Opinion

X-ray astronomy, Moon

P. Gandhi

# X-ray astronomy from the lunar surface

P. Gandhi1 1School of Physics & Astronomy, University of Southampton, SO17
1BJ, UK<EMAIL_ADDRESS>
((e.g. in the optical or infrared); reflecting off a mirror; We end with a
discussion of several key technical challenges that will need to be overcome
when operating X-ray facilities on the Moon, and relevant considerations for
choosing appropriate observatory sites. There have been previous proposals to
study the Solar wind and Earth’s magnetosheath from the vantage point of X-ray
observatories on the lunar surface [2], but here we go beyond to develop wider
cases for general X-ray astronomical studies.; Accurate metrology calibration
at X-ray wavelengths will be the key challenge in bringing lunar surface
interferometry to fruition. This is true for satellites in-orbit as well, but
with the added complication on the Moon of having to account for unwanted
shifts resulting from human, seismic or thermal activity. Regular calibration
and alignment will be essential. The horizon distance for, say, a 3 m tall
telescope structure on the Moon is about 3 km, which gives a first estimate of
the upper-limit to direct line-of-sight baseline lengths that could be
calibrated (if using lasers between various array elements, for instance).;
These challenges might be addressed through a combination of judicious site
selection, appropriate thermal control technologies, and developing strategies
for rapid path length recalibration and realignment. All of these are
discussed further in § 5.; Current surface polishing techniques remain far
from the ultimate goal of delivering smooth mirrors capable of diffraction-
limited imaging in X-rays. Better precision is possible, and commonly
employed, at longer optical wavelengths – but these are easily obscured and
scattered by foreground dust. X-rays offer the possibility to penetrate
through heavy obscuration. Thus, X-ray occultation studies could open up new
possibilities to study most of the obscured regions of the Milky Way disc, if
they can be implemented.; 0.6 $\sqrt{(L/1\,{\rm
km})(\lambda/6.2\,{\mbox{\AA}})/2}$ mm for 2 keV X-ray photons (say), with $L$
being the distance to the occulter, which would lengthen the occultation
shadow on the detector, in principle. However, small X-ray wavelengths render
diffraction effects much less important than those commonly seen in the radio
to optical regimes [24].; generation and cumulative refinement; near; far;
far; Lunar dust refers to the fine and loose particulate component of
regolith, smaller than pebbles. Dust typically comprises micron-size, abrasive
debris left over from impact disintegration of Silicate and Basaltic rocks
[29]. Exposed to high energy cosmic radiation and Solar wind, dust particles
on the Moon are easily charged, and can be electrostatically levitated above
the surface, thus posing a hazard to any surface equipment and habitation
[30].; Dust screens or shields could be deployed that prevent particulate
matter entering the mirrors, while being transparent to X-rays. Depending upon
the material used, screens may be opaque to certain energy ranges, especially
at soft X-rays. This will need to be factored in to design considerations, but
could also help screen out some of the particle-induced background counts from
the lunar surface (discussed further below). Active removal of dust should be
possible electrostatically, by lofting charged dust particles away from
surfaces using suitable electrically biased plates or wands. Dust particles
could also be actively charged using electron beams or plasma jets, thereby
allowing them to be lofted and removed [31]. Such devices may either be fitted
on the telescopes, or they could be outsourced to service modules and robots
in the longer term.; Ultimately, calibration and regular cleaning will need to
become a matter of routine. Slow accumulation of dust can be accounted for, as
long as appropriate calibration data are regularly taken. Once human
settlements are more developed, regular downtime for maintenance and cleaning
regimens can be implemented.; Another important factor to contend with will be
seismic activity, for which relatively few studies exist. The Apollo Seismic
Network operated for over a decade, and showed that the most common kind of
seismic activity results from deep moon quakes, with magnitudes $m_{b}$ $<$ 3
and a frequency of $\sim$ 500 events per year [32, 33, 34], on average, driven
in large part by tidal interactions with the Earth and the Sun. Shallow quakes
can be much more powerful, but occur only a few times per year. Meteoroid
impacts are also a intermittent source of seismic activity, and are observed
to be strongly clustered in time, possibly associated with meteor shower
streams [35]. Not enough is known about the locations and focal origin points
of deep and shallow quakes. These appear to be patchy and focused on the near
side according to the data obtained thus far, though this is likely to be a
selection effect of the current monitor sampling [32]. The Chandrayaan-3
Vikram lander, which recently touched down at a southern lunar latitude,
carried an instrument to detect seismic activity in these ill-explored zones
[36]. In the longer term, we should learn a lot more from ambitious concepts
such as the Lunar Gravitational-Wave Antenna, which will comprise a correlated
network of seismometers aimed at providing several orders-of-magnitude
improvement in sensitivity over the Apollo network in the mHz range [37].
Locally strong seismic activity will inevitably interrupt observations.
Focusing and pointing recalibration strategies will be important to implement,
to ensure speedy realignment following seismic events. Earth-based telescopes
now routinely include anti-seismic measures such as shock absorbers that
protect sensitive elements and provide some insulation from quake movements
[38]. Daily regimens of pointing calibration are also a standard part of most
ground observatory operations. Seismicity is likely to be less of a
complication for timing collimators, but will impact science cases with
stringent path length tolerances such as a long focal-length telescopes and
interferometers. These challenging concepts will likely benefit from more
conventional designs (e.g. monolithic, physically connected elements) in early
stage designs. The technology for advanced seismic isolation stages and
suspension systems now exists, and has been demonstrated in gravitational wave
interferometers [39]. Porting some of this technology could help to mitigate
environmental noise for lunar X-ray interferometry. ;

###  5.3 Thermal stresses

The lunar surface represents an extreme thermal environment, with temperature
variations of more than 250 K in exposed equatorial surface regions [40].
Directed and reflected Solar radiation, as well as reemitted infrared
radiation, all act as incident heat loads for any surface body during day
time. Conversely night-time operations require added heat retention capacity
against the bitterly low surface temperatures. The overall impact of these
variations is thermal stress on telescope frames and electronics, resulting in
long-term fatigue.

These are important considerations for X-ray optics, where component
tolerances can be more demanding than other wavelengths. Materials with
exceptionally low thermal expansion coefficients will be required, in addition
to development of novel radiators and thermal insulation. These would also
have to be tolerant to dust, which will inevitably accumulate on exposed
surfaces over time and change their thermal regulation properties. Conversely,
even thin layers of regolith can themselves act as very effective insulators
for moderating temperature extremes (variations are known to drop to less than
a few deg at shallow depths of just tens of cm [41]), and so could potentially
be incorporated into thermal protection designs of specific telescope and
electronic components. This would not solve the problem of thermal stresses on
the sky-facing telescope surfaces, of course.

Active cooling strategies adopted in X-ray telescopes now include Transition
Edge Sensors.

; For night time heat retention and operation, radioisotope heater units
(RHUs), radiothermal generators (RTGs) and thermal capacitors are some
technological solutions being developed. RHUs have been successfully used
since the early days of lunar exploration [45]. RTGs typically incorporate
highly toxic and difficult-to-obtain materials with short half lives (e.g.
radioactive isotopes of Plutonium and Polonium), and obviously also come with
radiation safety concerns. Other elemental isotopes are under active
consideration, e.g. Americium-241 [46].; For the more challenging science
cases still at the proof-of-concept stage (e.g. X-ray interferometry), smaller
monolithic structures that can be thermally controlled more efficiently would
be the obvious starting point.;

###  5.4 Background

The lunar surface will act as a source of external background. Particle
Induced X-ray Emission (PIXE), scattering and X-ray fluorescence (XRF) from
the surface will generate elevated environmental counts. Such emission has
been detected in a range of experiments dedicated to studying the lunar albedo
and surface chemistry [47, 48]. Solar-induced scattering is the primary driver
of this background, though extra-solar cosmic rays will also contribute. The
sunlit side scatters incident Solar X-rays with an efficiency of $\sim$ 10-5,
peaking at soft energies [49]. The ‘dark’ Moon (i.e. the surface not
illuminated by the Sun) is quieter by orders of magnitude, though its emission
remains to be characterised in detail [19].

Thus, observations at soft energies and at low, near-horizon altitudes will
suffer from scattered surface background during lunar day. For example,
topographic occultation observations will need care to screen out the
resultant soft scattered X-ray background, or to avoid observations in the
direction of sunlit topography. The detectors themselves will benefit from
collimators and shields (excluded from the schematic impression in Fig. 1 for
clarity). Judicious site selection may also help here, if appropriate shadowed
mountain sides or craters can be identified as observatory sites. XRF can also
fluoresce artificial materials, albeit concentrated in specific transitions
[50], which custom-made occulters will need to account for in their designs.

Additional weaker, non-negligible sources of background will include X-ray
scattering by the thin dusty plasma that has been elevated in Moon’s exosphere
[30], and charge exchange during passages during the Earth’s magnetotail on a
few days near full Moon [51]. Other natural sky background rates should not
differ substantially from the rates seen by telescopes in high Earth orbit,
and will benefit from the absence of the excess noise experienced in Earth’s
South Atlantic Anomaly. Knowledge of the background impacting other X-ray
facilities operating far from the Earth (e.g. at L2) would also be
informative. Detailed quantification of these background effects will be
important for full characterisation of the overall environmental noise.

; A constant power supply will need to be maintained on-site. As the
efficiency and capacity of industrial-scale rechargeable batteries improve,
these could plausibly be stacked together and deployed for lunar night
operations of low-power facilities. Monthly battery replacement, or recharging
during lunar day using solar panels (cf. Fig. 1), could be integrated into
facility logistical plans. Power demands will scale with the size and
‘complexity’ of the science cases discussed above, with the more ambitious
plans likely requiring the presence of developed human infrastructure to
sustain their power needs.; but our nearest cosmic neighbour has always had a
tantalizing relationship with X-ray astronomy. After all, the very first
rocket flight that opened up cosmic X-ray astronomy was, supposedly, aimed at
searching for X-rays from the Moon [1]. And the Moon enabled some of the first
detailed cosmic occultation studies in X-rays.; Operating on the lunar surface
will also introduce significant new challenges, including elevated background,
seismicity and thermal variations, which should not be underestimated.; , and
S. Mereghetti, C. Dashwood Brown, D. Buisson and J. Tomsick for supportive
comments and suggestions. Feedback from two anonymous reviwers helped to
strengthen the manuscript substantially)

###### Abstract

Motivated by efforts to return humanity to the Moon, three cases are reviewed
for X-ray astronomy from the lunar surface: (1) Facilitation of ambitious
engineering designs including high throughput telescopes, long focal length
optics and X-ray interferometery; (2) Occultation studies and the gain they
enable in astrometric precision; (3) Multimessenger time-domain coordinated
observations. The potential benefits of, and challenges presented by,
operating from the Moon are discussed. Some of these cases have relatively low
mass budgets and could be conducted as early pathfinders, while others are
more ambitious and will likely need to await improvements in technology or
well-developed lunar bases.

###### keywords:

X-ray astronomy, Moon, Interferometry, Microcalorimeters, Occultation,
Multimessenger

##  1 Introduction

X-ray astronomy is just over six decades old, and has been fundamental to
unlocking the hot and energetic universe on all scales [1]. Detection of high
energy cosmic photons can only be done from beyond the Earth’s atmosphere.
Focusing of X-rays also requires specialised designs different from the normal
incidence telescopes in wide use at lower energies . This has posed important
constraints on the development of the field so far.

For example, grazing photon incidence is the primary mode employed in X-ray
imaging optics. At the shallow angles necessary for bringing high energy
X-rays to a focus, the requisite focal lengths start to become impracticably
long for single satellites in orbit. Furthermore, deployment and control of
large area telescopes in orbit is not currently possible due to payload launch
constraints. As a result, X-ray focusing telescopes are restricted to
effective collecting areas of order a few hundred sq. cm, far smaller than
telescope mirrors commonly available at lower energies.

Unlike the Earth, lunar X-ray astronomy could be conducted from the surface.
Telescopes and their supporting infrastructure would have to contend with

surface gravity, but one that is low relative to Earth. This could
substantially ease, and even circumvent, the above engineering challenges, and
also open up other novel directions of scientific developments, though not
without introducing new challenges.

Humanity’s return to the Moon with the Artemis missions and proposed lunar
settlement plans currently being formulated (cf. Carpenter, this volume)
provide the opportunity to explore new science cases that may benefit from
operation on the lunar surface. Here, we outline three such cases for X-ray
astronomy. We discuss how operating from the lunar surface could facilitate
the implementation of complex engineering design concepts, including large
area and long X-ray focal length telescopes, as well as multi-telescope
interferometry. Following this, two science cases are developed, centred on
occultation studies of bright cosmic targets in order to enable X-ray
astrometric studies, and a case for coherent multiwavelength approaches to
coordinating simultaneous observations. Similar ambitions existed as early as
1990 [3], though the progress of the field has proven to be more challenging
than anticipated at that time. This article is not meant as an exhaustive
review. Instead, it should be read as a pilot proposal on the need to consider
and develop novel scientific exploitation opportunities offered by our nearest
cosmic neighbour, even for the kinds of science that we have been carrying out
from beyond the Earth for decades.

##  2 Facilitating Ambitious Engineering Designs

Figure 1: Artist’s impression of a focusing X-ray telescope in a dome on the
lunar surface. The dome is a mesh structure supporting and directing the
mirrors. X-ray detectors are situated at the centre of the dome on the ground,
separate from the mirror structure but tracking-locked. Additional collimators
and detector shielding material will be required, but has been stripped away
for clarity in this impression. Lunar topography and power units are visible
in the background, with much of the control electronics expected to be buried
below the surface regolith. An imagined autonomous servicing rover is depicted
in the foreground. Credit: S. Mandhai inc. @TheAstroPhoenix.

###  2.1 Larger Area, Longer Focal Length Optics

Focusing X-ray photons requires grazing incidence optics, with the current
dominant design being the Wolter type [4]. The relevant critical angles
($\theta$) are shallow, and scale inversely with photon energy $E$:

$\displaystyle\theta$ $\displaystyle=$ $\displaystyle
5.6\,{\lambda}\,{\sqrt{\rho}}\,\,{\rm arcmin}$ (1) $\displaystyle=$
$\displaystyle 1.2\,\frac{\sqrt{\rho}}{E}\,\,{\rm deg}$ (2)

where $\rho$ is the mass density of the mirror surface material in gm cm-3,
and $\lambda$ the photon wavelength in Angstroms [5]. Imaging around 10 keV
requires grazing incidence angles of small fractions of a degree. Bringing
these photons to a sharp focus then necessitates long focal lengths. The
NuSTAR mission, for instance, uses a deployable mast about 10 m in length [6].

Imagine, instead, a lunar X-ray observatory anchored to the surface. This
location should allow the requisite space and infrastructure deployment needed
for operation and control of long focal length telescopes. A rigid structure,
such as a dome or a basic skeletal frame, capable of supporting the mirrors,
orientating them, and tracking cosmic sources would be required. The X-ray
detectors need not be physically attached to the same structure, if they can
be tracking-locked to the mirrors, say. An artist’s impression of one such
conceptual schematic design in shown in Fig. 1. If such ambitious designs can
be realised, they would open up cosmological studies of the growth of cosmic
structure in the early universe, in addition to a multitude of other science
cases beyond the sensitivity and resolution of current facilities.

An important design consideration here is that mirror effective area will
scale down as the projected surface area ($\sim$ ${\rm sin}\,\theta)$
decreases, so shallower incident angles would correspond to smaller effective
areas. Detailed engineering simulations will thus be needed to verify the
practical limitations on useful focal length enhancements. Size will
ultimately be constrained by the mirror and dome body mass that can be
transported to the lunar surface. Future concept designs for large-area X-ray
mirrors with state-of-the-art focusing optics include mono-crystalline Silicon
as a low-density, high tensile strength substrate free of stress. The AXIS
telescope concept, for instance, aims to achieve an order-of-magnitude
improvement in effective area over Chandra, while maintaining similar imaging
quality, and a total optics mass of $\approx$ 1 tonne [7]. Materials such as a
Silicon would also be readily extractable from lunar mines for in-situ
manufacturing in the more distant future.

Similarly, lightweight, stiff materials (e.g. Carbon fiber or other
composites) would be preferred for the support structure. Larger payload
capacities such as with SpaceX’s Starship (promising up to 100 tonnes of
payload capacity111https://www.spacex.com/vehicles/starship/) would facilitate
larger, more ambitious designs for lunar installation [8]. The absence of
lunar winds mitigates the need for a fully covered protective dome, allowing
for structure construction to be relatively light, though some additional
shielding and collimation to protect the detectors from background X-rays will
be required.

Simpler designs with large array detectors that maximise collecting area
without optimising focusing optics have also been considered before [3, 9]. An
array of high throughput collimation detectors would act as excellent ‘photon
buckets’ for timing science, and could enable a wide range of transient
studies, discussed later in the article. Such designs ought to be amongst the
first X-ray pathfinders to be deployed on the lunar surface.

Another concept that could benefit is the lobster eye mirror design. These
mirrors mimic the reflecting tubes that have evolved naturally as part of
lobster eye vision, and provide the unique capability of ultra wide field
imaging. Such a design was conceptualised several decades ago [10], but only
recently demonstrated in orbit with an Einstein Probe pathfinder mission,
LEIA, covering an instantaneous field-of-view of 340 deg2 [11]. The typical
collecting areas of current designs is modest (a few cm2). Stacking large
arrays of microchannel plates is impractical in orbit, but could be done on
the Moon to boost the focal collecting area.

###  2.2 X-ray Interferometry

Normal incidence focusing telescopes can be (and have been) designed to be
diffraction-limited at all wavelengths from the ultraviolet to the radio
bands. A $d$ = 1 m diameter optical telescope operating at a wavelength
$\lambda$ = 6000 Å, say, yields an effective Airy disc size angular resolution
$\theta$ $\sim$ 0.15 arcsec. X-ray observations have the potential to deliver
enormous gains in angular resolution relative to longer wavelengths, given
that the Rayleigh diffraction criterion scales linearly with wavelength
$\lambda$. An X-ray telescope sampling 1 m of an incoming X-ray wavefront at
an energy $E$ = 2 keV (say, equivalent to $\lambda$ = 6.2 Å) should, in
principle, be capable of achieving an angular precision three orders of
magnitude better ($\theta$ $\sim$ 0.15 milli-arcsec). But X-ray telescope
designs are not, currently, diffraction-limited because of the lack of
technology capable of polishing mirror surfaces that are commensurately smooth
to within a fraction of a wavelength.

An alternative approach would be to utilise interferometry, which still
remains to be realised for cosmic X-ray studies. Bringing together signals
from multiple telescopes (fringe combining) results in effective gains in
angular resolution of $\sim$ $L$/$d$, where $L$ is the characteristic baseline
separation of the array. Sparse interferometry with just a few telescopes
primarily yields a complex ‘visibility’ – a measure of the spatial extent of
the target, but better sampling of the image plane over time can be used to
reconstruct source images. Interferometry thus allows $\theta$ to improve
indefinitely beyond the confines of single telescopes, in principle, being
ultimately limited by the sensitivity resulting from the finite collecting
area of an array.

Uttley et al. [12] outline a concept design of an orbiting facility that could
leverage X-ray interferometric gains even within a single spacecraft.222based
upon flat reflecting mirror design proposed for the MAXIM mission [13]. Phase
interferometry on baselines as short as $\sim$ 20 m would open up a vast suite
of cosmic X-ray source studies, including direct imaging and resolution of the
accretion discs around accreting supermassive black holes, the orbits of
Galactic X-ray binaries, and the coronae around nearby active stars, to name a
few. Expanding this design to a multi-spacecraft constellation would open up
studies at angular resolutions on sub-micro-arcsec scales of the ‘shadows’ of
massive black holes at the nuclei of many nearby galaxies, measurements of the
elongations of gamma-ray burst afterglows at cosmological distances, and allow
the capability to resolve exoplanet transits across X-ray emitting stellar
discs. For reference, with a 10 km baseline, observing 10 keV (1 Å) photons
yields $\theta$ $\sim$ 10 nanoarcsec. Alternate mission concepts also exist,
promising the delivery of a step change in X-ray angular resolution (e.g.,
[14]).

But cosmic X-ray interferometry remains to be demonstrated, and there are
enormous challenges associated with bringing this to fruition, specifically in
enabling path-length equalization at precisions of order nanometers, across
$\sim$ 1 m+ baselines relevant for realistic telescope collecting areas.
Highly rigid systems with stable pointing control will be needed in a single
spacecraft. Thermal stability will also be critical (discussed further in §
5). Some of these challenges are compounded for a constellation where
stabilisation is needs to be maintained across the array baseline connecting
free flying spacecraft, which may be separated by tens to hundreds of km or
more.

Placing such an interferometer on the Moon would alleviate some of these
extreme design challenges, while introducing others. Baselines can be fixed
rigid to the ground for multi-telescope arrays across several metres scales,
and gradually extended to several hundred metres or more in the long-term.
Filling the $u$–$v$ Fourier plane for imaging reconstruction can also be
easily done by taking advantage of the rotation of the Moon, as is now
standard for Earth-based interferometers. Tidal locking of the Moon means that
this rotation will be relatively slow, favouring observations of order $\sim$
1 Ms (a duration of about two weeks).

Conversely, customisation of baselines (as is the case for the radio Very
Large Array on Earth) could also be implemented on-demand for specific angular
resolution needs, without the restriction of a limited fuel supply that would
be relevant in-orbit.

Additional challenges will include stringent requirements on thermal
stability. Specifically, precise beam positioning as well as stability of
target staring will require exceptionally low thermal coefficients for the
telescope structural components, or will require isothermal controls at $\sim$
mK levels.

##  3 Astrometry with Occultation studies

Occultation of a background light source by a foreground body offers the
possibility of high angular resolution studies. This relies on accurate timing
in order to enhance spatial resolution, assuming that the instantaneous
location of the foreground body is known well enough, and that the occultation
time can be measured with sufficient accuracy. With enough signal, such
studies can be used to enhance spatial localisation of the background source,
study it size and shape, and also search for sub-structure within both bodies.

In the early days of X-ray astronomy, lunar occultations were utilised as a
means to characterise the spatial profile of extended sources. For instance,
Bowyer et al. [15] observed a lunar occultation of the Crab Nebula using an
Aerobee rocket, measuring the extent of the nebular emission. Point-source
astrometric measurements have also been improved through occultation
measurements (e.g., [16, 17]). The need for occultation studies was largely
alleviated as focusing X-ray observatories became mainstream, and as their
angular resolution improved. Achieving angular resolutions much better than a
few arcsec still remains challenging, though, with the few facilities such as
Chandra capable of high quality imaging being expensive to construct and
launch, and with pointed X-ray observations covering only a small field-of-
view.

A return to the Moon offers the possibility of a revival of X-ray occultation
studies. This is because large-area detectors can now be made relatively
cheaply. A good example here is NICER [18], which routinely observes transient
sources gathering hundreds of counts per second, but at the expense of spatial
resolution. This mitigates a key limitation of past occultation studies, which
were photon-starved. Being able to place large-area telescopes on the Moon
could then substantially improve on timing and occultation work.

Below, we consider two kinds of occultation events – due to natural, and
artificial, foreground occulters. The most ambitious of these will be a
challenge, but if feasible, could push X-ray studies towards accurate
(fractional arcsec) astrometry, a field that has so far been the domain of
only longer wavelengths.

###  3.1 Natural foreground occultation sources

The instantaneous projected area of the Earth as seen from the Moon is 2.8
deg2 on average, about 13 times larger than that of the Moon seen from the
Earth. Over the course of a sidereal month (27.3 days), the Earth would sweep
out $\approx$ 660 deg2 of the sky from a lunar vantage point. Due to
precession of the lunar orbit, over 18.6 years the full sweep will encompass
an area just shy of 20 % of the sky. For a lunar site with a view of the
Earth, this should allow a wide range of occultation studies using the Earth
as the foreground occulter.

Absorption and refraction through the Earth’s atmosphere will blur the event,
reducing astrometric precision, and would also be strongly energy dependent.
Solar Wind Charge eXchange (SWCX) radiation from the Earth’s magnetosphere and
sheath will also create an elevated background at soft energies around the
Earth’s limb [19, 20, 21]. A more interesting possibility could then be to
instead utilise lunar topography as the foreground occulter. A near equatorial
site would see the entire sky rise and then set once a month. All sources in
the sky would thus be occulted by the lunar landscape twice every lunar month.

Here, we explore positional precision in an occultation event. This will
depend upon the source ($C_{\rm S}$) and background ($C_{\rm B}$) count rates,
together with the defined signal-to-noise ratio (SNmin) threshold
corresponding to a significant detection. We model our estimates follow the
algorithm outlined by [22]. Let us assume a small time bin $\Delta T$ during
which counts are accumulated. The simplest case is to assume the high count
rate limit relevant for bright sources (e.g. Galactic transients), where
uncertainties are Gaussian, and with two consecutive time bins straddling the
instant of occultation (i.e. the target being visible in one bin and then
disappearing in the next). A source can be robustly claimed to have entered
occultation within a time interval $\Delta T$ if

$\displaystyle C_{\rm S}\,\Delta T\,\geq\,SN_{\rm min}\times\sqrt{C_{\rm
B}\Delta T+(C_{\rm S}+C_{\rm B})\Delta T}.$ (3)

In the small counts regime, this is modified by the need to incorporate
Poisson uncertainties instead. Here, we approximate the overall noise by
propagating the confidence limit formulae presented by Gehrels [23]. A
detailed assessment of timing precision will require proper simulations, and
will be able to utilise systemic differences in the light curve levels pre-,
and then post-, occultation, in order to improve statistics beyond the
information provided by the two consecutive bins straddling the occultation.
It will also be instrument-specific, dependent on deadtime effects,
variability in source flux, and limited by the background. These details are
beyond the scope of our evaluations here, which should thus be considered as a
baseline upon which to build future simulations.

The angular speed of the occulter ($\Delta s$/$\Delta T$) projected on the
direction of areal motion of the background source (at a relative angle
$\alpha$, say) then governs the angular precision $\Delta s$. In this
scenario, the angular speed is $\approx$ 0.55 arcsec sec-1 for both the Earth
and for lunar topographic features, as seen from the surface.

Fig. 2 shows the resultant precision $\Delta s$ possible, for a wide range of
count rates, assuming SNmin = 5, and $\alpha$ = 90∘ between the limb of the
occulter and the projected areal direction of motion. The solid curve
corresponds to $C_{\rm B}$ = 1 ct s-1, whilst the dotted and dashed curves
straddling it on either side correspond to background rates two dex higher and
lower, respectively.

The figure demonstrates the impressive gains in spatial localisation that can
be made at high count rates. Specifically, at $C_{\rm S}$ $\approx$ tens of ct
s-1, $\Delta s$ already matches the current best X-ray angular resolution of
$\approx$ 0.5 arcsec delivered by Chandra. At 500 ct s-1, $\Delta s$ is a few
$\times$ 10 mas. In this regime, it becomes possible to not only accurately
localise and associate source counterparts unambiguously, but also to start to
measure their proper motions, directly in X-rays. For reference, 10 ct s-1
with a telescope of the collecting area of NICER corresponds to an X-ray flux
$F_{0.5-10}$ = 2 $\times$ 10-11 erg s-1 cm-2 for a typical X-ray power-law
spectrum with photon index $\Gamma$ = 2.333Count rate $N_{\rm counts}$
$\propto$ $E^{-\Gamma}$. Larger collimator arrays could push this sensitivity
deeper without the need for focusing optics, if the background level can be
restricted to a reasonable level.

Some relevant limitations to consider here include the diffraction of photons
around any occulting limb. The relevant Fresnel scale in this case will be
Furthermore, physically extended emission regions such as scattering halos
surrounding compact accreting sources could lengthen the occultation event or
reduce its contrast. Conversely, occultations can be used to find and study
the substructures of such extended emission zones (e.g. [25]). Finally,
$\Delta s$ will ultimately by limited by the precision of local topographic
mapping. For a hill at distance $d_{\rm km}$ km from an observing site, the
height of the summit will need to be known to a precision better than $\Delta
h$ = 0.5 ($\Delta s$/0$.\\!\\!^{\prime\prime}$1) $d_{\rm km}$ mm. Utilising
artificial occultation devices, instead, could alleviate some of this
limitation, as discussed below.

Figure 2: Astrometric constraints possible with occultation studies from the
Moon. Localisation precision $\Delta s$ in arcsec is shown as a function of
source count rate. The solid curve is the prediction for a background rate of
$C_{\rm B}$ = 1 ct s-1. The dotted and dashed lines denote rates of 0.01 and
100 ct s-1, respectively. Time bins $\Delta T$ corresponding to a signal-to-
noise detection threshold of 5 are annotated.

###  3.2 Customised occultation devices

Having telescopes on the lunar surface within physical reach enables on-demand
customisation of hardware, tailored to specific observations. Artificial
occulters could be deployed such that an occultation will occur for a source
of interest at specific times and at any altitude and azimuth. These could be
simple metallic or other custom-designed opaque plates. Deciding the number,
shape, smoothness, and height above ground of the occulter then becomes a
(relatively easy) engineering problem to address.

Artificial occulters would be most beneficial for newly discovered transients,
when a natural occultation is not imminent. The expected astrometric gain
should be similar to that discussed in the previous section, and would
circumvent the challenge of high accuracy topographic relief mapping.

Occultation by a plane is a one-dimensional event, so the positional accuracy
is maximised in the direction normal to the occultation, and the calculations
above assume $\alpha$ = 90$.\\!\\!^{\circ}$ Arranging for two occultations,
with edges at orthogonal angles to each other (e.g. with a suitably oriented
wedge) would allow gains to be made in two directions, resulting in a better
positional reconstruction using two-dimensional astrometric information.

###  3.3 Astrometric Reference Frames

Astrometry requires cross-calibration to a reliable coordinate reference
frame. For orbiting satellites, default pointing uncertainties can be of order
arcsec or larger, unless fine guiding instrumentation is deployed. Even with
fine guiding, high astrometric accuracy can be difficult unless the
observations utilise instrumentation with large fields of view to encompass
multiple background objects within a single observation. Calibration then
relies upon tying some of these objects to known astrometric catalogues.

A fixed observing site on the Moon, together with occulters whose relative
location is unambiguously known, mitigates much of the uncertainty associated
with pointing inaccuracies. This will allow of an accurate absolute
astrometric reference frame relevant to any given site. Regular calibration
will undoubtedly be required to account for instrumental and background
deviations (discussed further in § 5).

##  4 Multimessenger Time-domain Coordination

Opening up new parameter space inevitably leads to new discoveries [26].
Multiwavelength and multimessenger studies have demonstrated this repeatedly
over the years through a wealth of astrophysical discoveries, because many
astrophysical phenomena are inherently broadband sources of radiation.

Sources that are spatially compact are additionally expected to show time-
domain correlations between various bands. Some of the most extreme changes in
time are associated with non-thermal radiation processes such as those found
in Gamma-ray bursts, kilonovae, active galactic nuclei, tidal disruption
events, magnetars, and X-ray binaries, to name a few examples. Their discovery
has often relied on having observing cadences sampling $\sim$ weekly
timescales, with some campaigns able to organise intensive daily monitoring.
But physical changes often occur faster than this. These could be state
transitions in X-ray binaries, fast radio bursts, periodic eruptions, jet
switches (on or off), and more.

Current monitoring strategies are often too patchy to catch these events,
leading to missing out on evidence for some of the key physics connecting the
various bands. The issue is compounded when the expected changes span more
widely separated bands. One example is shown in Fig. 3. This presents a rapid
state transition that occurred during the accretion outburst of the Galactic
black hole X-ray binary V404 Cyg. Two X-ray epochs are shown, separated by a
gap in the middle caused by one of the regular Earth occultations every $\sim$
90 minutes typical for orbiting satellites, when the target is hidden by our
planet. The source behaviour differs dramatically between the two epochs, with
the second one being significantly brighter and spectrally ‘harder’ (i.e.
peaked towards higher energies) in both X-rays and radio (panels b–d).
Significant inter-band timing correlations also appear in Epoch 2, whilst they
are absent in Epoch 1 (panels f–g). Earth occultation lasted just over 2,000
seconds, but deprived us of the exact instant of transition in X-rays. The
source went on to become one of the brightest cosmic sources in the sky some
hours later (for details, see [27]).

This example illustrates some of the challenges associated with current
multiwavelength coordination. Observers have to contend with visibility
constraints at multiple sites, typical $\sim$ 50%+ losses due to Earth
occultation (relevant for both ground and space telescopes), not to mention
time lost as a result of differing weather conditions at different sites. A
detailed discussion of the barriers to coordinating observational campaigns
can be found in Middleton et al. [28]. Additional human factors include the
current ‘double jeopardy’ associated with the need to have proposals accepted
by multiple time allocation committees for a single coordinated campaign, and
the lack of a coherent approach to multiwavelength proposing that can enable
such observations as a matter of course.

As we push the boundaries of time-domain astrophysics, the demands on the need
for strictly coordinated observations will rise. The campaign on V404 Cyg was
far from well planned, and emerged in near real-time as the spectacular nature
of the source became clear to astronomers worldwide. Opportunities for
coordinated tracking of the outburst evolution were missed as a result.
Despite the best will of all interested parties, having widely separated
facilities each with their own operational constraints can result in
disjointed efforts.

The Moon offers the possibility of surmounting these barriers, by allowing a
single site to host facilities capable of detecting photons across the entire
electromagnetic spectrum. Telescopes from radio to X-rays (and beyond) at a
single location would all be subject to (near) identical visibility
constraints, and free of atmospheric disturbances. This could allow
astronomers at all wavelengths to reconsider and optimise coherent approaches
to observation planning and delivery. Strict multiwavelength and
multimessenger coordination could become the default mode of operation.

The long lunar synodic day (29.5 Earth days) would allow lengthy uninterrupted
multiwavelength monitoring of transient and variable events, at least $\sim$
30 $\times$ longer than what is possible in Earth (ground and low orbit)
observations. This will open up new observation possibilities for transient
events in the Fourier frequency regime of $\sim$ $\mu$Hz–mHz. Dual
observatories on the and the sides could remove virtually all sampling gaps,
delivering unprecedented coordinated monitoring data.

Such a multiwavelength facility could also serve as an focus for international
collaborative efforts, if multiple agencies would agree to lead the delivery
of infrastructure required in individual bands (for example), with everyone
benefitting from the strict simultaneity of the data that are ultimately
gathered.

Figure 3: An example of the need for strictly simultaneous monitoring across
the electromagetic spectrum. The black hole X-ray binary V404 Cyg underwent a
spectacular but short-lived accretion outburst in 2015. This figure shows the
only set of coordinated optical (Panel a), X-rays (b, c) and radio (d, e)
observations that could be arranged at high time resolution with ULTRACAM,
NuSTAR, and AMI. Other opportunities were missed due to day/night visibility
constraints. Even here, Earth occultation of NuSTAR resulted in missing the
instant of a physical state transition apparent from the dramatic changes
visible between Epochs 1 and 2, separated by the empty gap of X-ray data in
the middle. Note the significant rise in X-ray brightness (b) and spectral
hardness (c) in Epoch 2, similar effects in the radio (d and e) and the
appearance of significant optical/X-rays timing correlations (DCF; panels f,
g), none of which is visible pre-occultation during Epoch 1. The instant of
transition may have been associated with the sudden appearance of rapid (sub-
second) optical fluctuations visible in the inset in panel (a) during the
X-ray data gap. From [27].

##  5 Site Considerations

Unlike low frequency radio telescopes targetting the early universe, X-ray
observatories do not need to be located on the side of the Moon. Power
requirements are not expected to be excessive either, at least in the various
case studies envisaged herein, though such requirements will inevitably scale
in proportion with the size of facility designs.

Though there will be important challenges to overcome, X-ray observatories
could thus benefit from the fact that they will be affected by different (and,
in some cases, fewer) site constraints as compared to other facilities. The
paucity of ‘prime’ observing sites on the lunar surface (cf. Krolikowski &
Elvis, this volume) due to a combination of human activity and stringent
requirements of other wavelengths and messengers is an issue that should not
be underestimated.

Below, we discuss several of the primary challenges and considerations that
will need to be addressed for any operating X-ray facility on the lunar
surface.

###  5.1 Lunar Dust

Contending with lunar dust will be an important consideration for all planned
lunar facilities. For nested shell X-ray mirrors, accumulation of dust will
need to be prevented within the narrow inter-shell gaps. So locations very
close to human habitation, mining and launch/landing ports would be best
avoided. But lunar regolith could also serve the useful purpose of acting as a
shield for any sensitive electronic equipment, with light layers of dust
capable of protecting from cosmic rays.

###  5.2 Seismicity

These are superconducting sensors with exquisite sensitivity to discriminate
the energies of incident photons. As a result, they are expected to deliver
orders-of-magnitude improvements in spectral resolution with upcoming
microcalorimeter arrays (e.g., [42, 43, 44]). When cryogenically cooled to
$\sim$ 50 mK, thermal noise is suppressed and results in spectral resolution
$E/\Delta E>1,000$ at astrophysically important energies around 6 keV.

Lunar observatories would offer the possibility of on-demand coolant
refilling, and any cryogenic cooling instruments could also benefit from the
mining 3He on the Moon. Alternatively, closed cycle coolers are increasingly
replacing cryogenic technology. These can generate microphonic noise which not
only worsens spectral resolution, but could also potentially destabilise beam
combination. Physically separating the cycling coolers from the detectors
would help to lessen any microphonic noise, and will be easier to implement on
the lunar surface, free from the confines of a spacecraft.

A side benefit of the high spectral resolution enabled by microcalorimeters is
enhanced fringe sampling and a larger field-of-view for X-ray interferometers
[12]. This would, in turn, help with interferometric source localisation,
overcoming the current deficiency of targets that have known coordinates
accurate at the sub-milliarcsec level, a requirement for fringe acquisition
and combination.

Ultimately, though, there will be limitations to the surface areas that can be
actively cooled (or heated). Any such design considerations will have to be
combined with judicious site selection. Thermal gradients could be mitigated
by locating facilities in either the warmer zones of the shadowed lunar
regions, or in the high illumination regions (the so-called peaks of eternal
light).

Beyond the issues discussed above, facility siting will be dependent upon the
desired science goals and calibration requirements.

Galactic astronomy has a long heritage in X-rays. The brightest X-ray sources
in the sky beyond the Solar system will be Galactic point sources, mostly
accreting compact objects. These would be the best targets for the astrometric
occultation science case, as well as the coordination science case, discussed
above. An optimal site would thus allow good visibility of the Galactic bulge
and centre region. Such a site would still undoubtedly be able to deliver
cosmological studies.

Some of the science cases considered above may best be undertaken from craters
with floors a few tens to hundred metres wide, or larger (e.g. to optimise
line-of-sight visibility of other unit telescopes in an interferometric array
for calibration purposes, or clear sightlines to foreground occultation
sources).

Calibration sources could plausibly be embedded in rocks and topography
surrounding the site,444Putting in place appropriate safety measures for human
presence, of course. providing bright emission sources, free of Doppler
variations, at a variety of focal lengths. In the longer term, construction,
testing, and calibration of large-scale X-ray telescopes could be moved
entirely to the lunar surface, because the absence of an atmosphere removes
the need for specialised evacuated beamlines as currently required on Earth.

##  6 Conclusion

X-ray astronomical observations can only be conducted from beyond the Earth’s
atmosphere. The benefits of the Moon in this regard may not be immediately
obvious,

The field has moved on a long way since then, but here we have presented
several science cases and engineering considerations that could reap benefits
from humanity’s inevitable return to the Moon, including reengineering designs
to surmount current practical restrictions to telescope focal lengths and
effective areas, development of X-ray interferometery, a revival of
occultation studies for improving astrometric precision, and developing a
coordinated approach to multimessenger time-domain observations.

And ultimately, cost considerations will likely be the governing factor for
adoption of any of these proposals. Several of the cases above can be
implemented without payload capacity being a restriction. For example, NICER
has a compact and light-weight design, with a launch mass of only 372
kg.555https://www.nasa.gov/sites/default/files/atoms/files/spacex_crs-11_mission_overview.pdf
With Starship promising a payload transport capability of $\sim$ 100 tonnes,
deployment of telescopes with effective area mass tens of times larger than
NICER should not pose a critical limitation.

Some of the ambitious design infrastructure (e.g. the dome from Fig. 1) would
benefit most from enhanced launch capabilities, and these likely need to await
advanced settlement of Moon bases. In the mean time, it should be feasible to
deploy lightweight pathfinders that can operate autonomously from the surface,
once initial lunar support capability becomes operational, via the Gateway
orbiting spaceport and leveraging the proposed regular landings of the
European Argonaut missions (Carpenter, this volume).

PG acknowledges support from UKRI, and thanks the organisers for the
opportunity to contribute this article. He also thanks R. L. Kelley, M. Elvis
for very useful discussions.

## References

  * [1] Giacconi R, Gursky H, Paolini FR, Rossi BB. 1962 Evidence for x Rays From Sources Outside the Solar System. PRL 9, 439–443. (10.1103/PhysRevLett.9.439)
  * [2] Collier MR, Abbey TF, Bannister NP, Carter JA, Choi M, Cravens T, Evans M, Fraser GW, Hills HK, Kuntz K, Lyons J, Omidi N, Porter FS, Read AM, Robertson I, Rozmarynowski P, Sembay S, Sibeck DG, Snowden SL, Stubbs T, Travnicek P. 2009 The Lunar X-ray Observatory (LXO)/Magnetosheath Explorer in X-rays (MagEX). In Smith RK, Snowden SL, Kuntz KD, editors, The Local Bubble and Beyond II vol. 1156American Institute of Physics Conference Series pp. 105–111. (10.1063/1.3211802)
  * [3] Gorenstein P. 1990 High throughput x-ray telescope on a lunar base. AIP Conference Proceedings 207, 382–391. (10.1063/1.39362)
  * [4] Wolter H. 1952 Spiegelsysteme streifenden Einfalls als abbildende Optiken für Röntgenstrahlen. Annalen der Physik 445, 94–114. (10.1002/andp.19524450108)
  * [5] . 2000 Soft x-rays and extreme ultraviolet radiation : principles and applications.
  * [6] Harrison FA, Craig WW, Christensen FE, Hailey CJ, Zhang WW, Boggs SE, Stern D, Cook WR, Forster K, Giommi P, Grefenstette BW, Kim Y, Kitaguchi T, Koglin JE, Madsen KK, Mao PH, Miyasaka H, Mori K, Perri M, Pivovaroff MJ, Puccetti S, Rana VR, Westergaard NJ, Willis J, Zoglauer A, An H, Bachetti M, Barrière NM, Bellm EC, Bhalerao V, Brejnholt NF, Fuerst F, Liebe CC, Markwardt CB, Nynka M, Vogel JK, Walton DJ, Wik DR, Alexander DM, Cominsky LR, Hornschemeier AE, Hornstrup A, Kaspi VM, Madejski GM, Matt G, Molendi S, Smith DM, Tomsick JA, Ajello M, Ballantyne DR, Baloković M, Barret D, Bauer FE, Blandford RD, Brandt WN, Brenneman LW, Chiang J, Chakrabarty D, Chenevez J, Comastri A, Dufour F, Elvis M, Fabian AC, Farrah D, Fryer CL, Gotthelf EV, Grindlay JE, Helfand DJ, Krivonos R, Meier DL, Miller JM, Natalucci L, Ogle P, Ofek EO, Ptak A, Reynolds SP, Rigby JR, Tagliaferri G, Thorsett SE, Treister E, Urry CM. 2013 The Nuclear Spectroscopic Telescope Array (NuSTAR) High-energy X-Ray Mission. ApJ 770, 103. (10.1088/0004-637X/770/2/103)
  * [7] Mushotzky R, AXIS. 2022 The Advanced X-ray Imaging Satellite (AXIS) a Probe mission concept. In American Astronomical Society Meeting #240 vol. 54American Astronomical Society Meeting Abstracts p. 127.03.
  * [8] Elvis M, Lawrence C, Seager S. 2023 Accelerating astrophysics with the SpaceX Starship. Physics Today 76, 40–45. (10.1063/PT.3.5176)
  * [9] Peterson LE. 1990 High energy astrophysics from the moon. AIP Conference Proceedings 207, 345–359. (10.1063/1.39301)
  * [10] Angel JRP. 1979 Lobster eyes as X-ray telescopes.. ApJ 233, 364–373. (10.1086/157397)
  * [11] Zhang C, Ling ZX, Sun XJ, Sun SL, Liu Y, Li ZD, Xue YL, Chen YF, Dai YF, Jia ZQ, Liu HY, Zhang XF, Zhang YH, Zhang SN, Chen FS, Cheng ZW, Fu W, Han YX, Li H, Li JF, Li Y, Liu PR, Ma XH, Tang YJ, Wang CB, Xie RJ, Yan AL, Zhang Q, Jiang BW, Jin G, Li LH, Qiu XB, Su DT, Sun JN, Xu Z, Zhang SK, Zhang Z, Zhang N, Bi XZ, Cai ZM, He JW, Liu HQ, Zhu XC, Cheng HQ, Cui CZ, Fan DW, Hu HB, Huang MH, Jin CC, Li DY, Pan HW, Wang WX, Xu YF, Yang X, Zhang B, Zhang M, Zhang WD, Zhao DH, Bai M, Ji Z, Liu YR, Ma FL, Su J, Tong JZ, Wang YS, Zhao ZJ, Feldman C, O’Brien P, Osborne JP, Willingale R, Burwitz V, Hartner G, Langmeier A, Müller T, Rukdee S, Schmidt T, Kuulkers E, Yuan W. 2022 First Wide Field-of-view X-Ray Observations by a Lobster-eye Focusing Telescope in Orbit. ApJL 941, L2. (10.3847/2041-8213/aca32f)
  * [12] Uttley P, den Hartog R, Bambi C, Barret D, Bianchi S, Bursa M, Cappi M, Casella P, Cash W, Costantini E, Dauser T, Diaz Trigo M, Gendreau K, Grinberg V, den Herder JW, Ingram A, Kara E, Markoff S, Mingo B, Panessa F, Poppenhäger K, Rózanska A, Svoboda J, Wijers R, Willingale R, Wilms J, Wise M. 2020 An x-ray interferometry concept for the ESA Voyage 2050 programme. In Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series vol. 11444Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series p. 114441E. (10.1117/12.2562523)
  * [13] Cash W. 2005 Imaging a black hole: MAXIM. Advances in Space Research 35, 122–129. (10.1016/j.asr.2003.09.057)
  * [14] Hayashida K, Asakura K, Ishikura A, Sakuma S, Hanasaka T, Kawabata T, Yoneyama T, Noda H, Okazaki K, Hanaoka M, Hattori K, Sawagami K, Kamogawa W, Nakajima H, Matsuhita Y, Mineta T, Yoshimoto M, Hakamata T, Ode Y, Matsumoto H, Tsunemi H, Awaki H, Terashima Y, Kawaguchi T. 2020 Sub-arcseconds to micro-arcsecond x-ray imaging with multi image x-ray interferometer method (MIXIM): concept and scalabe mission plans. In Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series vol. 11444Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series p. 114441C. (10.1117/12.2562193)
  * [15] Bowyer S, Byram ET, Chubb TA, Friedman H. 1964 Lunar Occultation of X-ray Emission from the Crab Nebula. Science 146, 912–917. (10.1126/science.146.3646.912)
  * [16] Janes AF, Pounds KA, Ricketts MJ. 1973 Correction to the Position of GX3 + 1. Nature 244, 349. (10.1038/244349a0)
  * [17] Davison PJN, Morrison LV. 1977 A lunar occultation position for GX 9+1.. MNRAS 178, 53P–56. (10.1093/mnras/178.1.53P)
  * [18] Arzoumanian Z, Gendreau KC, Baker CL, Cazeau T, Hestnes P, Kellogg JW, Kenyon SJ, Kozon RP, Liu KC, Manthripragada SS, Markwardt CB, Mitchell AL, Mitchell JW, Monroe CA, Okajima T, Pollard SE, Powers DF, Savadkin BJ, Winternitz LB, Chen PT, Wright MR, Foster R, Prigozhin G, Remillard R, Doty J. 2014 The neutron star interior composition explorer (NICER): mission definition. In Takahashi T, den Herder JWA, Bautz M, editors, Space Telescopes and Instrumentation 2014: Ultraviolet to Gamma Ray vol. 9144Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series p. 914420. (10.1117/12.2056811)
  * [19] Wargelin BJ, Markevitch M, Juda M, Kharchenko V, Edgar R, Dalgarno A. 2004 Chandra Observations of the “Dark” Moon and Geocoronal Solar Wind Charge Transfer. ApJ 607, 596–610. (10.1086/383410)
  * [20] Snowden SL, Collier MR, Kuntz KD. 2004 XMM-Newton Observation of Solar Wind Charge Exchange Emission. ApJ 610, 1182–1190. (10.1086/421841)
  * [21] Fujimoto R, Mitsuda K, Mccammon D, Takei Y, Bauer M, Ishisaki Y, Porter SF, Yamaguchi H, Hayashida K, Yamasaki NY. 2007 Evidence for Solar-Wind Charge-Exchange X-Ray Emission from the Earth’s Magnetosheath. PASJ 59, 133–140. (10.1093/pasj/59.sp1.S133)
  * [22] Mereghetti S, Bignami GF, Zaidins C. 1990 Future Possibilities for Lunar Occultation Studies of Faint X-Ray Sources. Experimental Astronomy 1, 165–171. (10.1007/BF00462038)
  * [23] Gehrels N. 1986 Confidence Limits for Small Numbers of Events in Astrophysical Data. ApJ 303, 336. (10.1086/164079)
  * [24] Roques F, Moncuquet M, Sicardy B. 1987 Stellar occultations by small bodies - Diffraction effects. AJ 93, 1549–1558. (10.1086/114438)
  * [25] Mitsuda K, Takeshima T, Kii T, Kawai N. 1990 Dust-Grain Scattering of X-Rays Observed during the Lunar Occultation of a Transient X-Ray Source near the Galactic Center. ApJ 353, 480. (10.1086/168637)
  * [26] Harwit M. 2019 Cosmic Discovery: The Search, Scope, and Heritage of Astronomy. Cambridge University Press. OCLC: 1099860228. e-ISBN: 9781108655088. (10.1017/9781108655088)
  * [27] Gandhi P, Bachetti M, Dhillon VS, Fender RP, Hardy LK, Harrison FA, Littlefair SP, Malzac J, Markoff S, Marsh TR, Mooley K, Stern D, Tomsick JA, Walton DJ, Casella P, Vincentelli F, Altamirano D, Casares J, Ceccobello C, Charles PA, Ferrigno C, Hynes RI, Knigge C, Kuulkers E, Pahari M, Rahoui F, Russell DM, Shaw AW. 2017 An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system. Nature Astronomy 1, 859–864. (10.1038/s41550-017-0273-3)
  * [28] Middleton MJ, Casella P, Gandhi P, Bozzo E, Anderson G, Degenaar N, Donnarumma I, Israel G, Knigge C, Lohfink A, Markoff S, Marsh T, Rea N, Tingay S, Wiersema K, Altamirano D, Bhattacharya D, Brandt WN, Carey S, Charles P, Díaz Trigo M, Done C, Kotze M, Eikenberry S, Fender R, Ferruit P, Fürst F, Greiner J, Ingram A, Heil L, Jonker P, Komossa S, Leibundgut B, Maccarone T, Malzac J, McBride V, Miller-Jones J, Page M, Rossi EM, Russell DM, Shahbaz T, Sivakoff GR, Tanaka M, Thompson DJ, Uemura M, Uttley P, van Moorsel G, van Doesburgh M, Warner B, Wilkes B, Wilms J, Woudt P. 2017 Paving the way to simultaneous multi-wavelength astronomy. NAR 79, 26–48. (10.1016/j.newar.2017.07.002)
  * [29] McKay DS, Heiken G, Basu A, Blanford G, Simon S, Reedy R, French BM, Papike J. 1991 The Lunar Regolith. In Heiken GH, Vaniman DT, French BM, editors, Lunar Sourcebook, A User’s Guide to the Moon , pp. 285–356.
  * [30] Rennilson JJ, Criswell DR. 1974 Surveyor Observations of Lunar Horizon-Glow. Moon 10, 121–142. (10.1007/BF00655715)
  * [31] Hirabayashi M, Hartzell CM, Bellan PM, Bodewits D, Delzanno GL, Hyde TW, Konopka U, Thomas E, Thomas HM, Hahn I, Israelsson UE. 2023 Electrostatic dust remediation for future exploration of the Moon. Acta Astronautica 207, 392–402. (10.1016/j.actaastro.2023.03.005)
  * [32] Nakamura Y, Latham GV, Dorman HJ. 1982 Apollo Lunar Seismic Experiment—Final summary. Lunar and Planetary Science Conference Proceedings 87, 117–A123. (10.1029/JB087iS01p0A117)
  * [33] Goins NR, Dainty AM, Toksoz MN. 1981 Seismic energy release of the moon.. JGR 86, 378–388. (10.1029/JB086iB01p00378)
  * [34] Lammlein DR. 1977 Lunar seismicity and tectonics. Physics of the Earth and Planetary Interiors 14, 224–273. (10.1016/0031-9201(77)90175-3)
  * [35] Dorman J, Evans S, Nakamura Y, Latham G. 1978 On the time-varying properties of the lunar seismic meteoroid population.. Lunar and Planetary Science Conference Proceedings 3, 3615–3626.
  * [36] Sinha RK, Rani A, Ruj T, Bhardwaj A. 2023 Geologic investigation of lobate scarps in the vicinity of Chandrayaan-3 landing site in the southern high latitudes of the moon. Icarus 402, 115636. (10.1016/j.icarus.2023.115636)
  * [37] Harms J, Ambrosino F, Angelini L, Braito V, Branchesi M, Brocato E, Cappellaro E, Coccia E, Coughlin M, Della Ceca R, Della Valle M, Dionisio C, Federico C, Formisano M, Frigeri A, Grado A, Izzo L, Marcelli A, Maselli A, Olivieri M, Pernechele C, Possenti A, Ronchini S, Serafinelli R, Severgnini P, Agostini M, Badaracco F, Bertolini A, Betti L, Civitani MM, Collette C, Covino S, Dall’Osso S, D’Avanzo P, DeSalvo R, Di Giovanni M, Focardi M, Giunchi C, van Heijningen J, Khetan N, Melini D, Mitri G, Mow-Lowry C, Naponiello L, Noce V, Oganesyan G, Pace E, Paik HJ, Pajewski A, Palazzi E, Pallavicini M, Pareschi G, Pozzobon R, Sharma A, Spada G, Stanga R, Tagliaferri G, Votta R. 2021 Lunar Gravitational-wave Antenna. ApJ 910, 1. (10.3847/1538-4357/abe5a7)
  * [38] Sutherland W, Emerson J, Dalton G, Atad-Ettedgui E, Beard S, Bennett R, Bezawada N, Born A, Caldwell M, Clark P, Craig S, Henry D, Jeffers P, Little B, McPherson A, Murray J, Stewart M, Stobie B, Terrett D, Ward K, Whalley M, Woodhouse G. 2015 The Visible and Infrared Survey Telescope for Astronomy (VISTA): Design, technical overview, and performance. A&A 575, A25. (10.1051/0004-6361/201424973)
  * [39] Matichard F, Lantz B, Mittleman R, Mason K, Kissel J, Abbott B, Biscans S, McIver J, Abbott R, Abbott S, Allwine E, Barnum S, Birch J, Celerier C, Clark D, Coyne D, DeBra D, DeRosa R, Evans M, Foley S, Fritschel P, Giaime JA, Gray C, Grabeel G, Hanson J, Hardham C, Hillard M, Hua W, Kucharczyk C, Landry M, Le Roux A, Lhuillier V, Macleod D, Macinnis M, Mitchell R, O’Reilly B, Ottaway D, Paris H, Pele A, Puma M, Radkins H, Ramet C, Robinson M, Ruet L, Sarin P, Shoemaker D, Stein A, Thomas J, Vargas M, Venkateswara K, Warner J, Wen S. 2015 Seismic isolation of Advanced LIGO: Review of strategy, instrumentation and performance. Classical and Quantum Gravity 32, 185003. (10.1088/0264-9381/32/18/185003)
  * [40] Williams JP, Paige D, Greenhagen B, Sefton-Nash E. 2017 The global surface temperatures of the Moon as measured by the Diviner Lunar Radiometer Experiment. Icarus 283, 300–325. Lunar Reconnaissance Orbiter - Part II (https://doi.org/10.1016/j.icarus.2016.08.012)
  * [41] Vasavada AR, Bandfield JL, Greenhagen BT, Hayne PO, Siegler MA, Williams JP, Paige DA. 2012 Lunar equatorial surface temperatures and regolith properties from the Diviner Lunar Radiometer Experiment. Journal of Geophysical Research (Planets) 117, E00H18. (10.1029/2011JE003987)
  * [42] Tashiro M, Maejima H, Toda K, Kelley R, Reichenthal L, Hartz L, Petre R, Williams B, Guainazzi M, Costantini E, Fujimoto R, Hayashida K, Henegar-Leon J, Holland M, Ishisaki Y, Kilbourne C, Loewenstein M, Matsushita K, Mori K, Okajima T, Porter FS, Sneiderman G, Takei Y, Terada Y, Tomida H, Yamaguchi H, Watanabe S, Akamatsu H, Arai Y, Audard M, Awaki H, Babyk I, Bamba A, Bando N, Behar E, Bialas T, Boissay-Malaquin R, Brenneman L, Brown G, Canavan E, Chiao M, Comber B, Corrales L, Cumbee R, de Vries C, den Herder JW, Dercksen J, Diaz-Trigo M, DiPirro M, Done C, Dotani T, Ebisawa K, Eckart M, Eckert D, Eguchi S, Enoto T, Ezoe Y, Ferrigno C, Fujita Y, Fukazawa Y, Furuzawa A, Gallo L, Gorter N, Grim M, Gu L, Hagino K, Hamaguchi K, Hatsukade I, Hawthorn D, Hayashi K, Hell N, Hiraga J, Hodges-Kluck E, Horiuchi T, Hornschemeier A, Hoshino A, Ichinohe Y, Iga S, Iizuka R, Ishida M, Ishihama N, Ishikawa K, Ishimura K, Jaffe T, Kaastra J, Kallman T, Kara E, Katsuda S, Kenyon S, Kimball M, Kitaguchi T, Kitamoto S, Kobayashi S, Kobayashi A, Kohmura T, Kubota A, Leutenegger M, Li M, Lockard T, Maeda Y, Markevitch M, Martz C, Matsumoto H, Matsuzaki K, McCammon D, McLaughlin B, McNamara B, Miko J, Miller E, Miller J, Minesugi K, Mitani S, Mitsuishi I, Mizumoto M, Mizuno T, Mukai K, Murakami H, Mushotzky R, Nakajima H, Nakamura H, Nakazawa K, Natsukari C, Nigo K, Nishioka Y, Nobukawa K, Nobukawa M, Noda H, Odaka H, Ogawa M, Ohashi T, Ohno M, Ohta M, Okamoto A, Ota N, Ozaki M, Paltani S, Plucinsky P, Pottschmidt K, Sampson M, Sasaki T, Sato K, Sato R, Sato T, Sawada M, Seta H, Shibano Y, Shida M, Shidatsu M, Shigeto S, Shinozaki K, Shirron P, Simionescu A, Smith R, Someya K, Soong Y, Sugawara K, Sugawara Y, Szymkowiak A, Takahashi H, Takeshima T, Tamagawa T, Tamura K, Tanaka T, Tanimoto A, Terashima Y, Tsuboi Y, Tsujimoto M, Tsunemi H, Tsuru T, Uchida H, Uchida Y, Uchiyama H, Ueda Y, Uno S, Vink J, Watanabe T, Witthoeft M, Wolfs R, Yamada S, Yamaoka K, Yamasaki N, Yamauchi M, Yamauchi S, Yanagase K, Yaqoob T, Yasuda S, Yoshida T, Yoshioka N, Zhuravleva I. 2020 Status of x-ray imaging and spectroscopy mission (XRISM). In den Herder JWA, Nikzad S, Nakazawa K, editors, Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray vol. 11444Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series p. 1144422. (10.1117/12.2565812)
  * [43] Nandra K, Barret D, Barcons X, Fabian A, den Herder JW, Piro L, Watson M, Adami C, Aird J, Afonso JM, Alexander D, Argiroffi C, Amati L, Arnaud M, Atteia JL, Audard M, Badenes C, Ballet J, Ballo L, Bamba A, Bhardwaj A, Stefano Battistelli E, Becker W, De Becker M, Behar E, Bianchi S, Biffi V, Bîrzan L, Bocchino F, Bogdanov S, Boirin L, Boller T, Borgani S, Borm K, Bouché N, Bourdin H, Bower R, Braito V, Branchini E, Branduardi-Raymont G, Bregman J, Brenneman L, Brightman M, Brüggen M, Buchner J, Bulbul E, Brusa M, Bursa M, Caccianiga A, Cackett E, Campana S, Cappelluti N, Cappi M, Carrera F, Ceballos M, Christensen F, Chu YH, Churazov E, Clerc N, Corbel S, Corral A, Comastri A, Costantini E, Croston J, Dadina M, D’Ai A, Decourchelle A, Della Ceca R, Dennerl K, Dolag K, Done C, Dovciak M, Drake J, Eckert D, Edge A, Ettori S, Ezoe Y, Feigelson E, Fender R, Feruglio C, Finoguenov A, Fiore F, Galeazzi M, Gallagher S, Gandhi P, Gaspari M, Gastaldello F, Georgakakis A, Georgantopoulos I, Gilfanov M, Gitti M, Gladstone R, Goosmann R, Gosset E, Grosso N, Guedel M, Guerrero M, Haberl F, Hardcastle M, Heinz S, Alonso Herrero A, Hervé A, Holmstrom M, Iwasawa K, Jonker P, Kaastra J, Kara E, Karas V, Kastner J, King A, Kosenko D, Koutroumpa D, Kraft R, Kreykenbohm I, Lallement R, Lanzuisi G, Lee J, Lemoine-Goumard M, Lobban A, Lodato G, Lovisari L, Lotti S, McCharthy I, McNamara B, Maggio A, Maiolino R, De Marco B, de Martino D, Mateos S, Matt G, Maughan B, Mazzotta P, Mendez M, Merloni A, Micela G, Miceli M, Mignani R, Miller J, Miniutti G, Molendi S, Montez R, Moretti A, Motch C, Nazé Y, Nevalainen J, Nicastro F, Nulsen P, Ohashi T, O’Brien P, Osborne J, Oskinova L, Pacaud F, Paerels F, Page M, Papadakis I, Pareschi G, Petre R, Petrucci PO, Piconcelli E, Pillitteri I, Pinto C, de Plaa J, Pointecouteau E, Ponman T, Ponti G, Porquet D, Pounds K, Pratt G, Predehl P, Proga D, Psaltis D, Rafferty D, Ramos-Ceja M, Ranalli P, Rasia E, Rau A, Rauw G, Rea N, Read A, Reeves J, Reiprich T, Renaud M, Reynolds C, Risaliti G, Rodriguez J, Rodriguez Hidalgo P, Roncarelli M, Rosario D, Rossetti M, Rozanska A, Rovilos E, Salvaterra R, Salvato M, Di Salvo T, Sanders J, Sanz-Forcada J, Schawinski K, Schaye J, Schwope A, Sciortino S, Severgnini P, Shankar F, Sijacki D, Sim S, Schmid C, Smith R, Steiner A, Stelzer B, Stewart G, Strohmayer T, Strüder L, Sun M, Takei Y, Tatischeff V, Tiengo A, Tombesi F, Trinchieri G, Tsuru TG, Ud-Doula A, Ursino E, Valencic L, Vanzella E, Vaughan S, Vignali C, Vink J, Vito F, Volonteri M, Wang D, Webb N, Willingale R, Wilms J, Wise M, Worrall D, Young A, Zampieri L, In’t Zand J, Zane S, Zezas A, Zhang Y, Zhuravleva I. 2013 The Hot and Energetic Universe: A White Paper presenting the science theme motivating the Athena+ mission. arXiv e-prints p. arXiv:1306.2307.
  * [44] Gandhi P, Kawamuro T, Díaz Trigo M, Paice JA, Boorman PG, Cappi M, Done C, Fabian AC, Fukumura K, García JA, Greenwell CL, Guainazzi M, Makishima K, Tashiro MS, Tomaru R, Tombesi F, Ueda Y. 2022 Frontiers in accretion physics at high X-ray spectral resolution. Nature Astronomy 6, 1364–1375. (10.1038/s41550-022-01857-y)
  * [45] Ulamec S, Biele J, Trollope E. 2010 How to survive a Lunar night. PlSS 58, 1985–1995. (10.1016/j.pss.2010.09.024)
  * [46] Ambrosi RM, Williams H, Watkinson EJ, Barco A, Mesalam R, Crawford T, Bicknell C, Samara-Ratna P, Vernon D, Bannister N, Ross D, Sykes J, Perkinson MC, Burgess C, Stroud C, Gibson S, Godfrey A, Slater RG, Reece MJ, Chen K, Simpson K, Tuley R, Sarsfield M, Tinsley TP, Stephenson K, Freis D, Vigier JF, Konings RJM, Fongarland C, Libessart M, Merrifield J, Kramer DP, Byrne J, Foxcroft B. 2019 European Radioisotope Thermoelectric Generators (RTGs) and Radioisotope Heater Units (RHUs) for Space Science and Exploration. SSRv 215, 55. (10.1007/s11214-019-0623-9)
  * [47] Kamata Y, Takeshima T, Okada T, Terada K. 1999 Detection of X-ray fluorescence line feature from the lunar surface. Advances in Space Research 23, 1829–1832. (10.1016/S0273-1177(99)00539-6)
  * [48] Bhardwaj A, Elsner RF, Randall Gladstone G, Cravens TE, Lisse CM, Dennerl K, Branduardi-Raymont G, Wargelin BJ, Hunter Waite J, Robertson I, Østgaard N, Beiersdorfer P, Snowden SL, Kharchenko V. 2007 X-rays from solar system objects. PlSS 55, 1135–1189. (10.1016/j.pss.2006.11.009)
  * [49] Schmitt JHMM, Snowden SL, Aschenbach B, Hasinger G, Pfeffermann E, Predehl P, Trumper J. 1991 A soft X-ray image of the Moon. Nature 349, 583–587. (10.1038/349583a0)
  * [50] Adler I, Trombka JI. 1977 Orbital chemistry - Lunar surface analysis from the X-ray and gamma ray remote sensing experiments. Physics and Chemistry of the Earth 10, 17–43. (10.1016/0079-1946(77)90004-0)
  * [51] Narendranath S, Athiray PS, Sreekumar P, Kellett BJ, Alha L, Howe CJ, Joy KH, Grande M, Huovelin J, Crawford IA, Unnikrishnan U, Lalita S, Subramaniam S, Weider SZ, Nittler LR, Gasnault O, Rothery D, Fernandes VA, Bhandari N, Goswami JN, Wieczorek MA, C1XS Team. 2011 Lunar X-ray fluorescence observations by the Chandrayaan-1 X-ray Spectrometer (C1XS): Results from the nearside southern highlands. Icarus 214, 53–66. (10.1016/j.icarus.2011.04.010)