# A cold-atom Ramsey clock with a low volume physics package

A. Bregazzi  SUPA and Department of Physics, University of Strathclyde, G4
0NG, Glasgow, United Kingdom  E. Batori  University of Neuchâtel, Institute of
Physics, Laboratoire Temps-Fréquence, Avenue de Bellevaux 51, 2000 Neuchâtel,
Switzerland  B. Lewis  SUPA and Department of Physics, University of
Strathclyde, G4 0NG, Glasgow, United Kingdom  C. Affolderbach  University of
Neuchâtel, Institute of Physics, Laboratoire Temps-Fréquence, Avenue de
Bellevaux 51, 2000 Neuchâtel, Switzerland  G. Mileti  University of Neuchâtel,
Institute of Physics, Laboratoire Temps-Fréquence, Avenue de Bellevaux 51,
2000 Neuchâtel, Switzerland  E. Riis  SUPA and Department of Physics,
University of Strathclyde, G4 0NG, Glasgow, United Kingdom  P. F. Griffin
<EMAIL_ADDRESS>SUPA and Department of Physics, University of
Strathclyde, G4 0NG, Glasgow, United Kingdom

###### Abstract

We demonstrate a Ramsey-type microwave clock interrogating the 6.835 GHz
ground-state transition in cold 87Rb atoms loaded from a grating magneto-
optical trap (GMOT) enclosed in an additively manufactured loop-gap resonator
microwave cavity. A short-term stability of $1.5\times 10^{-11}$ $\tau^{-1/2}$
is demonstrated, in reasonable agreement with predictions from the signal-to-
noise ratio of the measured Ramsey fringes. The cavity-grating package has a
volume of $\approx$67 cm3, ensuring an inherently compact system while the use
of a GMOT drastically simplifies the optical requirements for laser cooled
atoms. This work is another step towards the realisation of highly compact
portable cold-atom frequency standards.

Compact frequency standards based on the interrogation of both ions and
neutral atoms continue to receive much interest, with many different schemes
now reported in the literature Knappe  _et al._ (2004); Micalizio  _et al._
(2012); Esnault  _et al._ (2013); Kang  _et al._ (2015); Tjoelker  _et al._
(2016); Cao  _et al._ (2017); Elvin  _et al._ (2019); Maurice  _et al._
(2020); Carle  _et al._ (2021). While compact clocks based on thermal atomic
vapours and coherent population trapping (CPT) remain unrivalled in terms of
size, weight and power (SWaP) they typically exhibit stabilities that are
limited in the medium to long-term due to light-shift effects Hafiz  _et al._
(2018) and the buffer gasses required to reduce atomic collisions Knappe  _et
al._ (2004). In addition to this, CPT clocks struggle to reach atomic shot
noise due to the limited signal-to-noise ratio (SNR), a consequence of the low
detected signal photon count per atom. This limitation often requires complex
interrogation schemes to optimise the clock stability Liu  _et al._ (2013);
Yun, Guérandel, and de Clercq (2016); Yudin  _et al._ (2018).

The development in the last decade of pulsed optically pumped (POP) clocks in
thermal vapours has driven new research, with state-of-the-art stabilities
Micalizio  _et al._ (2012); Stefanucci  _et al._ (2012); Hao  _et al._ (2016).
However, these clocks still suffer from buffer gas shifts, providing the
ultimate limitation to their long-term stabilities. Achievable Ramsey times
within these systems are also limited to a few ms by spin relaxation of the
thermal atoms Micalizio  _et al._ (2012); Batori  _et al._ (2022), restricting
the short-term stability performance.

In an effort to combat these limitations several groups have now developed
compact cold atom microwave clocks based on spherical optical-integrating
sphere cavities Esnault  _et al._ (2010, 2011), cylindrical cavities Müller
_et al._ (2011); Liu  _et al._ (2015) and more recently loop-gap-resonator
cavities Lee  _et al._ (2021). Three examples of cold atom microwave clocks
are now even commercially available Muquans (2023); Spectra Dynamics (2023);
AOSense (2023). Different laser cooling schemes with varying optical
geometries have been used within these systems, with isotropic cooling of an
optical molasses Esnault  _et al._ (2010, 2011); Liu  _et al._ (2015); Muquans
(2023), pyramid MOTs Givon  _et al._ (2022) and larger traditional 6-beam MOTs
Müller  _et al._ (2011); Lee  _et al._ (2021) all being utilised.

Figure 1: Simplified schematic of the physics package. Trap light (red arrow)
propagates parallel to the magnetic bias field (black dashed arrow), 90:10
non-polarising beam splitter (NPBS) splits the optical pumping (blue arrow)
and readout (green arrow) light onto a reference photodiode (PDRef) and signal
photodiode (PDSig) after being retro-reflected by a mirror (M). Local
oscillator (LO) is connected via a SMA vacuum feedthrough.

In this letter we demonstrate operation and first stability measurement of a
cold-atom atomic clock based on an additively manufactured loop-gap resonator
(LGR) cavity with a volume of $\approx 67$ cm3 incorporating an integrated
GMOT Nshii  _et al._ (2013). Although vapour-cell atomic clocks based on LGR
structures with external diameters of order 1 cm have been demonstrated Batori
_et al._ (2022), such small devices will have limited benefit when using cold
atoms. Previous investigations with both GMOTsBregazzi  _et al._ (2021) and
6-beam MOTsHoth, Donley, and Kitching (2013) have shown that by constraining
the trap beam diameter the maximum trapped atom number is limited, therefore
constricting the potential stability of the final clock.

In the present study a sample of cold atoms is optically pumped into the clock
state, probed in a double resonance Ramsey-type scheme and the resulting state
populations read out through an absorptive method. The use of additive
manufacturing allows for complex electrode geometries, while maintaining a
highly scalable manufacturing process due to the lack of precision machining
and assembly required Affolderbach  _et al._ (2018); Batori  _et al._ (2023).
While the main aim of this work has been to demonstrate the feasibility of
integrating this cavity design and its fabrication technique with the GMOT
architecture, we envisage that further substantial reduction in size is
possible by adapting the cavity to form the bulk of the vacuum system. The
minimal optical access required of this scheme also allows for a reduction in
the number of apertures present in the cavity body which must be carefully
considered during the design process so as not to degrade the cavity mode and
field homogeneity Lee  _et al._ (2021); Batori  _et al._ (2023).

A commercial laser system (Muquans ILS) at 780 nm with an integrated electro-
optic modulator (EOM) for the creation of optical sidebands on the carrier
frequency is used throughout. The laser light is split into three distinct
optical paths, each with a double passed acousto-optic modulator (AOM) for
power and frequency control to enable trapping, optical pumping and state
readout. The trapping light is coupled into a fibre to be passed to the
physics package. The optical pumping and state readout beams are coupled into
a single additional fibre and likewise passed to the physics package.

The microwave cavity itself, described in detail in RefBatori  _et al._ (2023)
consists of a loop-gap structure with a four electrode geometry and has an
outer radius of 44 mm and a height of roughly 44 mm. The cavity operates in a
TE011-like mode, tuned to the ground-state-splitting of 87Rb with a quality
factor of Q$\approx$360 and is mounted within a stainless steel vacuum
chamber, maintained at ultra-high-vacuum (UHV) by an ion pump. Optical access
is enabled via viewports in the vacuum chamber along two orthogonal axes. The
first axis is parallel to the cylindrical symmetry axis of the cavity (in the
following this is referred to as the cavity axis) and allows the trap light to
be directed onto the grating chip after expansion and collimation from the
fibre. Optical pumping and state readout light, expanded from the fibre to a
1/e2 diameter of 7 mm, is directed onto the atoms through two 4 mm holes
drilled in the side of the cavity body. A retro-reflecting mirror for this
light is placed outside the vacuum chamber to decrease the acceleration
experienced by the atoms when interacting with the optical pumping and probe
beams and increase signal amplitudes. A simplified schematic of this is shown
in Fig.1. No significant degradation of the cavity field is observed by the
introduction of the holes in the cavity bodyBatori  _et al._ (2023).

A pair of anti-Helmholtz coils are mounted within the vacuum chamber, along
the cavity axis in order to create the quadrupole magnetic field required for
the trapping process. Three orthogonal pairs of Helmholtz shim coils, mounted
externally to the vacuum chamber are used for the cancellation of external
stray DC magnetic fields and to apply a magnetic bias along the cavity axis of
$\approx 100$ mG in order to lift the atomic degeneracy during optical
molasses and clock interrogation. We note that the current demonstration is a
proof of concept with no magnetic shielding of the experiment present,
limiting the potential stability of the system.

The experimental cycle is initiated by turning the trapping coils on with the
trap light tuned to be approximately $\Delta$=-2$\Gamma$ red detuned
($\Gamma/2\pi$=6.07 MHz) from the 87Rb D2 F=2$\rightarrow$F’=3 cycling
transition, with re-pump light generated by the EOM operating at 6.57 GHz to
produce 5% optical sidebands. A Rb vapour, maintained at the $1\times 10^{-9}$
Torr level, is produced by resistively heating a pair of alkali metal
dispensers. We then perform a 6 ms optical molasses by turning the trap coils
off and linearly ramping the light detuning to $\Delta$=-5$\Gamma$ while
simultaneously decreasing the trap light intensity. After molasses we measure
atomic temperatures of $\approx 10~{}\mu K$. In order to decrease the clock
cycle time and mitigate experimental dead time, we employ atom recapture
between experimental cycles Esnault  _et al._ (2013); Rakholia, McGuinness,
and Biedermann (2014). In steady state this allows the trapping of $>3\times
10^{6}$ atoms with a load time of 100 ms for a clock cycle operating at
$\approx$7 Hz. Once the atoms have been trapped and cooled, the trap light is
extinguished by an AOM and blocked by a mechanical shutter Zhang  _et al._
(2015) to ensure no light leakage during microwave interrogation. After
molasses, the atoms are assumed to be roughly evenly distributed between the
five $m_{F}$ levels of the $F=2$ hyperfine ground-state manifold. We therefore
employ a 1 ms optical pumping stage with 10 $\mu$W total power of linearly
polarised light, polarisation axis parallel to the quantization axis, tuned to
the F=2$\rightarrow$F’=2 and re-pump light tuned to the F=1$\rightarrow$F’=2
transition. Due to selection rules this pumps $>95\%$ of the atoms into the
52S1/2, $\ket{{F=2,m_{F}=0}}$ state Duan  _et al._ (2017) as seen by the
almost complete elimination of the $m_{F}=1\rightarrow m_{F}^{\prime}=1$
transitions, when scanning the microwave detuning, increasing the contrast of
the resulting clock signal.

Figure 2: Typical Ramsey fringe taken with a 10 ms free evolution time.

A Keysight E8257D microwave synthesizer is used as a local oscillator and
microwave source throughout. Square microwave $\pi/2$ pulses with a duration
of 200 $\mu$s and typical power levels of 0.04 mW are applied to the atoms.
The specified phase noise performance of the local oscillator allows the Dick
effect Santarelli  _et al._ (1998) to be reduced to the $6.12\times 10^{-13}$
level at 1 s for the experimental cycle time. After the desired microwave
pulses have been applied, the atomic states are read out by an absorption
method using 30 $\mu$W of optical power, measured by recording the
transmission on a photodiode of two subsequent probe pulses. First a short
pulse of readout light tuned to the F=2$\rightarrow$F’=3 transition is
applied, giving an measure of the number of atoms in the $F=2$ ground-state.
We then apply a second readout pulse with re-pump light present. This measures
the total number of atoms, providing a normalisation of the signal and reduced
sensitivity to atom number fluctuations. Intensity noise in the readout pulses
is reduced by the use of a reference photodiode.

An example Ramsey fringe taken with a Ramsey time, $T_{R}$=10 ms,
corresponding to a fringe linewidth of 50 Hz is shown in Fig.2. This fringe
exhibits an SNR of around 110, measured at the half-width points of the
central fringe, where SNR is defined as the ratio of the peak amplitude
divided by the amplitude noise. The predicted SNR limited short-term relative
frequency instability of a local oscillator stabilised to an atomic transition
in terms of Allan deviation is given by Riehle (2006):

$\sigma_{SNR}(\tau)=\frac{1}{\pi C}\frac{\Delta
f}{f_{0}}\frac{1}{SNR}\sqrt{\frac{T_{C}}{\tau}}$ (1)

where $C$ is the fringe contrast, $\Delta f$ is the signal linewidth ($\approx
1/2T_{R}$), $f_{0}$ is the central frequency (6.8346… GHz), $T_{C}$ is the
full experimental cycle time ($T_{C}=140$ms) and $\tau$ is the averaging time.
We use this relationship as a basis to optimise our experimental cycle to
maximise the potential stability of our clock. A plot of the predicted
stability, using the measured SNR, as a function of the Ramsey time is shown
in Fig.3. From this we find that as the Ramsey time is increased the predicted
stability also improves up to the level of $8.95\times 10^{-12}$ for a 10 ms
Ramsey time. After this time however the stability begins to decrease as the
SNR is degraded. This degradation in SNR is attributed to atomic losses due to
both the thermal expansion of the cloud and the atoms falling out of the probe
region under gravity. The extension of this Ramsey time should be possible by
moving the holes in the cavity body lower down, introducing elliptical holes
to maintain good probe-atom overlap along the path of gravity or by
implementing a grating-chip atomic fountainLewis  _et al._ (2022). This last
option is particularly attractive because as with traditional atomic fountains
it would be possible apply both $\pi/2$ pulses when the atoms are at the same
vertical point of the cavity. This will allow the phase difference observed by
the atoms between the two microwave pulses to be minimised, essential for high
contrast Ramsey fringes in a relatively low-Q cavity such as used here. A
grating-chip atomic fountain (using CPT interrogation) such as this has
already demonstrated Ramsey times out to 100 ms, with a corresponding fringe
linewidth of 5 HzLewis  _et al._ (2022).

Figure 3: Predicted short-term stability as a function of Ramsey time. Inset:
Typical central Ramsey fringe obtained at 10 ms free evolution time. Red
points indicate measured normalised probe absorption while the black line
shows a sinusoidal fit to the data. The fringe offset from 0 Hz detuning
corresponds to the frequency shift expected from the second-order Zeeman shift
at a 100 mG bias field. Figure 4: Red points: Overlapping Allan deviation of
local oscillator’s stability when locked to atomic signal. Blue solid line:
stability predicted by (1) and measured fringe SNR. Red dashed line:
$\tau^{-1/2}$ dependence of the measured 1 s stability. Black points:
stability limit due to the second-order Zeeman shift. Yellow points: measured
GPSDO reference oscillator stability. Error bars represent 1$\sigma$
confidence bound.

To assess the stability of our system we stabilise the local oscillator to the
atomic signal. The signal is modulated and demodulated at the fringe’s half
maximum to construct the error signal which is used to feedback onto the local
oscillator by voltage tuning its 10 MHz internal reference. Fig.4 shows an
overlapping Allan deviation of the resulting frequency stability when compared
to an oven controlled quartz crystal oscillator (Wenzel Associates 501-29647)
disciplined to GPS (GPSDO). From Fig.4 we see that the clock stability
averages down with a $1.5\times 10^{-11}~{}\tau^{-1/2}$ dependence out to 10
s, at which point it deviates slightly from the ideal $\tau^{-1/2}$
dependence. This is in reasonable agreement with the theoretical stability
obtained from Fig.3 at a 10 ms Ramsey time and is well above the ultimate
stability limit set by the quantum projection noise (QPN) of $4.9\times
10^{-13}~{}\tau^{-1/2}$, calculated by replacing the SNR term in (1) by
$\sqrt{N}$, where $N$ is the atom number.

As the experiment is currently operated in a magnetically unshielded
environment the instability contribution due to the second-order Zeeman shift
was expected to limit the clock stability in the medium-term. This was
confirmed by measuring the magnetic field stability via the
$\ket{F=1,m_{F}=1}\rightarrow\ket{F^{\prime}=2,m_{F}^{\prime}=1}$ microwave
transition, exhibiting a magnetic field sensitivity of $\beta_{1}$=1.4 MHz/G
Steck . The expected stability of the $m_{F}=0$ clock transition
($\beta_{0}$=575 Hz/G2 Steck ) could then be calculated, shown as black points
in Fig.4. This plot shows that the second-order Zeeman shift is indeed an
important limitation to the clock stability at $\tau>$10 s, due to a
pronounced hump in the magnetic field stability around this time, after which
the stability averages down slightly to the level of $<2\times 10^{-12}$. For
future iterations of this set-up it will be imperative to introduce magnetic
shielding for long-term stability performance.

Another limiting factor to the clock stability in the medium term is the GPSDO
reference. Three-corner-hat stability measurements of this reference against
two commercial Cs beam clocks (OSA 3235B and Microchip 5071A) indicates
excellent short-term performance of the GPSDO $<1\times 10^{-12}$. At 700 s
however, a peak in the instability is observed at $\approx 2\times 10^{-12}$.
This peak is associated with the time constant of the tuning loop used to
reference the oscillator to the GPS signal. When taken in conjunction with the
stability limit due to the Zeeman effect, these two effects help explain the
flattening of the clock stability for averaging times >100 s.

To more fully characterise the short-term stability ($\tau$=1 s) of the system
a stability budget of the main instability contributions to the clock was
constructed, shown in Table 1. The short-term clock performance is found to be
primarily limited by the Ramsey fringe SNR, discussed above. In future we
expect to be able to improve the SNR contribution to short-term stability
towards the projection noise limit. This will be achieved by increasing the
Ramsey time, optimisation of the optical detection process and suppression of
intensity and frequency fluctuations in the probe beam.

The next largest contributor to the 1 s clock stability after the fringe SNR
is electronic noise on the voltage line used to tune the 10 MHz local
oscillator. While this is not a limiting factor to the clock stability at
present, careful minimisation of this noise source will be required when
moving towards a magnetically shielded experiment where the expected clock
stability should improve below the $1\times 10^{-12}$ level. The total
predicted short term stability from Table 1 is found to be 1.6 times better
than the measured stability. Efforts are ongoing to identify the remaining
instability contributions in order to minimise them in future iterations of
the set-up.

Noise source |  |  $\sigma$ contribution $(\tau=1s)$
---|---|---
SNR |  | $8.95\times 10^{-12}$
Electronic noise |  | $1.14\times 10^{-12}$
Dick |  | $6.12\times 10^{-13}$
Zeeman |  | $1.07\times 10^{-12}$
QPN |  | $4.90\times 10^{-13}$
Total ($\sqrt{\sigma_{SNR}^{2}+\sigma_{LO-tune}^{2}+\sigma_{Dick}^{2}}$) |  | $9.04\times 10^{-12}$
Measured |  | $1.49\times 10^{-11}$
Table 1: Table of noise sources and their contributions to the 1 s Allan
deviations stability.

In conclusion, we have demonstrated a compact cold-atom microwave clock based
on an additively manufactured loop-gap-resonator cavity interacting with laser
cooled atoms loaded from a GMOT chip. In the present system the experimentally
optimised short-term clock stability is measured as
$\sigma_{y}(\tau)=1.5\times 10^{-11}~{}\tau^{-1/2}$, primarily limited by the
Ramsey fringe SNR. In the medium-term the clock stability is primarily limited
by the second-order Zeeman shift due to operating in an unshilded environment.
A secondary limit is placed on the medium-term stability performance by the
reference oscillator itself exhibiting a stability bump at around 700 s. By
addressing these issues, we expect to improve the short-term stability to be
more in-line with other cold-atom cavity clock demonstrations Esnault  _et
al._ (2010); Muquans (2023); Spectra Dynamics (2023); Lee  _et al._ (2021).

The current physics package is inherently compact and remains highly amenable
to further miniaturisation. The use of additive manufacturing also maintains a
highly scalable manufacturing process. Additively manufactured vacuum chambers
have also recently demonstrated compatibility with UHV Cooper  _et al._ (2021)
pressure levels. This raises the enticing possibility of the cavity body
itself simultaneously acting as a UHV chamber, allowing the entire physics
package to be manufactured as a single bulk component, drastically reducing
the size of the system. In addition to this, passively pumped vacuum chambers
have now been shown to maintain the UHV levels required for atom trapping for
extended periods Little  _et al._ (2021); Burrow  _et al._ (2021), allowing
for a reduction in power consumption by negating the continuous use of an ion
pump. While the stability of our clock is currently several orders of
magnitude below the state-of-the-art, the scope for improved performance and
further miniaturisation is very good. We therefore believe this work
represents a step towards highly compact and portable cold-atom frequency
standards.

###### Acknowledgements.

The authors would like to thank R. Elvin and J. P. McGilligan for useful
conversations and J. P. McGilligan for careful reading of the manuscript. A.B.
was supported by a Ph.D. studentship from the Defence Science and Technology
Laboratory (Dstl). E. B., C.A. and G. M. acknowledge funding from the European
space Agency (ESA) and the Swiss Space Office (Swiss Confederation). We
gratefully acknowledge funding from the International Network for
Microfabrication of Atomic Quantum Sensors (EPSRC EP/W026929/1).

## Data Availability Statement

The data that support the findings of this study are available from
https://doi.org/10.15129/bb0d3614-6394-4d0e-8649-b1bf66c71722

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# X-ray diagnostics of Cassiopeia A’s “Green Monster”: evidence for dense
shocked circumstellar plasma

Jacco Vink Anton Pannekoek Institute/GRAPPA, University of Amsterdam, Science
Park 904, 1098 XH Amsterdam, The Netherlands  SRON Netherlands Institute for
Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands  Manan
Agarwal Anton Pannekoek Institute/GRAPPA, University of Amsterdam, Science
Park 904, 1098 XH Amsterdam, The Netherlands  Patrick Slane Center for
Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138,
USA  Ilse De Looze Sterrenkundig Observatorium, Ghent University, Krijgslaan
281 - S9, B-9000 Gent, Belgium  Dan Milisavljevic Purdue University,
Department of Physics and Astronomy, 525 Northwestern Ave, West Lafayette, IN
47907  Integrative Data Science Initiative, Purdue University, West Lafayette,
IN 47907, USA  Daniel Patnaude Center for Astrophysics $|$ Harvard &
Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  Tea Temim Princeton
University, 4 Ivy Ln, Princeton, NJ 08544, USA

###### Abstract

The recent survey of the core-collapse supernova remnant Cassiopeia A (Cas A)
with the MIRI instrument on board the James Webb Space Telescope (JWST)
revealed a large structure in the interior region, referred to as the “Green
Monster”. Although its location suggests that it is an ejecta structure, the
infrared properties of the “Green Monster” hint at a circumstellar medium
(CSM) origin. In this companion paper to the JWST Cas A paper, we investigate
the filamentary X-ray structures associated with the “Green Monster” using
Chandra X-ray Observatory data. We extracted spectra along the “Green Monster”
as well as from shocked CSM regions. Both the extracted spectra and a
principal component analysis show that the “Green Monster” emission properties
are similar to those of the shocked CSM. The spectra are well-fit by a model
consisting of a combination of a non-equilibrium ionization model and a power-
law component, modified by Galactic absorption. All the “Green Monster”
spectra show a blueshift of around -2500 ${\rm km\ s^{-1}}$, suggesting that
the structure is on the near side of Cas A. The ionization age is around
$n_{\rm e}t\approx 1.4\times 10^{11}~{}{\rm cm^{-3}s}$. This translates into a
pre-shock density of $\sim 11~{}{\rm cm^{-3}}$, higher than previous estimates
of the unshocked CSM. The relatively high $n_{\rm e}t$ and relatively low
radial velocity suggest that this structure has a relatively high density
compared to other shocked CSM plasma. This analysis provides yet another piece
of evidence that the CSM around Cas A’s progenitor was not that of a smooth
steady wind profile.

Supernova remnants (1667) — X-ray astronomy (1810) — Stellar mass loss (1613)
— Circumstellar dust(236)

Figure 1:  Top panel: RGB color image using the narrow bands 1.2–1.4keV (red,
showing Fe-L and Mg XI line emission), 1.85-1.98 keV (green, Si XII), and
4.0–4.5keV (blue, continuum). The images are obtained by combining images from
several individual deep observations in 2004 (ObsID 4634, 4635, 4636, 4637,
4638, 4723). The outline of the “Green Monster” structure is shown with green
polygons. Spectral extraction regions along the “Green Monster” (GM 1–5) and
three CSM-related structures (CSM 1–3) are shown in white. The background
regions used are shown in cyan. Bottom panels: The central region of Cas A in
X-rays in 2004 (left), 2019 (ObsID19606, center), and IR using JWST/MIRI data
(right). The X-ray extraction regions are overlayed as well as a rough outline
of the “Green Monster”. The RGB channels are for the X-ray images identical to
those in the top panel. The JWST map consists of the F2550W (red), F1800W
(green), and F1000W (blue) filters (see Milisavljevic et al.). All images
employ square-root brightness scaling.

##  1 Introduction

The infrared imaging mosaics based on the 2022 James Webb Space Telescope
(JWST) observations with the MIRI and NIRCAM camera of the young core-collapse
supernova remnant Cassiopeia A (Cas A) revealed in exquisite detail various
spatial infrared (IR) components, such as numerous, small ejecta knots
emitting thermal dust radiation, a more diffuse circumstellar medium (CSM)
component in the outer shell, and, with NIRCAM, very diffuse emission likely
caused by synchrotron radiation (Milisavljevic et al. submitted).111See also
the press release image of April 7, 2023, https://www.nasa.gov/universe/webb-
reveals-never-before-seen-details-in-cassiopeia-a. These basic components were
known from previous IR and multiwavelength observations, but unexpectedly the
JWST images also revealed a giant loop structure across part of the interior
of Cas A. This IR structure was popularized under the name the “Green
Monster”. The location of this loop interior to the main shell seems to
indicate an ejecta origin, but its filamentary structure is morphologically
very distinct from the more prominent ejecta components, consisting of
numerous small knots.

The IR properties of the “Green Monster” (GM) are somewhat similar to a large
protrusion in the southern part, which is associated with a bright radio
feature (e.g., feature H in Braun, 1987), and with an arc of nitrogen-rich
knots, labeled quasi-stationary flocculi (QSFs, van den Bergh, 1971; Koo et
al., 2020). See the labeling in Fig. A1. The spectra of these QSFs show that
they find their origin in the helium- and nitrogen-enriched wind of the
progenitor star (e.g., Koo et al., 2023), and are hence associated with the
CSM.

However, its locations and small “bullet-hole”-like features in the GM
filaments prompted us to scrutinize the identification of the GM as a shocked
CSM rather than an ejecta structure. This letter is a companion paper to the
JWST results reported by Milisavljevic et al. —providing a general overview of
Cas A observations by JWST— and the paper by De Looze et al., which focuses on
the IR the morphology and spectroscopy of the GM. De Looze et al. report that
the IR spectroscopic properties of the GM has similarities with those regions
containing QSFs, and are spectroscopically distinct from other CSM-related
dust emission. They attribute this to a high carbon-over-silicate grain
abundance ratio for the GM/QSF regions, as compared to other CSM-related dust
components.

Here we show that X-ray filaments associated with the GM do indeed have X-ray
characteristics shared with shocked CSM. This analysis is based on archival
X-ray data gathered by the NASA Chandra X-ray Observatory (Chandra for
short).222Extracted data products as well as software will be made available
on https://doi.org/10.5281/zenodo.10301088

##  2 X-ray data

###  2.1 X-ray spectra of “Green Monster” region and the southern CSM

Chandra X-ray Observatory observed Cas A numerous times with the ACIS-S CCD
detectors in imaging mode. The last such observation concerned a 49ks deep
exposure made in 2019 (ObsID 19606). Much deeper exposures—1 Ms in total, but
individual exposure amounting to 170 ks—were made in 2004 as part of the
Chandra very-large program (VLP) (Hwang et al., 2004).

The X-ray maps using an RGB color representation—see Fig. 1—reveal reddish
filamentary structures that align well, but not perfectly, with the GM
identified in the JWST maps (Fig. 1 lower right). The reddish color in the
X-ray maps corresponds with the 1.2-1.4 keV band, which is characterized by Mg
XI/XII, and Fe-L line emission. The reddish color is in stark contrast to the
greenish color associated with Si XII line emission from Si-rich ejecta, and
the blue/purplish color of X-ray synchrotron radiation.

The RGB X-ray maps, therefore, already provide an hint that the X-ray emission
associated with the GM is not associated with Si-rich ejecta, or synchrotron
emission. In Fig. 1(top) two other components also appear red: the iron-rich
ejecta in the southeastern region (Hughes et al., 2000; Hwang & Laming, 2012),
and a very bright red filament just above the base of the eastern jet, known
as “Baade-Minkowski’s filament 1” (Baade & Minkowski, 1954), which contains
O/Ne/Mg rich ejecta (Vink, 2004).

The morphology of these filaments show some evolution between 2004 and 2019
(c.f. Fig. 1 bottom left and center). In the north of the GM some stronger Si-
lines show up in the 2019 data set, possibly coinciding with the regions of
ejecta knots seen with JWST/MIRI in the northern part of the GM. In addition,
the GM overlaps partially with nonthermal X-ray filaments, which seem to
evolve relatively rapidly in some regions (Patnaude et al., 2011; Vink et al.,
2022), causing some different spectral characteristics between 2004 and 2019.
These nonthermal filaments are either associated with the forward shock
(Gotthelf et al., 2001; Vink & Laming, 2003), but projected to the interior,
or they are associated with the reverse shock regions (Helder & Vink, 2008;
Uchiyama & Aharonian, 2008), requiring relative speeds between the reverse
shock and the unshocked ejecta of $\gtrsim 3000~{}{\rm km\ s^{-1}}$, a
requirement that seems to met in parts of the western half of Cas A (Vink et
al., 2022). Since the reddish-colored filaments associated with the GM itself
are relatively stable between 2004 and 2019, we opted for an investigation of
the deepest single observation run available, which is the 2004 observation
labeled ObsID 4638. We found that the extracted spectra have good statistics,
with most of the uncertainties arising from systematic effects, such as the
choice (or discarding) of background spectra. The data were processed with the
Chandra software package ciao-4.15, and employed the xspec package (Arnaud,
1996) for spectral analysis.

We extracted five spectra along the GM filaments, as well as a region
associated at the tip of the arc of QSFs (CSM3), a fainter region in the
south, outside the ejecta shell (CSM2), and also associated with QSFs. For
comparison we also extracted a relatively bright CSM region in the north
(CSM1), with no QSF counterpart. Background spectra were obtained from faint
nearby regions, assuming that the extracted spectra of filaments contain also
more diffuse components plus other background components. We note here that
even background spectra from extraction regions outside Cas A still seem to be
dominated by dust scattered X-ray emission from Cas A itself. All extraction
regions are displayed in Fig. 1. All the spectra were optimally binned
following the procedure described in Kaastra & Bleeker (2016) through the
FTool (Blackburn, 1995) task ftgrouppha.

For fitting spectra, we experimented with both a model using the standard non-
equilibrium ionization (NEI) model vnei and the vpshock model (Borkowski et
al., 2001). The vnei assumes a single ionization age $\tau$ (or $n_{\rm e}t$),
whereas vpshock assumes a superposition of ionization ages assuming a plane
parallel shock geometry. Since the vpshock is more realistic, we report here
only the vpshock model fits. The vpshock model specifies a lower and an upper
bound to the ionization age, $\tau_{\rm max}$. The lower bound was set to
zero, and the upper bound was left as a free parameter. Apart from the NEI
component the model includes Galactic absorption (tbabs Wilms et al., 2000),
and a power-law component to account for the X-ray synchrotron radiation. The
abundances were not well constrained for all spectra, in particular for those
spectra that showed evidence for a prominent synchrotron component (GM3). For
that reason we made simultaneous fits to all GM spectra, coupling the
abundances across all the GM spectra, but determining the other relevant
parameters for each individual spectrum. We used the same procedure for the
three CSM spectra. Apart from the electron temperature $kT_{\rm e}$,
$\tau_{\rm max}$, the normalizations and absorption columns $N_{\rm H}$, we
also allowed for line Doppler shifts. The abundance of Ni was coupled to Fe,
and the abundance of Ca was coupled to Ar. All abudances were measured in
units of the Lodders et al. (2009) proto-solar abundance values.

For the optimization of the fitted model we used the Bayesian scheme employed
in Ellien et al. (2023), based on the Python version of xspec combined with
the Python package BXA (Buchner et al., 2014), a Python implementation of the
Ultranest algorithm (Feroz et al., 2009). The versions used in this work were
xspec v12.13.1 and BXA v4.1.1. The Ultranest algorithm ensures extensive
coverage of the parameter space.

All fitted models provide reasonable fit to the spectra, given that these are
line-rich spectra with excellent statistics. The latter implies that
systematic uncertainties dominate. The spectra and best-fit models are shown
in Fig. 2, and the best-fit parameters are listed in Table. 1.

Table 1: Best-fit parameters for GM and CSM spectra.  Region: | GM1 | GM2 | GM3 | GM4 | GM5 | CSM1 | CSM2 | CSM3
---|---|---|---|---|---|---|---|---
$kT_{\rm e}$ (keV) | $1.84^{+0.05}_{-0.05}$ | $2.17^{+0.12}_{-0.12}$ | $1.89^{+0.73}_{-0.73}$ | $2.62^{+0.10}_{-0.10}$ | $1.15^{+0.06}_{-0.06}$ | $2.40^{+0.14}_{-0.14}$ | $1.48^{+0.19}_{-0.19}$ | $1.50^{+0.08}_{-0.08}$
O |  $4.98^{+0.20}_{-0.20}$ |  $3.97^{+0.52}_{-0.46}$
Ne |  $2.11^{+0.14}_{-0.13}$ |  $1.49^{+0.14}_{-0.13}$
Mg |  $1.65^{+0.08}_{-0.08}$ |  $0.88^{+0.05}_{-0.05}$
Si |  $1.63^{+0.08}_{-0.08}$ |  $0.73^{+0.04}_{-0.04}$
S |  $1.58^{+0.09}_{-0.09}$ |  $0.76^{+0.05}_{-0.05}$
Ar |  $1.40^{+0.14}_{-0.13}$ |  $0.37^{+0.16}_{-0.11}$
Fe |  $1.39^{+0.10}_{-0.09}$ |  $0.94^{+0.13}_{-0.12}$
$\tau_{\rm max}(10^{11}{\rm cm^{-3}s})$ | $1.29^{+0.07}_{-0.07}$ | $1.83^{+0.19}_{-0.17}$ | $1.12^{+0.36}_{-0.27}$ | $1.08^{+0.05}_{-0.04}$ | $1.85^{+0.30}_{-0.26}$ | $0.60^{+0.04}_{-0.04}$ | $0.71^{+0.13}_{-0.11}$ | $0.78^{+0.05}_{-0.05}$
$v_{\rm rad}({\rm km\,s^{-1}})$ | $-2.394^{+45}_{-45}$ | $-2.634^{+99}_{-99}$ | $-2.396^{+422}_{-422}$ | $-2.848^{+141}_{-141}$ | $-1.779^{+164}_{-164}$ | $657^{+110}_{-110}$ | $-2.544^{+257}_{-257}$ | $-1.266^{+549}_{-549}$
norm aaDefined as norm$=$EM$=10^{-14}/(4\pi d^{2})\int n_{\rm e}n_{\rm H}dV$. | $4.71^{+0.34}_{-0.32}$ | $1.91^{+0.13}_{-0.12}$ | $0.24^{+0.04}_{-0.03}$ | $3.30^{+0.24}_{-0.22}$ | $1.16^{+0.10}_{-0.09}$ | $1.37^{+0.09}_{-0.08}$ | $0.89^{+0.09}_{-0.08}$ | $3.04^{+0.14}_{-0.13}$
$\Gamma$ | $3.69^{+0.04}_{-0.04}$ | $2.98^{+0.04}_{-0.04}$ | $2.53^{+0.04}_{-0.04}$ | $3.66^{+0.02}_{-0.02}$ | $3.17^{+0.28}_{-0.28}$ | $3.40^{+0.15}_{-0.15}$ | $3.13^{+0.29}_{-0.29}$ | $2.54^{+0.31}_{-0.31}$
PL norm bbunits: $10^{-3}{\rm ph\,cm^{-2}\,keV^{-1}}$ @ 1 keV | $4.26^{+0.42}_{-0.38}$ | $3.38^{+0.17}_{-0.16}$ | $1.22^{+0.07}_{-0.06}$ | $4.20^{+0.33}_{-0.31}$ | $\left(9.71^{+68.9}_{-8.51}\right)\times 10^{-6}$ | $0.83^{+0.13}_{-0.11}$ | $0.14^{+1.31}_{-0.13}$ | $0.17^{+11.85}_{-0.17}$
$N_{\rm H}({\rm cm^{-2}})$ | $1.89^{+0.02}_{-0.02}$ | $1.78^{+0.02}_{-0.02}$ | $2.19^{+0.05}_{-0.05}$ | $1.81^{+0.02}_{-0.02}$ | $2.05^{+0.03}_{-0.03}$ | $1.48^{+0.03}_{-0.03}$ | $2.37^{+0.04}_{-0.04}$ | $1.93^{+0.03}_{-0.03}$
C-stat/bins | 261.39/83 | 157.34/79 | 90.30/75 | 235.28 /81 | 119.56 / 71 | 170.46/ 79 | 75.43/64 | 151.39/ 73

Figure 2: Left: X-ray spectra from the “Green Monster” region. Right: Spectra
of the three CSM regions. See Fig. 1 and Table 1 for the identification and
spectral fit parameters. The dashed and dotted lines show the model
contributions of the vpshock and powerlaw components, respectively.

###  2.2 Principal component analysis

The GM spectra are characterized by less-prominent Si-K and S-K lines as
compared to ejecta spectra, which have either bright Si-K, S-K lines, or
otherwise prominent Fe-L and Fe-K line complexes (e.g. Hughes et al., 2000;
Hwang & Laming, 2012). However, a low equivalent width of the Si-K and S-K
lines can also be caused by a dominant X-ray synchrotron composition. So it is
difficult from spectral fitting alone to establish whether the GM spectra are
originating from metal-rich ejecta plasma, with a strong nonthermal component,
or originate from CSM.

In order to better characterize the X-ray spectral content of Cas A, we
resorted to a principal component analysis (PCA). PCA is a well-known
statistical method in which a complex data set, represented as a set of data
vectors, is restructured into a set of orthogonal eigen vectors, ordered
according to their relative importance, i.e., the amount of variance they
cover. PCA has been used before to characterize X-ray spectra of young SNRs,
see Warren & Hughes (2004); Warren (2006).

The PCA of the data consists of representing the spatial-plus-energy data cube
as a two dimensional matrix, with the rows corresponding to all the spatial
pixels and the columns corresponding to discrete energy bands. For this PCA,
we combined the ObsIDs 4634, 4635, 4636, 4637, 4638, 4723, creating 28
combined images (Table A1) with a pixel resolution of 0.98″. The images were
rebinned to a pixel size of at least 1.97″ (4 by 4 Chandra pixels), using a
mesh-refinement scheme common to all energy bands. The columns were centered
(mean per band subtracted) and divided by the square root of the pixel
variance for each band. The PCA output consists of the eigenvectors (principal
components, PCs), the singular values, and the PC scores. The latter are
images indicating for each pixel the amount by which it is represented by a
PC.

The PC scores 2–5 are shown in Fig. A3. PC2 identifies mostly with the
nonthermal X-ray emission, and PC3 and PC4 seem to identify with the
(partially) iron-rich regions. PC5 appears to correlate best with some
filaments associated with the GM. However, PCA regularizes data as a set of
orthogonal components, whereas different physical components present in the
X-ray spectra may not necessarily be orthogonal. Physical components may,
therefore, be spread over different PC scores. Scatter plots of PC scores for
the total data set and just the GM region (red) are shown in Fig. 3. It shows
that the GM region is associated with positive values of PC2 and values of PC5
of less than 0.01.

We use these scatter plots to automatically select PC scores similar to the
GM. We can identify all pixels where the spectra have similar PC properties as
those of the GM. For this, we looked at the density of GM points in scatter
plots like the ones shown in Fig. A3. The selection threshold density is a
tuneable parameter. Fig. 3 shows masks of all pixels where the PC1–8
properties are similar to the GM for two different selection thresholds. For
the lenient threshold (Fig. 3, left), we see that the mask selects out all
regions associated with the main shell, which is dominated by Si-rich ejecta
emission. It also excludes the Fe-rich regions in the southeastern part (also
reddish in Fig. 1top). Instead, most of the outer regions, where we expect the
shocked CSM plasma to reside, are selected. For the more stringent threshold
(Fig. 3, right), some parts of the outer regions are not selected, which
includes, interestingly CSM1. But the region containing the arc of QSFs in the
south (CSM3) and CSM2 are shown to be “ GM-like”. Note that the threshold used
here is rather severe as many areas within the GM-region itself are blacked
out, perhaps due to contamination with ejecta emission or nonthermal emission.

No matter the selection threshold, the PCA analysis identifies the spectral
X-ray properties of the GM to be very dissimilar from the ejecta-rich shell,
and to be much more similar to regions in the outskirts of Cas A, where we
expect the emission to come from shocked CSM. The dependency on threshold
indicates some variations within both the GM-region as well as the outer
regions of Cas A, which may be related to differences in plasma properties.

Figure 3: Top panels: Scatterplot of PC scores for all pixels (black) and
those corresponding to the GM region (red), zooming in on where the majority
of the points are located. (See Fig. A4 for the full range of PC scores.)
Bottom panels: Mask created from the PC1 to PC8 scores using values
corresponding to the red colored regions in the top panels. The left panel
corresponds to a lose selection and the right panel to a more stringent
selection (see text).

##  3 Discussion

The GM is a striking, newly JWST/MIRI identified IR structure inside the main
shell of Cas A. The JWST/MIRI data by itself already provides some clues that
it is structure related to the shocked CSM. The X-ray analysis described here
confirms a CSM origin. It shows that the X-ray spectra of GM regions are
similar to the spectra of CSM regions, although for all these spectra there is
a variable level of contribution of X-ray synchrotron radiation. A perhaps
even more convincing argument that the GM X-ray spectra are CSM related is
provided by a principal component analysis (PCA), which shows that the
spectral properties of the GM region also identifies the outer regions of Cas
A, for which the most likely origin is shocked CSM (c.f. Fig. 5 in Hwang &
Laming, 2012).

From the X-ray spectral analysis a few commonalities, informing us on the
nature of the GM plasma, emerge:

  1. 1.

the ionization ages ($\tau$ or $n_{\rm e}t$) of both the GM and CSM are
relatively low, $\tau_{\rm max}\approx 0.5$–$2\times 10^{11}~{}{\rm
cm^{-3}s}$, compared to other regions of Cas A for which $\tau_{\rm
max}>2\times 10^{11}~{}{\rm cm^{-3}s}$ seems more typical (Hwang & Laming,
2012);

  2. 2.

the GM spectra are all blueshifted with radial velocities of
$\approx-2500~{}{\rm km\,s^{-1}}$;

  3. 3.

some spectra require a synchrotron contribution, which dominates the spectrum
for GM3;

  4. 4.

the CSM spectra are rather similar to the GM spectra in that the
silicon/sulfur line emission and/or iron-line emission are much less prominent
(i.e. have lower equivalent widths) than for typical spectra from Cas A (c.f.
spectra in Hwang & Laming, 2012).

In particular, points 1) and 4) reinforce the idea that the GM spectra are
indeed CSM-related. The presence of a strong synchrotron component in some
spectra (in particular GM3) suggests that these filaments may be close to the
forward shock, as the rapid synchrotron cooling times of the electrons with
$\gtrsim 10$ TeV only allows for X-ray synchrotron radiation within $10^{17}$
cm of the shock (e.g. Vink & Laming, 2003). But chance alignment with either
forward-shock or reverse-shock-related filaments cannot be excluded.

The abundance pattern does show relatively low abundances, but are not
completely consistent with solar. Optical studies indicate that the CSM of Cas
A is enhanced in He and N, which will affect the thermal continuum, and hence
the derived abundances of the other elements. In particular, the oxygen
abundance both for the GM and the CSM spectra is relatively high. It is not
clear whether the CSM of Cas A is indeed enhanced in oxygen or whether some
systematics are involved. We note that oxygen is nearly completely ionized,
and the low energy part of the spectrum with oxygen lines is heavily affected
by absorption. Both effects may leave the measured abundance vulnerable to
systematic effects. We also note that the best-fit GM abundances (except
oxygen) are a factor $\sim 2$ higher than the CSM abundances. We caution not
to over-interpret this effect. There may indeed be abundance variations
throughout the CSM in Cas A, but we cannot exclude some contamination by
ejecta emission to the spectra of GM regions.

The $\tau_{\rm max}$ values for GM regions are higher than for the CSM
regions. The typically $\tau_{\rm max}$ values of $1.4\times 10^{11}~{}{\rm
cm^{-3}s}$ translate into a post-shock electron density of $n_{\rm
e,1}\approx\tau_{\rm max}/t=44~{}{\rm cm^{-3}}(t/100~{}{\rm yr})(\tau_{\rm
max}/1.4\times 10^{11})$, with $t$ the time since the plasma was shocked. The
value for $\tau_{\rm max}$ corresponds to a pre-shock density—i.e. a factor 4
lower—of $n_{\rm e,0}\approx 11/(t/100~{}{\rm yr})(\tau_{\rm max}/1.4\times
10^{11})~{}{\rm cm^{-3}}$. This value for the pre-shock density is a factor
5–10 higher than previous estimates (Vink, 2004; Lee et al., 2014) but could
be marginally consistent if one takes plasma ages closer to the maximum age of
$\sim 350$ yr.

For the CSM fits we infer still a relatively high pre-shock density of $n_{\rm
e,0}\approx 5.5/(t/100~{}{\rm yr})(\tau_{\rm max}/0.7\times 10^{11})~{}{\rm
cm^{-3}}$. This is a factor two lower than for the GM regions, but still
higher than inferred by Lee et al. (2014). This may be caused by a bias in
finding suitable CSM extraction regions, for which we focussed on clear
structures, rather than very diffuse regions.

The blueshifts of the GM spectra indicate that the GM lies on the near-side of
Cas A, and, as shown in Fig. A2, are statistically significant. Interestingly,
the QSF regions are also on average blueshifted, with optically determined
Doppler shifts of approximately -600–100 ${\rm km\,s^{-1}}$ (van den Bergh,
1971; Alarie et al., 2014). Another hint that there may be a connection
between the QSFs and the GM (c.f. De Looze et al.).

The regions CSM2 and CSM3 analyzed here, are associated with the QSFs, and
appear also blueshifted, indicating surprisingly large radial velocities.
CSM3, at the tip of the arc of QSFs, may, in fact, have a smaller Doppler
shift than tabulated: the Doppler shift probability density distribution (Fig.
A2) shows two peaks of high likelihood, of which one is being consistent with
no Doppler shift. The blueshift of $\sim 2500~{}{\rm km\,s^{-1}}$ for CSM2 is
statistically significant. This could be possible if locally the shock
velocity is large and the X-ray emitting plasma received a relatively large
velocity. CSM2 is located in a region where the measured X-ray proper motions
indicate that the shock velocity changes from $\sim 6200~{}{\rm km\,s^{-1}}$
to $\sim 4000~{}{\rm km\,s^{-1}}$ (Vink et al., 2022). So a high shock
velocity around CSM2 may indeed be possible, but needs to be further
investigated.

CSM1 is not associated with QSFs, and is the only region analyzed here that
shows a redshift rather than a blueshift.

The typical GM’s radial velocity of $|v_{\rm rad}|=2500\cos i~{}{\rm km\
s^{-1}}$ corresponds to shock velocities of $V_{\rm s}\approx 3300\cos
i~{}{\rm km\ s^{-1}}$, given that the plasma downstream of a strong shock
running into a monatomic gas has $v_{\rm plasma}=4/3V_{\rm s}$. The factor
$\cos i$ takes into account projection effects. Since the GM lies close to the
center, the projection effects should be small —$\cos i\approx 0$—30∘—which
suggests that the GM is associated with parts of the CSM where the shock front
has decelerated to $V_{\rm s}\sim 3500$–$4000~{}{\rm km\ s^{-1}}$. X-ray
proper motion studies of Cas A show that the typical shock speeds are 5800 km
s-1 (Vink et al., 2022). But a value as low as 4038 km s-1 is reported for a
position angle of 190∘. Interestingly, this corresponds to the region
containing the arc of QSFs, for which a higher than average density is likely.
Since the shock velocities for a given postshock pressure are proportional to
$\rho_{0}V_{\rm s}^{2}$, we expect that the lower inferred shock velocities
are the result of a pre-shock density that is $V_{\rm
s,GM}^{2}/\overline{V_{\rm s}}^{2}\approx 3$ higher than the average CSM
density near the current shock front. So both the best-fit $\tau_{\rm max}$
values, as well as the relatively low plasma velocities of $\approx
2500~{}{\rm km\ s^{-1}}$ suggest that the filaments in the GM stand out due to
a higher than average density.

Some of the X-ray analysis presented here goes beyond the main topic of this
paper, i.e. the nature of the GM. For example, the PCA provides a yet
unexplored way to identify which parts of Cas A consist mostly of shocked CSM
and which parts consist of shocked ejecta. It is interesting that most of the
region interior to the main shell—not just the GM—seems to be
spectroscopically similar to the shocked CSM. This suggests a lack of ejecta-
related emission from the central regions of Cas A. According to the PCA CSM1
appears to have different X-ray properties than the GM regions and CSM2 and
CSM3, which may reflect some reveal variation in thermal X-ray emission from
the shocked CSM. This may be related to the variation in IR dust grain
emission properties between the northern CSM on the one hand, and the QSF and
GM-related emission on the other hand, which is due to a higher carbon-over-
silicate abundance in the QSF and GM regions (De Looze et al.).

Another noteworthy result of the PCA is that the main shell for position
angles between roughly 170∘and 200∘seems to have some CSM-like properties,
except for two narrow gaps. This is a region in which the ejecta shell is
relatively narrow in X-rays (Fig. 1top) and in the optical the ejecta-related
fast moving knots only started appearing since the 1970s (see Fig. 1 in
Patnaude & Fesen, 2014). Some of the IR emission properties in that region
seem to be similar to the GM emission (Milisavljevic et al., De Looze et al.).
In addition, this region is also relatively bright in [Fe II] $1.644~{}{\rm\mu
m}$, which it has in common with the arc of QSFs further to the south (Lee et
al., 2017, their Fig. 12). Lee et al. (2017) attribute this to a prominent CSM
component associated with the contact discontinuity in this part of the shell.

##  4 Conclusion

We analyzed the X-ray properties of the JWST-identified “Green Monster” (GM)
region in order to study the nature of this new structure. For the X-ray
investigation, we used a spectral analysis employing an absorbed vpshock plus
power-law model, the latter to account for nonthermal X-ray emission. For
optimizing the models we used a Bayesian analysis scheme. In addition, we
characterized the X-ray spectral properties of the GM in comparison to the
rest of Cas A using PCA.

Both types of X-ray analysis suggest that the GM is a structure that has
similar spectral characteristics as the outer regions of Cas A, which consist
most likely of shocked CSM. For the PCA this conclusion is based on the fact
that selecting PC scores associated with the GM also select for the outer
regions of Cas A, and exclude the Si-rich and Fe-rich ejecta components.
Spectroscopically we show that that GM spectra are relatively weak in Si XIII
emission—and line emission from other intermediate mass elements—and are well
characterized by abundances that are close to solar abundances, and quite
similar to other CSM-related spectra.

The analysis presented here supports the conclusion of De Looze et al., based
on JWST data alone, that the GM monster structure corresponds to a structure
in the CSM of Cas A, rather than to a ejecta-related structure.

In addition, the X-ray analysis shows that the GM spectra are blueshifted by
about 2500 km s-1, suggesting that the GM structure is on the near-side of Cas
A, and projected onto the interior. The radial velocity is relatively low,
given the average shock speed of Cas A of 5800 km s-1, which could be
explained if the pre-shock CSM corresponding to the GM was a factor 2–3 denser
than the average CSM density.

The infrared and X-ray properties of the GM provide yet another piece of the
puzzle regarding the CSM of Cas A. This will help to reconstruct the mass-loss
properties of the progenitor star, and how it ended up to become the Type IIb
supernova identified from light echo spectra (Krause et al., 2008; Rest et
al., 2011).

The authors thank the Lorentz Center Leiden and the main scientific organizer
Maria Arias for organizing the workshop “Supernova remnants in complex
environments”, where most of the authors were present. We thank Amaël Ellien
and Emanuele Greco for helping M.A. set up the Bayesian spectral fitting
procedures, and we thank Allessandra Mercuri for her help in understanding the
X-ray properties of the shocked CSM in Cas A. M.A. is supported by the
research program Athena with project No. 184.034.002, which is (partially)
financed by the Dutch Research Council (NWO). J.V. is supported by funding
from the European Unions Horizon 2020 research and innovation program under
grant agreement No. 101004131 (SHARP). D.M. acknowledges NSF support from
grants PHY- 2209451 and AST-2206532.

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## Supplementary material

This appendix shows supplementary material that support the main text. Fig. A1
identifies several regions of interest, mentioned in the text. Fig. A2
provides the sampling plots for the spectral fits for the Doppler shift
parameters versus $\tau_{\rm max}$. Table A1 shows the image energy bands used
for the PCA. The first eight PCA score images are shown in Fig. A3.

Figure A1:  Same as Fig. 1, but now with rough indications of important
regions mentioned in the text. In general, all Si-rich ejecta show up here in
green. Fe-rich ejecta show up as shades of red, but so are regions relatively
abundant in Ne IX/X and Mg XI/XII. Bright X-ray synchrotron filaments show up
in shades of purple.

Figure A2: Correlation plots of the Doppler shifts versus $\tau_{\rm max}$ for all GM and CSM regions. The histograms are normalized to a sum of one to form a probability density distribution. The GM analysis is based on 128263 nested sampling iterations and for the CSM analysis there were 41480 nested sampling iterations.  Table A1: Input energy bands used for principal component analysis.  | $E_{\rm min}$ | $E_{\rm max}$ |  | $E_{\rm min}$ | $E_{\rm max}$
---|---|---|---|---|---
| (keV) | (keV) |  | (keV) | (keV)
1 | 0.50 | 0.62 | 15 | 2.63 | 2.78
2 | 0.62 | 0.72 | 16 | 2.78 | 3.00
3 | 0.72 | 0.87 | 17 | 3.00 | 3.28
4 | 0.87 | 1.03 | 18 | 3.28 | 3.60
5 | 1.03 | 1.12 | 19 | 3.60 | 3.75
6 | 1.12 | 1.23 | 20 | 3.75 | 4.04
7 | 1.23 | 1.42 | 21 | 4.04 | 4.50
8 | 1.42 | 1.56 | 22 | 4.50 | 5.00
9 | 1.56 | 1.69 | 23 | 5.00 | 5.40
10 | 1.69 | 1.85 | 24 | 5.40 | 5.75
11 | 1.85 | 1.98 | 25 | 5.75 | 6.25
12 | 1.98 | 2.13 | 26 | 6.25 | 6.62
13 | 2.13 | 2.28 | 27 | 6.62 | 6.85
14 | 2.28 | 2.63 | 28 | 6.85 | 7.35

Figure A3: The PC scores 1 (top left) to 8 (bottom right). In particular in
the scores for PC 5, 7, 8 one recognizes some of the filaments associated with
the GM.

Figure A4:  Same as top panels of Fig. 3, but now with the full range of PC
scores.