The high-energy radiation from infalling matter that often helps identify black hole candidates is X-rays.
In an X-ray binary, a black hole pulls gas from a companion star or captures part of its stellar wind. The gas forms a rapidly rotating accretion disk, where friction and compression heat the material to millions of degrees. The inner disk and nearby hot plasma then emit intense X-rays, making the system detectable even though the black hole itself produces no light.
X-rays are strong evidence of energetic accretion, but they do not by themselves prove that the compact object is a black hole. A neutron star can also power an X-ray binary. Astronomers combine the X-ray signal with orbital measurements: if the unseen object’s mass exceeds the maximum plausible neutron-star mass, a black-hole interpretation becomes compelling.
Cygnus X-1 was the first widely identified black-hole candidate. Its system contains a massive companion star and a compact object fed partly by stellar wind, producing the characteristic high-energy emission.