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Title:Very High-Energy Gamma-Ray Episodic Activity of Radio Galaxy NGC 1275 in 2022–2023 Measured with MACE
**Authors:**S. Godambe et al.
First Author’s Institution: Bhabha Atomic Research Centre
Status: Published in ApJL
the MACE telescope
Figure 1: The MACE telescope. [Khurana et al. 2023]
The Major Atmospheric Cherenkov Experiment (MACE) (see Figure 1) is a new ground-based gamma-ray telescope (specifically the type of telescope called an imaging atmosphere Cherenkov telescope or IACT) located near Hanle, Ladakh, India. Though the atmosphere is opaque to gamma rays, this can actually be used to our advantage. The particles created after a gamma ray interacts with atmospheric molecules travel faster than the speed of light in the atmosphere, creating a flash of optical light called Cherenkov radiation that is analogous to a sonic boom. Large optical detectors like MACE can use this Cherenkov radiation to directly measure the direction and energy of a gamma ray on the ground, even though the original photon is long gone.
Several other IACTs exist around the world, including the High Energy Spectroscopic System, the Major Atmospheric Gamma Imaging Cherenkov telescopes, the Very Energetic Radiation Imaging Telescope Array System, and the new Large-Sized Telescope, a prototype for the next generation array of gamma-ray telescopes like the Cherenkov Telescope Array Observatory. However, MACE is the IACT at the highest altitude (4,270 meters above sea level!) and is the third largest (after the Large-Sized Telescope and one of the High Energy Spectroscopic System telescopes), with an impressive light-collecting diameter of 21 meters of segmented mirrors. This makes MACE sensitive to lower-energy gamma rays than typical IACTs since low-energy gamma rays attenuate higher in the atmosphere and are dimmer, requiring larger mirrors and higher elevation. It’s important to look at this energy range (less than a few hundred gigaelectronvolts) since it bridges the sensitivity ranges of the Fermi Large Area Telescope (Fermi-LAT), a space-based gamma-ray telescope, and other IACTs.
In MACE’s first research article, the authors discuss observations of NGC 1275, a nearby radio galaxy. In just a month of observations, they discovered two gamma-ray outbursts from the source!
Does Jet Inclination Kill the Radio Star~~~~ Galaxy?
Active galactic nuclei are understood to be supermassive black holes that accrete matter from their surroundings and often shoot out large jets that are larger than the galaxies that host them. For the very high energy (energies greater than 100 GeV) gamma rays, where IACTs operate, we expect to see mainly blazars, a class of active galactic nucleus where the jet points directly at Earth, beaming particles toward us through a process called Doppler boosting and producing gamma rays and other photons as they go. This beaming gives us bright and fast flares — bursts of gamma rays — whenever material falls into the jet.
What’s weird is that we also see flares from radio galaxies that are similar in brightness and speed to the flares from blazars. Since radio galaxies (named for having large radio lobes visible at the ends of the jets) are active galactic nuclei where the jet is misaligned from Earth and doesn’t beam directly at us, we’d expect them to be dimmer than they are. Think about looking sideways at a laser versus looking right into the laser. The latter is so bright that it will probably damage your eyesight (please don’t try this at home!), but the former is much dimmer. Observing and studying gamma-ray flares from radio galaxies is fun because they’re unexpectedly bright, and these observations are crucial for understanding how the flares are being powered.
spectral energy distributions
Figure 2: Spectral energy distributions composed of MACE, Fermi-LAT, and Swift Ultraviolet/Optical Telescope (ultraviolet) and X-Ray Telescope (X-ray) observations of the December flare (upper panel), January flare (middle panel), and the non-flaring period between the two flares (bottom panel). The dashed and solid lines represent the best-fit models for two different viewing angles of the jet. [Godambe et al. 2024]
New Year, New Flares
Today’s authors report on two flares from NGC 1275 that occurred in December 2022 and January 2023. Flares are unpredictable and often sporadic, so it’s impressive that two were seen within a month of each other!
Using data from Fermi-LAT and the X-ray/ultraviolet observatory Swift, they can construct a spectral energy distribution, which shows the brightness of the source across all observed energies (Figure 2). The authors fit different models to the spectral energy distribution to determine all sorts of parameters about the radio galaxy, like the size of the region that’s producing gamma rays, how tilted the jet is toward us, the maximum-energy gamma ray that can be accelerated in the jet, and more. These models are constructed by simulating the observed multi-wavelength photons from blobs of material that fall into jets of different configurations. The model that best fits the observed data should then be a good descriptor of the physical environment of the active galactic nucleus.
The authors find that the physical conditions for both flares are very similar. Consistent with other observations of NGC 1275 and other radio galaxies, they find a smaller Doppler factor and larger viewing angle (the “misalignment” of the jet with Earth’s line of sight). The “quiet” phase between flares seems to come from a reduction of either the Doppler factor — meaning that the particles in the jet aren’t being beamed as much — or an attenuation of the magnetic field strength, which makes it harder to accelerate particles to gamma-ray energies.
The authors conclude that more complicated jet processes must be happening for us to see these flares, such as different parts of the jet moving at different speeds or additional magnetic field acceleration before particles fall into the jet. The former scenario should increase the absorption of higher-energy gamma rays and make it impossible to see any gamma rays above 1 teraelectronvolt. The latter scenario requires particles to orbit large-scale magnetic fields that are the size of the active galactic nucleus’s event horizon (the radius at which light can no longer escape from the gravitational pull of the black hole), which makes it hard to see variability on timescales of less than a day.
Future observations are needed to see if higher-energy gamma-rays or faster flares are ever seen again from NGC 1275. If so, we’d need to go back to the drawing board to figure out other theories to explain how radio galaxies get so bright in gamma rays!
Original astrobite edited by Junellie Perez.
About the author, Samantha Wong:
I’m a graduate student at McGill University, where I study high energy astrophysics. This includes studying all sorts of extreme environments in the universe like active galactic nuclei, pulsars, and supernova remnants with the VERITAS gamma-ray telescope.