Note to editors:
The animation is available on the Internet in QuickTime and MPEG formats at http://xte.mit.edu/XTE/movie/, as well as VHS and Beta tapes.
ATLANTA -- If we could view the night sky with X-ray sensitive eyes, we would see a turbulent panorama of dying stars, active quasars and the enormous temperatures produced by matter being sucked into massive black holes at mind-boggling speeds.
An animated video released today (Jan. 13) by Massachusetts Institute of Technology researchers at the annual meeting of the American Astronomical Society (AAS) reveals with unprecedented clarity the changing brightnesses of X-ray sources in the sky over a four-year period.
The video was presented at the AAS meeting by Hale Bradt, professor of physics at MIT, co-recipient of the Rossi Prize of the High Energy Astrophysics Division of the AAS for his role in the RXTE project. He was assisted in its preparation by MIT graduate student Michael Muno, post-doctoral associate Donald Smith, and Alan Levine and Ronald Remillard, principal research scientists at MIT's Center for Space Research and Robin Corbet of NASA's Goddard Space Flight Center.
The All-Sky Monitor (ASM) experiment on the Rossi X-ray Timing Explorer (RXTE) satellite, launched in December 1995, provided data on about 150 X-ray sources for the animation. X-ray cameras in space like the Rossi are our window to these extremely energetic and often violent processes in the universe. The seven-minute video "demonstrates dramatically the extreme brightening, fading, disappearances and sudden appearances of these stars from 1996 to 1999," Bradt said. "By contrast, the visible sky we usually see is relatively unchanging and seemingly quite peaceful."
CHAOS IN THE HEAVENS
Neutron stars and black holes in the Milky Way are the final stages in the lives of stars like our sun. They are incredibly small and massive. A neutron star is about 500,000 times more massive than Earth and it is squeezed into a sphere no larger than Manhattan. The force of gravity at its surface is 200 billion times stronger than what we feel on the Earth's surface.
If such a compact star is in a close orbit with a normal gaseous star, gas from the latter star will be pulled toward the neutron star or black hole and will become accelerated to extremely high speeds. This motional energy is converted to heat, which raises the temperature to tens of millions of degrees Fahrenheit. At these huge temperatures, the gas becomes a plasma of mostly protons and electrons that radiates mostly in X-rays. The energy output of X-ray stars is typically 10,000 to 100,000 times that of the sun.
"This process of mass transfer from one star to another can be highly chaotic, leading to large variations in the X-ray brightness," Muno said. Some X-ray stars, usually too faint to be seen by most instruments, will flare up to great brightnesses for a few hours, days, weeks or months and then disappear. These are called transient sources or X-ray novae. Other X-ray sources will be steady for long periods and only occasionally become so faint they cannot be detected.
Changes in energy output, or brightness, may be due to unstable winds of gas from the donor star in a binary star system. Changes also may be caused by instabilities in the disk of whirling gas surrounding the neutron star or black hole. "It is the objective of scientists studying the data from RXTE to better understand these processes, which cannot be duplicated on Earth," Bradt said.
In contrast, the visible sky is relatively benign. Most stars, like our sun, are quite stable as they burn nuclear fuel and produce light. Even the sun's violent solar flares only cause minuscule changes in the output of visible light.
VIEWING THE ANIMATION
The animation depicts X-ray stars on a map of the entire sky. The X-ray stars are pinpoints, just like visible stars, but in the animation they are indicated with colored circles. The size of a circle represents the brightness. The color of the circle is the "X-ray color:" Blue generally represents higher-temperature gases and red represents lower-temperature gases.
The animation runs about 350,000 times faster than actual time. Four days of time pass in about one second, and the entire four years passes by in around seven minutes. The rectangular image below the sky image is a magnification of the center of our Milky Way galaxy, which has a high density of X-ray stars.
Objects other than compact X-ray binaries are shown. Supernova remnants such as the Crab nebula are the leftover nebulae from the collapse of a star. They are quite steady emitters of X-rays. Active galactic nuclei such as quasars are most likely massive blackholes in the centers of distant galaxies. They undergo violent flaring, which can be seen in a few cases, but because of their great distances, they are faint in the ASM data.
The dramatic variability of the galactic "microquasars" is evident in the animation. These sources exhibit intense jets of material seen by radio observers. The material in the jets is probably the same infalling material that causes X-rays.
IN PURSUIT OF X-RAY SOURCES
RXTE's primary objective is to study variations in brightness of X-ray stars. Of its three instruments, the ASM developed at MIT and operated by MIT and NASA's Goddard Space Flight Center (GSFC) continuously surveys the entire sky about once every two hours, snapping an X-ray "picture" every two minutes as it rotates. ASM is the Rossi's eyes and ears. The other two instruments on board focus on specific stars for detailed studies.
Scientists at MIT and GSFC use data from the ASM to calculate the intensities of the 150 sources in near real time. The data are immediately made available to the public, so scientists using RXTE or other observatories such as the Italian BeppoSAX or the recently launched NASA Chandra Observatory will know the state of X-ray sources they may wish to study.
"The ASM data reveal the existence of previously unknown sources, alert astronomers to new types of behaviors and help us understand the energetic processes in the sources, such as the creation of jets of material being ejected from black holes," Bradt said.
The ASM's resolution in time and angle far exceeds that of the only other X-ray experiment that lead to an X-ray movie of the whole sky, which was created in the 1970s by William Priedhorsky and James Terrell of the Los Alamos National Laboratory with data from the Vela B satellite. An animation of the more restricted region at the center of our galaxy was released last June. It was constructed with data from another instrument on the Rossi satellite by Craig Markwardt and Frank Marshall of NASA's GSFC.
The intensities of the ASM data may be found in graphical form at http://heasarc.gsfc.nasa.gov/xte_weather/ for the past week and http://xte.mit.edu/XTE/ASM_lc.html for the past year.
This work is funded by NASA.
