X-Ray and Optical Polarization Aligned with the Radio Jet Ejecta in GX 339–4

Journal article

We present the first X-ray polarization measurements of GX 339–4. IXPE observed this source twice during its 2023–2024 outburst, once in the soft-intermediate state and again during a soft state. The observation taken during the intermediate state shows a significant (4σ) polarization degree PX = 1.3% ± 0.3% and polarization angle θX = −74° ± 7° only in the 3–8 keV band. FORS2 at the Very Large Telescope observed the source simultaneously, detecting optical polarization in the B, V, R, and I bands (between ∼0.1% and ∼0.7%), all roughly aligned with the X-ray polarization. We also detect a discrete jet knot from radio observations with the Australia Telescope Compact Array taken later in time; this knot would have been ejected from the system around the same time as the hard-to-soft X-ray state transition, and a bright radio flare occurred ∼3 months earlier. The proper motion of the jet knot provides a direct measurement of the jet orientation angle on the plane of the sky at the time of the ejection. We find that both the X-ray and optical polarization angles are aligned with the direction of the ballistic jet.

Authors: Mastroserio, G; De Marco, B; Baglio, MC; Carotenuto, F; Fabiani, S

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal Letters
• Volume: 978
• Issue: 2
• Article number: L19
• Publication date: 2025-01-03
• Acceptance date: 2024-11-28
• DOI: 10.3847/2041-8213/ad9913
• EISSN: 2041-8213
• ISSN: 2041-8205
• Language: English
• Keywords: X-ray astronomy; Radio astronomy; Low-mass x-ray binary stars; Accretion; Astrophysical black holes