Abstract
The central parsec of the Galaxy contains dozens of massive stars with a cumulative mass loss rate of ~ 10^{-3} solar masses per year. Shocks among these stellar winds produce the hot plasma that pervades the central part of the galaxy. We argue that these stellar wind shocks also efficiently accelerate electrons and protons to relativistic energies. The relativistic electrons inverse Compton scatter the ambient ultraviolet and far infrared radiation field, producing high energy gamma-rays with a roughly constant luminosity from \~ GeV to ~ 10 TeV. This can account for the TeV source seen by HESS in the Galactic Center. Our model predicts a GLAST counterpart to the HESS source with a luminosity of ~ 10^{35} ergs/s and cooling break at ~ 4 GeV. Synchrotron radiation from the same relativistic electrons should produce detectable emission at lower energies, with a surface brightness ~ 10^{32} B^2_{-3} ergs/s/arcsec^2 from ~ THz to ~ keV, where B_{-3} is the magnetic field strength in units of mG. The observed level of diffuse thermal X-ray emission in the central parsec requires B < 300 micro-G in our models. Future detection of the diffuse synchrotron background in the central parsec can directly constrain the magnetic field strength, providing an important boundary condition for models of accretion onto Sgr A*.
Highlights
In this Letter we investigate the possibility that stellar wind shocks in the Galactic center efficiently accelerate electrons and protons to ultrarelativistic energies
Note that equation assumes UB ! Uph; if UB 1 Uph the electrons lose their energy to synchrotron radiation, not inverse Compton (IC) scattering, and the total synchrotron power would be given by equation (3) instead of equation (8); as we show B must be Շ300 mG in the context of our model, so this possibility is not realized
We do not include the predicted radiation from pion decay estimated in § 2.2 because it is much less certain, we reiterate that the g-ray flux from pion decay in the circumnuclear disk (CND) could be nonnegligible
Summary
The young stellar cluster in the central parsec of the Galaxy contains nearly two dozen luminous blue supergiants and WolfRayet stars, in addition to a larger population of lower mass O and B stars (Krabbe et al 1991; Genzel et al 2003). Shocks produced by colliding stellar winds can account for the hot (≈1–2 keV) X-ray–emitting plasma that fills the central few parsecs of the Galaxy (Quataert 2004; Rockefeller et al 2004; Cuadra et al 2005). In this Letter we investigate the possibility that stellar wind shocks in the Galactic center efficiently accelerate electrons and protons to ultrarelativistic energies. The parameter h measuring the fraction of energy in shock-accelerated electrons is adjusted to reproduce the HESS TeV luminosity (h ≈ 5 # 10Ϫ3) With this normalization the g-ray spectrum is uniquely determined, including the predicted cooling break in the GLAST band at ≈4 GeV; the flux of the synchrotron radiation depends primarily on the magnetic field strength in the central parsec. For E ! Emin, the electrons do not lose all of their energy before flowing out of the central parsec and so the IC power is suppressed, with nLn ∝ (E/Emin)1/2
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