You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
the mechanisms that produce gamma - ray bursts ( grbs ) have long been among the mysteries of modern astrophysics .
there have been a wide variety of models ( see mszros 2002 for a review of the theories of grbs ) .
the discovery of optical afterglows opened a new window on the field ( see , e.g. , van paradijs , kouveliotou , & wijers 2000 ) .
early identification of the optical afterglows led to the measurement of redshifts for several grbs ( e.g. ,  grb  970508 : metzger et  al .
1997 ) , providing definitive evidence for their cosmological origin .
observations at other wavelengths , especially radio , have revealed many more details about the bursts ( e.g. , berger et al .
2000 ; frail et al .
this only applies to the long - duration bursts at this point .
models that use supernovae ( sne ) to explain grbs have been part of the discussion from the start ( e.g. , colgate 1968 ; woosley 1993 ; macfadyen & woosley 1999 ) . strong evidence for the grb - sn association was first provided by grb  980425 ; the optical afterglow was not found , but an unusual type ic sn was seen in the error box of the grb ( galama et al .
1998 ; patat & piemonte 1998 ) .
many other circumstantial arguments were made to connect grbs with sne .
direct evidence , where an afterglow is seen to transform into an sn , was first seen with grb  030329 ( stanek et  al .
2003 ; matheson et  al .
2003 ; hjorth et  al . 2003 ; kawabata et  al . 2003 ; kosugi et  al .
subsequently , two other cases have been identified : grb  021211 ( della valle et  al .
2003 ) and grb  031203 ( malesani et  al .
2004 ) . in this paper
, i will review the evidence for the grb - sn association and describe the nature of the sne that are associated with grbs .
there were many indirect pieces of evidence that showed an association between grbs and sne .
most of these actually related grbs to massive stars , but , since massive stars do undergo core collapse to become sne , the connection was implicit . on the large scale ,
the host galaxies of grbs showed strong star formation , indicating that massive stars were present ( e.g. , hogg & fruchter 1999 ) .
more detailed studies found that the locations of grbs within their host galaxies were associated with the regions that contained massive stars ( e.g. ,  bloom , kulkarni , & djorgovski 2002 ) .
other circumstantial arguments derived from evidence related to the interaction of the grb with its local environment .
massive stars in general have stellar winds , and the observations of the afterglows are consistent with models of interaction with these winds ( e.g. , chevalier & li 2000 ) .
a few grb afterglows have shown strong absorption features at relatively high velocities ( a few thousand km s@xmath0 ) , the best case being grb  021004 .
for that grb , these features were interpreted as mass - loss shells from a wolf - rayet progenitor , again implying a massive star ( mirabal et  al .
2003 ; schaefer et  al .
another claim of evidence for association with sne came from reported detections of line features in the x - ray afterglows of some grbs ( e.g. , piro et  al . 1999 ; reeves et  al .
2002 ) that would result from material synthesized in the collapse , followed by a grb after a period of time .
recent reanalysis of all observations of x - ray afterglows , however , indicates that these earlier claims are not statistically significant ( sako , harrison , & rutledge 2004 ) .
perhaps the strongest element in the circumstantial case for the grb - sn association was the presence of bumps in the afterglows of many grbs .
the traditional optical afterglow of a grb decays as a power - law .
late - time deviations from this power law were consistent in their timing and brightness with an sn having exploded at the same time as the grb ( e.g. , bloom et  al .
rebrightening , though , is not necessarily direct evidence of a supernova .
that would require a spectroscopic detection .
the first evidence for an sn associated with a grb came when sn  1998bw was found in the error box of grb  980425 ( galama et  al .
the sn was of type ic ( see filippenko 1997 for a review of sn types ) , but it was peculiar , with evidence for unusually high velocities ( patat et al .
this event was odd in many ways .
the grb itself was relatively weak ( woosley , eastman , & schmidt 1999 ) and the radio emission from the sn was extraordinary , both in luminosity and the rapidity of its appearance ( kulkarni et al .
this was clearly an object of great interest , but it was not conclusive proof that sne and grbs are related .
it is , though , the framework upon which all further claims of grb - sn connections are built .
a weaker case is grb  011121 ( garnavich et  al .
2003a ; bloom et al .
a late - time bump in the light curve was observed from the ground and with _ hst_. at @xmath1 , the supernova component would have been relatively bright .
a spectrum of the grb taken during the rebrightening does not show any obvious features of an sn , but the color evolution derived from broad - band photometry is consistent with a supernova ( designated sn  2001ke ; garnavich et  al .
while sn  1998bw was a strong hint of the grb - sn association , no traditional optical afterglow was seen . without that direct association ,
the link between grbs and sne was still in question .
the ` monster burst ' of 2003 , grb  030329 was to provide that link .
the burst was unusually bright in gamma - rays , implying that it was relatively close .
spectroscopy of the afterglow soon confirmed a low redshift of 0.1685 ( greiner et  al .
2003 ) . as the afterglow faded , subtle features appeared in the normally flat power - law spectra of the afterglow .
by subtracting a continuum based upon the early shape of the spectrum , this structure was revealed as the spectrum of an unusual type ic sn similar to sn  1998bw , designated sn  2003dh ( garnavich et  al .
2003b ) . within a few days ,
the sn became the dominant component in the spectrum ( stanek et  al .
2003 ; hjorth et al . 2003
; kawabata et  al .
2003 ) .    using the early power - law continuum spectrum as a model
, one could decompose the observed spectra at later times into two separate components : grb afterglow and sn spectrum . using a least - squares technique , the best match for the sn among the low - redshift sample
was sn  1998bw ( matheson et  al .
in fact , taking into account cosmological time dilation , the spectroscopic evolution of sn  2003dh almost exactly matched sn  1998bw ( figure 1 ) .
models of these spectra are presented by mazzali et al .
( 2003 ; see also mazzali s contribution in this volume ) .
an important point about the appearance of the sn was that the light curve did not show the bump that is supposed to be the characteristic of a rebrightening caused by the sn ( see matheson et  al .
2003 and lipkin et  al .
2004 for a discussion of the light curve ) . without the spectroscopic confirmation ,
the sn in grb  030329 would still be a matter of contention .
nebular - phase spectra of sn  2003dh show a spectrum similar to a typical type ic sn .
kosugi et  al .
( 2004 ) present a spectrum at an age of @xmath23 months .
a spectrum obtained with the keck telescope by filippenko , chornock , & foley ( 2004 ) in december of 2003 is much like a normal type ic sn ( bersier et  al .
, in preparation ) .
following the discovery of sn  2003dh , reexamination of spectra of an earlier burst yielded some evidence for a sn component .
della valle et  al . ( 2003 ) found that a spectrum of the afterglow of grb  021211 had structure similar to an sn . in this case , though , the sn did not match sn  1998bw or any other peculiar type ic sn . rather , it was most similar to sn  1994i , a relatively normal type ic .
a more clear example came with grb  031203 . despite high foreground reddening , spectroscopy with
the vlt revealed an sn component , designated sn  2003lw ( malesani et  al .
2004 ) . for this sn , sn
1998bw was again a good match .
of the four sne with clear grb associations , three show remarkably similar spectra .
the classification scheme for sne ( see filippenko 1997 ) is based upon spectroscopic features .
the type ii sne show hydrogen , while those of type i do not .
type ia sne have a distinctive silicon feature and show an elemental pattern consistent with the theory that they arise from thermonuclear disruption of a white dwarf .
the other type i sne ( i.e. , without the strong silicon absorption ) are distinguished either by the presence ( ib ) or absence ( ic ) of helium .
a strong circumstantial case has tied types ii , ib , and ic to the same underlying mechanism of core collapse . a transition between type ii and something like a ib
was observed for sn  1993j ( e.g. , filippenko , matheson , & ho 1993 ) , strengthening this connection and providing an explanation for the subclasses .
models of the progenitor of sn  1993j postulated that a massive star in a binary had lost most of its hydrogen layer , leaving a small amount that produced the early spectrum of a type ii , and then revealing the helium layer below ( see matheson et  al .
2000 for a summary and an extensive list of references ) .
study of a large number of type ib / c sne indicated that the sne ic had less massive envelopes than the sne ib ( matheson et  al .
this implied that the ib sne have lost their hydrogen , leaving the helium layer , while the sne ic have lost hydrogen and helium , leaving a carbon / oxygen core . the spectrum itself , though derived from the same underlying mechanism , can be very different depending on the amount of the envelope stripped from the star at the time of the explosion .
if the grb mechanism does entail a jet ( e.g. , macfadyen , woosley , & heger 2001 ) , then a smaller progenitor with a less massive envelope would make it easier for the jet to punch through the stellar atmosphere and still have the energy to produce the observed burst and afterglow .
in addition to sn  1998bw , there are several low - redshift peculiar type ic sne . these are objects that show high expansion velocities and sometimes , but not always , large luminosity ( they are occasionally referred to as `` hypernovae '' ) .
two well - studied such examples are sn  1997ef and sn  2002ap ( iwamoto et al .
1998 , 2000 ; foley et  al . 2003 ) .
none of these other high - velocity sne has been directly associated with a grb .
there is a wide diversity in their spectroscopic and photometric development , but only the ones like sn  1998bw have been related to grbs .
it is not yet clear if this is significant .    even though the overall energy budget may be different for grbs and these other peculiar type ic sne , they are clearly unusual objects .
the high velocities set them apart from a typical sn , with the only similar objects being the sne associated with grbs .
to study the sn component of a grb requires it to be at low redshift , a condition that also makes them rare .
the peculiar ic sne , though , are relatively more common , and are bright enough to enable detailed studies with even modest telescopes . study of these objects could be the first step to understanding the grb mechanism .
while circumstantial arguments had linked sne with grbs , spectroscopic data was needed for irrefutable proof . this crucial piece of evidence arrived in the form of grb  030329 , wherein a direct transformation from grb afterglow to peculiar type ic sne was thoroughly observed .
later that same year , grb  031203 provided a further link .
these two , along with sn  1998bw , showed similar spectra . that they were type ic implied that the progenitor was a stripped - envelope star , lending credence to jet - based models for grbs .
although these grb - sne are rare , low - redshift examples of peculiar type ic sne are not , providing a laboratory to understand stellar explosions in extreme circumstances .
supernovae ( sne ) were long suspected as possible progenitors of gamma - ray bursts ( grbs ) . 
 the arguments relied on circumstantial evidence . 
 several recent grbs , notably grb  030329 , have provided direct , spectroscopic evidence that sne and grbs are related . 
 the sne associated with grbs are all of type ic , implying a compact progenitor , which has implications for grb models . 
 other peculiar type ic sne may help to expand understanding of the mechanisms involved .