BL Lac is brightening, increasing by a magnitude over the last week or two.
Jim Roe [ROE]
Please somebody, reveal the physics of this active galactic nucleus' magnitude variations by up to ~1 per day! Do any other objects in this class show such variations?
BL Lac objects have rapid variations as part of their class definition. All of the AGNs have black holes as their central power source, and variations between one type of AGN and another are often due to the orientation of the jet toward our line of sight. In the BL Lacs/Blazars, the beam of the jet is oriented close to our line of sight, so that we're able to see down into the central engine. The timescale of variability is related to the light-crossing timescale for the objects that we're seeing, and since the black hole is on the order of light hours to light days in size, those are the variations that we sometimes see.