XRT News (Image and Extended Source Analysis)

Release of new XRT responses (July, 1996)

• The GIS Crab spectrum was not perfectly fitted with a power-law, and 5 % residuals remained.
• SIS and GIS did not give consistent normalizations. SIS normalization was considered to be more realistic, and the GIS gave 15 - 20 % smaller normalizations.
• The GIS Crab spectrum gave the photon index 2.15, which is a little higher than generally believed values (2.08-2.12; e.g., Toors and Seward; Ast.J.79,995).
• The GIS Crab flux was 18 % smaller than the standard flux (e.g., Toors and Seward; Ast.J.79,995).

These problems were corrected in ascaarf v2.62 and xrt_ea_v2_0.fits/xrt_psf_v2_0.fits. Essentially, two changes have been made:

• XRT effective area (xrt_ea_v2_0.fits) and the point spread function (xrt_psf_v2_0.fits) are revised based on a new Nagoya ray-tracing code. The gold optical constant ( ) was tuned above 2.7 keV so that the residuals of the GIS Crab spectral fit become minimum. The PSF was re-calibrated based on the new GIS Cyg X-1 observations, resulting in a slightly wider PSF and 13 % decrease of the effective area within a 6 arcmin radius from the image center (hence increased GIS flux).
• Only modifying the XRT calibration files did not solve the problems shown above satisfactorily, hence additional correction filters are introduced so that the GIS Crab spectrum can be fitted with a power-law, and the four detectors give the same normalization.

Please note that ascaarf v2.62 has to be used with xrt_ea_v2_0.fits and xrt_psf_v2_0.fits to make the latest and most reliable ARFs. The correction filters are included in the ascaarf v.2.62 source code, and can be turned off by specifying the hidden parameter "arffil = no". By turning off the ARF filter and combining with older XRT calibration files (xrt_ea_v1_1.fits and xrt_psf_v1_1.fits), ascaarf v2.62 is able to make the same old ARFs as were made with ascaarf v2.53 and earlier.

Analyzing extended sources with ASCA data (February 15, 1994)

As you may know, the point spread function (PSF) of ASCA's X-ray telescopes (XRT) has a relatively sharp core (FWHM of 50 arcsec) but broad wings (half-power diameter of 3 arcmin). In addition, the GIS has its own PSF which is comparable in width to that of the XRT but of a different shape (a Gaussian with a sigma of 30 arcsec at ~6 keV and a 1/sqrt(E) dependence).

For analyzing point sources, the practical consequence of the broad PSF is that a generous extraction radius should be used to capture most of the counts (e.g., 4 arcmin for the SIS, 6 arcmin for the GIS).

For extended sources, however, the astrophysical implications of the broad PSF are more complicated and harder to deal with. If you are analyzing an extended source, then please note the following effects which, if not taken into account, could lead to spurious scientific results. It is important to realize that these effects are not calibration uncertainties or instrumental defects, but features intrinsic to the design of the XRT which emphasizes effective area as much as angular resolution.

1. General redistribution of counts.

The XRT PSF has a sharp core and broad wings. If your extended source has a brightness distribution which is peaked on the same scale as the PSF, then the outer parts of the image will contain a significant proportion of counts from the core. For example, the moderate redshift cluster A2218 has a core radius of about 1 arcmin. If the image is divided into annuli (3 arcmin wide centered on the core) then the second annulus (between 3 and 6 arcmin) will contain more emission from the cluster core than from the parts of the cluster which actually lie within the annulus.

2. XRT vignetting

The effect described above will be compounded by telescope vignetting if the core of your object is close the optical axis of the XRT.

3. Energy dependence of the XRT PSF

The XRT scatters high-energy X-rays more than low-energy X-rays. In other words, the PSF broadens with increasing energy. One consequence of this effect is to introduce spurious (outwardly increasing) temperature gradients in isothermal distributions. And if there is a negative temperature gradient present, its size will appear reduced.

4. Off-axis ARF (Auxiliary Response File)

If an off-axis region is specified and an ARF constructed for it then if this region really contains counts from closer to the optical axis then the ARF will not be correct and the difference will be in the sense of increasing the measured temperature.

5. Energy dependence of the GIS PSF

Unlike the SIS, the GIS has an intrinsic PSF which is of comparable size to that of the XRT. The energy dependence of the GIS PSF is a further complication.

In view of these effects, extracting spatially resolved spectral information from ASCA observations is not trivial, especially in the case of sources which are of order 10 arcminutes or less in size. It is likely, therefore, that extensions to present analysis techniques are needed to address the issue.

The ASCA instrument teams are calibrating the PSF as accurately and completely as practicable. To this end, the ASCA Team recently conducted a series of supplementary calibration observations of the bright X-ray binary Cygnus X-1. Images extracted from these observations, which placed the target at various positions in the field of view, are available for several energy bands in the directories: