Possible sources of high energy particles

Thanks to radio and γ-ray observations, we know that supernova remnants (SNR), distinguished in shell-like and Crab-like remnants, are possible sources of high energy particles originating within the galaxy whereas cosmic rays coming from outside the Galaxy could be produced by Active Galactic Nuclei (AGN). We will give a brief overview of these astrophysical objects.

Shell-like supernova remnant

A spectacular image of a shell-like supernova, the intensively studied Cassiopeia, is shown in Fig. 1.3. X-ray photons are the result of bremsstrahlung of very hot gas associated to the interstellar medium that encounters the supernova wave blast (the blue outer ring in Fig. 1.3). Some of the elements inside the SNR are clearly visible:

silicon (red), sulfur (yellow), calcium (green) and iron (purple). They produce X-rays making possible the creation of their location maps. The radio emission is associated to the synchrotron radiation of electrons accelerated by the shock wave and it is so intense

CHAPTER 1. COSMIC RAYS IN THE UNIVERSE

Figure 1.3: X-ray image of the supernova remnant Cassiopeia. Picture taken from [4].

that the relativistic particles have to originate inside the supernova. Theγ-ray spectrum coming from shell-like SNRs, indicates that they are sources of cosmic-rays up to and above 100 TeV, given the coincidence of the γ-ray image with those from X-ray and radio wavelengths [7].

Crab-like supernova remnant

Five supernovae were observed in our galaxy in the past millennium. After their explosion they became the brightest objects in the sky: pulsars. Pulsars are spinning neutron stars with enormously strong magnetic fields. While stars typically have radii of 10⁶ km, during a supernova explosion they shrink under a gravitational collapse to a size of just about 20 km. This process leads to densities of 6·10¹³ g/cm³, where electrons and protons are close enough to produce neutrons through the weak process:

e⁻+ p→n +ν_e.

In neutron stars, neutron decay will be prevented by the Pauli principle, because all the quantum states that can be reached by the electron and the proton are already filled. One of these five supernova remnants is the Crab Nebula, a young pulsar, that was observed already from the original supernova explosion in 1054 by the astronomers of that time and that is shown in Fig. 1.4 with todays techniques. The bright central X-ray source is the young pulsar which has a pulse period of 33.2 ms and is the energy source for the nebula. Jets of material are ejected perpendicular to the disc of the pulsar, possibly producing cosmic rays. The other four are very similar to the Crab and are close to it.

Active Galactic Nuclei (AGN)

At the core of a galaxy there is in general a supermassive black hole, and in 1%

of the cases this black hole is active as it emits radio waves and is surrounded by an 14

Figure 1.4: X-ray image of the supernova remnant Crab. Picture taken from [4].

accretion disk. In this case one speaks of an active galactic nucleus and it ejects high energy particles perpendicularly with respect to the disk. A scheme is shown in Fig. 1.5.

Following the so-called₅₇₈ unified model, there are various type of AGN with the same basic10 Messengers from the High-Energy Universe

Fig. 10.36 Schematic diagram for the emission by an AGN. In the “unified model” of AGN, all share a common structure and only appear different to observers because of the angle at which they are viewed. Fromhttp://www.astro-photography.net, adapted

respect to the line of sight. In blazars, emission is modified by relativistic effects due to the Lorentz boost.

Blazars. Observationally, blazars are divided into two main subclasses depending on their spectral properties.

• FSRQs. They show broad emission lines in their optical spectrum.

• BL Lacertae objects (BL Lacs). They have no strong, broad lines in their optical spectrum. BL Lacs are moreover classified according to the energies of the peaks of their SED; they are called accordingly low-energy peaked BL Lacs (LBLs), intermediate-energy peaked BL Lacs (IBL) and high-energy peaked BL Lacs (HBL). Typically FSRQs have a synchrotron peak at lower energies than LBLs.

Blazar population studies at radio to X-ray frequencies indicate a redshift distribution for BL Lacs that seems to peak atz∼0.3, with only few sources beyondz∼0.8, while the FSRQ population is characterized by a rather broad maximum between z∼0.6–1.5.

Non-AGN Extra Galactic Gamma Ray Sources. At TeV energies, the extragalactic γray sky is completely dominated by blazars. At present, more than 50 objects have been discovered and are listed in the online TeV Catalog. The two most massive close by starburst galaxies NGC 253 and M82 are the only non-AGN sources detected at TeV energies. Only 3 radio galaxies have been detected at TeV energies (Centaurus

Figure 1.5: Scheme of an AGN in the various parts, seen from several sides from the Earth.

Picture taken from [2].

structure but depending on the jets emitting direction the name of the AGN is different.

If the observer sees only the jet emission of the AGN we speak of Blazars. As soon as one looks at the AGN less perpendicularly, the toroidal structure is visible and we speak of Quasars. Parallel to the AGN jets, the black hole is hidden and one essentially observes

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CHAPTER 1. COSMIC RAYS IN THE UNIVERSE

the jets and the associated radio emission. In this case we speak of radio Galaxies. There is an additional division into Seyfert 1 and 2 Galaxies that are connected to AGN of type I and II: type I AGN has little or no obscuration from the central source of radiation, while in type II AGN the line of sight to the central source is completely obscured. For more details see [8].

1.2.2 Second order Fermi acceleration mechanism and diffusion model