GRB 970228
Encyclopedia
GRB 970228 was the first gamma-ray burst (GRB) for which an afterglow was observed. It was detected on 28 February 1997 at 02:58 UTC. Since 1993, physicists had predicted GRBs to be followed by a lower-energy afterglow (in wavelengths such as radio waves
, x-rays, and even visible light), but until this event, GRBs had only been observed in highly luminous
bursts of high-energy gamma ray
s (the most energetic form of electromagnetic radiation
).
The burst had multiple peaks in its light curve
and lasted approximately 80 seconds. Peculiarities in the light curve of GRB 970228 suggested that a supernova
may have occurred as well. The position of the burst coincided with a galaxy about 8.1 billion light-year
s away (a redshift
of z = 0.695), providing early evidence that GRBs occur well beyond the Milky Way
.
flash of gamma ray
s, the most energetic form of electromagnetic radiation
. GRBs were first detected in 1967 by the Vela satellites, a series of spacecraft designed to detect nuclear explosions.
GRB 970228 was detected on 28 February 1997 at 02:58 UTC by the Gamma-Ray Burst Monitor (GRBM) and one of the Wide Field Cameras (WFCs) on board BeppoSAX
, an Italian–Dutch satellite originally designed to study X-rays. Within a few hours, the BeppoSAX team determined the burst's position with an error box—a small area around the specific position to account for the error
in the position—of 3 arcminutes. The burst was also detected by the Ulysses space probe.
The burst was located at a right ascension
of and a declination
of . It lasted around 80 seconds and had multiple peaks in its light curve. Gamma-ray bursts have very diverse time profiles, and it is not fully understood why some bursts have multiple peaks and some have only one. One possible explanation is that multiple peaks are formed when the source of the gamma-ray burst undergoes precession
.
and James E. Rhoads published an article arguing that, regardless of the type of explosion that causes GRBs, the extreme energetics of GRBs meant that matter from the host body must be ejected at relativistic speed
s during the explosion. They predicted that the interaction between the ejecta and interstellar matter would create a shock front
. Should this shock front occur in a magnetic field, accelerated electrons in it would emit long-lasting synchrotron radiation
in the radio frequencies, a phenomenon that would later be referred to as a radio afterglow. Jonathan Katz later concluded that this lower-energy emission would not be limited to radio waves, but should range in frequency from radio waves to x-ray
s, including visible light.
The Narrow Field Instruments on board BeppoSAX began making observations of the GRB 970228's position within eight hours of its detection. A transient x-ray source was detected which faded with a power-law slope in the days following the burst. This x-ray afterglow was the first GRB afterglow ever detected. Power-law decays have since been recognized as a common feature in GRB afterglows, although most afterglows decay at differing rates during different phases of their lifetimes.
Optical images were taken of GRB 970228's position on 1 and 8 March using the William Herschel Telescope
and the Isaac Newton Telescope
. Comparison of the images revealed an object which had decreased in luminosity in both visible light and infrared light. This was the burst's optical afterglow. The predicted radio afterglow was never observed for this burst. At the time of this burst's discovery, GRBs were believed to emit radiation isotropically. The afterglows from this burst and several others—such as GRB 970508
and GRB 971214
—provided early evidence that GRBs emit radiation in collimated jets, a characteristic which lowers the total energy output of a burst by several orders of magnitude.
Daniel Reichart of the University of Chicago
and Titus Galama of the University of Amsterdam independently analyzed GRB 970228's optical light curve, both concluding that the host object may have undergone a supernova
explosion several weeks before the gamma-ray burst occurred.
Galama analyzed the light curve of the burst and found that its luminosity decayed at different rates at different times. The luminosity decayed more slowly between March 6 and April 7 than it did before and after these dates. Galama concluded that the earlier light curve had been dominated by the burst itself, whereas the later light curve was produced by the underlying Type Ic supernova. Reichart noted that the late afterglow was redder
than the early afterglow, an observation which conflicted with the then-preferred relativistic fireball model for the gamma-ray burst emission mechanism. He also observed that the only GRB with a similar temporal profile was GRB 980326, for which a supernova relation had already been proposed by Joshua Bloom
.
An alternative explanation for the light curves of GRB 970228 and GRB 980326 involved dust echoes
. Although GRB 980236 did not provide enough information to definitively rule out this explanation, Reichart showed that the light curve of GRB 970228 could only have been caused by a supernova. Definitive evidence linking gamma-ray bursts and supernovae was eventually found in the spectrum of GRB 020813
and the afterglow of GRB 030329
. However, supernova-like features only become apparent in the weeks following a burst, leaving the possibility that very early luminosity variations could be explained by dust echoes.
. He discovered a faint nebula
r patch at the burst's position, almost certainly a distant galaxy. Although there was a remote chance that the burst and this galaxy were unrelated, their positional coincidence provided strong evidence that GRBs occur in distant galaxies rather than within the Milky Way
. This conclusion was later supported by observations of GRB 970508
, the first burst to have its redshift
determined.
The position of the burst's afterglow was measurably offset from the centroid
of the host galaxy, effectively ruling out the possibility that the burst originated in an active galactic nucleus
. The redshift of the galaxy was later determined to be z = 0.695, which corresponds to a distance of approximately . At this distance, the burst would have released a total of assuming isotropic emission.
Radio waves
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...
, x-rays, and even visible light), but until this event, GRBs had only been observed in highly luminous
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
bursts of high-energy gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s (the most energetic form of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
).
The burst had multiple peaks in its light curve
Light curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band...
and lasted approximately 80 seconds. Peculiarities in the light curve of GRB 970228 suggested that a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
may have occurred as well. The position of the burst coincided with a galaxy about 8.1 billion light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s away (a redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
of z = 0.695), providing early evidence that GRBs occur well beyond the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
.
Observations
A gamma-ray burst (GRB) is a highly luminousLuminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
flash of gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s, the most energetic form of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
. GRBs were first detected in 1967 by the Vela satellites, a series of spacecraft designed to detect nuclear explosions.
GRB 970228 was detected on 28 February 1997 at 02:58 UTC by the Gamma-Ray Burst Monitor (GRBM) and one of the Wide Field Cameras (WFCs) on board BeppoSAX
BeppoSAX
BeppoSAX was an Italian–Dutch satellite for X-ray astronomy which played a crucial role in resolving the origin of gamma-ray bursts , the most energetic events known in the universe...
, an Italian–Dutch satellite originally designed to study X-rays. Within a few hours, the BeppoSAX team determined the burst's position with an error box—a small area around the specific position to account for the error
Measurement uncertainty
In metrology, measurement uncertainty is a non-negative parameter characterizing the dispersion of the values attributed to a measured quantity. The uncertainty has a probabilistic basis and reflects incomplete knowledge of the quantity. All measurements are subject to uncertainty and a measured...
in the position—of 3 arcminutes. The burst was also detected by the Ulysses space probe.
The burst was located at a right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
of and a declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
of . It lasted around 80 seconds and had multiple peaks in its light curve. Gamma-ray bursts have very diverse time profiles, and it is not fully understood why some bursts have multiple peaks and some have only one. One possible explanation is that multiple peaks are formed when the source of the gamma-ray burst undergoes precession
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
.
Afterglow
In 1993, Bohdan PaczyńskiBohdan Paczynski
Bohdan Paczyński or Bohdan Paczynski was a Polish astronomer, a leading scientist in theory of the evolution of stars, accretion discs and gamma ray bursts....
and James E. Rhoads published an article arguing that, regardless of the type of explosion that causes GRBs, the extreme energetics of GRBs meant that matter from the host body must be ejected at relativistic speed
Relativistic speed
A Relativistic speed is a speed which is a significant proportion of the speed of light. Therefore scientific analysis must take the consequences of special relativity into account...
s during the explosion. They predicted that the interaction between the ejecta and interstellar matter would create a shock front
Shock Front
Shock Front is the first full length album by Converter, released November 22, 1999 . The album is released in two editions, the first featuring a special metal plate packaging , the second featuring a regular cardboard booklet format .-Track listing:# "Conqueror"–8:05# "Shock Front"–7:15#...
. Should this shock front occur in a magnetic field, accelerated electrons in it would emit long-lasting synchrotron radiation
Synchrotron radiation
The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced in synchrotrons using bending magnets, undulators and/or wigglers...
in the radio frequencies, a phenomenon that would later be referred to as a radio afterglow. Jonathan Katz later concluded that this lower-energy emission would not be limited to radio waves, but should range in frequency from radio waves to x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s, including visible light.
The Narrow Field Instruments on board BeppoSAX began making observations of the GRB 970228's position within eight hours of its detection. A transient x-ray source was detected which faded with a power-law slope in the days following the burst. This x-ray afterglow was the first GRB afterglow ever detected. Power-law decays have since been recognized as a common feature in GRB afterglows, although most afterglows decay at differing rates during different phases of their lifetimes.
Optical images were taken of GRB 970228's position on 1 and 8 March using the William Herschel Telescope
William Herschel Telescope
The William Herschel Telescope is a optical/near-infrared reflecting telescope located at the Observatorio del Roque de los Muchachos on the island of La Palma in the Canary Islands, Spain. The telescope, which is named after William Herschel, is part of the Isaac Newton Group of Telescopes...
and the Isaac Newton Telescope
Isaac Newton Telescope
The Isaac Newton Telescope or INT is a 2.54 m optical telescope run by the ING at Roque de los Muchachos Observatory on La Palma in the Canary Islands since 1984....
. Comparison of the images revealed an object which had decreased in luminosity in both visible light and infrared light. This was the burst's optical afterglow. The predicted radio afterglow was never observed for this burst. At the time of this burst's discovery, GRBs were believed to emit radiation isotropically. The afterglows from this burst and several others—such as GRB 970508
GRB 970508
GRB 970508 was a gamma-ray burst detected on May 8, 1997, at 21:42 UTC. A gamma-ray burst is a highly luminous flash associated with an explosion in a distant galaxy and producing gamma rays, the most energetic form of electromagnetic radiation, and often followed by a longer-lived...
and GRB 971214
GRB 971214
GRB 971214 is a gamma-ray burst observed in 1997. It originated 12 billion light years away. For a brief period this was thought by some researchers to have been the most energetic event observed in the universe, but this claim has since been discredited.In 1998, it was hypothesized by George...
—provided early evidence that GRBs emit radiation in collimated jets, a characteristic which lowers the total energy output of a burst by several orders of magnitude.
Supernova relation
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...
and Titus Galama of the University of Amsterdam independently analyzed GRB 970228's optical light curve, both concluding that the host object may have undergone a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
explosion several weeks before the gamma-ray burst occurred.
Galama analyzed the light curve of the burst and found that its luminosity decayed at different rates at different times. The luminosity decayed more slowly between March 6 and April 7 than it did before and after these dates. Galama concluded that the earlier light curve had been dominated by the burst itself, whereas the later light curve was produced by the underlying Type Ic supernova. Reichart noted that the late afterglow was redder
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
than the early afterglow, an observation which conflicted with the then-preferred relativistic fireball model for the gamma-ray burst emission mechanism. He also observed that the only GRB with a similar temporal profile was GRB 980326, for which a supernova relation had already been proposed by Joshua Bloom
Joshua Bloom
Joshua Simon Bloom is an American astrophysicist and associate professor of astronomy at the University of California, Berkeley. He received a Bachelor of Arts in astronomy and astrophysics and physics from the Harvard College in 1996, an M.Phil from Cambridge University in 1997, and a Ph.D...
.
An alternative explanation for the light curves of GRB 970228 and GRB 980326 involved dust echoes
Light echo
thumb|right|250px|Reflected light following path B arrives shortly after the direct flash following path A but before light following path C. B and C have the same apparent distance from the star as seen from [[Earth]]....
. Although GRB 980236 did not provide enough information to definitively rule out this explanation, Reichart showed that the light curve of GRB 970228 could only have been caused by a supernova. Definitive evidence linking gamma-ray bursts and supernovae was eventually found in the spectrum of GRB 020813
GRB 020813
GRB 020813 was a gamma-ray burst that was detected on 13 August 2002 at 02:44 UTC. A gamma-ray burst is a highly luminous flash associated with an explosion in a distant galaxy and producing gamma rays, the most energetic form of electromagnetic radiation, and often followed by a longer-lived...
and the afterglow of GRB 030329
GRB 030329
GRB 030329 was a gamma-ray burst that was detected on 29 March 2003 at 11:37 UTC. A gamma-ray burst is a highly luminous flash associated with an explosion in a distant galaxy and producing gamma rays, the most energetic form of electromagnetic radiation, and often followed by a longer-lived...
. However, supernova-like features only become apparent in the weeks following a burst, leaving the possibility that very early luminosity variations could be explained by dust echoes.
Host galaxy
During the night between 12 and 13 March, Jorge Melnick made observations of the region with the New Technology TelescopeNew Technology Telescope
The New Technology Telescope or NTT is an Alt-Az, 3.58-metre Richey-Chretien telescope part of the European Southern Observatory and began operations in 1989. It is located in Chile at the La Silla Observatory and was an early pioneer on the use of active optics...
. He discovered a faint nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
r patch at the burst's position, almost certainly a distant galaxy. Although there was a remote chance that the burst and this galaxy were unrelated, their positional coincidence provided strong evidence that GRBs occur in distant galaxies rather than within the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
. This conclusion was later supported by observations of GRB 970508
GRB 970508
GRB 970508 was a gamma-ray burst detected on May 8, 1997, at 21:42 UTC. A gamma-ray burst is a highly luminous flash associated with an explosion in a distant galaxy and producing gamma rays, the most energetic form of electromagnetic radiation, and often followed by a longer-lived...
, the first burst to have its redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
determined.
The position of the burst's afterglow was measurably offset from the centroid
Centroid
In geometry, the centroid, geometric center, or barycenter of a plane figure or two-dimensional shape X is the intersection of all straight lines that divide X into two parts of equal moment about the line. Informally, it is the "average" of all points of X...
of the host galaxy, effectively ruling out the possibility that the burst originated in an active galactic nucleus
Active galactic nucleus
An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...
. The redshift of the galaxy was later determined to be z = 0.695, which corresponds to a distance of approximately . At this distance, the burst would have released a total of assuming isotropic emission.