High Energy emission from the Galactic Center

Jason Ybarra

The Galactic Center

 Contains a supermassive black hole M = 3.6 × 10 6 M   3 main radio sources Sgr B, SgR C, SgR A  Very dense molecular clouds  Supernova remnants

Observations

 INTEGRAL IBIS/ISGRI  (20-400 keV) HESS (0.1-20 TeV)

INTEGRAL observations

 IBIS/ISGRI imager  4.6 Ms total exposure time for observations between 2003-2004  Range 20-400 keV (Bélanger et al 2006)

INTEGRAL

IBIS/ISGRI mosaic of GC in in the 20–40 keV range. (Bélanger et al 2006)

20-30 keV 40-56 keV

INTEGRAL

(Bélanger et al 2006) 30-40 keV 56-85 keV

Spectrum of IGR J17456−2901

Red is the ISGRI data. Green is a power law fit with index Γ = 3.04 ± 0.08

(Bélanger et al 2006)

Spectrum of IGR J17456−2901

1-10 keV from XMM-Newton. 20-400 keV from ISGRI

Power law Γ 2 =3.22

Spectrum High-temp plasma (6.6 keV) Power law Γ=1.51

Low temp plasma (1 keV) 6.4 keV Fe line

(Bélanger et al 2006)

Spectrum of IGR J17456−2901

The two-temperature plasma component does a decent job of modeling the 1-10 keV spectrum, but cannot account for the emission flux > 20 keV (Bélanger et al 2006)

Possible Sources?

 X-Ray Transients  Sgr A* flares  Charged-Particle Acceleration

X-Ray Transients

   Large number of X-Ray transients near Sgr A* 4 within 30″ of Sgr A* Light curves were constructed from Chandra and XMM Newton data INTEGRAL CXOGC J174535.5−290124 CXOGC J174540.0−290005 CXOGC J174540.0−290031 CXOGC J174538.0−290022 (Bélanger et al 2006)

X-Ray Transients

   Contemporaneous XMM-Newton data for J174540.0−290031 Estimated flux ~ 5 x 10 34 erg s -1 is still an order of magnitude too low.

Spectrum of transient unlikely to be a pure power law > 100 keV IGR J17456-2901 CXOGC J174535.5−290124 CXOGC J174540.0−290005 CXOGC J174540.0−290031 CXOGC J174538.0−290022 (Bélanger et al 2006)

Possible Sources?

 X-Ray Transients  Sgr A* flares  Charged-Particle Acceleration  Sgr A East

X-Ray Flares

 Flares occur on average once per day  Average L ~ 10 35 thousand seconds.

ergs s -1 , but last for a few  The constant luminosity of IGR J17456-2901 cannot result from successive flares (Bélanger et al 2006)

Possible Sources?

 X-Ray Transients  Sgr A* flares  Charged-Particle Acceleration

Charged Particle Acceleration

 Perhaps same origin as the HESS TeV source  The TeV emission is thought to come from the acceleration of particles (protons) to very high energies (Bélanger et al 2006)

Proton-Proton Collisions

Accelerated protons are thought to collide with ambient protons p p

Proton-Proton Collisions

Accelerated protons are thought to collide with ambient protons p p This interaction produces neutral pions p π 0 p

Proton-Proton Collisions

p p p p Neutral pions decay very quickly into two gamma rays

γ

π 0

γ

Proton-Proton Collisions

p p p n π +

Proton-Proton Collisions

p p p n π + μ +

ν μ

Proton-Proton Collisions

p p p Secondary electrons and positrons can produce gamma-rays through bremsstrahlung or inverse Compton scattering

ν e

e + n μ +

ν μ

π +

ν μ

Diffuse Emission

 1) 2) Belanger et al. (2006) argue that the emission is diffuse Absence of variability Not detected by JEM-X (3′ resolution)

HESS

 High Energy Stereoscopic System (HESS)  This is an array of 4 atmospheric Cherenkov Telescopes

HESS

 High Energy Stereoscopic System (HESS)  This is an array of 4 atmospheric Cherenkov Telescopes

 HESS detected a point like source of very-high energy gamma rays a the galactic center (HESS J1745-290).

SNR/Pulsar Wind Nebula Galactic center (Aharonian et al 2006)

  The white contours indicate molecular gas traced by CS emission The correlation between molecular material and the faint γ-ray emission indicates cosmic ray origin SNR/Pulsar Wind Nebula Galactic center (Aharonian et al 2006)

(Aharonian et al 2006)

Energy distribution

  The diffuse material exhibits the same power law index as HESS J1745-290 This suggests that J1745 290 is the source of cosmic rays that slowly diffuse out (Aharonian et al 2006)

What can accelerate the particles?

 1) 2) Two possibilities: Supernova Remnant Sgr A East SMBH Sgr A*

Inverse Compton scattering

 If a high-energy photon and a low-energy electron interact, the electron receives energy  If a low-energy photon and a high-energy electron interact, the photon will increase it energy.

Inverse Compton scattering

Average energy lost by the photon ΔE γ /E γ = - E γ / m e c 2 Average energy gained by the photon ΔE γ /E γ = 4/3 β 2 γ 2 ΔE γ /E γ = 4/3 β 2 γ 2 - E γ / m e c 2

Inverse Compton Scattering

(Hinton & Aharonian 2007)

Inverse Compton Scattering

The magnetic field strength is fixed at 105 μG (Hinton & Aharonian 2007)

Inverse Compton

Solid line – very young source with B = 50μG, electron spectrum α =0.3

Dashed line – old source B = 110 μG, α =1.5

(Hinton & Aharonian 2007)

Dark Matter Annihilation

Green - Minimal Supersymmetric Standard Model annihilation of 14 TeV neutralinos

Dark Matter Annihilation

Blue – mixed final state, DM masses 6-30 TeV

Summary

 1.

2.

Two leading theories Gamma rays from accelerated particle interactions (p-p → p + p + π 0 , π 0 → 2γ ) Inverse Compton scattering

References

 Aharonian et al (HESS Collaboration) 2006, PRL, 97, 221102  Belanger et al 2006, ApJ, 636, 275  Hinton, J. A. & Aharonian F. A. 2007, ApJ, 657, 302

Diffuse TeV Emission from the Galactic Center

Jason Ybarra High Energy Astrophysics Seminar April 30, 2008

Previously …

 HESS detected a point-like source of very-high energy gamma rays at the galactic center (HESS J1745-290).

SNR/Pulsar Wind Nebula Galactic center (Aharonian et al 2006)

SNR/Pulsar Wind Nebula  The white contours indicate molecular gas traced by CS emission  The correlation between molecular material and the faint γ-ray emission indicates cosmic ray origin (Aharonian et al 2006) Galactic center

Energy distribution

 The diffuse material exhibits the same power law index as HESS J1745-290  This suggests that J1745-290 is the source of cosmic rays that slowly diffuse out (Aharonian et al 2006)

Can protons accelerated by Sagittarius A* account for the diffuse emission seen by HESS?

Simulations

 Distribution of molecular clouds  Magnetic field modeled with Kolmogrov turbulence

Wommer et al. 2008 (arXiv:0804.3111v1 [astro-ph] 18 Apr 2008)

Energy loss rates

 p-p scattering  p γ scattering  Synchrotron cooling  Compton scattering (Wommer et al. 2008)

Energy loss rates

p-p scattering Compton scattering Synchrotron cooling

Cooling rates within the clouds

(Wommer et al. 2008)

Energy loss rates

p-p scattering Compton scattering Synchrotron cooling

Cooling rates between the clouds

(Wommer et al. 2008)

Proton propagation

 F = qv × B  Diffusion equation  1,000 protons were followed with the Lorentz force equation in order to determine diffusion coefficients (Wommer et al. 2008)

Proton Distribution

(Wommer et al. 2008)

Simulated Gamma-ray intensity map

Intensity assuming Sagittarius A* as source of relativistic protons, B ~ 10 μG (Wommer et al. 2008)

Simulated Gamma-ray intensity map

Intensity assuming Sagittarius A* as source of relativistic protons matched to intensity range of HESS (Wommer et al. 2008)

Simulated Gamma-ray energy map

 Diffuse emission from Sagittarius A* in simulation extends only a fraction of a degree  Diffusion from galactic center is too slow to account for emission beyond a fraction of a degree  Morphology is inconsistent with HESS data (Wommer et al. 2008)

Simulated Gamma-ray intensity map – multiple injection sites

Intensity assuming 5 distinct sources of protons (from HESS observations), B = 10 μG, intensity range of HESS (Wommer et al. 2008)

Simulated Gamma-ray intensity map – multiple injection sites

Intensity assuming 5 distinct sources of protons (from HESS observations), B = 100 μG, intensity range of HESS (Wommer et al. 2008)

Simulated Gamma-ray emission map – multiple injection sites

 Emission is concentrated at the injection sites  Morphology is centrally peaked and inconsistent with HESS data (Wommer et al. 2008)

Simulated Gamma-ray intensity map

Protons injected throughout the inter-cloud medium accelerated through second-order Fermi acceleration, B ~ 10 μG, intensity range of HESS (Wommer et al. 2008)

Simulated Gamma-ray intensity map – protons accelerated throughout inter-cloud medium

 Protons accelerated throughout the inter cloud medium by second-order Fermi acceleration can produce a diffuse emission consistent with the HESS data (Wommer et al. 2008)