Unique view of neighbouring galaxy provides pulsar bonanza

Scientists from the University of Southampton, together with colleagues at NASA, have detected for the first time the activity cycles of 20 X-ray pulsars outside the Milky Way galaxy. The discovery is the product of a weekly three-year observation campaign that also uncovered five new X-ray pulsars and captured an unprecedented snapshot of seven 'bursting' pulsars in one single viewing.

The pulsars' unobscured location in the Small Magellanic Cloud (SMC), a neighbouring galaxy, provide a unique insight into the X-ray pulsar phenomenon, in which these objects erupt suddenly and randomly with an outpouring of X-ray radiation for days, weeks or months at a time. Each outburst glows with the intensity of 10,000 suns.

Results from the campaign are presented today (9 January) at the 199th Meeting of the American Astronomical Society in Washington, D.C. The observation team - from the Department of Physics and Astronomy at the University of Southampton in England and the NASA Goddard Space Flight Center in Greenbelt, Maryland - used NASA's Rossi X-ray Timing Explorer.

"The SMC is an X-ray pulsar laboratory," said Silas Laycock, a postgraduate student at the University of Southampton. "The pulsars there are likely to be similar to the ones in our Milky Way galaxy, yet they are above and beyond the dusty galactic plane which obscures "local" pulsars and are also laid out at roughly equal distances from Earth. This makes for convenient, long-term analysis."

"X-ray pulsars are not always "turned on", so when we saw seven in one viewing, we felt a bit like the peasant in the folktale who killed seven giants with one blow," said Dr Robin Corbet of NASA Goddard.

Dr Corbet estimates that the tiny SMC has 10 times the concentration of these types of pulsars, perhaps created during a burst of star formation a few million years ago when the Milky Way galaxy and the SMC were at their closest.

An X-ray pulsar is a type of spinning neutron star, the core remains of a star once several times more massive than our sun. The observed X-ray pulsars in the SMC reside in binary star systems, in which the compact pulsar orbits around a young and unstable hydrogen-burning star (called a Be star). This neighbouring star is surrounded by a large disk of hydrogen, providing a reservoir of fuel to power the X-ray pulsar.

The tiny pulsar suddenly becomes visible and 'pulses' in X-ray radiation when the orbit brings the two stars close to each other. The pulsar's enormous magnetic field then channels the gas towards its magnetic poles, where the gas attains speeds up to 20 per cent that of light and heats to temperatures far hotter than when it was part of the neighbouring star. This gas now glows predominantly in the X-ray band.

Scientists see the pulsar 'pulse' in X-ray radiation with every spin, as the pole flashes towards Earth's view - but only when there is a fuel supply. These pulsars are transient, and what makes similar X-ray binary systems light up at such different rates is a mystery. For example, one pulsar, named XTE 172sec, underwent seven separate X-ray outbursts, while other sources were seen less frequently.

The three-year Rossi Explorer campaign has provided some clues into this mechanism. Several sources exhibited recurring outbursts, with approximately regular intervals between recurrences. This provides an estimate of orbital periods in these binary systems. Also, the team found that the recurrent pulsars (lighting up at least once a year) have relatively long spin rates, from 46 to 755 seconds.

The petite, irregularly shaped SMC is about 200,000 light years away and is the second closest galaxy to the Milky Way galaxy, visible to the naked eye from the Southern Hemisphere. The SMC's total mass is only 1/50th that of the Milky Way galaxy.

Other members of the observation team are Drs Malcolm Coe (Silas Laycock's PhD supervisor at the University of Southampton), Frank Marshall (NASA Goddard) and James Lochner (NASA Goddard). Drs Corbet and Lochner join NASA Goddard through the Universities Space Research Association.