Gamma rays diffuse

PALS is based on the injection of positrons into investigated sample and measurement of their lifetimes before annihilation with the electrons in the sample. After entering the sample, positron thermalizes in very short time, approx. 10"12 s, and in process of diffusion it can either directly annihilate with an electron in the sample or form positronium (para-positronium, p-Ps or orto-positronium, o-Ps, with vacuum lifetimes of 125 ps and 142 ns, respectively) if available space permits. In the porous materials, such as zeolites or their gel precursors, ort/zo-positronium can be localized in the pore and have interactions with the electrons on the pore surface leading to annihilation in two gamma rays in pick-off process, with the lifetime which depends on the pore size. In the simple quantum mechanical model of spherical holes, developed by Tao and Eldrup [18,19], these pick-off lifetimes, up to approx. 10 ns, can be connected with the hole size by the relation ... 

Figure 5.3. Left. The gamma-ray emission from XX annihilation in a rich, Coma-like, nearby galaxy cluster is shown Mx = 70 — 500 GeV (from top down). The integral flux is compared to the sensitivity of ongoing and planned gamma-ray experiments, as labelled. Right. The diffuse synchrotron emission spectrum of secondary electrons produced in XX annihilation is shown to fit the Coma radio-halo spectrum the green area represent the prediction of a model in which the x annihilates predominantly into fermions, while the blue area represent the gauge-boson dominated x annihilation (from Colafrancesco Mele 2001).

There is not yet, however, a definite detection of diffuse gamma-ray emission from galaxy clusters. While there is a preliminary evidence of gamma-ray emission from a dozen bright, radio-active clusters which host powerful radio galaxies and Blazars and are associated to unidentified EGRET sources (Co-lafrancesco 2002), many of the quiet, X-ray selected clusters only have upper limits for their emission at E > 100 MeV. 

Figure 6.7. Predictions of the diffuse gamma-ray emission of Coma as expected in different models for the CR origin bremsstrahlung for = 0.3 (short dashes) and 1 (long dashes) neutralino annihilation for Mx = 100 GeV (yellow area an enhancement factor 3 has been choosen) and p-p collision (blue area) (from ).

After losing their kinetic energy the penetrated positrons may either directly annihilate with surrounding electrons into two gamma rays, or combine with an electron to form a Ps atom. Although both positrons and Ps are known to localize within the free volumes, a certain fraction of them may diffuse back to the surface and escape to the vacuum. The probability of positrons and Ps annihilating in the polymer depends on their diffusion coefficients. 

The implication of Eq. 17 is that for E1 70 MeV the diffuse gamma-ray spectrum should have the same power law behavior as the proton spectrum, a ss 2.7. What is observed, however, is that the spectrum of gamma-rays from the inner galaxy is harder than this, having a power-law behavior of approximately E 2A (Hunter et al., 1997). This is currently not fully understood. One possibility is that the cosmic-ray spectrum producing the gamma rays is harder than observed locally near Earth (Hunter et al., 1997). 

The Milagro detector s large field of view and continuous duty cycle make it an ideal instrument for the discovery of previously unknown sources. Recent publications cover topics including detection of the Crab Nebula[l], limits on TeV emission from GRB [2] and a TeV all-sky survey of the northern celestial hemisphere[3]. Recently we have presented papers on the detection of diffuse TeV emission from the Galactic plane[4], limits on TeV emission from satellite detected GRB[5], a study of nearby AGN[6] and limits on relic neutralino annihilation derived from TeV flux limits from the sun[7]. The focus of this paper is the search for extended sources of TeV gamma rays with the Milagro detector. 

The optimal square bin size for detection of a point gamma-ray source with Milagro is 2.1° on a side corresponding to an angular resolution of 0.75 °. To search for diffuse gamma-ray sources with angular extent greater than or equal... 

Presented here is evidence for the first detection of TeV emission from the galactic plan and two previously unknown sources of TeV gamma rays, one coincident with 3EG J0520+2556, and another coincident with the Cygnus Arm. The new sources are found to be diffuse. The two source candidates have probabilities of 5.9a and 5.5a respectively. When all the trials of the all-sky search with a range of 20 bin sizes are counted the probabilities of ob-... 

Discovery of Diffuse Gamma Ray Emission from the Galactic Plane using the Milagro Detector, Heidelberg Workshop 2004. 

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