Multi-Zone Modeling of Spatially Multi-Zone Modeling of Spatially Non-uniform Cosmic Ray SourcesNon-uniform Cosmic Ray Sources

Armen Atoyan

Concordia University, Montreal

FAA60Barcelona, 7 November 2012

Multi-zone modeling of nonthermal radiation of Cas A in collaboration with F. Aharonian, R. Tuffs and H. Voelk.

(Paper-I: A&A 354, p.915; Paper-II A&A 355, p.211; 2000)

Basic motivation of the study (done in 1998-1999): VHE fluxes to expect from Cas A known as a prominent source of

synchrotron radiation (by that time not detected yet in -rays).

Main results : for Cas-A :

prediction of -ray fluxes that could be expected, and what it would mean for the origin (leptonic vs hadronic )

General:Multi-zone model for CR sources that allows

• (a) modification of CR energy spectra on timescales t < tloss . (b) relaxing constrains on IC -ray fluxes from synchrotron X-rays

(e.g. for Tycho).

(Stage et al. 2006)

Chandra X-rays 0.5-6 keV

Chandra -4-6 keV

(Uchiyama & Aharonian, 2008)

Cas A radio map

Likely acceleration sites:bow shocks, bright radio ring & knots in the shell,(flux variability on ~yrs scale detected from knots in radio, and recently also in X-rays)

Single-zone approach: cannot explain synchrotron (X-rays vs synchrotron self-absorption), problems with breamsstahlung

Spectra for B=100 G, 350 G & 700 G (Paper I)

Multi-zone modeling: simplest case - two zones. Basic assumptions: (a) the source consists of compact regions with high B field (zone 1), and much larger region with lower B field (zone 2).

(b) particles are accelerated (mostly) in compact zone 1, but can escape into zone 2 (by diffusion and/or convection with plasma)

Equations derived for the two zone-model:

Possibility for steepening of the energy spectra in zone 1 by diffusive propagation, D(E) ~ E ; hard spectra in zone 2.

Energy distributions of electrons in zone 1 and zone 2

(a)Spectral energy density

(b)Total energy distributions

Spectral modifications by diffusion: possible only if the energy densities in zone 1 and zone 2 are different (!)

Radiation spectra

Two-zone model:B1 = 0.4 mG, B2 =1.5 mG

Ecut = 35 TeV

Secular decline of radio(assuming B-field in zone 1 declines on timescales ~ 130 yr)

Model predictions for -rays

ICompton (target - CMB and FIR) Bremsstrahlung (ngas ~ 10-15 pcc)

+ pp (hadronic) (Wp ~ 2 1049 ergs)

TeV was later on detected (HEGRA, MAGIC, VERITAS)The spectrum is much harderthan expected for leptonic origin.

Protons accelerated to ~200 TeV explain the data.

Data: HEGRA 2001, VERITAS 2010

Tycho SNR: type Ia

Radio (NRAO) X-rays (Chundra)

Two-zone model for Tycho(Atoyan & Dermer, 2012)

Difference: acceleration mostly in the rim,lower B field, fast escape into zone II (no spectral modifications)

Model parameters:B1 = 100G, B2 =34 GEcut = 40 TeV d = 2.8 kpc

Gamma-rays: Leptonic vs Hadronic Models

Leptonic: bremsstrahlung (dashed) + Compton (solid); (contribution from zone 1 only ~10%)

Conclusion: neither leptonic nor hadronic origins can be excluded at this time.Detection of a cutoff at low energies could help confirm pp.

Wp ~ 2 1049 ergs

Ee ~ 4.6 1048 ergs

Ep ~ 3 1049

(no = 0.6 pcc)

Conclusions

Single-zone model can miss most of the electrons which may accumulate in radio-dim regions with low B field.

Multi-zone modeling of sources is need for robust predictions.