[vsnet-alert 7550] SAX J1808.4-3658 optical observations

["Berto Monard" <LAGMonar@csir.co.za>]

The outbursting msec accreting pulsar SAX J1808.4-3658 (reported in ATEL
#110, RXTE observations / CB Markwardt et al) was observed with an
unfiltered CCD camera on 16 Oct 2002 over a time period of more than 2
hours at the Bronberg Observatory near Pretoria. 

The observatory is located at 25deg 55' South, 28deg 27' East, altitude
1590m.
The observing instrumentation: SCT 0.3m f/4.2 with ST-7E, unfiltered.

A composition of 19 individual images taken around Oct 16.90 showed the
optical counterpart of the XRay transient very clearly. The derived
(average) magnitude of the object on the composed image was 16.6CR.
Images of the digitized sky survey do not show the object to estimated
limiting magnitudes of around 19.5 and 20.5 for the generation 1 and
generation 2/red respectively. 
 
Astrometry traceable to UCAC1 coordinates gave the following (2000)
position:
18 08 27.67 -36 58 43.9
which agrees very well with that from SIMBAD.

Derived timeseries photometry (1 minute cycles) showed the object's
light flux evolving (not chaotically) between magnitudes (CR) 16.3 and
17.5.

The faintness of the object, the crowded starfield and the limitations
of the observing instrumentation did not allow to get any more detailed
or precise information.

The composed image and the timeresolved photometry can be made
available. 

Regards,

Berto Monard / MLF
Bronberg Observatory / CBA Pretoria

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