The phrase "narrow beam" refers to a concentrated and focused stream of light, sound, or any other type of energy that is confined to a limited space or direction. Full definition
One dramatic consequence is that some of the star's material, stripped from the star and collected around the black hole, can be ejected in extremely narrow beams of particles at speeds approaching the speed of light. (sciencedaily.com)
The radio lobes are produced from electrons shot out from the nucleus in narrow beams called jets. (astronomynotes.com)
To avoid those problems, critics have noted that small black holes could appear very bright in x-rays by channeling all of their energy into narrow beams. (sciencemag.org)