. That same year, she earned her Ph.D. in radio astronomy from Cambridge University.

However, in 1974, only Hewish and Ryle received the Nobel Prize for Physics for their work. Many in the scientific community raised their objections, believing that Bell Burnell had been unfairly snubbed. However, Bell Burnell humbly rejected the notion, feeling that the prize had been properly awarded given her status as a graduate student, though she has also acknowledged that gender discrimination may have been a contributing factor.

Life on the Electromagnetic Spectrum

Nobel Prize or not, Bell Burnell’s depth of knowledge regarding radio astronomy and the electromagnetic spectrum has earned her a lifetime of respect in the scientific community and an esteemed career in academia. After receiving her doctorate from Cambridge, she taught and studied gamma ray astronomy at the University of Southampton. Bell Burnell then spent eight years as a professor at University College London, where she focused on x-ray astronomy.

During this same time, she began her affiliation with Open University, where she would later work as a professor of physics while studying neurons and binary stars, and also conducted research in infrared astronomy at the Royal Observatory, Edinburgh. She was the Dean of Science at the University of Bath from 2001 to 2004, and has been a visiting professor at such esteemed institutions as Princeton University and Oxford University.

Array of Honors and Achievements

In recognition of her achievements, Bell Burnell has received countless awards and honors, including Commander and Dame of the Order of the British Empire in 1999 and 2007, respectively; an Oppenheimer prize in 1978; and the 1989 Herschel Medal from the Royal Astronomical Society, for which she would serve as president from 2002 to 2004. She was president of the Institute of Physics from 2008 to 2010, and has served as president of the Royal Society of Edinburgh since 2014. Bell Burnell also has honorary degrees from an array of universities too numerous to mention.

Personal Life

In 1968, Jocelyn married Martin Burnell, from whom she took her surname, with the two eventually divorcing in 1993. The two have a son, Gavin, who has also become a physicist.

A documentary on Bell Burnell’s life, Northern Star, aired on the BBC in 2007.

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.

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

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The pulse profiles of the five newly discovered millisecond radio pulsars in Omega Centauri’s core. [Dai et al. 2020]

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.

Citation

“Discovery of Millisecond Pulsars in the Globular Cluster Omega Centauri,” Shi Dai et al 2020 ApJL 888 L18.

https://iopscience.iop.org/article/10.3847/2041-8213/ab621a

See the full article here .


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