Fastest known X-ray emissions from a collapsed star support existence of exotic photon bubbles in an X-ray pulsar

By Robert Sanders, Public Affairs

Charleston, S.C. -- Astrophysicists from the University of California, Berkeley, and Lawrence Livermore National Laboratory today (Wednesday, April 14) report evidence that strongly supports their exotic theory about the bizarre goings-on at the surface of a neutron star.

Neutron stars already are considered among the most unusual beasts in the stellar zoo. With the mass of a normal star compressed into a ball a dozen miles across, a neutron star has a gravitational field so strong it creates extremely violent conditions on the star's surface.

These stars also spin at rates up to hundreds of times a second, sending out pulses of radiation that have earned them the name "pulsar."

The 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 produce a shower of X-rays that should be detectable by an X-ray telescope above the Earth's surface.

The opportunity to test this hypothesis came with the launch of the Rossi X-ray Timing Explorer satellite in December 1995. The NASA satellite was designed to look for short-lived or periodic X-ray emissions in space, such as X-ray and gamma-ray bursters and pulsars.

In 1997 the team, composed of observational astrophysicist J. Garrett Jernigan of UC Berkeley's Space Sciences Laboratory, theoretical astrophysicist Richard I. Klein of the Lawrence Livermore National Laboratory and an adjunct professor of astronomy at UC Berkeley, and theoretical astrophysicist Jonathan Arons, chair of the astronomy department and professor of astronomy and physics at UC Berkeley, spent a total of three days observing the X-ray pulsar known as Centaurus X-3.

After extensive analysis of the data, they 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 prior to the launch of the Rossi X-ray Timing Explorer satellite."

Their discovery and the supporting theory were presented at a press conference today in South Carolina at a meeting of the High Energy Astrophysics Division of the American Astronomical Society.

"This is a rare example where both the seminal observations and new supporting theory have been carried out by the same people," Klein added.

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, which is 20 kilometers (13 miles) in diameter. The polar region is about 20 square kilometers (eight square miles) - roughly one fourth the area of San Francisco. This is the first time anyone has clearly observed phenomena on the surface of a neutron star.

Neutron stars are surrounded by intense magnetic fields that are 10 billion times the strength of the magnetic field of the sun. The energy generated on the surface of Centaurus X-3 is equivalent to several billion nuclear explosions occurring each second on every square meter of the star's polar caps.

About 10 years ago, Arons and Klein developed a detailed theory of what happens when a spinning and highly magnetized neutron star - an X-ray pulsar - draws matter in from a nearby companion star. The numerical calculations and theory show that rapid, oscillating X-ray emissions originate near the surface of such neutron stars as the infalling matter from a companion star 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 kilometers above the surface of the neutron star only to fall and disintegrate, releasing their energy in a thousandth of a second.

Calculations by the 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.

Jernigan, who has been involved with RXTE since its inception in 1979, approached Arons and Klein three years ago about using the satellite X-ray detectors to look for photon bubbles. They 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 about 30,000 light years from our solar system. Its radius is about 10 kilometers, its mass is equivalent to that of the sun, and it rotates on its axis once every 4.8 seconds.

Using Centaurus X-3's known observed properties - the magnetic field, the mass and 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.

This search resulted in discovery of two distinct quasi-periodic oscillation frequencies at about three thousandths and one thousandth of a second in the emergent radiation from Centaurus X-3, as well as the presence of a broad spectrum that extends to yet higher frequencies than have been found in any known collapsed object.

The quasi-periodic oscillation frequencies and the properties of the broad spectrum they discovered matched predictions of their theory for photon bubbles in Centaurus X-3, providing the strongest support yet for the existence of photon bubbles in X-ray pulsars.

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 identification of photon bubbles in Centaurus X-3 opens the door to future X-ray observations with high resolution timing that will allow the team to probe the physics of the surface of highly luminous accreting X-ray pulsars.

The team's work was supported by NASA and the U.S. Department of Energy.


Garrett Jernigan and Richard Klein will be in South Carolina during the meeting, reachable through the press room at (843) 722-0600 x7100.

After April 19 Jernigan can be reached at (510) 642-1070 or Klein is at (510) 642-3379 on Tuesdays, and the remainder of the week can be reached at LLNL, (925) 422-3548. His email address is

Jonathan Arons can be reached at (510) 642-4730 or

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