A beacon at the edge of the galaxy blinks without a schedule: sometimes silent for years, sometimes emitting hundreds of flares in a single day. Such a beacon needs a watchman who never sleeps and never gets distracted. Since 2008, the gamma-ray monitor Fermi GBM in Earth orbit has been keeping that watch, and its log has just been updated with the most complete record in the history of observations.
Magnetars are neutron stars with magnetic fields a quadrillion times stronger than Earth's. From time to time, their crusts crack or their magnetospheres reconfigure, and the star emits a short pulse of gamma radiation lasting fractions of a second. To catch such flares, you need to watch the entire sky at once without blinking. Fermi GBM does exactly that: 12 detectors made of sodium iodide crystals continuously record every photon with energies from 8 to 200 kiloelectronvolts.
The new catalog is not just a list of dates and numbers. For each burst, the duration, spectrum, and total energy have been measured. It turns out that two sources dominate: SGR J1935+2154 produced 567 bursts, while SGR J1550–5418 produced 459. The average burst duration for these stars is about 200 milliseconds, shorter than a lightning strike. The spectra of most bursts are best described not by one but by two "blackbodies"—what physicists call ideal emitters, akin to a glowing furnace that radiates strictly according to its temperature.
But the most interesting part is the deviation from the simple law. If the emitting region were an ordinary hot surface, its area would grow with temperature according to a known rule, just as the area of a frying pan grows as it heats up. Instead, the relationship is different: the power-law index is about −4.5 to −5.4 instead of −4. This suggests that the flare region is not like a calm furnace but changes rapidly, possibly contracting or expanding during the emission. The energy distribution of bursts is not random either: it follows a power law, which often appears in systems where events of different scales are self-similar, like cracks in drying mud or avalanches in a sandpile.
The watch log is now open to everyone. It can be used to test models, plan observations, and prepare new instruments. And the beacons keep blinking, with each new flare adding a line to this long chronicle.
On a single day, January 22, 2009, the magnetar SGR J1550–5418 emitted 341 bursts, and accounting for detector dead time, the actual number could have been around 541.