RSSArchive for September, 2013

The ‘missing link’ pulsar

AN INTERNATIONAL TEAM of astronomers using CSIRO radio telescopes in Australia and other ground and space-based instruments, has caught a small star called a pulsar undergoing a radical transformation, described in a paper in the journal Nature.

“For the first time we see both X-rays and extremely fast radio pulses from the one pulsar. This is the first direct evidence of a pulsar changing from one kind of object into another – like a caterpillar turning into a butterfly,” said Dr Simon Johnston, Head of Astrophysics at CSIRO’s Astronomy and Space Science division.

The pulsar and its companion star

The pulsar and its companion star. The ageing pulsar rotates slower and slower, then matter from its companion spins it up again. As the pulsar is spun up, it alternates between emitting X-rays (white) and radio waves (pink). Credit: ESA

The cosmic drama is being played out 18,000 light-years away, in a small cluster of stars (called M28) in the constellation of Sagittarius.

The pulsar (called PSR J1824-2452I) has a tiny companion star, with about a fifth the mass of the Sun. Although small, the companion is fierce, pounding the pulsar with streams of matter.

Normally the pulsar shields itself from this onslaught, its magnetic field deflecting the matter stream into space.

But sometimes the stream swells to a flood, overwhelming the pulsar’s protective ‘force field.’ When the stream hits the pulsar’s surface its energy is released as blasts of X-rays.

Eventually the torrent slackens. Once again the pulsar’s magnetic field re-asserts itself and fends off the companion’s attacks.

“We’ve been fortunate enough to see all stages of this process, with a range of ground and space telescopes. We’ve been looking for such evidence for more than a decade,” said Dr Alessandro Papitto, the paper’s lead author. Dr Papitto is an astronomer of the Institute of Space Studies (ICE, CSIC-IEEC) of Barcelona, Spain.

‘Teenage’ behaviour

The pulsar and its companion form what is called a ‘low-mass X-ray binary’ system. In such a system, the matter transferred from the companion lights up the pulsar in X-rays and makes it spin faster and faster, until it becomes a ‘millisecond pulsar’ that spins at hundreds of times a second and emits radio waves. The process takes about a billion years, astronomers think.

In its current state the pulsar is exhibiting behaviour typical of both kinds of systems: millisecond X-ray pulses when the companion is flooding the pulsar with matter, and radio pulses when it is not.

“It’s like a teenager who switches between acting like a child and acting like an adult,” said Mr. John Sarkissian, who observed the system with CSIRO’s 64-m (210-ft) Parkes radio telescope in eastern Australia.

“Interestingly, the pulsar swings back and forth between its two states in just a matter of weeks.”

This video shows an artist’s impression of the pulsar and its companion star. Credit: ESA

A global effort

The pulsar was initially detected as an X-ray source with the INTEGRAL satellite. X-ray pulsations were seen with another satellite, ESA’s XMM-Newton; further observations were made with NASA’s Swift. NASA’s Chandra X-ray telescope got a precise position for the object.

Then, crucially, the object was checked against the pulsar catalogue generated by CSIRO’s Australia Telescope National Facility, and other pulsar observations. This established that it had already been identified as a radio pulsar.

The source was detected in the radio with CSIRO’s Australia Telescope Compact Array, and then re-observed with CSIRO’s Parkes radio telescope, NRAO’s Robert C. Byrd Green Bank Telescope in the USA, and the Westerbork Synthesis Radio Telescope in The Netherlands. Pulses were detected in a number of these later observations, showing that the pulsar had ‘revived’ as a normal radio pulsar only a couple of weeks after the last detection of the X-rays.

The astronomers involved in these investigations work at institutions in Australia, Canada, Germany Italy, The Netherlands, Spain, Switzerland, and the USA.

Adapted from information issued by CSIRO.

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Galactic explosion betrays black hole

TWO MILLION YEARS AGO a supermassive black hole at the heart of our galaxy erupted in an explosion so immensely powerful that it lit up a cloud 200,000 light years away, a team of researchers led by the University of Sydney has revealed.

The finding is an exciting confirmation that black holes can ‘flicker’, moving from maximum power to switching off over, in cosmic terms, short periods of time.

An artist's conception of a black hole generating a jet

An artist’s conception of a black hole generating a jet. Two million years ago the supermassive black hole at the centre of our Galaxy was 100 million times more powerful than it is today. Credit: NASA / Dana Berry / SkyWorks Digital

“For 20 years astronomers have suspected that such a significant outburst occurred, but now we know when this sleeping dragon, four million times the mass of the Sun, awoke and breathed fire with 100 million times the power it has today,” said Professor Joss Bland-Hawthorn from the University’s School of Physics, and lead author of an article on the research to be published in The Astrophysical Journal.

Professor Bland-Hawthorn unveiled the research at the international Galaxy Zoo science conference on 24 September in Sydney.

“It’s been long suspected that our Galactic Centre might have sporadically flared up in the past. These observations are a highly suggestive ‘smoking gun’,” said Martin Rees, Astronomer Royal, who was one of the first scientists to suggest that massive black holes power quasars.

Fossil record

The evidence for the findings comes from a lacy filament of hydrogen gas called the Magellanic Stream. It trails behind our galaxy’s two small companion galaxies, the Large and Small Magellanic Clouds.

“Since 1996, we’ve been aware of an odd glow from the Magellanic Stream, but didn’t understand the cause. Then this year, it finally dawned on me that it must be the mark, the fossil record, of a huge outburst of energy from the supermassive black hole at the centre of our galaxy.”

The region around the galaxy’s supermassive black hole and the black hole is called Sagittarius A* (pronounced Sagittarius A-star). It emits radio, infrared, ultraviolet, x-ray and gamma ray emissions. Flickers of radiation rise up when small clouds of gas fall onto the hot cloud of matter that swirls around the black hole.

The video below show a computer simulation of a black hole in real time showing how gas falling in forms a disc that spins around the black hole. The friction causes the gas to become so hot it produces beams of UV radiation. Credit: McKinney (UMD), Tchekhovskoy (Princeton), Blandford (KIPAC), Kaehler (KIPAC)

In stark contrast to this current inactivity, evidence is emerging that there was a cataclysmic event in the past.

“In particular, in 2010 NASA’s Fermi satellite discovered two huge bubbles of hot gas billowing out from the centre of the galaxy, covering almost a quarter of the sky,” said Professor Bland-Hawthorn.

On-and-off black holes

Earlier this year, computer simulations of the Fermi bubbles made by the University of California Santa Cruz controversially suggested that they were caused by a colossal explosion from Sagittarius A* within the last few million years.

“When I saw this research I realised that this same event would also explain the mysterious glow that we see on the Magellanic Stream,” Professor Bland-Hawthorn said.

“Together with Dr Ralph Sutherland from Mount Stromlo Observatory and Dr Phil Maloney, from the University of Colorado, I calculated that to explain the glow it must have happened two million years ago because the energy release shown by the Santa Cruz group perfectly matched, to our delight, that from the Magellanic Stream.”

“The galaxy’s stars don’t produce enough ultraviolet to account for the glow, nor could they have in the past,” said Dr Maloney. “The Galactic Centre never formed stars at a high enough rate. There had to be another explanation.”

Professor Bland-Hawthorn said, “In fact the radiation from stars is one hundred times too little to account for the radiation now or at any time. The galaxy could never have produced enough UV radiation to account for it. So the only explanation was it had to be produced from our dragon, the massive black hole.”

“The realisation that these black holes can switch on and off within a million years, which given the universe is 14 billion years old means very rapidly, is a significant discovery.”

Will such a colossal explosion ever happen again?

“Yes, absolutely! There are lots of stars and gas clouds that could fall onto the hot disk around the black hole,” says Professor Bland-Hawthorn. “There’s a gas cloud called G2 that astronomers around the world are anticipating will fall onto the black hole early next year. It’s small, but we’re looking forward to the fireworks!

Professor Bland-Hawthorn is a Fellow of the Australian Astronomical Observatory.

Adapted from information issued by the University of Sydney.

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