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Extraordinary Gamma-Ray Burst
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HUBBLE 2011: SCIENCE YEAR IN REVIEW
Investigating an Extraordinary Gamma-Ray Burst
On March 28, 2011, NASA’s Swift satellite detected the first in a string of powerful gamma-ray and X-ray outbursts from an
unknown source located in the constellation Draco. The source was initially cataloged as Gamma-Ray Burst (GRB) 110328A,
and astronomers worldwide were informed of its discovery.
As detailed in the sidebar on page 113, the Swift spacecraft was specifically designed to detect and provide near real-time
notification of GRB events since these celestial phenomena are typically very short-lived. Though Swift sees an average
of two or three per week, GRBs and their attendant “afterglows,” which are visible in X-ray and visible-light wavelengths,
typically disappear from view within hours to a few days.
As multiple telescopes turned to the location of GRB 110328A, it was soon apparent that this was no routine find. Swift ’s
gamma-ray detector triggered three additional times from this same location during the next 48 hours—something never
seen before. Even months after its discovery, this mysterious source, renamed Swift 1644+57, continued to radiate X-rays
copiously. These emissions fluctuated in brightness on short timescales while slowly fading with time. It was clear, by then,
that this was no ordinary GRB.
What could it be? To answer this question, astronomers were eager to know more about its surroundings. Was it inside or
outside of a galaxy? If inside, was the galaxy normal or “disturbed” in some manner? Was the galaxy colliding with another?
Was it distant or nearby?
Within two hours of Swift 1644+57’s discovery, astronomers took the first ground-based observations. These views were
hampered by poor weather, so no optical counterpart could be seen at the reported location of the source. They made
another attempt from a different observatory 12 hours later. This effort detected a very faint object, found at a spot on the
This artist’s rendering illustrates what astronomers believe produced the bright and unusual gamma-ray and X-ray source named Swift
1644+57. In this scenario, a passing star is stripped of material by the strong gravitational pull of a supermassive black hole. As clouds of
material fall inward, high-energy jets form a beam across space from the vicinity of the black hole.
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1
A star passes near a
supermassive black hole.
Black
Hole
2
The star is torn apart
by the gravitational pull
of the black hole.
3
Fragments of the star
feed the accretion disk
around the black hole.
4
A narrow, powerful beam of
radiation blasts toward Earth
along the black hole’s spin axis.
Star
Disk
Jet
This diagram illustrates astronomers’ hypothesized explanation for a series of intense bursts of energy seen from Swift 1644+57. Scientists believe the
bursts originated from the center of a dwarf spheroidal galaxy located 3.8 billion light-years away.
sky consistent with an X-ray afterglow obtained by Swift. Meanwhile, astronomers searched previously archived images of
this area of the sky and found several taken roughly a year earlier. They showed the same faint object, a tiny galaxy looking
surprisingly similar to its current appearance.
Subsequent views by observatories around the world led to the general finding that the light from its host galaxy dominated the
signal from the GRB source so that, at optical wavelengths, Swift 1644+57 looked no different over time. In contrast, at infrared
wavelengths, the observed brightness of Swift 1644+57 varied by a factor of three and was approximately 2.5 times as bright as
its host galaxy in this wavelength band. Nevertheless, ground-based observations could not pinpoint whether the source came
from the galaxy’s nucleus or further out from its center.
Maryland-based astronomer Andrew Fruchter led a team that
was allocated time on Hubble to examine Swift 1644+57. On
April 4, 2011, they captured a detailed view of the mysterious
source and its surrounding environment using both optical and
near-infrared wavelengths. They too found a single, small galaxy
at the position of Swift 1644+57. Additionally, they discovered a
point-like infrared source at the same spot where Swift saw the
Images from Swift’s Ultraviolet/Optical Telescope (in purple and white) and
X-ray Telescope (in yellow and red) were combined in this view of Swift
1644+57 (GRB 110328A).
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HUBBLE 2011: SCIENCE YEAR IN REVIEW
Swift
Swift is the first multi-wavelength observatory dedicated to the study of gamma-ray-burst (GRB) science. Its three
instruments work together to observe GRBs and afterglows in the gamma-ray, X-ray, ultraviolet, and optical wavebands.
Swift monitors the sky for new GRBs with a wide-field detector, localizes their positions onboard, and autonomously
reorients itself to observe the new burst quickly with its other telescopes.
The satellite facilitates community-wide contributions to GRB science in two ways. First, it disseminates the positions
of the GRBs it detects as soon as they are determined. This allows operators of other space- and ground-based
telescopes to schedule immediate follow-up observations. Second, if a GRB is discovered by another telescope, the
coordinates can be uploaded to Swift so that it can perform rapid observations of these bursts itself.
Swift
Like many other astronomical spacecraft (including Hubble ), Swift uses heavy, spinning wheels located near its center
of mass (called momentum wheels) to reorient (slew) the spacecraft from one target to another. The name “Swift ” is
not a mission-related acronym, but rather refers to this rapid slew capability built into the spacecraft and the nimble,
speedy bird of the same name. NASA launched Swift into orbit on November 20, 2004.
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flaring X-rays. With very high accuracy, they determined that this source was located at the very heart of the galaxy. Multiple,
concurrent, ground-based observations of its spectrum indicated that the galaxy is approximately 3.8 billion light-years
away and likely a galaxy type known as a dwarf spheroidal. This faint celestial system is intrinsically no more luminous than
the Milky Way’s small neighbor, the Large Magellanic Cloud.
Fruchter’s team also used NASA’s Chandra X-ray Observatory on April 4 to conduct a four-hour-long exposure of Swift
1644+57. The resultant image, which located the X-ray afterglow 10 times more precisely than the original Swift data, also
indicated that the source lies at the very core of the galaxy. This was significant, as the location of the flaring source is an
important detail in identifying its nature. A second set of Hubble observations taken on April 20 showed that the source had
faded at infrared wavelengths, but remained constant in visible light.
GRBs are generally believed to signal the gravitational collapse of a massive “normal” star, or perhaps in certain cases, the
merger of two highly dense “degenerate” neutron stars. The intense bursts of high-energy gamma rays that result from these
violent events can be as brief as 10 milliseconds and as long as several minutes, but typically only last about 30 seconds.
Likewise, the bright X-ray and optical emissions that follow from these explosions are only visible for a few hours to days
after the burst. These bursts normally appear at the edges or elsewhere in galaxies, not at their centers.
The observed characteristics of Swift 1644+57—its
multiple gamma-ray outbursts, its persistent and
fluctuating X-ray emissions, its invariant opticalwavelength brightness, and its confirmed location at the
core of its host galaxy—all signaled to the team that a
different physical process was at work to produce it than
for a more typical GRB.
Dr. Andrew Levan of the United Kingdom was the
principal author of the team’s 2011 Science article that
described their multi-wavelength observations of Swift
The Chandra X-ray Observatory completed this four-hour exposure
of Swift 1644+57 on April 4, 2011.
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Chandra
NASA’s Chandra X-ray Observatory is an orbiting telescope specially designed to detect X-ray emission from very hot
objects or regions of the universe. Its targets are typically highly energetic or “exotic” in nature. Such objects include
active galactic nuclei, quasars, neutron stars, and supernovas, as well as the hot gaseous regions within clusters of
galaxies and unusual, transient events around black holes.
Because the atmosphere absorbs X-rays, Chandra must orbit far above the Earth. In its highly elliptical, 64.2-hour
orbit, the spacecraft’s closest approach to Earth is 9,942 miles (16,000 kilometers) above the surface. At its farthest
point, Chandra swings out to 83,000 miles (133,000 kilometers)—more than a third of the way to the Moon. This
orbital shape also keeps the spacecraft above the Earth’s charged-particle (Van Allen) belts for approximately 85
percent of the time.
The Chandra mission, like Hubble ’s, issues an annual call for proposals to scientists from around the world, inviting
them to submit detailed requests for telescope time. These requests are then peer reviewed, ranked for their scientific
merit, and awarded telescope time.
Since its launch on July 23, 1999, the Chandra X-ray Observatory has been NASA’s flagship mission for X-ray
astronomy, taking its place with the other two active “Great Observatories,” Hubble and the Spitzer
Space Telescope .
Chandra
Chandra was named in honor of Indian-American physicist Subrahmanyan
Chandrasekhar, who is known for determining the maximum mass for white
dwarfs. “Chandra” also means “moon” or “luminous” in Sanskrit.
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1644+57. He details the group’s belief that the flaring source is associated with a supermassive black hole at the center of
its host galaxy. Though too far away to be observationally confirmed, such black holes exist at the hearts of virtually all large
galaxies that are close to the Milky Way. These black holes produce quasars, highly luminous jets that beam prodigious
amounts of energy into space.
The observed X-ray luminosity of Swift 1644+57, however, is well beyond that seen from typical quasars, while its optical
luminosity is approximately 10,000 times fainter. Additionally, to produce the observed X-ray luminosity in the initial
outburst detected by Swift, the black hole at the center of its host galaxy would have to be much larger than typically found
in a small galaxy. Evidence like this led them to conclude that Swift 1644+57 is a rare astronomical object or occurrence, the
most likely being the tidal disruption of a single star as it passes close to the central galactic black hole in its host galaxy.
Computer models suggest that in such a scenario, a star is ripped apart by tidal forces as it nears the black hole. As the star’s
gas falls inward, both its velocity and temperature dramatically increase. As a result, the environment around the black hole
forms a narrow beam of electromagnetic radiation, along with charged particles circling in a tightly spun magnetic field that
form an extremely high-velocity jet. As pieces of material are torn away from the star and accreted onto the black hole, flares
of radiation from gamma rays to radio waves erupt and energy is routed into the jet. To explain the intense X-ray brightness
and variability seen, the outward-directed beam from Swift 1644+57 must point virtually straight at Earth.
Other astronomical objects have shown somewhat similar bursts of radiation at X-ray, ultraviolet, and optical wavelengths.
These have led researchers to believe they too were witnessing the tidal disruption of stars in these systems on some scale.
With Swift 1644+57, however, astronomers for the first time have seen gamma-ray energies released from such an object.
Swift 1644+57’s X-ray luminosity is also at least 10,000 times brighter and much more variable than other cases where
stellar tidal disruption is thought to be at play. If indeed this is a case of stellar disruption into a black hole, researchers
expect the intensity of the jet will slowly and predictably fade with time as the star’s material is gradually exhausted.
Astronomers estimate that within a particular galaxy, the tidal disruption of a star into its central black hole occurs only
once in approximately 100 million years. The black hole in the middle of our own galaxy does emit rapidly variable X-rays,
but these are a trillion times fainter than the flares seen from Swift 1644+57. This is comforting, as even though our solar
system is 26,000 light-years away from the galaxy’s core, a high-energy beam from an event like Swift 1644+57 could have
dire consequences for life on our planet. The energy from such a beam would act to destroy the protective ozone layer in our
atmosphere, exposing life on Earth to harmful ultraviolet and X-ray radiation from the Sun.
Fortunately for us on Earth, even if a high-energy jet like Swift 1644+57 arises from the center of the Milky Way, the
likelihood of being directly in the beam of the jet and experiencing its high-energy pummeling is less than one percent.
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Further Reading
Baldwin, E. “Star Eaten by Black Hole.” Astronomy Now Online , June 16, 2011. http://www.astronomynow.com/news/n1106/16flare/.
“Black Hole Eats Star, Triggers Gamma-ray Flash.” COSMOS Magazine , June 17, 2011.
http://www.cosmosmagazine.com/news/4428/black-hole-eats-star-produce-gamma-ray-flash.
Bloom, J. S., et al. “A Possible Relativistic Jetted Outburst from a Massive Black Hole Fed by a Tidally Disrupted Star.” Science 333,
no. 6039 (July 8, 2011): 203–206.
Fazekas, A. “Black Hole Caught Eating a Star, Gamma-Ray Flash Hints,” National Geographic Daily News , June 16, 2011.
http://news.nationalgeographic.com/news/2011/06/110615-black-holes-eat-star-galaxy-nasa-swift-gamma-rays-space-science/
Gehrels, N., et al. “The Brightest Explosions in the Universe.” Scientific American 287, no. 6 (December 2002).
Levan, A. J., et al. “An Extremely Luminous Panchromatic Outburst from the Nucleus of a Distant Galaxy.” Science 333, no. 6039
(July 8, 2011): 199–202.
Shao, L., et al. “Swift 1644+57/GRB 110328A: The Physical Origin and the Composition of the Relativistic Outflow.” The Astrophysical
Journal Letters 734 (June 20, 2011): L33.
Dr. Andrew Fruchter is an associate astronomer at the Space Telescope Science Institute in Baltimore, Maryland.
He received his doctorate from Princeton University and took a Teagle Fellowship at the Carnegie Institution of
Washington, followed by a Hubble Fellowship at the University of California at Berkeley before moving full time
to his current position. Dr. Fruchter was one of the principle scientists behind the Hubble Deep Field (HDF)
observation and co-developed the image processing software that was used to create the HDF image. He also
shared the Gruber Prize in cosmology as a member of one of the two teams that discovered the acceleration of the
expansion of the universe. These teams were also recognized with the 2011 Nobel Prize in Physics.
Dr. Andrew Levan was born in Middlesbrough, England, and was educated in physics and astronomy at the
University of Leicester. He first became interested in gamma-ray bursts and high-energy transients during a
summer internship at the Space Telescope Science Institute in 2001. He then went on to make these events
the core of his doctoral thesis, also undertaken at the University of Leicester. Upon graduation, Dr. Levan won
a postdoctoral fellowship from the United Kingdom funding council PPARC, which he held at the University
of Hertfordshire before moving to become an assistant professor at the University of Warwick in 2007 and an
associate professor in 2010.
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