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We use Chandra data to map the gas temperature in the central region of the merging cluster A2142. The cluster is markedly nonisothermal; it appears that the central cooling flow has been disturbed but not destroyed by a merger. The X-ray image exhibits two sharp, bow-shaped, shock-like surface brightness edges or gas density discontinuities. However, temperature and pressure profiles across these edges indicate that these are not shock fronts. The pressure is reasonably continuous across these edges, while the entropy jumps in the opposite sense to that in a shock (i.e. the denser side of the edge has lower temperature, and hence lower entropy). Most plausibly, these edges delineate the dense subcluster cores that have survived a merger and ram pressure stripping by the surrounding shock-heated gas.

Two classes of rotating neutron stars-soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars-are magnetars, whose X-ray emission is powered by a very strong magnetic field (B approximately 10(15) G). SGRs occasionally become 'active', producing many short X-ray bursts. Extremely rarely, an SGR emits a giant flare with a total energy about a thousand times higher than in a typical burst. Here we report that SGR 1806-20 emitted a giant flare on 27 December 2004. The total (isotropic) flare energy is 2 x 10(46) erg, which is about a hundred times higher than the other two previously observed giant flares. The energy release probably occurred during a catastrophic reconfiguration of the neutron star's magnetic field. If the event had occurred at a larger distance, but within 40 megaparsecs, it would have resembled a short, hard gamma-ray burst, suggesting that flares from extragalactic SGRs may form a subclass of such bursts.

Gamma Ray Bursts (GRBs) are bright, brief flashes of high energy photons that have fascinated scientists for 30 years. They come in two classes 1 : long (>2 s), softspectrum bursts and short, hard events. The major progress to date on understanding GRBs has been for long bursts which are typically at high redshift (z ~ 1) and are in sub-luminous star-forming host galaxies. They are likely produced in core-collapse explosions of massive stars 2 . Until the present observation, no short GRB had been accurately (<10") and rapidly (minutes) located. Here we report the detection of X-ray afterglow from and the localization

Two short (<2 s) γ-ray bursts (GRBs) have recently been localized 1-4 and fading afterglow counterparts detected 2-4 . The combination of these two results left unclear the nature of the host galaxies of the bursts, because one was a starforming dwarf, while the other was probably an elliptical galaxy. Here we report the X-ray localization of a short burst (GRB 050724) with unusual γ-ray and X-ray properties. The X-ray afterglow lies off the centre of an elliptical galaxy at a redshift of z=0.258 (ref. 5), coincident with the position determined by groundbased optical and radio observations 6-8 . The low level of star formation typical for elliptical galaxies makes it unlikely that the burst originated in a supernova explosion. A supernova origin was also ruled out for GRB 050709 (ref. 3), even though that burst took place in a galaxy with current star formation. The isotropic energy for the short bursts is 2-3 orders of magnitude lower than that for the long

We report the best evidence to date of a jet break in a short Gamma-Ray Burst (GRB) afterglow, using Chandra and Swift XRT observations of the Xray afterglow of GRB 051221A. The combined X-ray light curve, which has three breaks, is similar to those commonly observed in Swift observations of long GRBs. A flat segment of the light curve at ∼ 0.1 days after the burst represents the first clear case of strong energy injection in the external shock of a short GRB afterglow. The last break in the light curve occurs at ∼ 4 days post-burst and breaks to a power-law decay index of ∼ 2. We interpret this as a jet break, with important implications for models of short GRBs, since it requires collimation of the afterglow into a jet with an initial opening angle θ 0 ∼ 4 • − 8 • and implies a

We report the results of the Chandra observations of the Swift-discovered short gamma-ray burst GRB 050724. Chandra observed this burst twice, about 2 days after the burst and a second time 3 weeks later. The first Chandra pointing occurred at the end of a strong late-time flare. About 150 photons were detected during this 49.3 ks observation in the 0.4Y10.0 keV range. The spectral fit is in good agreement with spectral analysis of earlier Swift XRT data. In the second Chandra pointing the afterglow was clearly detected with eight background-subtracted photons in 44.6 ks. From the combined Swift XRT and Chandra-ACIS-S light curve we find significant flaring superposed on an underlying power-law decay slope of ¼ 0:98 þ0:11 À0:09 . There is no evidence for a break between about 1 ks after the burst and the last Chandra pointing about 3 weeks after the burst. The nondetection of a jet break places a lower limit of 25 on the jet opening angle, indicating that the outflow is less strongly collimated than most previously reported long GRBs. This implies that the beaming corrected energy of GRB 050724 is at least 4 ; 10 49 ergs.

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