Detection of Extended X-Ray Emission around the PeVatron Microquasar V4641 Sgr with XRISM
Hiromasa Suzuki, Naomi Tsuji, Yoshiaki Kanemaru, M. Shidatsu, Laura Olivera-Nieto, Samar Safí-Harb, Shigeo S. Kimura, E. De la Fuente, S. Casanova, Kaya Mori, X. Wang, Sei Kato, Dai Tateishi, Hideki Uchiyama, Takaaki Tanaka, Hiroyuki Uchida, Shun Inoue, Dezhi Huang, M. Lemoine‐Goumard, Daiki Miura, Shoji Ogawa, Shōgo Kobayashi, Chris Done, Maxime Parra, María Díaz Trigo, T. Muñoz‐Darias, M. Armas Padilla, Ryota Tomaru, Yoshihiro Ueda
Abstract
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.