View source for GRB 970228

{{short description|Gamma-ray burst detected on 28 Feb 1997, the first for which an afterglow was observed}}
{{Infobox astronomical event
| onlysourced = yes
| ra = {{RA|05|01|46.7}}
| dec = {{DEC|+11|46|53.0}}<ref name="Groot"/>
| epoch = J2000
| energy = {{val|5.2|e=44|ul=J}}
}}

'''GRB 970228'''<ref name="nomenclature">"GRB" indicates that the event was a gamma-ray burst, and the numbers follow a YYMMDD format corresponding to the date on which the burst occurred: 28 February 1997.</ref> was the first [[gamma-ray burst]] (GRB) for which an [[Afterglow (gamma-ray burst)|afterglow]] was observed.<ref>[[#Schilling|Schilling 2002]], p. 101</ref> 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 [[luminosity|luminous]] bursts of high-energy [[gamma ray]]s (the most energetic form of [[electromagnetic radiation]]); this resulted in large positional uncertainties which left their nature very unclear.  

The burst had multiple peaks in its [[light curve]] and lasted approximately 80&nbsp;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&nbsp;billion&nbsp;[[light-year]]s<ref name=nwjcc/> away (a [[redshift]] of z&nbsp;=&nbsp;0.695), providing early evidence that GRBs occur well beyond the [[Milky Way]]; this was proven decisively two months later with a subsequent burst [[GRB 970508]].

== Observations ==
A gamma-ray burst (GRB) is a highly [[luminosity|luminous]] 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.<ref>[[#Schilling|Schilling 2002]], pp. 12–16</ref>

GRB 970228<ref name="nomenclature"/> 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]],<ref>[[#Varendoff|Varendoff 2001]], p. 381</ref><ref name="CostaJournal">[[#CostaJournal|Costa 1997b]]</ref> an Italian–Dutch satellite originally designed to study X-rays.<ref>[[#Schilling|Schilling 2002]], pp. 58–60</ref> The burst lasted around 80&nbsp;seconds and had multiple peaks in its light curve.<ref name="Costa 1997a">[[#CostaCirc|Costa 1997a]]</ref> 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]].<ref>[[#Zwart|Zwart 2001]]</ref>
Within a few hours, the BeppoSAX team used the X-ray detection to determine the burst's position with an error box—a small area around the specific position to account for the [[Measurement uncertainty|error]] in the position—of 3&nbsp;[[arcminute]]s.<ref name="CostaJournal"/> The burst was also detected by the [[Ulysses (spacecraft)|Ulysses]] space probe.<ref>[[#Hurley|Hurley 1997]]</ref>

About one and nine days later, optical images of the error box were taken with the [[William Herschel Telescope]] on La Palma; comparison of the images revealed a fading point source located at a [[right ascension]] of {{RA|05|01|46.7}} and a [[declination]] of  {{DEC|+11|46|53.0}}, providing the first arcsecond-accuracy localization of any Gamma-ray burst.<ref name="Groot"/> 

Later images after the point source faded revealed a faint galaxy at almost the same position, the presumed host galaxy of the burst; a chance position coincidence was unlikely but possible, so the cosmological origin of GRBs was not conclusive until observations of [[GRB 970508]] about two months later.

== Afterglow ==
In 1993, [[Bohdan Paczyński]] 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.<ref>[[#Pac|Paczyński 1993]]</ref> 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]].<ref>[[#Katz|Katz 1994]]</ref>

The Narrow Field Instruments on board BeppoSAX began making observations of the GRB 970228's position within eight hours of its detection.<ref name="Costa 1997a"/> A transient x-ray source was detected which faded with a [[power-law]] slope in the days following the burst. This x-ray [[Afterglow (gamma-ray burst)|afterglow]] was the first GRB afterglow ever detected.<ref name="CostaJournal"/> 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.<ref>[[#Panaitescu|Panaitescu 2007]], §2</ref>

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]].<ref name="Groot">[[#GrootCirc|Groot 12 March 1997]]</ref> This was the burst's optical afterglow. Deeper follow-up observations using the [[New Technology Telescope]] showed that the afterglow coincided with a distant, small galaxy: the first evidence of the extragalactic, cosmological nature of Gamma-ray bursts.<ref name="Groot2">[[#GrootCirc2|Groot 14 March 1997]]</ref><ref name="Van Paradijs">[[#VanParadijs97|Van Paradijs et al., 1997]]</ref> After the gamma-ray bursts itself had faded away, very deep observations taken with the [[Keck telescopes]] showed the underlying galaxy to have a redshift of 0.695. The predicted radio afterglow was never detected for this burst.<ref name="Bloom"/> At the time of this burst's discovery, GRBs were believed to emit radiation [[isotropic]]ally. 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]].<ref>[[#Huang|Huang 2002]]</ref>

== Supernova relation ==
[[File:Gamma ray burst.jpg|thumb|right|Artist's illustration showing the life of a [[star#Massive stars|massive star]] as it goes supernova, collapses into a black hole, and emits a gamma-ray burst along its axis of rotation ''Credit: Nicolle Rager Fuller/NSF''|alt=From top left to right, a blue sphere grows larger and gains more layers. At far right, the sphere explodes. From bottom right to left, the exploded sphere flattens out to a swirling disk with two bright beams of light coming out of the axis of rotation.]]
[[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.<ref>[[#Schilling|Schilling 2002]], p. 173</ref><ref name="Reichart99">[[#Reichart99|Reichart 1999]]</ref>

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]].<ref>[[#Galama|Galama 2000]]</ref> Reichart noted that the late afterglow was [[visual spectrum|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]],<ref name="Reichart99"/> for which a supernova relation had already been proposed by [[Joshua Bloom]].<ref>[[#Bloom99|Bloom 1999]]</ref>

An alternative explanation for the light curves of GRB 970228 and GRB 980326 involved [[light echo|dust echoes]]. Although GRB 980326 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.<ref>[[#Reichart01|Reichart 2001]]</ref> Definitive evidence linking gamma-ray bursts and supernovae was eventually found in the [[stellar spectrum|spectrum]] of [[GRB 020813]]<ref>[[#Butler|Butler 2003]]</ref> and the afterglow of [[GRB 030329]].<ref>[[#Stanek|Stanek 2003]]</ref> 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.<ref>[[#Moran|Moran 2005]]</ref>

== Host galaxy ==
During the night between 12 and 13 March, [[Jorge Melnick]] made observations of the region with the [[New Technology Telescope]]. 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]].<ref>[[#Schilling|Schilling 2002]], p. 102</ref> This conclusion was later supported by observations of [[GRB 970508]], the first burst to have its [[redshift]] determined.<ref>[[#Reichart98|Reichart 1998]]</ref>

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&nbsp;=&nbsp;0.695,<ref name="Bloom">[[#Bloom|Bloom 2001]]</ref> which corresponds to a distance of approximately {{val|8.123|e=9|ul=ly}}.<ref name=nwjcc>Converting of the redshift into the distance done by on-line tools: <br>{{cite web | first=Edward L. | last=Wright | date=9 May 2008 | title=Ned Wright's Javascript Cosmology Calculator | publisher=UCLA Division of Astronomy & Astrophysics | url=http://www.astro.ucla.edu/~wright/CosmoCalc.html | access-date=2010-06-11 }}</ref> At this distance, the burst would have released a total of {{val|5.2|e=44|ul=J}} assuming [[isotropic]] emission.<ref>[[#Djorgovski|Djorgovski 1999]]</ref>

== Notes ==
{{reflist|colwidth=20em}}

== References ==
{{Refbegin|30em}}
* {{cite journal | author = Bloom, J. S. | title =  The redshift and the ordinary host galaxy of GRB 970228 | journal = Astrophysical Journal | volume = 554 | issue = 2 | date = 2001 | pages = 678–683 | arxiv = astro-ph/0007244 | doi = 10.1086/321398 | ref = Bloom | bibcode=2001ApJ...554..678B| last2 =  Djorgovski | first2 =  S. G. | last3 =  Kulkarni | first3 =  S. R. | s2cid = 16648604 }}
* {{cite journal | author = Bloom, J. S. | title = The unusual afterglow of the γ-ray burst of 26 March 1998 as evidence for a supernova connection | journal = Nature | volume = 401 | issue = 6752 | pages = 453–456 | doi = 10.1038/46744 | date = 30 September 1999 |arxiv=astro-ph/9905301 | ref = Bloom99|bibcode = 1999Natur.401..453B | last2 = Kulkarni | first2 = S. R. | last3 = Djorgovski | first3 = S. G. | last4 = Eichelberger | first4 = A. C. | last5 = Côté | first5 = P. | last6 = Blakeslee | first6 = J. P. | last7 = Odewahn | first7 = S. C. | last8 = Harrison | first8 = F. A. | last9 = Frail | first9 = D. A. | last10 = Filippenko | first10 = A. V. | last11 = Leonard | first11 = D. C. | last12 = Riess | first12 = A. G. | last13 = Spinrad | first13 = H. | last14 = Stern | first14 = D. | last15 = Bunker | first15 = A. | last16 = Dey | first16 = A. | last17 = Grossan | first17 = B. | last18 = Perlmutter | first18 = S. | last19 = Knop | first19 = R. A. | last20 = Hook | first20 = I. M. | last21 = Feroci | first21 = M. | s2cid = 205058997 }}
* {{cite journal | title = The X-ray Afterglows of GRB 020813 and GRB 021004 with Chandra HETGS: Possible Evidence for a Supernova prior to GRB 020813 | journal = The Astrophysical Journal | volume = 597 | issue = 2 | pages = 1010–1016 | date = 10 November 2003 | author = Butler, Nathaniel R. | ref = Butler | doi = 10.1086/378511 | bibcode=2003ApJ...597.1010B|arxiv = astro-ph/0303539 | last2 = Marshall | first2 = Herman L. | last3 = Ricker | first3 = George R. | last4 = Vanderspek | first4 = Roland K. | last5 = Ford | first5 = Peter G. | last6 = Crew | first6 = Geoffrey B. | last7 = Lamb | first7 = Donald Q. | last8 = Jernigan | first8 = J. Garrett | s2cid = 6171688 }}
* <cite id = CostaCirc>Costa, E. ''et al''. (1997a) "[http://www.cbat.eps.harvard.edu/iauc/06500/06572.html IAU Circular 6572: GRB 970228; 1997aa]". ''International Astronomical Union''.</cite>
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* {{cite journal | title = GRB 970228: Redshift and properties of the host galaxy | author = Djorgovski, George | journal = GCN Circulars | volume = 289 | date = 3 May 1999 | url =http://gcn.gsfc.nasa.gov/gcn3/289.gcn3 |page=1 | ref = Djorgovski| bibcode = 1999GCN...289....1D }}
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* <cite id = GrootCirc>Groot, P. J. ''et al''. (12 March 1997) "[http://www.cbat.eps.harvard.edu/iauc/06500/06584.html IAU Circular 6584: GRB 970228]". ''International Astronomical Union''.</cite>
* <cite id = GrootCirc2>Groot, P. J. ''et al''. (14 March 1997) "[http://www.cbat.eps.harvard.edu/iauc/06500/06588.html IAU Circular 6588: GRB 970228]". ''International Astronomical Union''.</cite>
* {{cite journal | title = Are Gamma-ray Bursts Due to Isotropic Fireballs or Cylindrical Jets? | author = Huang, Yong-feng | journal = Chinese Astronomy and Astrophysics | volume = 26 | issue = 4 | date = 2002 | pages = 414–423 | doi = 10.1016/S0275-1062(02)00092-9 | ref = Huang | last2 = Tan | first2 = Chang-yi | last3 = Dai | first3 = Zi-gao | last4 = Lu | first4 = Tan |bibcode = 2002ChA&A..26..414H }}
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* <cite id = Pedichini>Pedichini, F. ''et al''. (22 April 1997) "[http://www.cbat.eps.harvard.edu/iauc/06600/06635.html IAU Circular 6635: GRB 970228; C/1995 O1]". ''International Astronomical Union''.</cite>
* {{cite journal|title=The Redshift of GRB 970508|author=Reichart, Daniel E.|date=19 February 1998|publisher=University of Chicago|journal=Astrophysical Journal Letters|volume=495|issue=2|pages=L99–L101|doi=10.1086/311222|arxiv=astro-ph/9712134 |ref = Reichart98|bibcode=1998ApJ...495L..99R|s2cid=119394440 }}
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* {{cite journal | author = Reichart, Daniel E. | title = Light Curves and Spectra of Dust Echoes from Gamma-Ray Bursts and Their Afterglows: Continued Evidence That GRB 970228 Is Associated with a Supernova | journal = Astrophysical Journal | volume = 554 | issue = 2 | pages = 649–659 | doi = 10.1086/321428 | date = 2001 | ref = Reichart01 | bibcode=2001ApJ...554..643R|arxiv = astro-ph/0012091 | s2cid = 7492485 }}
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{{refend}}

== External links ==
*[http://www.mpe.mpg.de/~jcg/grb970228.html Gamma-ray Burst 970228]
*[http://www.asdc.asi.it/bepposax/first/grb970228.html BeppoSAX follow-up observations of the region of the Gamma-ray burst GRB 970228] {{Webarchive|url=https://web.archive.org/web/20110303114630/http://www.asdc.asi.it/bepposax/first/grb970228.html |date=2011-03-03 }}

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{{DEFAULTSORT:Grb 970228}}
[[Category:Astronomical objects discovered in 1997|19970228]]
[[Category:Long-duration gamma-ray bursts|970228]]
[[Category:February 1997]]
[[Category:Orion (constellation)]]