2. Mission Description

This chapter is a brief introduction to the satellite and its instruments and is intended to provide a quick overview. Reading it thoroughly should provide the reader with the necessary information to understand the capabilities of the instruments at a level sufficient to judge if XRISM is suitable for a scientific project, and to prepare the feasibility section of a XRISM proposal.

Figure 2.1: The 96 minute XRISM orbit.

2.1 Spacecraft: Orbit and Attitude

XRISM is in many ways similar to previous Japanese X-ray astronomy satellites (e.g., Suzaku) in terms of orbit, pointing, and tracking capabilities. XRISM is placed in a near-circular orbit with an apogee of $\sim$575 km, an inclination of $\sim$31 degrees, and an orbital period of about 96 minutes (Figure 2.1). From this altitude, most targets are occulted by the body of the Earth for about 1/3 of each orbit, except those near the orbital poles. Additionally, during passages through the South Atlantic Anomaly (SAA), the instruments will not produce usable data. Thus a 1 day observation of a target typically results in about 40 ks of good on-source time.

The maximum slew rate of the spacecraft is 4.5 degrees/min, and settling to the final attitude takes $\sim 10$ minutes, using the star trackers. Given the relatively long slew time, the normal mode of operation is to point at a single target for at least 1/4 day (10 ks net exposure time) and often several days, without interleaving multiple targets.

Objects near the spacecraft orbital poles will not suffer Earth occultations (i.e., they are in the continuous viewing zone, CVZ), thus can be observed at a much higher efficiency. The orbit is predicted to precess with a period of about 2 months. The orbital poles will be at declinations of $\pm 59$ ($90-31$) degrees and a right ascension that changes with the precession phase. Because the evolution of XRISM orbit is influenced in part by the atmospheric drag, accurate long-term prediction for the times when a given target is in CVZ is not feasible.

The XRISM instruments are oriented perpendicular to the solar panels, which are fixed to the spacecraft, and must remain pointed to within 30 degrees of the Sun. Therefore, targets can be observed only when their Sun angle is in the 60–120 degree range. This also implies a constraint on the roll angle: when the target is at a Sun angle close to 90 degrees, there is a $\pm$30 degree range of possible roll angles. When observed close to the limits of Sun angles, the roll angle is tightly constrained.

The overall observing efficiency of the satellite is expected to be about 45%. Proposals should specify the desired good on-source exposure but without accounting for instrumental dead times due to high count rates, if relevant.

2.2 Overview of XRISM Instruments

XRISM carries two instruments, Resolve and Xtend, that are co-aligned and operate simultaneously, each behind its own X-ray Mirror Assembly (XMA). Here we provide a brief overview, leaving further details to chapters dedicated to these in turn. We show a schematic of the XRISM spacecraft in Figure 2.2.

Figure 2.2: A schematic of the XRISM spacecraft. Resolve dewar and its XMA are highlighted in blue, while the Xtend instrument and its XMA are highlighted in pink. Image credit: JAXA.

2.2.1 X-ray Mirror Assembly (XMA)

The two near-identical X-ray Mirror Assemblies (XMAs) are used to focus X-ray photons on the two detectors. Both XMAs are composed of 203 thin reflector shells that use conical approximations of Wolter-I type optics, thereby resulting in a light-weight, large collecting area ($\sim$580 cm$^{-2}$ at 1.5 keV and $\sim$420 cm$^{-2}$ at 6.4 keV) X-ray telescopes. In terms of imaging quality, XMAs represent an incremental improvement over previous telescopes of the same type, such as those used for Suzaku. Half-power diameter (HPD) of 1.3$'$ is measured on ground for Resolve-XMA, and 1.5$'$ for Xtend-XMA. Stray light, or contaminating X-rays from outside of the field of view (FOV), is reduced by a stray-light baffle also called a Pre-Collimator (PC) placed above each reflector. A Thermal Shield (TS) is attached in front of the pre-collimator to stabilize the thermal environment of the telescope.

A more detailed descriptions of XMA can be found in Chapter 4.

2.2.2 Resolve

Resolve consists of an X-ray micro-calorimeter array of 6x6 pixels placed at the focus of an XMA, covering a $\sim 3' \times 3'$ arcmin$^2$ region of the sky. It is almost identical to the Hitomi Soft X-ray Spectrometer. Its unprecedented non-dispersive high spectral resolution ($\Delta$E around 5 eV) capability is achieved by cooling the detector to 50 milli-Kelvin and measuring the temperature rise when an X-ray photon is absorbed by the detector element.

The ability of the Resolve instrument to determine the photon energy depends on recording the complete history of temperature rise and decay associated with each photon, isolated from all other events, to be compared with templates. This leads to the concept of event grade. When the count rate is high, the fraction of high resolution events will decrease. Moreover, for very bright sources, there is a limit imposed by the Pulse Shape Processor (PSP) which is unable to process events at a rate above $\sim$200 per second (see Chapter 8 for more details). For both these reasons, it may be necessary and/or advisable to reduce the count rate of the brightest X-ray sources using a filter or offset pointing. For the former, both a neutral density and a (soft X-ray absorbing) beryllium filters are provided.

The effective area is $\sim$180 cm$^2$ at the 6–7 keV K-band, considerably larger than any other high-resolution X-ray spectrometers. The imaging capability is limited both by the pixel size and the imaging performance of the XMA. The calorimeter array is backed by an anti-coincidence detector that enables rejection of background events. The milli-Kelvin temperature in the Dewar is achieved by a cooling system that includes adiabatic demagnetization refrigerators, He coolants, and mechanical coolers. The cooling system allows redundancy and even cryogenic-free operation. For calibration, $^{55}$Fe sources (one continuously illuminating a dedicated calibration pixel, and one on the filter wheel that can be inserted into the light path) and Modulated X-ray Sources (MXS) are available. The best strategy to utilize these calibration sources, as well as the in-orbit performance of Resolve in terms of gain and line spread function, are currently being investigated, but gain calibration accuracy of better than $\sim$1 eV is expected.

In comparison with grating instruments, the two key advantages of Resolve are:

• High spectral resolution for spatially extended sources: the energy resolution of Resolve is unchanged regardless of spatial extent, whereas those of grating instruments become degraded.
• High spectral resolution in its bandpass, particularly the Fe K region. Note that the energy resolution of Resolve is a slowly rising function of photon energy and is better than $\sim$5 eV for much of its bandpass except well above the Fe K band, resulting in a higher resolving power at higher energies. The opposite is the case for grating instruments whose wavelength resolution is roughly constant for a given order of a given instrument.

A more detailed descriptions of Resolve can be found in Chapter 5.

2.2.3 Xtend

Xtend is an array of four X-ray CCDs behind the other XMA. It has an imaging-spectroscopic capability of a wide field (38$\times$38 arcmin$^2$) and a medium energy resolution in the 0.4–13 keV band (comparable to Suzaku XIS, Chandra ACIS, and XMM-Newton EPIC). There is a gap of $\sim$20 arcsecond between the chips (see Figure 2.3 for the layout). The nominal pointing position is placed at the center of Resolve FoV, which is offset from the center of Xtend by $\sim$ 5$'$.

Figure 2.3: Schematic layout of the two scientific instruments on-board XRISM, Resolve and Xtend. This is the view looking toward the sky from the back of the focal plane.

The background level of Xtend is expected to be low and stable benefiting from the low-Earth orbit of the satellite. Above $\sim$7 keV, the use of a thick depletion layer should lead to a significant background reduction compared to the Back-Illuminated (BI) CCD in Suzaku XIS. All four CCDs on-board XRISM are BI devices and are more resistant to micro-meteorite impacts than Front Illuminated (FI) devices .

A more detailed descriptions of Xtend can be found in Chapter 6.

Table 2.1: XRISM capabilities at a glance.

S/C	Orbital apogee	575 km
	Orbital period	96 minutes
	Observing efficiency	$\sim 45$%
XMA	Energy Range	0.3–15 keV
	Effective Area	$\sim$580 cm$^{-2}$ at 1.5 keV
		$\sim$420 cm$^{-2}$ at 6.4 keV
	Angular Resolution	1.3$'$ HPD for Resolve-XMA
		1.5$'$ HPD for Xtend-XMA
Resolve	Energy Range	1.7–12 keV
	Efective Area	$\sim$180 cm$^2$ at 6 keV
	Energy Resolution	$\sim$5 eV FWHM
	FOV	$3.05' \times 3.05'$ arcmin$^2$
	Pixels	6$\times$6 array
	Filters	Open, ND, and Be
	Background	$0.8 \times 10^{-3}$ ct s$^{-1}$ keV${-1}$
Xtend	Energy Range	0.4–13 keV
	Effective Area	$\sim$350 cm$^{-2}$ at 1.5 keV
		$\sim$300 cm$^{-2}$ at 6.0 keV
	Energy Resolution	$\sim$180 eV FWHM at 6 keV
	FOV	$38' \times 38'$ arcmin$^2$

.