Large-scale radio bubbles around the black hole transient V4641 Sgr

TL;DR

We report the discovery of a large-scale bow-tie radio structure (~$35~\\mathrm{pc}$) around the black hole X-ray binary V4641 Sgr, spatially coincident with XRISM’s extended X-ray emission but not with the TeV gamma-ray bubble. Deep MeerKAT imaging indicates diffuse, symmetric radio lobes consistent with synchrotron emission from particles accelerated by past jet/disk-wind activity, requiring electron energies up to ~$100$ TeV and a two-zone emission scenario for the multiwavelength spectrum. Gaia DR3 proper-motion analysis supports a physical association, with the bow-tie center traceable back to the source on ~10 kyr timescales, while the TeV bubble likely stems from separate particle populations or histories. Free-free disk-wind emission struggles to reproduce the radio flux and energetics, reinforcing a jet-driven origin and highlighting V4641 Sgr as a laboratory for particle acceleration and microquasar feedback on the ISM.

Abstract

Black holes (BHs) in microquasars can launch powerful relativistic jets that have the capacity to travel up to several parsecs from the compact object and interact with the interstellar medium. Recently, the detection of large-scale very-high-energy (VHE) gamma-ray emission around the black hole transient V4641 Sgr and other BH-jet systems suggested that jets from microquasars may play an important role in the production of galactic cosmic rays. V4641 Sgr is known for its superluminal radio jet discovered in 1999, but no radio counterpart of a large-scale jet has been observed. The goal of this work is to search for a radio counterpart of the extended VHE source. We observed V4641 Sgr with the MeerKAT radio telescope at the L and UHF bands and produced deep maps of the field using high dynamic range techniques. We report the discovery of a large-scale (35 pc), bow-tie-shaped, diffuse, radio structure around V4641 Sgr, with similar angular size to the extended X-ray emission discovered by XRISM. However, it is not spatially coincident with the extended VHE emission. After discussing the association of the structure with V4641 Sgr, we investigate the nature of the emission mechanism. We suggest that the bow-tie structure arose from the long-term action of large-scale jets or disk winds from V4641 Sgr. If the emission mechanism is of synchrotron origin, the radio/X-ray extended structure implies acceleration of electrons up to more than 100 TeV as far as tens of parsecs from the black hole.

Figures (7)

• Figure 1: V4641 Sgr field as observed by the MeerKAT interferometer in the L band (central frequency of 1.217 GHz), with an angular resolution of $8"$. The background noise level is $4~\mu\mathrm{Jy~beam^{-1}}$, while the noise level in the region surrounding the bow-tie is on the order of $10~\mu\mathrm{Jy~beam^{-1}}$. The position of V4641 Sgr is marked by a black dot inside a white circle. The inset shows the central region of the field, with the bow-tie outlined in white. The white arrow indicates the proper motion of V4641 Sgr with respect to nearby stars, in the north-northeastern (NNE) direction (position angle of $15\degr$ E of N; see Sect. \ref{['sec:proper_motion']}).
• Figure 2: V4641 Sgr field as observed by the MeerKAT interferometer in the L band, along with H.E.S.S. gamma-ray contours (energy range of 0.8 to 22 TeV) at the levels of 3, 4, ..., 9 $\sigma$HESS2025. The dashed circle and the shaded area represent the radius of the XRISM X-ray source and its uncertainty Suzuki2025.
• Figure 3: Proper motion of stars in the vicinity of V4641 Sgr, in the direction of increasing right ascension (x-axis) and declination (y-axis), as measured by Gaia. Only stars within a $3'$ radius and a $p_\star \pm 2\sigma_{p_\star}$ parallax range are shown. The differential parallax between V4641 Sgr and individual stars is color-coded. The black star $\star$ corresponds to the optical counterpart of V4641 Sgr, while the dotted gray lines indicate the average proper motion in the sample.
• Figure 4: Broadband spectral energy distribution. Emission from the central $10'$ around V4641 Sgr is shown (coincident MeerKAT and XRISM detections), together with Fermi-LAT, H.E.S.S., HAWC and LHAASO spectra Suzuki2025Neronov2025HESS2025Alfaro2024LHAASO2025 at larger scales ($\sim 1\degr$). The gray points correspond to gamma-ray fluxes, scaled down to the XRISM source region by relative area. Synchrotron and IC models (dotted lines), as well as the pion decay model (black solid line) are overlaid.
• Figure 5: Schematic illustration of the bow-tie around V4641 Sgr in the plane of the sky. In our model, the geometry of the bow-tie is a cone of radius, $R$, and height, $H$, with V4641 Sgr at its vertex. The symmetry axis of the cone has a position angle of $-50\degr$ E of N. $\varphi$ is the projected opening angle of the structure. The black dot corresponds to the position of V4641 Sgr, while the direction of the 1999 radio jets Hjellming2000 is represented in dark blue. The pink shaded areas symbolize the gamma-ray bubbles Alfaro2024LHAASO2025HESS2025.