Alignment of radio jets in the microquasar V4641 Sagittarii with its high-energy structures

TL;DR

V4641 Sgr hosts a stellar-mass black hole with relativistic jets and unexpectedly aligned high-energy structures. The authors reanalyzed archival VLA data to test whether the radio jet and the VHE/UHE gamma-ray emissions share a common axis, finding a P.A. near 163 degrees and a near-sky-plane orientation. This alignment supports a co-spatial, in-situ acceleration scenario in a single relativistic outflow, with hadronic processes providing the TeV–PeV emission. The work positions V4641 Sgr as a Galactic PeVatron and offers a benchmark for jet composition and magnetic structure in microquasars.

Abstract

V4641 Sagittarii (V4641 Sgr) is a unique Galactic microquasar system featuring a stellar-mass black hole accreting matter from a massive companion. One of its intriguing features is the presence of relativistic radio jets almost perpendicular to the observed extended gamma-ray emission, implying significant propagation effects or interactions with the Galactic magnetic field. Here we report observational evidence that the radio jet and the very high-energy (VHE) and ultra high-energy (UHE) gamma-ray emission could be aligned along a common axis, indicating a co-spatial or co-directional origin. This alignment supports a model where synchrotron radio emission, VHE and UHE gamma rays are produced within a single, highly collimated relativistic outflow. Our findings favor scenarios of in-situ particle acceleration up to hundreds of TeV, challenge previous interpretations involving large-scale particle diffusion, and simplify the geometric modeling of the source. This case highlights the potential of V4641 Sgr as a PeVatron candidate within our Galaxy and provides a benchmark for understanding jet composition and magnetic structure in microquasars.

Figures (3)

• Figure 1: Left: Visibilities as a function of baseline length at a position angle of $162^\circ$, using a total of 100 bins. Error bars are smaller than the symbol size. Right: Over-resolved map of V4641 Sgr showing diffuse extended emission. The restoring beam is circular with a FWHM of $0.3^{\prime\prime}$. The horizontal bar indicates a scale of $1^\prime$. Colour bar on the right gives the flux density scale in units of Jy beam$^{-1}$, with the rms noise of the map being 0.15 mJy beam$^{-1}$. North is up, and East is to the left.
• Figure 2: Zoom-in on the clean components of the over-resolved radio map shown in Fig. \ref{['fig:uvplot']}, revealing an alignment suggestive of a bipolar, elongated structure. The area of each circle is proportional to the flux density of the corresponding clean component. The small cross marks the position of the central core component, which is too bright to be represented using the same brightness scale.
• Figure 3: The same over-resolved map of V4641 Sgr as in Fig. \ref{['fig:uvplot']} but with the central clean component subtracted at the cross position as described in the text. The circular synthesized beam shown in the bottom right corner also has a FWHM of $0.3^{\prime\prime}$.