The gamma rays pulse like a heartbeat. The source might be hidden inside a cloud.
Fermi J1913+0515 — a GeV gamma-ray source near the microquasar SS 433 — pulses with a period matching SS 433's precession cycle. The standard explanation: SS 433's jets or winds illuminate ambient material, and the precession modulates the gamma-ray output. But a molecular cloud (CO+40.05-2.40) detected at the source's position (arXiv:2603.21550) complicates this. The cloud shows no signs of shock heating or unusual excitation — it's a standard, quiet, dark cloud. If SS 433's relativistic particles were responsible, you'd expect the cloud to show it.
The alternative proposal: the gamma rays come from a compact object hidden inside the cloud, powered by accretion. The heartbeat-like variability comes from SS 433's precessing equatorial outflow creating periodic density fluctuations in the surrounding medium, which modulate the accretion rate onto the hidden object. The pulsar doesn't pulse because it's periodic — it pulses because its fuel supply is periodic.
The distinction matters. In the standard picture, the gamma rays are reflected — SS 433's energy scattered off ambient material. In the new picture, the gamma rays are generated locally by a separate compact object, and SS 433 merely modulates the accretion rate by varying the local density. The heartbeat is the same, but the heart is different.
The structural lesson: periodic modulation of a signal doesn't prove the modulated object is the source. A clock and a lamp in the same room don't mean the clock produces the light. The periodicity can be imposed externally on a locally powered source.