BeppoSAX observation of the X-ray emission of gamma-ray bursts

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P R O C E E D I N G S SUPPLEMENTS

ELSEVIER Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 646-655

BeppoSAX observation of the X-ray emission of Gamma-Ray Bursts Enrico Costa a*

aIst i tuto di Astrofisica Spaziale - CNR Area di Ricerca di Roma - Tor Vergata, Via Fosso del Cavaliere - 1-00133 Roma, I ta ly

For more than 20 years the difficulty, to locate the direction of a beam, intrinsic to the V ray detection techniques, and the rapidity of the phenomenon itself has been the insurmountable limit to the association of Gamma-Ray Bursts with an already established class of sources. The combination of different instruments aboard BeppoSAX has turned out to be a real breakthrough for the study of this outstanding phenomenon. From January to May 1997 four gamma ray bursts were detected with Wide Field Instruments and a rapidly pointed with Narrow Field Instruments. In the direction of three of these burst (and likely also of the fourth one) faint, fading sources, have been found. These fading sources decay according to a power law but with the evidence of a flaring activity. The spectra are hard and non-thermal. Some data suggest that the burst itself continuously evolves into the decaying source and the energy associated to the source is comparable to that of the burst itself. This discovery, followed by the detection of optical transients associated with two of the X-ray sources, one with a nebulosity ad another with redshifted absorption features and a scintillating radio source, has substantially enriched the phenomenological landscape of orders of magnitude in wavelength, positioning and time starting a new season of theoretical interpretations. The perspectives of this research of BeppoSAX and the new contribution of other missions, with particular regard to RossiXTE, are also discussed.

1. GRB Astrophysics before BeppoSAX

Accidentally discovered in 1973 [12] G a m m a Ray Bursts (GRB) are probably the most dra- matic phenomenon originating from non solar as- trophysical objects. In a detector of Hard X/Soft G a m m a Rays (such as a NaI(T1) scintillator) put on a platform orbiting outside Ear th atmosphere, the counting rate from a GRB can exceed of an order of magni tude the counting rate from the (huge) particle and electromagnetic radiation background. In spite of this prominence GRBs have been for 25 years one of the most intriguing subjects in High Energy Astrophysics[15,16,36]. This contradiction is not the result of a cosmic conspiracy but the straightforward consequence of instrumental limits. In a domain where optics have been (so far) not applicable, Compton ef- fect is significant or dominant , the positioning of a source is based on the restriction of the field of by collimating or anticollimating devices more or less coded. This is almost impossible for sources which can be everywhere in the sky and tha t fade and disappear after few seconds from their

*On behalf of the BeppoSAX Team

0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0920-5632(98)00315-6

appearance. As a consequence the hope to per- form a fast positioning of GRBs has been mainly commit ted to their possible detection in other energy ranges. A wise, alternative method has been tha t of positioning the burst with the re- spective delays of detection from many satellites on a very long baseline (Interplanetary Network, IPN). This method arrived to a good efficiency in the late eighties, when G a m m a Burst detectors aboard various satellites, some of which devoted to interplanetary research, connected in the Third Interplanetary Network produced a first rich cat- alogue[7] of small (10 - 40 arcmin 2) error boxes.

The quest in other wavelengths for sources in the smallest of these error boxes was long. In particular a few candidate sources were found by deep Einstein and ROSAT pointing but the more important result was in terms of upper limits se- riously constraining all the interpretat ion of future single measurements. Upper limits are often neglected after the first positive detection of a new phenomenon. Actually the comparison with these da ta was relevant also to interpret the first uncertain outcome of BeppoSAX and to prevent overinterpretation of the first da ta from pointing

E. Costa~Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 646-655 647

GRB fields. The lack of any conclusive result in the localization of the single GRB was only a par t of the story. An impressive collection and interpretat ion of da ta from numerous missions and an overwhelming theoretical effort were spent to t ry to solve the mystery of GRBs. A systematic review is not the goal of this presentation. Of course positioning of the bursts was one of the various approach. Another as well important is the study of spatial distribution, which is usually a good indicator of the nature of the sources. Since the beginning the discussion on the nature of GRBs has divided the part isans of the galactic models from those of the extragalactic ones. This dia- tribe closely reminds that on the distance of the spiral shaped nebulae in the first 20 years of the century. For GRBs the part isan of the galactic location have been for long time the majori ty but the positions have reversed with time and new data. After a long collection of da ta from the Pioneer Venus Orbiter, PVO the breakthrough came when the Burst And Transient Source Ex- periment on the Gamma-Ray Observatory, designed to have a high sensitivity and a uniform coverage of the sky, has detected so far about two thousands GRBs. But since the first cata- logues three outstanding results where achieved: i) GRB position distribution is isotropic in the sky. ii) GRB luminosity distribution is cut-off at low luminosity instead of growing indefinitely to the instrumental limit, as expected from a homo- geneous distribution in Euclidean tridimensional space, iii) GRBs never come twice from the same direction, i) and iii) are true only after remov- ing the Soft G a m m a Repeaters from the list. ii) means tha t the GRB sources have a spherical dis- t r ibut ion and we are in the center of the sphere. The combination of i) and ii) requires tha t the sources are at least in an extended halo of 100 kpc radius. Another explanation that requires less ad hoc hypothesis is that the bursts come from sources distr ibuted in the universe at dis- tances where the redshift effects are significant. The apparent luminosity distribution would be due to a uniform distribution in an expanding universe, possibly combined with evolution effects. Although extragalactic hypothesis copes in ~ more straightforward manner the BATSE data

galactic models were still very popular through 1996. This means that , although BA TSE data put strict constraints to any model, the solution of tile mystery still largely relies on the direct identification of sources connected to the GRB. Be- ing elusive the identifications so far performed of GRBs with permanent sources (new or old), the quest was oriented to the fast follow-up observations in other wavelengths. Small size telescopes were equipped with automat ic pointing systems in order to grasp a possible new fading source a few minutes after the GRB. A new IPN was in the making with the contribution of an experiment at high ecliptic lati tude onboard Ulysses and a GRB detector aboard Mars96. But due to the intrinsic slowness of the IPN (more due to management reasons than to technical ones), expectations where mainly concentrated on the HETE satellite, equipped with a small Gamma- Ray Burst Monitor, wide field optical and X-ray instruments and a dedicated fast download system for the coordinates of a burst.

2. BeppoSAX

Tile Beppo Satellite di Astronomia X[3] has some of the features needed to this search combined aboard the same satellite. The Wide Field Cameras of BeppoSAX[26] have the capabili ty to locate a source with a precision of 3 to 5 arcminutes depending on the source intensity and position in the field of view. The G a m m a - R a y Burst Monitor[33,10] has the capability to detect aboard a GRB and trigger a fast da ta acquisition. Actually this is not a self standing experiment. Four anticoincidence detectors of the Phoswich Detector System experiment [22], have been equipped with a dedicated elec- tronics but no requirements (e.g. of clearance) have been put on the rest of the satellite. Since the PDS is the center of a complex payload [41,2] the exposition to different sky directions is irregular. Thence BeppoSAX cannot contribute to the study of isotropy. The directions parallel to the axis of the two WFCs are relatively clean since only the carbon-fiber sustain of the LECS-MECS telescopes obstruct tile GRBM field of view. On

648 E. Costa~Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 646-655

Table 1 Summary of the BeppoSAX NFI observation of GRBs. Target of Oppor tun i ty Pointing

TOO Flux refers to the Average in the first

GRB G a m m a - r a y X- ray BeppoSAX Peak Flux Peak Flux TOO Flux

erg cm2 s -1 erg cm2 s -1 GRB960720 1.7 x 10 - s 2.5 x 10 -8 - GRB970111 5.6 x 10 - s 1.4 x 10 -7 1.2 x 10 -13 GRB970228 3.7 x 10 -6 1.4 x 10 -7 3.0 x 10 -12 GRB970402 3.2 x 10 -7 1.6 x 10 -8 2.0 x 10 -13 GRB970508 5.6 x 10 -7 3.5 x 10 - s 6.0 x 10 -13

X- ray Optical Radio Transient Transient Transient

Y N N Y Y N Y N N Y Y Y

the reverse since the photons from a source in the field of a WFC arrive almost orthogonal to the corresponding detector the coincidence of a signal from two detectors, which is one of the trigger conditions, is less easily fulfilled. But with its detectors of 1050 cm 2 is probably the most sensitive G a m m a Burst Monitor, beside BATSE. To unders tand some other peculiarities of Bep- poSAX for a GRB program we must outline some positive and some negative da ta of the mission. i) BeppoSAX records all da ta aboard. Downloads them in Malindi, where from they are forwarded to Roma. This means tha t before the Quick Look Analysis is performed on any BeppoSAX data a t ime from 2 to 4 hours from the aboard detection is elapsed. This is negative for GRB research. ii) The GRBM and WFC are mounted on the same platform: this means tha t they have all the same constraints (Earth occultation, South Atlantic Geomagnet ic Anomaly passage, Satel- lite failures or maintenance interruptions). This makes the overlap of the two very high. iii) The GRBM and WFC are analysed from the same Sci- ence Operat ion Center where all the da ta poten- tially useful for the decision are present simulta- neously, iv) One of the most powerful tool for a follow-up observations, the package of BeppoSAX Narrow Field Instruments is present aboard the same satellite and under the same management is the package tha t quickly detected the GRB. While i) makes BeppoSAX inadequate for detec- tions as fast as those foreseen for HETE, ii) and most of all iii) have allowed for a search faster and faster culminated with the detection of an X-ray afterglow.

The real breakthrough of BeppoSAX is based on the pinpointing capability of Wide Field Cam- eras. Actually BeppoSAX is the first real ex- ploitation of the technique of Coded Masks in the X-ray range, namely the first mission tha t uses a Wide Field Camera with a suitable allocation of weight, power and telemetry resources, with a good priority rat ing and with a long observing program. A frequently asked question is whether the GRBM trigger is really needed and WFC could detect GRBs by itself. The answer is complex. The WFCs of BeppoSAX are a fully passive de- vice. Photons are marked with the energy and (one out of four) time, downloaded as a list to- gether with ratemeters and housekeeping data. No image or t ime series analysis is performed onboard. Until now no GRB has been detected by WFCs without a trigger by GRBM or by BATSE. This is mainly because GRBM has been conceived as an instrument to provide a real t ime trigger- ing capability while WFCs are not. This is also because the signal is much more outstanding (in terms of s tandard deviations) in 7 than in X- rays and, moreover the astronomical sources of false triggers are much stronger in X-rays than in 7. But for sure most of these problems can be worked out in onground software and we can hope that sometimes the first X-ray self triggered GRB will arrive.

E. Costa~Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 646-655 6 4 9

3. T h e G a m m a R a y B u r s t s o b s e r v e d b y BeppoSAX

BeppoSAX was launched on May 1st 1996. Photomult ipl iers of the Lateral Shields of Phoswich Detector System were fully operative on June. During the Commissioning Phase priority was given to the checks of the main function of the detectors as anticoincidence of the PDS. A first analysis of da ta from the GRBM was s tar ted in August and by the end of this month a GRB oc- curring in both GRBM and Wide Field Cameras was found: GRB960720. Since this moment the GRBM program of BeppoSAX proceeded on two parallel rails. On one side the "regular" work to arrive to the opt imal performance of the instrument included the choice of the optimal parame- ters for the GRBM (long integration time, short integration time, threshold). On the other side a detailed analysis of the available instrumenta- tion, software and of the operation sequences was s tar ted aimed to define the optimal procedure for the identification of a GRB in the WFCs start ing from a trigger of the GRBM and to point as soon as possible NFIs to the position of the GRB. In Table 1 I review the GRBs detected by G R B M / W F C s by November 1997. GRB960720 was found during the Science Veri- fication Phase. The field was pointed with NFI about 5 weeks after. A few faint sources, typical of any SAX pointing, so no clear candidate as an associated source was found. After the improve- ment by Wide Field Cameras a bright, radio-loud, quasar 4C49.29 (z=1.038) was found in the error box[53] with a faint X-ray emission. But, since no such a source was ever present in any previous or subsequent error box, it is likely that the association is serendipitous, unless we consider it as the tracer of an host. Beside this result, the simultaneous detection of this GRB from WFCs and GRBM has allowed for a broad band, t ime resolved spectral analysis[37,39]. The GRB is a typical single pulse with fast rise t ime and ex- ponential decay. The X-ray pulse is slower and longer more and more at lower energies. The duration (fig.l) of the pulse is f(E) ,-. E -0'46. This is consistent with what observed at higher energies[13] and is the clear signature of radiative

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cooling by synchrotron losses[45]. GRB970111 is the first GRB tha t was pointed

in a short t ime (about 16 hours). T h e X-ray flux is as well delayed and slower than he Gamma-ray flux (fig.2) so tha t the general t rend is to a soft- ening of the spectrum. Many flares are present and in coincidence with them the spect rum be- comes harder again. The search fo r an associated source with NFIs[14] was somehow deceiv- ing since it is the strongest of the GRBs detected in the WFCs. At the t ime the procedure to re- construct error boxes with WFCs da ta was not yet well assessed and when the error box arrived to its final dimensions (about 7 arcmi:n2), only a faint source, 1SAX J1528.1+1937 was included in it, that faded along the pointing. T h e source has a probabili ty in the 1070 range to be a field source and is at the edge of the IPN annulus made with BATSE/SAX/UIysses data. But the: fading behaviour, on the basis of the present knowledge on afterglow sources encourages (but does not certi- fies), the identification of the source as the rem- nant of GRB970111. GRB970228 marked the turning point of the GRB


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Figure 3. Temporal evolution of the u f u distribution for GRB970228 A and B are the first pulse C to G the following train of pulses (from [21]

Astrophysics since it was the first GRB with a clearly detected X-ray afterglow [8,9]. The narrow field instruments pointed to the GRB field 8 hours after the GRB detected a relatively bright source SAX J0501.7+1146 that faded during the observation. The source was pointed again for about 10 days by BeppoSAX, by ASCA[52] and ROSAT[20]. The source faded following a power law Kt -1"33 ( see the figure in[39]). Moreover the extrapolation of the decay curve to the time of the GRB is consistent with the X-ray flux in the train of three pulses following the first main pulse. A time resolved spectral analysis has been performed[21]. The first pulse quickly evolves from a very hard to a soft spectrum as in the GRB960720 with a"bending" of the spectrum that could cor- respond to the break of the synchrotron spectrum entering in the range(fig.3). The spectrum of the following three pulses is hard (flat in the u f u distribution). In the band 2 to 10 keV where MECS and WFCs overlap we can compare the

spectrum during the pulses following with the afterglow. This is a power law with photon index of 2.1 4- 0.3: surprisingly not only the last part f the GRB is in the same decay curve of the afterglow but also all data from 5 s to 10 days are compatible with a constant hard power law spectrum. If we assume this continuity, namely that the three pulses are the beginning of the afterglow, and integrate to infinity we find a total energy comparable to that of the main ")'-ray event. This means that the afterglow is not a simple re- verberation of the first burst but is dissipating a relevant fraction of the total energy.

GRB970402 is the less bright of the bursts detected by GRBM/WFCs. It lasted about 50 seconds and no strong evidence was found of spectral evolution. In r iga I show the spectrum of the burst obtained by combining WFCs and GRBM data. Data are fitted in the 3-700 keV band with a single power law with photon index -1.4. The burst field was pointed in about 8 hours with


ORB970402 SPECTRUM

loo Energy (keV)

Figure 4. The broad band spectrum of GRB970402

NFIs and a fading afterglow source was found. The overall behaviour resembles the GRB970228 scaled of an order of magnitude.

GRB970508 is the burst tha t was pointed in the shortest t ime with NFIs and tha t was a further shock again any illusion tha t GRB afterglow phenomenology could be less fuzzy than tha t of the main "), events. The burst is intrinsically faint and simply structured. The source[38,1] 1SAXJ0653.8+7916 was found but it did not show a monotone decay. During the first TOO pointing, when the statistics is better, the source decays and, at the middle of the observation, flares again. In the second pointing, 3 days after the burst , the source was still relatively bright, then in the two last pointing the source decayed much faster. Actually the source seems to burst repeatedly around a template power law decay. Also from the spectral point of view this burst shows some significant difference from GRB970228 as its spectrum softens at the and of the observation[40]. Time histories of the different afterglow sources is shown in fig.5. The first result of the comparison is tha t afterglow curves have different slopes.

The hierarchy of luminosity itself afteT one day is much different than tha t 100 s after the GRB. In the plot we displayed at the GRB t ime the flux averaged on the GRB duration. But also if we take the GRB peak flux as quoted in TAB.1 we do not see an evident correlation with the afterglow. On the contrary if we look to the X-ray peak flux of the main event we see t h a t bursts with strong afterglow were already rich in X-ray content at the main event time. In the discussion of the afterglow decay the role of GRB970508 is somehow peculiar: and this im- pacts on the discussion on the optical afterglow source. From the statistical point of view we eval- uate to 10 -3 the probabili ty of a serendipitous detection of a source of this intensity in the WFC error box. This should be considerably decrease if we consider the fading behaviour of the source.

4. Follow-up in other wavelengths

An alert system was set up during the develop- ment of the program. Started in a rear manner after GRB960720 it arrived to a good level of efficiency during GRB970508. As mentioned before the expectations on the sources in other wavelength to be associated with the GRB was either oriented to the search of an optical flash in a few seconds or minutes after the GRB or on the search of faint persistent sources to be positionally associated. Also in this domain BeppoSAX results triggered observations yielding surprising results. The fast detection and imaging of GRB970228 triggered many optical and radio observations. These where significantly accelerated when the X-ray transient was found. By comparing two plates taken at the William Herschel Telescope a fading optical source was found[47]. The collection of all da ta show a power law decay with a possible flat start[24]. Tile WHT and Keck observed and lIST comfirmed[43,23] the presence of a nebulosity. This was suggested immediately as an irregular host galaxy. The field of GRB970508 was pointed in a very short time[34]. A transient source was found[4] with a not monotone behaviour (similar to the X-ray afterglow). The source was carefully mon-

652 E. Costa/Nuclear Physics B (Proc. Suppl.) 69/1-3 (1998) 646-655

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the GRB fluenre and duration

itored[ll.l,l], lq,ck porfi,rmed a slwctrum[31] showing two al,sort)~i,m fi,ature.s at rt~(lshift of 0.835 and 1).767 respectively. This would clo~ the del,ato on tit,, distant,, scale ~f GRBs unless w(" think lh:tl fJlo s,~11r('t-, (tlt;tt. in f ~ t ]I;L~ a be- |l;IViOtlr dif~(,ront froll! otllpr aftorghm's) is jllS! a

field ~rendipitous source. As I ~ d befi,re the probability is not v,.,ry high. A further develop- ment is the finding with VLA of a h in t , quickly wLriable radio source{18]. If this is interpreted ;L~ scintillation data wouht support the presence an expastding small object. But detection of the


source in the m m range[5] does not support this interpretation. H S T observation[35] showed the absence of a host nebular object down to very deep level. In other terms the two detected optical transients showed two situations very different. More- over, the lack of detection from GRB97011116] and GRB970402 supports the idea that optical afterglows may be absent in some cases (maybe for absorption) or follow different decay laws.

5. W h a t w e k n o w m o r e o n X - r a y e m i s s i o n

f r o m G R B s

Results from BeppoSAX, other satellites and telescopes have produced a flood of interpretation activity tha t cannot be accounted here. From the first days the difficulty for some scenarios to match da ta was outlined. For instance the mis- matching of the afterglow curve with the cooling neutron star[46]. The theory of internal and ex- ternal shocks[30,48], was assessed to fit the newly discovered timescale[50,27]. With the strong pre- vailing of extragalactic distance scale the fireball models where most developed an, in particular the a t t empt to match experimental data, with models of NS-NS or NS-BH, or with the inter- action of the shocks with the interstellar medium [49,28]. Another relevant discussion s tar ted mainly from the optical da ta is ne the "host" . While the nebulosity detected around GRB970228 could be con- sidered the host of the burst the lack of an host in GRB970508 was somehow intriguing if coupled with the measured redshift[35] suggested the association of the GRB with Starburst Galaxies to justify a potential very faint object. On the other side the possibility that NS-NS mergers receive such a kick from the second collapse to drift out of galaxies in a t ime shorter than the merging t ime was also outlined to support the opposite thesis that GRBs do not occur within galaxies or tha t GRB are not mergers. The presence of optical transient only in a fraction of GRBs has been interpreted as the evidence of the existence of hypernovae[32] located in dense star forming regions where the optical afterglow could suffer a high extinction.

I want just remind what are experimental facts in this new GRB afterglow science. 1) P rompt X-ray emission may be a very different fraction of G a m m a - R a y emission (few % to 10%) and has typically lower t ime constants. 2) Some bursts (100%?) have afterglow X-ray sources. The total energy dissipated in the afterglow could be of the same order of the energy in the main gamma event.

3) The afterglow sources fade according to a power law: f ( t ) ~ t -~ with a in the range 1.1 to 1.5. 4) The GRB slowly evolves into the afterglow. No gap seems present between the two. 5) The GRB may show flaring or bursting activity after the first main event. The power law index in 3) is a template. 7) Anyway the power law index is different from a GRB to another. 8) The afterglow shows a power law spectra with a photon index of the order of 1.4 - 2.0. Black Body models are not supported by data.. 9) The afterglow spectrum may be constant over a long t ime (10d). 10) The afterglow spectra may also change. 11) Some bursts (50%?) with X-ray afterglow have an optical afterglow. One of these optical transients has a nebular host; the other one has redishifted absorption t~atures. The lat ter has also a radio afterglow. 12) We have no evidence of a change of slope or of a stop of the decay curve. Most of these s tatements are based on a marginal statistics. A few on one case only. But da ta from ASCA, XTE and ROSAT completely confirm the BeppoSAX data. GRB970828 is apparent ly quite similar to BeppoSAX afterglow sources. Some of the results can in the future be questioned or improved the general picture will hold.

6. T h e f u t u r e o f B e p p o S A X a n d O t h e r M i s s i o n s

Unfortunately a few days after the observation of GRB970508 a gyro of the BeppoSAX Atti tude Control System went out of order. BeppoSAX did no science until September when a new pointing mode was made operative. Now SAX capabilities


are comparable to the previous ones. With respect to the expectations of the GRB Community before BeppoSAX launch two major negative facts occurred: i) the failure of HETE ii) the failure of Mars96 But this does not mean that the search of X- ray afterglow sources is completely committed to BeppoSAX. In fact the detection of the X-ray afterglows, fixing a time scale different from that of most predictions, suggested that existing resources could be converted to this search of GRB remnants. The GRB Coordinates Network was set up trans- ferring the function of refining the localization of the GRBs from an off-line activity to a fast one. Coordinates are now available and distributed few minutes after the GRB. The Rossi X-Ray Timing Explorer can be pointed to the direction and scan the error box with PCA collimator. Morover the All Sky Monitor aboard the same satellite are now checking BATSE-GCN boxes to search for bursts in the field of view of the experiment. In this case an X-ray position of the same quality of that of BeppoSAX WFCs can be achieved. The best result was achieved with GRB970828 that was pin- pointed by ASM and tracked by ASCA one day after the GRB for one more day[51]. This mea- surement was very important since ASCA with its large collecting area could detect an impressive flare with evidence of spectral change, on the scale of 4000s. No optical transient was detected in this case down to the 24 V magnitude. More results can be expected from further combination of data of BeppoSAX, ASM, BATSE and from an improved rapidity of IPN. In the future the launch of HETE-H with a X / G a m m a sensitivity lower than BeppoSAX but with much wider field of view, the presence of optical sensors and designed on the basis of a fast alert system will give the high throughput needed for a systematic study[42], but we can expect that before that day a few tens of GRBs will be observed by the existing satellites.. But what do we really expect from these measurements? Leaving the door open to the possibility of completely new and unexpected discov- eries I can t ry to summarize which are the major open questions immediate consequence of the last

months results. 1) Fill the gap from the end of GRB to the first pointing of telescopes. Wide field cameras can follow the fading burst down to about 1×10 - l ° ergcrn-2s -1 and this level is arrived by the GRB in a few minutes. The NFI, pointed to the GRB in about 6 to 8 hours can expect to detect fluxes of the order of a few × 10 -12 ergcrn-2s -1. What happens in between is of the highest importance. Data from all the GRBs suggest that the GRB is evolving, continuously or by a sequence of flares, into the afterglow. But this is far to be demonstrated and the inter- polated piece of the light curve is a good deal of the involved energy. Two ways can be followed to fill this gap. a) BeppoSAX digging data of WFCs (in progress but depends on the position of the burst in the orbit), b)Faster X-Ray TOOs: X T E / P C A can, BeppoSAX (with the one-gyro pointing mode) probably can only in very particular cases. 2) See more bursts by a more combinations of satellites SAXGRBM-WFC BATSE-SAXWFCs, BATSE-ASM, SAXGRBM-ASM.. A few could arrive from BATSE/PCA selected in intensity. A few more can arrive from OSSE. Some may jump out from the past satellites archives (e.g.a GRB was recently identified in HEAO1 data). 3) Check the reality of the flaring and spectral evolution behaviour of afterglow. A strong afterglow is needed to BeppoSAX. ASCA can do much better, of the orbit (15 to 20%) To resume the capabilities of BeppoSAX to con- tinue the GRB afterglow science is comparable to the one in the first part of 1997.

7. A d d e d N o t e

After the Conference and before the Final Re- lease of these Proceedings three GRBs have been detected by BeppoSAX WFCs. GRB971214, GRB971227, GRB980109. The first two have been pointed with NFIs. In the error box of GRB971214 a bright afterglow source was detected and an optical transient, much redder than GRB970228 and GRB970508 OTs, of R magnitude 22.1 fading of about 2 magnitudes in two days[25] and suspect redhift and host[29]. In the


field of 971227 a faint afterglow candidate source was detected by BeppoSAX while no optical transient was found. The position of GRB980109 was poorly known (10 arcminutes radius) so that no pointing of NFI was started. These data is enriched but not upset the picture. We can state that BeppoSAX detected afterglow sources or likely candidate in all the GRB fields pointed with NFIs. We can also say that one burst out of two has an optical Transient detectable at a level of 22d magnitude.

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BeppoSAX observation of the X-ray emission of gamma-ray bursts

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