Hadronic Emissions from the Microquasar V4641 Sgr, SS433, and its implications in the Diffuse Galactic Emission

Abstract

Microquasars (MQs) are Galactic binary systems, consisting of a star and a compact object, a neutron star or a stellar mass black hole, which accretes matter from its companion star and gives rise to relativistic jets. Recent detection of very-high-energy (VHE; $E \gtrsim 100,\text{GeV}$) and ultra-high-energy (UHE; $E \gtrsim 100,\text{TeV}$) gamma-rays by LHAASO, HAWC and HESS from the MQ V4641 Sgr and SS 433 suggests them as Galactic PeVatrons. In this work, we studied a hadronic origin of the observed TeV-PeV gamma-ray emission from these MQs. We considered the hadronic scenario where the gamma-rays are produced by the interaction of relativistic protons in the MQ jet with the stellar wind. We fitted our model with observed data and constrained physical parameters like the hadronic jet power fraction, the proton spectral index, the maximum proton energy and the jet bulk Lorentz factor. Our best-fit model shows hard proton spectra ($1.84-2.44$) and maximum proton energies between 1 and 5 PeV. We also estimated the all-flavor neutrino fluxes corresponding to the gamma-ray fluxes from the hadronic model and found that V4641 sgr can be detected by next-generation neutrino telescopes like KM3NeT-ARCA and TRIDENT. Furthermore, we modeled a synthetic population of Galactic MQs and estimated their contribution to the diffuse TeV-PeV gamma-ray flux. For the inner Galaxy PSR contribution dominates in the range 10-100 TeV, and above 100 TeV diffused cosmic ray interactions with the molecular clouds is most dominant. We find that a population $\sim 14$ MQs is required to explain the LHAASO data above 100 TeV. For the outer Galaxy, we show that MQs are the dominant class of sources, and we constrain their population $\sim$14. Our findings strongly suggest that MQs are efficient particle accelerators, contributing to Galactic PeVatrons and potential multimessenger sources in our Galaxy.

Figures (6)

• Figure 1: Spectral energy distribution of VHE and UHE gamma-ray emission from the hadronic model for the single extended source (a), northern site (b) and southern site (c) of HAWC ROI of the microquasar V4641 Sgr. The blue line represents the best-fit hadronic model predicted gamma-ray flux. The light blue band indicates the corresponding $68\%$ (equivalent to $1\sigma$) credible intervals derived from the MCMC posterior samples. The solid red line shows the predicted all-flavor neutrino flux corresponds to the hadronic gamma-ray flux. Blue, orange and green data points with error bars are the observed gamma-ray data reported by HAWC alfaro2024ultra, LHAASO Lhaaso_2024arL and HESS neronov2025multimessenger respectively. The Dashed green and black lines indicate neutrino flux sensitivity level by KM3NeT-ARCA, 10 years and TRIDENT $5\sigma$ discovery for 10 years respectively, and the solid black line for 10 years of TRIDENT data (extracted from neronov2025multimessenger). Both the sensitivity lines have been plotted by dashed lines with gaps for visual distinction.
• Figure 2: Spectral energy distribution of VHE and UHE gamma-ray emission from the hadronic model for the east lobe(a), west lobe(b) and the total summed up emission(d) of SS 433. The blue line represents the best-fit hadronic model predicted gamma-ray flux. The light blue band indicates the corresponding $68\%$ (equivalent to $1\sigma$) credible intervals derived from the MCMC posterior samples. The solid red line shows predicted all-flavor neutrino flux corresponds to the modeled gamma-ray flux. Blue, orange and green data points with error bars are the observed gamma-ray data reported by HAWCalfaro2024spectral, LHAASOLhaaso_2024arL and HESSneronov2025multimessenger respectively. The Dashed green and black lines indicate neutrino flux sensitivity level by KM3NeT-ARCA, 10 years and TRIDENT $5\sigma$ discovery for 10 years respectively, and the solid black line for 10 years of TRIDENT data (extracted from neronov2025multimessenger). Both the sensitivity lines have been plotted by dashed lines with gaps for visual distinction.
• Figure 3: Gamma ray flux contribution from a population of Galactic microquasars (MQs), Molecular Clouds (MCs), Pulsars (PSRs) and Supernova Remnants (SNRs) for the Galactic diffuse gamma ray emission of inner (a) and outer galaxy (b). The black points with error bars are the LHAASO observed diffuse gamma ray data for inner and outer galaxy respectively (extracted from Cao2023PhRvL_01C and increased by 61% and 2% for inner and outer galaxy respectively). The pink solid line with lightpink shaded region represents the population modeled gamma-ray flux of MQs for $E'^{\max}_p$ up to 10 PeV. The blue solid line is the total flux contribution from MQS, MCs, PSRs and SNRs with corresponding shaded region indicating the $68\%$ credible intervals derived from 100 simulations. The red dashed line and green region show contributions from SNRs and PSRs (extracted from kaci2025microquasars). The purple dashed line represents the contribution from MCs (taken from Roy24).
• Figure 4: Gas density profile along the jet axis for the microquasars V4641 Sgr (blue line) and Ss 433 (orange line). The density, $n(z)$ as discussed in Section \ref{['model']} is computed for $z_0 \le z \le z_{max}$, where $z_0$ is base of the jet (values listed in table \ref{['tab:model_parameters']}) and $z_{max}$ is the jet extension, taken up to $10^{14}\,\,\text{cm}$. Other parameters adopted to calculate $n(z)$ are listed in table \ref{['tab:model_parameters']}
• Figure 5: Posterior distributions of free parameters of the hadronic model for the production of gamma-rays from V4641 Sgr (fig. a) and SS 433 (fig. b). The vertical dashed lines indicate median values with $68\%$ credible intervals for each distribution.