Kavli Affiliate: Herman L. Marshall
| First 5 Authors: Yash Bhargava, Thomas D. Russell, Mason Ng, Arvind Balasubramanian, Liang Zhang
| Summary:
We present the first X-ray polarization measurement of the neutron star
low-mass X-ray binary and Z-source, GX 340$+$0, in the normal branch (NB) using
a 200 ks observation with the Imaging X-ray Polarimetric Explorer (IXPE). This
observation was performed in 2024 August. Along with IXPE, we also conducted
simultaneous observations with NICER, AstroSat, Insight-HXMT, ATCA, and GMRT to
investigate the broadband spectral and timing properties in the X-ray and radio
wavelengths. During the campaign, the source traced a complete Z-track during
the IXPE observation but spent most of the time in the NB. We measure X-ray
polarization degree (PD) of $1.22pm0.25%$ in the 2-8 keV energy band with a
polarization angle (PA) of $38pm6^circ$. The PD in the NB is observed to be
weaker than in the horizontal branch (HB) but aligned in the same direction.
The PD of the source exhibits a marginal increase with energy while the PA
shows no energy dependence. The joint spectro-polarimetric modeling is
consistent with the observed X-ray polarization originating from a single
spectral component from the blackbody, the Comptonized emission, or reflection
feature, while the disk emission does not contribute towards the X-ray
polarization. GMRT observations at 1.26 GHz during HB had a tentative detection
at 4.5$pm$0.7 mJy while ATCA observations a day later during the NB detected
the source at 0.70$pm$0.05 mJy and 0.59$pm$0.05 mJy in the 5.5 & 9 GHz bands,
respectively, suggesting an evolving jet structure depending on the Z-track
position.
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