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We describe the AGILE gamma-ray astronomy satellite which has recently been selected as the first Small Scientific Mission of the Italian Space Agency. With a launch in 2002, AGILE will provide a unique tool for high-energy astrophysics in the 30 MeV - 50 GeV range before GLAST. Despite the much smaller weight and dimensions, the scientific performances of AGILE are comparable to those of EGRET., Comment: 7 pages, latex, 4 figures, Invited paper presented at the GeV-TeV Gamma-ray Astrophysics Workshop "Towards a Major Atmospheric Cerenkov Detector VI'' - Snowbird, Utah - August 13-16, 1999

We investigated the spectral evolution in the 2--700 keV energy band of Gamma-Ray Bursts (GRBs) detected by the Gamma-Ray Burst Monitor (GRBM) and localized with the Wide Field Cameras (WFCs) aboard the BeppoSAX satellite before May 1998. Most of them have been followed-up with the Narrow Field Instruments aboard the same satellite. In the light of these results we discuss open issues on the GRB phenomenon. We find that the optically thin synchrotron shock model (SSM) provides an acceptable representation of most of the time-resolved GRB spectra extending down to 2 keV, except in the initial phases of several bursts and during the whole duration of the quite strong GRB970111, where a low-energy photon depletion with respect to the thin SSM spectrum is observed. We find that the X-ray afterglow starts at about 50% of the GRB duration, and that its fluence, as computed from the WFC light curve, is consistent with the decay law found from the afterglow NFI observations. We also investigate the hydrodynamical evolution of the GRB in our sample and their associated afterglow, when it was detected. We find that the photon index of the latest spectrum of the GRB prompt emission is correlated with the index of the afterglow fading law, when available, as expected on the basis of an external shock of a relativistic fireball., Comment: 35 pages, 1 LaTeX file, 20 postscript figures, 1 postscript table, accepted for pubblication in Astrophysical Journal Supplement Series. Corrected error bars in Fig.2/GRB980425/panel B and GRB980425 fluence in Tab. 3

We present BeppoSAX follow-up observations of GRB980425 obtained with the Narrow Field Instruments (NFI) in April, May, and November 1998. The first NFI observation has detected within the 8' radius error box of the GRB an X-ray source positionally consistent with the supernova 1998bw, which exploded within a day of GRB980425, and a fainter X-ray source, not consistent with the position of the supernova. The former source is detected in the following NFI pointings and exhibits a decline of a factor of two in six months. If it is associated with SN 1998bw, this is the first detection of X-ray emission from a Type I supernova above 2 keV. The latter source exhibits only marginally significant variability. The X-ray spectra and variability of the supernova are compared with thermal and non-thermal models of supernova high energy emission. Based on the BeppoSAX data, it is not possible to firmly establish which of the two detected sources is the GRB X-ray counterpart, although probability considerations favor the supernova., Comment: 16 pages, Latex, 6 PostScript figures and 1 GIF figure, 2 tables, submitted to The Astrophysical Journal

Simultaneous X-ray (BeppoSAX, 0.1-300 keV) and radio (Ryle Telescope, 15 GHz) observations of the superluminal galactic source GRS 1915+105 in high flux state were carried out starting October 1996. Here we present the simultaneous radio/X-ray light curves of the flare event and the spectral and timing analysis of the relevant BeppoSAX data. We obtain evidence for a temporary disappearance and subsequent restoring of the inner portion of the accretion disk during the flare from the spectral analysis of the X-ray data. We find a variable power law photon index positively correlated to the variability of the exponential cut-off energy. In addition, frequency and intensity variations of a $\sim$4-5~Hz quasi-periodic oscillation (QPO) are detected during the same time interval. The centroid frequency of the QPO is found to be correlated to the spectral power law photon index, and to the X-ray flux from the disk and the high energy flux from the power law. These observational results are discussed in the framework of the disk-instability model proposed for this source., Comment: 10 pages, 9 figures; A&A Main Journal in press

We have added the BeppoSAX Gamma-Ray Burst Monitor to the 3rd Interplanetary Network of burst detectors. We analyze 16 bursts whose positions are known to good accuracy from measurements at other wavelengths. We show that there is excellent agreement between the Ulysses/BeppoSAX triangulation annuli and the known positions of these events, and that these annuli can in many cases provide useful constraints on the positions of bursts detected by the BeppoSAX Wide Field Camera and Narrow Field Instruments., Comment: 16 pages, 18 figures, submitted to the Astrophysical Journal

We report the possible detection (99.3% of statistical significance) of redshifted Fe iron line emission in the X-ray afterglow of Gamma-ray burst GRB970508 observed by BeppoSAX. Its energy is consistent with the redshift of the putative host galaxy determined from optical spectroscopy. In contrast to the fairly clean environment expected in the merging of two neutron stars, the observed line properties would imply that the site of the burst is embedded in a large mass of material ($>0.5 \Ms$), consistent with pre-explosion ejecta of a very massive star. This material could be related with the outburst observed in the afterglow 1 day after the GRB and with the spectral variations measured during this phase. We did not find evidence of Fe line in two other GRB with known redshift (GB971214 and GB980613), but we note that the upper limits are of the same order of the intensity measured in GB97508 and that none of these afterglows shows rebursting activity., Comment: 2 pages, 1 figure, proc. of Gamma-Ray Bursts in the Afterglow Era, Nov. 1998, Rome, F. Frontera & L. Piro ed.s., A&A Suppl. Ser., in press

We report optical observations of the GRB 980329 error box which represent the second earliest detection of the GRB 980329 optical transient. We determine R = 23.7 $\pm$ 0.5 mag on 29.89 March, which is consistent with R = 23.6 $\pm$ 0.2 mag as reported by Palazzi et al. (1998) on 29.99 March. Based on extrapolations of the light curve we claim that the R-band magnitude of the GRB 980329 host galaxy should lie in the range 26.8 mag $<$ R $<$ 29 mag. We also discuss the similarities with GRB 970111., Comment: 4 pages with three encapsulated PostScript figure included. Accepted for publication as a letter in Astronomy & Astrophysics

Cosmic gamma-ray bursts (GRBs) have been firmly established as one of the most powerful phenomena in the Universe, releasing electromagnetic energy approaching the rest-mass energy of a neutron star in a few seconds. The two currently popular models for GRB progenitors are the coalescence of two compact objects (such as neutron stars or black holes) or collapse of a massive star. An unavoidable consequence of the latter model is that a bright supernovae should accompany the GRB. The emission from this supernova competes with the much brighter afterglow produced by the relativistic shock that gives rise to the GRB itself. Here we present evidence for an unusual light curve for GRB 980326 based on new optical observations. The transient brightened ~3 weeks after the burst to a flux sixty times larger than that extrapolated from the rapid decay seen at early time. Furthermore, the spectrum changed dramatically and became extremely red. We argue that the new source is the underlying supernova. If our hypothesis is true then this would be the first evidence for a supernova connection with GRBs at cosmological distances. We suggest that GRBs with long durations are associated with death of massive stars., Comment: Submitted to Nature (restricted embargo). Sixteen pages of TeX. Uses macros reforder.tex, citmac.tex and psfig.sty (included). Three Postscript figures

We report on X-ray, optical and infrared follow-up observations of GRB 980703. We detect a previously unknown X-ray source in the GRB error box; assuming a power law decline we find for its decay index alpha < -0.91 (3-sigma). We invoke host galaxy extinction to match the observed spectral slope with the slope expected from `fireball' models. We find no evidence for a spectral break in the infrared to X-ray spectral range on 1998 July 4.4, and determine a lower limit of the cooling break frequency: nu_c > 1.3 x 10^{17} Hz. For this epoch we obtain an extinction of A_V = 1.50 +/- 0.11. From the X-ray data we estimate the optical extinction to be A_V = 20.2 +12.3 -7.3, inconsistent with the former value. Our optical spectra confirm the redshift of z = 0.966 found by Djorgovski et al. (1998). We compare the afterglow of GRB 980703 with that of GRB 970508 and find that the fraction of the energy in the magnetic field, epsilon_B < 6x10^{-5}, is much lower in the case of GRB 980703, which is a consequence of the high frequency of the cooling break., Comment: Accepted for publication in Part 1 of The Astrophysical Journal

Over a total of 20 gamma-ray bursts localized with arcmin accuracies, GRB980519 represents the 13th detected by the BeppoSAX Wide Field Cameras. An X-ray TOO observation performed by the BeppoSAX Narrow Field Instruments, starting about 9.5 hours after the high energy event, revealed X-ray afterglow emission in the 0.1--10 keV energy range. The flux decay was particularly fast with a power-law index of ~ 1.8. This is the fastest decay so far measured. Signs of bursting activity are evident. The power-law spectral index of 2.8 (+0.6, -0.5) is quite soft but not unique among GRB afterglows., Comment: 2 pages; 3 EPS figures; requires aa.cls, psfig.sty; accepted for publication on A&AS (Proc. of ``Gamma-Ray Bursts in the Afterglow Era'', Rome 3-6 Nov. 1998.)

We report the discovery of a radio counterpart to GRB 990123. In contrast to previous well-studied radio afterglows which rise to peak flux on a timescale of a week and then decay over several weeks to months, the radio emission from this GRB was clearly detected one day after the burst, after which it rapidly faded away. The simplest interpretation of this ``radio flare'' is that it arises from the reverse shock. In the framework of the afterglow models discussed to date, a forward shock origin for the flare is ruled out by our data. However, at late times, some radio afterglow emission (commensurate with the observed late-time optical emission, the optical afterglow) is expected from the forward shock. The relative faintness of the observed late-time radio emission provides an independent indication for a jet-like geometry in this GRB. We use the same radio observations to constrain two key parameters of the forward shock, peak flux and peak frequency, to within a factor of two. These values are inconsistent with the notion advocated by several authors that the prompt optical emission detected by ROTSE smoothly joins the optical afterglow emission. Finally, with hindsight we now recognize another such radio flare and this suggests that one out of eight GRBs has a detectable radio flare. This abundance coupled with the reverse shock interpretation suggests that the radio flare phenomenon has the potential to shed new light into the physics of reverse shocks in GRBs., Comment: pages including 2 figures. Accepted by the Astrophys. J. (Letters)

We present BeppoSAX GRBM and WFC light curves of GRB980425 and NFI follow-up data taken in 1998 April, May, and November. The first NFI observation has detected within the 8' radius error box of the GRB an X-ray source positionally consistent with the supernova SN 1998bw, exploded within a day of GRB980425, and a fainter X-ray source, not consistent with the position of the supernova. The former source is detected in the following NFI pointings and exhibits a decline of a factor of two in six months. If it is associated with SN 1998bw, this is the first detection of hard X-ray emission from a Type I supernova. The latter source exhibits only marginally significant variability. Based on these data, it is not possible to select either source as a firm candidate for the GRB counterpart., Comment: 2 pages, 1 PostScript figure, submitted to A&AS, Proc. of the Conference "Gamma-Ray Bursts in the Afterglow Era", held in Rome, 1998 November 3-6. Results concerning 'Source 2' have been updated

Afterglow, or long-lived emission, has now been detected from about a dozen well-positioned gamma-ray bursts. Distance determinations made by measuring optical emission lines from the host galaxy, or absorption lines in the afterglow spectrum, place the burst sources at significant cosmological distances, with redshifts ranging from ~1--3. The energy required to produce the bright gamma-ray flashes is enormous: up to ~10^{53} erg or 10 percent of the rest mass energy of a neutron star, if the emission is isotropic. Here we present the discovery of the optical afterglow and the redshift of GRB 990123, the brightest well-localized GRB to date. With our measured redshift of >1.6, the inferred isotropic energy release exceeds the rest mass of a neutron star thereby challenging current theoretical models for the origin of GRBs. We argue that the optical and IR afterglow measurements reported here may provide the first observational evidence of beaming in a GRB, thereby reducing the required energetics to a level where stellar death models are still tenable., Comment: Revised version (27 Feb 99). 19 pages, 3 figures. Original version submitted to Nature 2/11/99. Uses macros reforder.sty and citmac.sty (supplied)

We present deep images of the field of gamma-ray burst (GRB) 990123 obtained in a broad-band UV/visible bandpass with the Hubble Space Telescope, and deep near-infrared images obtained with the Keck-I 10-m telescope. Both the HST and Keck images show that the optical transient (OT) is clearly offset by 0.6 arcsec from an extended object, presumably the host galaxy. This galaxy is the most likely source of the metallic-line absorption at z = 1.6004 seen in the spectrum of the OT. With magnitudes V_{C} ~ 24.6 +/- 0.2 and K = 21.65 +/- 0.30 mag this corresponds to an L ~ 0.7 L_* galaxy, assuming that it is located at z = 1.6. The estimated unobscured star formation rate is SFR ~ 6 M_sun/yr, which is not unusually high for normal galaxies at comparable redshifts. The strength of the observed metallic absorption lines is suggestive of a relatively high metallicity of the gas, and thus of a chemically evolved system which may be associated with a massive galaxy. It is also indicative of a high column density of the gas, typical of damped Ly-alpha systems at high redshifts. We conclude that this is the host galaxy of GRB 990123. No other obvious galaxies are detected within the same projected radius from the OT. There is thus no evidence for strong gravitational lensing magnification of this burst, and some alternative explanation for its remarkable energetics may be required. The observed offset of the OT from the center of its apparent host galaxy, 5.5 +/- 0.9 proper kpc (projected) in the galaxy's rest-frame, both refutes the possibility that GRBs are related to galactic nuclear activity and supports models of GRBs which involve the death and/or merger of massive stars. Further, the HST image suggests an intimate connection of GRB 990123 and a star-forming region., Comment: Updated references. 12 pages including 3 Postscript figures. Camera- ready reproductions of the figures can be found at http://astro.caltech.edu/~jsb/GRB/grb990123.html

We report the detection by the Gamma Ray Burst Monitor onboard BeppoSAX of the strongest and longest ever detected outburst from SGR 1900+14. Oscillations are detectable with a period of ~5.16 s for the entire duration of the event ~300 s. The temporal analysis reveals also a remarkable periodic substructure: after about 35 s from the event onset each 5.16-s pulse shows a pattern of four subpulses and a dip, each separated by ~1 s. Significant spectral variation is detected during the event and for each individual oscillation. The first and most intense part of the outburst is quite hard, and similar to what previously detected from the `March 5th event'. A hard non-thermal spectral component persists for ~200 s. SGR 1900+14 was proposed to be a strongly magnetized neutron star (B>10^14 G) undergoing violent instabilities by internal magnetic/crustal stresses. However, the onset of an apparent 1-s periodicity within the 5.16-s pulsations and the observed spectral properties show a complex behaviour that is not satisfactorily modelled yet., Comment: 16 pages, 3 figures, TeX uses aasms4.sty; ApJ Letters in press

We report the possible detection (99.3% of statistical significance) of redshifted Fe iron line emission in the X-ray afterglow of Gamma-ray burst GRB970508 observed by BeppoSAX. Its energy is consistent with the redshift of the putative host galaxy determined from optical spectroscopy. The line disappeared about 1 day after the burst. We have also analyzed the spectral variability during the outburst event that characterizes the X-ray afterglow of this GRB. The spectrum gets harder during the flare, turning to steep when the flux decreases. The variability, intensity and width of the line indicate that the emitting region should have a mass approximately greater than 0.5 solar masses (assuming the iron abundance similar to its solar value), a size of about 3 times 10^15 cm, be distributed anisotropically, and be moving with sub-relativistic speed. In contrast to the fairly clean environment expected in the merging of two neutron stars, the observed line properties would imply that the site of the burst is embedded in a large mass of material, consistent with pre-explosion ejecta of a very massive star. This material could be related with the outburst observed in the afterglow 1 day after the GRB and with the spectral variations measured during this phase., Comment: To appear in The Astrophysical Journal Letters, AASTEX LateX, 2 PostScript figures

We present an analysis of the X-ray absorption properties of 6 gamma-ray burst (GRB) afterglows measured with BeppoSAX. Between 8 hrs and 20 hrs after the initial GRB trigger, individual spectra can be described by a power-law with a photon index of ~2 and absorption marginally consistent with the galactic value. Taken collectively, the data are inconsistent with zero absorption at the >99.999 confidence level. The data are only marginally consistent with a distribution of column densities varying as the total galactic absorption in the direction of each of the bursts (chi^2 =9.6 for 6 degrees of freedom). The data are consistent with cosmological models in which GRB occur within host galaxies. By simultaneously fitting a power-law spectral model with absorption fixed at the galactic value and additional, redshifted, absorption to all 6 afterglow spectra, the best-fit average absorption within the host objects is found to be (1.01 ^0.28_0.51) 10^22 atom cm^-2. This value is compatible with the host galaxy column densities inferred from optical data for GRB970508, GRB971214 and GRB980329, supporting the hypothesis that GRB occur within heavily absorbed star forming regions of their host galaxies., Comment: 4 pages. 1 Figure. Accepted for publication in A&AL)

We present the results of the monitoring of the GRB 970111 field that started 19 hours after the event. This observation represents the fastest ground-based follow-up performed for GRB 970111 in all wavelengths. As soon as the detection of the possible GRB 970111 X-ray afterglow was reported by Feroci et al. (1998) we reanalyzed the optical data collected for the GRB 970111 field. Although we detect small magnitude variability in some objects, no convincing optical counterpart is found inside the WFC error box. Any change in brightness 19 hours after the GRB is less than 0.2 mag for objects with B < 21 and R < 20.8. The bluest object found in the field is coincident with 1SAXJ1528.8+1937. Spectroscopic observations revealed that this object is a Seyfert-1 galaxy with redshift z=0.657, which we propose as the optical counterpart of the X-ray source. Further observations allowed to perform multicolour photometry for objects in the GRB 970111 error box. The colour-colour diagrams do not show any object with unusual colours. We applied a photometric classification method to the objects inside the GRB error box, that can distinguish stars from galaxies and estimate redshifts. We were able to estimate photometric redshifts in the range 0.2 < z < 1.4 for several galaxies in this field and we did not find any conspicuous unusual object. We note that GRB 970111 and GRB 980329 could belong to the same class of GRBs, which may be related to nearby sources (z ~1) in which high intrinsic absorption leads to faint optical afterglows., Comment: 10 pages with 11 encapsulated PostScript figures included. Uses Astronomy & Astrophysics LaTeX macros. Accepted for publication in Astronomy & Astrophysics

GRB970402 is the fourth gamma-ray burst detected by BeppoSAX simultaneously in the Gamma Ray Burst Monitor (GRBM) and one of the two Wide Field Cameras (WFCs). A rapid pointing of the BeppoSAX Narrow Field Instruments (NFIs) 8 hours after the GRB led to the identification of an unknown weak X-ray source: 1SAX J1450.1-6920. Its position was approximately at the center of the 3 arcmin error circle derived from the WFC image. Both the Medium Energy (MECS, 2-10 keV) and Low Energy (LECS, 0.1-10 keV) concentrators detected the source. A follow-up observation performed 1.5 days later and lasting 54 ks showed that the source had faded almost to, but not below the detectability threshold. The flux decrease between the two observations was a factor ~= 2.5. 1SAX J1450.1-6920 was the second X-ray afterglow associated with a GRB. Searches promptly started at other wavelengths (optical, IR, radio) did not reveal any transient event within the 3 arcmin error circle., Comment: 4 pages, Latex, 3 PS figures, requires l-aa.sty, accepted for publication in Astronomy and Astrophysics Letters

GRB 980329 is the brightest gamma-ray burst detected so far with the Wide Field Cameras aboard BeppoSAX, both in gamma-rays and X-rays. With respect to its fluence (2.6 X 10**-5 erg/s/cm**2 in 50 to 300 keV) it would be in the top 4% of gamma-ray bursts in the 4B catalog (Meegan et al. 1998). The time-averaged burst spectrum from 2 to 20 and 70 to 650 keV can be well described by the empirical model of Band et al. (1993). The resulting photon index above the break energy is exceptionally hard at -1.32 +/- 0.03. An X-ray afterglow was detected with the narrow-field instruments aboard BeppoSAX 7 h after the event within the error box as determined with the Wide Field Cameras. Its peak flux is (1.4 +/- 0.2) X 10**-12 erg/s/cm**2 (2 to 10 keV). The afterglow decayed according to a power law function with an index of -1.35 +/- 0.03. GRB 980329 is characterized by being bright and hard, and lacking strong spectral evolution., Comment: 13 pages with 3 figures. Accepted for publication in ApJ Lett

We report on Hubble Space Telescope (HST) observations of the host galaxy of GRB 971214, about four months after the burst. The redshift of the proposed host galaxy at z=3.418, combined with optical and radio observations of the burst afterglow, implies the extremely large isotropic energy release from the burst in gamma-rays of approximately 3x10^53 ergs, some two orders of magnitude higher than the previously commonly assumed numbers. The positional offset between the optical transient observed at the Keck telescope and the centroid of the proposed host galaxy in the HST image is 0.14 -+0.07 arcsec. We find no evidence in our deep HST image for a chance foreground galaxy superposed along the line of sight to the proposed host at z=3.418. The morphology and photometric properties of this galaxy, such as the total flux, morphology, radial surface profile and scale length, are typical as compared to other, spectroscopically confirmed z>3 galaxies., Comment: LaTex, 10 pages, 3 figures, accepted to ApJL

The discovery of X-ray, optical and radio afterglows of gamma-ray bursts (GRBs) and the measurements of the distances to some of them have established that these events come from Gpc distances and are the most powerful photon emitters known in the Universe, with peak luminosities up to 10^52 erg/s. We here report the discovery of an optical transient, in the BeppoSAX Wide Field Camera error box of GRB980425, which occurred within about a day of the gamma-ray burst. Its optical light curve, spectrum and location in a spiral arm of the galaxy ESO 184-G82, at a redshift z = 0.0085, show that the transient is a very luminous type Ic supernova, SN1998bw. The peculiar nature of SN1998bw is emphasized by its extraordinary radio properties which require that the radio emitter expand at relativistical speed. Since SN1998bw is very different from all previously observed afterglows of GRBs, our discovery raises the possibility that very different mechanisms may give rise to GRBs, which differ little in their gamma-ray properties., Comment: Under press embargo at Nature (submitted June 10, 1998)

We report on the discovery of a variable radio source, VLA J070238.0+385044, associated with the proposed x-ray counterpart, 1SAX J0702.6+3850 of GRB 980329. The source was monitored from one day after the burst to one month later at centimeter wavelengths (1.4, 4.9, 8.3 and 15 GHz; Very Large Array -- VLA), and in the 3-mm band (90 GHz; Owen Valley Radio Observatory -- OVRO). Based on its variability, compactness and spectrum, we identify VLA J070238.0+385044 as the afterglow from GRB 980329. We interpret the rapid flux density variations as interstellar scintillation and the sharp turnover in the radio spectrum below 13 GHz as arising from synchrotron self-absorption. This suggests that the angular diameter of the fireball from GRB 980329 was of order a few microarcseconds in the first two weeks. The absence of a readily detectable afterglow in the optical, but clear detections in both the radio and infrared, can be understood as the result of extinction by dust. In this context it is interesting to note that half of all well-localized gamma-ray bursts have no optical counterpart. Based on our study of GRB 980329 we suggest that the progenitors of a sizable fraction of gamma-ray bursts are associated with regions of moderate to high gas density., Comment: in press at ApJ Letters, 12 page LaTeX document includes 2 postscript figures

We report the detection of the GRB 971214 counterpart in the near-infrared by means of two images in the K'-band taken at Calar Alto only ~3.5 and ~5 hours after the gamma-ray event. We detect the transient at K'=18.03+/-0.18 and K'=18.00+/-0.22 respectively. Our data seem to indicate the existence of a plateau with duration ~1.5 < T < 6.7 hours (between 3.5 and 10.2 hours after the high-energy event). Moreover the power-law decline should be steeper than the one given by the index ak'=0.45. There is also a change in the slope of the broad-band spectrum at some wavelength between the J and K' bands (possibly around the H-band)., Comment: 5 pages with 3 encapsulated PostScript figures included. Uses Astronomy & Astrophysics LaTeX macros. Accepted for publication as a Letter in Astronomy & Astrophysics

The BeppoSAX satellite has recently opened a new way towards the solution of the long standing gamma-ray bursts' (GRBs) enigma, providing accurate coordinates few hours after the event thus allowing for multiwavelength follow-up observational campaigns. The BeppoSAX Narrow Field Instruments observed the region of sky containing GRB970111 16 hours after the burst. In contrast to other GRBs observed by BeppoSAX no bright afterglow was unambiguously observed. A faint source (1SAXJ1528.1+1937) is detected in a position consistent with the BeppoSAX Wide Field Camera position, but unconsistent with the IPN annulus. Whether 1SAXJ1528.1+1937 is associated with GRB970111 or not, the X-ray intensity of the afterglow is significantly lower than expected, based on the properties of the other BeppoSAX GRB afterglows. Given that GRB970111 is one of the brightest GRBs observed, this implies that there is no obvious relation between the GRB gamma-ray peak flux and the intensity of the X-ray afterglow., Comment: 5 pages, LaTeX A&A style, 3 postscript figures; A&A Letters accepted

We present the results of photometric observations of the variable optical source associated to the remnant of the gamma-ray burst GRB970508 performed at SAO RAS (Russia) from May to August 1997. The observations were carried out with the standard (Johnson-Kron-Cousins) photometric BVRcIc system using the 1-meter and 6-meter telescopes. The brightness of the optical source increased from Rc=21.19 (May 9.75 UT) to Rc=19.70 (May 10.77 UT), whereupon it was decaying in all the four BVRcIc bands during about one month after the burst. Between the 3rd and the 6th night after the GRB970508 event, the flux decrease of the optical source follows an exponential law in all the four bands. In that period the broad-band spectrum of the object does not change and can be approximately described by a power with index -1.1. The subsequent observations showed a reduction of the source brightness decrease rate in all bands. The source flux decay after the maximum in the Rc band is well described by a single power-law with index -1.171 for the whole set of observations during 86 days. In the other bands the decay seems to slow down from the 31th day onwards. In particular, in the Ic band the source magnitude is about 23.1 from the 36th day after the GRB to the end of the observations (August 4, 1997)., Comment: 8 pages, Latex, 5 PS figures, requires l-aa.sty, accepted for publication in Astronomy and Astrophysics

In about one year, five gamma-ray bursts were simultaneously observed with the Wide Field Cameras and Gamma Ray Burst Monitor aboard the BeppoSAX satellite. From some of them X-ray afterglow emission has been clearly detected with the same satellite. In order to understand how GRB emission is related to the X-ray afterglow, we are performing a systematic study of the spectral properties of these events. We report here preliminary results of this study., Comment: 5 pages, 3 figures. To appear in the Proceedings of the 4th Huntsville Gamma-ray Burst Symposium

We report on Hubble Space Telescope observations of the optical transient discovered in the error box of GRB970508. The object was imaged on 1997 June 2 with the Space Telescope Imaging Spectrograph and Near-Infrared Camera and Multi-Object Spectrometer. The observations reveal a point-like source with R = 23.1 +- 0.2 and H = 20.6 +- 0.3, in agreement with the power-law temporal decay seen in previous ground-based monitoring. Unlike the case of GRB970228, no nebulosity is detected surrounding the OT of GRB970508, although Mg I absorption and [O II] emission seen in Keck spectra at a redshift of z = 0.835 suggest the presence of a dense, star-forming medium. The HST observations set very conservative upper limits of R ~24.5 and H ~22.2 on the brightness of any underlying extended source. If this subtends a substantial fraction of an arcsecond, then the R band limit is ~25.5. Subsequent photometry suggests a flattening of the light curve at later epochs. Assuming the OT decline follows a pure power-law and ascribing the flattening to the presence of an underlying component of constant flux, we find that this must have R = 25.4, consistent with the upper limits determined by HST. At z = 0.8, this would correspond to an absolute magnitude in the U band of ~-18, similar to that of the Large Magellanic Cloud. We propose a scenario in which the host galaxy of the GRB is of Magellanic type, possibly being a ``satellite'' of one of the bright galaxies located at few arcseconds from the optical transient., Comment: 5 pages, 1 figure, to appear in the Proceedings of the Fourth Gamma Ray Bursts Symposium, eds. C. A. Meegan, R. Preece, and T. Koshut

The X-ray pulsar Her X-1 was observed for more than two orbital cycles near the maximum of the 35 day X-ray intensity cycle by the Narrow Field Instruments on-board the BeppoSAX satellite. We present the first simultaneous measurement of the 0.1-200 keV spectrum. Three distinct continuum components are evident in the phase averaged spectrum: a low energy excess, modeled as a 0.1 keV blackbody; a power-law and a high energy cut-off. Superposed on this continuum are Fe L and K emission features at 1.0 and 6.5 keV, respectively, and a ~40 keV cyclotron absorption feature. The cyclotron feature can be clearly seen in raw count spectra. We present the properties of the cyclotron feature with unprecedented precision and discuss the indications given by this measurement on the physical properties of the emitting region., Comment: 5 pages, 2 figures, accepted for publication in A&A Letters

We report on the timing analysis of the observation of the X-ray binary pulsar Her X-1 performed during the BeppoSAX Science Verification Phase. The observation covered more that two full orbital cycles near the maximum of the main-on in the 35 day cycle of Her X-1. We present the pulse profiles from 0.1 to 100 keV. Major changes are present below 1 keV, where the appearance of a broad peak is interpreted as re-processing from the inner part of the accretion disk, and above 10 keV, where the pulse profile is less structured and the main peak is appreciably harder. The hardness ratios show complex changes with pulse phase at different energies., Comment: 5 pages, 2 PostScript figures, uses aipproc.sty, to appear in Proceedings of Fourth Compton Symposium

We report high-energy spectral data of the prompt emission of GRB970228 and its X-ray afterglow. We establish that the nature of the X-ray afterglow emission is non-thermal and similar to the later portion of GRB970228. Our data can be used to discriminate different emission models of GRB afterglows. While cooling of excited compact objects can be ruled out, fireball models are constrained in the physics of the radiation emission processes and their evolution., Comment: 12 pages, 4 figures Accepted for publication in ApJ Letters

The gamma-ray burst GB970508 was observed simultaneously by the Gamma Ray Burst Monitor (GRBM) and one of the X-ray Wide Field Cameras (WFC) aboard BeppoSAX. The latter provided a position within 1.9' radius. A series of follow-up observations with the Narrow Field Intruments (NFI) was then performed in a period from 6 hours to 6 days after the main event. A previously unknown source, which we associate with the afterglow of the GRB, was discovered in the error box. We find that, after the initial burst, X-ray emission is still present and decays as t^(-1.1) up to 6 times 10^4 s. This is followed by a burst of activity with a duration 10^5 s. The energy produced in this event is a substantial fraction of the total energy of the GRB, which means that the afterglow is not a remnant of the initial burst (the GRB) that fades away smoothly. Our results support the idea that the processes generating the GRB and its afterglow are the same., Comment: 5 pages, LaTeX, 4 postscript figures; A&A, accepted revised version

Optical Transients from gamma-ray burst sources, in addition to offering a distance determination, convey important information on the physics of the emission mechanism, and perhaps also about the underlying energy source. As the gamma-ray phenomenon is extremely diverse, with time scales spanning several orders of magnitude, some diversity in optical counterpart signatures appears plausible. We have studied the Optical Transient, which accompanied the gamma-ray burst of May 8, 1997 (GRB 970508). Observations conducted at the 2.5-m Nordic Optical Telescope (NOT) and the 2.2-m telescope at the German-Spanish Calar Alto observatory (CAHA) cover the time interval starting 3 hours 5 minutes to 96 days after the high energy event. This brackets all other published observations, including radio. When analyzed in conjunction with optical data from other observatories, evidence emerges for a composite light curve. The first interval, from 3 to 8 hours after the event was characterized by a constant, or slowly declining brightness. At a later moment the brightness started increasing rapidly, and reached a maximum approximately 40 hours after the GRB. From that moment the GRB brightness decayed approximately as a power-law of index -1.21. The last observation, after 96 days, m_R = 24.28+-0.10, is brighter than the extrapolated power-law, and hints that a constant component, m_R = 25.50+-0.40 is present. The OT is unresolved (FWHM 0.83") at the faintest magnitude level. The brightness of the optical transient, its duration and the general shape of the light curve sets this source apart from the single other optical transient known, that of the February 28, 1997 event., Comment: 13 pages, LaTeX (aaspp4.sty) 2 figures. To appear in ApJ 496 (March 20, 1998)

Until recently the positions of gamma ray bursts were not sufficiently well known within a short timescale to localize and identify them with known celestial sources. Following the historical detection of the X-ray afterglow of the burst GRB970228, extending from 30 s to 3 days after the main peak, by the Beppo-SAX satellite and that of an optical transient 21 hr after the burst, we report here the detection of the same optical transient, in images obtained only 16 hours after the burst., Comment: Accepted for publication in Astronomy and Astrophysics Letters, Springer LateX needed

We report results of follow-up multicolor photometry of the optical variable source that is a probable remnant of the gamma-ray burst GRB970508 discovered by the BeppoSAX satellite (IAUC 6649). Observations were carried out in Johnson-Kron-Cousins $ BVR_cI_c system with the 1-m and 6-m telescopes of SAO RAS. Between the 2nd and the 5th day after the burst a fading of the remnant is well fitted with an exponential law in all four bands. During this period the `broadband spectrum' of the object was unchanged and can be approximated by a power-law, F_{\nu}\propto\nu^{-1.1}. After the 5th day the decline of brightness is slowed down. In the R_c band until the 32nd day, the light curve can be described by a power-law relation, F_t \propto t^{-1.2}., Comment: 14 pages, Postscript figures, Latex, psfig.sty included, poster to Fourth Huntsville Symposium on Gamma-Ray Bursts

The Italian-Dutch satellite for X-ray Astronomy BeppoSAX is successfully operating on a 600 km equatorial orbit since May 1996. We present here the in-flight performances of the Gamma Ray Burst Monitor experiment during its first year of operation. The GRBM is the secondary function of the four CsI(Na) slabs primarily operating as an active anticoincidence of the PDS hard X-ray experiment.. It has a geometric area of about 4000 cm2 but, due to its location in the core of the satellite its effective area is dependent on the energy and direction of the impinging photons. A dedicated electronics allows to trigger on cosmic gamma-ray bursts. When the trigger condition is satisfied the light curve of the event is recorded from 8 s before to 98 s after the trigger time, with a maximum time resolution of 0.48 ms, in an energy band of 40-700 keV., Comment: 12 pages, Postscript; Proc. SPIE Conference 3114 (San Diego, 1997)

In this paper we present the observations performed by the BeppoSAX Gamma-Ray Burst Monitor (GRBM) and Wide Field Cameras (WFC) of GB960720. We derive a precise localization (3 arcmin radius) and fast broad band (2-700 keV) spectral evolution of the event. A search in the catalogues at all wavelengths in the error box yields a unique outstanding source: the bright radio quasar 4C 49.29. Although the probability of finding such a source by chance is very low (0.0002), the absence of similar counterparts in other small error boxes suggests a chance occurrence. We also find that the duration-energy relationship for bursts previously observed above 25keV (Fenimore et al. 1995) extends down to 1.5 keV. This result suggests that the same radiation mechanism is operating from X-rays to gamma-rays and is in agreement with radiative cooling by synchotron emission. A fast evolution of the spectrum is found, in which the ratio of X- to gamma-ray intensities varies over three orders of magnitude. Furthermore, the spectrum in the initial phase of the event betrays the presence of an optically thick source rapidly evolving in a thin configuration.No other class of sources in the universe shows such a fast and extreme evolution. These results pose new and tighter constraints on theoretical models for gamma-ray bursts., Comment: 5 pages, LaTeX, 5 postscript figures; A&A, Accepted 15 July 1997, updated reference Costa 1997b

The optical counterpart of the Gamma-Ray Burst GRB 970228, discovered by Groot et al. (IAU Circ. 6584), is also detected in the B and R frames obtained about 4 hours earlier at the Bologna Observatory. Our observations indicate a very likely rise of the optical emission within these 4 hours. The R luminosity of the transient at maximum was about 15 times that of an underlying extended object. Follow-up data show that the maximum optical emission was delayed of not less than 0.7 days with respect to the gamma-ray peak and that no new big flares were seen after the main one. The optical transient became significantly redder once it has reverted to quiescence., Comment: 4 pages, no figures, LaTeX A&A style file L-AA version 3, accepted by Astronomy and Astrophysics Letters, also available at http://www.bo.astro.it/bap/BAPhome.html (file bap97-21-text.ps) or via anonymous ftp at boas3.bo.astro.it/bap/files/bap97-21-text.ps

The high energy experiment PDS is one of the Narrow Field Instruments aboard the X-ray astronomy satellite BeppoSAX. It covers the energy band from 15 to 300 keV. Here we report results on its in-flight performance and observations of galactic and extragalactic X-ray sources obtained during the Science Verification Phase of the satellite: in particular Crab, Cen X-3, 4U1626-67 and PKS2155-305., Comment: 5 pages, 4 PostScript figures, uses aipproc.sty, to appear in Proceedings of Fourth Compton Symposium

Here we report the discovery in the X-ray band of the first afterglow of a gamma-ray burst. It was detected and quickly positioned by the Beppo-SAX satellite on 1997 February 28 (GRB970228). The X-ray afterglow source was detected with the X-ray telescopes aboard the same satellite about eight hours after the burst and faded away in a few days with a power law decay function. The energetic content of the X-ray afterglow results to be a significant fraction of gamma-ray burst energetics. The Beppo-SAX detection and fast imaging of GRB970228 started a multiwavelength campaign that lead to the identification of a fading optical source in a position consistent with the X-ray source., Comment: 10 pages, LaTeX, 3 postscript figures; Nature, issue of 19 June 1997

The gamma-ray burst of 1997 February 28 was localized using the arrival-time analysis method with the Ulysses, BeppoSAX, and WIND spacecraft. The result is a plus-or-minus 31.5 arcsec (3 sigma) wide annulus of possible arrival directions which intersects both the position of the burst determined independently by the SAX Wide Field Camera, and the position of a fading X-ray source detected by the SAX focussing X-ray telescopes, and reduces these source location areas by factors of 7 and 1.5 respectively. The combination of the annulus and the SAX locations, a 0.76 square arcminute error box, is consistent with that of an optical transient source and an extended object, possibly a galaxy. We also present the time history, peak flux, and fluence of this event, and derive a model-independent lower limit to the source distance of ~11000 AU., Comment: 11 pages, postscript, 2 figures. Accepted for publication in ApJ Letters

We report on a comprehensive radio monitoring program of the bright gamma-ray burster GRB970111. These VLA observations were made at a frequency of 1.4 GHz and span a range of post-burst timescales between 28 hours and one month. Despite extensive sampling at sub-milliJansky sensitivities, no radio source was detected above 0.5 mJy in the current best error box (~14 arcmin^2) for GRB970111. A highly unusual radio source, VLA J1528.7+1945, was seen to drop in flux density by a factor of two in our monitoring period but it lies outside the error box and thus it is unlikely to be related to GRB970111. Cosmological fireball models of gamma-ray bursts make predictions of late-time emission occurring at longer wavelengths. The absence of a flaring or fading radio counterpart to GRB970111 provides strong constraints on these models., Comment: ApJ Let (accepted)

We describe a probe-class mission concept that provides an unprecedented view of the X-ray sky, performing timing and 0.2-30 keV spectroscopy over timescales from microseconds to years. The Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X) has three key science drivers: (1) measuring the spin distribution of accreting black holes, (2) understanding the equation of state of dense matter, and (3) exploring the properties of the precursors and electromagnetic counterparts of gravitational wave sources. To perform these science investigations, STROBE-X comprises three primary instruments. The first uses an array of lightweight optics (3-m focal length) that concentrate incident photons onto solid state detectors with CCD-level (85-130 eV) energy resolution, 100 ns time resolution, and low background rates to cover the 0.2-12 keV band. This technology is scaled up from NICER, with enhanced optics to take advantage of the longer focal length of STROBE-X. The second uses large-area collimated silicon drift detectors, developed for ESA's LOFT, to cover the 2-30 keV band. These two instruments each provide an order of magnitude improvement in effective area compared with its predecessor (NICER and RXTE, respectively). Finally, a sensitive sky monitor triggers pointed observations, provides high duty cycle, high time resolution, high spectral resolution monitoring of the X-ray sky with ~20 times the sensitivity of the RXTE ASM, and enables multi-wavelength and multi-messenger studies on a continuous, rather than scanning basis. The STROBE-X mission concept is a rapidly repointable observatory in low-Earth orbit, similar to RXTE or Swift, and will be presented to the 2020 Astrophysics Decadal Survey for consideration as a probe-class mission.