1. Introduction

The X-Ray Imaging and Spectroscopy Mission (XRISM; Tashiro et al. 2025) is a mission led by the Institute of Space and Astronautical Science (ISAS) of Japan Aerospace Exploration Agency (JAXA) in collaboration with the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and other partners in Japan and around the world. Its main purpose is to perform high spectral resolution, high throughput imaging X-ray spectroscopy of celestial X-ray sources. XRISM was lanched on 2023 September 6 (UT) on a JAXA HII-A rocket. The initial in-orbit check-out and commissioning phase of XRISM was followed by Performance Verification (PV) observations, whose targets have been chosen by the XRISM Science Team. Subsequently, XRISM became a general purpose observatory open to all astronomers, with annual calls for proposals issued in parallel by JAXA, NASA, and ESA. In addition, all PV data entered public archive in August, 2025, 1 year after the conclusion of the PV phase; all subsequent observational data will enter public archive after an initial 1-year exclusive use period.

The scientific payload of XRISM consists of two instruments, Resolve and Xtend.

• Resolve (Ishisaki et al., 2025; Kelley et al., 2025), a soft X-ray spectrometer, which combines a lightweight X-ray Mirror Assembly (XMA) paired with an X-ray microcalorimeter spectrometer, and provides non-dispersive $\sim$5 eV energy resolution in the 1.7–12 keV bandpass with a field of view of 3 $\times$ 3 arcmin$^2$.

  The Resolve instrument has a Gate Valve (X-ray aperture door) that protects the detector on the ground as well as during launch and early operations, and is meant to be opened during commissioning (Sato et al., 2025). However, the Gate Valve has not opened, thereby shifting the nominal energy band of Resolve from 0.3–12 keV to 1.7–12 keV and lowering the effective area (Midooka et al., 2021). While the XRISM team will continue to assess different approaches to opening the Gate Valve, the Cycle 3 program will be determined assuming the closed Gate Valve configuration, and therefore this document describes Resolve in this configuration.

• Xtend (Noda et al., 2025; Mori et al., 2022), a soft X-ray imager, consisting of an array of four CCD detectors that extend the field of the observatory to $38.5 \times 38.5$ arcmin$^2$ over the energy range 0.4–13 keV. Xtend uses a lightweight XMA identical in design to that used for Resolve.

  Note that the Resolve Gate Valve does not affect the Xtend instrument.

1.1 Purpose and Scope of this Document

This document is intended for researchers who have some general understanding of space-based astronomy and X-ray observations. It should provide the readers with a general understanding of the characteristics of Resolve and Xtend, including on-board data handling, calibration, and operational limitations. This document also explains the basics of how XRISM will be operated, and how the observing time will be allocated. Based on this document, the reader should be able to determine whether XRISM is the appropriate instrument to address their scientific question, and how to design an appropriate observing program for the proposal.

This document is not intended as a data analysis guide. Neither is it intended to provide in-depth descriptions of the instruments. In addition, the details of proposal submission mechanisms will be explained elsewhere.

The document is organized as follows. Chapter 2 summarizes the principal characteristics of the detectors on-board XRISM. Chapter 3 covers how the observing time will be allocated. This includes data rights and time available for Guest Observers (GOs), as well as policies regarding Targets of Opportunity (TOOs). In addition, observational constraints due to the orbit, Sun angle, and the pointing accuracy are described. The proposal process is reviewed, including how to submit a proposal, how they will be evaluated, and how observations will be scheduled, performed, and the results disseminated.

The subsequent three chapters describe XMA (Chapter 4), Resolve (Chapter 5), and Xtend (Chapter 6). Chapter 7 (new in Cycle 3) and Chapter 8 focus on particular caveats associated with, respectively, extended sources and bright sources, which may thus be of importance for many proposers. Chapter 9 provides in-depth discussion on spatial-spectral mixing and consequences. Finally, Chapter 10 (new in Cycle 3) covers the list of tools and resources for proposal planning.

1.2 Cycle 3 Updates to the POG

The POG has gone through substantial changes from Cycle 2 to Cycle 3. Below we summarize the important changes.

• New joint XRISM observation opportunities with NuSTAR and XMM-Newton are now available and policies are explained in Section 3.4.
• A new chapter, Chapter 7, is added to POG to provide proposers with guidance and recommendations on how to assess and account for possible spatial-spectral mixing in their proposers.
• New information from the XRISM research and instrumentation papers are now included in POG in relevant sections to provide the latest findings for the proposers.
• A chapter composed of a complete list of proposal tools, Chapter 10 is now added to the POG. The indicated website links should be checked for the most updated recommendations and tools.