Journal Article

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

Ambrifi, A, Covino, S, Vincentelli, F. M, Baglio, M. C, Shaw, A. W, Fabiani, S, Alabarta, K, Saikia, P, Wang, J, Mariani, I, Kara, E, Miceli, C, Cavecchi, Y, Casella, P, Lodato, G, Carotenuto, F, Campana, S, Lai, E. V, De Marco, B, Capitanio, F, Motta, S, Del Santo, M, Dovčiak, M, Russell, T. D, Illiano, G, Mastroserio, G, Russell, D. M, Svoboda, J, Marino, A, Manca, A

2025

Overview

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.

• Publisher: IOP Publishing
• Subject: Accretion disks / Astronomy / Astrophysics / Black holes / Ejecta / Knots / Optical polarization / Polarization / Radiation / Radio jets (astronomy) / Radio observation / Soft x rays / Stars & galaxies / Very Large Telescope / X-rays