Introductory Lecture on Astrophysics Part II: non-thermal phenomena Pasquale D. Serpico
Recap & & Intro Intro Recap In Lecture I, I argued that modern astrophysicists do not study only “visible light” to infer properties of celestial bodies. They use also “invisible” photon bands, but are more and more interested in particles different from photons, too. This field of “High Energy Astrophysics” has witnessed a very rapid growth over the last decades, stimulated by the possibilites opened by the space age, but also “contamination” with particle physics technology and techniques Nonetheless, some of the key questions at its core, remain only partially answered despite a century of investigation! I ’ ll try to introduce you the main pillars of the current paradigm on these topics. Most of these topics will be covered more in detail in other Lectures, in particular by Johannes Blumer and Abelardo Moralejo, of which hopefully this represent a “natural” introduction
Non-thermal messengers Non-thermal messengers
Cosmic Rays: A : A century-old Problem century-old Problem Cosmic Rays direct observation indirect observation (EAS) A A long-standing issue is long-standing issue is the the origin origin of the of the I II III IV I II III IV non-thermal particle spectra of of “ “cosmic cosmic non-thermal particle spectra rays rays” ” hitting hitting the the Earth from outer Earth from outer space space main problem main problem: : charged particles charged particles are are deflected while propagating in the in the deflected while propagating magnetized ISM: ISM: they they do do not not track back track back magnetized to their sources to their sources! ! How to identify them How to identify them? ? Possible ways to attack the problem: Compare what observed at the Earth with theoretical models accounting for production and propagation of CRs. Try to identify the source processes via the photons (& ν ’s) emitted by the CR E-losses in/near the sources
How do do cosmic rays cosmic rays propagate? propagate? How Charged particles deflected in a in a B-field B-field. . Their Their “ “Larmor Radius Larmor Radius” ” is is Charged particles deflected r L r L B B ⊗ ⊗ Even for protons, , this distance is comparable to distance between neighboring stars this distance is comparable to distance between neighboring stars Even for protons up to ~PeV up to ~PeV and and smaller than Galactic Sizes smaller than Galactic Sizes up up to EeV to EeV. . CRs probe probe thus thus “ “small-scale inhomogeneities small-scale inhomogeneities” ” CRs in the field field, , changing changing direction direction by what appear by what appear in the “random kicks “ random kicks” ”, , similar to brownian motion similar to brownian motion Macroscopically, , this is described as this is described as “ “diffusion diffusion” ” Macroscopically Continuity Equation Continuity Equation Fick’ Fick ’s law s law For homogeneous For homogeneous medium medium properties properties, the , the flux flux can can only change only change due due to gradients to gradients
Diffusion coefficient Diffusion coefficient In a diffusive motion motion, in a time , in a time t t a a particle moves by particle moves by a a rms distance rms distance: : In a diffusive what CRs show show does not reflect necessarily present picture given by does not reflect necessarily present picture given by “ “light light” ” what CRs The functional form for D depends on the structure of magnetic field The functional form for D depends on the structure of magnetic field inhomogeneities, in turn , in turn depending depending also on the flux of also on the flux of CRs CRs themselves (non- themselves (non- inhomogeneities linear, non perturbative linear, non perturbative problem!) For example in quasi-linear theory one finds problem!) For example in quasi-linear theory one finds β =1, 3/2, 5/3... respectively for Bohm respectively for Bohm, , Kraichnan Kraichnan, , Kolmogorov turbulence spectra Kolmogorov turbulence spectra β =1, 3/2, 5/3... In general, D is parameterized as D~K E In general, D is parameterized as D~K E δ δ ; free-parameters are fitted from data ; free-parameters are fitted from data The CR propagation problem is further complicated by other processes changing The CR propagation problem is further complicated by other processes changing Energy, nuclear species (spallation/decay spallation/decay), ), “ “winds winds” ” in the Galaxy in the Galaxy… … Energy, nuclear species (
Diffusion-loss equation: : from sources to observables from sources to observables Diffusion-loss equation Source term (time, space, momentum dep dep.) .) Source term (time, space, momentum Diffusion Diffusion Includes dec Includes dec. ./frag /frag. for heavier nuclei . for heavier nuclei Energy loss Energy loss � � � � � ) � � • � t = Q + � � ( D sp � � p ( p � ) + Convection velocity Convection velocity � � ( p � 2 � ) � � p � + � � � � ) + � � � � p p 2 D mom � ( V 3 ( � � V ) � � � � � + � � � p � p � � � � � � � � � frag � decay Adiabatic flow term Adiabatic flow term Fragmentation and decay terms Fragmentation and decay terms Diffusive reacceleration Diffusive reacceleration (negligible/vanishing for protons) (negligible/vanishing for protons) Arrival directions ~isotropic! Production Site Energy Spectrum ~featurless! Production Mechanism Chemical Composition Propagation
Learning about sources via via ‘ ‘ neutral neutral ’ ’ E-loss particles E-loss particles Learning about sources The alternative strategy is to look at “potential sources” and try to learn something from visibile, straight propagating channels, at present photons from radio to γ (in principle also ν and GW) Why do we expect them to “trace” what happens to non-thermal particles? That’s because Cosmic Rays unavoidably undergo energy-loss while being confined, accelerated, and while escaping the sources. We shall then study of HE particle lose energy, and show some examples of how their “waste particles” allow one to make diagnostics
Make an educated guess about the the origin origin of of CRs CRs… … Make an educated guess about Most Galactic sources are SNRs Most Galactic sources are SNRs & PSR/PWN, most extragalactic ones AGN & PSR/PWN, most extragalactic ones AGN
Electron losses losses in in matter matter Electron Interactions with matter are mostly relevant as E-losses at “low energy”, MeV-GeV Up to hundreds of MeV, the dominant loss is by Coulomb scattering with electrons & ions and atomic ionization, using the ISM as target. These effects have little dependence on energy and are not very useful for diagnostics! On the other hand, deceleration of e when deflected in the field of a nucleus provides energetic γ as “loss channel”. Although this Bremsstrahlung process is the dominant E- loss only at GeV energies (at least in the Galaxy) it provides a useful diagnostic tool even at lower energies
Bremsstrahlung at work at work Bremsstrahlung X-ray emission from Clusters of Galaxies dominated by by Bremsstrahlung Bremsstrahlung from from X-ray emission from Clusters of Galaxies dominated hot gas filling space between galaxies, not by Galaxies themselves! gas filling space between galaxies, not by Galaxies themselves! hot The virial theorem implies ~keV temperatures for these huge bound objects! Measuring X-ray emission allows one to: - track hot gas (main baryonic component of clusters) - infer mass of the cluster (if virial theorem can be applied)
e e losses losses in in radiation fields radiation fields: Inverse : Inverse Compton Compton Scattering Scattering It is the process by which a HE e e transfers energy & momentum transfers energy & momentum It is the process by which a HE to a low energy “ to a low energy “target target” ” photon in the environment photon in the environment electron loses some energy high energy electron photon gains energy lower energy photon Final γ energy can be understood as log 10 I( ν ) “double frame change”: arbitrary unit In the HE e frame, γ of energy ε seen having ε ’~ ~ γ ε ε . ε ’ γ In this frame, if little recoil is involved, ν max / ν 0 the scattering leaves ε ’ unchanged unchanged ε ’ (not momentum direction, of course!) ν / ν 0 Back in the the Lab frame this means ε ’’~ ~ γ ε ’ ~ ~ γ ε ε ’’ γ ε ’ 2 γ 2 ε arbitrary unit E max = (h ν ) max ≈ 4 γ 2 h ν 0
e e losses losses in in B-fields B-fields: : Synchrotron radiation Synchrotron radiation Charge in a B-field moves along an “ “helix helix” ”, , Charge in a B-field moves along an an accelerated charge does radiate an accelerated charge does radiate doedds For relativistic e , the cyclotron frequency slows down by a factor Γ the radiation is beamed in a cone of size 1/ Γ a time-dilation effect exists btw e & observer frame As a result, and when performing the frame transformation, it can be shown that a beamed, continuum spectrum arises, with a peak at
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