While ESA's satellite INTEGRAL was observing the sky, it spotted a burst of gamma-rays—high-energy photons—coming from the nearby galaxy M82. Only a few hours later, ESA's XMM-Newton X-ray space telescope searched for an afterglow from the explosion but found none.

Magnetars are among the universe's most extreme objects - a class of the compact stellar remnants called neutron stars that possess immensely strong magnetic fields. Once in a while, they produce enormous eruptions of gamma rays in the strongest nondestructive release of energy known in the cosmos.

Gamma rays are a broad category of high-energy photons, including everything with more energy than an X-ray. While they are often created by processes like radioactive decay, few astronomical events produce them in sufficient quantities that they can be detected when the radiation originates in another galaxy.

While all neutron stars boast powerful magnetic fields, magnetars stand out with fields exceeding 10,000 times that of a typical neutron star.