Scientists Find Exotic X-Ray Bubbles in Their First Detailed Observation of a Neutron Star Surface
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453April 14, 1999
Charleston, S.C. -- The Centaurus X-3 neutron star spends its day blowing bubbles, but that's not to say it's lazy. In fact, this collapsed star produces extreme gravitational and electromagnetic fields and must spin completely about its axis in less than five seconds to produce these photon bubbles that burst with X-ray radiation up to 2,000 times per second, the fastest pops ofX-ray light ever recorded.
The same group that predicted the exotic bubble phenomenon ten years ago also found the first evidence for their existence this year, peering at the polar region of a star only 13 miles in diameter and nearly 30,000 light years away. The group reports its findings at a press conference Wednesday, April 14, at the meeting of the High Energy Astrophysics Division of the American Astronomical Society in Charleston, S.C.
A neutron star is the core remains of an exploded star several times more massive than the sun. Centaurus X-3 is also a pulsar, sending out pulses of visible and X-ray radiation as it spins. The photon bubbles are cavities of X-ray light in the sea of electrically charged gas (plasma) on the neutron star surface.
Researchers proposed some 10 years ago that when the neutron star is part of a binary system, the matter sucked onto the neutron star from the companion creates an unusual field of radiation bubbles that dance around the polar regions. When these bubbles burst, they predicted, they would produce a shower of X-rays that could be detected by an X-ray telescope above the Earth's surface.
In 1997, these researchers took a long, hard look (a total of 72 hours) at Centaurus X-3 using the Rossi X-ray Timing Explorer (RXTE), a NASA X-ray satellite capable of detecting X-ray emissions over micro-second timescales. The researchers include J. Garrett Jernigan of UC Berkeley's Space Sciences Laboratory, Richard I. Klein of the Lawrence Livermore National Laboratory and adjunct professor of astronomy at UC Berkeley, and Jonathan Arons, chair of the astronomy department and professor of astronomy and physics at UC Berkeley.
After extensive analysis of the data, the researchers discovered the X-ray emissions flickering at rates between 100 and 2,000 times per second -- a range the team had predicted with large scale supercomputer calculations.
"This is the fastest known X-ray emission of any collapsed star in the universe, be it a white dwarf, black hole or neutron star," Klein said. "The discovery lends strong support to our theory that the origin of these rapid X-ray fluctuations are the exotic photon bubbles we predicted earlier."
The team's new observational discovery, combined with their recent theoretical work, also permitted them to deduce for the first time the actual size of the X-ray emitting polar cap region on the surface of the neutron star Centaurus X-3 -- about 20 square kilometers (eight square miles) -- roughly one fourth the area of San Francisco.
The Arons and Klein theory details what happens when a spinning and highly magnetized neutron star -- in this case, a pulsar emitting X-rays but not visible light -- draws matter in from a nearby companion star. Rapid, oscillating X-ray emissions originate near the surface of such neutron stars as the infalling matter from a companion star (attracted by the neutron star's gravitational pull) crashes down onto the small polar cap regions on the neutron star's surface. The infalling matter, channeled by the intense magnetic fields onto the polar caps, moves at one third the speed of light and converts its energy into intense radiation.
"The rain of hot matter and radiation onto the polar cap of the neutron star is like an extremely violent version of the Northern Lights," Jernigan said.
This radiation creates a strong pressure near the surface of the neutron star and pushes the infalling matter aside, poking holes in the matter and creating empty bubbles that fill with 100 million degree X-rays.
These bubbles of X-ray light, called photon bubbles, rise up like hot fingers to a few miles above the surface of the neutron star only to fall and disintegrate, releasing their energy in a thousandth of a second.
Calculations by collaborators, performed on supercomputers at the Lawrence Livermore National Laboratory, show that the photon bubble fingers release radiation energy in a more or less regular fashion, causing the neutron star to flicker or oscillate. They named these "photon bubble oscillations" or PBOs.
Arons and Klein predicted that PBOs and photon bubble turbulence would have the best chance of being discovered in the highly luminous X-ray pulsar Centaurus X-3, which resides in our galaxy. Knowing Centaurus X-3's observed properties -- the magnetic field, the mass, the radius, the luminosity and distance to the Earth -- the astrophysicists deduced the rate that matter falls onto the polar cap regions. They then calculated a predicted oscillation frequency of the PBOs that would be present in Centaurus X-3 and other behavior of the photon bubbles, including a type of turbulence. They used the RXTE satellite to obtain three days of data for Centaurus X-3 to search for the presence of photon bubbles.
The results are of special importance because they are the first such example of rapid millisecond variations of energy in a highly magnetized rotating X-ray pulsar. Similar high frequency oscillations were discovered in 1996 in an X-ray binary neutron star -- Sco X-1, a so-called low mass X-ray binary -- characterized by a low magnetic field and no clear evidence of polar caps. This new evidence that oscillations in Centaurus X-3 are caused by photon bubbles is much stronger, Klein said.
The team's work was supported by NASA and the U.S. Department of Energy.
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