The black hole in V4641 Sagittarii behaves like a vacuum cleaner running at full power, but neighbors only see a faint indicator light. The LHAASO and HAWC telescopes caught gamma rays from it with energies up to 0.8 PeV — millions of times more energetic than hospital X-rays. To accelerate particles to such energies, you need power hundreds of times greater than what the system has shown in X-rays over recent decades. This creates an "energy crisis": where does the power come from?
Astrophysicists simulated the evolution of this binary system on a computer, trying thousands of initial conditions. It turned out that all scenarios leading to the current parameters of V4641 Sagittarii include a long phase of slow matter transfer from the star to the black hole. On average, during this time the system should shine in X-rays with a power of about $10^{38}$ erg/s, like a hundred billion suns shining only in X-rays. But we see only a small part: a dense shell of gas and dust, ejected during past outbursts, hides most of the radiation, like a curtain hiding a bright lamp.
So there is no crisis: the black hole gets enough fuel to work as a particle accelerator. The energy is sufficient for the model where fast electrons produce gamma rays, and almost sufficient for the proton model. V4641 Sagittarii is a rare example of a galactic accelerator that boosts particles to energies comparable to those arriving from deep space.
A gamma-ray photon with an energy of 0.8 PeV is a trillion times more energetic than visible light. A single such photon could toss a grain of sand several centimeters into the air.
In Stanislaw Lem's novel "The Invincible," the heroes encounter a swarm of microscopic machines that evolved on a barren planet. V4641 Sagittarii resembles such a swarm: hidden power that bursts out only as a narrow beam of gamma rays.