Astronomen beobachten Röntgenblitze eines nahegelegenen supermassiven Schwarzen Lochs, die auf mysteriöse Weise beschleunigt werden

The summary title is misleading. It’s not that the x-rays are accelerating, because they already propagate at the speed of light, but that the frequency of bursts of x-rays has increased. The pulses are happening more quickly; the particles are still moving at the same velocity.