Stray Light

Figure 28: MOS1 count image showing the stray-light arcs produced by a bright point source outside of the FOV.

The XMM-Newton mirrors have a small sensitivity to stray light originating just out of the field of view (0.4–1.4 degrees) due to unbaffled single reflections (two reflections are necessary for X-ray focusing). In most cases the effect has little significance, however, if a bright source is located just outside of the FOV then the effect can be spectacular. This can be seen in the LMC data where LMC X-1 produces spectacular arcs in neighboring pointings. There is nothing that can be done to model and subtract this additional background component, and excision of the affected region is the only option. Extended sources, like the SNR N132D in the LMC can complicate the identification of stray light as the clear arcs due to a point source become blurred and merge into a bright emission region. Figure 28 shows the effect from a point source for a single exposure and instrument. As demonstrated in Figure 29, the effect of stray light can considerably stronger in the hard band when the source is hard but the surrounding diffuse emission is relatively soft.

Figure 29: Images of the LMC X-1 region in the $2.0-7.2$ keV band (upper) and $0.4-1.25$ keV band (lower). The stray light from LMC X-1 is relatively much stronger in the hard band.