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

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2. Observations and results

We found several high quality Schmidt plates taken very close in time to the 5 March event. The first plate (ESO 3188) was taken on 8 March 1979 at the European Southern Observatory (ESO) Schmidt Telescope at La Silla (Chile). The plate was exposed through a [FORMULA] filter on 8 March 1979 (from 06:43 to 09:13 UT), 2.6 days after the extraordinary event of 5 March, and only two days after the second burst that took place on 6 March. See Fig. 1. Another five plates were obtained from four to six days after, at the United Kingdom Schmidt Telescope Unit (UKSTU) at Siding Springs Observatory, New South Wales, Australia (in the UBVR broad bands). Table 1 reports the plate material we have examined.

[FIGURE] Fig. 1. Left: N49 on 8 March 1979. The circle marks the position of the ROSAT source, near the error box of SGR 0526-66 (from Rothschild et al. 1994). Right: The same region, showing the N49 area on 17 February 1976. Both plates were exposed through an [FORMULA] filter.


Table 1. Schmidt plates fortuitously taken the week following the 5 March 1979 event

A visual inspection using a microscope performed for a [FORMULA] 100 square arcmin region around N49 did not revealed any variable object on the images. Any variability was smaller than 0.5 mag, for objects brighter than U = 18, B = 18, V = 18 and R = 20.

In any case, the region including N49 on the ESO plate 3188 was digitised by means of the PDS machine at ESO headquaters, Garching, with a resolution of 5 µm. An identical region on a second plate taken on 17 Feb 1976 was also scanned. All the UKSTU plates were scanned via the fast, high precision microdensitometer SuperCOSMOS at the Royal Observatory, Edimburgh (Hambly et al. 1998, McGillivray 1998). The sampling interval was 10 µm, achieving a resolution of 15 µm.

All the images were reduced by using the standard MIDAS and IRAF routines. The two regions on ESO plates (see Fig. 1) were shifted in order to align the centroids, and one image was divided by the other, in order to search for variability. Fig. 2 shows the UBVR images of the N49 region on the UKSTU plates.

[FIGURE] Fig. 2. UBVR images of the N49 region taken by the 1.2-m UKSTU, four to six days after the 5 March event. Limiting magnitudes are U = 18, B = 18, V = 18 and R = 20. North is at the top and east to the left.

Thus, the main result of our search is that no variable object or variable patch in the nebula was detected either within the SGR 0526-66 error box given by Mazets et al. (1982) or at the position of the point X-ray source observed by ROSAT (Rothschild et al. 1994). We also examined the surrounding area with an identical negative result, as we discussed elsewhere (Castro-Tirado 1994).

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998