Twenty years ago, an X-ray image of the Arches cluster resembled a photo of a person in thick fog: the cluster itself was drowned in the glow of a neighboring cloud. The cloud acted like a mirror, reflecting hard radiation from the vicinity of the black hole, and that reflection swamped the faint signal from the stars. Now the fog has lifted — the reflected glow has faded, and astronomers have finally seen the cluster itself.
It turns out that the X-ray emission from the Arches hides in the very center of the cluster, where stars are packed most tightly. It is faint but persistent: the spectrum shows that it is produced by fast electrons spiraling in magnetic fields and losing energy as they emit X-rays. Such electrons are prime suspects in the production of cosmic rays — particles that, at near-light speed, thread through the Galaxy and reach Earth.
This discovery settles a long-standing debate: previously, one could not tell whether the cluster itself was shining or whether everything visible was merely reflection. Now it is clear: inside the Arches, a particle accelerator of its own is at work, albeit not as powerful as those in supernova remnants. But there are many such clusters in the Galaxy, and together they could make a significant contribution to the cosmic ray flux.
Iron in the neighboring cloud gave itself away by a line at an energy of 6.4 keV, like a fingerprint. When this line vanished from the spectrum, astronomers knew the reflected background was gone and they could study the cluster itself.