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Astron. Astrophys. 341, 487-490 (1999)

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1. Introduction

Soft Gamma-ray Repeaters (SGRs) emit inttermitent bursts of soft gamma-rays. Only four SGRs have been observed so far. The Soft Gamma-ray Repeater 0525-66 (hereafter SGR 0526-66) was first detected by the Konus experiment on VENERA 11 and 12 at 15:50 UT on 5 March 1979, when it produced an extraordinary outburst reaching a peak intensity of several times [FORMULA] in 0.2 ms rise time (Mazets et al. 1982, Cline et al. 1982, Golenetskii et al. 1988), at least one order of magnitude greater than any other soft gamma-ray event ever observed. The 0.2 ms risetime implied an emitting region with a diameter [FORMULA] 60 km. Successive bursts took place on 6 March (06:14 UT), 4 April (00:43 UT) and 24 April (18:28 UT).

The main event on March 5 was observed by several experiments, allowing to determine a precise error box (0.05 arcmin2) which was positionally coincident with the 2 arcmin2 plerionic supernova remnant N49 in the Large Magellanic Cloud, at a distance of 55 kpc. Therefore it was thought that the observed bursts were pressumably associated with the neutron star within the SN remnant, as it has been proven for another soft gamma-ray repeater, SGR 1806-20 that lies in a plerion (Murakami et al. 1994, Kulkarni et al. 1994).

The SGR 0526-66 error box has been widely studied over the years. Since the work of Fishman, Duthie and Dufour (1981), a possible detection of optical bursts were reported (Pedersen et al. 1984), and a limit of [FORMULA] = 19.4 was set for any periodic component on the basis of a search for an optical 8 s periodicity (Boïr et al. 1989), similar to the periodicity observed in the 5 March gamma-ray event (Barat et al. 1979, Terrell et al. 1989).

N49 has been observed four times by ROSAT , and a point X-ray source has been detected within the SGR error box (Rothschild et al. 1994). The same point source was already seen by EINSTEIN 45 days after the 5 March event. The overall countrate from the point source has been found to be constant during the [FORMULA]2-yr duration of the ROSAT observations (Marsden et al. 1996), similarly to another soft gamma-ray repeater, SGR 1806-20 (Sonobe et al. 1994).

It has been proposed that SGRs are produced in "magnetars", neutron stars with magnetic fields B [FORMULA] 1014 G, when the magnetic field drifts through the neutron star crust, causing a starquake during which the starquake energy is released as an intense SGR burst (Thompson & Duncan 1996, Cheng et al. 1996, Duncan et al. 1998). This seems to have been confirmed in the case of SGR 1806-20 (Kouveliotou et al. 1998). The thermal radiation is enhanced by a factor of 10 or more due to the strong magnetic field, producing the observed persistent X-ray emission (Usov 1997).

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998