The long-term accretion luminosity of V4641 Sgr through binary evolution simulations: implications for its ultrahigh-energy gamma-ray emission

Abstract

Recent observations by LHAASO and HAWC have revealed extended ultrahigh-energy (UHE; $E>100$ TeV) gamma-ray emission assoicated to the black-hole X-ray binary (BHXRB) V4641 Sgr, with a spectrum extending up to $\sim0.8$ PeV. Interpreting this emission requires a very {high time-averaged non-thermal particle power}, significantly exceeding the long-term observed X-ray luminosity which is commonly used as a proxy for the accretion power, leading to an apparent ``energy crisis''. To address this, we perform detailed binary-evolution simulations with \textit{MESA}, constrained by the known system parameters. The simulations suggest that V4641 Sgr is likely in a long-lasting, slow mass-transfer phase, with a time-averaged intrinsic X-ray luminosity of over evolutionary timescales of order $L_X\sim10^{38}$ erg/s, far above the observed luminosity over the last few decades. This is consistent with earlier suggestions of an extended obscuring/reprocessing envelope or outflow in V4641 Sgr. The inferred intrinsic accretion power can then readily supply the energy required to explain the UHE emission under the leptonic model, and is also marginally consistent with the requirement from the hadronic model, resolving the energy crisis. This supports V4641 Sgr as a Galactic PeV particle accelerator.

Figures (3)

• Figure 1: Evolutionary track for a binary system initially composed of a $5\,M_\odot$ BH and a $7.9\,M_\odot$ donor in a 1-day orbit. Upper-left panel: Mass-transfer rate $\dot{M}_{\rm tr}$ (black curve) and X-ray luminosity $L_{\rm X}$ (red curve) as a function of orbital period $P_{\rm orb}$. The vertical dashed line marks the observed orbital period of V4641 Sgr ($P_{\rm orb}=2.82$ days). The thick magenta curves denote a 20 Myr time interval centered on this orbital period. The horizontal dashed line indicates the Eddington accretion rate $\dot{M}_{\rm Edd}$ for the BH. The slanted dashed line represents the critical mass-transfer rates below which the accretion disk becomes unstable and the binary appears as a transient source Lasota2008. Upper-right panel: Mass-transfer rate as a function of donor mass $M_{\rm d}$. The observed donor mass range for V4641 Sgr ($2.5-3.3\,M_\odot$) is delineated by two vertical dashed lines. Lower-left panel: Orbital period versus donor effective temperature $T_{\rm eff}$. The red symbol indicates the observed position of V4641 Sgr. At this position, the modeled donor has $\log\,g\sim3.4$, consistent with spectroscopic constraints Orosz2001. Lower-right panel: Orbital period versus donor mass. The four blue dots represent initial binary configurations (selected from our evolutionary library) that evolve into systems resembling V4641 Sgr.
• Figure 2: Similar to Figure \ref{['V4641sgrf1']}, but for the binary initially containing a $5\,M_\odot$ BH and a $6.3\,M_\odot$ donor in a 1.26-day orbit.
• Figure 3: Similar to Figure \ref{['V4641sgrf1']}, but for the binary initially containing a $4\,M_\odot$ BH and a $6.7\,M_\odot$ donor in a 1.58-day orbit.