ISSN 0253-2778

CN 34-1054/N

Open AccessOpen Access JUSTC Astronomy 18 January 2023

Investigating Galactic cosmic rays with γ-ray astronomy

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https://doi.org/10.52396/JUSTC-2021-0269
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  • Author Bio:

    Ruizhi Yang is currently a Professor at the University of Science and Technology of China (USTC). He received his bachelor’s degree from USTC in 2007 and his Ph.D. degree in Astrophysics from the Purple Mountain Observatory, CAS in 2013. His research mainly focuses on high-energy astrophysics and γ-ray astronomy

  • Corresponding author: E-mail: yangrz@ustc.edu.cn
  • Received Date: 20 December 2021
  • Accepted Date: 15 June 2022
  • Available Online: 18 January 2023
  • Cosmic rays (CRs) are one of the most important components in the interstellar medium (ISM), and the origin of CRs remains a mystery. The diffusion of CRs in turbulent magnetic fields erases the information on the distribution of CR accelerators to a large extent. The energy dependent diffusion of CRs also significantly modifies the initial (acceleration) spectra of CRs. In this regard, γ-rays, the secondary products of interactions of CRs with gas and photons in the ISM, provide us with more information about the origin of CRs. More specifically, the γ-ray emissions associated with gas, can be used to study the distribution of CRs throughout the Galaxy; discrete γ-ray sources can elucidate the locations of individual CR accelerators. Here, the current status and prospects in these fields are reviewed.
    Due to the unprecedented sensitivity in the ultrahigh energy γ-ray band (red curve in lower panel), LHAASO (layout shown as the upper panel) will play a leading role in the cosmic ray study.
    Cosmic rays (CRs) are one of the most important components in the interstellar medium (ISM), and the origin of CRs remains a mystery. The diffusion of CRs in turbulent magnetic fields erases the information on the distribution of CR accelerators to a large extent. The energy dependent diffusion of CRs also significantly modifies the initial (acceleration) spectra of CRs. In this regard, γ-rays, the secondary products of interactions of CRs with gas and photons in the ISM, provide us with more information about the origin of CRs. More specifically, the γ-ray emissions associated with gas, can be used to study the distribution of CRs throughout the Galaxy; discrete γ-ray sources can elucidate the locations of individual CR accelerators. Here, the current status and prospects in these fields are reviewed.
    • γ-rays are an important tool to study cosmic rays (CRs).
    • γ-rays can be used to probe the CR accelerator and the CR distributions.
    • LHAASO and other future experiments will shed light on the origin of CRs.

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Catalog

    Figure  1.  The layout of three arrays in LHAASO. The figure is from Ref. [27].

    Figure  2.  The one year LHASSO sensitivity for $ \gamma $-rays, compared with the sensitivities of other instruments. The sensitivities of other instruments are from Refs. [21, 26, 28, 29].

    Figure  3.  The differential spectrum of several TeV-bright SNRs; the data points are from Refs. [37, 3944]. The figure is from Ref. [38].

    Figure  4.  The differential luminosities of in extended regions around the star clusters Cyg OB2 (Cygnus Cocoon), Westerlund 1, Westerlund 2, NGC 3603, as well as in the Central Molecular Zone (CMZ) of the Galactic Centre assuming that CMZ is powered by CRs accelerated in Arches, Quintuplet and Nuclear clusters. The error bars contain both the statistical and systematic errors. The differential $ \gamma $-ray luminosities, $\dfrac{{\rm d}L}{{\rm d}E} = 4\pi {\rm d}^2 Ef(E)$. The luminosities of all sources have similar energy dependence close to $ E^{1.2} $ as shown in the curve. We also show the $ \gamma $-ray spectra expected from interactions of parent proton population with a spectrum of $E^{-2.3}{\rm e}^{-\tfrac{E}{E_b}}$ with $ E_b $ = 0.2 PeV and 1 PeV, respectively. The data points are from Ref. [38].

    Figure  5.  The CR proton radial distributions in the Cygnus Cocoon, Wd 1 Cocoon and CMZ above 10 TeV. For the Cygnus Cocoon, the energy density of protons above 10 TeV is derived from the extrapolation of the Fermi-LAT $ \gamma $-ray data to higher energies. The flux reported by the ARGO collaboration at 1 TeV supports the validity of this extrapolation. For comparison, the energy densities of CR protons above 10 TeV based on the measurements by AMS-02 are also shown in Ref. [75]. The figure is from Ref. [38].

    Figure  6.  The surface brightness profile of both the TeV halo (electron) and CR continuous injection case (1/r). The figure is from Ref. [77].

    Figure  7.  Top panel: The Fermi-LAT $ \gamma $-ray counts map above 1 GeV towards Orion A. Bottom panel: The Planck dust opacity ($ \tau_{353} $) map, which is proportional to the gas column density.

    Figure  8.  The derived CR spectrum in Orion A (shaded area) compared with the direct measurement of the CR spectrum from PAMELA (data points)[84]. The figure is from Ref. [82].

    Figure  9.  The derived $ \gamma $-ray emissivities per H atom (proportional to CR density) Galactocentric distributions compared with the distribution of SNRS[85] and OB stars[86]. The CR radial distributions are derived in Ref. [59].

    Figure  10.  The energy distribution of $ \gamma $-rays in the Sgr B complex. The data points are from Refs. [93, 94].

    Figure  11.  The derived CR density and index in different position of the Galaxy using GMCs in Ref. [93].

    Figure  12.  The significance map above 25 TeV from the LHAASO-KM2A observations on LHAASO J1825-1326. The green circle in the right bottom is the point spread function of LHAASO-KM2A. The white circles are the extensions of HESS J1825-137 at lower (large) and higher (smaller) energies. The purple circle is the extension of HESS J1826-130. The two black diamonds label the position of two pulsars while the cyan ellipse is the dense molecular gas clump identified in Ref. [97]. The figure is from Ref. [98].

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