[2] The authors favour a scenario in which a white dwarf was shredded by a so-called intermediate-mass black hole. A white dwarf is the small, slowly-cooling core that is left behind after a star like our Sun dies. Intermediate-mass black holes are between 100 and 100 000 times more massive than the Sun. Most known black holes have masses significantly greater or lower than that, and intermediate-mass black holes remain a poorly understood type of object.

This may be not so powerful gamma ray source as expected, considering that it may radiate in one or two narrow beams, which isn't unusual.

Probably aliens. That's my first thought on articles like this.