Extreme galaxies flinging out the highest energy photons

by Amy Furniss

Gamma-ray blazars (also known as BL Lac objects) are among the most extreme galaxies, whipping up and then flinging out into intergalactic space particles at energies far beyond those attainable by the most powerful particle accelerators on Earth. The study of the variable gamma-ray emission from these energetic galaxies is possible through observation with Cherenkov light telescopes (also discussed previously in the KIPAC blog here) such as the Very Energetic Radiation Imaging Telescope Array System (or VERITAS, described further below). VERITAS recently detected a gamma-ray flare from the galaxy 1ES 1727+502 (while another mysterious flare from a different blazar was recently discussed in this KIPAC blogpost and notably, the VERITAS observations of this galaxy in a bright state were made possible through the recent development of an innovative observing setup which enables ground-based gamma-ray telescopes to observe during bright moonlight. In fact, in this case this was not a major technological advance, it was just one of those lightbulb moments—something which just hadn't been tried before, and turned out to be a unique idea that worked out excellently when actually implemented.

Above: An artist's representation of a blazar, which is a type of active galaxy that emits a jet aligned so that it is beaming towards us. (Credit: NASA/JPL-Caltech.)

VERITAS, mentioned above, is a system of four ground-based gamma-ray telescopes which observe the most extreme astrophysical processes at work within the Universe. Located in Southern Arizona, the telescopes detect gamma-ray photons with energies greater than 85 GeV—i.e., those falling in the most energetic part of the electromagnetic spectrum. The observations of the gamma-ray photons are made indirectly, through the detection of atmospheric particle showers which result when a very energetic gamma ray hits the Earth’s atmosphere, and which then in turn emit a collective cone of dim blue Cherenkov light. The blue light is so dim, however, that the cameras of the VERITAS telescopes require 499 sensitive photomultiplier tubes to make sure the photons are seen at all.

Due to the extreme sensitivity of the photomultiplier tubes, ground-based gamma-ray telescopes such as VERITAS have (up until recently) operated during short observational windows when the skies are dark (no Moon) and clear (no clouds). This is different than the space-based gamma-ray Fermi Large Area Space Telescope (which is sensitive to slightly lower energy photons than VERITAS)—a huge benefit to space-based observatories is the freedom from weather and Moon constraints suffered by ground-based instruments. Fermi, for example, operates by surveying the entire sky every three hours, whereas until recently VERITAS was restricted to making observations of small patches of sky relatively far from the Moon when the weather was clear (for numerical comparison, the "field of view" of VERITAS is 3 square degrees, while the full sky is about 41,000 square degrees).

Above: VERITAS (the Very Energetic Radiation Imaging Telescope Array System) is a ground-based gamma-ray instrument located in southern Arizona. It is an array of four 12 meter reflectors made up of many smaller mirrors. The telescopes, known as imaging atmospheric Cherenkov telescopes, are used for gamma-ray astronomy in the GeV–TeV energy range. This very high energy gamma-ray observatory effectively complements the NASA Fermi mission, which is sensitive to gamma-ray photons at MeV to GeV energies.

The restricted observing conditions for VERITAS have challenged gamma-ray studies of blazars, as these sources are known to show variability on nearly every timescale, from years down to minutes. However, the new and innovative strategy mentioned above has allowed observations during high Moon-brightness by dialing down the high voltage inputs of the photomultiplier tubes which make up the VERITAS cameras, and allowing more regular monitoring of gamma-ray galaxies. This is a very new observation technique that has only been explored and utilized in the last three years of a 40-year-old field, and promises great discovery potential into the future.

The discovery that VERITAS is capable of operating under bright moonlight conditions with no significant loss of sensitivity has enabled a much more informative observing strategy for gamma-ray galaxies such as 1ES 1727+502. Not only does the more regular observing program increase the chances of catching these galaxies undergoing bright states, but the VERITAS observations can more easily occur simultaneously with observations in other wavebands that are not constrained by the presence of a bright Moon.

Above, we show a snapshot measurement of the power output of 1ES 1727+502 at many different wavelengths. This is one of the most effective ways to study the relativistic particle population within a blazar jet, by observing the source across the entire broadband spectrum, from radio through gamma-ray energies. At every photon energy (shown in terms of frequency along the x-axis, and increasing from left to right), the blazar puts out a specific relative power (represented on the y-axis). Blazars are known for their double-peaked power spectra, with a low-energy peak in the infrared-optical range (the peak on the left at lower frequency), and a high-energy peak in the gamma-ray range (the peak on the right). Here, the VERITAS observations are shown by the data points on the far right portion of the plot, and are matched with observations from Fermi, X-ray and UV observations with the NASA space-based satellite Swift, as well as ground-based optical and radio measurements. The model used to represent the full emission of the source is represented by the blue and red curves. By finding the correct model to reproduce these curves, we can gain insight into how the blazar is producing the emission from radio through gamma-ray energies.

Minimally interrupted observations of gamma-ray blazars in different states, or undergoing variations, can provide powerful insight into the relativistic particles thought to be at work within the galaxies. Although gamma-ray galaxies such as 1ES 1727+502 are understood to be active galactic nuclei that are powered by accretion onto supermassive black holes and have a relativistic jet of photons and particles pointed along our line of sight, the details about how the relativistic particles entered or formed the jet, or how and why they behave as they do is still an open topic which can only be addressed through regular observation at gamma-ray energies.

Based on more consistent VERITAS observations of 1ES 1727+502, the galaxy was found to be in a bright state. Matching the VERITAS observations with observations in other bands enables the modeling of the particle population thought to be responsible for the variable emission. The modeling shows that the relativistic particles in the galaxy’s jet are possibly accelerated to ultra-relativistic energies via "shock acceleration," perhaps within the jet at a location where two regions of particles moving at different velocities collide. The modelling of the observed emission from radio through the gamma-ray energies also indicates that 1ES 1727+502 has one of the most energetic particle populations known when compared to other gamma-ray galaxies. Further, it tells us that the gamma-ray emission observed by VERITAS can be derived from a jet which does not have any significant photon density in the immediate vicinity of the part of the jet which is producing the emission, such as, e.g., a dusty torus around the accretion disk, or molecular clouds farther out from the compact inner region of the jet base.

We emphasize again that bright moonlight observing time is particularly useful for studying rapidly variable gamma-ray sources such as galaxies like 1ES 1727+502. The merging of data taken under multiple observing conditions will be an important technique for future large, ground-based gamma-ray telescope arrays, such as the Cherenkov Telescope Array. VERITAS will continue to use this new observing mode to obtain the deepest observations of many gamma-ray sources yet, building an archive of different states of variable gamma-ray emitters such as the blazar 1ES 1727+502.

This is all to the good, because one thing that has been proven over and over in astronomy is that novel and unforeseen discoveries are made precisely when our instruments open up new domains of observation to us—and seeing farther and deeper into the Universe is always bound to open our senses to magic and mysteries previously unknown.