Infrared Space Observatory
Space Infrared Telescope Facility
National Radio Astronomy Observatory Homepage
2MASS Homepage at UMass-Amherst
Farhad Yusef-Zadeh's combination images; see Orion especially
The launch of the Space Infrared Telescope Facility (SIRTF) is scheduled on an uncrewed rocket, a Delta, from Cape Canaveral on August 18, 2003. Checkout of the instruments is to take about four months, and the new name for SIRTF is to be announced with the release of the first observations after that time.
Gamma-ray bursts (GRBs) are the most violent explosions in the Universe, but little is known about them. In recent years, several theories have been put forward to explain these elusive explosions, but the mystery still remains. Now, two recent bursts observed by astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) provide unique data that can not only help test previous models but also help theorists come up with a better picture of what GRBs really are.
GRB 020405 was observed with the MMT 6.5-meter telescope at Mount Hopkins, Arizona, on April 6, 2002, a day after the burst was first detected. Data from this burst indicated a polarization level of almost 10 percent, the highest level ever measured. A day later, a second group measured a polarization level of about two percent. Interestingly, astronomers also observed a two percent polarization only hours before the 10 percent measurement, implying a rapid change in polarization on either side of the peak.
Utilizing the MMT was crucial to gathering enough light for the measurements. The telescope was outfitted with a very sensitive digital camera and a set of filters used to measure polarization. These filters are made of the same material used to make polarized sunglasses.
Said Smithsonian astronomer Brian McLeod, who developed the camera equipment, "The key to making this measurement was having the camera installed on the MMT telescope for many different projects. GRBs are discovered only about once a month, so we can't just wait around with the telescope idle. When the GRB went off, we called the astronomers who happened to be using the telescope that night and asked them to point the telescope at the GRB."
GRBs are believed to come from either the merger of two black holes or neutron stars, or from the explosion of a very massive star. Models show that these explosions appear very energetic because much of their energy is blasted outward in two narrow jets in opposite directions.
In a broad sense, these recent observations support such models, which predict some amount of varying polarization. But the group's observations also demonstrate that many details still need to be worked out. For instance, polarization from a GRB afterglow is expected to be highest when viewed from the edge of the jet. In some cases the polarization can be as high as 20 percent, implying that GRB 020405 was indeed seen from near the edge of its jet. At this extreme viewing angle, calculations predict a gradual decrease in polarization. Instead, the astronomers saw a significant decrease in the span of just one day.
One by one, the group has ruled out errors resulting from observing instruments, dust (either in the host galaxy or in the Milky Way), and microlensing (the temporary brightening in light from a distant object when a dim star comes between it and the Earth). Bersier hopes that comparing his results with those of other groups that observed this burst will help produce a more robust model of GRBs.
Bersier and his colleagues wanted to see if the GRB light curve would show the same short-term variations seen in a burst the previous year. Sure enough, their observations demonstrated that the light from the burst fluctuated on a timescale of 15 - 30 minutes. Over the course of several hours, the brightness of the afterglow repeatedly decreased and increased. Since several nearby stars did not exhibit this highly unusual behavior, the team concluded the variations to be genuine and intrinsic to the burst.
The rapid variations in the light curve, or "wiggles," are believed to be due to density variations in the interstellar matter. Since they appeared within hours of the GRB, astronomers theorize that the matter must be close to the GRB itself. This is a clue that the likely source of the GRB was a hypernova, or exploding star.
According to Bersier, "This second burst has provided us with the best-sampled light curve to date." The more than 100 data points revealed a gradually fading burst with a significant bump in the light curve. This sudden increase in energy while the afterglow was fading has puzzled astronomers. Though several models can help explain the surge of energy at the start of the blast and minor surges in the middle, no single model has been found to explain this extra energy during fading. Bersier says more detailed work is needed before a completely accurate model emerges and suggests accounting for energy distributions in future models.
The rapid brightness fluctuations were not the only thing that caught astronomers' interest. Watching this burst, Bersier and his colleagues were surprised to see the afterglow change its intrinsic color as it faded. While one other burst has shown a similar color change, that burst is believed to have been affected by microlensing. No model can explain the color change seen in GRB 021004 yet.
On a fundamental level, findings from these two bursts will help answer some basic questions about the Universe. Light from these bursts began its journey billions of years ago, when the Universe itself was a teenager. It was the time when clouds of dense gas combined violently to form new stars and new galaxies. Scientists hope that by observing the oldest visible phenomenon in the Universe, they will some day be able to answer how life itself began.
This research was reported within papers in the February 1, 2003, and February 20, 2003, issues of The Astrophysical Journal Letters.
Chandra was able to obtain an unusually long observation (approximately 21 hours) of the afterglow of GRB 020813 (so named because the High-Energy Transient Explorer, HETE, discovered it on August 13, 2002.) A grating spectrometer aboard Chandra revealed an overabundance of elements characteristically dispersed in a supernova explosion. Narrow lines, or bumps, due to silicon and sulfur ions (atoms stripped of most of their electrons) were clearly identified in the X-ray spectrum of GRB 020813.
"Our observation of GRB 020813 supports two of the most important features of the popular supra-nova model for gamma-ray bursts," said Butler. "An extremely massive star likely exploded less than two months prior to the gamma-ray burst, and the radiation from the gamma-ray burst was beamed into a narrow cone."
An analysis of the data showed that the ions were moving away from the site of the gamma-ray burst at a tenth the speed of light, probably as part of a shell of matter ejected in the supernova explosion. The line features were observed to be sharply peaked, indicating that they were coming from a narrow region of the expanding shell. This implies that only a small fraction of the shell was illuminated by the gamma-ray burst, as would be expected if the burst was beamed into a narrow cone. The observed duration of the afterglow suggests a delay of about 60 days between the supernova and the gamma ray burst.
The supra-nova model involves a two-step process: the first step is the collapse of the core of an extremely massive star accompanied by the ejection of the outer layers of the star. The collapsed core forms a rapidly rotating black hole surrounded by a swirling disk of matter. In the second step this black hole-disk system produces a jet of high-energy particles. Shock waves within the jet produce the burst of X-rays and gamma rays that is observed to last only a few minutes. Interaction of the jet with the ejected supernova shell produces the X-ray afterglow, which can last for days or even months. The reason for the delay between the formation of the black hole and the production of the jet is not understood.
Earlier observations with Japan's ASCA, the Italian-Netherlands Beppo-SAX, and the European Space Agency's XMM-Newton satellites, as well as Chandra had given some indication of the presence of elements expected in a shell ejected by a supernova. However, the number of X-rays detected in those observations was small, and the possibility remained that the reported lines were an instrumental effect or statistical fluctuation. Since Chandra was able to observe X-ray lines from GRB 020813 for almost an entire day, the number of X-rays detected was five times larger than for previous observations. This enabled the team to make a definitive identification of the silicon and sulfur lines.
Chandra observed GRB 020813 for about 77,000 seconds, approximately 21 hours after the initial burst. Other members of the research team included Herman Marshall, George Ricker, Roland Vanderspeak, Peter Ford, Geoffrey Crew (MIT), and Donald Lamb (University of Chicago).
The High Energy Transmission Grating Spectrometer was built by MIT. NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program, and TRW, Inc., Redondo Beach, Calif., is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Mass., for the Office of Space Science at NASA Headquarters, Washington.
Images and additional information about this result are available at: http://chandra.harvard.edu
Astronomers using the National Science Foundation's 12 Meter Telescope at Kitt Peak, Ariz., have discovered the complex organic molecule vinyl alcohol in an interstellar cloud of dust and gas near the center of the Milky Way Galaxy. The discovery of this long-sought compound could reveal tantalizing clues to the mysterious origin of complex organic molecules in space.
"The discovery of vinyl alcohol is significant," said Barry Turner, a scientist at the National Radio Astronomy Observatory (NRAO) in Charlottesville, Va., "because it gives us an important tool for understanding the formation of complex organic compounds in interstellar space. It may also help us better understand how life might arise elsewhere in the Cosmos." Vinyl alcohol is an important intermediary in many organic chemistry reactions on Earth, and the last of the three stable members of the C2H4O group of isomers (molecules with the same atoms, but in different arrangements) to be discovered in interstellar space.
Turner and his colleague A. J. Apponi of the University of Arizona's Steward Observatory in Tucson detected the vinyl alcohol in Sagittarius B - a massive molecular cloud located some 26,000 light-years from Earth near the center of our Galaxy. The astronomers were able to detect the specific radio signature of vinyl alcohol during the observational period of May and June of 2001. Their results have been accepted for publication in Astrophysical Journal Letters.
Of the approximately 125 molecules detected in interstellar space, scientists believe that most are formed by gas-phase chemistry, in which smaller molecules (and occasionally atoms) manage to "lock horns" when they collide in space. This process, though efficient at creating simple molecules, cannot explain how vinyl alcohol and other complex chemicals are formed in detectable amounts.
For many years now, scientists have been searching for the right mechanism to explain how the building blocks for vinyl alcohol and other chemicals are able to form the necessary chemical bonds to make larger molecules - those containing as many as six or more atoms. "It has been an ongoing quest to understand exactly how these more complex molecules form and become distributed throughout the interstellar medium," said Turner.
Since the 1970s, scientists have speculated that molecules could form on the microscopic dust grains in interstellar clouds. These dust grains are thought to trap the fast-moving molecules. The surface of these grains would then act as a catalyst, similar to a car's catalytic converter, and enable the chemical reactions that form vinyl alcohol and the other complex molecules. The problem with this theory, however, is that the newly formed molecules would remain trapped on the dust grains at the low temperature characteristic of most of interstellar space, and the energy necessary to "knock them off" would also be strong enough to break the chemical bonds that formed them.
"This last process has not been well understood," explained Turner. "The current theory explains well how molecules like vinyl alcohol could form, but it doesn't address how these new molecules are liberated from the grains where they are born."
To better understand how this might be accomplished, the scientists considered the volatile and highly energetic region of space where these molecules were detected. Turner and others speculate that since this cloud lies near an area of young, energetic star formation, the energy from these stars could evaporate the icy surface layers of the grains. This would liberate the molecules from their chilly nurseries, depositing them into interstellar space where they can be detected by sensitive radio antennas on Earth.
Astronomers are able to detect the faint radio signals that these molecules emit as they jump between quantum energy states in the act of rotating or vibrating.
Turner cautions, however, that even though this discovery has shed new light on how certain highly complex species form in space, the final answer is still not in hand.
"Although vinyl alcohol and its isomeric partners may well have formed on grains," said Turner "another important possibility has been found. The grain evaporative processes near star formation appear to release copious amounts of somewhat simpler molecules such as formaldehyde (H2CO) and methanol (CH3OH), which may be reacting in the gas phase to produce detectable amounts of vinyl alcohol and its isomers." A program to search for other families of isomers is planned, which the astronomers believe could distinguish between these two possibilities.
The astronomers used 2- and 3-mm band radio frequencies to make their observations with the 12 Meter Telescope. This telescope was taken off-line by the NRAO to make way for the Atacama Large Millimeter Array, and is now operated by the Steward Observatory of the University of Arizona. Built in 1967, the telescope has had a long and productive history in detecting molecules in space.
Image of vinyl alcohol molecule and its isomers:
John Gaustad (Swarthmore), Peter McCullough (U. Illinois), Wayne Rosling (Las Cumbres Obs.), and Dave Van Buren (Extra Solar Research Corp.) with what may be the world's smallest telescope, located at the Cerro Tololo Inter-American Observatory. It is available at:
The atlas consists of 2168 images covering 542 fields south of +15 degrees declination. Each image is approximately 13 degrees on a side, has a resolution of approximately 0.8 arcminutes, and reaches a sensitivity of approximately 0.5 rayleigh.