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Register a new userScience Drivers for AGN Observations with the Cherenkov Telescope Array
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Henric Krawczynski
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The current generation of Imaging Atmospheric Cherenkov Telescopes (IACTs), including the H.E.S.S., MAGIC, and VERITAS telescope arrays, have made substantial contributions to our knowledge about the structure and composition of the highly relativistic jets from Active Galactic Nuclei (AGNs). In this paper, we discuss some of the outstanding scientific questions and give a qualitative overview of AGN related science topics which will be explored with the next-generation Cherenkov Telescope Array (CTA). CTA is expected to further constrain the structure and make-up of jets, and thus, to constrain models of jet formation, acceleration, and collimation. Furthermore, being the brightest well-established extragalactic sources of TeV {gamma}- rays, AGNs can be used to probe the EBL, intergalactic magnetic fields, and the validity of the Lorentz Invariance principle at high photon energies.
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The Cherenkov Telescope Array, CTA, will be the major global observatory for very high energy gamma-ray astronomy over the next decade and beyond. The scientific potential of CTA is extremely broad: from understanding the role of relativistic cosmic particles to the search for dark matter. CTA is an explorer of the extreme universe, probing environments from the immediate neighbourhood of black holes to cosmic voids on the largest scales. Covering a huge range in photon energy from 20 GeV to 300 TeV, CTA will improve on all aspects of performance with respect to current instruments. The observatory will operate arrays on sites in both hemispheres to provide full sky coverage and will hence maximize the potential for the rarest phenomena such as very nearby supernovae, gamma-ray bursts or gravitational wave transients. With 99 telescopes on the southern site and 19 telescopes on the northern site, flexible operation will be possible, with sub-arrays available for specific tasks. CTA will have important synergies with many of the new generation of major astronomical and astroparticle observatories. Multi-wavelength and multi-messenger approaches combining CTA data with those from other instruments will lead to a deeper understanding of the broad-band non-thermal properties of target sources. The CTA Observatory will be operated as an open, proposal-driven observatory, with all data available on a public archive after a pre-defined proprietary period. Scientists from institutions worldwide have combined together to form the CTA Consortium. This Consortium has prepared a proposal for a Core Programme of highly motivated observations. The programme, encompassing approximately 40% of the available observing time over the first ten years of CTA operation, is made up of individual Key Science Projects (KSPs), which are presented in this document.
The Cherenkov Telescope Array (CTA) observatory will probe the non-thermal universe above 20 GeV up to several hundreds of TeV with a significant improvement in sensitivity and angular resolution compared to current experiments. Its outstanding capabilities will allow to increase the number of extragalactic cosmic accelerators detected at very high energy (VHE) and therefore to better constrain the population of VHE accelerators and the gamma-ray absorption processes in the intergalactic medium. For the first time in the history of imaging atmospheric Cherenkov telescopes (IACTs), CTA will be an open observatory and high-level data will be made available to the astronomical community. Gammapy is an open-source Python package developed by the Cherenkov telescope community that provides tools to simulate the gamma-ray sky and analyse IACT data. The versatile architecture of, and steady user contributions to Gammapy enable a large variety of high-level data analyses. Examples of Gaammapy applications are presented, particularly in the context of extragalactic science with CTA.
Surveys open up unbiased discovery space and generate legacy datasets of long-lasting value. One of the goals of imaging arrays of Cherenkov telescopes like CTA is to survey areas of the sky for faint very high energy gamma-ray (VHE) sources, especially sources that would not have drawn attention were it not for their VHE emission (e.g. the Galactic dark accelerators). More than half the currently known VHE sources are to be found in the Galactic plane. Using standard techniques, CTA can carry out a survey of the region |l|<60 degrees, |b|<2 degrees in 250 hr (1/4th the available time per year at one location) down to a uniform sensitivity of 3 mCrab (a Galactic Plane survey). CTA could also survey 1/4th of the sky down to a sensitivity of 20 mCrab in 370 hr of observing time (an all-sky survey), which complements well the surveys by the Fermi/LAT at lower energies and extended air shower arrays at higher energies. Observations in (non-standard) divergent pointing mode may shorten the all-sky survey time to about 100 hr with no loss in survey sensitivity. We present the scientific rationale for these surveys, their place in the multi-wavelength context, their possible impact and their feasibility. We find that the Galactic Plane survey has the potential to detect hundreds of sources. Implementing such a survey should be a major goal of CTA. Additionally, about a dozen blazars, or counterparts to Fermi/LAT sources, are expected to be detected by the all-sky survey, whose prime motivation is the search for extragalactic dark accelerators.
The Cherenkov Telescope Array (CTA) will be the major global observatory for VHE gamma-ray astronomy over the next decade and beyond. It will be an explorer of the extreme universe, with a broad scientific potential: from understanding the role of relativistic cosmic particles, to the search for dark matter. Covering photon energies from 20 GeV to 300 TeV, and with an angular resolution unique in the field, of about 1 arc min, CTA will improve on all aspects of the performance with respect to current instruments, surveying the high energy sky hundreds of times faster than previous TeV telescopes, and with a much deeper view. The very large collection area of CTA makes it an important probe of transient phenomena. The first CTA telescope has just been inaugurated in the Canary Islands, Spain, and as more telescopes are added in the coming years, scientific operation will start. It is evident that CTA will have important synergies with many of the new generation astronomical and astroparticle observatories. In this talk we will review the CTA science case from the point of view of its synergies with other instruments and facilities, highlighting the CTA needs in terms of external data, as well as the opportunities and strategies for cooperation to achieve the basic CTA science goals.
Misaligned AGN (MAGNs), i.e., radio-loud AGNs with the jet not pointing directly towards us, represent a new class of GeV emitters revealed by the Fermi space telescope. Although they comprise only a small fraction of the high-energy sources, MAGNs are extremely interesting objects offering a different perspective to study high-energy processes with respect to blazars. The aim of this work is to evaluate the impact of the new-generation Cherenkov Telescope Array (CTA) on the MAGN class and propose possible observational strategies to optimize their detection.
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