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piro et al . ( 1999 ) and yoshida et al . ( 1999 ) report the detection of an iron emission line feature in the x  ray afterglow spectra of grb  970508 and grb  970828 , respectively . both lines are characterized by a large flux and equivalent width ( ew ) compared with the theoretical previsions made in the framework of the hypernova and compact merger grb progenitor models ( ghisellini et al . 1999 , bttcher et al . the line detected in grb  970508 is consistent with an iron @xmath0 line redshifted to the rest  frame of the candidate host galaxy ( @xmath1 , metzger et al . 1997 ) , while grb  970828 has no measured redshift and the identification of the feature with the same line would imply a redshift @xmath2 . the line fluxes ( equivalent widths ) are @xmath3  erg  @xmath4  s@xmath5 ( @xmath6  kev ) and @xmath7  erg  @xmath4  s@xmath5 ( @xmath8  kev ) for grb  970508 and grb  970828 , respectively . as indicated by the flux uncertainties , these line features are revealed at a statistical level of @xmath9 . a strong iron emission line unambiguously points towards the presence , in the vicinity of the burster , of a few per cent of iron solar masses concentrated in a compact region . thus the presence of such a line in the x  ray afterglow spectrum would represent the `` rosetta stone '' for unveiling the burst progenitor .    to date , three main classes of models have been proposed for the origin of gamma  ray bursts ( grb ) : neutron star  neutron star ( ns  ns ) mergers ( paczyski 1986 ; eichler et al . 1989 ) , hypernovae or failed type ib supernovae ( woosley 1993 ; paczyski 1998 ) and supranovae ( vietri & stella 1998 ) . in the ns ns model the burst is produced during the collapse of a binary system composed of two neutron stars or of a neutron star and a black hole . interacting neutrinos or the spin  down of the remnant black hole should power a relativistic outflow and , eventually , the production of @xmath10ray photons . hypernovae are the final evolutionary stage of very massive stars ( @xmath11 ) , whose core implodes in a black hole without the explosion of a classical supernova . again , the rotational energy of the remnant black hole is tapped by a huge magnetic field , powering the burst explosion . while ns ns mergers should take place outside the original star formation site , due to the large peculiar velocities and long lifetime of the coalescing binary systems , hypernovae take place in the dense and possibly iron rich molecular cloud where the massive star was born . the supranova scenario ( vietri & stella 1998 ) assumes that , following a supernova explosion , a fast spinning neutron star is formed , with a mass that would be supercritical in the absence of rotation . as radiative energy losses spin it down in a time  scale of months to years , it inevitably collapses to a kerr black hole , whose rotational energy can then power the grb . a supernova remnant ( snr ) is naturally left over around the burst location . since the detected lines are both characterized by a flux of several @xmath12  erg  s@xmath5 , we adopt as reference a line with a flux as @xmath13 and adopt cgs units . ] @xmath14 erg @xmath4 s@xmath5 . this in itself constrains both the amount of line  emitting matter and the size of the emitting region . we assume that the emitting region is a homogeneous spherical shell centered in the grb progenitor , with radius @xmath15 and width @xmath16 . a limit to the size of the emitting region can be set , _ independently from the flux variability _ , imposing the flux level times the minimum duration of the emission ( @xmath17 ) to be less than the total available energy supplied by the burst fluence . allowing for an efficiency @xmath18 of the burst photon reprocessing into the line ( @xmath19 , see ghisellini et al . 1999 ) we have : @xmath20 where @xmath21 is the total grb fluence . note that this result , obtained with independent arguments , agrees with the @xmath22  day variability time scales of the detected features .    to obtain a lower limit to the total amount of mass ( see lazzati et al . 1999 ) we consider a parameter @xmath23 given by the ratio between the total number of iron line photons divided by the number of iron nuclei : @xmath23 is the number of photons produced by a single iron atom . this number can be constrained to be less than the total number of ionizations an ion can undergo when illuminated by the burst or x  ray afterglow photons . for a grb located at @xmath24 , we obtain  km  s@xmath5  mpc@xmath5 , @xmath25 and @xmath26 . ] : @xmath27 where @xmath28 it the total energy of the burst photons , @xmath29 the energy of an ionizing photon and @xmath30 the ionization cross  section of the iron k shell . the total mass is given by the ratio of the total number of produced photons divided by @xmath23 times the mass of the iron atom . we have : @xmath31 where @xmath32 is the time during which the line is observed in units of @xmath33 seconds and @xmath34 is the iron abundance in solar units . if such a large amount of mass were uniformly spread around the burst location , it would completely stop the fireball . in fact ( see e.g. wijers , rees & mszros 1997 ) the fireball is slowed down to sub  relativistic speeds when the swept up mass equals its initial rest mass . with a typical baryon load of @xmath35 , the mass predicted in eq . [ minmass ] would stop the fireball after an observer time @xmath36  s , i.e. almost one day . any surviving long wavelength emission should then decay exponentially in the absence of energy supply . the only way to reconcile a monthly lasting power law optical afterglow with iron line emission is through a particular geometry , in which the line of sight is devoid of the remnant matter . this implies ( see also fig . [ fig1 ] ) that the burst emission is only moderately ( if at all ) beamed , since the burst photons must illuminate the surrounding matter and the line of sight simultaneously . the limits on the mass discussed in the above section are very general and independent of the mechanism producing the line photons . lazzati et al . 1999 give a complete analysis of three mechanisms that are able to generate such an intense line feature . in this scenario , burst ( or x  ray afterglow ) photons keep all the iron in the vicinity of the burst fully ionized . if the density of iron and free electrons is large and the plasma is cool , recombination of free electrons with iron nuclei is efficient and iron line photons are produced . however , we require that both @xmath37  @xmath38 and that @xmath39  k. during the burst inverse compton interactions of free electrons with @xmath10ray photons heat the plasma to @xmath40  k , and recombination is inefficient . this scenario may , however , be marginally consistent with grb  970508 data if the ionization flux is provided by an afterglow with an high energy tail but with typical compton temperature of @xmath41  k.      a supernova remnant of several solar masses , some months after the explosion of the supernova , has an emission integral of the same order of magnitude of the intergalactic medium of a galaxy cluster . these systems emit @xmath42  kev iron lines due to collisional excitation of hydrogenoid iron ions . in addition , the iron richness of a supernova remnant can be up to ten times larger than in a cluster of galaxies , allowing for the emission of iron line with several kev equivalent widths . in addition to the line , the heated snr produces bremsstrahlung radiation at a level @xmath43  erg  @xmath4  s@xmath5 , compatible with a standard x  ray afterglow at @xmath44 . if some dense matter ( e.g. a snr with less than a month of life ) is present in close vicinity with the burster , part of the incident radiation is reflected toward the observer . this same physical process is efficient in the disc of seyfert galaxies , and a strong iron line is produced since the cross section of the iron atom is much larger than that of free electrons . for a @xmath44 burst , lines with up to @xmath45  erg  @xmath4  s@xmath5 can be produced . in steady state , @xmath46  ev ew lines are produced ( if the material covers half of the sky ) , but in gamma ray bursts the reflected component is seen when the burst emission has already faded and the ew can be much higher ( even a few kev ) . given the fast variability properties of the detected lines , this model is favored with respect to recombination and thermal emission . the presence of dense matter at rest in the reference frame of the burster ( and not of the fireball ) is a discriminating parameter for the main models of burst progenitor . here we analyze the two main models , binary neutron star mergers and hypernovae , and a particular model , the supranova , that naturally accounts for the surrounding iron rich matter . the production of lines in the fireball has been discussed in mszros & rees 1998a . they find that lines ( particularly iron lines ) can be produced inside the fireball , but we should observe them at a frequency @xmath47 , where @xmath48 is the frequency of the line in the laboratory . in the case of grb  970508 the observed frequency of the line is consistent with an iron line at the same redshift of the burster . an incredible fine  tuning is necessary to explain the feature as an optical  uv line blueshifted in the x  ray band due to the fireball expansion . the line must hence be produced by the interaction of the burst photons with cold matter that surrounds the burster . the lifetime of a binary system composed of a couple of compact objects ( ns  ns or bh  ns ) ranges from 100 million to 10 billion years , mainly depending on the initial separation and on the mass of the binary system components ( see e.g. lipunov et al 1995 ) . due to the kick velocity that a compact object inherits from the supernova explosion that generated it , @xmath49  km  s@xmath5 on average ( kalogera et al . 1998 ) , these binary systems merge well outside their formation site and , possibly , even outside the host galaxy . assuming an hydrogen density @xmath50  @xmath38 and a solar iron abundance , a sphere of radius : @xmath51 is necessary to meet the requirements of eq . [ minmass ] . however , the radius of the above equation is five orders of magnitude larger than the maximum allowed value ( cf . [ maxrad ] ) . if we insert in eq . [ radio ] the density appropriate for a molecular cloud , in which a very massive star is thought to be located , we obtain @xmath52  cm for @xmath53  @xmath38 that , for an iron rich cloud , is marginally consistent with the limit of eq . [ maxrad ] . to firmly rule out the possibility of an intense iron line produced in the hypernova scenario , we analyze each emission mechanism discussed in sect . reflection can be immediately excluded since a radius @xmath54  cm cloud is necessary to have a thomson thick mirror . the efficiency of thermal emission is proportional to the square of the density . lazzati et al . 1999 estimate that both a density @xmath55  @xmath38 and a mass of several solar masses ( see also sect .  [ emec ] ) are necessary to produce such an intense iron line . in the hypernova scenario , the density is 5 orders of magnitude smaller , giving a flux 10 orders of magnitude fainter . the recombination time in a medium with the considered density is too long ( see bttcher et al . 1999 ) , however a fluorescence line may be emitted due to ionization of the inner shell of almost neutral iron atoms . the more optimistic estimate of the line flux produced by fluorescence in a molecular cloud is given in ghisellini et al . 1999 , with an upper limit @xmath56  erg  @xmath4  s@xmath5 . even if we consider the presence of a pre  hypernova wind with @xmath57/yr , the total mass contained in a radius of @xmath58  cm would be a few percent of a solar mass , leaving the above discussion unaffected . we hence conclude that in a standard hypernova scenario it is not possible to produce an intense iron line . mszros & rees 1998b discussed the possible production of an intense iron line if a compact companion of the hypernova has left a dense torus of material ( @xmath59 ; @xmath60  @xmath38 ) at a distance @xmath61 cm from the burster . another progenitor model can account for this matter in a more natural way . in the model presented by vietri & stella 1998 the burst explosion is preceded by a supernova . the neutron star left over by the supernova explosion is supermassive , i.e. its mass exceeds the limiting mass for a neutron star , but the implosion in a black hole is prevented by the fast rotation . as the system spins down , due to radiative losses , the neutron star shrinks until a limiting state is reached and the star collapses in a black hole . the duration of this transient phase depends from the strength of the magnetic field embedded in the star and from the initial spin velocity , but should typically last between several months and a dozen of years . if we consider a supernova exploded a year before the burst , the density of its shell remnant at the burst onset is : @xmath62 where @xmath63 is the mass of the ejecta in solar units , @xmath64 is their speed in units of @xmath65  cm  s@xmath5 and the width of the shell has been assumed @xmath66 . the shell radius is @xmath67  cm , in good agreement with the constraints derived above . if we consider that a snr can have an iron richness ten times the solar value ( woosley 1988 ) , this scenario is the most natural one to explain the intense iron line observed in the afterglow of grb  970508 and grb  970828 . the recent explosion of grb  990123 has gathered the attention of the gamma ray astronomers due to the huge fluence and large distance of this burst . the early x  ray afterglow flux was measured by the _ beppo_-sax satellite at a level of @xmath68  erg  s@xmath5 ( heise et al . 1999 ) about 6 hours after the burst trigger . the first results on the spectral analysis gives a `` featureless spectrum '' ( heise et al . 1999 ) . if an iron line feature with the same ew of those of grb  970508 and grb  970828 were present in the afterglow of grb  990123 , a firm detection would have been established : since the underlying continuum flux of the latter burst is @xmath69 times larger than in the other two cases , the signal to noise ratio of the line would have been 10 times larger . moreover , due to the larger redshift of grb  990123 ( z=1.6004 , kulkarni et al . 1999 ) the redshifted line energy lies in a region where the detector efficiency is the largest . this could have led to a @xmath70 line feature . there are , however , two reasons why an iron line is not expected in the afterglow of grb  990123 . first , ( lazzati et al . 1999 ) the line intensity is limited by the recombination time and not by the ionization time . this means that , even in the case of grb  970508 - a weak burst - the ionizing flux was more than enough to fully exploit the matter surrounding the progenitor . if an iron line with flux @xmath71  erg  @xmath4  s@xmath5 was present in the x  ray afterglow of the 23 january burst , its ew would be decreased to the very low value of @xmath72  ev , hence well below the detection threshold of the _ beppo_-sax . finally , the break in the power  law decrease of the optical light curve of grb  990123 afterglow has been interpreted as a sign of beaming in the burst itself ( kulkarni et al . 1999 ; fruchter et al 1999 ) . if this interpretation is correct , even if a cloud were present in close vicinity with the burster , its illumination would have been minimal , preventing the formation of an intense line . we have analyzed how the detection of an iron line redshifted to the rest frame of the burster can be used to constrain the progenitor models . indeed , due to the relativistic outflows involved in the fireball model , very poor information of the progenitor is contained in the gamma  ray burst photons , and the only mean to investigate the progenitor nature is through the interaction of the photons with the ambient medium . in particular , we have analyzed three different models predicting very different conditions of the external medium : the merging of two compact objects in a binary system , the hypernova and the supranova . we derived a ( model  independent ) lower limit to the mass required to produce the line , ruling out the ns ns merger model and , with a careful analysis , the hypernova . the supranova , which naturally accounts for a shell remnant of iron rich material surrounding the burster seems the most promising model capable to produce an intense iron line .    due to the paucity of the statistical significance of the line detections , more observations are needed to draw firm conclusions . we show that , despite its luminosity , grb  990123 is not the ideal test for the line detection and more observations with more efficient satellites are needed . bttcher , m. , dermer , c. d. , crider , a. w. , & liang , e. p. 1999 , , 343 , 111 eichler , d. , livio , m. , piran , t. , & shramm , d. n. 1989 , nature , 340 , 126 fruchter a. s. , et al . 1999 , apj submitted ( astro - ph/9902236 ) heise , j. , et al . 1999 , iauc n. 7099 kalogera , v. , kolb , u. , & king , a. r. 1998 , , 504 , 967 kulkarni , s. , et al . 1999 , nature , 398 , 389 lazzati , d. , campana , s. , & ghisellini , g. 1999 , , 304 , l31 lipunov , v. m. , postnov , k. a. , prokhorov , m. e. , & panchenlo , i. e. 1995 , , 454 , 593 mszros , p. , & rees , m. j. 1998a , , 502 , l105 mszros , p. , & rees , m. j. 1998b , , 299 , l10 metzger , m. r. , et al . 1997 , nature , 387 , 878 paczyski , b. 1986 , , 308 , l43 paczyski , b. 1998 , , 494 , l45 piro , l. , et al . 1999 , , 514 , l73 vietri , m. , & stella . l. 1998 , , 507 , l45 wijers , r. a. m. j. , rees , m. j. , & mszros , p. 1997 , , 288 , l51 woosley , s. e. 1988 , , 330 , 218 woosley , s. e. 1993 , , 405 , 273 yoshida , a. , et al . 1999 ,  in press
the discovery of a powerful and transient iron line feature in the x  ray afterglow spectrum of grb  970508 and grb  970828 , if confirmed , would be a major breakthrough for understanding the nature the progenitor of gamma 
 ray bursts . 
 we show that a large mass of iron very close to the burster is necessary to produce the emission line . 
 this in itself strongly limits the possible progenitor of the gamma  ray event , suggesting the former explosion of a supernova , as predicted in the supranova model ( vietri & stella 1998 ) . the line emission process and the line intensity depend strongly on the age , density and temperature of the remnant . 
 the simultaneous observation of the iron line and of a power  law optical afterglow lasted for one year strongly suggest that the burst emission is isotropic . 
 recent observations of grb  990123 are also discussed .