X-Ray and Gamma-Ray Emission from the Galactic Center

The inner 10 pc of our Galaxy is a source of the very high energy γ-rays (E γ > 100 GeV) coincident with the position of the supermassive black hole Sgr A * (Acero et al. 2009).

The EGRET telescope (see Mayer-Hasselwander et al. 1998) found a gammaray flux toward the Galactic Center of the order of 2 • 10 37 erg s -1 for energies E> 500 MeV in an error circle of 0.2 degree radius.

The INTEGRAL team measured a 511 keV line of width 2.2 keV, with a flux 1.01 • 10 -3 photons cm -2 s -1 . The bulge annihilation emission is highly symmetric with an extension of 5-8 degree. (see e.g., Churazov et al. 2005).

Last Suzaku observations of Koyama et al. (2007) found a clear evidence for a hot plasma in the GC with the diameter about 20 acrminutes (i.e. ∼ 50 -60 pc). The total X-ray flux from this region in the range 2 to 10 keV is F X ∼ 2•10 36 erg s -1 , and the total energy of plasma in this region is about 3 • 10 52 erg. Such a high plasma temperature is surprising, since it could not be gravitationally confined and a very high amount of energy (∼ 10 42 erg s -1 is required to maintain the plasma outflow (see, e.g. Koyama et al. 1996)). This energy supply cannot be produced by SN explosions and other more powerful sources of energy are required to support the energy balance there.

Intensive emission in X-ray iron lines is observed from the Galactic center, which is often explained that the gas there was exposed in the past by sources of intensive X-ray emission e.g., from a supernova or from the galactic nucleus (Sunyaev et al. 1993;Koyama et al. 1996).

The IBIS/ISGRI imager on the INTEGRAL observatory detected for the first time a hard continuum X-ray emission located within 1 ′ of Sgr A* over the energy range 20-100 keV (Belanger et al. 2006).

Latter Koyama et al. (2007) also found that the continuum flux from the GC contained an additional hard component. Yuasa et al. (2008) performed analysis of Suzaku data and showed a prominent hard X-ray emission in the range from 14 to 40 keV whose spectrum is a power law with the spectral index ranging from 1.8 to 2.5. The total luminosity of the power-law component from the central region is about 4 • 10 36 erg s -1 . The spatial distribution of hard X-rays correlates with the distribution of hot plasma.

We assume that this activity of the Galactic center in different ranges of waves has common origin, namely, it is due to processes of star accretion onto the central black hole.

In Cheng et al. (2006Cheng et al. ( , 2007) ) we discussed the origin of the 511 keV annihilation flux from the GC region and production of continuum gamma-ray emission in the range E γ > 100 MeV. The origin of the 511 keV line emission from the GC region is supposed to be due to annihilation of secondary positrons generated by pp collisions. Below we present a model of X-ray and de-excitation gamma-ray line emission which can also be produced by this activity.

Energy release of black holes due to processes of accretion can be observed in different wave ranges.

X-rays. The maximum flux of X-rays can be estimated from the equation for the Eddington emissivity,

For a black hole with the mass M bh ∼ 10 6 M ⊙ it give about L Edd ∼ 10 44 erg s -1 . Flux of relativistic charged particles. Flux of relativistic particles in the form of jets (electrons or protons). The origin of relativistic protons in jets is still rather speculative but we have evidences in favour of their production near black holes (see Abraham et al. 2007;Istomin & Sol 2009). The energy carried away by relativistic protons is estimated as (see Cheng et al. 2006)

(

where η p is the conversion efficiency from accretion power into the the energy of jet motion and M * is the star mass.

Flux of subrelativistic protons. Once passing the pericenter, the star is tidally disrupted into a very long and dilute gas stream. The outcome of tidal disruption is that some energy is extracted out of the orbit to unbind the star and accelerate the debris. Initially about 50% of the stellar mass becomes tightly bound to the black hole , while the remainder 50% of the stellar mass is forcefully ejected (see, e.g. Ayal et al. 2000). The kinetic energy carried by the ejected debris is a function of the penetration parameter b -1 and can significantly exceed that released by a normal supernova (∼ 10 51 erg) if the orbit is highly penetrating (see Alexander 2005),

X and Gamma-Ray Emission

For the star capture time τ s ∼ > 10 4 years (see Alexander 2005) it gives a power input W ∼ < 3 • 10 42 erg s -1 . The mean kinetic energy per escaping nucleon is given by

where ηM * is the mass of escaping material, b is the ratio of r p -the periapse distance (distance of closest approach) to the tidal radius R T . For the blackhole mass M bh = 4.31 × 10 6 M ⊙ the energy of escaping particles is E esc ∼ 68(η/0.5) -1 (b/0.1) -2 MeV nuc

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