posted 04-29-2009 06:06 PM               

New Gamma-Ray Burst Smashes Cosmic Distance Record

NASA's Swift satellite and an international team of astronomers have found a gamma-ray burst from a star that died when the universe was only 630 million years old, or less than five percent of its present age. The event, dubbed GRB 090423, is the most distant cosmic explosion ever seen.

"Swift was designed to catch these very distant bursts," said Swift lead scientist Neil Gehrels at NASA's Goddard Space Flight Center in Greenbelt, Md. "The incredible distance to this burst exceeded our greatest expectations -- it was a true blast from the past."

At 3:55 a.m. EDT on April 23, Swift detected a ten-second-long gamma-ray burst of modest brightness. It quickly pivoted to bring its ultraviolet/optical and X-ray telescopes to observe the burst location. Swift saw a fading X-ray afterglow but none in visible light.

"The burst most likely arose from the explosion of a massive star," said Derek Fox at Pennsylvania State University. "We're seeing the demise of a star -- and probably the birth of a black hole -- in one of the universe's earliest stellar generations."

Gamma-ray bursts are the universe's most luminous explosions. Most occur when massive stars run out of nuclear fuel. As their cores collapse into a black hole or neutron star, gas jets -- driven by processes not fully understood -- punch through the star and blast into space. There, they strike gas previously shed by the star and heat it, which generates short-lived afterglows in many wavelengths.

"The lack of visible light alone suggested this could be a very distant object," explained team member Edo Berger of Harvard University.

Beyond a certain distance, the expansion of the universe shifts all optical emission into longer infrared wavelengths. While a star's ultraviolet light could be similarly shifted into the visible region, ultraviolet-absorbing hydrogen gas grows thicker at earlier times. "If you look far enough away, you can't see visible light from any object," he noted.

Within three hours of the burst, Nial Tanvir at the University of Leicester, U.K., and his colleagues reported detection of an infrared source at the Swift position using the United Kingdom Infrared Telescope on Mauna Kea, Hawaii. "Burst afterglows provide us with the most information about the exploded star and its environs," Tanvir said. "But because afterglows fade out so fast, we must target them quickly."

At the same time, Fox led an effort to obtain infrared images of the afterglow using the Gemini North Telescope on Mauna Kea. The source appeared in longer-wavelength images but was absent in an image taken at the shortest wavelength of 1 micron. This "drop out" corresponded to a distance of about 13 billion light-years.

As Fox spread the word about the record distance, telescopes around the world slewed toward GRB 090423 to observe the afterglow before it faded away.

At the Galileo National Telescope on La Palma in the Canary Islands, a team including Guido Chincarini at the University of Milan-Bicocca, Italy, determined that the afterglow's so-called redshift was at least 7.6. Tanvir's team, gathering nearly simultaneous observations using one of the European Southern Observatory's Very Large Telescopes on Cerro Paranal, Chile, found a redshift of 8.2, later confirmed by the Italian group. This means the burst exploded 13.035 billion light-years away.

"It's an incredible find," Chincarini said. "What makes it even better is that a telescope named for Galileo made this measurement during the year in which we celebrate the 400th anniversary of Galileo's first astronomical use of the telescope."

The previous record holder was a burst seen in September 2008. It showed a redshift of 6.7, which places it 190 million light-years closer than GRB 090423.

NASA's Goddard Space Flight Center manages Swift. It was built and is being operated in collaboration with Pennsylvania State University, the Los Alamos National Laboratory in New Mexico, and General Dynamics of Gilbert, Ariz., in the United States. International collaborators include the University of Leicester and Mullard Space Sciences Laboratory in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy, and additional partners in Germany and Japan.

posted 01-10-2018 03:39 PM               

NASA's Newly Renamed Swift Mission Spies a Comet Slowdown

Observations by NASA's Swift spacecraft, now renamed the Neil Gehrels Swift Observatory after the mission’s late principal investigator, have captured an unprecedented change in the rotation of a comet. Images taken in May 2017 reveal that comet 41P/Tuttle-Giacobini-Kresák — 41P for short — was spinning three times slower than it was in March, when it was observed by the Discovery Channel Telescope at Lowell Observatory in Arizona.

The abrupt slowdown is the most dramatic change in a comet's rotation ever seen.

"The previous record for a comet spindown went to 103P/Hartley 2, which slowed its rotation from 17 to 19 hours over 90 days," said Dennis Bodewits, an associate research scientist at the University of Maryland (UMD) in College Park who presented the findings Wednesday, Jan. 10, at the American Astronomical Society (AAS) meeting in Washington. "By contrast, 41P spun down by more than 10 times as much in just 60 days, so both the extent and the rate of this change is something we've never seen before."

The comet orbits the Sun every 5.4 years, traveling only about as far out as the planet Jupiter, whose gravitational influence is thought to have captured it into its present path. Estimated to be less than 0.9 mile (1.4 kilometers) across, 41P is among the smallest of the family of comets whose orbits are controlled by Jupiter. This small size helps explain how jets on the surface of 41P were able to produce such a dramatic spindown.

As a comet nears the Sun, increased heating causes its surface ice to change directly to a gas, producing jets that launch dust particles and icy grains into space. This material forms an extended atmosphere, called a coma. Water in the coma quickly breaks up into hydrogen atoms and hydroxyl molecules when exposed to ultraviolet sunlight. Because Swift's Ultraviolet/Optical Telescope (UVOT) is sensitive to UV light emitted by hydroxyl, it is ideally suited for measuring how comet activity levels evolve throughout the orbit.

Ground-based observations established the comet's initial rotational period at about 20 hours in early March 2017 and detected its slowdown later the same month. The comet passed 13.2 million miles (21.2 million km) from Earth on April 1, and eight days later made its closest approach to the Sun. Swift's UVOT imaged the comet from May 7 to 9, revealing light variations associated with material recently ejected into the coma. These slow changes indicated 41P's rotation period had more than doubled, to between 46 and 60 hours.

UVOT-based estimates of 41P's water production, coupled with the body's small size, suggest that more than half of its surface area contains sunlight-activated jets. That's a far greater fraction of active real estate than on most comets, which typically support jets over only about 3 percent of their surfaces.

"We suspect that the jets from the active areas are oriented in a favorable way to produce the torques that slowed 41P's spin," said Tony Farnham, a principal research scientist at UMD. "If the torques continued acting after the May observations, 41P's rotation period could have slowed to 100 hours or more by now."

Such a slow spin could make the comet's rotation unstable, allowing it to begin tumbling with no fixed rotational axis. This would produce a dramatic change in the comet’s seasonal heating. Bodewits and his colleagues note that extrapolating backward suggests the comet was spinning much faster in the past, possibly fast enough to induce landslides or partial fragmentation and exposing fresh ice. Strong outbursts of activity in 1973 and 2001 may be related to 41P's rotational changes.

A less extreme relationship between a comet's shape, activity and spin was previously seen by the European Space Agency's Rosetta mission, which entered orbit around comet 67P/Churyumov-Gerasimenko in 2014. The comet's spin sped up by two minutes as it approached the Sun, and then slowed by 20 minutes as it moved farther away. As with 41P, scientists think these changes were produced by the interplay between the comet's shape and the location and activity of its jets.

A paper detailing these findings will be published in the journal Nature on Jan. 11.

NASA's Swift spacecraft has conducted a broad array of science investigations for 13 years — monitoring comets, studying stars hosting exoplanets, and catching outbursts from supernovas, neutron stars and black holes — and it continues to be fully operational. NASA announced at the AAS meeting that the mission has now been renamed in honor of Neil Gehrels, who helped develop Swift and served as its principal investigator until his death on Feb. 6, 2017.

Swift's rapid scheduling capability, plus a trio of telescopes covering optical to gamma-ray wavelengths, continues to deliver important contributions in the study of gamma-ray bursts — the most powerful explosions in the universe — while maintaining a critical role in monitoring how astronomical objects as diverse as comets, stars and galaxies change over time.

"The Neil Gehrels Swift Observatory is a name that reflects Swift's current status as the go-to facility for rapid-response, multiwavelength follow-up of time-variable sources," said Paul Hertz, director of NASA's Astrophysics Division in Headquarters, Washington. "With Swift, Neil helped usher in the era of time-domain astronomy. He would have been very excited about today’s discovery."

"Swift is still going strong, and we continue to receive four urgent 'target-of-opportunity' observing requests from the broader astronomical community each day," said S. Bradley Cenko, who was recently appointed as the mission’s principal investigator. "Neil’s leadership and vision continue to guide the project, and we can think of no better way to honor this legacy than with the new name."

Above: NASA's Swift spacecraft, now renamed the Neil Gehrels Swift Observatory after the mission's late principal investigator, has become the go-to facility for rapid-response, multiwavelength follow-up of time-variable sources. This illustration highlights the diversity of Swift's work, which ranges from comets in our solar system to observations of variable sources in our galaxy and beyond.

Goddard manages the Swift mission in collaboration with Penn State in University Park, the Los Alamos National Laboratory in New Mexico and Orbital Sciences Corp. in Dulles, Virginia. Other partners include the University of Leicester and Mullard Space Science Laboratory in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy.

posted 03-20-2024 07:09 PM               

NASA's Swift Temporarily Suspends Science Operations

On March 15, NASA's Neil Gehrels Swift Observatory entered into safe mode, temporarily suspending science operations due to degrading performance from one of its three gyroscopes (gyros), which are used to point the observatory for making observations. The rest of the spacecraft remains in good health.

Swift is designed to successfully operate without one of its gyros if necessary; however, a software update is required. The team is working on the flight software update that would permit the spacecraft to continue science operations using its two remaining gyros. The team is working to return Swift to science observations as soon as possible.

Launched in 2004, Swift has been observing the high-energy universe for nearly 20 years. Stay tuned to nasa.gov/swift for more updates.