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How could we measure high energy photons, whithout measuring them ?

I can't understand how we can "see" those Gamma Ray Bubbles if they are not reaching here

In this graph from Nasa you can see those "bubbles" are not reaching solar system:

Art about bubles

Then how could be measure that Gamma Ray without the Gamma Rays

Thanks for any answer!

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  • $\begingroup$ please split this into two separate questions since they are independent of one another $\endgroup$ Commented Mar 3, 2011 at 12:53
  • $\begingroup$ @Tobias Kienzler right, done! $\endgroup$
    – HDE
    Commented Mar 3, 2011 at 12:57
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    $\begingroup$ @HDE I went to the archive link that Lubos provided below, and I cannot understand why you think they are not measuring photons from there. The system measures converted into e+e- pairs gamma rays in GeV. Also one "sees" the bubbles without their envelope reaching here, in a similar way that you see the sun, without being cooked in the heliosphere. The detector detects the gammas that reach its angular opening. $\endgroup$
    – anna v
    Commented Mar 3, 2011 at 13:39
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    $\begingroup$ @anna v, we have a causality problem here, I can't read Lubos answer before doing the question, so if you take account time, or even some causal order, you will understand it. $\endgroup$
    – HDE
    Commented Mar 3, 2011 at 13:42
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    $\begingroup$ @hde :) OK then $\endgroup$
    – anna v
    Commented Mar 3, 2011 at 15:43

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Concerning the 8-shaped bubbles around the galaxy, see

http://motls.blogspot.com/2010/11/fermi-milky-way-cutting-x-ray-infinity.html
http://arxiv.org/abs/arXiv:1005.5480

They're not pictures of photons - X-rays themselves. The infinity symbol is a picture of X-ray sources: we are observing the X-rays that came from those sources here. Note that the whole structure is smaller than 100,000 light years or so - very tiny when compared to the cosmological distances. So if the 8-shaped sources were created 10 million years ago, the time needed for the photons to get here is negligible. They're here "immediately".

It's hard to measure the distance from which an X-ray is approaching us. However, you should understand that the Sun and the Earth are not in the middle of the Milky Way. They're not in the middle of the 8-shaped figure. We're looking at the situation from the "side" (the Solar System is somewhere between the center and the visible edge of our Galaxy) so we literally see something that is 8-shaped in the skies. Assuming that the distribution of the sources is rotationally symmetric - with respect to the Milky Way's axis - one can actually reconstruct the shape of the sources in 3D from the 2D picture we see (because the 3D picture only depends on 2 dimensions, because of the axial symmetry).

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  • $\begingroup$ Gamma and X-rays are close in frequency, it's said X are emitted by electrons and gamma by the nucleus (but we know nucleus can even emit electrons so no much sense to make a difference), simplifying : both are electromagnetic waves with photons. Why not get both here? Are you saying is that those gamma ray are not reaching here But what we are measuring are some X ray from there and calculating the distance of the source we get those "static" bubbles, very confused about why are they called "gamma ray bubbles"? $\endgroup$
    – HDE
    Commented Mar 3, 2011 at 16:51
  • $\begingroup$ Hi, the interior of the bubbles is emitting gamma rays. I have probably misstated the statement. The edges exhibit some X-rays, too. This is just how the things are. X-rays and gamma rays are two (intervals of) frequencies or colors, if you wish, and the bubbles have variable colors, depending on the direction. The Milky Way is transparent so any photon below the GZK cutoff that is emitted gets here. $\endgroup$ Commented Mar 3, 2011 at 17:40

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