1. Background rejection

During the observation the Space Shuttle passed through the South Atlantic Anomaly (SAA). The SAA is an area of low geomagnetic rigidity over the Atlantic ocean off the east coast of Brazil. It is due to the misalignment between the earth's magnetic and rotational axes. Because the earth's magnetic field is lower than the average in this region charged particles trapped in the inner radiation belt can penetrate down to an altitude of 250 km. X-Ray detectors are usually switched off or placed in a safe mode during SAA passages because of the high charged-particle background.

The anticoincidence features of BBXRT worked well, allowing useful data to be obtained during the SAA transit. Contaminating events were removed by rejecting those for which any of the quality flags were set and the analysis was restricted to energies above 1 keV. The latter also removed geocoronal contamination but caused no loss of information as the source is heavily absorbed below 1 keV. This is the first time a celestial X-ray source has been successfully observed through an SAA passage.

Contamination from the galactic ridge is a concern for sources which are near the galactic equator. Assuming a background flux of 8 x 10^-8 erg cm^-2 s^-1 sr^-1 in the 2-11 keV band (Koyama et al. (1986)), the flux detected by the A0 pixel should be $\sim$ 3 x 10^-14 erg cm^-2 s^-1. This is much smaller than the continuum and iron line fluxes detected by BBXRT ( 4 x 10^-9 and 8 x 10^-11 erg cm^-2 s^-1, respectively) so emission from the galactic ridge can safely be neglected.