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Soft gamma repeaters are high-energy transient sources associated with neutron stars in young supernova remnants 1 . They emit sporadic, short (∼ 0.1 s) bursts with soft energy spectra during periods of intense activity. The event of March 5, 1979 was the most intense and the only clearly periodic one to date 2,7 . Here we report on an even more intense burst on August 27, 1998, from a different soft gamma repeater, which displayed a hard energy spectrum at its peak, and was followed by a ∼ 300 s long tail with a soft energy spectrum and a dramatic 5.16 s period. Its peak and time integrated energy fluxes at Earth are the largest yet observed from any cosmic source. This event was probably initiated by a massive disruption of the neutron star crust, followed by an outflow of energetic particles rotating with the period of the star. Comparison of these two bursts supports the idea that magnetic energy plays an important role, and that such giant flares, while rare, are not unique, and may occur at any time in the neutron star's activity cycle.Four soft gamma repeaters (SGRs) are known. All appear to be associated with radio supernova remnants, indicating that they are young 4 (<20,000 y). SGRs are probably strongly magnetized neutron stars ('magnetars' 5 ), in which, unlike the radio pulsars, the magnetic energy dominates the rotational energy. SGR0525-66 produced both the unusual, energetic and periodic burst of March 5 1979 6,7,8 and a series of subsequent, much smaller bursts 9,10 . It lies towards the N49 supernova remnant in the Large Magellanic Cloud 11,12 . A quiescent soft X-ray source has been identified which may be the neutron star 13 . SGR1900+14, first detected in 1979, was, until recently, the least prolific SGR 14,15 , hindering attempts to locate it precisely. Several lines of evidence suggested that it was associated with the galactic supernova remnant G42.8+0.6 16 and a quiescent soft X-ray source 17 . This possible association was strengthened by a source location obtained with the network synthesis method 18 , and more recently by triangulation 19,20,21 , although since this X-ray source lies outside the remnant, the connection between the two could still be considered to be unresolved.

We present a possible Cepheid-like luminosity estimator for the long gamma-ray bursts based on the variability of their light curves. To construct the luminosity estimator, we use CGRO/BATSE data for 13 bursts, W ind/Konus data for Ðve bursts, Ulysses/GRB data for one burst, and NEAR/XGRS data for one burst. Spectroscopic redshifts, peak Ñuxes, and high-resolution light curves are available for 11 of these bursts ; partial information is available for the remaining nine bursts. We Ðnd that the isotropic equivalent peak luminosities L of these bursts positively correlate with a rigorously constructed measure V of the variability of their light curves. We Ðt to these data a model that accommodates both intrinsic scatter (statistical variance) and extrinsic scatter (sample variance). We Ðnd that If one L D V 3.3~0 1 . . 9 1 . excludes GRB 980425 from the Ðt, on the grounds that its association with SN 1998bw at a redshift of z \ 0.0085 is not secure, the luminosity estimator spans B2.5 orders of magnitude in L , and the slope of the correlation between L and V is positive with a probability of 1 [ (1.4 ] 10~4) (3.8 p). Although GRB 980425 is excluded from this Ðt, its L and V values are consistent with the Ðtted model, which suggests that GRB 980425 may well be associated with SN 1998bw and that GRB 980425 and the cosmological bursts may share a common physical origin. If one includes GRB 980425 in the Ðt, the luminosity estimator spans B6.3 orders of magnitude in L , and the slope of the correlation is positive with a probability of 1 [ (9.3 ] 10~7) (4.9 p). In either case, the luminosity estimator yields best-estimate luminosities that are accurate to a factor of B4, or best-estimate luminosity distances that are accurate to a factor of B2. Regardless of whether GRB 980425 should be included in the Ðt, its light curve is unique in that it is much less variable than the other B17 light curves of bursts in our sample for which the signal-tonoise ratio is reasonably good.

We describe and discuss the global properties of 45 gamma-ray bursts (GRBs) observed by HETE-2 during the first three years of its mission, focusing on the properties of X-Ray Flashes (XRFs) and X-ray-rich GRBs (XRRs). We find that the numbers of XRFs, XRRs, and GRBs are comparable. We find that the