The last two days of the 32nd General Assembly for the International Astronomical Union (IAU GA; see this bite for an overview) contained Focus Meeting 12, called: The High-Energy Gamma-ray Universe: Results and perspectives with wide-field ground-based facilities. This meeting contained highlights from many of the major operating and future ground-based gamma-ray observatories, which detect the highest energy light on the electromagnetic spectrum.
Ground-based gamma-ray observatories fill in the very high energy (VHE; E > 100 GeV) and ultra high energy (UHE; E > 100 TeV) parts of the electromagnetic spectrum. The instruments used to detect them can broadly be separated into two categories:
- Imaging atmospheric Cherenkov telescopes (IACTs), which are pointing instruments sensitive to VHE gamma-rays;
- Particle detector arrays (also called extensive air shower arrays), which scan the full sky in survey mode and are sensitive to both VHE and UHE gamma-rays.
The history and status of VHE gamma-ray astronomy
The meeting opened with a historical overview of the field by Dr. Razmik Mirzoyan, who outlined the past century of very high energy astrophysics, from the discovery of cosmic radiation by Victor Hess to the first ground-based gamma-ray telescopes in the 1960s to future observatories that will be coming online within the next few years.
Dr. Mirzoyan highlights the difficult endeavour that was the initial detection of the Crab nebula in very high energy (VHE) gamma-rays by the Whipple telescope in 1988. This detection marked the beginning of the field of ground-based gamma-ray astronomy. Though VHE gamma-ray sources are not plentiful and many decades of work go into a single detection, Dr. Mirzoyan jokes about the early days of the field,
“I remember in the ‘80s, everyone was reporting 3-4 sigma [detection significance] sources and it was a shame if you went to a conference and you didn’t have your own source. If you look at conference proceedings from back then, they were full of discoveries; on the other hand, many of them didn’t turn out to be true, but it kept the science alive and people kept working in the right direction.”
There were 49 total contributions to the session, which consisted of 31 oral presentations and 18 posters. Unfortunately, with so many contributions, it is difficult to summarize all of them in enough detail to do each presentation justice. The presentations can roughly be grouped into three categories: current experiment updates, future experiment updates, and science results. In this bite, I’ll focus mostly on experimental updates, which are the main drivers of the science we can do and will form the basis of future gamma-ray science over the next few decades.
Current instruments
Africa’s own (and only) gamma-ray observatory, H.E.S.S., an IACT located in Namibia, attended the meeting as well as the ASTRI mini-array of IACTs and particle array detectors LHAASO, HAWC, and Tibet ASγ.
The major update from H.E.S.S. is from director Prof. Stefan Wagner, who reports on H.E.S.S.’ plans to conduct more sky surveys (including extragalactic, Large Magellanic Cloudcloud, and deeper Galactic surveys), following their very successful Galactic plane survey (HGPS) that discovered lots of new Galactic gamma-ray sources. Although H.E.S.S. is technically a pointing instrument, it still boasts a 5-degree field of view and has much better angular resolution than particle detector arrays. Even after almost twenty years of operations, H.E.S.S. still has a lot of science left to do and will be starting their extragalactic sky survey soon!
Prof. Hao Zhou of the LHAASO Collaboration highlights some of the exciting work that they’ve been doing since its start of operations in 2019. In particular, he discusses the LHAASO detection of the brightest of all time (BOAT) gamma-ray burst, GRB 221009A, where they detected the highest energy photon ever! Additionally, he highlights the Cygnus star-forming region, from which LHAASO detects a lot of ultra high energy photons and discusses their viability as PeVatrons – the most powerful accelerators in our Galaxy.
HAWC’s Dr. Alberto Carramiñana presented an update which included a detection of the Sun in VHE gamma-rays for the first time, as well as a new upgraded array which will improve the detector’s sensitivity to be competitive with newer instruments.
HAWC’s Dr. Alberto Carramiñana presented an update which included a novel detection of the Sun in VHE gamma-rays (see Figure 2), as well as a new upgraded array which will improve the detector’s sensitivity to remain competitive with new instruments, even after over a decade of operations.
Future instruments
This meeting was attended by representatives of two of the most anticipated new gamma-ray instruments of the next decade, the Cherenkov Telescope Array Observatory (CTAO) and the Southern Wide Field Gamma-ray Observatory (SWGO).
SWGO is an upcoming particle detector array that will directly detect particles that come from the products that a gamma-ray splits into when it hits our atmosphere. This technique is also used by HAWC and LHAASO. SWGO’s co-spokesperson, Dr. Petra Huentemeyer, explains that SWGO will be able to detect even more particles than existing detectors (so will be able to detect much dimmer sources!) by going “higher and bigger” – i.e., moving up in altitude and having a larger collecting area of detectors. SWGO is also going to be the first particle detector array located in the Southern Hemisphere, and it’s expected to detect many new sources in our own Galaxy, which is much more easily seen in the Southern Hemisphere. They’ve just finished the design phase of the project (picking the detector types and configurations based on science goals and in this presentation, Dr. Huentemeyer announces that they’ve selected a site for the array at the Atacama Astronomical Park (where ALMA is located), which is a critical step toward starting the construction phase of the instrument.
CTAO is a huge IACT array whose northern site in La Palma, Spain, is planned to come online in 2028 and will be a huge array of IACTs that will be ~10x more sensitive than current instruments. Dr. Roberta Zanin, CTAO’s project scientist, discusses how CTAO can work together with particle array detectors, like HAWC and LHAASO, to get a really detailed and complete picture of the gamma-ray sky.
Prof. Nikolay Budnev of the Tunka Advanced Instrument for cosmic rays and Gamma Astronomy (TAIGA) collaboration also presented plans for a new low-cost hybrid particle detector array/IACT array planned for Siberia, where detectors are being deployed this summer! TAIGA plans to look for PeVatrons.
Finally, Dr. Sei Kato of the Andes Large area PArticle detector for Cosmic ray physics and Astronomy (ALPACA) collaboration between scientists in Bolivia, Japan and Mexico. ALPACA is also a Southern hemisphere particle detector array which will detect ~PeV scale gamma-rays and cosmic rays. They’ve completed a mini-array called ALPAQUITA and have been operating since 2023, with many energetic gamma-rays and cosmic rays already detected! In particular, they’re looking to understand the gamma-ray excess seen by Fermi-LAT at the Galactic center, which is an ongoing mystery in the field.
Very high energy gamma-ray sources
The remaining talks in the session covered science updates that aimed to understand the highest energy sources in our universe. For gamma-ray astronomers, detecting new sources isn’t quite enough! It’s important to understand how particles are being accelerated to produce the observed gamma-rays and if these are also possible birthplaces of ultra high energy cosmic rays.
Several of these talks investigate sources like gamma-ray binaries and supernova remnants as possible PeVatrons. Others are looking at star-forming regions to try and see if it’s feasible to make cosmic rays along with gamma-rays in these regions. Additionally, presenters discussed the broad array of sources, like GRBs, active galactic nuclei, and even unidentified sources, to try and better understand their gamma-ray emission.
Since observational gamma-ray astronomy often falls in the cracks between astrophysics and particle physics, I wasn’t expecting such a great representation of the field at an IAU meeting. The scope of this focus meeting was enormous and covered so many interesting topics in the ever-growing field of gamma-ray astronomy. Although it wasn’t possible to fully discuss each presentation or even each theme of the meeting in detail, recordings of both days are freely available on YouTube (Day 1 and Day 2).
Astrobite edited by: Sowkhya Shanbhog
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