Introduction

Purpose and Scope

The ASCA Technical Description provides proposers with information about ASCA and its scientific instruments (see also Tanaka, Inoue, & Holt 1994 Publ. Astron. Soc. Japan, 46, L37 for an overview). By reading this book and by using the associated software, proposers should be able to:

1.

Determine when ASCA can observe a given source

2.

Identify the capabilities of ASCA for scientific studies

3.

Determine and demonstrate whether a proposed observation is feasible

4.

Determine how best to use ASCA's capabilities.

The first item is covered in Chapter 4 which discusses the various constraints on observing with ASCA. Also described is the typical sampling window which observers can expect.

The second item is covered partly in Chapters 5, 6 & 7 which contain technical descriptions of the scientific instruments, the X-ray telescope (XRT), the Gas Imaging Spectrometer (GIS) and the Solid-state Imaging Spectrometer (SIS), respectively. The various operating modes of the GIS and SIS are described in their respective chapters.

Chapters 8 & 9 discuss the feasibility of carrying out a given observation with the GIS and SIS, respectively. These chapters deal with the various limitations of the two instruments.

Given the complexity of the ASCA instruments, care must be taken to demonstrate that a proposed observation is feasible. This is best done by creating and analyzing simulated data. Chapter 10 contains extensive documentation on how to do this using the programs XSPEC and PIMMS. Note that the energy resolution of the SIS is now only a factor of $\sim 2$ better than the GIS in the Fe K band; any feasibility study must take account of this.

Quick Comparison of the GIS & SIS

Although the GIS and SIS have broadly similar effective areas, pass bands and angular resolution, they do have significant and complementary differences. So, for a given investigation, the GIS could be more suitable than the SIS, and vice versa. But whichever instrument they choose to emphasize in their proposals, Guest Observers will always get data from the two GIS and two SIS instruments: during normal operation, the four focal plane instruments will always be collecting data. The detailed properties of the GIS and SIS are given in Chapters 6, 7, 8 & 9: here, to aid proposers, we give a quick comparison of the fundamental properties of the two instruments:

• Energy resolution: The SIS has better energy resolution than the GIS (at 5.9 keV $\sim 8\%$ for the GIS and for the SIS, $\sim 4\%$, reduced from $\sim 2\%$ since launch; see also §3.3.1, §7.6.1, and §9.6.2 ).
• Effective area: In the range 1.5-5 keV the two instruments have similar effective areas. Below 1.5 keV, the SIS has more effective area; above 5 keV the GIS has more.
• Field of view: The GIS field of view--circular, with a diameter of 50 arcmin--covers four times more area than the SIS field of view--square, $22\times 22$ arcmin². Note that the presence of hot and flickering pixels in the SIS does not now allow the full four-chip SIS FOV to be used without sacrificing the low-energy response.

• Angular resolution: Unlike the SIS, the GIS significantly broadens the intrinsic PSF of the XRT.
• Maximum count rate: The GIS can observe much brighter sources than the SIS. How much brighter depends on mode.
• Temporal resolution: In general, the GIS has much better temporal resolution than the SIS. How much better depends on mode.

To demonstrate two of these differences--effective area and energy resolution--Figure 2.2a&b show the same set of Raymond-Smith spectra convolved with the responses of the GIS+XRT and SIS+XRT, respectively. Four temperatures are shown ( kT = 0.25, 1, 4 , 16 keV); the absorbing column is 10²⁰ cm^-2; the abundance is solar; and the redshift is zero. All the models are normalized to give 10^-11 erg cm^-2 s^-1 in the 0.5-12 keV band (note that the detector area is set here, arbitrarily, to 1 cm²).

Latest Information

Investigators are strongly encouraged to keep abreast with the latest news and information by regularly consulting the ASCA GOF home page which has the URL:

http://heasarc.gsfc.nasa.gov/docs/asca/ascagof.html

In particular, the properties of the SIS are changing with time and this may affect the feasibility of a particular proposal.

Figure Captions

Figure 2.2a: Raymond-Smith models of various temperatures ( kT = 0.25, 1, 4, 16 keV) convolved with the combined response of one XRT and one GIS. Each model has an absorbing column of 10²⁰ cm^-2, solar abundance and zero redshift. All the models are normalized to give 10^-11 erg cm^-2 s-1 in the 0.5-12 keV band. Note that the instrument response used is appropriate to the earliest observations performed during the PV and AO-1 phases.

Figure 2.2b: Same as Figure 2.2a, but for one XRT and one SIS.