Title: Minute time scale variability in γ-ray flare of BL Lacertae
Authors: Joysankar Majumdar, Raj Prince, and Sakshi Maurya
First Author’s Institution: Department of Physics, Institute of Science, Banaras Hindu University, Varanasi-221005, India
Status: Submitted to ApJ
Active Galactic Nuclei (AGN) are understood to be supermassive black holes at the centres of galaxies, with accretion disks of material falling into them and expelling huge jets of material into their surroundings, even beyond the galaxies that host them. Blazars are a special class of AGN where the jet is pointed directly at Earth, allowing us to observe the particles being accelerated in these jets. Despite detecting many blazar jets in gamma rays wavelengths (the highest energy light), we still aren’t sure why jets are formed and how they accelerate particles to some of the highest energies observed in the universe.
Blazars are known to flare – meaning they show periods of brightening and dimming, where each period can happen over timescales of minutes to weeks, depending on the source. Flares are thought to come from material falling into the jet or getting caught up in clumps or knots in the jet. Today’s authors look at one of the brightest and fastest flares ever seen from a blazar, observed just a few weeks ago, and investigate the possible ways such a flare could’ve been made.
Minute to win it!
In October 2024, the blazar called BL Lacertae, nicknamed BL Lac (which, somewhat confusingly, is also the namesake of the broader class of BL Lac objects) was seen to flare by the Fermi-LAT gamma-ray telescope. This flare was the brightest we’ve ever seen from BL Lac and this flare was possibly the fastest of any gamma-ray blazar observation ever, with increases and decreases in the brightness of the jet spanning a few minutes. The only known competition is a BL Lac flare observed a few years earlier in April 2021 which also showed minute-scale variability but was dimmer than this year’s. It remains uncertain whether the flare reported in today’s paper is the brightest ever BL Lac flare and the brightest ever blazar flare seen from BL Lac. Further studies of the variability and analysis of the multi-wavelength data taken by other gamma-ray and optical instruments will be important to confirm these superlatives.
Minute-scale flares are particularly interesting because they indicate that whatever’s causing the bright emission needs to be really small – much smaller than the scale of the black hole’s event horizon. This argument makes use of “light travel time”: if a large region (e.g., the event horizon of a black hole) is flashing very quickly, light from the middle part of the region that’s closest to Earth will arrive first, then light from the edges will arrive later (think about the hypotenuse vs. the straight edge lengths of a right angled triangle). Seeing the continuous arrival of fast pulses at slightly different times will smear the rapid variations with each other and just look like a wider pulse (see Figure 2). Using this argument, we know that by seeing quick flashes, the diameter of whatever is emitting that light must have a diameter smaller than the speed of light times the variability timescale.
A closer look at the flare
Following an initial alert from the Fermi-LAT Collaboration reporting a factor of 20 increase in gamma-ray flux (how bright the object is as a rate of photons per second per area) from the average value, several observatories observed BL Lac in October 2024, including very high energy (VHE; higher energy gamma-rays than Fermi-LAT) gamma-ray telescopes VERITAS and MAGIC and optical telescopes DFOT and LAST. A multi-wavelength picture of the flare is really important for understanding where the flare came from and why BL Lac is the fastest flaring blazar we’ve seen so far.
Today’s authors take a closer look at the Fermi-LAT data to find the fastest variability timescale and the maximum flux from BL Lac. The authors take search for variability in the flare using a method called Bayesian blocks to identify when changes in flux are significantly different from previous blocks of time (see Figure 3). They use this method to identify the main flaring period, using the time bins widths adapted to the flux observations to identify how fast the blazar flux is actually varying.
They find that the maximum flux value (the highest point in Figure 3) is roughly 10 times higher than the estimate initially reported in the Fermi-LAT alert and they find variability on the scale of ~ 1 minute, which is very similar to the variability found in the 2021 flare.
Recipes for a super fast flare
The authors propose several scenarios to explain the minute-scale variability in the flare. Wherever it came from has to be much smaller than BL Lac’s event horizon – the closest light can get to the black hole, while still being able to escape its gravity. The smallest variability that’s possible caused by an object greater than this size, according to measurements of BL Lac’s event horizon, is over thirty minutes. They propose the following three scenarios that could produce flares that are only a minute long from origins outside the event horizon:
- The flare originates from a small hole in the magnetosphere (where particles are affected by the black hole’s magnetic field) near the event horizon. This would essentially create a small hole that particles can stream through and escape toward Earth.
- The magnetic field lines in the jet could undergo magnetic reconnection, where they merge and reconfigure, releasing energy and accelerating bunches of particles in the process.
- The jet could hit another object in its path, like a star or cloud of gas. This would add a “blob” of material to the jet and create particle collisions that rapidly accelerate bunches of particles.
To verify if any of these predictions are realistic, future work can be done to see if theoretical predictions of any of the above scenarios match the flares seen by Fermi-LAT and other multi-wavelength observations. The Fermi-LAT data can also be re-analyzed using algorithms that are more precise for extremely short variations than the Bayesian block technique used in today’s paper. This super fast flare from BL Lac is nonetheless very exciting in its own right, and will help us better understand how supermassive black holes form jets to produce particle accelerators in some of the universe’s most extreme environments.
Astrobite edited by Storm Colloms
Featured image adapted from: Event Horizon Telescope Collaboration, ESO/WFI
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