APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

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APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby APOD Robot » Fri Apr 29, 2016 4:06 am

Image Fermi's Gamma-ray Moon

Explanation: If you could only see gamma-rays, photons with up to a billion or more times the energy of visible light, the Moon would be brighter than the Sun! That startling notion underlies this novel image of the Moon, based on data collected by the Fermi Gamma-ray Space Telescope's Large Area Telescope (LAT) instrument during its first seven years of operation (2008-2015). Fermi's gamma-ray vision doesn't distinguish details on the lunar surface, but a gamma-ray glow consistent with the Moon's size and position is clearly found at the center of the false color map. The brightest pixels correspond to the most significant detections of lunar gamma-rays. Why is the gamma-ray Moon so bright? High-energy charged particles streaming through the Solar System known as cosmic rays constantly bombard the lunar surface, unprotected by a magnetic field, generating the gamma-ray glow. Because the cosmic rays come from all sides, the gamma-ray Moon is always full and does not go through phases. The first gamma-ray image of the Moon was captured by the EGRET instrument onboard the Compton Gamma-ray Observatory, launched 25 years ago.

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby Ann » Fri Apr 29, 2016 5:08 am

APOD Robot wrote:
Why is the gamma-ray Moon so bright? High-energy charged particles streaming through the Solar System known as cosmic rays constantly bombard the lunar surface, unprotected by a magnetic field, generating the gamma-ray glow. Because the cosmic rays come from all sides, the gamma-ray Moon is always full and does not go through phases. The first gamma-ray image of the Moon was captured by the EGRET instrument onboard the Compton Gamma-ray Observatory, launched 25 years ago.


Okay.

Sorry. I couldn't resist. It's not that I don't find the gamma-ray bright Moon interesting, even though I already knew about its gamma-ray brightness and the reason for it (the Moon's lack of a magnetic field). But most people probably haven't heard about the gamma-ray bright Moon, and it is a good thing if the Astronomy Picture of the Day can educate people.

It's just that I'm underwhelmed by the picture. Yes, I know that this is what scientific pictures look like, except that this one has been colorized yellow and red, for the benefit of people who like to look at yellow and red. (And possibly for clarity, though I'm not so sure about that.)

I guess the message of this APOD is that it is good that the Earth has global magnetic field, and since a magnetic field may also be the prerequisite for a healthy atmosphere, it is good that the Earth has a magnetic field that gives our favorite planet a robust atmosphere. (Of course, Venus has a whopper of an atmosphere, which is so thick that it prevents the habitality of life as we know it on our sister planet. Yet Venus does not have much of a magnetic field, or at least I don't think so, so it's a mystery why it has such an enormous atmosphere.)

Oh well. I guess the lesson that can be learned from today's APOD is that it is good that the Earth is the Earth.

Okay.

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby geckzilla » Fri Apr 29, 2016 5:18 am

Ann wrote:. . . except that this one has been colorized yellow and red, for the benefit of people who like to look at yellow and red. (And possibly for clarity, though I'm not so sure about that.

Look at the intensity bar under the picture. This could be a graph straight out of a scientific paper and it probably is. And yes, the colorization makes it much easier to match the intensity scale to the picture.

Edit: Not sure if intensity can be interchangeably used with sigma/significance, here. Probably not? If not, I goofed.
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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby alter-ego » Fri Apr 29, 2016 5:36 am

Ann wrote:. . . except that this one has been colorized yellow and red, for the benefit of people who like to look at yellow and red. (And possibly for clarity, though I'm not so sure about that.

This is from the linked paper:

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby alter-ego » Fri Apr 29, 2016 6:19 am

geckzilla wrote:...
Look at the intensity bar under the picture. This could be a graph straight out of a scientific paper and it probably is. And yes, the colorization makes it much easier to match the intensity scale to the picture.
Edit: Not sure if intensity can be interchangeably used with sigma/significance, here. Probably not? If not, I goofed.

The graphic is a time-integrated, photon count distribution. I think the higher the σ, the more the photons are localized to the pixel coordinates. Looking at all the pixels you end up with a low-resolution, 3°blob of gamma rays centered on the Moon with more photons nearer the center than 1°away. So in this context, an "intensity bar" is probably a reasonable analogy, though maybe not linear with the σ scale. Still, one gets the desired impression of a concentrated source of gamma rays.
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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby heehaw » Fri Apr 29, 2016 8:49 am

So tell me again why the Sun is fainter? Isn't the Sun bombarded with cosmic rays too? Of course the Sun is also the source of some cosmic rays.

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby GuLi » Fri Apr 29, 2016 8:52 am

APOD wrote
[cut] up to a billion or more times [cut]


which sounds "as heard on TV".

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby geckzilla » Fri Apr 29, 2016 9:02 am

GuLi wrote:APOD wrote
[cut] up to a billion or more times [cut]

which sounds "as heard on TV".

Don't mess with gamma rays. They're seriously energetic. There's a reason people don't go near the Chernobyl site and why astronauts need shielding from them!

(Although apparently humans are more harmful to wildlife than gamma rays, given the environmental resurgence around Chernobyl. Imagine that.)
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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby neufer » Fri Apr 29, 2016 1:24 pm

heehaw wrote:
So tell me again why the Sun is fainter? Isn't the Sun bombarded with cosmic rays too?
Of course the Sun is also the source of some cosmic rays.

It is the nuclei of heavy elements like iron, titanium and marigold
that emit high energy gamma-rays when stimulated by cosmic rays.

http://www.lpi.usra.edu/lunar/missions/ ... gamma_ray/

https://en.wikipedia.org/wiki/The_Effec ... _Marigolds wrote:
<<The Effect of Gamma Rays on Man-in-the-Moon Marigolds is a play written by Paul Zindel, a playwright and science teacher. The play revolves around the dysfunctional family consisting of single mother Beatrice and her two daughters, Ruth and Tillie, who try to cope with their abysmal status in life. The play is a lyrical drama, reminiscent of Tennessee Williams' style.

Shy Matilda "Tillie" Hunsdorfer prepares her experiment, involving marigolds raised from seeds exposed to radioactivity, for the science fair. She is, however, constantly thwarted by her mother Beatrice, who is self-centered and abusive, and by her extroverted and unstable sister Ruth, who submits to her mother's will. Over the course of the play, Beatrice constantly tries to stamp out any opportunities Tillie has of succeeding, due to her own lack of success in life. As the play progresses, the paths of the three characters diverge: Tillie wins the science fair through perseverance; Ruth attempts to stand up to her mother but has a nervous collapse at the end of the play, and Beatrice—driven to the verge of insanity by her deep-seated enmity towards everyone—kills the girls' pet rabbit Peter and ends up wallowing in her own perceived insignificance. Despite this, Tillie (who is much like her project's deformed but beautiful and hardy marigolds) secretly continues to believe that everyone is valuable.>>
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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby Chris Peterson » Fri Apr 29, 2016 2:29 pm

Ann wrote:Yes, I know that this is what scientific pictures look like, except that this one has been colorized yellow and red, for the benefit of people who like to look at yellow and red. (And possibly for clarity, though I'm not so sure about that.)

Very much for clarity. The reason that intensity (or other single-channel values) is often mapped to a false-color table is because our eyes can see a much wider range of colors than we can a range of grays (white to black). So false-color intensity maps allow us to see many more intermediate intensity levels than we could see in grayscale alone.
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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby pferkul » Fri Apr 29, 2016 2:54 pm

The cited paper "http://arxiv.org/pdf/1604.03349v2.pdf" has 115 authors! Just a bit absurd...

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby Chris Peterson » Fri Apr 29, 2016 3:02 pm

pferkul wrote:The cited paper "http://arxiv.org/pdf/1604.03349v2.pdf" has 115 authors! Just a bit absurd...

While superficially it might seem so, this is common with "big science" papers- results that come from billion dollar projects with thousands of scientists involved. There really can be hundreds of researchers involved in all the little pieces that have to come together in a paper like this.
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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby MarkBour » Fri Apr 29, 2016 4:21 pm

So, the Fermi-LAT is capable of looking into space and focusing in on the gamma-ray portion of the spectrum. The APOD (caption) is saying that from Earth orbit, the Moon is brighter in this band than the Sun. What about absolute luminosity? Is the Moon only brighter to Fermi-LAT because it is so close? Or would an instrument sitting on Kepler 22-b see our Moon as brighter than our Sun in this band? If so, would it be able to see our Moon as brighter than the surrounding background radiation? If so, then vice versa ... could Fermi-LAT ever detect an exoplanet?
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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby BMAONE23 » Fri Apr 29, 2016 4:59 pm

geckzilla wrote:
GuLi wrote:APOD wrote
[cut] up to a billion or more times [cut]

which sounds "as heard on TV".

Don't mess with gamma rays. They're seriously energetic. There's a reason people don't go near the Chernobyl site and why astronauts need shielding from them!

(Although apparently humans are more harmful to wildlife than gamma rays, given the environmental resurgence around Chernobyl. Imagine that.)


Humans are truly only more harmful when they allow their Hubris to dictate over common sense

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby pferkul » Fri Apr 29, 2016 5:01 pm

Chris Peterson wrote:
pferkul wrote:The cited paper "http://arxiv.org/pdf/1604.03349v2.pdf" has 115 authors! Just a bit absurd...

While superficially it might seem so, this is common with "big science" papers- results that come from billion dollar projects with thousands of scientists involved. There really can be hundreds of researchers involved in all the little pieces that have to come together in a paper like this.


I understand this is the current (unfortunate) practice. But it is better to acknowledge them in some other way, rather than claim they authored the paper. There is a big distinction! And why stop at 115 authors? Why not include their families, teachers, friends, supporters, benefactors, vendors, suppliers, etc. (I'm done ranting now ;) )

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby BMAONE23 » Fri Apr 29, 2016 5:03 pm

This would have been a much better image if it were taken near Solar Eclipse and had both the Sun and Moon in the same image.
But not bad though...
Why, the Moon is just teeming with Energy. All we need to do is find a way to gather that Gamma and put it to use.

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby Chris Peterson » Fri Apr 29, 2016 5:12 pm

pferkul wrote:
Chris Peterson wrote:
pferkul wrote:The cited paper "http://arxiv.org/pdf/1604.03349v2.pdf" has 115 authors! Just a bit absurd...

While superficially it might seem so, this is common with "big science" papers- results that come from billion dollar projects with thousands of scientists involved. There really can be hundreds of researchers involved in all the little pieces that have to come together in a paper like this.


I understand this is the current (unfortunate) practice. But it is better to acknowledge them in some other way, rather than claim they authored the paper. There is a big distinction! And why stop at 115 authors? Why not include their families, teachers, friends, supporters, benefactors, vendors, suppliers, etc. (I'm done ranting now ;) )

Well, we simply don't have a good way to do that. Paper authorship is a universal currency in science. Every person there played a role in the publication- contributing data and editing the paper.
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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby Chris Peterson » Fri Apr 29, 2016 5:17 pm

BMAONE23 wrote:This would have been a much better image if it were taken near Solar Eclipse and had both the Sun and Moon in the same image.
But not bad though...
Why, the Moon is just teeming with Energy. All we need to do is find a way to gather that Gamma and put it to use.

This image wasn't "taken". It's the digital summation of seven years of data. So some of it may well have been collected with the Sun in the field. And the total energy of the Moon in this very short wavelength is low- otherwise we wouldn't need a 7-year exposure to see it!
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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby Ann » Fri Apr 29, 2016 5:20 pm

Chris Peterson wrote:
Ann wrote:Yes, I know that this is what scientific pictures look like, except that this one has been colorized yellow and red, for the benefit of people who like to look at yellow and red. (And possibly for clarity, though I'm not so sure about that.)

Very much for clarity. The reason that intensity (or other single-channel values) is often mapped to a false-color table is because our eyes can see a much wider range of colors than we can a range of grays (white to black). So false-color intensity maps allow us to see many more intermediate intensity levels than we could see in grayscale alone.


Well, the original paper that alter-ego linked to showed the intensity scale as blue, green, yellow and red. If the APOD settled for yellow and red only, I have to assume that it was done for aesthetic reasons.

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby Chris Peterson » Fri Apr 29, 2016 6:18 pm

Ann wrote:
Chris Peterson wrote:
Ann wrote:Yes, I know that this is what scientific pictures look like, except that this one has been colorized yellow and red, for the benefit of people who like to look at yellow and red. (And possibly for clarity, though I'm not so sure about that.)

Very much for clarity. The reason that intensity (or other single-channel values) is often mapped to a false-color table is because our eyes can see a much wider range of colors than we can a range of grays (white to black). So false-color intensity maps allow us to see many more intermediate intensity levels than we could see in grayscale alone.

Well, the original paper that alter-ego linked to showed the intensity scale as blue, green, yellow and red. If the APOD settled for yellow and red only, I have to assume that it was done for aesthetic reasons.

I wouldn't assume that. There are a number of semi-standardized false-color maps that are commonly used, and when studying an object it is useful to cycle through them, as each is useful for picking up details that others can't. The scale chosen for the APOD is a good choice here because it sweeps from black to white, meaning it maintains the intuitive sense of intensity, while at the same time expanding the intensity detail. Other scales that utilize more colors provide more detail, but at the expense of making the absolute intensity less obvious. For a non-specialized audience such as APOD viewers, the chosen scale is going to make more sense, which has nothing to do with aesthetics.

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby Boomer12k » Sat Apr 30, 2016 1:25 am

I remember up in Washington State... In Seattle, there is a science pavilion, or such, I think it was near the Space Needle. There was a spot where you stood, and a light blinked... it blinked every time a Cosmic Ray went through your body standing on that spot.... how many spots have I stood on since the mid 60's???????
So.... do I give off Gamma Rays when hit with a Cosmic Ray???? Well, I AM positively GLOWING!!!! :lol2:

Interesting that is glows from Cosmic Rays, and brighter than the Sun... would be interesting to see the difference.

And it has been doing so for a long time.

Science is SO AMAZING....

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby BMAONE23 » Sat Apr 30, 2016 2:59 am

While the gamma ray sun may be quiet when compared to the energetic gamma ray moon, it can be far brighter than the gamma ray moon when flaring

http://heasarc.gsfc.nasa.gov/docs/objects/heapow/archive/solar_system/solarflare_fermi.html

At the flare's peak, the LAT detected gamma rays with two billion times the energy of visible light, or about four billion electron volts (GeV), easily setting a record for the highest-energy light ever detected during or immediately after a solar flare. The flux of high-energy gamma rays, defined as those with energies beyond 100 million electron volts (MeV), was 1,000 times greater than the sun's steady output


http://asterisk.apod.com/viewtopic.php?f=31&t=28949
Normally the sun is no brighter than the Vela Pulsar

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby neufer » Sat Apr 30, 2016 3:37 am

Boomer12k wrote:
I remember up in Washington State... In Seattle, there is a science pavilion, or such, I think it was near the Space Needle. There was a spot where you stood, and a light blinked... it blinked every time a Cosmic Ray went through your body standing on that spot.... how many spots have I stood on since the mid 60's???????
So.... do I give off Gamma Rays when hit with a Cosmic Ray???? Well, I AM positively GLOWING!!!! :lol2:

Most likely those were relativistic secondary radiation muons from atmospheric air showers rather than the original primary cosmic (proton & alpha particle) rays themselves. And since these low energy (though relativistic) secondary radiation muons easily passed through your body they clearly were NOT generating high energy gamma rays.

https://en.wikipedia.org/wiki/Cosmic_ra ... osmic_rays wrote:
<<When cosmic rays enter the Earth's atmosphere they collide with atoms and molecules, mainly oxygen and nitrogen. The interaction produces a cascade of lighter particles, a so-called air shower secondary radiation that rains down, including x-rays, muons, protons, alpha particles, pions, electrons, and neutrons. All of the produced particles stay within about one degree of the primary particle's path.

Typical particles produced in such collisions are neutrons and charged mesons such as positive or negative pions and kaons. Some of these subsequently decay into muons, which are able to reach the surface of the Earth, and even penetrate for some distance into shallow mines. The muons can be easily detected by many types of particle detectors (e.g., scintillation detectors.

Cosmic rays impacting other planetary bodies in the Solar System are detected indirectly by observing high energy gamma ray emissions by gamma-ray telescope. These are distinguished from radioactive decay processes by their higher energies above about 10 MeV.>>
https://en.wikipedia.org/wiki/Muons#Muo ... tomography wrote:
<<Since muons are much more deeply penetrating than X-rays or gamma rays, muon imaging can be used with much thicker material or, with cosmic ray sources, larger objects. One example is commercial muon tomography used to image entire cargo containers to detect shielded nuclear material, as well as explosives or other contraband. The technique of muon transmission radiography based on cosmic ray sources was used in the 1960s to search for possible hidden chambers in the Pyramid of Chephren in Giza.>>
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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby Ann » Sat Apr 30, 2016 5:20 am

Thanks, BMAONE, for the interesting info and picture of the gamma-ray bright Sun.

You wrote:
Normally the sun is no brighter than the Vela Pulsar


Why is the Vela Pulsar so bright in gamma rays? I believe that it is pretty bright in X-rays too, although not strikingly bright in X-rays compared with everything else in the Milky Way. (The X-ray Vela Pulsar is probably the yellow dot located about halfway to the right edge from the center.) But what is that makes the Vela Pulsar so gamma-ray bright? Why is it so much brighter in gamma rays than the pulsar in the Crab Nebula, for example?

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Re: APOD: Fermi's Gamma-ray Moon (2016 Apr 29)

Postby alter-ego » Sat Apr 30, 2016 6:36 am

Chris Peterson wrote:
BMAONE23 wrote:This would have been a much better image if it were taken near Solar Eclipse and had both the Sun and Moon in the same image.
But not bad though...
Why, the Moon is just teeming with Energy. All we need to do is find a way to gather that Gamma and put it to use.

This image wasn't "taken". It's the digital summation of seven years of data. So some of it may well have been collected with the Sun in the field. And the total energy of the Moon in this very short wavelength is low- otherwise we wouldn't need a 7-year exposure to see it!

Integrating the Moon's photon counts over the LAT energy detection range (30MeV to 10GeV), the photon flux density ≈ 0.0002 photons per cm2 per second, or on average, about 1 photon per square cm2 every 1.7 hours. An interesting comparison is the eye's threshold sensitivity to 550nm (green) ≈ 250 photons per cm2 per second.

Also, there were specific observing protocols followed to minimize unnecessary gamma rays which included a minimum angular separation between the Sun and Moon = 20°.
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