Vignetting and Stray Light


Thanks to the efforts of Masatoshi Ebara, Shunsaku Okada, Hideyuki Mori, Yoshitomo Maeda, and the XRT team, we can now release more information about the response of the Suzaku XRTs, especially regarding stray light and diffuse sources. This information will be of particular interest to those considering observations of sources with nearby (within 2 degrees) bright sources, or diffuse sources which fill the field of view.

Stray light flux of XRT-I/XIS

We show here the angular responses of the XRT-I at 1.5 and 4.5 keV for sources up to 2 degrees off-axis. The effective area is normalized to the on-axis values. The integration area corresponds to the detector size of XIS (17'.8x17'.8). The plots will help users planning to observe diffuse source or faint emission near bright sources, such as outskirts of cluster of galaxies, diffuse objects in the Galactic plane, or SN 1987A.

The three solid lines in the plots correspond to different parameters of ray-tracing program while the crosses are the normalized effective area using the Crab pointings. For example, the effective area of the stray light (again relative to an on-axis observation) at 1.5 keV is approximately 0.001 at angles smaller than 70 arcmin off-axis and less than 0.001 at angles larger than 70 arcmin off-axis.

Although we have not finished optimizing the parameters of the ray-tracing, the measured flux of stray light is in good agreement with that of raytracing. This ray-tracing routine will be incorporated in the arf generator in future.

1.49 keV Stray Light plot 4.51 keV Stray Light plot

(Click images to enlarge; or download the PDF versions of the 1.49 keV and the 4.51 keV stray light plots. Further details about the stray light can be found in Mori et al. 2005, PASJ 57, 245.)

Effective area for diffuse objects (Vignetting)

The following images plot the vignetting curves of XRT-I at three different energies: 1.5, 4.5 and 8.0 keV. The three solid lines in the plots correspond to different parameters for the Suzaku ray-tracing program while the crosses are the preliminary XRT-I effective area (EA) inferred from the Crab pointings with some assumptions. The XRT-I EA shown here does not include either the quantum efficiency of the detector or transmissivity of the thermal shield and the optical blocking filter.

The rough numbers of the effective area averaged over the detector size of XIS (17.8'x17.8') is listed as "Average EA" in the following table. The average EA is about 50-60% of the on-axis EA.

EnergyOn-axis EAAverage EAAverage/On-axis Ratio
[keV][cm2][cm2] 
1.54852970.6
4.53352080.6
8.02651340.5

The released ARF files for Cycle 1 reflects the on-axis EA. We suggest that users wishing to simulate the spectrum for point and very extended objects follow these procedures:

For a point source,

  1. Simply fake the spectrum with the arfs provided.

For a very extended emission over the field of view of XIS, such as the Galactic ridge emission, Local Bubbles or nearby CGs.

  1. Estimate the spectrum model in the field of 17.8' x 17.8'.
  2. Reduce the normalization (or flux) of the model by 50%.
  3. Fake the spectrum with the arfs provided.
Vignetting plotted against SATX Vignetting plotted against SATY

(Click images to enlarge; or download the PDF versions of the vignetting graphs plotted against SATX or SATY.)