The Prototypical Blazar BL Lacertae Shows New Behavior

Astronomers have spotted a distant blazar behaving in a new way. Understanding its behaviour could lead to a deeper knowledge of these immensely powerful objects.

The Quintessential Blazar

BL Lacertae is the active centre of a distant galaxy almost one billion light-years from Earth. Despite this large distance, it can match the brightness of objects within our own solar system such as Pluto. When it was first discovered in 1929, BL Lacertae was thought to be a star in our own Milky Way. As such, it is an example of a quasi-stellar object — a quasar.

plot of BL Lacertae's magnitude over time

An example of BL Lacertae’s optical variability over less than one day in 2020. [Adapted from Kalita et al. 2023]

BL Lacertae was also the first known blazar — a quasar that varies its brightness. In fact, the BL from its name was combined with quasar to coin the term blazar in the first place.

Quasars and blazars are so bright because they are powered by a supermassive black hole that holds sway over the centre of the galaxy. It’s devouring, ripping, and swirling material around. Its brightness varies depending upon what it is snacking on. Starting in August 2020, and lasting for over a year, BL Lacertae was particularly bright in wavelengths stretching from the optical to very high-energy gamma rays.

plot of BL Lacertae's color

Illustration of how BL Lacertae’s color changed over time. The color bar indicates the number of days since the modified Julian date 2459122. [Adapted from Kalita et al. 2023]

Old Blazar, New Tricks

A team led by Nibedita Kalita (Shanghai Astronomical Observatory and Polar Research Institute of China) used the 1.26-metre telescope at the Xinglong Observatory to observe BL Lacertae between October and November 2020. The team found that even within this short period it got up to 30% brighter. Sometimes these brightness variations unfolded in less than an hour.

This short variation time implies that the source is fairly compact. The team estimated the source area to measure about 100 times the radius of the black hole. As well as intra-night variability, they also saw a longer variation pattern that lasted approximately 11 days. They also noticed colour changes in the blazar’s spectrum as it brightened that had never been seen in other blazars before.

composite optical and radio image of the galaxy Hercules A and its jets

Magnetic fields can help shape and power narrow jets, like those of the radio galaxy Hercules A shown here. [NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA)]

Reasons for Change

Kalita’s team suspects that this longer-term variability is due to changes in the jet of material associated with the black hole. Strong magnetic fields corral material into a narrow column that if pointing at Earth can be particularly bright. It can quickly get brighter if particles within it are accelerated to relativistic speeds by some sudden change. This could be shock waves within the material or perhaps a process called magnetic reconnection. Also responsible for flares on the Sun, reconnection happens when two oppositely orientated magnetic fields are forced together. They snap into a new configuration releasing a lot of stored energy.


“Optical Flux and Spectral Variability of BL Lacertae During Its Historical High Outburst in 2020,” Nibedita Kalita et al 2023 ApJ 943 135. doi:10.3847/1538-4357/aca801