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Uncovering the innermost jet in the hidden ultra-luminous X-ray source Cygnus X-3

Jun Yang


As a Galactic high-mass X-ray binary, Cyg X-3 hosts a compact object accreting matter from a Wolf-Rayet star. X-ray polarization observations show that Cyg X-3 is most likely a hidden ultra-luminous X-ray (ULX) source with a luminosity above the Eddington limit along the direction of an optically thin X-ray funnel. The latest EVN observations of Cyg X-3 at 22 GHz reveal an elongated jet structure in the quiescent radio state for the first time. More interestingly, the jet position angle is fully consistent with the X-ray tunnel direction, indicating that the radio jet propagates along and within the X-ray funnel. The finding provides an unprecedented view of the complex central engines of ULXs.


Figure 1 Left: Schematic view of the inner structure of the Galactic hidden ULX Cyg X-3. Right: the innermost jet observed with the EVN at 22 GHz. The yellow arrow marks the direction of the radio jet and the optically thin X-ray funnel. The black ellipse shows the best-fitting elliptical Gaussian model and the sizes of the inner cross represent the full width at half maximum  (FWHM). The map beam has an FWHM of 1.60 x0.94 mas at PA = -61.5 degree and is also plotted in the bottom-left corner. The white contours plot the levels of (-1, 1, 4, 16, 64) x 0.24 mJy/beam. The map peak brightness is 17.3 mJy/beam.


Cyg X-3 was discovered more than five decades ago. In the binary system, the compact object is likely a black hole. Recently, Cyg X-3 has been recognized by Veledina et al. (2023) as a hidden Galactic ULX source with the IXPE (Imaging X-ray Polarimetry Explorer) observations. The X-ray emission of Cyg X-3 shows a very high (about 25 %) linear polarization and a relatively low apparent luminosity because of strong Thomson scattering along the line of sight. If there exists an optically thin X-ray funnel (Veledina et al. 2023), its origin would be tightly related to potential innermost jet activity. The proposed inner structure of Cyg X-3 is shown in the left panel.

The innermost jet of Cyg X-3 is shown in the right panel. The EVN map was observed with seven European telescopes. Thanks to 4-Gbps data rate and a very high observing frequency of 22 GHz, the pilot EVN experiment successfully minimized the negative effect of scatter broadening. Based on the simultaneous X-ray observations with the space telescope NICER, Cyg X-3 was found in the quiescent radio state. In the EVN map, the jet direction is consistent with the direction of the optically thin X-ray funnel. This supports that the radio jet propagates roughly along the central axis of the X-ray funnel. Moreover, the jet is marginally resolved in the transverse direction, possibly because of the strong stellar winds and the rapid orbital motion.


Veledina, A. et al., 2023, arXiv e-prints, p. arXiv:2303.01174
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Published in Yang et al. 2023, MNRAS, 526, L1-L7

Jun Yang
Onsala Space Observatory, Sweden