Prime View of Neighbouring Galaxy Yields Pulsar Bonanza, Seven in One Blow
January 9, 2002
Washington -- Scientists have detected for first time the activity cycles of a population of X-ray pulsars outside the Milky Way galaxy, the product of a weekly three-year observation campaign that uncovered five new X-ray pulsars and captured an unprecedented snapshot of seven "flaring" pulsars in one single viewing.
One surprise discovery has been that the X-ray pulsars with slower spin rates seem to flare more frequently than faster-spinning pulsars. This adds a new dimension to the popular theory on pulsar evolution.
Results from the campaign are presented today at the 199th Meeting of the American Astronomical Society in Washington, D.C. The observation team is from the University of Southampton in England and NASA Goddard Space Flight Center in Greenbelt, Md. They used NASA's Rossi X-ray Timing Explorer for these observations.
The 20 observed pulsars, with their unobscured location in the Small Magellanic Cloud (SMC), provide for unique insight into the X-ray pulsar phenomenon, in which these objects suddenly erupt 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.
"The SMC is an X-ray pulsar laboratory," said Silas Laycock, a graduate student at the University of Southampton. "The pulsars there are likely 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."
Dr. Robin Corbet of NASA Goddard estimates that the petite 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 once-massive star. The X-ray pulsars in this observation reside in binary star systems, in which the compact pulsar orbits around a young and unstable hydrogen-burning star (called a Be star.) The pulsar system suddenly becomes visible and "pulses" in X-ray radiation when the orbit brings the two stars close to each other.
The pulsar's gravity pulls gas away from its neighbor, and enormous magnetic fields channel this gas toward the pulsar's poles. The fast-moving gas 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 in the SMC underwent seven separate X-ray outbursts, while other sources were seen less frequently.
Two variables in this picture are the pulsar's spin period (measured in seconds) and its orbital period (taking months to years). Scientists have thought that faster-spinning pulsars have shorter orbits. The idea here is that pulsars with shorter orbits would come in contact more often with their neighbor; the furious gas transfer would "spin up" the pulsar; and the flaring would be seen (depending on the state of the Be star's disk) with perhaps each orbit, at the periastron.
The three-year Rossi Explorer campaign uncovered what seems to be the opposite: The slower-spinning pulsars were flaring much more frequently than the fast-spinning pulsars. Laycock said the physics behind this apparent contradiction must involve at least the following two effects:
Perhaps systems with smaller orbits have larger tidal forces. This would truncate the disk of gas around the bulging Be star, limiting the fuel source to power an X-ray flare. Also, perhaps fast-spinning pulsars find it much harder to accrete gas, due to centrifugal forces acting to fling away incoming gas before it crashes towards the poles.
Exactly what allows some pulsars to break through these twin barriers remains a mystery. As the Rossi Explorer campaign continues, the scientists hope to measure the orbital periods of the pulsars, which could help provide an explanation for their current observations. Indeed, observations show that a number of sources exhibit recurring outbursts, with approximately regular intervals between recurrences that provide an estimate of orbital periods.
A second surprise in the campaign was the opportunity to see seven flares at once. "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 Corbet.
The 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 (Laycock's thesis advisor 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.