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

TL;DR

This paper reports the discovery of a large-scale radio structure around the black hole transient V4641 Sgr, suggesting long-term jet or wind activity that accelerates electrons to very high energies, with implications for cosmic ray production.

Contribution

It presents the first detection of a large-scale radio bow-tie-shaped structure around V4641 Sgr, linking it to jet or wind activity and high-energy electron acceleration.

Findings

Discovered a 35 pc diffuse radio structure around V4641 Sgr.

The structure is not spatially coincident with the VHE gamma-ray emission.

Electrons are accelerated to over 100 TeV within the structure.

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.