# X-ray and Radio campaign of the Z-source GX 340$+$0: discovery of X-ray
polarization and its implications

Yash Bhargava Department of Astronomy and Astrophysics, Tata Institute of
Fundamental Research,
1 Homi Bhabha Road, Colaba, Mumbai 400005, India
<EMAIL_ADDRESS>Mason Ng MIT Kavli Institute for Astrophysics
and Space Research, Massachusetts Institute of Technology, Cambridge, MA
02139, USA  Liang Zhang Key Laboratory of Particle Astrophysics, Institute of
High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
Arvind Balasubramanian Department of Astronomy and Astrophysics, Tata
Institute of Fundamental Research,
1 Homi Bhabha Road, Colaba, Mumbai 400005, India  Thomas D. Russell INAF,
Istituto di Astrofisica Spaziale e Fisica Cosmica, Via U. La Malfa 153,
I-90146 Palermo, Italy  Aman Kaushik Department of Astronomy and Astrophysics,
Tata Institute of Fundamental Research,
1 Homi Bhabha Road, Colaba, Mumbai 400005, India  Vishal Jadoliya Department
of Physics, Indian Institute of Technology Hyderabad, IITH main road, Kandi
502284  Swati Ravi MIT Kavli Institute for Astrophysics and Space Research,
Massachusetts Institute of Technology, Cambridge, MA 02139, USA  Sudip
Bhattacharyya Department of Astronomy and Astrophysics, Tata Institute of
Fundamental Research,
1 Homi Bhabha Road, Colaba, Mumbai 400005, India  Mayukh Pahari Department of
Physics, Indian Institute of Technology Hyderabad, IITH main road, Kandi
502284  Jeroen Homan Eureka Scientific, Inc., 2452 Delmer Street, Oakland, CA
94602, USA  Herman L. Marshall MIT Kavli Institute for Astrophysics and Space
Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Deepto Chakrabarty MIT Kavli Institute for Astrophysics and Space Research,
Massachusetts Institute of Technology, Cambridge, MA 02139, USA  Francesco
Carotenuto Astrophysics, Department of Physics, University of Oxford, Keble
Road, Oxford OX1 3RH, UK

###### Abstract

We present the discovery of X-ray polarization from the neutron star low-mass
X-ray binary and Z-source, GX 340$+$0, using an Imaging X-ray Polarimetry
Explorer (IXPE) observation in March 2024. Along with the IXPE observation, we
conducted an extensive X-ray and radio monitoring campaign to ascertain the
source properties during and around the IXPE observation. The source appeared
to be on the horizontal branch throughout the multi-wavelength campaign. We
measured a significant X-ray polarization in 2–8 keV with polarization degree
(PD) = $4.02\pm 0.35$% and polarization angle (PA) = $37.6\pm 2.5\arcdeg$. The
energy-dependent polarization indicates that in the 2–2.5 keV energy range,
the PA is much lower, $\sim 9\pm 8\arcdeg$, while other energy bands are
consistent with the PA found over 2.5–8 keV. The simultaneous AstroSat-IXPE
spectro-polarimetric observations provide some evidence for independent
polarization from various spectral components, hinting at a disparity in the
PA from the accretion disk and the Comptonized emission while suggesting an
unpolarized emission from the blackbody component. Radio observations in the
0.7–9 GHz frequency range reveal a non-detection of radio emission in 0.7–1.5
GHz and a significant detection in 5.5–9 GHz, suggesting the presence of a
spectral break in 1.5–5.5 GHz. Using ATCA observation, we place upper limits
on the radio polarization at $<$6% on the linear polarization and $<$4% on the
circular polarization at 3$\sigma$ level. We discuss the origin of the X-ray
polarization and its implications on the geometry of the spectral components.

X-ray binary stars (1811); Accretion (14); Stellar accretion disks (1579);
Polarimetry (1278); X-ray astronomy (1810)

††facilities: IXPE, AstroSat, NuSTAR, NICER, Insight-HXMT, GMRT,
ATCA††software: astropy (Astropy Collaboration et al., 2018), ixpeobssim
(Baldini et al., 2022), CASA (CASA Team et al., 2022), Stingray (Huppenkothen
et al., 2019a, b)

##  1 Introduction

Z-sources are bright, low-mass X-ray binaries (LMXB) with a neutron star (NS)
as the compact object. They show a unique ‘Z’ shaped track on their X-ray
hardness-intensity diagram (HID) and the color-color diagram (CCD; Hasinger &
van der Klis, 1989). The track itself is divided into the Horizontal branch
(HB), the Normal branch (NB) and the Flaring branch (FB), with the hard apex
joining the HB and the NB and the soft apex joining the NB and the FB. These
sources host a weak magnetic field NS (typically $10^{8-9}$ G) and accrete
close to the Eddington rate. The transitions across different branches are
smooth, always passing through the apexes. There are two subclasses of the
Z-sources, depending on the shape of the ‘Z’ traced: Cyg-like sources (which
include Cyg X-2, GX 340$+$0, and GX 5$-$1) and Sco-like sources (which include
Sco X-1, GX 17$+$2, and GX 349$+$2). The evolution along the track is often
linked to strong changes in the mass accretion rate (Homan et al., 2007; Lin
et al., 2009; Homan et al., 2010; Chakraborty et al., 2011; Ng et al., 2024).

The repetitive track of a Z-source on the HID indicates that there is a clear
and specific way of spectral evolution as the source evolves. To understand
what is causing the spectral evolution, one needs to investigate the
components involved in the emission. The persistent Z sources mentioned above
have been extensively probed using the spectro-timing analysis, which yielded
a crude picture with many degeneracies in the geometrical configuration.
Historically, there has been a dichotomy in the modeling of the X-ray
spectrum. The emission in 2–20 keV energy range is well modeled as a
combination of a thermal and a Comptonization component with thermal component
being an accretion disk (Mitsuda et al., 1989), a blackbody component (Church
et al., 2006) or a combination of two thermal components with harder non-
thermal component (Lin et al., 2007, 2009, 2012). The Comptonized emission
(also referred to as corona) arises from the Compton up-scattering of photons
by a hot plasma of electrons (Mitsuda et al., 1989; Zdziarski et al., 1996;
Church et al., 2006; Seifina et al., 2013; Bhargava et al., 2023). The
investigations of the source using simultaneous soft X-ray instruments
demonstrate the presence of a tail of soft thermal component, which can be
modeled as a blackbody (e.g. Seifina et al., 2013) or a disk emission (e.g.
Bhargava et al., 2023).

The polarimetric observation of Z-sources has already provided new insights
into the orientation of various emission components. In the case of Cyg X-2,
only the NB was observed, and a 2–8 keV polarization degree (PD) of $1.8\pm
0.8$% (Farinelli et al., 2023) was measured. The spectro-polarimetric
investigation of Cyg X-2 indicates that the polarization angles (PA) of the
emissions from the disk and the corona may have an angle of 90$\arcdeg$
between them. GX 5$-$1 was observed in HB and NB, and the PD in HB ($4.3\pm
0.3$%) was observed to be significantly higher than that in NB ($2.0\pm 0.3$%;
Fabiani et al., 2024). This is possibly due to a higher contribution of the
coronal emission in HB as compared to NB (Church et al., 2012). GX 5$-$1 also
hinted towards a difference in the PA of the spectral components, which is not
as distinct as observed in Cyg X-2. The transient Z-source XTE J1701$-$462
demonstrated a variable X-ray polarization when observed during the HB and NB
transition of the source (Cocchi et al., 2023). During the HB, the source
showed a PD of $4.6\pm 0.4$%, while during the NB, the PD dropped to $\sim
0.6$%. Sco X-1 was observed with IXPE in NB and FB, and the emission from the
source has a lower polarization (La Monaca et al., 2024) as compared to NB
polarization in either Cyg X-2 or GX 5$-$1 indicating that as we go along the
Z-track the X-ray polarization declines. Cir X-1 (which also shows
intermittent Z-source-like behavior) indicated a variation in the PA as a
function of the X-ray hardness, indicating a clear interplay of spectral
components (Rankin et al., 2024). Additionally, spectro-polarimetric
simulations for weakly magnetized sources predict a typical X-ray polarization
of $\approx$4% with the energy dependence of PD/PA breaking the degeneracies
in the spectral decomposition of such sources (Gnarini et al., 2022).

GX 340$+$0 is one of the bright, persistent sources which demonstrate a
complete trace of the Z-track (Mitsuda et al., 1989; Jonker et al., 1998,
2000; Gilfanov et al., 2003; Lavagetto et al., 2004; Iaria et al., 2006;
Bałucińska-Church et al., 2010; Seifina et al., 2013) The source is known to
traverse through all of the branches in a relatively short (a few days) period
(Jonker et al., 2000). The source is obscured in optical wavelengths but can
be associated with a possible infra-red counterpart (Miller et al., 1993). The
radio emission from the source has been detected (Penninx et al., 1993) and
was found to be strongly correlated to the X-ray flux (Oosterbroek et al.,
1994; Berendsen et al., 2000). The binary inclination of the source is unknown
due to a lack of optical measurements, but since the source doesn’t exhibit
any dips (in contrast with Cyg X-2), the inclination is expected to be
$\lesssim 70\arcdeg$ (Frank et al., 1987). The reflection modeling of the Fe
K$\alpha$ feature indicate that the inclination of the source is $\approx
37\arcdeg$ (D’Aì et al., 2009; Miller et al., 2016). In contrast to the
estimates of inclination from reflection, Seifina et al. (2013) suggest that
the inclination of the system is high (i.e. we view the system close to edge-
on), and the Z-track evolution is caused by the changing size of the
Comptonizing medium.

One of the important unanswered questions about this source is the nature of
the corona and its geometry, whether it covers the disk (Bhargava et al.,
2023) or it is a puffed up optically thin medium close to the NS surface
(Seifina et al., 2013) or it aligns with the radio emission (Oosterbroek et
al., 1994). To understand the geometry of various components of the system, we
conducted the first IXPE observation of GX 340$+$0 supported by an extensive
campaign using various X-ray and radio observatories. In this letter, we
report the results from the X-ray polarimetric, spectral, and timing
observations of the source, as well as supporting radio observations.

##  2 Data analysis & Results

IXPE observed the source during the latter half of March 2024 as detailed in
Table 1. Since GX 340$+$0 is variable on a much shorter timescale, it is
paramount to have simultaneous observations with instruments with spectro-
timing capabilities to identify the exact state and investigate the branch-
resolved polarization. We coordinated a multi-wavelength campaign of the
source to observe the source simultaneously or at least contemporaneously with
IXPE. The details of these observations are noted in Table 1, and a combined
lightcurve of the campaign is shown in Figure 1. To plot in the same scale,
the X-ray lightcurves have been background-subtracted and normalized with the
Crab count rates for each respective instrument, except for the IXPE
lightcurve, which is scaled by a factor of 15 (the reason for scaling the IXPE
observation is described in section A.1) The details of the data reduction for
individual observatories and instruments are mentioned in various subsections
of Appendix A.

Table 1: Observation log for the multi-wavelength campaign of GX 340$+$0 Epoch | Observatory | Instrument | Observation ID | Start Time | Stop Time | Exposure (in ks)
---|---|---|---|---|---|---
0 | NICER | XTI | 7108010101-2 | 2024-03-20 22:14:20 | 2024-03-21 03:01:08 | 2.4
GMRT | – | ddtC332 | 2024-03-20 22:30:00 | 2024-03-21 01:30:00 | 3.54
1 | IXPE | GPD (DU1–3) | 03003301 | 2024-03-23 13:56:24 | 2024-03-25 13:09:44 | 99.0
AstroSat | SXT | 9000006138 | 2024-03-23 12:23:48 | 2024-03-24 00:15:28 | 14.2
LAXPC | 2024-03-23 12:24:50 | 2024-03-24 00:13:10 | 23.1
2 | NuSTAR | FPMA/B | 91002313002 | 2024-03-27 06:24:26 | 2024-03-27 16:53:46 | 12.4
Insight-HXMT | LE | P061437700101- | 2024-03-28 01:33:25 | 2024-03-29 06:04:41 | 6.46
ME | P061437700109 | 2024-03-28 01:32:54 | 2024-03-29 06:53:30 | 13.98
GMRT | – | ddtC332 | 2024-03-27 22:30:00 | 2024-03-28 01:30:00 | 3.54
NuSTAR | FPMA/B | 91002313004 | 2024-03-28 11:15:40 | 2024-03-28 21:45:00 | 12.5
ATCA | – | CX566 | 2024-03-29 11:29:40 | 2024-03-29 13:18:20 | 6.0
2+ | NuSTAR | FPMA/B | 91002313006 | 2024-03-31 04:57:17 | 2024-03-31 16:13:33 | 11.4
NuSTAR | FPMA/B | 91002313008 | 2024-04-01 13:00:41 | 2024-04-01 21:54:01 | 10.9
ATCA | – | CX566 | 2024-04-01 12:43:20 | 2024-04-01 20:25:30 | 14.04
Figure 1: Top panel: The orbit-wise Crab-normalized lightcurve from MAXI/GSC
overplotted with lightcurves from various missions (indicated in different
symbols and colors as per the legend) during and around the IXPE observation.
All the lightcurves are background subtracted and Crab-normalized except for
the IXPE lightcurve, which is scaled by a factor of 15. The radio fluxes from
ATCA are indicated using the right axis while GMRT did not detect the source
in both Band 4 (750 MHz) and Band 5 (1260 MHz), and therefore, 3$\sigma$ upper
limits are depicted for both bands. Bottom panel: The IXPE DU1 observation
lightcurve (in 2–8 keV) is overplotted with simultaneous AstroSat-LAXPC 20 (in
3–20 keV) lightcurve. Both lightcurves are binned at 100 s. The AstroSat and
NuSTAR observations indicated that the source was in the HB throughout. Since
the source has to pass through the hard apex in its Z-track to transition to
the NB, which we did not observe in our observations, we conclude that the
source was in the HB throughout the duration of the IXPE observation.
Therefore for the duration of IXPE observation, the source is in HB.

###  2.1 Model-independent X-ray polarimetric analysis

We investigated the polarimetric properties of the source by extracting the
pcube using ixpeobssim in various energy bands from the filtered events after
appropriate weighting (see section A.1 for details). The pcube encapsulates
the normalized Stokes Q and U information as a function of energy. In the full
energy band (2–8 keV), we detected a significantly polarized emission with PD
= $4.02\pm 0.35$% and PA = $37.6\pm 2.5\arcdeg$ (black point in the left panel
of Figure 2). The confidence intervals reported in the text correspond to
$1\sigma$ intervals unless indicated otherwise. In a model-independent
fashion, we probed the energy dependence of the polarization by extracting the
pcube in six logarithmically spaced bins in 2–8 keV (depicted in the left
panel of Figure 2 and noted in Table 2). Most of the energy bands have a
similar PA as the full bands, with the exception of 2–2.5 keV, which has a
slightly lower PA. The PD of different energy bands hint at a slight increase
towards higher energies but are consistent within the $2\sigma$ confidence
level. The larger error on the PA in 2–2.5 keV energy band is due to a lower
modulation fraction of individual detector units (Di Marco et al., 2022).
Similar uncertainties have been seen in previous IXPE observations of other
sources (e.g. Rawat et al., 2023).

Figure 2: Left panel: Polar plot of model-independent polarization of GX
340$+$0 measured in various energy bands (see section 2.1). The marker denotes
the observed PD in radial coordinates and PA in azimuthal coordinates, while
the ellipses show 1$\sigma$ & 2$\sigma$ confidence intervals for each
measurement. The colours correspond to different energy bands. The PA of 2–2.5
keV emission is marginally different from the rest of the energy bands. Right
panel: Polar plot of component-wise polarization as estimated from the
spectro-polarimetric analysis (see section 2.3). We depict the estimated PD/PA
from the accretion disk and Comptonized emission for two cases: a) The
blackbody component has a contribution to the polarization, and b) The
blackbody emission is unpolarized. The contours shown correspond to
1–3$\sigma$ confidence levels Table 2: Polarization properties of GX 340$+$0
Model-independent-PCUBE
---
Energy band (keV) | PD (%) | PA ($\arcdeg$)
2.0–8.0 | $4.02\pm 0.35$ | $37.5\pm 2.5$
2.0–2.5 | $4.3\pm 1.1$ | $9\pm 8$
2.5–3.2 | $3.2\pm 0.6$ | $35\pm 5$
3.2–4.0 | $3.0\pm 0.5$ | $34\pm 5$
4.0–5.0 | $4.4\pm 0.5$ | $38\pm 3$
5.0–6.3 | $4.5\pm 0.7$ | $37\pm 4$
6.3–8.0 | $5.8\pm 1.2$ | $48\pm 6$
Count rate resolved in 2–8 keV
Count rate range | PD (%) | PA ($\arcdeg$)
5.1–6.8 | $3.69\pm 0.53$ | $36\pm 4$
6.8–9.5 | $4.35\pm 0.47$ | $38\pm 3$
Spectro-polarimetric analysis
Component | PD (%) | PA ($\arcdeg$)
Disk | $19\pm 6$ | $-21\pm 10$
Blackbody | $3.0\pm 1.7$ | $-14\pm 18$
Corona | $11\pm 3$ | $53\pm 8$
Assuming blackbody emission is unpolarized
Disk | $20.0\pm 6.5$ | $-21\pm 9$
Corona | $9.0\pm 0.6$ | $39\pm 2$

We investigated the polarization properties of the source as a function of the
count rate by dividing the IXPE observation into two subsets (high and low),
using the median count rate ($\approx 6.8$ counts/s in DU1). The 2–8 keV
polarization for these subsets (PD/PA for high: $4.35\pm 0.47$%/$38\pm
3\arcdeg$ and for low: $3.69\pm 0.53$%$/36\pm 4\arcdeg$) are consistent with
each other within 2$\sigma$ confidence interval (also noted in Table 2).

###  2.2 Identification of the branch

GX 340$+$0 shows a strong evolution of the spectro-timing properties as a
function of the Z-track position. Thus, to compare the properties of the
source across various observations, we constructed the HID of the source in
various energy bands for different X-ray instruments (Figure 3). In all the
cases, the source is observed to follow a track similar to the HB (see
Bhargava et al., 2023, for example). We verify the nature of the branch by
investigating the timing properties of the source, in particular the 3–20 keV
power density spectrum (PDS) in from AstroSat-LAXPC and the 3–30 keV co-
spectrum from NuSTAR-FPMA/B. The details of the HID-resolved timing analysis
are discussed in section B.1. The presence of the quasi-periodic feature in
20–42 Hz (see Table 3) is a clear indication that the source is in HB (Jonker
et al., 2000; Bhargava et al., 2023; Pahari et al., 2024). Additionally, after
the end of the AstroSat observation, the IXPE DU1 count rate never goes beyond
the 9 counts/s. For a transition to the NB the source _has_ to go through the
hard apex (Bhargava et al., 2023). Comparing the Crab-normalized NuSTAR count
rates and scaled IXPE count rates, it is evident that during the IXPE
observation, the source stayed in zone 6 or lower (see the zone divisions in
the HID; Figure 3). Therefore, for the duration of IXPE observation _GX 340
$+$0 stayed in HB_.

Figure 3: X-ray Hardness-intensity diagram from various contemporaneous
observations of GX 340$+$0 highlighting that the source was observed in the
HB. The vertical lines demarcate the zones used to investigate the HID-
resolved spectro-timing properties of the source (see sections B.1 and B.2)

###  2.3 Spectro-polarimetric investigation using AstroSat and IXPE

The marginally different PA from 2–2.5 keV emission suggests that different
spectral components could have different PA (also seen for other Z-sources;
Farinelli et al., 2023; Fabiani et al., 2024). Motivated by this, we conducted
a spectro-polarimetric analysis using the simultaneous AstroSat and IXPE
observations. The last three orbits of the SXT observations indicated an
abnormal excess in count rate, which is not seen in the same bands in LAXPC or
IXPE. Thus, we excluded those orbits from both LAXPC and SXT spectra. For
spectro-polarimetric modeling, we used the $\chi^{2}$ statistic and have
reported $1\sigma$ confidence intervals unless stated otherwise. We use xspec
v12.14.0 to model the spectra. We modeled the SXT+LAXPC spectra with the
spectral decomposition from Bhargava et al. (2023) (which corresponds to a
similar source state). The spectral model, tbabs*(diskbb+bbodyrad+nthcomp),
assumed that the seed photon temperature for Comptonization is tied to the
diskbb temperature and the input spectrum for nthcomp assumed to be disk-like.
Similar to (Bhargava et al., 2023), we note that the inner temperature of the
accretion disk ($\sim 0.2$ keV) is lower than the blackbody component ($\sim
1$ keV). The absorption column is found to be consistent with 1023 cm-2
(Bhargava et al., 2023) with the abundances from Wilms et al. (2000) and
cross-sections from Verner et al. (1996). The authors would like to refer the
readers to Figure 2 of (Bhargava et al., 2023), which depicts the spectral
decomposition of the source assumed for this work. This model is sufficient to
describe the spectra (with a $\chi^{2}$/degrees of freedom or dof of
153.0/109). The parameters of the additive components are found to be
consistent with those in Bhargava et al. (2023). The reduced $\chi^{2}$ here
is slightly higher than Bhargava et al. (2023), possibly due to spectral
information from multiple HID zones being averaged for the spectro-
polarimetric analysis. We also conducted f-tests to determine the necessity of
three additive components to describe the spectra (as prescribed in Bhargava
et al. (2023)) and concluded that for an optimal description of the spectra,
tbabs*(diskbb+bbodyrad+nthcomp) is sufficient. We also include cross-
normalization constants between LAXPC and SXT, which are consistent within 10%
(Bhargava et al., 2019, 2023).

We included the Stokes Q and U spectra for all the detector units to conduct
the spectro-polarimetric tests. We included these spectra by grouping them
with LAXPC spectra. We checked if the observation is consistent with a single
polarization for all the components by fitting the spectra to
tbabs*polconst*(diskbb+bbodyrad+nthcomp). Since we intended to estimate
polarization properties assuming the inherent spectral model as discussed
above, we kept the spectral parameters fixed while estimating the PD/PA. We
found that assuming a constant polarization results in an acceptable fit
($\chi^{2}$/dof=941.3/1009) and a PD/PA of $2.8\pm 0.2$%/$35\pm 2\arcdeg$.
Since the model-independent analysis indicated a possible change in the PA as
a function of energy, we replaced the constant polarization with linear
polarization (pollin) and kept the slope of the PD fixed and the slope of the
PA free. It resulted in a slightly lower $\chi^{2}$/dof = 935/1008. To
determine which model is statistically better, we computed the Akaike
Information Criterion (AIC) and Bayesian Information Criterion (BIC) for both
models. The AIC prefers pollin with positive evidence for polconst
($\Delta$AIC = 4.78) while the BIC suggests that both models are equally good
($\Delta$BIC=0.12, with BIC of polconst lower than pollin). Since either
criterion is not able to distinguish between the models overwhelmingly (i.e.
$\Delta$AIC or $\Delta$BIC $\gtrsim 10$), we prefer the simpler model, i.e.
the polarization is constant with energy.

Other Cyg-like Z-sources have hinted at a possible difference in PA of
additive components (Farinelli et al., 2023; Fabiani et al., 2024). Therefore,
we tested the same hypothesis by applying polconst individually to diskbb,
bbodyrad and nthcomp. We found that there is a $2\sigma$ detection of
polarization from accretion disk (PD = $19\pm 6.5$%, PA = $-22\pm 10\arcdeg$)
and a $>3\sigma$ detection from the Comptonizing medium (PD = $11\pm 3$%, PA =
$53\pm 8\arcdeg$) while the PD of blackbody component ($3.0\pm 1.7$%) is
consistent with zero within $2\sigma$ confidence interval
($\chi^{2}$/dof=923.3/1005). Notably, the PA of the accretion disk component
is significantly different from that of the Comptonizing medium (at a
confidence of 5.7$\sigma$). If we assume that there is no polarization from
the blackbody emission, then we obtain stricter limits on the PD/PA for disk
(PD = $20.0\pm 6.5$%, PA = $-21\pm 9\arcdeg$) and Comptonized emission (PD =
$9.0\pm 0.6$%, $38\pm 2\arcdeg$) for a similar fit
($\chi^{2}$/dof=926.2/1007). The confidence on the disk polarization increased
to $\sim 3\sigma$ level while the Comptonized emission shows a highly
significant polarization ($\sim 15\sigma$), and the PA between them is
different at $6\sigma$ confidence level. We depict the count spectra from
AstroSat (equivalent to Stokes I), Q and U spectra from IXPE with residuals
for the model assuming an unpolarized blackbody emission in the Figure 4.

Figure 4:  Top panels: Spectra from AstroSat and IXPE from the spectro-
polarimetric analysis (section 2.3). The Stokes Q and U spectra were grouped
with LAXPC spectra for spectro-polarimetric modeling. The spectra were jointly
modeled with tbabs*(polconst*diskbb+polconst*bbodyrad+polconst*nthcomp), with
the PD for polconst on bbodyrad fixed at zero. Bottom panels: Residuals
(estimated from $\chi=$ (data$-$model)/error) for all spectra are depicted in
respective panels.

To compare if the polarization from individual components is statistically
different than single polarization from all components, we compute the AIC and
BIC for case a) free blackbody PD and case b) blackbody PD fixed at 0% and
compare it with the AIC and BIC for model
tbabs*polconst*(diskbb+bbodyrad+nthcomp). For case a), the AIC overwhelmingly
prefers individual polarization components ($\Delta$AIC=11.5) over a single
polarization component, while the BIC prefers a single polarization component
over individual components strongly ($\Delta$BIC=8.1). For case b), both AIC
and BIC prefer individual components over the single component but with
varying degrees of acceptance ($\Delta$AIC=12.3 and $\Delta$BIC=2.5).
Additionally, comparing case a) and b) with AIC and BIC indicates that both
criteria prefer case b) with different degrees of acceptance ($\Delta$AIC=0.8
and $\Delta$BIC=10.6). Based on the information criteria we claim that there
is reasonable evidence for individual components having different polarization
properties and the blackbody emission being unpolarized.

###  2.4 Modeling the Iron-line

For each of the zone-resolved NuSTAR spectra defined in Figure 3, we employed
the same phenomenological model described above, i.e., with
tbabs*(diskbb+bbodyrad+nthcomp) (the details of the continuum modeling are
described in section B.2). This allows for a fair comparison with the AstroSat
and IXPE results above. However, we found that there was a $\sim
6.4{\rm\,keV}$ residual asymmetric feature in all of the six zone-resolved
spectra, which we interpret as a relativistic Fe emission line. Previous
observations with XMM-Newton and Chandra/HETGS have also revealed the presence
of the relativistic Fe K emission line (D’Aì et al., 2009; Cackett et al.,
2010; Miller et al., 2016). To fit for the emission line, we performed
preliminary fits and tried a simple Gaussian, a diskline model, a relline
model, and replaced the nthcomp component with a relxillCp to account for the
Comptonized component and Fe line feature (Fabian et al., 1989; Zdziarski et
al., 1996; Życki et al., 1999; Dauser et al., 2010). We found that the relline
model is the best description for the asymmetric line, as determined by the
$\Delta\chi^{2}$ between the inclusion and exclusion of the model (Dauser et
al., 2010). The high-resolution X-ray observations with Chandra/HETGS
constrained the disk inclination to be $i=35\arcdeg$, so in what follows, we
fix the disk inclination parameter in the relline model to $i=35\arcdeg$
(Miller et al., 2016).

Across the six zone-resolved NuSTAR spectra, we found that the rest-frame
centroid energy did not significantly shift as a function of the HID zone. We
found energies between 6.36 keV to 6.58 keV, consistent within $2\sigma$.
However, the line normalization decreased (though not monotonic) from $(5.9\pm
0.6)\times 10^{-3}{\rm\,photons\,cm^{-2}\,s^{-1}}$ in zone 2 to
$(1.8_{-1.2}^{+1.3})\times 10^{-3}{\rm\,photons\,cm^{-2}\,s^{-1}}$ in zone 7.

##  3 Discussion

During the campaign, the source stayed primarily in the HB. The HID from
various X-ray instruments, the timing analysis of AstroSat and NuSTAR
observations (which showed QPOs at frequencies typical of HB oscillations) and
the spectral properties of the source provide ample evidence for the state of
the source. The HB oscillation evolved across the HB in a typical manner,
i.e., going down the HB, from $\sim$21.2–23.4 Hz to $\sim$43.2 Hz while
fractional rms amplitude of the QPO decreased. Similar timing evolution have
been reported using various fast-timing instruments earlier (Jonker et al.,
2000, 2002; Bhargava et al., 2023; Pahari et al., 2024). The hard apex is
expected to show a frequency of $\sim 45$ Hz (Jonker et al., 2000) and all
QPOs we observe are lower in frequency, which indicates during AstroSat,
NuSTAR, and by extension IXPE epochs, the source was in the HB. The broadband
spectra also seem to reiterate previous findings, i.e., the detection of three
spectral components varying along the Z-track (Iaria et al., 2006; Seifina et
al., 2013; Bhargava et al., 2023). GX 340$+$0 staying in one state for a
duration of $\sim$ 13 days is quite unusual as the source has traced the
complete HID over a time scale of few days (Jonker et al., 2000) or flit
between HB and NB in the same time scale (Bhargava et al., 2023). It is
possible that during the intervals where we did not have pointed X-ray
observations, it may have visited NB/FB, but the MAXI/GSC monitoring of the
source is unable to discern such visits.

###  3.1 Discovery & Nature of X-ray Polarization in GX 340+0

The first IXPE observation of GX 340$+$0 revealed a significant X-ray
polarization in 2–8 keV. The PD of source in HB is $4.02\pm 0.35$% in the HB,
and a similar PD was observed in GX 5$-$1 (Fabiani et al., 2024) and XTE
J1701$-$462 (Cocchi et al., 2023). The PA for polarized emission in the full
band is measured to be $37.6\pm 2.5\arcdeg$.

The energy-resolved polarization study indicates that the lowest energy bin
(i.e. 2–2.5 keV) has a different PA as compared to the PA of the rest of the
emission (albeit a marginal difference, see Figure 2 left panel). Previous
spectral studies of the source revealed that in softer X-rays (typically below
3 keV), the emission has three active components which could contribute to the
polarization in this band (e.g. Lavagetto et al., 2004; Iaria et al., 2006;
Lin et al., 2007, 2009, 2012; Seifina et al., 2013; Bhargava et al., 2023). In
the higher energy bands, two of these components dominate mainly. The model-
independent observation of polarization indicates that in the higher bands
($\gtrsim 2.5$ keV) all the emission aligns in PA. These components have been
modeled as a combination of accretion disk+Comptonized emission (for Cyg X-2;
Done et al. 2002), blackbody+Comptonized emission (Church et al., 2006;
Bhargava et al., 2023), or a combination of two Comptonized emissions of
different electron temperatures (Seifina et al., 2013). Since we do not
observe a significant variation of PA with energy in 2.5–8 keV, we can infer
that either both components have an aligned emission or one component has a
stronger contribution to the polarization, which dictates the observed
polarization properties. In the 2–2.5 keV band, due to the presence of a third
component (i.e. accretion disk), the net PA of the emission changes
marginally. Simulations by Gnarini et al. (2022) for a slab-like corona
predicts a change in the observed PA similar to what is observed in GX 340$+$0
(i.e. lower PA in softer X-rays) for various inclination angles.

For a spectro-polarimetric analysis, we have assumed that the emission
comprises of accretion disk, blackbody and a corona, which is fed by photons
from the accretion disk. Assuming constant polarization from individual
components, we estimate that the disk emission has a relatively high degree of
polarization albeit with low detection significance (Table 2). The simulations
of the accretion disk predict that due to the light bending effect in the
strong gravity regime, the inner emission of the accretion disk is expected to
have a polarization of a few per cent at the high inclination of the accretion
disk (Schnittman & Krolik, 2009). The PA of the disk emission in GX 340$+$0 is
also observed to deviate by 59–75$\arcdeg$ with respect to the Comptonized
emission. Similar differences in the PA of disk and Comptonized emission are
also seen in other Z-sources (Cocchi et al., 2023; Fabiani et al., 2024),
which could indicate that it might be an intrinsic property of the HB of the
Cyg-like Z-sources. We note that the present observation seems to indicate
that the blackbody emission is consistent with zero PD, and if the PD from the
blackbody component is assumed to be zero, then we obtain strong constraints
on the PD from the coronal emission, and the disk emission is estimated to
have a polarization at $>3\sigma$ confidence. This would favor an
interpretation of the blackbody component as a symmetrical emission from the
surface of the NS, perhaps a hotspot, while the comptonised emission is
covering the accretion disk in a slab like geometry (see figure 1 of Gnarini
et al., 2022, for a schematic).

###  3.2 Radio emission from GX 340$+$0

Our IXPE observations of GX 340$+$0 were accompanied by a supporting quasi-
simultaneous radio campaign with the upgraded Giant Metrewave Radio Telescope
(GMRT) and the Australia Telescope Compact Array (ATCA). See Appendix A for
complete details on the observations, data reduction, and analysis. The GMRT
observations were carried out on two epochs, on 2024-03-20/21 and
2024-03-27/28 at central frequencies of 750 MHz and 1.26 GHz (see Table 1). On
both dates, in both frequency bands, no radio counterpart to GX 340$+$0 was
detected by the GMRT.

ATCA observations occurred on 2024-03-29 and 2024-04-01, where data were taken
at 5.5 GHz and 9 GHz (under project code CX566). A bright radio source was
detected at both frequencies on both dates with flux densities of $720\pm 60$
$\mu$Jy at 5.5 GHz and $385\pm 50$ $\mu$Jy at 9 GHz on 2024-03-29, and $720\pm
30$ $\mu$Jy at 5.5 GHz and $595\pm 30$ $\mu$Jy at 9 GHz on 2024-04-01. These
fluxes are brighter than limits placed at similar frequencies previously
(Berendsen et al., 2000). On 2024-04-01, we were also able to place
constraints on the linear and circular polarization (LP and CP, respectively)
of the source. While no LP or CP was detected, stacking the two frequencies
together gives 3$\sigma$ upper limits of $<$6% on the LP and $<$4% on the CP
(centred at 7250 MHz). Such constraints are consistent with expectations for
the radio emission from an accreting X-ray binary (e.g., Fender, 2006).

The non-detection of GX 340$+$0 in the GMRT observations but detection by the
ATCA observations suggest a break in the radio spectrum, lying between the
1.26 GHz GMRT band and the 5.5 GHz ATCA band. Such a break can be a result of
either synchrotron self-absorption or free-free absorption (e.g., Gregory &
Seaquist, 1974; Seaquist, 1976; Miller-Jones et al., 2004; Chauhan et al.,
2021). However, due to the non detection in the lower frequency bands ($<$2000
MHz) we are unable to determine which mechanism is responsible for the
spectral turnover (e.g., Rybicki & Lightman, 1986). We also note that the
radio source would be expected to vary as the accretion properties evolve on
relatively short timescales, as seen in the evolution of radio flux at 9 GHz
across two ATCA observations. As the GMRT and ATCA observations are not
exactly simultaneous, it is, therefore, possible that the source state has
changed between the observations. However, during both GMRT observations, we
have simultaneous X-ray coverage (NICER during the first and Insight-HXMT
during the second, see Figure 1) and the X-ray HID in Figure 3 clearly
indicates during both these epochs the source was on the HB. The second ATCA
observation is strictly simultaneous with NuSTAR and thus determines that the
source was on the HB as well. Previous investigations in radio wavelengths
have suggested that radio emission is also highly variable (and correlated
with X-ray variability). A non-detection (with upper-limits of 200 $\mu$Jy)
with ATCA (Berendsen et al., 2000) and with current non-detection in 0.7–1.5
GHz with GMRT suggests that the radio emission is variable between states (as
shown by historical Very Large Array detections of 1.0–1.5 $\mu$Jy at 1.4 GHz
on the NB Oosterbroek et al. 1994 and current detection by ATCA in 5.5–9 GHz
on the HB). Hence, simultaneous observations of GX 340$+$0 are paramount to
determine the behavior of the source.

##  4 Conclusion

Here, we summarize the key results from our X-ray and radio investigation of
GX 340$+$0 primarily with IXPE to investigate the polarization properties.

  * •

The source stayed in the horizontal branch throughout the epochs of
observations, which spanned roughly 13 days.

  * •

For the first time, we detect X-ray polarization in GX 340$+$0 in 2–8 keV with
the PD = $4.02\pm 0.35$% and PA = $37.6\pm 2.5\arcdeg$. We also detect
significant polarization in various energy bands, with the 2–2.5 keV
polarization indicating a different PA (at a $2.78\sigma$ confidence) as
compared to the PA of other energy bands (which all have similar PA).

  * •

The spectral parameters are typically consistent with previously reported
works, with broadband spectra requiring 3 components (accretion disk,
blackbody, and Comptonised emission) to describe the spectra. The accretion
disk is found to have a low temperature ($\sim$0.2 keV) and is assumed to be
the source of the seed photons for Comptonization. The blackbody component is
roughly at $\sim$1 keV. Additionally, NuSTAR spectra show an asymmetric iron
line feature consistent with previous reports of reflection features which
varies as the source move along the HB.

  * •

Due to almost all energy bands depicting a similar PA, a constant polarization
(i.e. no dependence of PD/PA on energy) is able to fit the spectro-
polarimetric data. If we assume that the blackbody component is not polarized,
then disk polarization is greater than 3$\sigma$ while Comptonised emission
has a highly significant detection ($\sim 15\sigma$).

  * •

In radio wavelengths, the source is not detected at 700-1500 MHz (3$\sigma$
upper limit at 0.9 mJy) and is significantly detected in higher frequencies
(5.5-9 GHz). This indicates that there is a spectral break between 1.5-5.5
GHz.

This research used data products provided by the IXPE Team (MSFC, SSDC, INAF,
and INFN). This work makes use of data from the AstroSat mission of the Indian
Space Research Organisation (ISRO), archived at the Indian Space Science Data
Centre (ISSDC). The article has used data from the SXT and the LAXPC developed
at TIFR, Mumbai, and the AstroSat POCs at TIFR are thanked for verifying and
releasing the data via the ISSDC data archive and providing the necessary
software tools. This work has made use of data from the NuSTAR mission, a
project led by the California Institute of Technology, managed by the Jet
Propulsion Laboratory, and funded by the National Aeronautics and Space
Administration. We thank the NuSTAR Operations, Software and Calibration teams
for support with the execution and analysis of these observations. This
research has made use of the NuSTAR Data Analysis Software (NUSTARDAS) jointly
developed by the ASI Science Data Center (ASDC, Italy) and the California
Institute of Technology (USA). This work was supported by NASA through the
NICER mission and the Astrophysics Explorers Program. This work has made use
of the data from the Insight-HXMT mission, a project funded by the China
National Space Administration (CNSA) and the Chinese Academy of Sciences
(CAS). This research has also made use of data and/or software provided by the
High Energy Astrophysics Science Archive Research Center (HEASARC), which is a
service of the Astrophysics Science Division at NASA/GSFC and the High Energy
Astrophysics Division of the Smithsonian Astrophysical Observatory. We thank
the staff of the GMRT that made these observations possible. GMRT is run by
the National Centre for Radio Astrophysics of the Tata Institute of
Fundamental Research. The Australia Telescope Compact Array is part of the
Australia Telescope National Facility (https://ror.org/05qajvd42), which is
funded by the Australian Government for operation as a National Facility
managed by CSIRO. We acknowledge the Gomeroi people as the Traditional Owners
of the ATCA observatory site. We thank Jamie Stevens and the ATCA staff for
making the observations possible. YB and AB would like to especially thank Dr
Shriharsh Tendulkar for valuable discussion on radio observations and insights
into the emission from the neutron stars. LZ acknowledges support from the
National Natural Science Foundation of China (NSFC) under grant 12203052.

##  Appendix A Data Reduction

###  A.1 IXPE

IXPE (Weisskopf et al., 2022) observed GX 340$+$0 for an exposure of $\approx
99$ ks from 2024-03-23 13:56:24 to 2024-03-25 13:09:44 using all three gas
pixel detector units (referred as DU1, DU2 and DU3). Due to a spacecraft
anomaly, some of the engineering data could not be recovered, and the data was
estimated by extrapolating the rest of the data. Hence, for the observation, a
special processing was conducted by the instrument team, and thus, level 2
files as provided by the instrument team were utilized for the analysis.

We use ixpeobssim software (v31.0.1) to process the observations and extract
the polarization properties of the source (Kislat et al., 2015; Baldini et
al., 2022). We select the source events from a circular region of radius
1.6$\arcmin$ and the background region from an annular region of inner radius
of 2.5$\arcmin$ and outer radius of 5$\arcmin$. We bin the selected events
using different algorithms viz. pcube, phaq, phau to generate polarization
cubes, Q spectra, and U spectra, respectively, for each DU. The calibration
files version v13 obssim20240101_alpha075 were used to extract appropriately
weighted data products. For the model-independent analysis, the polarization
information from the detectors was combined and depicted in Figure 2.

To plot the IXPE lightcurve in the same scale as all instruments in Figure 1,
we scale the IXPE count rates by a factor of 15. GX 340$+$0 appears as a point
source in the IXPE image, while the Crab observation shows a structure of a
pulsar wind Nebula. This causes a strong dependence of the count rate on the
region size, and using a region of 1.6$\arcmin$ in Crab yielded a
significantly higher count rate, resulting in a lower Crab-normalized count
rate for IXPE. Since the purpose of the figure is to show the simultaneous
evolution of the source, we scale the light curve by a factor of 15. We depict
the actual count rate of the source in the bottom panel of Figure 1.

###  A.2 NICER

NICER (Gendreau et al., 2016) observed GX 340$+$0 simultaneously with the
first epoch of GMRT observation (i.e. 2024 March 20-21) to characterize the
source state. The source was observed for 4 segments of $\sim 700$ s each per
International Space Station orbit ($\sim 90$ min). The data was processed
using the standard filtering criteria: a) excluding detectors 14 and 34, b)
excluding South Atlantic Anomaly passages, c) Elevation angle for the bright
Earth limb $>30\arcdeg$, and d) Undershoot and Overshoot rates of 0–500 and
0–30, respectively, with the tool nicerl2 distributed as part of nicerdas
2024-02-09_V012 (heasoft v6.33). The light curves and spectra were extracted
using the nicerl3-lc and nicerl3-spect with the background model scorpeon
(Markwardt et al., 2024) which also allows for estimation of the background
count rate in various energy bands. For normalizing the light curve with Crab,
a similar analysis was conducted on a Crab observation from December 2023 21
(observation id: 6013010117).

###  A.3 AstroSat

Coordinating with the epoch of IXPE observation, AstroSat (Singh et al., 2014)
observed the source with Large Area X-ray Proportional Counter (LAXPC; Yadav
et al., 2016, 2017) as a primary instrument and Soft X-ray telescope (SXT;
Singh et al., 2016, 2017) simultaneously. The AstroSat observation covered a
total duration of 45 ks with a net exposure of 23.1 ks in LAXPC and 14.2 ks in
SXT. The details of data reduction procedures for each instrument are
summarized in the following sub-sections.

####  A.3.1 LAXPC

The orbit-wise level 1 data was downloaded from
AstroBrowse111https://astrobrowse.issdc.gov.in/astro_archive/archive/Home.jsp
and the data was processed using the LAXPCsoftware22Aug15222http://astrosat-
ssc.iucaa.in/uploads/laxpc/LAXPCsoftware22Aug15.zip (Antia et al., 2021; Misra
et al., 2021). LAXPCsoftware22Aug15 includes the relevant calibration files,
responses and software to generate higher-level data products, including the
merged event file for the full observation (laxpc_make_event). The intervals
corresponding to Earth occultation and South Atlantic anomaly passages were
excluded using the routine laxpc_make_stdgti. Using the built-in routines, the
energy-dependent light curves, spectra, associated responses, background
spectrum, and background light curves were extracted. For the analysis, we
consider the data from the top layer (layer 1) due to its minimal background
in 3–20 keV thus limiting the spectral analysis to the same energy range. For
the analysis, the data from LAXPC 20 was considered as the other two units
were not suitable for spectral analysis (LAXPC 10 has shown abnormal gain, and
LAXPC 30 suffered a gas leak early in the mission). For the spectral analysis
of the data, we assume a 3% systematic error (Bhargava et al., 2019, 2023).

####  A.3.2 SXT

The orbit-wise level 2 processed data was procured from
AstroBrowse333https://astrobrowse.issdc.gov.in/astro_archive/archive/Home.jsp
which has been processed with the latest version of the SXT pipeline
(AS1SXTLevel2-1.5). The individual orbits were merged using the Julia tool
SXTMerger as suggested by the SXT Payload Operation Centre (POC). The standard
products (i.e. spectra and lightcurves) were extracted using xselect (v2.5b
from heasoft v6.32.1) for a circular region of 14$\arcmin$. For the extraction
of the spectral products, only the grade 0 events were selected, and the
appropriate response matrix was obtained from the POC. The standard ancillary
response file was modified for the selected region using the sxtARFModule tool
provided by the POC. For the spectral analysis, we assume a 1.5% systematic
error (Sridhar et al., 2019; Bhargava et al., 2023).

###  A.4 NuSTAR

NuSTAR (Harrison et al., 2013) carried out four Target of Opportunity (ToO)
observations of GX 340$+$0 on 2024 March 27, 28, 31, and April 1, for a total
of 47.3 ks of filtered exposure (details of the observations are mentioned in
Table 1). The data were processed using the standard NuSTAR Data Analysis
Software (NuSTARDAS) version 2.1.2 available under HEASoft 6.33, and with the
CALDB version 20240405. We extracted the source events with a circular region
of 120$\arcsec$ in radius with the centroid coordinates RA = 16h 45m 47.7s,
DEC = -45$\arcdeg$ 36$\arcmin$ 40$\arcsec$.0. We also defined a background
region in a source-free region with a circular region of radius 120$\arcsec$
with the centroid coordinates RA = 16h 46m 20.1184s, DEC = -45$\arcdeg$
29$\arcmin$ 33$\arcsec$.231. We ran the nupipeline task to process the data
and nuproducts to generate light curves and spectra. The light curves were
binned with 128 s bins, and the spectra were regrouped to have a minimum of 30
counts per spectral bin. To facilitate comparison between the different
instruments, we generated Crab-normalized light curves in different energy
bands. We utilized the observation from 2024 March 18 (ObsID 11002303004,
exposure=4.983 ks) and extracted the source region with centroid coordinates
RA = 5h 34m 30.9s, DEC = +22$\arcdeg$ 00$\arcmin$ 53$\arcsec$.0 with a
200$\arcsec$ radius, and a background region with centroid coordinates RA = 5h
34m 21.1461s, DEC = +21$\arcdeg$ 52$\arcmin$ 55$\arcsec$.149, with a
150$\arcsec$ radius.

###  A.5 Insight-HXMT

Insight-HXMT (Zhang et al., 2020) observed GX 340$+$0 from 2024-03-28 01:21:40
to 2024-03-29 07-28:35. The data are extracted from all three instruments
using the Insight-HXMT Data Analysis software (HXMTDAS) v2.06444The data
analysis software is available from http://hxmten.ihep.ac.cn/software.jhtml.,
and filtered with the following standard criteria: (1) pointing offset angle
less than $0.04^{\circ}$; (2) Earth elevation angle larger than $10^{\circ}$;
(3) the value of the geomagnetic cutoff rigidity larger than 8 GV;(4) at least
300 s before and after the South Atlantic Anomaly passage. To avoid possible
contamination from the bright Earth and nearby sources, we only use data from
the small field of view (FoV) detectors (Chen et al., 2018; Yang et al.,
2022). The HE data are not used in our spectral analysis as the HE spectra are
dominated by the background. The energy bands adopted for spectral analysis
are 2-10 keV for LE and 10-20 keV for ME.

###  A.6 GMRT

We observed GX 340$+$0 with the wideband receiver backend of GMRT in two
frequency bands - in the frequency band 4 (central frequency 750 MHz,
bandwidth 400 MHz) and band 5 (central frequency 1260 MHz, bandwidth 400 MHz)
spread over two epochs on 2024 March 20-21 and 2024 March 27-28 (Proposal
ddtC332). The raw data were downloaded in the FITS format and converted to the
CASA (CASA Team et al., 2022) measurement set format. Then the data was
calibrated (3C286 was used for flux calibration, and J1717-398 was used for
phase calibration) and imaged using the automated continuum imaging pipeline
CASA-CAPTURE (Kale & Ishwara-Chandra, 2021). Eight rounds of self-calibration
were done within each pipeline run. No sources were detected at the position
of GX 340$+$0 in both frequency bands and at both epochs. So, we place
3$\sigma$ upper limits of 900 $\mu$Jy in band 4 ($\approx$750 MHz) and 300
$\mu$Jy in band 5 ($\approx$1.3 GHz) by computing the RMS flux density in a
large source-less region in the final image.

###  A.7 ATCA

GX 340$+$0 was observed by the Australia Telescope Compact Array (ATCA) on
2024-03-29 and 2024-04-01 under project code CX566. Observations on 2024-03-29
ran between 11:29:40 UT and 13:18:20 UT, while on 2024-04-01, GX 340$+$0 was
observed between 12:43:20 UT and 20:25:30 UT. Data were recorded at central
frequencies of 5.5 GHz and 9 GHz, with 2 GHz of bandwidth at each frequency.
For both epochs, we used PKS B1934$-$638 for primary bandpass and flux
calibration, and J1631$-$4345 for gain calibration. Data were calibrated and
imaged, following standard procedures with the Common Astronomy Software
Application (CASA version 5.1.2; CASA Team et al., 2022). Imaging used a
Briggs robust parameter of zero, balancing sensitivity and angular resolution,
as well as suppressing sidelobes from bright sources in the field. GX 340$+$0
was detected on both epochs. Fitting for a point source in the image plane, we
measure flux densities of $720\pm 60$ $\mu$Jy at 5.5 GHz and $385\pm 50$
$\mu$Jy at 9 GHz on 2024-03-29 and $720\pm 30$ $\mu$Jy at 5.5 GHz and $595\pm
30$ $\mu$Jy at 9 GHz on 2024-04-01. Our ATCA observation taken on 2024-04-01
had sufficient hour angle coverage that we can measure the radio polarization.
To do so, we used the unpolarised PKS B1934$-$638 to solve for antenna
leakages and the CASA task qufromgains to model the polarization angle of the
gain calibrator. No linear (or circular) polarization was detected at either
frequency. Stacking the 5.5 GHz and 9 GHz data together to provide our deepest
limits gives 3$\sigma$ upper limits of $<$6% on linear polarization and $<$4%
on the circular polarization (at 7.25 GHz).

##  Appendix B HID-resolved analysis

To verify the branch, we divide all instrument HID into multiple zones based
on the Crab-normalized count rates (shown as vertical lines in Figure 3).
Various observations sample a subset of these zones, e.g. NICER samples zones
0–4 while NuSTAR samples zones 2–7. Based on the HID division, we determined
the time intervals corresponding to each zone and used them to extract the
HID-resolved products using the techniques mentioned in section A.

###  B.1 HID-resolved timing analysis

From AstroSat-LAXPC observation, we have extracted rms-normalized and Poisson-
noise subtracted power density spectrum (PDS) in the 3–30 keV energy range
after removing data gaps and count drops due to telemetry losses. Each PDS is
fitted separately using a combination of power-law and multiple Lorentzians in
the frequency range of 0.05–100 Hz. From the best-fit model components, total
fractional rms, QPO frequencies, and their fractional rms are calculated.
Fitted parameter values for PDS corresponding to different zones are provided
in Table 3.

To ensure consistency and to corroborate the branch-resolved spectroscopic
results from NuSTAR, we analyzed the 3–30 keV NuSTAR cospectra for each of the
zones (Huppenkothen & Bachetti, 2018). The cospectrum is the real part of the
cross-spectrum, and these were generated using 32 s segments with Stingray
utilizing data from the two independent detectors (FPMA and FPMB); in this
case, uncorrelated variability such as dead time effects, which can be
significant for NuSTAR, is eliminated (Huppenkothen & Bachetti, 2018;
Huppenkothen et al., 2019a, b). The cospectra were well described by the
combination of a zero-centred Lorentzian (i.e., centroid frequency of $0$ Hz)
to describe the broadband noise and a Lorentzian to describe the QPO. In the
case of Zone 6, where QPO parameters were unconstrained, perhaps due to low
statistics. We report the timing parameters from the cospectra in Table 3.

Table 3: Power density spectral properties of five HID zones (see Figure 3). Observed QPOs, their fractional rms, and the total PDS rms in the frequency range of 0.05–100 Hz are provided. Zone | Observed QPO Frequency (Hz) | Observed QPO rms (%)
---|---|---
Number | AstroSat | NuSTAR | AstroSat | NuSTAR
Zone 2 | $23.37^{+0.18}_{-0.17}$ | $21.2\pm 0.2$ | $8.45^{+0.46}_{-0.56}$ | $6.8\pm 0.2$
Zone 3 | $26.35^{+0.23}_{-0.24}$ | $27.9\pm 0.3$ | $7.39^{+0.29}_{-0.30}$ | $5.5\pm 0.3$
Zone 4 | $38.14^{+1.12}_{-1.26}$ | $32.8\pm 0.5$ | $3.27^{+1.14}_{-0.83}$ | $4.2\pm 0.3$
| $29.15^{+0.87}_{-0.76}$ |  $\cdots$ | $4.96^{+0.87}_{-0.73}$ |  $\cdots$
Zone 5 | $36.42^{+0.87}_{-0.76}$ | $38.3\pm 0.6$ | $3.73^{+0.38}_{-0.46}$ | $3.5\pm 0.3$
Zone 6 | $43.23^{+2.15}_{-2.11}$ |  $\cdots$ | $3.37^{+1.35}_{-1.14}$ |  $\cdots$

###  B.2 HID-resolved spectral modeling

In section 2.4, we described the modeling of the Fe K emission line. Here, we
describe briefly the modeling of the continuum, where we fit with
tbabs*(diskbb+bbodyrad+nthcomp) as with the AstroSat and IXPE joint fit (and
relline for the Fe K emission line). We jointly fit the FPMA and FPMB spectra
with the same model and included a cross-calibration constant. The cross-
calibration constant was around 0.98 for all six zone-resolved spectra. Given
the higher energy coverage of NuSTAR, we fixed the inner disk temperature
($T_{\rm in}$) at 0.28 keV. We also set the temperature of the seed photons
for the Comptonization component to be the inner disk temperature and assumed
that the seed photons were provided by the accretion disk. We found a higher
disk blackbody normalization value of ($1.0$–$1.6)\times 10^{6}(R_{\rm
in}/D_{10})^{2}\text{cos}\,\theta$, where $R_{\rm in}$ is the apparent inner
disk radius in km, $D_{\rm 10}$ is the source distance in units of 10 kpc, and
$\theta$ is the disk inclination. We also found a higher blackbody temperature
of $kT=1.4$–$1.6{\rm\,keV}$ as opposed to values around $kT=1.1{\rm\,keV}$
(Bhargava et al., 2023). Finally, while we also found an electron temperature
of around $kT_{\rm e}\approx 3.0$–$3.2{\rm\,keV}$, we found a higher power-law
photon index of $\Gamma\approx 2.0$ instead of $\Gamma\approx 1.5$–$1.7$. In
all individual spectral fits for the six zones, the full model reasonably
describes the spectra well, with $\chi^{2}/{\rm d.o.f.}=1.08$–$1.26$.

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