Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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!

share|cite|improve this question
please split this into two separate questions since they are independent of one another – Tobias Kienzler Mar 3 '11 at 12:53
@Tobias Kienzler right, done! – HDE Mar 3 '11 at 12:57
@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. – anna v Mar 3 '11 at 13:39
@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. – HDE Mar 3 '11 at 13:42
@hde :) OK then – anna v Mar 3 '11 at 15:43
up vote 2 down vote accepted

Concerning the 8-shaped bubbles around the galaxy, see

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).

share|cite|improve this answer
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"? – HDE Mar 3 '11 at 16:51
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. – Luboš Motl Mar 3 '11 at 17:40
My first thought looking at the artistic depiction of the Milky Way with the two gamma-ray bubbles was that I was staring a dz^2 atomic orbital, or near enough. I've itching to find out if this is just a coincidence made likely by the axial symmetry, or if there is perhaps some deep connection between the overall mass/energy distribution and spherical harmonics (as a wild guess I thought maybe some kind of resonance of the galaxy's angular momentum with the motions of the surrounding distribution of dark matter...). Can you perhaps tell me if I'm staring at unicorns here? – David H Dec 30 '12 at 16:54

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.