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during the last 15 years , thanks to satellite missions like _ bepposax _
@xcite , _ swift _ @xcite , and _ fermi _
@xcite , our progress in the understanding of gamma - ray bursts ( grbs ) has been quite spectacular .
we now know that grbs are cosmological events related to a catastrophic energy release in stellar mass objects .
energy dissipation within a highly relativistic `` fireball '' , presumably emitted in the form of a jet ( e.g. , * ? ? ?
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traditionally , grbs have been divided in two main categories , long and short ones ( e.g. * ? ? ?
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these two populations of bursts are thought to be related to two different progenitor models : `` collapsars '' for the long - soft bursts ( e.g. , * ? ? ?
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* and references therein ) , and the merger of binary systems of compact objects such as neutron star ( ns)-ns or black - hole ( bh)-ns systems , for the short - hard ones ( e.g. , * ? ? ?
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despite the recent issues raised in the classification of short and long grbs based solely on their prompt emission properties ( e.g , * ? ? ?
* ) , the general picture of two classes of bursts related to two main progenitor models , still holds .
the collapsar scenario is observationally supported by the fact that long grbs occur in galaxies with high specific star formation rate ( e.g. , * ? ? ?
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indeed , the question of what makes some stars die as sne and some other as relativistic grbs , is not solved yet ( e.g. , * ? ? ?
* and references therein ) .
indications that progenitors of short bursts belong , on average , to old stellar populations with a typical lifetime of several gyr @xcite , provide support to the binary merger scenario .
collapsars and binary mergers leading to the formation of a bh plus an accretion disk ( e.g. , * ? ? ?
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* ) , have the potential to power the grb fireball via the energy released from the accretion of the disk onto the newly formed bh . while the formation of a bh plus disk system is common to both progenitor models , the more compact scale of the ns - ns ( or bh - ns systems ) , and the less massive debris left over after merger , are invoked to explain the shorter duration and the smaller isotropic energies of these bursts with respect to long ones .    within the standard fireball model ,
once the fireball is launched from the central engine , the observed radiation is explained as synchrotron and/or inverse compton emission from electrons accelerated in internal and external shocks ( e.g. , * ? ? ?
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* ) , taking place at distances @xmath3 cm from the central source .
high - energy ( gev ) tails observed in some grbs have challenged the internal - external shock fireball model in its simplest formulation @xcite .
however , it remains true that the electromagnetic emission from grbs , being produced at large distances from the central engine , provides indirect information on the progenitor . on the other hand , gravitational waves ( gws ) emitted from the progenitor
could directly probe its nature .
being related to catastrophic events involving stellar - mass objects , grbs are good candidates for the detection of gws @xcite . coalescing binaries , thought to be associated with short bursts , are one of the most promising gw sources ( e.g. , * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * and references therein ) for detectors like the laser interferometer gravitational - wave observatory ( ligo ; * ? ? ? * ) and virgo @xcite .
for such systems , a chirp signal should be emitted in gws during the in - spiral , followed by a burst - type signal associated with the merger , and subsequently a signal from the ring - down phase of the newly formed bh ( e.g. , * ? ? ?
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* and references therein ) .
the last , initially deformed , is expected to radiate gws until reaching a kerr geometry @xcite .    in the collapsar scenario ,
relevant for long grbs , the high rotation required to form the centrifugally supported disk that powers the grb , should produce gws via bar ( e.g. , * ? ? ?
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* ) or fragmentation instabilities that might develop in the collapsing core ( see e.g. , * ? ? ?
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* for recent reviews ) and/or in the disk @xcite
. moreover , asymmetrically infalling matter is expected to perturb the final bh geometry , leading to a ring - down phase @xcite .
ligo and virgo have been carrying out electromagnetically triggered searches for gws @xcite over the past decade ( for bar detectors electromagnetically triggered searches , see e.g. * ? ? ?
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the ligo scientific collaboration operates two ligo observatories in the u.s .
along with the geo600 detector @xcite in germany .
together with virgo , located in italy , they form a detector network capable of detecting gw signals arriving from all directions .
grb - triggered searches for gws by ligo and virgo have targeted both the chirp signal expected in the case of short grbs during the ns - ns or bh - ns in - spiral , and short unmodeled pulses of gws that may be expected during the merger / collapse , and ring - down phases of short / long grbs @xcite .
these searches have adopted on - source time windows of few minutes ( long grbs ) or few seconds ( short grbs ) around the grb trigger time .
in fact , for long grbs , the time delay between the gw signal and @xmath0-ray trigger is thought to be dominated by the time necessary for the fireball to push through the stellar envelope of the progenitor ( 10 - 100s ; * ? ? ? * ) . on the other hand , for short grbs ,
the ns - ns / bh - ns merger is believed to occur quickly , and be over within a few seconds ( naturally accounting for the short nature of these bursts ) .
it is estimated that triggered searches for gws in few minutes time - windows yield a factor of @xmath4 improvement in sensitivity with respect to untriggered ones @xcite .
the most exciting results from ligo grb - triggered searches of gws are probably represented by the cases of the short grbs 070201 @xcite and 051103 @xcite , whose error boxes overlap with nearby galaxies ( m31 for grb070201 ; m81 for grb051103 ) . a ns - ns binary merger scenario occurring in such hosts
was excluded by ligo with rather high confidence @xcite .
however , the possibility that grb070201 and grb051103 are related to ( extra - galactic ) soft gamma - ray repeaters ( sgr ) giant flares ( for a recent review , see * ? ? ? * and references therein ) , could not be ruled out .
indeed , ligo upper - limits for short unmodeled pulses of gws from grb070201 and grb051103 , are above the maximum gw energy emissible in sgr giant flares @xcite .
a very appealing prospect is represented by the possibility of using gws to trigger electromagnetic ( radio , optical , x - ray ) follow - ups of gw sources ( e.g. , * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) . the discovery of off - axis optical or radio afterglows @xcite triggered via the ( non - beamed ) gw emission from the grb progenitors , would yield a dramatic confirmation of the `` jet model '' , map out the beaming distribution , and provide fundamental inputs to models of relativistic outflows .
radio follow - ups , in particular , are an effective tool to identify relativistic and mildly relativistic outflows ( e.g. , * ? ? ?
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finding electromagnetic counterparts to gw triggers is technically challenging due to imperfect localization of the gw signal and uncertainty regarding the relative timing of the gw and electromagnetic emissions .
the localization of ligo - virgo triple - coincidence gw triggers can yield error - areas of @xmath5deg@xmath6 , possibly spread over different patches of the sky ( see e.g. fig . 3 in * ? ? ?
* ) . the problem of following - up with optical ( or radio , or x - ray ) telescopes such a large error - area can be partially mitigated by : ( i ) restricting the search for electromagnetic counterparts to transients in nearby galaxies ( within the ligo - virgo horizon distance ) ; ( ii ) by noticing that the most promising electromagnetic counterparts of gw events detectable by ligo and virgo
are expected to be `` exotic '' ( rare ) ones ( e.g. , the orphan afterglow of a grb , and/or the  kilonova  from a binary merger - see * ? ? ?
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in 2009 - 2010 , ligo and virgo , together with partner electromagnetic observatories , performed their first `` looc - up '' - locating and observing optical counterparts to unmodeled pulses of gravitational waves - experiment ( * ? ? ? * ; * ? ? ? * and references therein ) . at the time , there were two operating ligo interferometers @xcite , each with 4-km arms ( one near hanford , washington , the other in livingston parish , louisiana ) .
the virgo 3-km arms detector @xcite located near cascina ( italy ) , was also operating .
the looc - up search has established a baseline for low - latency analyses with the next - generation gw detectors ( advanced ligo and advanced virgo ; * ? ? ?
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the collaboration between gw and electromagnetic observatories is likely to continue to develop over the next few years , as the scientific community gets ready for a global network of advanced gw detectors .
the forthcoming years may see the development of new gw searches in coincidence with grbs , aimed at answering some of the questions opened by recent observations .
in particular , a compelling result from _ swift _ has been the discovery that the `` normal '' power - law behavior of long grb x - ray light curves is often preceded at early times by a steep decay , followed by a shallower - than - normal decay ( e.g. * ? ? ? * ; * ? ? ? * ) .
the steep - to - shallow and shallow - to - normal decay transitions are separated by two break times , @xmath7  s and @xmath8  s  @xmath9  s. it has been suggested that the shallow phase may be attributed to a continuous energy injection by a long - lived central engine , with progressively reduced activity @xcite .
newborn magnetars , besides being candidate grb progenitors ( e.g. , * ? ? ?
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independent support for the magnetar scenario comes from the observation of sn2006aj , associated with the nearby sub - energetic grb060218 , suggesting that the sn - grb connection may extend to a much broader range of stellar masses than previously thought , possibly involving two different mechanisms : a `` collapsar '' for the more massive stars collapsing to a bh , and a newborn ( highly - magnetized ) ns for the less massive ones @xcite .
several studies have shown how magnetars dipole losses may indeed explain the flattening observed in grb afterglows @xcite . @xcite
have explored a scenario in which the newly born magnetar left over after the grb explosion undergoes a secular bar - mode instability @xcite , thus producing a bar - like gw signal associated to the electromagnetic plateau , potentially detectable by the advanced ground - based interferometers like ligo and virgo ( up to distances of @xmath5mpc ) . compared to current analyses that gw detectors are carrying out ( see section [ triggered ] ) ,
this scenario @xcite involves a new class of gw signals , with a longer duration ( @xmath10 s ) and a different frequency evolution .
data analysis techniques for the search of longer duration gw signals possibly applicable to grb searches , are being developed ( e.g. , * ? ? ?
the ligo interferometers are being upgraded to the next - generation advanced detectors @xcite , that are expected to become operational around 2015 .
virgo will also be upgraded to become advanced virgo @xcite .
additionally , the new lcgt detector @xcite is being built in japan .
these advanced detectors are expected to detect compact binary coalescences , possibly at a rate of dozens per year after reaching design sensitivity @xcite , so that the short - grb progenitor scenario may finally be probed directly .
long - standing open questions ( e.g. , is the jet model for grbs correct ? why do some massive stars die as sne and others as relativistic grbs ? ) , or other issues raised by more recent observations ( such as the difficulties in the long - short grb classification ; the role of magnetars as grb progenitors ; the link between long grbs and sgrs , etc . ) , will greatly benefit from joint gw studies .
the advanced gw detectors will provide a totally new view of the transient sky @xcite .
the prospects for this new era of astronomy are exciting , and promise a return of big scientific impact .
gamma - ray bursts are likely associated with a catastrophic energy release in stellar mass objects . 
 electromagnetic observations provide important , but indirect information on the progenitor . on the other hand , gravitational waves emitted from the central source , 
 carry direct information on its nature . in this context , 
 i give an overview of the multi - messenger study of gamma - ray bursts that can be carried out by using electromagnetic and gravitational wave observations . 
 i also underline the importance of joint electromagnetic and gravitational wave searches , in the absence of a gamma - ray trigger . 
 finally , i discuss how multi - messenger observations may probe alternative gamma - ray burst progenitor models , such as the magnetar scenario .