Detection of Extended X-Ray Emission around the PeVatron Microquasar V4641 Sgr with XRISM

Abstract

A recent report on the detection of very-high-energy gamma rays from V4641 Sagittarii (V4641 Sgr) up to ≈0.8 PeV has made it the second confirmed “PeVatron” microquasar. Here we report on the observation of V4641 Sgr with X-Ray Imaging and Spectroscopy Mission (XRISM) in 2024 September. Thanks to the large field of view and low background, the CCD imager Xtend successfully detected for the first time X-ray extended emission around V4641 Sgr with a significance of ≳4.5 σ and >10 σ based on our imaging and spectral analysis, respectively. The spatial extent is estimated to have a radius of 7′ ± 3′ (13 ± 5 pc at a distance of 6.2 kpc) assuming a Gaussian-like radial distribution, which suggests that the particle acceleration site is within ~10 pc of the microquasar. If the X-ray morphology traces the diffusion of accelerated electrons, this spatial extent can be explained by either an enhanced magnetic field (∼80 μ G) or a suppressed diffusion coefficient (∼10 ^27 cm ^2 s ^−1 at 100 TeV). The integrated X-ray flux, (4–6) × 10 ^−12 erg s ^−1 cm ^−2 (2–10 keV), would require a magnetic field strength higher than the Galactic mean (≳8 μ G) if the diffuse X-ray emission originates from synchrotron radiation and the gamma-ray emission is predominantly hadronic. If the X-rays are of thermal origin, the measured extension, temperature, and plasma density can be explained by a jet with a luminosity of ∼2 × 10 ^39 erg s ^−1 , which is comparable to the Eddington luminosity of this system.

Keywords

• Low-mass x-ray binary stars
• Gamma-ray sources
• Radio jets
• Non-thermal radiation sources