Introduction

This document provides detailed technical information about the Spectrum-X Gamma Observatory and the scientific instruments available to U.S. astronomers who wish to propose for SXG observations. This information is intended to allow proposers to identify sources appropriate for scientific study with SXG, determine when specific sources can be observed, and assess the feasibility of proposed observations.

Spectrum-X-Gamma, or SXG, (also known as Spectrum-Röntgen-Gamma or SRG) is a large, multi-wavelength, orbiting astronomical observatory, developed under the sponsorship of the Russian Academy of Sciences, with instruments contributed by a number of European countries and the US. The SODART X-ray Telescope System is one of the main SXG facilities. It consists of twin, thin-foil, conical X-ray telescopes (hereafter called FM1 and FM2), developed by DSRI, and several focal plane instruments. A set of UV telescopes, TAUVEX, are co-aligned with SODART. The SXG spacecraft is shown in Figure 1.

Moveable sleds allow the selection of four different focal plane instruments for each telescope.

The focal plane instruments that may be selected with FM1 are:

• Low Energy (0.2 - 8 keV) Proportional Counter (LEPC-1), an imaging proportional counter with a circular FOV with diameter $30~{\prime}$, 13% energy resolution at 6 keV and $\sim 0.5^{\prime}$ position resolution. Developed by DSRI.
• High Energy (2 - 25 keV) Proportional Counter (HEPC-1) with a circular FOV with diameter $60~{\prime}$ and other properties similar to the LEPC. Developed by DSRI.
• Silicon X-ray Array (SIXA), an array of solid state detectors for moderate resolution spectroscopy from 0.5 - 20 keV. Each detector is circular with a FOV of $3.9~{\prime}$ and the array is hexagonal with a total FOV diameter of $25^{\prime}$. Developed by the University of Helsinki and the Russian Space Research Institute (IKI).
• Focal Plane X-ray Detector (FRD-A), a high energy (2 - 25 keV) imaging proportional counter with a circular FOV with diameter $50^{\prime}$, 17% energy resolution at 6 keV and $\sim 0.5^{\prime}$ position resolution. Developed by IKI.

The focal plane instruments that may be selected with FM2 are:

• LEPC-2, HEPC-2 and FRD-2 (duplicates of LEPC-1, HEPC-1 and FRD-A1).
• X-ray Polarimeter (SXRP), developed by Columbia University.
• Objective Crystal Spectrometer (OCS), allowing high energy spectroscopy ( $E/\Delta E \sim 100-1000$ in four energy ranges: 0.20 - 0.285 keV, 0.60 - 1.2 keV, 2.50 - 5.0 keV and 5.0 - 10.0 keV. OCS is developed by DSRI, the Max-Planck-Institute and IKI.

The SODART system with all the telescopes and instruments is represented in Fig. 2.

SXG will be launched into a deep, highly eccentric orbit with a period of approximately four days. This choice of orbit enables long duration ( $\mathrel{\copy\simlessbox}$ 80 hours) observations to be made. The orbital parameters at the beginning of the mission are listed in Table 1. It is expected that orbital circularization will lead to an $\sim$40,000 km altitude at perigee after 2 years.

 

Table 1: SXG Initial Orbital Parameters

Semimajor axis	106000 km
Eccentricity	0.935
Inclination	51.6$^{\circ}$
Longitude of Ascending Node	190$^{\circ}$
Argument of Perigee	317$^{\circ}$
Orbital Period	96 hours
Altitude at Perigee	$\sim$500 km
Altitude at Apogee	$\sim$199,000 km

Ground station operations will take place every 18 to 30 hours. As experience grows, it should be possible to plan for at least 10 re-orientations of the spacecraft per 24 hour period. With all the operational and pointing constraints (see below) taken into account, $\sim$ 80% of the celestial sphere should be visible during any orbit.

The spacecraft pointing is controlled by a sun sensor, which stabilizes one axis, and two star sensors, one of which stabilizes a second axis. The star sensors are sensitive to only a small set of $\sim15$ bright stars. The pointing system provides an absolute pointing accuracy of $\sim 2^{\prime}$ in each coordinate (at a probability level of 90%). A proposed feedback system using TAUVEX could improve the pointing accuracy to $\sim 3^{{\prime}{\prime}}$, but implementation of this system has not yet been agreed upon. The settling time after a slew is at most 5 minutes. The pointing stability is $\mathrel{\copy\simlessbox}5^{{\prime}{\prime}}$ over a one minute interval with a limit cycle of 30 $^{{\prime}{\prime}}$ from the nominal attitude. The maximum drift rate is $1.0\times10^{-4}~^{{\prime}{\prime}}$ per second during pointing mode and the slew rate is at least 0.35$^{\circ}$per second. The post-facto aspect reconstruction will be $\mathrel{\copy\simlessbox}35 ^{{\prime}{\prime}}$ for the spacecraft. Additional aspect information will be provided by the TAUVEX UV telescope which is co-aligned with SODART and mounted to the SODART tube. The details of the aspect solution to be provided by TAUVEX are not yet specified.

  

Figure 1: The Spectrum-X-Gamma Satellite

  

Figure 2: The SODART X-ray Telescope System