Astronomy Picture of the Day Search Results for ""Gamma ray burst""

APOD: 2022 October 17 - X-Ray Rings Around a Gamma Ray Burst
Explanation: Why would x-ray rings appear around a gamma-ray burst? The surprising answer has little to do with the explosion itself but rather with light reflected off areas of dust-laden gas in our own Milky Way Galaxy. GRB 221009A was a tremendous explosion -- a very bright gamma-ray burst (GRB) that occurred far across the universe with radiation just arriving in our Solar System last week. Since GRBs can also emit copious amounts of x-rays, a bright flash of x-rays arrived nearly simultaneously with the gamma-radiation. In this case, the X-rays also bounced off regions high in dust right here in our Milky Way Galaxy, creating the unusual reflections. The greater the angle between reflecting Milky Way dust and the GRB, the greater the radius of the X-ray rings, and, typically, the longer it takes for these light-echoes to arrive.

APOD: 2022 October 15 - GRB 221009A
Explanation: Gamma-ray burst GRB 221009A likely signals the birth of a new black hole, formed at the core of a collapsing star long ago in the distant universe. The extremely powerful blast is depicted in this animated gif constructed using data from the Fermi Gamma Ray Space Telescope. Fermi captured the data at gamma-ray energies, detecting photons with over 100 million electron volts. In comparison visible light photons have energies of about 2 electron volts. A steady, high energy gamma-ray glow from the plane of our Milky Way galaxy runs diagonally through the 20 degree wide frame at the left, while the transient gamma-ray flash from GRB 221009A appears at center and then fades. One of the brightest gamma-ray bursts ever detected GRB 221009A is also close as far as gamma-ray bursts go, but still lies about 2 billion light-years away. In low Earth orbit Fermi’s Large Area Telescope recorded gamma-ray photons from the burst for more than 10 hours as high-energy radiation from GRB 221009A swept over planet Earth last Sunday, October 9.

APOD: 2017 October 23 - NGC 4993: The Galactic Home of an Historic Explosion
Explanation: That reddish dot -- it wasn't there before. It's the dot to the upper left of galaxy NGC 4993's center, do you see it? When scanning the large field of possible locations of an optical counterpart to the unprecedented gravitational wave event GW170817 in August, the appearance of this fading dot quickly became of historic importance. It pinpointed GW170817's exact location, thereby enabling humanity's major telescopes to examine the first ever electromagnetic wave counterpart to a gravitational wave event, an event giving strong evidence of being a short gamma-ray burst kilonova, the element-forming explosion that occurs after two neutron stars merge. The featured image of lenticular galaxy NGC 4993 by Hubble shows the fading dot several days after it was discovered. Analyses, continuing, include the physics of the explosion, what heavy elements formed, the similarity of the speeds of gravitational radiation and light, and calibrating a new method for determining the distance scale of our universe.

APOD: 2017 October 16 - GW170817: A Spectacular Multiradiation Merger Event Detected
Explanation: Both gravitational and electromagnetic radiations have been detected in rapid succession for an explosive merging event for the first time. Data from the outburst fit well with a spectacular binary neutron-star death-spiral. The explosive episode was seen on August 17 in nearby NGC 4993, an elliptical galaxy only 130 million light years distant. Gravitational waves were seen first by the ground based LIGO and Virgo observatories, while seconds later the Earth-orbiting Fermi and INTEGRAL observatories detected gamma-rays, and hours after that Hubble and other observatories detected light throughout the electromagnetic spectrum. Pictured is an animated illustrative movie of the event's likely progenitors. The video depicts hot neutron stars as they spiral in toward each other and emit gravitational radiation. As they merge, a powerful jet extends that drives the short-duration gamma-ray burst, followed by clouds of ejecta and, over time, an optical supernova-type episode called a kilonova. This first coincident detection confirms that LIGO events can be associated with short-duration gamma-ray bursts. Such powerful neutron star mergers are thought to have seeded the universe with many heavy nuclei including the iodine needed for life and the uranium and plutonium needed for nuclear fission power. You may already own a souvenir of one of these explosions -- they are also thought to be the original creators of gold.

APOD: 2014 June 3 - WR 104: A Pinwheel Star System
Explanation: Might this giant pinwheel one-day destroy us? Probably not, but investigation of the unusual star system Wolf-Rayet 104 has turned up an unexpected threat. The unusual pinwheel pattern has been found to be created by energetic winds of gas and dust that are expelled and intertwine as two massive stars orbit each other. One system component is a Wolf-Rayet star, a tumultuous orb in the last stage of evolution before it explodes in a supernova -- an event possible anytime in the next million years. Research into the spiral pattern of the emitted dust, however, indicates the we are looking nearly straight down the spin axis of the system -- possibly the same axis along which a powerful jet would emerge were the supernova accompanied by a gamma-ray burst. Now the WR 104 supernova itself will likely be an impressive but harmless spectacle. Conversely, were Earth really near the center of the powerful GRB beam, even the explosion's 8,000 light year distance might not be far enough to protect us. Currently, neither WR 104 nor GRB beams are understood well enough to know the real level of danger.

APOD: 2013 May 8 - Earth's Major Telescopes Investigate GRB 130427A
Explanation: A tremendous explosion has occurred in the nearby universe and major telescopes across Earth and space are investigating. Dubbed GRB 130427A, the gamma-ray burst was first detected by the Earth-orbiting Fermi and Swift satellites observing at high energies and quickly reported down to Earth. Within three minutes, the half-meter ISON telescope in New Mexico found the blast in visible light, noted its extreme brightness, and relayed more exact coordinates. Within the next few minutes, the bright optical counterpart was being tracked by several quickly re-pointable telescopes including the 2.0-meter P60 telescope in California, the 1.3-meter PAIRITEL telescope in Arizona, and the 2.0-meter Faulkes Telescope North in Hawaii. Within two hours, the 8.2-meter Gemini North telescope in Hawaii noted a redshift of 0.34, placing the explosion about 5 billion light years away -- considered nearby in cosmological terms. Previously recorded images from the RAPTOR full-sky monitors were scanned and a very bright optical counterpart -- magnitude 7.4 -- was found 50 seconds before the Swift trigger. The brightest burst in recent years, a signal from GRB 130427A has also been found in low energy radio waves by the Very Large Array (VLA) and at the highest energies ever recorded by the Fermi satellite. Neutrino, gravitational wave, and telescopes designed to detect only extremely high energy photons are checking their data for a GRB 130427A signal. Pictured in the above animation, the entire gamma-ray sky is shown becoming momentarily dominated by the intense glow of GRB 130427A. Continued tracking the optical counterpart will surely be ongoing as there is a possibility that the glow of a classic supernova will soon emerge.

APOD: 2011 November 24 - Caught in the Afterglow
Explanation: In this artist's illustration, two distant galaxies formed about 2 billion years after the big bang are caught in the afterglow of GRB090323, a gamma-ray burst seen across the Universe. Shining through its own host galaxy and another nearby galaxy, the alignment of gamma-ray burst and galaxies was inferred from the afterglow spectrum following the burst's initial detection by the Fermi Gamma Ray Space Telescope in March of 2009. As seen by one of the European Southern Observatory's very large telescope units, the spectrum of the burst's fading afterglow also offered a surprising result - the distant galaxies are richer in heavy elements than the Sun, with the highest abundances ever seen in the early Universe. Heavy elements that enrich mature galaxies in the local Universe were made in past generations of stars. So these young galaxies have experienced a prodigious rate of star formation and chemical evolution compared to our own Milky Way. In the illustration, the light from the burst site at the left passes successively through the galaxies to the right. Spectra illustrating dark absorption lines of the galaxies' elements imprinted on the afterglow light are shown as insets. Of course, astronomers on planet Earth would be about 12 billion light-years off the right edge of the frame.

APOD: 2011 September 11 - On the Origin of Gold
Explanation: Where did the gold in your jewelry originate? No one is completely sure. The relative average abundance in our Solar System appears higher than can be made in the early universe, in stars, and even in typical supernova explosions. Some astronomers have recently suggested that neutron-rich heavy elements such as gold might be most easily made in rare neutron-rich explosions such as the collision of neutron stars. Pictured above is an artist's illustration depicting two neutron stars spiraling in toward each other, just before they collide. Since neutron star collisions are also suggested as the origin of short duration gamma-ray bursts, it is possible that you already own a souvenir from one of the most powerful explosions in the universe.

APOD: 2011 April 19 - The GRB 110328A Symphony
Explanation: A symphony of planet-wide observations began abruptly on March 28 when the Earth-orbiting Swift satellite detected a burst of high-frequency gamma-rays from GRB 110328A. When the same source flared again after a 45 minute pause it was clear this event was not a typical gamma-ray burst. Twelve hours after the initial fanfare astronomers using the 2.5-meter Nordic Optical Telescope chimed in with a mid-range observation of the optical counterpart. Early the next day the explosion was picked up in baritone low-frequencies of radio waves by the EVLA radio dishes in the USA. Later many optical telescopes, including the 8-meter Gemini North telescope in Hawaii, began playing along by tracking the optical counterpart. The unusual source was spotted at a higher register in X-rays by the Chandra X-ray Observatory and was intermittently followed in the even more soprano-like gamma-ray range for a week. Joining the chorus, Hubble Space Telescope recorded this image in optical and infrared light, confirming that the flash was located along the path of a galaxy at redshift 0.351. If associated with the galaxy, this explosion occurred when the universe was about two thirds of its present age. There is much speculation that the unusual gamma-ray burst was a star being ripped apart by a supermassive black hole in the center of a galaxy and the puzzling features of the distant detonation are still being explored.

APOD: 2009 April 29 - GRB 090423: The Farthest Explosion Yet Measured
Explanation: An explosion so powerful it was seen clear across the visible universe was recorded in gamma-radiation last week by NASA's orbiting Swift Observatory. Farther than any known galaxy, quasar, or optical supernova, the gamma-ray burst recorded last week was clocked at redshift 8.2, making it the farthest explosion of any type yet detected. Occurring only 630 million years after the Big Bang, GRB 090423 detonated so early that astronomers had no direct evidence that anything explodable even existed back then. The faint infrared afterglow of GRB 090423 was recovered by large ground telescopes within minutes of being discovered. The afterglow is circled in the above picture taken by the large Gemini North Telescope in Hawaii, USA. An exciting possibility is that this gamma-ray burst occurred in one of the very first generation of stars and announced the birth of an early black hole. Surely, GRB 090423 provides unique data from a relatively unexplored epoch in our universe and a distant beacon from which the intervening universe can be studied.

APOD: 2008 August 28 - Fermi's First Light
Explanation: Launched on June 11 to explore the universe at extreme energies, the Gamma-ray Large Area Space Telescope has been officially renamed the Fermi Gamma-ray Space Telescope, in honor of Nobel Laureate Enrico Fermi (1901-1954), pioneer in high-energy physics. After testing, Fermi's two instruments, the Gamma-ray Burst Monitor (GBM) and the Large Area Telescope (LAT), are now regularly returning data. Fermi's first map of the gamma-ray sky from the LAT is shown in this false-color image, an all-sky view that looks toward the center of our Milky Way Galaxy with the galactic plane projected across the middle. What shines in the gamma-ray sky? Along the galactic plane, energetic cosmic rays collide with gas and dust to produce the diffuse gamma-ray glow. Strong emission from spinning neutron stars or pulsars, and distant active galaxies known as blazars, can be identified by placing your cursor over the map. A prelude to future discoveries, the remarkable result combines only 4 days of observations, equivalent to a year of observations with the Compton Gamma-ray Observatory mission of the 1990s. In addition to the ability to monitor gamma-ray bursts, the greatly improved sensitivity will allow Fermi to look deeper into the high-energy Universe.

APOD: 2008 March 28 - Across the Universe
Explanation: How far can you see? Even the faintest stars visible to the eye are merely hundreds or thousands of light-years distant, all well within our own Milky Way Galaxy. Of course, if you know where to look you can also spot the Andromeda Galaxy as a pale, fuzzy cloud, around 2.5 million light-years away. But staring toward the northern constellation Bootes on March 19th, even without binoculars or telescope you still could have witnessed a faint, brief, flash of light from a gamma-ray burst. The source of that burst has been discovered to lie over halfway across the Universe at a distance of about 7.5 billion light-years. Now holding the distinction of the most distant object that could be seen by the unaided eye and the intrinsically brightest object ever detected, the cosmic explosion is estimated to have been over 2.5 million times more luminous than the brightest known supernova. The monster burst was identified and located by the orbiting Swift satellite, enabling rapid distance measurements and follow-up observations by large ground-based telescopes. The fading afterglow of the gamma-ray burster, cataloged as GRB080319B, is shown in these two panels in X-rays (left) and ultraviolet light (right).

APOD: 2008 January 18 - Supernova Factory NGC 2770
Explanation: The stellar explosions known as supernovae are among the most powerful events in the universe. Triggered by the collapsing core of a massive star or the nuclear demise of a white dwarf, supernovae occur in average spiral galaxies only about once every century. But the remarkable spiral galaxy NGC 2770 has lately produced more than its fair share. Two still bright supernovae and the location of a third, originally spotted in 1999 but now faded from view, are indicated in this image of the edge-on spiral. All three supernovae are now thought to be of the core-collapse variety, but the most recent of the trio, SN2008D, was first detected by the Swift satellite at more extreme energies as an X-ray flash (XRF) or possibly a low-energy version of a gamma-ray burst on January 9th. Located a mere 90 million light-years away in the northern constellation Lynx, NGC 2770 is now the closest galaxy known to host such a powerful supernova event.

APOD: 2006 February 27 - GRB 060218: A Mysterious Transient
Explanation: What is it? Something is happening in a small portion of the sky toward the constellation of Aries. Telescopes around the globe are tracking an unusual transient there as it changes day by day. No one is sure what it will do next. The entire space mystery began on February 18 when the Earth-orbiting robot Swift satellite noticed an unusual transient began to glow dimly in gamma rays. Dubbed GRB 060218, the object is a type of gamma ray burst (GRB) but the way its brightness changes is very unusual. Since detection, GRB 060218 has been found to emit light across the electromagnetic spectrum, including radio waves and visible light. Pictured above, the Sloan Digital Sky Survey (SDSS) image of the field of GRB 060218 well prior to its Swift trigger is shown on the left, while the same field, taken by the orbiting Swift satellites' ultraviolet telescope after the Swift trigger, is shown on the right. The oddball GRB is visible in the center of the right image. Subsequent observations found a redshift for the transient of z=0.033, showing it to be only about 440 million light years away, relatively nearby compared to typical GRBs. Whether GRB 060218 represents a new type of gamma ray burst, a new type of supernova, or an unusual link between the GRBs and supernovas has become an instant topic of research.

APOD: 2005 October 15 - Dusty Environs of Eta Carinae
Explanation: Eta Car is a massive star, but it's not as bright as it used to be. Now only easily visible in binoculars or a small telescope, Eta Carinae has a history of spectacular flaring and fading behavior. In fact, in April of 1843 Eta Car briefly became second only to Sirius as the brightest star in planet Earth's night sky, even though at a distance of about 7,500 light-years, it is about 800 times farther away. Surrounded by a complex and evolving nebula, Eta Carinae is seen near the center of this false-color infrared image, constructed using data from the Midcourse Space Experiment (MSX). The MSX satellite mapped the galactic plane in 1996. In the picture, wispy, convoluted filaments are clouds of dust glowing at infrared wavelengths. Astronomers hypothesize that Eta Car itself will explode as a supernova in the next million years or so. Massive Eta Car has even been considered a candidate for a hypernova explosion and the potential source of a future gamma-ray burst.

APOD: 2005 April 2 - Cyg X-1: Can Black Holes Form in the Dark?
Explanation: The formation of a black hole from the collapsing core of a massive star is thought to be heralded by a spectacular supernova explosion. Such an extremely energetic collapse is also a leading explanation for the mysterious cosmic gamma-ray bursts. But researchers now suggest that the Milky Way's most famous black hole, Cygnus X-1, was born when a massive star collapsed -- without any supernova explosion at all. Their dynamical evidence is summarized in this color image of a gorgeous region in Cygnus, showing Cyg X-1 and a cluster of massive stars (yellow circles) known as Cygnus OB3. Arrows compare the measured direction and speed of Cyg X-1 and the average direction and speed of the massive stars of Cyg OB3. The similar motions indicate that Cyg X-1's progenitor star was itself a cluster member and that its path was not altered at all when it became a black hole. In contrast, if Cyg X-1 were born in a violent supernova it would have likely received a fierce kick, changing its course. If not a supernova, could the formation of the Cyg X-1 black hole have produced a dark gamma-ray burst in the Milky Way?

APOD: 2005 February 21 - Galactic Magnetar Throws Giant Flare
Explanation: Was the brightest Galactic blast yet recorded a key to connecting two types of celestial explosions? Last December, a dense sheet of gamma rays only a few times wider than the Earth plowed through our Solar System, saturating satellites and noticeably reflecting off the Moon. A magnetar near our Galactic Center, the source of Soft Gamma Repeater (SGR) 1806-20, had unleashed its largest flare on record. The brightness and briefness of the tremendous explosion's initial peak made it look quite similar to another type of tremendous explosion if viewed from further away -- a short duration gamma-ray burst (GRB). Short duration GRBs are thought by many to be fundamentally different than their long duration GRB cousins that are likely related to distant supernovas. Illustrated above is a series of drawings depicting an outgoing explosion during the initial SGR spike. A fast moving wave of radiation is pictured shooting away from a central magnetar. The possible link between SGRs and GRBs should become better understood as more and similar events are detected by the Earth-orbiting Swift satellite.

APOD: 2004 December 24 - Swift RocketCam
Explanation: A forward-facing RocketCam (TM) mounted inside the payload fairing of a Delta II rocket captured these dramatic video frames on November 20th -- as the Swift satellite observatory journeyed to an orbit 600 kilometers above planet Earth. Some frames were interpolated to correct for transmission problems. The sequence shows the fairing separation, the second stage rotating past the Earth's limb, and finally the 1500 kilogram satellite itself separating from the second stage. Observing at optical, ultraviolet, x-ray and gamma-ray energies, Swift is designed to locate the sources of energetic gamma-ray bursts and watch as their afterglows fade in the distant Universe. Still in its checkout phase, the observatory is already detecting the high energy flashes from these awe-inspiring cosmic blasts.

APOD: 2004 November 22 - Swift Launches
Explanation: Where do gamma ray bursts occur? To help find out, NASA launched the Swift satellite on Saturday, as pictured above. What Swift is designed to do better than any previous satellite is to quickly locate these enigmatic explosions in both sky position and distance. Once a gamma ray burst (GRB) is located, Swift itself will rotate to face it head-on and determine its distance directly. Swift locations will be immediately beamed down to Earth-based telescopes operating around the world and across the electromagnetic spectrum that are just waiting for a place on the sky to point. Subsequently collected GRB and afterglow information will help astronomers not only determine the nature of the initial explosions, but also the nature of the expanding shockwaves. Some Swift-located GRBs could turn out to be the most distant transient events ever detected, holding unique clues to the nature and rate of the very first wave of star formation in the early universe.

APOD: 2004 January 30 - X-Ray Rings Expand from a Gamma Ray Burst
Explanation: Why do x-ray rings appear to emanate from a gamma-ray burst? The surprising answer has little to do with the explosion itself but rather with light reflected off sheets of dust-laden gas in our own Milky Way Galaxy. GRB 031203 was a tremendous explosion -- a gamma-ray burst that occurred far across the universe with radiation just arriving in our Solar System last December 3. Since GRBs can also emit copious amounts of x-rays, a bright flash of x-rays likely arrived simultaneously with the gamma-radiation. In this case, the x-rays also bounced off two slabs of cosmic dust nearly 3500 light-years distant and created the unusual reflections. The longer path from the GRB, to the dust slab, to the XMM-Newton telescope caused the x-ray light echoes to arrive well after the GRB.

APOD: 2003 June 13 - Neptune: Still Springtime After All These Years
Explanation: In the 1960s spring came to the southern hemisphere of Neptune, the Solar System's outermost gas giant planet. Of course, since Neptune orbits the Sun once every 165 earth-years, it's still springtime for southern Neptune, where each season lasts over four decades. Astronomers have found that in recent years Neptune has been getting brighter, as illustrated in this Hubble Space Telescope image made in 2002. Compared to Hubble pictures taken as early as 1996, the 2002 image shows a dramatic increase in reflective white cloud bands in Neptune's southern hemisphere. Neptune's equator is tilted 29 degrees from the plane of its orbit, about the same as Earth's 23.5 degree tilt, and Neptune's weather seems to be dramatically responding to the similar relative seasonal increase in sunlight -- even though sunlight is 900 times less intense for the distant gas giant than for planet Earth. Meanwhile, summer is really just around the corner, coming to Neptune's southern hemisphere in 2005.

APOD: 2003 June 12 - Cyg X-1: Can Black Holes Form in the Dark?
Explanation: The formation of a black hole from the collapsing core of a massive star is thought to be heralded by a spectacular supernova explosion. Such an extremely energetic collapse is also a leading explanation for the mysterious cosmic gamma-ray bursts. But researchers now suggest that the Milky Way's most famous black hole, Cygnus X-1, was born when a massive star collapsed -- without any supernova explosion at all. Their dynamical evidence is summarized in this color image of a gorgeous region in Cygnus, showing Cyg X-1 and a cluster of massive stars (yellow circles) known as Cygnus OB3. Arrows compare the measured direction and speed of Cyg X-1 and the average direction and speed of the massive stars of Cyg OB3. The similar motions indicate that Cyg X-1's progenitor star was itself a cluster member and that its path was not altered at all when it became a black hole. In contrast, if Cyg X-1 were born in a violent supernova it would have likely received a fierce kick, changing its course. If not a supernova, could the formation of the Cyg X-1 black hole have produced a dark gamma-ray burst in the Milky Way?

APOD: 2003 April 14 - A Gamma Ray Burst Supernova Connection
Explanation: New evidence has emerged that a mysterious type of explosion known as a gamma ray burst is indeed connected to a supernova of the type visible in the above image. Two weeks ago, the orbiting HETE satellite detected gamma-ray burst GRB030329. The extremely bright burst was found hours later to have an extremely bright afterglow in visible light, and soon set the record for the closest measured distance at redshift 0.17. The afterglow brightness allows unprecedented coverage of its evolution. Just this week, as many astronomers suspected would happen, the afterglow began to appear as a fading Type II Supernova. Type II Supernovas might not appear coincident with gamma-ray bursts, however, when the gamma-ray beam goes in another direction. The above spiral galaxy, NGC 3184, was home to a Type II Supernova in 1999 at the position of the arrow. Astronomers are currently pressing hard to find the host galaxy for GRB030329.

APOD: 2003 March 25 - A Slow Explosion
Explanation: Why would a gamma ray burst fade so slowly? This behavior, recorded last October, is considered a new clue into the cause of gamma-ray bursts, the most powerful explosions known in the universe. The burst, first detected by the orbiting HETE satellite and later tracked by numerous ground-based telescopes, showed an unusually slow and tumultuous decay in visible light. Speculations on the cause of the unusual light curve include a blast wave striking a windy circumburst medium, a blast wave energetically refreshed by a faster outgoing shock, and non-uniformity in a fast moving jet. Pictured above is the massive Wolf-Rayet star WR124, a star itself undergoing a slow explosion by producing a very powerful but tumultuous wind. Popular candidate progenitor sources for GRBs include supernova or hypernova explosions from massive stars, possibly ones with similarities to Wolf-Rayet stars.

APOD: 2002 December 19 - RAPTOR Images GRB 021211
Explanation: On December 11 astronomers found one of the brightest and most distant explosions in the Universe - a gamma-ray burst - hiding in the glare of a relatively nearby star. The earliest image of the burst's visible light was caught by an earthbound RAPTOR (RAPid Telescopes for Optical Response). The two exposures inset above were taken by a RAPTOR unit about 65 seconds (left) and 9 minutes (top right) after high-energy radiation from the burst, dutifully cataloged as GRB 021211, was identified by the orbiting HETE-2 satellite. One of only two optical transients (OTs) ever found at times so close to a burst's gamma-ray emission, the fading visible light source is indicated by arrows, blended with the image of foreground stars toward the constellation Canis Minor. The RAPTOR unit (lower inset) is designed with peripheral low resolution cameras and a central, sensitive high resolution imager, in analogy with a predator's vision. In the future, the RAPTOR project expects its innovative instruments to be able to independently discover and catalog a host of cosmic things that go bump in the night.

APOD: 2002 October 10 - Dusty Environs of Eta Carinae
Explanation: Eta Car is a massive star, but it's not as bright as it used to be. Now only easily visible in binoculars or a small telescope, Eta Carinae has a history of spectacular flaring and fading behavior. In fact, in April of 1843 Eta Car briefly became second only to Sirius as the brightest star in planet Earth's night sky, even though at a distance of about 7,500 light-years, it is about 800 times farther away. Surrounded by a complex and evolving nebula, Eta Carinae is seen near the center of this false-color infrared image, constructed using data from the Midcourse Space Experiment (MSX). The MSX satellite mapped the galactic plane in 1996. In the picture, wispy, convoluted filaments are clouds of dust glowing at infrared wavelengths. Astronomers hypothesize that Eta Car itself will explode as a supernova in the next million years or so. Massive Eta Car has even been considered a candidate for a hypernova explosion and the potential source of a future gamma-ray burst.

APOD: 2002 May 17 - Gamma Ray Burst, Supernova Bump
Explanation: On the 21st of November 2001, satellites detected yet another burst of gamma-rays from the cosmos. While this flash of high-energy photons lasted for less than a minute, eager astronomers have been following the fading optical light from the location of the burst source ever since. Seen above in a series of Hubble Space Telescope images recorded from December 4, 2001 to May 5, 2002 (13 through 161 days after the burst), the fading transient lies to the right of a fuzzy, distant galaxy, likely home to the gamma-ray burster. Two constant point-like objects to the left of the galaxy are foreground stars within our own Milky Way. The transient did not not simply fade away, though. Observations from the Hubble, OGLE, and the large Magellan telescope in Chile indicate that it bumped up or brightened again days after the burst in a convincing display characteristic of a supernova - the death explosion of a massive star. These results add to the mounting evidence that at least some of the mysterious cosmic gamma-ray bursts are produced in the violent event which ends the lives of massive stars.

APOD: 2002 April 5 - Gamma Ray Burst Afterglow: Supernova Connection
Explanation: What causes the mysterious gamma-ray bursts? Indicated in this Hubble Space Telescope exposure of an otherwise unremarkable field in the constellation Crater, is the dwindling optical afterglow of a gamma-ray burst first detected by the Beppo-SAX satellite on 2001 December 11. The burst's host galaxy, billions of light-years distant, is the faint smudge extending above and to the left of the afterglow position. After rapidly catching the fading x-ray light from the burst with the orbiting XMM-Newton observatory, astronomers are now reporting the telltale signatures of elements magnesium, silicon, sulphur, argon, and calcium - material most likely found in an expanding debris cloud produced by the explosion of a massive star. The exciting result is evidence that the gamma-ray burst itself is linked to a very energetic supernova explosion which may have preceded the powerful flash of gamma-rays by up to a few days.

APOD: 2002 February 28 - ESO 184-G82: Supernova - Gamma Ray Burst Connection
Explanation: Modern astronomers keep a long list of things that go bump in the night. Near the top are supernovae - the death explosions of massive stars, and gamma-ray bursts - the most powerful explosions seen across the Universe. Intriguingly, the galaxy in the above Hubble Space Telescope image may have been host to both a supernova and a gamma-ray burst which were one and the same event. ESO 184-G82 is a spiral galaxy with a prominent central bar and loose spiral arms dotted with bright star-forming regions. The inset shows an expanded view of one of the star-forming regions, about 300 light-years across. Indicated is the location of an extraordinarily powerful supernova explosion whose light first reached planet Earth on April 25, 1998. That location and date also correspond to the detection of an unusual gamma-ray burst, which may be representative of a peculiar class of these cosmic high-energy flashes. So far, this combination is unique and makes barred spiral ESO 184-G82, at a distance of only 100 million light-years, the closest known gamma-ray burst host galaxy.

APOD: 2001 November 12 - Is Mystery Object an Orphan Afterglow
Explanation: What is that unusual object? Astronomers can identify most objects that are imaged on the sky, but not all. Pictured above is one that currently defies classification. Attributes of the object include that it has unusual colors, appears to be fading as months go by, and appears to be associated with a distant galaxy. Its discoverers hold hope that they have uncovered the first known orphan afterglow, an explosion that would have been classified as a gamma-ray burst if the gamma-rays were beamed in our direction. Orphan afterglows, if they exist, could have unparalleled brightness, and hence be visible so far away that they yield key information about the early years of our universe. A bit of caution might be merited, however, as the last well-publicized mystery object turned out not to be a new member of the astronomical zoo, but rather an unusual type of quasar. Follow-up observations and analysis over the next year may find more objects like this and/or solve this mystery.

APOD: 2001 April 13 - GRB010222: Gamma Ray Burst, X Ray Afterglow
Explanation: A fading afterglow from one of the most powerful explosions in the universe is centered in this false color image from the spacebased Chandra X-ray Observatory. The cosmic explosion, an enormously bright gamma-ray burst (GRB), originated in a galaxy billions of light-years away and was detected by the BeppoSAX satellite on February 22. GRB010222 was visible for only a few seconds at gamma-ray energies, but its afterglow was followed for days by x-ray, optical, infrared and radio instruments. These Chandra observations of the GRB's x-ray glow hours after the initial explosion suggest an expanding fireball of material moving at near light speed has hit a wall of relatively dense gas. While the true nature of gamma-ray bursters remains unknown, the mounting evidence from GRB afterglows does indicate that the cosmic blasts may be hypernovae -- the death explosions of very massive, short-lived stars embedded in active star forming regions. As the hypernova blasts sweep up dense clouds of material in the crowded star forming regions they may also trigger more star formation.

APOD: 2000 October 19 - The Farthest Explosion Yet Measured
Explanation: It happened so far away that common human distance measures are inadequate to describe it. Furthermore, astronomers do not even claim to know exactly what happened. What is known is that satellites across our Solar System reported on 2000 January 31 a tremendous explosion of gamma rays had occurred towards some previously uninteresting direction. Soon one of the largest optical telescopes on Earth, a VLT in Chile, began to peer in the direction of the gamma ray burst. The VLT not only recorded an optical counterpart, shown above, but also was able to estimate that the cosmologically-induced redshift was an astonishing 4.5 -- placing GRB000131 farther across the universe than any explosion so measured. This vast distance indicates that GRB000131 occurred just as galaxies like our Milky Way were forming, and so qualifies gamma ray bursts as unique probes of this ancient epoch. This result bodes well for the recently launched HETE-2 satellite, which may record and help place more explosions in this distant and mysterious time-period of our universe.

APOD: 2000 October 12 - HETE-2 Rides Pegasus
Explanation: The Stargazer, a modified Lockheed L-1011 aircraft, soared into the skies above Kwajalein Atoll in the pacific on October 9th. A small satellite observatory known as the High Energy Transient Explorer - 2 (HETE-2) was tucked into Stargazer's winged Pegasus rocket, slung beneath the large trimotor jet's fuselage. Dropped from its mother ship, the Pegasus then successfully flew HETE-2 into orbit. HETE-2's mission is to hunt gamma-ray bursts, brief, random flashes of high energy photons from the distant cosmos. Gamma-ray bursts are impressive, believed to be the most powerful explosions in the Universe, but so few have been well located and studied that the nature of the bursters themselves is still shrouded in mystery. HETE-2's x-ray and gamma-ray instruments will be able to rapidly alert ground-based observatories to point toward ongoing, bright gamma-ray bursts. Communications antennae and solar panels neatly folded, HETE-2 is seen here being carefully enclosed in the Pegasus nose fairing.

APOD: 2000 July 2 - Gamma Ray Burst: A Milestone Explosion
Explanation: Gamma-Ray Bursts (GRBs) were discovered by accident. Thirty three years ago today, satellites first recorded a GRB. The data plotted here show that the count rate of the satellite gamma-ray instrument abruptly jumped indicating a sudden flash of gamma-rays. The Vela satellites that detected this and other GRBs were developed to test technology to monitor nuclear test ban treaties. With on board sensors they watched for brief x-ray and gamma-ray flashes, the telltale signatures of nuclear explosions. As intended, the Velas found flashes of gamma-rays - but not from nuclear detonations near Earth. Instead, the flashes were determined to come from deep space! Dubbed "cosmic gamma-ray bursts" they are now known to be the most powerful explosions originating in distant galaxies. What could power a gamma-ray burst?

APOD: 2000 June 28 - BATSE GRB Final Sky Map
Explanation: What causes the most powerful explosions in the universe? The BATSE modules that flew on the Compton Gamma Ray Observatory allowed more insight into enigmatic gamma-ray burst (GRB) explosions than ever before. From 1991-2000, BATSE detected 2704 GRBs, much more than ever previously recorded. The above final sky map of GRB locations (and fluence) shows them to occur at random locations on the sky - strong evidence that GRBs occur across our universe and not in sky bands indicative of our Solar System or our Galaxy. As with any successful mission, answers create more questions, and astronomers continue to puzzle over what object creates a GRB, and what happens in the initial stages of the explosion. BATSE's legacy includes recording 1190 gamma-ray flares from the Sun and the discovery of Terrestrial Gamma Flashes, unusual bursts of gamma rays that emanate from the Earth itself. To protect people from an uncontrolled re-entry, Compton was recently crashed into the Pacific Ocean.

APOD: November 4, 1999 - Gamma Ray Bursting
Explanation: Using graphics and data from NASA's Compton Gamma Ray Observatory, this animation illustrates one of the most exciting mysteries of modern astrophysics, gamma-ray bursts. Incredibly gamma-ray bursts, sudden flashes of radiation with over 100,000 times the energy of visible light photons, occur several times a day. They typically last from fractions of a second to many minutes and appear from random directions, unexpectedly triggering space-based gamma-ray instruments. At left a burst suddenly appears, flickers and fades in a false-color gamma-ray all-sky map, briefly overwhelming all other sources of celestial gamma-rays. The graph at right shows the corresponding response of an orbiting gamma-ray detector as its counting rate suddenly climbs and falls recording the passage of the mysterious burst. Originating far across the Universe, gamma-ray bursts are now known to be the most powerful explosions since the big bang and may yet prove to be useful tools for exploring the distant cosmos. Future space and ground-based observatories will also work to discover the nature of the bursters and the source of their extreme energy.

APOD: October 4, 1999 - The 220 Mirrors of CRTF
Explanation: Even the largest of modern optical telescopes are small when compared with the light gathering power of the Central Receiver Test Facility (CRTF) located in New Mexico, USA. CRTF has 220 mirrors each over 7-meters in diameter all focused on a single tower. CRTF's main use is to investigate methods of collecting and using solar power. CRTF does not create focussed sky images like a normal astronomical telescope, but collects light from a much larger area. Recently, astronomers with the STACEE project have begun using CRTF to collect secondary light emitted when high-energy gamma rays strike the Earth's atmosphere. Photons in this range might allow further exploration of enigmatic gamma ray burst explosions, and might be emitted when cosmic rays from supernovae impact gas clouds in the interstellar medium.

APOD: May 26, 1999 - GRB 990510: Another Unusual Gamma Ray Burst
Explanation: Another huge explosion has lit up the universe, and astronomers are studying it as best they can before the light fades away. Two weeks ago, the BATSE instrument on the orbiting NASA Great Observatory Compton detected unusually bright flashes of gamma-rays from a point deep in the southern sky. This gamma-ray burst was also recorded by the orbiting Beppo-SAX satellite, which downlinked an accurate position followed by the world's largest optical telescopes. The subsequent fading optical transient, pictured above, is so far out in the universe its light is measured to be redshifted by factor of at least 1.6. The type of powerful explosion that caused this gamma-ray burst is not only still unknown, but found to be fading in an unusual way.

APOD: April 20, 1999 - Candidates for a Hypernova
Explanation: What created these huge explosion remnants? Speculation has been building recently that outbursts even more powerful than well-known supernovae might occur. Dubbed hypernovae, these explosions might result from high-mass stars and liberate perhaps ten times more energy than conventional supernovae. A hypernova was originally postulated to explain the great amount of energy seemingly liberated in a gamma-ray burst. A search for visible remnants of hypernovae has now yielded the above two candidates. Nearby spiral galaxy M101, shown on the right, has two large expanding shells that might have originated from a hypernova. Remnant NGC 5471B on the upper left and MF83 below were identified by the unusually high amount of X-ray radiation they emit. MF83 is also one of the largest expanding shells ever found. Research continues into the possible nature and visibility of hypernovae and the gas shells they likely leave behind.

APOD: January 25, 1999 - Galaxy And Gamma Ray Burst
Explanation: Gamma-ray bursts rule the high-energy sky and Saturday another brief, intense flash of gamma-rays from the cosmos triggered space-based detectors. The orbiting Compton Observatory's BATSE instrument quickly relayed the burst's approximate location to fast-slewing, ground-based cameras primed to search for an elusive optical flash. The result -- the GCN coordinated ROTSE-I telephoto array recorded a breakthrough detection only 22 seconds after the burst trigger. Larger telescopes too captured the burster's optical counterpart and at right is an image identifying the candidate from the 60 inch Palomar reflector. At left is a prior sky survey image of the region which astonishingly shows a discernible smudge near the same position, likely the burster's host galaxy. The galaxy and bright burst suggest that this is the closest yet localized gamma-ray burst! Still, believed to be the most powerful explosions in the Universe, the source of the incredible energy of gamma-ray bursts remains a mystery.

Update: Reported redshift measurements now suggest that the burst is instead a very distant one. The galaxy in the survey image may not be the gamma-ray burst host but a foreground galaxy which by chance lies along the line of sight from Earth to the burster.

APOD: December 28, 1998 - NEAR to Asteroid Eros
Explanation: On December 23, the NEAR spacecraft flew by asteroid 433 Eros. NEAR was originally scheduled to brake and orbit Eros, but an unexpected shutdown of its main engine caused this plan to be aborted. NEAR will now be reset and attempt to return to orbit Eros in early 2000. The above image sequence was taken as NEAR approached Eros. The rotation of the asteroid is visible in the successive frames. When NEAR is fully operational, it will likely provoke the world's largest telescopes to point not toward asteroid Eros but to positions indicated by another of NEAR's instruments: its gamma-ray burst (GRB) detector. NEAR's distant GRB detector happens now to be in a unique position to contribute information crucial to the rapid acquisition of accurate GRB positions.

APOD: September 3, 1998 - SGR 1900+14 : Magnetar
Explanation: On August 27th an intense flash of X-rays and gamma-rays swept through our Solar System. Five spacecraft of the Third InterPlanetary gamma-ray burst Network, Ulysses, WIND, RXTE, NEAR, and BeppoSAX, recorded the high energy signal -- a signal so strong that it saturated detectors on WIND and RXTE and triggered the safety mode automatic shut-off of the NEAR gamma-ray instrument! As plotted here, the count rate for the Ulysses detector abruptly spiked to a high level and then slowly tailed off showing smaller peaks roughly every 5 seconds. The signal and location provided by these spacecraft observations leads researchers to identify the source as a dramatic flare-up from one of only four previously known "Soft Gamma Repeaters" . These exotic sources of gamma-rays are believed to be highly magnetized spinning neutron stars called Magnetars. Imaginatively cataloged as SGR 1900+14, this magnetar is estimated to have been born in a supernova explosion about 1,500 years ago and to have a magnetic field 500,000,000,000,000 times stronger than Earth's.

APOD: July 13, 1998 - GRB 980703: A Reassuring Redshift
Explanation: In the old days, just over a year ago, astronomers had little idea of the true distance to gamma-ray bursts. Did these enigmatic explosions occur in our outer Galaxy, or in the outer Universe? Last May, a first telling distance measure was made - GRB 970508 showed an absorption line with a redshift of about 0.8 - indicating that this gamma-ray burst (GRB) was an enormous distance away. Skeptics, however, are not always convinced by an unrepeated measurement. Since then, though, other tantalizing coincidences have occurred: GRB 971214 occurred unusually near a galaxy with the enormous redshift of 3.4, and GRB 980425 occurred unusually near a peculiar low-redshift supernova. Skeptics were intrigued. Now, the potentially definitive implications of the above-pictured optical transient might impress even the cautious. GRB 980703's optical transient shows a well-measured redshift from both an absorption line and an emission line: 0.97. The above negative highlights the uncommon transient source with the label "OT", while letters designate common comparison stars.

APOD: May 28, 1998 - Afterglow
Explanation: This sequence of three false color X-ray pictures from the Italian/Dutch BeppoSAX satellite follows the fading glow from a gamma-ray burster. This burster triggered orbiting gamma-ray observatories on December 14, 1997 and within 6.5 hours the sensitive X-ray cameras onboard BeppoSAX had been turned to record the first image (left) of the afterglow. Each image covers a field about the size of the full moon with the position of the afterglow indicated by the white circle. The first two pictures were taken 6 hours apart, while the final picture was made 2 days after the gamma-ray burst. Initiated by an unknown but immensely powerful explosive event, gamma-ray bursts are thought to be caused by blast waves of particles moving at nearly the speed of light. The expanding cosmic fireball produces seconds-long bursts of gamma-rays and then as it slows and sweeps up surrounding material, generates an afterglow visible for many days at X-ray, optical, and radio energies. Evidence indicates that this burst originated at a distance of 12 billion light-years requiring a fantastic and extreme energy source. What could power a gamma-ray burst?

APOD: May 8, 1998 - A Gamma Ray Burst Supernova
Explanation: Did a gamma-ray burst precede this supernova? This intriguing suggestion came to light yesterday with the discovery of an evolving supernova that is potentially coincident with the position of gamma-ray burst GRB 980425, which occurred just two weeks ago. If true, this would tie together the two most violent phenomena known in the universe. The supernova, indicated by the arrow, appears to be somewhat unusual, for one reason because of its extremely bright radio emission. The host galaxy has a redshift of 0.0085, placing it at the relatively close distance of about 125 million light years away. Today it remains undetermined whether the two events are related - perhaps the evolution of the supernova over the next few weeks will provide some clues.

APOD: May 7, 1998 - A Powerful Gamma Ray Burst
Explanation: Gamma-ray bursts are thought to be the most powerful explosions in the Universe, yet the cause of these high-energy flashes remains a mystery. Blindingly bright for space-based gamma-ray detectors the burst sources are so faint at visible wavelengths that large telescopes and sensitive cameras are required to search for them. The faint optical flash from a relatively intense gamma-ray burst detected on December 14th of last year seems to have originated in the galaxy indicated in this Hubble Space Telescope image - taken months after the burst had faded from view. Astronomers have recently announced that this galaxy's spectrum, recorded using the large Keck telescope atop Hawaii's Mauna Kea, indicates that it lies at a distance of about 12 billion light-years. The energy required to produce the observed flash of gamma-rays from this distance would be staggering! Some estimates suggest that in a few seconds the burster released the equivalent energy of several hundred supernovae (exploding stars). The eruption of such a large amount of energy in such a short time is so extreme that even exotic theoretical models of the bursters are being challenged. Could the bursts be caused by the cataclysmic merger of neutron stars with black holes ... or something as yet unknown?

APOD: December 18, 1997 - Gamma-ray Burster
Explanation: Gamma-ray bursts seem to be the most powerful explosions in the Universe. Yet their sources continue to elude researchers who stand in awe and frustration at the bursts' transient, enigmatic behavior. The blinking gif above illustrates the latest hard-won result in the quest to identify and understand the nature of the bursters. These Apache Point Observatory optical images from Monday and Tuesday this week have helped identify a faint, fading object (red arrow) near the position of a gamma-ray burst. The gamma-ray burst triggered satellite observatories on Sunday, December 14th. Faint stars near the constellation Ursa Major (the Big Dipper) also appear in these "negative" images of the sky. Though thousands of bursts have been detected by satellites sensitive to gamma rays, it is likely that this object represents only the third known optical counterpart to a gamma-ray burst.

APOD: September 17, 1997 - GRB Fireball Persists
Explanation: It's still there. The optical counterpart to the instantly famous gamma-ray burst (GRB) that occurred last February 28th has faded - but not completely. The astronomical community had waited patiently for months for the Earth to proceed far enough along in its orbit so that the Sun's glare no longer ruined inspection of the location of this historic flash. The above picture of GRB 970228, taken September 5th and released just yesterday, shows an extended structure in the center that hasn't changed. However, the arrow indicates a point on the upper right that is now five times dimmer than in preceding months. This fading point fits a model where the GRB originated in a fireball across the universe. The constant brightness of the extended feature indicates that it is not reflected emission from the GRB and might be the host galaxy. Even so, the great GRB mystery is not yet over: for one thing, how come other GRBs don't appear to occur in galactic hosts?

APOD: July 2, 1997 - Gamma-Ray Burst: A Milestone Explosion
Explanation: Gamma-Ray Bursts (GRBs) were discovered by accident. In fact, GRBs always seem to be where scientists least expect them. Thirty years ago today, satellites first recorded a GRB. The burst data plotted in this histogram show that the count rate of the gamma-ray instrument abruptly jumped indicating a sudden flash of gamma-rays. The Vela satellites that detected this and other GRBs were developed to test technology to monitor nuclear test ban treaties. With on board sensors they watched for brief X-ray and gamma-ray flashes, the telltale signs of nuclear explosions from the vicinity of the Earth. As intended, the Velas found flashes of gamma-rays - but not from nuclear detonations near Earth. Instead, the flashes came from deep space! Dubbed "cosmic gamma-ray bursts" their origin was then unknown and is still controversial. However, the gamma-ray surprises were not over. Exploring the high-energy sky nearly 25 years later, the orbiting Compton Observatory's Burst and Transient Source Experiment (BATSE), intentionally designed to detect cosmic gamma-ray bursts, was searching for clues to the GRB mystery. But the second burst BATSE recorded did not come from deep space. It came from near the Earth! Don't worry, these terrestrial GRBs are not nuclear bombs exploding. They are a new phenomenon now thought to be related to a recently discovered type of high altitude lightning. Exploring new horizons continues to yield unexpected results.

APOD: June 16, 1997 - APOD is Two Years Old Today
Explanation: The first Astronomy Picture of the Day (APOD) appeared two years ago today. Pictured above is a scene surrounding the creation of an early APOD, depicting the famous astronomer Tycho Brahe demonstrating a celestial globe to Emperor Rudolph II. The image of a possible optical counterpart to a gamma-ray burst appears on the back wall. In Tycho's day, humanity discovered the nature of the Earth and the geometry of the Solar System. The times we live in are even more fascinating as we explore the nature of our Solar System and the geometry of our whole universe. APOD continues to chronicle these events by finding, presenting ,and annotating the most important astronomical pictures of our time, and cataloging them in an indexed and searchable archive. Link to APOD and discover the cosmos! With over five million pages served, we thank NASA, Michigan Tech, USRA, and most of all our readers, for their continued support.

APOD: May 13, 1997 - Optical Transient Near GRB970508 Shows Distant Redshift
Explanation: The GRB distance scale controversy may have just ended with a flash. Gamma Ray Bursts (GRBs) are powerful explosions occurring in seemingly random positions on the sky. They are so featureless and so poorly resolved, however, that their distances could not be determined. Last Thursday, May 8th, the orbiting Beppo-Sax Satellite detected and precisely located a GRB (GRB970508), quickly relaying its position to astronomers. Within hours, many of the world's most powerful telescopes were re-pointed in the direction of the new GRB. There they found a faint but variable optical source: a potential counterpart to the gamma-ray burst that was getting brighter. Most importantly, continued scrutiny showed that this optical transient had absorption lines with a redshift of about 0.8 - the distance scale of galaxies and quasars. If this source and GRB970508 are related, the GRB itself must be many billions of light-years away and the 30-year controversy on the distance scale to GRBs will draw dramatically to a close. GRBs could move from the realm of astronomical mystery to useful beacons of the early universe. Above is a "negative" image of the GRB970508 field, located near the north celestial pole, taken hours after the initial flash of gamma rays. The faint optical transient source is inside the box. North is up and the image is 2.5 arcminutes across (about a tenth the size of the full moon).

APOD: April 22, 1997 - Historic Optical Flash Fades
Explanation: The largest telescopes in the world have scrambled to point toward this faint, fading object. Why? Because it may well be the first active optical counterpart ever found for a gamma-ray burst, and could hold the clue to the distance scale to this most enigmatic class of astronomical objects. During the past two months, multiwavelength observations, claims, and counterclaims have been exchanged at a torrential pace, as astronomers wait impatiently to see what setting houses the fading transient. Above are two pictures of the optical transient (OT), one taken on February 28^th, the other about a week later on March 8^th. The OT arrows show the source that faded from view during this time. The OT has been examined by large telescopes including the Hubble Space Telescope and Keck, yet researchers are still scrambling to get answers to fundamental questions about its true nature. Does the OT actually move (exhibit proper motion)? Is there a faint galaxy superposed behind the OT or is this extended emission fading too? In the face of intense scrutiny with the world's most capable telescopes, the origin of gamma-ray bursts so far remains mysterious!

APOD: April 7, 1997 - GRB970228: What's There?
Explanation: Could this fuzzy blob be the key to the whole gamma-ray burst (GRB) mystery? Astronomers the world over are now scrambling to determine the true nature of the extended emission seen to the lower right of the bright source in the above image. The bright object in the center is rapidly fading - and thought to be the first true optical counterpart to a GRB. But is it housed in a galaxy? If so, after the central emission has faded, this galaxy should be identifiable. Today, follow up observations of this blob are planned with the Hubble Space Telescope. If the extended emission does come from a galaxy it would bolster indications that the February 28th GRB occurred in that galaxy, across the universe from us. This, in turn, would imply that GRBs are truly the most powerful explosions ever known.

APOD: March 19, 1997 - Gamma-Ray Burster
Explanation: What and where are the Gamma-Ray Bursters? Since their discovery in the early 1970s, nobody has been able to explain the cause of mysterious flashes of gamma rays that come from seemingly random directions on the sky. Worse yet, it is even unclear whether these high energy explosions originate in our own Galaxy or in distant galaxies across the Universe. Until late last month, these bursters were known only by their gamma-ray flashes - no counterpart had been seen at any other wavelength. But on February 28, an Italian/Dutch satellite known as BeppoSAX detected what may well be X-rays from a burster, eight hours after the gamma-ray flash. The discovery image is shown above. Still hours later, using the position provided by this X-ray image, ground-based telescopes recovered an even better located variable optical source which also seems to be related to the burster. Dramatically, this optical transient has faded now. In its place lies a steady source that appears to be a dim, distant galaxy. Did this Gamma-Ray Burst originate in the distant galaxy? If so, it answers one facet of one of modern astronomy's greatest controversies. If not, this would not be the first fortuitous coincidence to mislead astronomers. Future satellite and ground-based observations will tell.

APOD: October 1, 1996 - BATSE's Biggest Gamma Ray Burst (Yet)
Explanation: Something big exploded but astronomers have no idea what. On September 24^th, the Burst and Transient Source Experiment (BATSE) onboard the orbiting Compton Gamma Ray Observatory recorded the most intense gamma ray burst in its five year history. During its mission, BATSE has recorded many gamma ray bursts - about one per day, but none were this powerful. Since their discovery over 25 years ago, the origin and even distance to gamma ray bursts remains hotly debated. Bursts are surely mysterious phenomena: they flash into existence suddenly dominating the entire gamma-ray sky, then typically fade in a few seconds never to be seen again. It is possible that this intense wave of gamma radiation caused other satellites to glitch and may even have been the cause of unusual noise in the Earth's atmosphere. If you know of such an anomaly, please report it.

APOD: September 30, 1996 - Exploring The Universe With IUE (1978-1996)
Explanation: How do planets, stars, and galaxies form? How do they evolve? The International Ultraviolet Explorer (IUE) satellite was launched in 1978 and operated by NASA/ ESA/ PPARC to help provide answers to some of the most fundamental questions about the contents of our universe. IUE served as a geostationary orbiting observatory for the next 18 years acquiring over 100,000 observations with its onboard ultraviolet spectrographs. Its prodigious scientific output is only hinted at by the all-sky plot above which shows the locations of IUE's many pointed observations over its long operational history. The brightness of a point on the sky represents the the number of IUE observations. The plot is in galactic coordinates (the plane of our Galaxy runs horizontally through the middle) and reveals the positions of distant quasars, galaxies, stars, star clusters, nebulae, novae, and supernovae - testifying to IUE's broad range of capabilities. The ecliptic plane is also visible running diagonally through the center, traced out by many observations of solar system objects. After over 18 years, IUE's science operations officially end today as the final commands are transmitted to the aging satellite by controllers at NASA's Goddard Space Flight Center. Astronomers from around the world whose careers have been touched by IUE have expressed their fond farewells to this most productive and longest lived astronomical satellite and their thanks to all who have made IUE's remarkable voyage of exploration possible.