Summer 2000 -- Vol. 17, No. 1

ACIS X-Ray Camera Productive Aboard Chandra Space Telescope

With discoveries in the Orion Nebula, Milky Way, and a starburst galaxy, the ACIS X-ray Camera has completed a successful first six months aboard the Chandra Space Telescope.

Gordon Garmire, Evan Pugh Professor of Astronomy and Astrophysics and principal investigator for the camera, outlined many discoveries made possible by the Advanced CCD Imaging Spectrometer (ACIS) during its first months on board the telescope at a meeting of the American Astronomical Society.

Some of the most recent were discoveries in the Orion Nebula, where astronomers have found nearly a thousand faint X-ray-emitting stars in a single observation of young stars.  The discovery is the richest field of X-ray sources ever obtained in the history of X-ray astronomy.  "This Chandra image is a milestone in the field--the first time X-ray astronomy has resolved individual massive stars still embedded in their natal cloud," Garmire said.  "Chandra's sensitivity is 20 times better than achieved with the best previous X-ray telescopes."

The brilliant Orion region is a dense congregation of about 2,000 very young stars formed during the past few million years.  The most massive and brightest of these stars are in the Orion Trapezium, which can be seen with the unaided eye in the winter sky in the "sword" of the Orion constellation.  The discovery of such a wealth of X-ray stars in the closest massive star-forming region to Earth (only 1,500 light years away) is expected to have a profound impact on our understanding of star formation and evolution.

"Chandra's superb angular resolution has resolved this dense cluster of stars with arcsecond accuracy and unsurpassed sensitivity, revealing--among other things--some fantastic objects including three stars with masses so small (roughly 1/20th of the Sun's mass) that they will evolve into brown dwarfs rather than true stars," Garmire said.  "With this new wealth of X-ray sources, we now can begin to do powerful statistical studies about the early formation and X-ray emission of these nearby young stars."

Young stars, such as those found in Orion, are known to be much brighter in X-rays than middle-aged stars such as the Sun--possibly as a result of violent flares in strong magnetic fields near their surfaces--but the physical causes and evolution of the activity are poorly understood.  The Sun itself was probably thousands of times brighter in X-rays during its first few million years.  With hundreds of stars observed simultaneously, possessing a wide range of properties such as mass and rotation rates, the astronomers hope the Orion observation will help unravel the astrophysical principles underlying this phenomenon.

The ACIS team studying the Orion X-ray source is lead by Eric Feigelson, professor of astronomy and astrophysics at Penn State, and includes Garmire and research scientists Patrick Broos, Leisa Townsley, and Yohko Tsuboi at Penn State; Steven Pravdo at the Jet Propulsion Laboratory; and Lynne Hillenbrand at the California Institute of Technology.

Another discovery, a faint X-ray source newly detected by Chandra, may be the long-sought X-ray emission from a known supermassive black hole at the center of our galaxy.  The precise positional coincidence between the new X-ray source and the radio position of a point-like, variable radio source at the center of our galaxy called Sagittarius A* indicates that the two are likely the same object, according to the researchers.  (The asterisk indicates that this object is a point source.)

The race to be the first to detect X-rays from Sagittarius A* has been described as one of the hottest and longest-running in all of X-ray astronomy.  According to the researchers, Chandra's detection of this X-ray source has placed astronomers within a couple of years of the coveted prize of measuring the spectrum of energy produced by Sagittarius A* to determine in detail how the supermassive black hole that powers it works.

Sagittarius A* looks like a faint quasar and is believed to be powered by gaseous matter falling into a supermassive black hole with 2.6 million times the mass of our Sun.  Astronomers believe that most galaxies harbor such massive black holes at their centers and that many of these black holes produce powerful and brilliant quasars and active galactic nuclei, so why the center of our galaxy is so dim is a long-standing puzzle.  The newly detected X-ray source close to Sagittarius A* has taken researchers by surprise because it is much fainter than expected, indicating that there must be something unusual about its environment.

Because there are a million times more stars in a given volume in the galactic center than elsewhere in the galaxy, researchers cannot yet say definitively that Sagittarius A* is the source of the X-rays.  Astronomers need more data to build up a spectrum they can use to rule out various classes of objects and either confirm or deny Sagittarius A* as the origin of the X-ray emission.  "Chandra's incredible sensitivity, plus the superior spatial resolution of Chandra's mirrors, make Chandra the perfect tool for studying this faint X-ray source in its crowded field," Garmire said.

Optical telescopes such as the Hubble Space Telescope cannot see the center of our galaxy, which is enshrouded by thick clouds of dust and gas in the plane of the galaxy. However, hot gas and charged particles moving at nearly the speed of light produce X-rays that penetrate this shroud.

The ACIS team studying the galactic-center source is lead by Dr. Frederick K. Baganoff, a postdoctoral research associate at Massachusetts Institute of Technology and the lead scientist for the "Sagittarius A* and the Galactic Center" project.  The team includes Garmire, Yoshitomo Maeda, and Niel Brandt at Penn State and Mark Morris of the University of California at Los Angeles.

The Chandra Observatory also imaged the core of the nearest starburst galaxy, Messier 82 (M82), a galaxy where stars are being born and are dying much more dramatically and at a rate far greater than in our own galaxy.  The image has revealed intricate structures missed by earlier satellite observatories, including a seething cauldron of exploding stars, neutron stars, black holes, 100-million-degree gas, a powerful galactic wind, and more than twenty powerful X-ray binary systems that contain a normal star in a close orbit around a neutron star or a black hole.  "Several sources are so bright that they are probably black holes, perhaps left over from past starburst episodes," Garmire said.

Short-lived massive stars in a starburst galaxy produce X-ray-emitting supernova explosions, which heat the interstellar gas to millions of degrees and leave behind neutron stars and black holes.  Astronomers need an X-ray-detecting telescope with the highest focusing power (spatial resolution) to clearly discriminate the complex and sometimes confusing superhot structures within a starburst galaxy.  "Chandra is the perfect tool for studying starburst galaxies because it has the critical combination of high-resolution optics and good sensitivity to penetrating X-rays," Garmire said.

The image of M82 was taken with the ACIS spectrometer in an observation that lasted about 10 hours.  At a distance of 11-million light years, M82 is the closest starburst galaxy to our galaxy and provides the best view of this type of galactic structure, which may have played a critical role in the early history of the universe.

The ACIS team has found that the X-ray-emitting gas in this galaxy's core region has a surprisingly hot temperature and that determining the source of high-energy X-rays from M82 may reveal whether starburst galaxies throughout the universe contribute significantly to the X-ray background radiation that pervades intergalactic space.  The image also shows a chimney-like structure at the base of the galactic wind, which may help astronomers understand how metal-rich starburst gas, which now pervades the universe, is dispersed into intergalactic space.

"What we don't see may be as important as what we do see:  Why do we not see any high-luminosity, compact X-ray source from a supermassive black hole or an X-ray binary pair at the very center of the M82 galaxy, although considerable evidence exists that such central black holes are present in many or most galaxies?" Garmire asked.  "We would like to determine what is different about conditions in this galaxy that has so dramatically affected its structure in order to better understand the conditions that affect the formation and evolution of structures throughout the universe."

The ACIS team studying the M82 galaxy is lead by Dr. Richard Griffiths, professor of astrophysics at Carnegie Mellon University, and includes Garmire and Brandt at Penn State and Andrew Ptak at Carnegie Mellon.

Images from Chandra also revealed previously unobserved features in the remnants of three different supernova explosions.  "It is as though we have a set of Russian dolls, with structures embedded within structures," Garmire said.

Two of the remnants, G21.5-0.9 and PSR 0540-69, show dramatic details of the prodigious production of energetic particles by a rapidly rotating, highly magnetized neutron star, as well as the enormous shell structures produced by the explosions.  The image of the third remnant, E0102-72, reveals puzzling spoke-like structures in its interior.

G21.5-0.9, in the constellation of Scutum, is about 16,000 light years (1 light year = 6 trillion miles) from Earth.  Chandra's image shows a bright nebula surrounded by a much larger diffuse cloud.  Inside the inner nebula is a bright central source that is thought to be a rapidly rotating highly magnetized neutron star.  A rotating neutron star acts like a powerful generator, creating intense electric voltages that accelerate electrons to speeds close to the speed of light.   The total output of this generator is greater than a thousand suns.  The fluffy appearance of the central nebula is thought to be due to magnetic field lines which constrain the motions of the high energy electrons.

A previously known pulsar is observed directly in the Chandra image of PSR 0540-69.  This pulsar, located in a satellite galaxy to the Milky Way that is 180,000 light years distant, emits pulses of radio, optical, and X radiation at a rate of 50 per second.  These pulses, which come from a neutron star rotating at this incredible rate, comprise only a few percent of the total energy output of the neutron-star powerhouse.

The third Chandra supernova image is E0102-72.  Located in the Small Magellanic Cloud, another satellite galaxy of the Milky Way, E0102-72 is 190,000 light years from Earth.  This object, like G21.5-0.9 and PSR 0540-69, is believed to have resulted from the explosion of a massive star several thousand years ago.  Stretching across 40 light years of space, the multi-million-degree source resembles a flaming cosmic wheel.

Launched 23 July 1999 aboard the Space Shuttle Columbia and activated 27 July 1999, ACIS serves as the primary X-ray camera of the Chandra Observatory.

"At the beginning of the mission, the instrument was partially damaged by a brief and unexpected dose of radiation.  Laboratory work revealed that the very high proton flux in the van Allen Belts caused this damage," Garmire said.  "We therefore are now moving the ACIS instrument to a safe location every time the spacecraft passes through the heart of the van Allen Belts.  This procedure has proven to be completely successful in that no further degradation of the instrument has been measured.  Lab work also has shown that, by changing the way the instrument is operated, we may be able to recover most of its original performance."

Chandra carries an X-ray telescope to focus the X-rays from objects in the sky. An X-ray telescope cannot work on the ground because X-rays are absorbed by the Earth's atmosphere.  The Chandra X-ray Observatory represents the third of NASA's "Great Observatories," following the Hubble Space Telescope and the Compton Gamma-Ray Observatory, which was decommissioned in June 2000.

Back to Science Journal Summer 2000 Index

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