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Time series analysis

The analysis of time series produced with the XRT PSPC is severely complicated by the window structure in front of the detector. This window support structure consists of 3 major components.
  1. Wide ribs which reduce the transmission for a point source to zero
  2. Thick wires with a 4 arcminute spacing and a width of about 12 arcseconds
  3. Thin wires with a spacing of 50 arcseconds and a width of 3 arcsecs
Further to this the spacecraft is usually wobbled over a distance of 3 arcmins in a time of 3 minutes. So the effect on a time series is the convolution of the window structure and this wobble. The mirror structure causes the beam incident on the detector to be focused to a HWHM of ~ 3 arcseconds on axis hence all the structural components can have a significant effect on the time series.

The three effects will cause.

  1. Nulls in the time series when a source passes under a rib (NB: most sources will not lie near a rib.
  2. A dip in the time series every time the source passes beneath a thick wire. This may occur 0,1 or 2 times every wobble (e.g. every 6 minutes) depending on the proximity of the source to a wire and the angle of the wobble relative to the wires. The intensity of this dip will be dependent on the beam size from the mirror at the off-axis angle of the source. A source at a large off-axis angle will be spread out by the mirror and so a small fraction of the total counts from the source will be obscured by each pass under a thick wire. A source near the centre of the field of view, however, will have most or all of its counts obscured at each pass under the thick wire.
  3. A smaller dip every time the source passes under a thin wire. This will occur upto 8 times a wobble cycle. Because these wires are only 3 arcsecs in diameter, at the centre of the field the reduction in count rate will be about 50 percent. Away from the inner ring these wires may not contaminate a time series at all.
The software at present makes no attempt to compensate for these effects within a time series. The user should be aware that these effects exist and take them into account when performing time series analysis.


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Next: Barycentric correction Up: XRT ROSAT Prev: The Boron filter