document: thanks to a fortunate coincidence of observations by agile , _ fermi _ , and _ swift _ satellites , together with the optical observations by the vlt / fors2 and the nordic optical telescope , it has been possible to obtain an unprecedented set of data , extending from the optical - uv , through the x - rays , all the way up to the high energy ( gev ) emission , which allowed detailed temporal / spectral analyses on grb 090510 @xcite . 
   in contrast with this outstanding campaign of observations , a theoretical analysis of the broadband emission of grb 090510 has been advanced within the synchrotron / self - synchrotron compton ( ssc ) and traditional afterglow models ( see , e.g. , sections 5.2.1 and 5.2.2 in * ? ? ? * ) . 
   paradoxically , this same methodology has been applied in the description of markedly different type of sources : e.g. , @xcite for the low energetic long grb 060218 , @xcite for the high energetic long grb 130427a , and @xcite for the s - grf 051221a .    in the meantime , it has become evident that grbs can be subdivided into a variety of classes and sub - classes @xcite , each of them characterized by specific different progenitors which deserve specific theoretical treatments and understanding . in addition 
   every sub - class shows different episodes corresponding to specifically different astrophysical processes , which can be identified thanks to specific theoretical treatments and data analysis . in this article , we take grb 090510 as a prototype for s - grbs and perform a new time - resoved spectral analysis , in excellent agreement with the above temporal and spectral analysis performed by , e.g. , the _ fermi _ team . 
   now this analysis , guided by a theoretical approach successfully tested in this new family of s - grbs @xcite , is directed to identify a precise sequence of different events made possible by the exceptional quality of the data of grb 090510 . 
   this include a new structure in the thermal emission of the p - grb emission , followed by the onset of the gev emission linked to the bh formation , allowing , as well , to derive the structure of the circumburst medium from the spiky structure of the prompt emission . 
   this sequence , for the first time , illustrates the formation process of a bh . 
   already in february 1974 , soon after the public announcement of the grb discovery @xcite , @xcite presented the possible relation of grbs with the vacuum polarization process around a kerr - newman bh . 
   there , evidence was given for : a ) the formation of a vast amount @xmath2-baryon plasma ; b ) the energetics of grbs to be of the order of @xmath11  erg , where @xmath12 is the bh mass ; c ) additional ultra - high energy cosmic rays with energy up to @xmath13  ev originating from such extreme process . 
   a few years later , the role of an @xmath2 plasma of comparable energetics for the origin of grbs was considered by @xcite and it took almost thirty years to clarify some of the analogies and differences between these two processes leading , respectively , to the alternative concepts of  fireball " and  fireshell " @xcite . in this article we give the first evidence for the formation of a kerr newman bh , in grb 090510 , from the merger of two massive nss in a binary system . 
   grbs are usually separated in two categories , based on their duration properties . 
   short grbs have a duration @xmath14 s while the remaining ones with @xmath15 s are traditionally classified as long grbs . 
   short grbs are often associated to ns - ns mergers ( see e.g.  @xcite ; see also @xcite for a recent review ) : their host galaxies are of both early- and late - type , their localization with respect to the host galaxy often indicates a large offset @xcite or a location of minimal star - forming activity with typical circumburst medium ( cbm ) densities of @xmath16@xmath17 @xmath18 , and no supernovae ( sne ) have ever been associated to them . 
   the progenitors of long grbs , on the other hand , have been related to massive stars @xcite . however , in spite of the fact that most massive stars are found in binary systems @xcite , that most type ib / c sne occur in binary systems @xcite and that sne associated to long grbs are indeed of type ib / c @xcite , the effects of binarity on long grbs have been for a long time largely ignored in the literature . 
   indeed , until recently , long grbs have been interpreted as single events in the jetted _ collapsar _ fireball model ( see e.g.  @xcite and references therein ) . 
   multiple components evidencing the presence of a precise sequence of different astrophysical processes have been found in several long grbs ( e.g.  @xcite , @xcite ) . following this discovery , further results led to the introduction of a new paradigm expliciting the role of binary sources as progenitors of the long grb - sn connection . 
   new developments have led to the formulation of the induced gravitational collapse ( igc ) paradigm @xcite . 
   the igc paradigm explains the grb - sn connection in terms of the interactions between an evolved carbon - oxygen core ( co@xmath19 ) undergoing a sn explosion and its hypercritical accretion on a binary ns companion @xcite . 
   the large majority of long bursts is related to sne and are spatially correlated with bright star - forming regions in their host galaxies @xcite with a typical cbm density of @xmath20 @xmath18 @xcite . 
   a new situation has occurred with the observation of the high energy gev emission by the _ 
   fermi_-lat instrument and its correlation with both long and short bursts with isotropic energy @xmath21 erg , which has been evidenced in @xcite and @xcite , respectively . on the basis of this correlation 
   the occurrence of such prolonged gev emission has been identified with the onset of the formation of a bh @xcite . 
   as recalled above , the long grbs associated to sne have been linked to the hypercritical accretion process occurring in a tight binary system when the ejecta of an exploding co@xmath19 accretes onto a ns binary companion ( see , e.g. , * ? ? ? 
   * ; * ? ? ? 
   * ; * ? ? ? 
   when the hypercritical accretion occurs in a widely separated system with an orbital separation @xmath22  cm @xcite , the accretion is not sufficient to form a bh . for these softer systems with rest - frame 
   spectral peak energy @xmath23 kev the upper limit of their observed energy is @xmath24 erg , which corresponds to the maximum energy attainable in the accretion onto a ns @xcite . 
   such long a burst corresponds to an x - ray flash ( xrf ) . 
   the associated x - ray afterglow is also explainable in terms of the interaction of the prompt emission with the sn ejecta ( fryer et al . 
   , in preparation ) . in these systems 
   no gev emission is expected in our theory and , indeed , is not observed . 
   interestingly , a pioneering evidence for such an x - ray flash had already been given in a different context by @xcite , @xcite , and @xcite . 
   for tighter binaries ( @xmath25  cm , @xcite ) , the hypercritical accretion onto the companion ns leads to the formation of a bh . for these harder systems with @xmath26 kev the lower limit of their observed energy is @xmath24 erg , which necessarily needs the accretion process into a bh . 
   an associated prolonged gev emission occurs after the p - grb emission and at the beginning of the prompt emission , and originates at the onset of the bh formation @xcite . 
   these more energetic events are referred to as binary - driven hypernovae ( bdhne ) . 
   specific constant power - law behaviors are observed in their high energy gev , x - rays , and optical luminosity light curves @xcite .    in total analogy , the formation of a bh can occur in short bursts , depending on the mass of the merged core of the binary system . 
   when the two ns masses are large enough , the merged core can exceed the ns critical mass and the bh formation is possible . in the opposite case , 
   a massive ns ( mns ) is created , possibly , with some additional orbiting material to guarantee the angular momentum conservation . 
   we then naturally expect the existence of two short bursts sub - classes : authentic short grbs ( s - grbs ) , characterized by the formation of a bh @xcite , with @xmath21 erg , a harder spectrum ( see section [ epeakeiso ] ) and associated with a prolonged gev emission ( see section [ gevemission ] ) ; short gamma - ray flashes ( s - grfs ) , producing a mns @xcite , with @xmath27 erg . in this second sub - class , of course , the gev emission should not occur and , indeed , is never observed .    , where @xmath28 is the critical value for vacuum polarization and @xmath29 is the electric field strength . 
   the plot assumes a black hole mass energy @xmath30 . 
   figure reproduced from @xcite with their kind permission . ]    following the discovery of the first prototype of this s - grb class , namely grb 090227b @xcite , the first detailed analysis of such a genuine short grb originating from a binary ns merger leading to a bh was done for grb 140619b by @xcite , determining as well the estimated emission of gravitational waves . 
   the latter has been estimated following the method applied by @xcite for grb 090227b . 
   from the spectral analysis of the early @xmath31 s , they inferred an observed temperature @xmath32 kev of the @xmath2 plasma at transparency ( p - grb ) , a theoretically derived redshift @xmath33 , a total burst energy @xmath34 erg , a rest - frame peak energy @xmath35 mev , a baryon load @xmath36 , and an average cbm density @xmath37 @xmath18 . 
   we turn in this article to the most interesting case of grb 090510 which has , in addition to very similar properties of the members of this new class of s - grb sources , a spectroscopically determined value of the redshift and represents one of the most energetic sources of this family both in the @xmath38-ray and in the gev ranges . 
   actually , a first attempt to analyze grb 090510 was made by interpreting this source as a long grb @xcite . 
   an unusually large value of the cbm density was needed in order to fit the data : this interpretation was soon abandoned when it was noticed that grb 090510 did not fulfill the nesting conditions of the late x - ray emission typical of long grbs @xcite , see also section [ xray ] and figure [ episode3 ] .    in light of the recent progress in the understanding of the fireshell theory , 
   we address the interpretation of grb 090510 as the merging of a binary ns . 
   we give clear evidence for the validity of this interpretation . in view of the good quality of the data both in @xmath38- rays and in the gev range 
   , we have performed a more accurate description of the p - grb , best fitted by a convolution of thermal spectra . 
   this novel feature gives the first indication for the existence of an axially symmetric configuration of the dyadotorus emitting the @xmath2 plasma which had been previously theoretically considered and attentively searched for . 
   this gives the first indication that indeed the angular momentum plays a role and a dyadotorus is formed , as theoretically predicted in a series of papers ( see * ? ? ? 
   * and figure [ dyadotorus ] ) . 
   this naturally leads to the evidence for the formation of a rotating bh as the outcome of the gravitational collapse . 
   we turn then to the main new feature of grb 090510 which is the high energy @xmath6@xmath7 gev emission ( see figure [ 090510gev ] ) . 
   the direct comparison of the gev emission in this source and in the bdhne 130427a shows the remarkable similarities of these two gev components ( see figure [ 090510gev ] ) . the fact that the s - grb 090510 originates from a binary ns merger and the bdhn 130427a from the igc of a sn hypercritical accretion process onto a companion ns clearly points to the bh as originating this gev emission , the reason being that these two astrophysical systems are different in their progenitors and physical process and have in the formation of a bh their unique commonality . 
   this paper is structured as follows : in section 2 we summarize the relevant aspects of the fireshell theory and compare and contrast it with alternative approaches . in section 3 we discuss the recent progress on the ns equilibrium configuration relevant for s - grbs and bdhne . in section 4 
   we move on to describe the observations of grb 090510 and their analysis . 
   the s - grb nature of grb 090510 is justified in section 5 , and we offer an interpretation of our results in section 6 . section 7 concludes this work . 
   a standard flat @xmath39cdm cosmological model with @xmath40 and @xmath41 km s@xmath42 mpc@xmath42 is adopted throughout the paper . 
   the fireshell scenario @xcite , has been initially introduced to describe a grb originating in a gravitational collapse leading to the formation of a kerr - newman bh . 
   a distinct sequence of physical and astrophysical events are taken into account :    * an optically thick pair plasma  the fireshell of total energy @xmath43  is considered . as a result , it starts to expand and accelerate under its own internal pressure @xcite . 
   the baryonic remnant of the collapsed object is engulfed by the fireshell  the baryonic contamination is quantified by the baryon load @xmath44 where @xmath45 is the mass of the baryonic remnant @xcite . 
   * after the engulfment , the fireshell is still optically thick and continues to self - accelerate until it becomes transparent . 
   when the fireshell reaches transparency , a flash of thermal radiation termed proper - grb ( p - grb ) is emitted @xcite . * in grbs , the @xmath2-baryon plasma evolves from the ultra - relativistic region near the bh all the way reaching ultra - relativistic velocities at large distances . 
   to describe such a dynamics which deals with unprecedentedly large lorentz factors and also regimes sharply varying with time , in @xcite it has been introduced the appropriate relative spacetime transformation paradigm . 
   this paradigm gives particular attention to the constitutive equations relating four time variables : the comoving time , the laboratory time , the arrival time , and the arrival time at the detector corrected by the cosmological effects . 
   this paradigm is essential for the interpretation of the grb data : the absence of adopting such a relativistic paradigm in some current works has led to a serious misinterpretation of the grb phenomenon . * in compliance with the previous paradigm , the interactions between the ultra - relativistic shell of accelerated baryons left over after transparency and the cbm have been considered . 
   they lead to a modified blackbody spectrum in the co - moving frame @xcite . 
   the observed spectrum is however non - thermal in general ; this is due to the fact that , once the constant arrival time effect is taken into account in the equitemporal surfaces ( eqts , see * ? ? ? 
   * ; * ? ? ? 
   * ) , the observed spectral shape results from the convolution of a large number of modified thermal spectra with different lorentz factors and temperatures . * 
   all the above relativistic effects , after the p - grb emission , are necessary for the description of the prompt emission of grbs , as outlined in @xcite . the prompt emission originates in the collisions of the accelerated baryons , moving at lorentz factor @xmath46@xmath47 , with interstellar clouds of cbm with masses of @xmath48@xmath49  g , densities of @xmath50@xmath3  @xmath18 and size of @xmath51@xmath52  cm , at typical distances from the bh of @xmath53@xmath54  cm ( see , e.g. , @xcite for long bursts ) . 
   our approach differs from alternative tratments purporting late activities from the central engine ( see , e.g. , the _ collapsar _ model in @xcite , @xcite , @xcite and references therein , and the _ magnetar _ model in @xcite , @xcite , @xcite , @xcite , @xcite , and references therein ) . * @xmath43 and @xmath55 are the only two parameters that are needed in a spherically symmetric fireshell model to determine the physics of the fireshell evolution until the transparency condition is fulfilled . 
   three additional parameters , all related to the properties of the cbm , are needed to reproduce a grb light curve and its spectrum : the cbm density profile @xmath56 , the filling factor @xmath57 that accounts for the size of the effective emitting area , and an index @xmath58 that accounts for the modification of the low - energy part of the thermal spectrum @xcite . 
   they are obtained by running a trial - and - error simulation of the observed light curves and spectra that starts at the fireshell transparency . * 
   a more detailed analysis of pair cration process around a kerr - newman bh has led to the concept of dyadotorus @xcite . there , the axially symmetric configuration with a specific distribution of the @xmath2 , as well as its electromagnetic field , have been presented as function of the polar angle . 
   the total spectrum at the transparency of the @xmath2plasma is a convolution of thermal spectra at different angles . 
   this formalism describing the evolution of a baryon - loaded pair plasma is describable in terms of only three intrinsic parameters : the @xmath2 plasma energy @xmath59 , the baryon load @xmath55 , and the specific angular momentum @xmath60 of the incipient newly - formed bh . 
   it is , therefore , independent of the way the pair plasma is created . 
   in addition to the specific case , developed for the sake of example , of the dyadotorus created by a vacuum polarization process in an already formed kerr - newman bh , more possibilities have been envisaged in the meantime :    * the concept of dyadotorus can be applied as well in the case of a pair plasma created via the @xmath61 mechanism in a ns merger as described in @xcite , @xcite , @xcite , @xcite , assuming that the created pair plasma is optically thick . 
   the relative role of neutrino and weak interactions vs. the electromagnetic interactions in building the dyadotorus is currently topic of intense research . * equally important are the relativistic magneto - hydrodynamical process leading to a dyadotorus , indicated in the general treatment of @xcite , and leading to the birth of a kerr - newman bh , surrounded by an opposite charged magnetosphere in a system endowed with global charge neutrality . 
   active research is ongoing . 
   * progress in understanding the ns equilibrium configuration imposing the global charge neutrality condition , as opposed to the local charge neutrality usually assumed @xcite . 
   a critical mass for a non - rotating ns @xmath62 has been found for the nl3 nuclear equation of state @xcite . 
   the effects of rotation and of the nuclear equation of state on the critical mass is presented in @xcite and in @xcite . 
   the existence of electromagnetic fields close to the critical value has been evidenced in the interface between the core and the crust in the above global neutrality model , as well as very different density distributions in the crust and in the core , which could play an important role during the ns 
   ns mergers ( see figure  [ nanda ] and * ? ? ? 
   ( _ middle panel _ ) in the core - crust transition layer normalized to the @xmath63-meson compton wavelength @xmath64  fm . _ 
   lower panel _ : density profile inside a ns star with central density @xmath65 , where @xmath66 is the nuclear density , from the solution of the tov equations ( locally neutral case ) and the globally neutral solution presented in @xcite . 
   the density at the edge of the crust is the neutron drip density @xmath67  g  @xmath68 . 
   reproduced from @xcite with their kind permission . ] 
   the above three possibilities have been developed in recent years , but they do not have to be considered exaustive for the formation of a dyadotorus endowed by the above three parameters 
   .    in conclusion the evolution in the understanding of the grb phenomenon , occurring under very different initial conditions , has evidenced the possibility of using the dyadotorus concept for describing sources of an optically thick baryon - loaded @xmath2 plasma within the fireshell treatment in total generality . 
   the key role neutrino emission in the hypercritical accretion process onto a ns has been already examined in the literature ( see , e.g. , * ? ? ? * ; * ? ? ? 
   the problem of hypercritical accretion in a binary system composed of a co@xmath19 and a companion ns has been studied in @xcite ( see also references therein ) . the energy released during the process , in form of neutrinos and photons , 
   is given by the gain of gravitational potential energy of the matter being accreted by the ns and depends also on the change of binding energy of the ns while accreting both matter and on the angular momentum carried by the accreting material ( see , e.g. , @xcite and @xcite ) . for a typical ns mass of @xmath70  m@xmath71 , a value observed in galactic ns binaries , and a ns critical mass @xmath72 in the range from @xmath73  m@xmath71 up to @xmath74 depending on the equations of state and angular momentum ( see * ? ? ? 
   * ; * ? ? ? 
   * ; * ? ? ? 
   * for details ) , the accretion luminosity can be as high as @xmath75@xmath76  erg  s@xmath42 for accretion rates @xmath77@xmath78  s@xmath42 ( see * ? ? ? 
   * ; * ? ? ? 
   * for details ) . for binary systems with a separation @xmath79  cm ( @xmath80  min ) , 
   our numerical simulations indicate that : a ) the accretion process duration lasts @xmath81  s ( see , e.g. , * ? ? ? 
   * ; * ? ? ? 
   * ) , b ) the ns collapses to a bh , and c ) a total energy larger than @xmath69  erg is released during the hypercritical accretion process . 
   these systems correspond to the bdhne @xcite . for systems with larger separations 
   the hypercritical accretion is not sufficient to induce the collapse of the ns into a bh and the value of @xmath69  erg represents a theoretical estimate of the upper limit to the energy emitted by norm in the hypercritical accretion process . 
   this sub - class of sources corresponds to the xrfs @xcite .    the same energetic considerations do apply in the analysis of the hypercritical accretion occurring in a close binary ns system undergoing merging @xcite . 
   therefore , in total generality , we can conclude that the energy emitted during a ns 
   ns merger leading to the formation of a bh should be larger than @xmath69  erg ( see figure [ rt_gw ] ) . 
   the limit of @xmath69  erg clearly depends on the initial ns mass undergoing accretion , by norm assumed to be @xmath70  m@xmath71 , and on the yet unknown value of @xmath72 , for which only an absolute upper limit of @xmath82  m@xmath71 has been established for the non - rotating case @xcite . as already pointed out in @xcite , for ns 
   ns mergers , the direct determination of the energy threshold of @xmath69  erg dividing s - grfs and s - grbs , as well as xrfs and bdhne , provides fundamental informations for the determination of the actual value of @xmath83 , for the minimum mass of the newly - born bh , and for the mass of the accreting ns .    ] ) . 
   the binary orbit gradually shrinks due to energy loss through gravitational waves emission ( yellow - brown ) . at point 
   a , the merger occurs : the fireshell ( in red ) is created and starts its expansion . 
   it reaches transparency at point b , emitting the p - grb ( light purple ) . the prompt emission ( deep purple ) 
   then follows at point c. the dashed lines represent the gev emission ( delayed relative to the start of the grb ) originating in the newly - born bh . 
   this space - time diagram well illustrates how the gev emission originates in the newly - born bh and follows a different space - time path from the prompt emission , contrary to what stated in @xcite . 
   the prompt emission originates from the interactions of the baryons , accelerated to ultrarelativistic lorentz factors during the pair - baryon electromagnetic pulse , with the clumpy circumburst medium ( see section 2 ) . 
   the analysis of the spiky structure of the prompt emission allows to infer the structure of the circumburst medium ( see figure  [ 090510simlc ] ) . 
   there is the distinct possibility that the gev emission prior to @xmath84 s in the arrival time may interact with the prompt emission . in this sense 
   the work by @xcite may become of interest . ] 
   in this section , we summarize the observations of grb 090510 as well as the data analysis . we used _ 
   ( gbm and lat ) and _ swift_/xrt data for the purposes of this work .      the _ fermi_/gbm instrument @xcite was triggered at @xmath8500:22:59.97 ut on may 10 , 2009 by the short and bright burst grb 090510 ( @xcite , trigger 263607781 / 090510016 ) . 
   the trigger was set off by a precursor emission of duration 30 ms , followed @xmath86 0.4 s later by a hard episode lasting @xmath87 s. this grb was also detected by swift @xcite , _ 
   fermi_/lat @xcite , agile @xcite , konus - wind @xcite , and suzaku - wam @xcite . 
   the position given by the gbm is consistent with that deduced from _ 
   swift _ and lat observations .    during the first second after lat trigger at 00:23:01.22 ut , _ 
   fermi_/lat detected over 50 events ( respectively over 10 ) with an energy above 100 mev ( respectively above 1 gev ) up to the gev range , and more than 150 ( respectively more than 20 ) within the first minute @xcite . 
   this makes grb 090510 the first bright short grb with an emission detected from the kev to the gev range . 
   observations of the host galaxy of grb 090510 , located by vlt / fors2 , provided a measurement of spectral emission lines . 
   this led to the determination of a redshift @xmath88 @xcite . 
   the refined position of grb 090510 obtained from the nordic optical telescope @xcite is offset by 0.7 " relative to the center of the host galaxy in the vlt / fors2 image . at @xmath89 , 
   this corresponds to a projected distance of 5.5 kpc . 
   the identified host galaxy is a late - type galaxy of stellar mass @xmath90 , with a rather low star - forming rate sfr @xmath91 ( @xcite and references therein ) .      . 
   lower panel : comptonized + power law best fit of the corresponding spectrum ( from @xmath92 to @xmath93 s).,title="fig : " ] . 
   lower panel : comptonized + power law best fit of the corresponding spectrum ( from @xmath92 to @xmath93 s).,title="fig : " ]    [ cols="<,^,^,^,^,^,^,^",options="header " , ]     in order to determine the profile of the cbm , a simulation of the prompt emission following the p - grb has been performed . 
   the simulation starts at the transparency of the fireshell with the parameters that we determined above . 
   a trial - and - error procedure is undertaken , guided by the necessity to fit the light curve of grb 090510 . 
   the results of this simulation ( reproduction of the light curve and spectrum , in the time interval from @xmath94 to @xmath95 s , and cbm profile ) are shown in figure [ 090510simlc ] . 
   the average cbm density is found to be @xmath96 . 
   this low value , typical of galactic halo environments , is consistent with the large offset from the center of the host and further justifies the interpretation of grb 090510 as a short grb originating in a binary ns merger . 
   our theoretical fit of the prompt emission ( see red line in the middle plot of figure [ 090510simlc ] ) predicts a cut - off at @xmath97 mev . 
   the spectrum at energy @xmath98 mev could be affected by the onset of the high energy power - law component manifested both in the data of the mini - calorimeter on board agile ( see top panel of figure 4 in @xcite ) and in the data points from the _ 
   fermi_-gbm bgo - b1 detector . 
   there is a weak precursor emission about 0.4 s before the p - grb ( or @xmath99 s in the cosmological rest frame ) . 
   two gev photons have been detected during the precursor emission . 
   precursors are commonly seen in long bursts : @xcite found that @xmath100 20% of them show evidence of an emission preceding the main emission by tens of seconds . 
   short bursts are less frequently associated with precursors . 
   no significant emission from the grb itself is expected prior to the p - grb  since it marks the transparency of the fireshell  but the precursor may be explainable in the context of a binary ns merger by invoking the effects of the interaction between the two nss just prior to merger . indeed 
   , it has been suggested that precursor emission in short bursts may be caused by resonant fragmentation of the crusts @xcite or by the interaction of the ns magnetospheres @xcite . 
   the timescale ( @xmath100 0.21 s between the precursor and the p - grb ) is consistent with a pre - merger origin of the precursor emission . from its formation to its transparency , the fireshell undergoes a swift evolution . 
   the thermalization of the pair plasma is achieved almost instantaneously ( @xmath101 s , @xcite ) ; and the @xmath102 plasma of grb 090510 reaches the ultra - relativistic regime ( i.e.  a lorentz factor @xmath103 ) in a matter of @xmath104 s , according to the numerical simulation . 
   the radius of the fireshell at transparency , @xmath105 cm , corresponds to more than a hundred light - seconds ; however relativistic motion in the direction of the observer squeezes the light curve by a factor @xmath106 , which makes the fireshell capable of traveling that distance under the observed timescale . 
   the spectral analysis of this precursor is limited by the low number of counts . 
   @xcite interpreted the spectrum with a blackbody plus power law model . 
   this leads to a blackbody temperature of @xmath107 kev . 
   the isotropic energy contained in the precursor amounts to @xmath108 erg . 
   an interesting feature of the fireshell model is the possibility to infer a theoretical redshift from the observations of the p - grb and the prompt emission . in the case of grb 090510 , 
   a comparison is therefore possible between the measured redshift @xmath88 and its theoretical derivation . 
   an agreement between the two values would in particular strengthen the validity of our p - grb choice , which would in turn strengthen our results obtained with this p - grb . 
   the feature of redshift estimate stems from the relations , engraved in the fireshell theory , between different quantities computed at the transparency point : the radius in the laboratory frame , the co - moving frame and blue - shifted temperatures of the plasma , the lorentz factor , and the fraction of energy radiated in the p - grb and in the prompt emission as functions of @xmath55 ( see figure 4 in @xcite ) . 
   thus , the ratio @xmath109 implies a finite range for the coupled parameters @xmath43 and @xmath55 ( last panel of figure 4 in @xcite ) . 
   assuming @xmath110 , this ratio is known since it is equal to the ratio between the observed fluences of the respective quantities : @xmath111    with the measured values @xmath112 erg @xmath113 and @xmath114 erg @xmath113 , we find @xmath115 . 
   in addition , knowing the couple [ @xmath116 , @xmath55 ] gives the ( blue - shifted towards the observer ) temperature of the fireshell at transparency @xmath118 ( figure 4 in @xcite , second panel ) . 
   but we also have the following relation between @xmath118 and the observed temperature at transparency @xmath119 , linking their ratio to the redshift : @xmath120    finally , since we assume that @xmath110 , we also have an expression of @xmath43 as a function of @xmath121 using the formula of the k - corrected isotropic energy : @xmath122    where @xmath123 is the photon spectrum of the grb and the fluence @xmath124 is obtained in the full gbm energy range 8  40000 kev . 
   the use of all these relations allows a redshift to be determined by an iterative procedure , testing at every step the value of the parameters @xmath125 and @xmath118 . 
   the procedure successfully ends when both values are consistent according to the relations described above . in the case of grb 090510 
   , we find @xmath126 , which provides a satisfactory agreement with the measured value @xmath88 . 
   grb 090510 is associated with a high - energy emission , consistently with all other observed s - grbs , i.e. energetic events with @xmath21 erg . 
   the only case of a s - grb without gev emission , namely grb 090227b , has been explained by the absence of alignment between the lat and the source at the time of the grb emission . 
   nevertheless evidence of some gev emission in this source has been recently obtained ( ruffini et al . 
   , in preparation ) . 
   the gev light curve of grb 090510 is plotted in figure [ 090510gev ] together with other s - grb light curves and showing a common power - law behavior , which goes as @xmath127 , similar to the clustering of the gev light curves found by @xcite . 
   these s - grbs are compared with that of the bdhne 130427a which shares a similar behavior . @xcite 
   suggest and argue that the gev emission is related to the presence of a bh and its activity . 
   this view is supported by the fact that the gev emission is delayed with respect to the @xmath38-ray emission : it starts only after the p - grb is over . 
   the gev emission of grb 090510 is particularly intense , reaching @xmath128 erg . 
   such a large value , one of the largest observed among s - grbs , is consistent with the large angular momentum of the newborn bh . 
   this energetic can not be explained in terms of nss in view of the lower value of the gravitational binding energy . 
   the absence of gev emission in s - grfs is also confirmed from the strong upper limit to the gev emission for s - grbs imposed by the fermi - lat sensitivity . 
   we assume for a moment that the gev emission of a s - grf is similar to that of s - grb . 
   we then compute the observed gev flux light curve of s - grb 090510 at different redshifts , e.g. , @xmath129 and @xmath130 , which correspond to the redshifts of the s - grb 081024b and of the s - grb 140402a , respectively ( aimuratov et al . 
   , in preparation ) . 
   the result is that if we compare these computed flux light curves with the _ 
   fermi_-lat sensitivity of the pass 8 release 2 version 6 instrument response functions , which is approximately @xmath131 erg @xmath113s@xmath42 , all of them are always well above the lat broadband sensitivity by a factor @xmath132 ( see figure [ gevz ] ) . 
   this result does not depend on the choice of the source . in their rest - frame all the s - grb gev light curves follow a similar behavior . therefore , the gev emission of s - grb 090510 is always above @xmath132 times to the lat sensitivity , even at higher redshifts . 
   if we now assume that s - grfs do conform to the same behavior of s - grbs , the absence of detection of gev emission implies that the s - grfs have necessarily fluxes at least @xmath133@xmath134 times smaller than those of s - grbs . 
   @xmath7 gev flux light curve of the s - grbs 090510 ( red squares ) , and the corresponding ones obtained by translating this s - grb at @xmath129 ( blue circles ) and at @xmath135 ( green diamonds ) . ]      in order to estimate the energy requirement of the @xmath6@xmath7 gev emission of figure [ 090510gev ] we consider the accretion of mass @xmath136 onto a kerr - newman bh , dominated by its angular momentum and endowed with electromagnetic fields not influencing the geometry , which remains approximately that of a kerr bh . 
   we recall that if the infalling accreted material is in an orbit co - rotating with the bh spin , up to @xmath137 of the initial mass is converted into radiation , for a maximally rotating kerr bh , while this efficiency drops to @xmath138 , when the infalling material is on a counter - rotating orbit ( see ruffini & wheeler 1969 , in problem 2 of @xmath139 104 in @xcite ) . 
   therefore , the gev emission can be expressed as @xmath140 and depends not only on the efficiency @xmath141 in the accretion process of matter @xmath136 , but also on the geometry of the emission described by the beaming factor @xmath142 ( here @xmath143 is the half opening angle of jet - like emission ) . 
   depending on the assumptions we introduce in equation [ accretion ] , we can give constraints on the amount of accreted matter or on the geometry of the system . 
   for an isotropic emission , @xmath144 , the accretion of @xmath145  m@xmath71 , for the co - rotating case , and of @xmath146  m@xmath71 , for the counter - rotating case , is required . 
   alternatively , we can assume that the accreted matter comes from the crustal material from an @xmath147  m@xmath71 ns 
   ns binary progenitor . 
   the crustal mass from the nl3 nuclear model for each of these nss is @xmath148  m@xmath71 ( see , e.g. , * ? ? ? 
   * and figure [ nanda ] ) . 
   assuming that crustal material accounts also for the baryon load mass , e.g. , @xmath149 m@xmath71 , the total available mass for accretion is @xmath150 m@xmath71 . 
   then , the presence of a beaming is necessary : from equation [ accretion ] , a half opening beaming angle @xmath151 , for co - rotating case , and @xmath152 , for the counter - rotating case , would be required . 
   the above considerations are clearly independent from the relativistic beaming angle @xmath153 , where the lower limit on the lorentz factor @xmath154 has been derived , in a different context , by @xcite to the gev luminosity light curve ( see figure [ 090510gev ] ) . 
   further consequences on these results for the estimate of the rate of these s - grbs will be presented elsewhere ( ruffini et al . in preparation ) . 
   it is interesting to recall some of the main novelties introduced in this paper with respect to previous works on grb 090510 . particularly noteworthy 
   are the differences from the previous review of short bursts by @xcite , made possible by the discovery of the high energy emission by the fermi team in this specific source @xcite . 
   a new family of short bursts characterized by the presence of a bh and associated high energy emission when lat data are now available , comprehends grbs 081024b , 090227b , 090510 , 140402a , and 140619b ( see , e.g. , figure [ 090510gev ] ) . 
   the excellent data obtained by the fermi team and interpreted within the fireshell model has allowed to relate in this paper the starting point of the high energy emission with the birth of a bh . 
   our fireshell analysis assumes that the @xmath38-ray and the gev components originate from different physical processes . 
   first , the interpretation of the prompt emission differs from the standard synchrotron model : we model the collisions of the baryon accelerated by the grb outflow with the ambient medium following a fully relativistic approach ( see section 2 ) . 
   second , we assume that the gev emission originates from the matter accretion onto the newly - born bh and we show that indeed the energy requirement is fulfilled 
   . this approach explains also the delayed onset of the gev emission , i.e. , it is observable only after the transparency condition , namely after the p - grb emission . 
   the joint utilization of the excellent data from the _ 
   fermi_-gbm nai - n6 and n7 and the bgo - b1 detectors and from the mini - calorimeter on board agile @xcite has given strong observational support to our theoretical work . 
   grb 090510 has been analyzed in light of the recent progress achieved in the fireshell theory and the resulting new classification of grbs . 
   we show that grb 090510 is a s - grb , originating in a binary ns merger ( see figure  [ rt_gw ] ) . 
   such systems , by the absence of the associated sn events , are by far the simplest grbs to be analyzed . 
   our analysis indicates the presence of three distinct episodes in s - grbs : the p - grb , the prompt emission , and the gev emission . by following the precise identification of successive events predicted by the fireshell theory , we evidence for the first indication of a kerr bh or , possibly , a kerr - newman bh formation :    * the p - grb spectrum of grb 090510 , in the time interval from @xmath92 to @xmath94 s , is best - fitted by a comptonized component ( see figures  [ pgrb ] and [ comp ] and table  [ tab : fit ] ) , which is interpreted as a convolution of thermal spectra originating in a dyadotorus ( see @xcite and @xcite , figure  [ dyadotorus ] , and section 2 ) . * 
   the prompt emission follows at the end of the p - grb ( see figure  [ spectotal ] ) . 
   the analysis of the prompt emission within the fireshell model allows to determine the inhomogeneities in the cbm giving rise to the spiky structure of the prompt emission and to estimate as well an averaged cbm density of @xmath155  @xmath18 obtained from a few cbm clouds of mass @xmath48 g and typical dimensions of @xmath53 cm ( see figure  [ 090510simlc ] ) . 
   such a density is typical of galactic halos where binary ns are expected to migrate due to large natal kicks . * the late x - ray emission of grb 090510 
   does not follow the characteristic patterns expected in bdhn events ( see figure  [ episode3 ] and @xcite ) . * 
   the gev emission occurs at the end of the p - grb emission and is initially concurrent with the prompt emission . 
   this sequence occurs in both s - grbs @xcite and bdhne @xcite . 
   this delayed long lasting ( @xmath156 s ) gev emission in grb 090510 is one of the most intense ever observed in any grb ( see figure  [ 090510gev ] and * ? ? ? 
   * ; * ? ? ? 
   * we then consider accretion on co - rotating and counter - rotating orbits ( see ruffini & wheeler 1969 , in problem 2 of @xmath139 104 in @xcite ) around an extreme kerr bh . assuming the accretion of the crustal mass @xmath157  m@xmath71 from a @xmath147  m@xmath71 ns 
   ns binary , fulfilling global charge neutrality ( see figure  [ nanda ] ) , geometrical beaming angles of @xmath152 , for co - rotating case , and @xmath151 , for the counter - rotating case , are inferred . in order to fulfill the transparency condition , 
   the initial lorentz factor of the jetted material has to be @xmath158 ( see section 6.6 ) . 
   * while there is evidence that the gev emission must be jetted , no beaming appears to be present in the p - grb and in the prompt emission , with important consequence for the estimate of the rate of such events @xcite . 
   * the energetic and the possible beaming of the gev emission requires the presence of a kerr bh , or a kerr - newman bh dominated by its angular momentum and with electromagnetic fields not influencing the geometry ( see also section 6.5 ) . * 
   the self - consistency of the entire procedure has been verified by estimating , on the ground of the fireshell theory , the cosmological redshift of the source . 
   the theoretical redshift is @xmath159 ( see section  6.4 ) , close to and consistent with the spectroscopically measured value @xmath10 @xcite . * 
   the values of @xmath160 and @xmath161 of grb 090510 fulfill with excellent agreement the muruwazha relation ( see section  5.2 , figure  [ calderone ] and * ? ? ? 
   the main result of this article is that the dyadotorus manifests itself by the p - grb emission and clearly preceeds the prompt emission phase , as well as the gev emission originating from the newly - formed bh . 
   this contrasts with the usual assumption made in almost the totality of works relating bhs and grbs in which the bh preceeds the grb emission . in conclusion , in this article 
   , we take grb 090510 as the prototype of s - grbs and perform a new time - resoved spectral analysis , in excellent agreement with that performed by the agile and the _ fermi _ teams . now 
   this analysis , guided by a theoretical approach successfully tested in this new family of s - grbs , is directed to identify a precise sequence of different events made possible by the exceptional quality of the data of grb 090510 . 
   this include a new structure in the thermal emission of the p - grb emission , followed by the onset of the gev emission linked to the bh formation , allowing , as well , to derive the strucutre of the circumburst medium from the spiky structure of the prompt emission . 
   this sequence , for the first time , illustrates the formation process of a bh . 
   it is expected that this very unique condition of generating a jetted gev emission in such a well defined scenario of a newly - born bh will possibly lead to a deeper understanding of the equally jetted gev emission observed , but not yet explained , in a variety of systems harboring a kerr bh . among these systems we recall binary x - ray sources ( see , e.g. , * ? ? ? * and references therein ) , microquasars ( see , e.g. , * ? ? ? * and references therein ) , as well as , at larger scale , active galactic nuclei . 
   we thank the editor and the referee for their comments which helped to improve the presentation and the contextualization of our results . 
   we are indebted to marco tavani for very interesting comments , as well as for giving us observational supporting evidences . 
   this work made use of data supplied by the uk _ swift _ data center at the university of leicester . 
   m. e. , m. k. , and y. a. are supported by the erasmus mundus joint doctorate program by grant numbers 2012 - 1710 , 2013 - 1471 , and 2014 - 0707 respectively , from the eacea of the european commission . c.c . 
   acknowledges indam - gnfm for support . 
   m.m . acknowledges the partial support of the project n 3101/gf4 ipc-11 , and the target program f.0679 of the ministry of education and science of the republic of kazakhstan . 
   , j. 2003 , in american institute of physics conference series , vol . 
   662 , gamma - ray burst and afterglow astronomy 2001 : a workshop celebrating the first year of the hete mission , ed . 
   g.  r. ricker & r.  k. vanderspek , 229236                                                                                                                                            , i.  b. , klebesadel , r.  w. , & evans , w.  d. 1975 , in annals of the new york academy of sciences , vol . 
   262 , seventh texas symposium on relativistic astrophysics , ed . 
   p.  g. bergman , e.  j. fenyves , & l.  motz , 145158
summary: in a new classification of merging binary neutron stars ( nss ) we separate short gamma - ray bursts ( grbs ) in two sub - classes . the ones with @xmath0  erg coalesce to form a massive ns and are indicated as short gamma - ray flashes ( s - grfs ) . 
   the hardest , with @xmath1  erg , coalesce to form a black hole ( bh ) and are indicated as genuine short - grbs ( s - grbs ) . within the fireshell model , s - grbs exhibit three different components : the p - grb emission , observed at the transparency of a self - accelerating baryon-@xmath2 plasma ; the prompt emission , originating from the interaction of the accelerated baryons with the circumburst medium ; the high - energy ( gev ) emission , observed after the p - grb and indicating the formation of a bh . 
   grb 090510 gives the first evidence for the formation of a kerr bh or , possibly , a kerr - newman bh . 
   its p - grb spectrum can be fitted by a convolution of thermal spectra whose origin can be traced back to an axially symmetric dyadotorus . 
   a large value of the angular momentum of the newborn bh is consistent with the large energetics of this s - grb , which reach in the @xmath3@xmath4  kev range @xmath5  erg and in the @xmath6@xmath7  gev range @xmath8  erg , the most energetic gev emission ever observed in s - grbs . 
   the theoretical redshift @xmath9 that we derive from the fireshell theory is consistent with the spectroscopic measurement @xmath10 , showing the self - consistency of the theoretical approach . 
   all s - grbs exhibit gev emission , when inside the _ fermi_-lat field of view , unlike s - grfs , which never evidence it . 
   the gev emission appears to be the discriminant for the formation of a bh in grbs , confirmed by their observed overall energetics .