Astronomers from MIT and McGill University reported on March 23 that they have taken a major step toward categorizing rare, strange-acting stars called Anomalous X-ray Pulsars (AXP) as neutron stars.
Victoria M. Kaspi, assistant professor of physics in the Center for Space Research (CSR) on leave from McGill University in Montreal, has found that one such AXP has experienced an "earth" quake -- a sudden, catastrophic shifting of the star's interior -- that is similar to quakes seen in regular neutron stars. This provides strong confirmation that the AXP is indeed a neutron star and has properties surprisingly similar to its "non-anomalous" cousins.
Professor Kaspi's finding may also support the magnetar hypothesis, which predicts the existence of neutron stars up to a thousand times more magnetic than the already strongly magnetic neutron star. She presented her work, based on observations with the Rossi X-ray Timing Explorer (RXTE) satellite, at the Rossi 2000 meeting at NASA Goddard Space Flight Center in Greenbelt, MD.
Her collaborators include her UROP student and physics senior Jessica Lackey, whose senior thesis is the basis for this work, and Assistant Professor of Physics Deepto Chakrabarty, also with the CSR.
"We have thought that earthquake-like events might occur on these stars for some time now, but we have lacked the right instrument to check this," said Professor Kaspi. "No X-ray astronomy satellite in the past had the agility to observe these objects as often and as regularly as we needed. Thanks to RXTE, we now know for certain that glitches occur in AXPs, and we can study the interiors of these unusual objects using a form of 'seismology,' like the way geologists study the Earth from earthquakes."
A neutron star is the skeletal remnant of a star once several times more massive than the sun that exhausted its nuclear fuel and subsequently exploded its outer shell. The remaining core, still possessing about a sun's worth of mass, collapses to a sphere about 10 miles in diameter. The collapse sets the neutron star spinning quickly, much like an ice skater spins faster as she "collapses" her body by pulling her arms inward.
A pulsar is a neutron star that "pulses" with radiation with every spin. The radiation comes largely from the neutron star's polar regions, channeled by strong magnetic fields. We on Earth see pulses each time the polar regions spin our way, in the same way that we see the rotating beacon of a lighthouse.
Since they were discovered in 1967, pulsars have been known to be strong emitters of radio waves. With the arrival of X-ray satellites, however, astronomers found that many pulsars also emitted X-ray radiation. A handful of pulsars emit their light exclusively as X-rays. The slow-spinning AXPs are among these X-ray-only pulsars, and the puzzle astronomers are trying to unravel is just how AXPs are related to the well-studied radio pulsars, if at all.
To solve this mystery, the MIT/McGill team needed to find evidence that AXPs had many of the same quirks as the well-studied radio pulsars. One such quirk is that some radio pulsars occasionally, without warning, start spinning faster than normal. These sudden spin-up events are called "glitches."
The glitch, or change in spin-down rate, is likely due to the earthquake-like phenomenon, technically called vortex line upspinning, which occurs within the stellar interior, beneath the stellar crust. Professor Kaspi's group set out to carefully monitor the spins of several AXPs in the hope of detecting a similar spin-up event.
After two years of watching and waiting, their patience was rewarded: one of their targets, called 1RXS J1708-4009, suddenly started spinning faster.
"We were delighted! The glitch we saw was practically identical to glitches that have been seen in the Vela radio pulsar," said Professor Kaspi. (The Vela pulsar is a well-studied neutron star in a nearby system that exploded some 20,000 years ago.) "This is clear evidence that the AXP is a neutron star with an internal structure just like the radio pulsars."
There are only five known AXPs. Professor Kaspi said many more of these objects may exist but that others are hard to identify because they are isolated and emit only X-ray radiation. The plethora of radio and optical telescopes, therefore, cannot identify them. RXTE is one of only three or four satellites best optimized for AXP hunts.
Magnetars, if proven to exist, would be neutron stars born with immense magnetic fields so powerful they could swipe clean the information on a credit card at a distance halfway to the moon. The galaxy holds a handful of magnetar candidates. The AXP glitch observation supports the magnetar hypothesis, Professor Kaspi said, but does not provide irrefutable proof.
"That can only come with patience and continued observations of these strange beasts," she said.
Professor Kaspi and her colleagues' work represents the latest chapter in understanding these strange types of objects first seen in the 1990s by a Japanese satellite named the Advanced Satellite for Cosmology and Astrophysics. She hopes that identifying the nature of AXPs will pave the way in understanding equally rare and bewildering objects, such as the magnetars and soft gamma-ray repeaters, which may be related to AXPs.
Rossi 2000 is the first meeting to bring together the diverse pool of observational astronomers and theorists utilizing RXTE, which was launched by NASA in December 1995. Images are available on a NASA web site.
A version of this article appeared in MIT Tech Talk on April 5, 2000.