Key Targets

Multi-purpose observations to address CTAO's themes

Key Targets

 

The key targets are multi-purpose observations designed to efficiently address the broad-ranging science questions of CTAO’s study themes.

 

Top Background Image Credit: NASA/ESA/SSC/CXC/STScI

The Galactic Centre

The region within a few degrees of the Galactic Centre (GC) is a complex and rich region, with a wide variety of known and potential high-energy emitters. The GC hosts the closest supermassive black hole, dense molecular clouds, strong star-forming activity, several supernova remnants and pulsar wind nebulae, arc-like radio structures, as well as the base of what may be large-scale galactic outflows. It is arguably one of the most studied regions of the sky in nearly every wavelength and has some of the deepest exposures with other major observatories (for example, Chandra and Spitzer).The CTAO’s deep observations of the GC region will add precise measurements at the highest energies to more accurately investigate the properties of the central source (which is coincident with the supermassive black hole Sgr A*), characterize the spectrum and morphology of the extended emission and probe the population of astrophysical particle accelerators in the region. 

 

Additionally, the GC region is the prime target for the CTAO search for the annihilation signature of dark matter particles.

 

Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is a satellite galaxy of our Milky Way, glowing with its several star-forming regions, such as the star-forming region 30 Doradus, the star cluster RMC 136, the remnant of supernova SN1987A and the puzzling 30 Dor C super-bubble. The star formation rate of the LMC is one-tenth the Milky Way, distributed in only about two percent of its volume. This activity is attested by more than 60 supernova remnants, dozens to hundreds of H II regions (clouds of luminous interstellar gas), bubbles and shells observed at various wavelengths in the LMC. 

 

The current Fermi-LAT and H.E.S.S. instruments have revealed a small number of sources, some of an uncertain nature. The CTAO will be able to cover the entire LMC with deeper exposure and sensitivity, probing the population of GeV-to-TeV (very high-energy, VHE) emitters, and their connection to the  global galaxy properties. Observations of this star-forming galaxy will address many CTAO science objectives: population studies of supernova remnants and pulsar wind nebulae; transport of cosmic rays on large scales — from their release into the interstellar medium to their escape from the system; and the search for a signature of dark matter.

 

»A field of view of eight degrees will allow CTA to survey the sky much faster and measure very extended regions of gamma-ray emission.«

The Galactic Plane

Astronomical surveys of our own Galaxy provide essential, large-scale data sets, which are the premises and provide the legacy for the Galactic science in all photon energies. The proposed Galactic Plane survey using the CTAO has the following important scientific goals: 

  

  1. Detection of hundreds of new VHE galactic sources, particularly pulsar wind nebulae and supernova remnants, to substantially increase the Galactic source count and permit the first high statistics population studies.  
  2. Discovery of new and unexpected phenomena in the Galaxy, such as new source classes and new types of transient and variable behaviour. 
  3. Discovery of new VHE gamma-ray binaries and PeVatron candidates. 
  4. Production of a multi-purpose legacy data set, comprising the complete Galactic Plane at VHE, tracing diffuse emission, as well as discrete sources. 

The CTAO plane survey will be uniform, comprehensive and much deeper than any previous gamma-ray survey, a key asset for the scientific strategy of the CTAO. 

 

Above, a simulated sky map showing the inner region (-90° < l < 90°) of the galactic plane, expected by  the CTAO Galactic Plane survey. The image comprises Galactic supernova remnants and pulsar wind nebulae as well as diffuse emission that could be detected during the 1,600 hours of observing time. Credit: CTAC

Galaxy Clusters

Galaxy clusters are expected to contain substantial populations of cosmic-ray (CR) electrons and protons accelerated by structure formation processes and by member galaxies and active galactic nuclei (AGN). The detection in several clusters of diffuse synchrotron radio emission confirms the presence of CR electrons and magnetic fields permeating the intra-cluster medium (ICM). CR protons and nuclei may play a significant role in the suppression of cooling flows in galaxy clusters, and while no direct proof for the existence of these particles in clusters exists yet, gamma rays can provide such proof unambiguously. CR protons can produce significant high-energy gamma-ray emission through interaction in cluster gas and subsequent pion decays. Clusters typically host hundreds of galaxies, some of which may be detected individually through gamma-ray observations (due to AGN and/or star-formation activity). Additionally, about 80 percent of the mass of clusters is in the form of dark matter (DM), making galaxy clusters a natural target for indirect DM searches. 

 

The CTAO will be able to detect diffuse gamma-ray emission from one of the nearest galaxy clusters, Perseus, or set stringent limits on the CR proton content, triggering a substantial revision of the current paradigm of proton acceleration and confinement in galaxy clusters. Additionally, due to the large mass of this cluster, the CTAO can significantly improve the constraints on decaying dark matter models with respect to those of Fermi-LAT from the Galactic Halo.

 

Cosmic Ray PeVatrons

The energy spectrum of cosmic rays (CRs) extends without any major feature until particle energies of a few PeVs, where it steepens into a feature called the knee. This implies our Galaxy hosts PeVatrons – extreme particle accelerators that can reach such PeV energies. However, the only known PeVatron found so far – located in the Galactic Centre and possibly connected to the supermassive black hole Sgr A* – does not provide enough power to explain the entirety of the CRs arriving on Earth. Thus, there are more Galactic PeVatrons yet to be found. 

 

The CRs accelerated by these PeVatrons interact in the interstellar medium producing gamma rays, which provide a powerful probe to search for such accelerators. The recent advances of ground-based gamma-ray astronomy have resulted in the discovery of tens of Galactic particle accelerators at energies beyond 100 TeV. Most notably, shell-type supernova remnants (SNRs) are commonly believed to supply most Galactic cosmic rays. The CTAO is proposing to carry out deep observations on known gamma-ray sources with a particularly hard spectrum and/or with hints for a possible spectral extension in the multi-TeV energy domain. The objective is to detect and analyze gamma-ray emission caused by cosmic rays at PeV energies and, with it, to identify PeVatrons. Some of the proposed targets include diffuse gamma-ray emissions from the vicinity of prominent SNRs and massive stellar clusters. 

Star Forming Systems

Cosmic rays may play a major role in the regulation of the star-formation process, so it is important to understand where they are accelerated, how they propagate and where they interact in the interstellar medium. The interaction of cosmic rays in the star-forming environments yields to the emission of gamma rays. Hence, CTAO observations of star-forming systems are expected to give insight into the relationship between high-energy particles and the star-formation process. The study of individual star-forming regions and star-forming galaxies will furthermore help to disentangle source-specific properties from global properties. For example, the CTAO will be able to measure the fraction of interacting high-energy particles as a function of the star formation rate and, hence, investigate to which extent cosmic rays, magnetic fields and radiation are in equipartition. 

 

Gamma-ray studies with the CTAO will be performed through deep observation of several classes of objects, spanning over six orders of magnitude in the star formation rate. The proposed targets include Galactic star-forming regions (the Carina nebula, Cygnus region and the Westerlund 1 stellar cluster), star-forming galaxies (Andromeda/M 31), starburst galaxies (NGC 253 and M 82) and ultra-luminous infrared galaxies (Arp 220). 

Active Galactic Nuclei

Galaxies hosting an actively accreting supermassive black hole are amongst the most luminous objects in the extragalactic sky and emit time-variable radiation across the entire electromagnetic spectrum up to multi-TeV energies, with fluctuations on time scales from many years down to a few minutes. These objects contribute a substantial fraction of the electromagnetic energy output of the Universe. Feedback from the Active Galactic Nucleus (AGN) is believed to significantly influence the evolution of the host galaxy and even galaxy cluster. It is in the TeV band that some of the most dramatic variability is seen, providing information on the very heart of the AGN. Regular and externally triggered CTAO observations of radio-loud AGN, in particular blazars and radio-galaxies, will provide an unprecedented data set relevant to the physics and phenomenology of AGN, to gamma-ray cosmology and to the study of cosmic rays and to fundamental physics. In particular TeV blazars can be used as a probe of cosmic voids, the relic background light and magnetic fields, and of space-time. 

 

AGN will be the main source class detected in the proposed blind survey of the extragalactic sky in the energy range from 100 GeV to 10 TeV. It will be the first time that such a large portion of the sky is observed uniformly in this energy range with a sensitive instrument. The major expected outcome is the unbiased study on the gamma-ray blazars and the search for new source classes. This survey will naturally connect to extragalactic survey in other wavelengths performed by actual and planned space and ground–based observatories.  

»The large collection area and high sensitivity in the wide energy range from 20 GeV up to 300 TeV will allow the CTAO to probe transient and time-variable gamma-ray phenomena in the very distant Universe with unprecedented precision.«

Transient Phenomena

The Universe hosts a diverse population of astrophysical objects, within our Galaxy as well as beyond, that explode, flare up or intensify activity in dramatic and unpredictable fashion across the entire electromagnetic spectrum and over a broad range of timescales spanning milliseconds to years. Collectively designated “transients,” many are known to be prominent emitters of high-energy gamma rays and are also likely sources of non-photonic, multi-messenger signals such as cosmic rays, neutrinos and/or gravitational waves. They are of great scientific interest, being associated with catastrophic events involving relativistic compact objects such as neutron stars and black holes that manifest the most extreme physical conditions in the Universe. The present generation of Cherenkov telescopes demonstrated the relevant impact of TeV observations on the understanding of classes of transients like gamma-ray bursts, putative neutrino emitters and gravitational wave counterparts.  

 

One of the CTAO’s greatest strengths will be its unprecedented sensitivity in VHE gamma rays for transient phenomena and short-timescale variability — far greater compared than satellite-based instruments such as Fermi-LAT. Hence, the CTAO has potential to break new ground in elucidating the physics of cosmic transients and, with its relatively large field of view, following up alerts of transient events issued by monitoring instruments, as well as discovering transients on its own. The proposed targets comprise six classes of objects: gamma-ray bursts, galactic transients, neutrino alerts, gravitational wave alerts, selected optical/radio transient factory events and serendipitous VHE transients, along with a VHE transient survey.