Corresponding Authors: S. Godambe, N. Mankuzhiyil, C. Borwankar (see paper for full author list)
First Author’s Institution: Astrophysical Sciences Division, Bhabha Atomic Research Centre
Status: Accepted to ApJ [open access]
The Major Atmospheric Cherenkov Experiment (MACE) 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), 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 particle is long gone.
Several other IACTs exist around the world, including H.E.S.S., MAGIC, VERITAS and the new LST-1, a prototype for the next generation array of gamma-ray telescopes (CTAO). However, MACE is the IACT at the highest altitude (4270m above sea level!) and the third largest (after LST-1 and one of the H.E.S.S. 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 (< a few hundred GeV) since it bridges the sensitivity ranges of Fermi-LAT, a space-based gamma-ray telescope, and other IACTs.
In MACE’s first science paper, 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 (AGN) 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. In very high energy (VHE; E > 100 GeV) gamma-rays, where IACTs operate, we expect to see mainly blazars, a class of AGN 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 similarly bright and fast to blazars. Since radio galaxies (named for having large radio lobes visible at the ends of the jets). are AGN where the jet is misaligned from the 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 vs. 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 crucial for understanding how they’re being powered.
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 X-ray/UV telescope Swift, they can construct a spectral energy distribution (SED), which shows the brightness of the source across all observed energies. The authors fit different models to the SED 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 multiwavelength 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 AGN.
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 the 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 TeV. The latter scenario requires particles to orbit large-scale magnetic fields that are the size of the AGN’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!
Featured image adapted from: M. Khurana et al., 2022
Edited by: Junellie Perez
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