---
title: 'Xtend: Wide-Field Soft-X-ray Telescope'
url: https://www.emergentmind.com/topics/xtend
type: topic
---

# Xtend: Wide-Field Soft-X-ray Telescope

Xtend is the wide-field soft-X-ray imaging telescope on board the X-Ray Imaging and Spectroscopy Mission (XRISM). It comprises the X-ray Mirror Assembly (XMA) and the Soft X-ray Imager (SXI), covers the $0.4$–$13$ keV band, and provides a field of view reported as either $38'\times38'$ or $38.5'\times38.5'$ in the instrument literature. Within XRISM, Xtend is the wide-field complement to the narrow-field, high-resolution Resolve microcalorimeter: it supplies imaging spectroscopy over a large solid angle, contextual imaging around Resolve targets, stray-light monitoring, and stand-alone observations of extended and time-variable sources [2303.07575] [2406.19911] [2503.20180].

## 1. System definition and mission role

Xtend is one of the two telescopes onboard XRISM, launched in September 2023, and was developed in the context of XRISM as a recovery mission for Hitomi [2406.19911] [2502.08030]. Its stated science role is to recover and extend the soft-X-ray imaging capability associated with cluster studies, supernova remnants, AGN, and extended hot plasmas, while also providing wide-field support for Resolve. The required use cases include mapping extended sources, guiding Resolve observations, and accumulating survey-style data with moderate spectral resolution [2303.07575] [2406.19910].

A recurring point of terminology is that *Xtend* denotes the telescope system, whereas *SXI* denotes the CCD focal-plane camera installed at the focus of the Xtend optics. The system-level combination of XMA and SXI is central to the published performance claims: the mirror establishes the collecting area, PSF, FoV, and vignetting behavior, while the CCD subsystem determines the sampling, event recognition, energy response, CTI behavior, and operational modes [2406.19911] [2502.08030].

Relative to Resolve, Xtend trades calorimetric resolution for solid angle and grasp. Resolve covers a $3'\times3'$ or $3.1'\times3.1'$ central field, whereas Xtend spans the full Moon on the sky and covers almost the same energy band. This division of labor is explicit in the instrument requirements: Xtend was required to support wide-field spectroscopy and stray-light monitoring around Resolve targets, while also enabling its own imaging and spectroscopic science [2502.08030].

## 2. Optical and detector architecture

The XMA is described as a thin-foil-nested, conically approximated Wolter-I optic with a focal length of $5.6$ m [2303.07575] [2406.19911]. Published descriptions report 203 nested foils per module, Au-coated reflecting surfaces, and a pre-collimator for stray-light reduction. The mirror was designed to support an on-axis HPD of approximately $1.3'$–$1.7'$ and an on-axis FWHM of approximately $7.2''$–$10''$, depending on the performance summary and calibration context [2303.07575] [2406.19911] [2503.20180].

At the focal plane, SXI uses four P-channel, back-illuminated CCDs arranged in a $2\times2$ array, each with a $200~\mu$m-thick depletion layer [2303.07575] [2406.19911] [2502.08030]. The CCDs employ frame-transfer architecture with a $30.720$ mm $\times 30.720$ mm imaging area and $1280\times1280$ physical pixels of $24~\mu$m square per device [2502.08030]. Default on-chip $2\times2$ binning produces $48~\mu$m logical pixels. Most performance papers describe a $640\times640$ logical array, while an initial-operations summary reports a $1040\times1040$ logical format for the full-window mode; the published record therefore contains multiple instrument-format descriptions [2406.19910] [2406.19911] [2502.08030].

The CCD design incorporates a $200$ nm Al optical blocking layer, implemented as two $100$ nm films, plus an additional buried Al layer around the device edges [2502.08030]. These changes were introduced specifically to eliminate the light-leak artifacts encountered on Hitomi. The same redesign cycle added a notch implant in the transfer channel, which confines signal packets and improves radiation tolerance; flight-model tests report approximately $3\times$ higher radiation tolerance than in un-notched devices [2502.08030]. Charge injection is executed every 160 physical rows, equivalently every 80 logical rows, to mitigate CTI [2303.07575] [2406.19910].

The instrument requirements tie the optical and detector subsystems together. Xtend was required to achieve at least $22'\times22'$ FoV, but in practice the delivered FoV is reported as $38'\times38'$ or $38.5'\times38.5'$; the required effective area is $>300$ cm$^2$ at $1.5$ keV and $>270$ cm$^2$ at $6$ keV; the required energy resolution is $<200$ eV FWHM at Mn K$\alpha$ at beginning of life and $<250$ eV at end of life [2502.08030]. Ground and in-flight reports state that these requirements were met [2406.19911] [2502.08030].

## 3. Readout modes, thermal control, and CTI mitigation

Xtend is operated in several readout modes designed to balance FoV, time resolution, live-time fraction, and pile-up tolerance. The full-window mode reads the entire array with a frame exposure of $3.9631$ s and a live-time fraction of $0.99$. The $1/8$-window mode uses a $0.4631$ s frame exposure and a live-time fraction of $0.93$. The $1/8$-window mode with burst option reduces the frame exposure to $0.0620$ s, with photon arrival time stamps accurate to $0.062$ s within each $0.5$ s cycle, at a live-time fraction of $0.12$ [2511.19967]. Initial operations verified full-window, $1/8$-window, burst, and $1/8$-window-plus-burst operation, together with time-tagged and automated command sequences for South Atlantic Anomaly passages and dark-level handling [2406.19910].

Thermal control is provided by a single-stage Stirling cooler with fine heater control. The nominal CCD operating temperature is $-110^\circ$C, with demonstrated operation down to $-120^\circ$C [2303.07575] [2406.19911]. Spacecraft thermal-vacuum tests and in-orbit commissioning showed stable operation at the design temperature, with reported stability of $\pm1^\circ$C in TVAC conditions and $\Delta T \le 0.3^\circ$C peak-to-peak, or control within $\pm0.2^\circ$C, during early operations [2406.19910] [2406.19911]. Pre-flight thermal-vacuum tests yielded Mn K$\alpha$ resolutions of approximately $175$–$178$ eV at $-110^\circ$C, consistent with subsystem tests [2406.19911].

CTI calibration is a major aspect of the Xtend CCD system. Experimental studies identified at least three trap populations with characteristic time constants of approximately 1 pixel, 10 pixels, and 100 pixels, respectively [2009.06246]. The shortest-time-constant traps dominate immediate trailing charge, while deeper traps contribute more to cumulative CTI. The published model distinguishes four transfer phases—$F_I'$, $F_I$, $F_S$, and $S_S$—and parameterizes the pulse-height degradation as a product of per-phase losses. A key empirical result is that the fast transfer in the imaging area is approximately $3$–$5\times$ worse than the fast transfer in the storage area, despite identical clock periods [2009.06246]. With charge injection enabled, the apparent flux dependence of CTI becomes negligible within the tested range, and the instrument is intended to use charge injection in all science modes [2009.06246].

Operational experience also refined several details after launch. The split threshold was kept at 25 channels after balancing energy resolution against good-event ratio; bad columns were masked to suppress pseudo-events; and the charge-injection row pattern was shifted on 2024-03-10 after the original pattern overlapped the nominal aim point and caused approximately $10\%$ photon loss for point sources [2406.19910]. These adjustments illustrate that the delivered in-orbit behavior is not only a consequence of hardware design, but also of command sequencing, calibration, and event-screening policy.

## 4. In-orbit performance and calibration status

The in-orbit performance literature reports that Xtend meets or exceeds its primary engineering requirements. First-light observations demonstrated the full FoV on the galaxy cluster Abell 2319, with all CCDs active except for known bad columns and with the onboard $^{55}$Fe calibration spots visible in the corners [2406.19911]. On-axis image quality in orbit is reported as HPD $\simeq1.3'$ or $1.4'$ and FWHM $\simeq10''$ or $7.2''$, depending on the calibration summary. Published examples state that Xtend resolves structures down to approximately $20''$ and localizes point sources to $\lesssim5''$ [2406.19911] [2503.20180].

The in-orbit spectral resolution is reported as $170$–$180$ eV FWHM at $5.9$–$6$ keV, consistent with ground measurements and comfortably inside the mission requirement [2406.19911] [2503.20180]. This resolution is sufficient to separate He-like and H-like Fe K$\alpha$ lines, a point emphasized in the in-orbit performance summary [2503.20180]. Cross-calibration observations of 3C 273 with other major X-ray observatories yielded on-axis effective areas of approximately $420$ cm$^2$ at $1.5$ keV and $310$ cm$^2$ at $6.0$ keV, matching pre-launch expectations and ground tests within the reported uncertainties [2503.20180]. Earlier ground calibration summaries quoted approximately $430$ cm$^2$ at $1.5$ keV and $350$ cm$^2$ at $6$ keV [2406.19911].

Background performance is a defining feature of Xtend. The mission requirements specified NXB below $1\times10^{-6}$ counts keV$^{-1}$ s$^{-1}$ arcmin$^{-2}$ cm$^{-2}$ in $5$–$10$ keV, and early in-orbit operations reported that this requirement was met [2406.19910] [2502.08030]. The in-orbit performance paper quotes an NXB level of $1.5\times10^{-7}$ counts s$^{-1}$ keV$^{-1}$ arcmin$^{-2}$ cm$^{-2}$ at $6$ keV, while an earlier status paper describes a continuum of approximately $5\times10^{-4}$ counts s$^{-1}$ arcmin$^{-2}$ keV$^{-1}$ at the same energy and emphasizes orbital stability to $\pm10\%$ over one month [2406.19911] [2503.20180]. Both accounts converge on the same instrumental conclusion: Xtend has a low, stable, and relatively line-sparse particle background in low-Earth orbit.

Several known instrumental issues from Hitomi were explicitly addressed in Xtend and verified in orbit. Light leakage was suppressed by sealing panel openings, applying low-reflectivity coatings, and thickening the Al optical-blocking layer; in-orbit checks found no significant light-leak events [2503.20180]. Crosstalk events caused by capacitive coupling were mitigated through threshold retuning and pipeline masking, with pseudo-crosstalk events reported as suppressed by more than $90\%$ above $0.6$ keV [2503.20180]. Health monitoring through overclocking data, calibration-source spectra, and day-Earth observations indicates stable readout noise, CTI growth rates of approximately $(3$–$5)\times10^{-6}$ yr$^{-1}$ per transfer, and negligible contamination even one year after launch [2503.20180].

A widely used systems metric for diffuse-emission work is the grasp, reported as $\Omega_{\rm eff}\sim60$ cm$^2$ deg$^2$ at $6$ keV [2503.20180]. This value, together with the low NXB and wide FoV, underlies many of the instrument’s extended-source use cases.

## 5. Analysis methodology, extended-emission sensitivity, and pile-up treatment

Xtend’s published analysis workflows for faint diffuse emission rely on the combination of wide FoV, low NXB, and explicit instrumental modeling. In the V4641 Sgr study, the data-reduction sequence consisted of the standard XRISM pipeline with HEAsoft 6.32 and CALDB v20240815, good-time filtering to exclude Earth eclipse, SAA, and limb intervals, removal of flickering pixels, production of a raw $1.2$–$7.0$ keV image, subtraction of a particle-background map derived from night-Earth data and rescaled to the $9$–$13$ keV count rate, and application of a vignetting correction derived from day-Earth flat-field data [2412.08089]. This sequence is representative of how Xtend’s low-background design is operationalized for diffuse-source work.

For imaging analysis, the PSF at $6$–$7$ keV was simulated with the `xrtraytrace` ray-tracing tool and calibrated in orbit, with a stated systematic uncertainty in the PSF tail of less than $80\%$ inside $r<8'$ [2412.08089]. In that same analysis, the extended component was modeled with a Gaussian-like radial surface-brightness profile,
$$
I(r)=I_0\exp\!\left[-\frac{r^2}{2\sigma^2}\right],
$$
and the imaging significance was assessed either by an F-test or through annular-bin excess significance relative to PSF plus background [2412.08089]. For spectral work, RMFs were generated with `xtdrmf`, ARFs with `xaarfgen` under a uniform-sky assumption, and source-plus-background spectra were fitted simultaneously with sky, NXB, and extended-emission components [2412.08089]. These details matter because Xtend’s moderate angular resolution is offset by a calibration strategy specifically tailored to low-surface-brightness analyses.

Pile-up is the main limiting effect for bright sources. A dedicated pile-up simulator for XRISM/Xtend was built on ComptonSoft and Geant4, with charge-cloud formation, diffusion, frame formation, and grade reconstruction treated in Monte Carlo [2511.19967]. The pile-up fraction is defined as
$$
F_p = 1 - \frac{r_{\rm out}}{r_{\rm in}},
$$
and, for a single pixel with Poisson arrival rate $\lambda$ and frame time $\tau$, the pile-up probability is
$$
P_{\rm pileup} = 1 - e^{-\lambda \tau}.
$$
Under a Crab-spectrum assumption for a point source, the reported $10\%$ pile-up limits are $7.8$ counts s$^{-1}$ in full-window mode, $66.2$ counts s$^{-1}$ in $1/8$-window mode, and $447.9$ counts s$^{-1}$ in $1/8$-window mode with burst option [2511.19967]. For a diffuse source in full-window mode, the reported $10\%$ threshold is $3.1$ counts s$^{-1}$ arcmin$^{-2}$, corresponding to approximately $5.0\times10^{-11}$ erg s$^{-1}$ cm$^{-2}$ arcmin$^{-2}$ in $0.4$–$13$ keV [2511.19967]. This quantitatively defines the regime in which Xtend can operate as a high-grasp wide-field imager without significant flux distortion.

## 6. Scientific applications and demonstrated results

Xtend’s science program spans both support for Resolve and stand-alone imaging spectroscopy. The in-orbit performance summary emphasizes its role in identifying contaminating point sources and foreground or background structure outside Resolve’s FoV, and notes that Xtend’s soft-band coverage compensated for Resolve during early mission phases [2503.20180]. The same report identifies first-light cluster and SNR observations, reduced-pile-up window modes for bright rapidly varying sources, transient searches, and tiling-mode follow-up of gravitational-wave and high-energy-neutrino alerts as core applications [2503.20180]. This suggests that Xtend’s operational niche is defined less by single-point-source acuity than by a combination of grasp, FoV, background control, and flexible timing modes.

A major demonstration of its extended-source capability is the detection of diffuse X-ray emission around the PeVatron microquasar V4641 Sgr [2412.08089]. Xtend detected the emission with significance greater than $4.5\sigma$ in imaging and greater than $10\sigma$ in spectral analysis. The radial extent was fitted with $\sigma = 7 \pm 3$ arcmin, corresponding to $(13 \pm 5)$ pc at a distance of $6.2$ kpc, and the extracted annular spectrum yielded a non-thermal fit with $N_{\rm H} = (0.6 \pm 0.4)\times10^{22}$ cm$^{-2}$, photon index $\Gamma = 1.8 \pm 0.2$, surface brightness $(9.2 \pm 1.1)\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$ arcmin$^{-2}$, and integrated $2$–$10$ keV flux $(5.1 \pm 0.6)\times10^{-12}$ erg s$^{-1}$ cm$^{-2}$ [2412.08089]. The interpretation offered in that work is that the observed scale can be matched either by an enhanced magnetic field of approximately $80~\mu$G or by a suppressed diffusion coefficient of approximately $10^{27}$ cm$^2$ s$^{-1}$ at $100$ TeV. More generally, the paper argues that Xtend’s very large FoV and well-characterized NXB enable the first detection of faint extended X-ray halos around Galactic PeVatrons [2412.08089].

Xtend has also been used in an unconventional observing configuration: day-Earth occultations that capture solar-flare X-rays reflected in the Earth’s atmosphere [2509.05029]. Over roughly one year of data, Xtend measured abundances of Mg, Si, S, Ar, Ca, and Fe during M- and X-class flares and found an inverse-FIP effect consistent with Suzaku-based results. The large effective area and FoV were reported to permit abundance tracking in several X-class flare loops on timescales of a few 100 s, and the neutral or low-ionized Fe-K$\alpha$ equivalent width was found to show an anti-correlation with hard-X-ray flux with best-fit power-law slope $-0.14 \pm 0.09$ [2509.05029]. Although this is not a primary astrophysical use case of the telescope design, it demonstrates that Xtend’s calibration, throughput, and cadence are sufficiently stable to support quantitative spectroscopy even in a scattering-dominated geometry.

A common misconception is to treat Xtend primarily as an auxiliary instrument to Resolve. The published in-orbit and science-performance record does not support that reduction. Xtend was designed to complement Resolve, but the literature also presents it as a stand-alone wide-field imaging spectrometer with low and stable background, large grasp, multiple readout modes, and demonstrated sensitivity to extended halos, flare-reflection spectra, diffuse backgrounds, transient phenomena, and other contexts in which solid angle and background systematics are more decisive than sub-eV spectroscopy [2503.20180].

Source: https://www.emergentmind.com/topics/xtend