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Fraser
2005-May-04, 06:41 PM
SUMMARY: Astronomers aren't stuck looking at plain old light any more. There's a whole electromagnetic spectrum out there, ready to be explored, from 21cm hydrogen radio-waves through microwave, infrared, ultraviolet, x-rays, to gamma rays and beyond, astronomers have instruments for all seasons and all wavelengths. High-energy gamma rays are blocked by the Earth's atmosphere - which is good for our health - but astronomers can still see them thanks to a process called Cherenkov radiation telescopy.

View full article (http://www.universetoday.com/am/publish/hess_observations.html)

What do you think about this story? Post your comments below.

antoniseb
2005-May-04, 07:37 PM
I remember reading Sky and Telescope articles about this stuff from similar projects in Australia and the New Mexico Institute of Mining and Technology (IIRC). I think that the Max Planck Intitute innovation was the improved use of computers in analyzing the data. Even back in the 1970s there was an awareness that the highest end CRs were at about 10^21 eV. What is interesting about the news in THIS story is that for the first time we are doing some real observations of known sources at these super-high energies. (Except the Crab Nebula, which has been an observed source from almost the beginning).

wstevenbrown
2005-May-06, 07:55 PM
Followup: http://arxiv.org/astro-ph/0504380

This paper lists 8 new sources discovered by H.E.S.S., all in the Galactic Plane. Optical counterparts, where they have been determined, are SN remnants and pulsars. That the sources are local is not terribly surprising. Steve

VanderL
2005-May-06, 08:06 PM
Hi Steve,


Optical counterparts, where they have been determined, are SN remnants and pulsars.

The crux of the story is not the "recognizable" sources, but the ones without an optical or X-ray counterpart. That was totally unexpected, hence the term "dark accelerators", whatever that is. There was some discussion on this, but other than electric fields as accelerators there's just not enough data.


That the sources are local is not terribly surprising

Could you explain that? Why would gamma-rays be local?

Cheers.

antoniseb
2005-May-06, 08:13 PM
Originally posted by VanderL@May 6 2005, 08:06 PM
Why would gamma-rays be local?
Extremely energetic gamma-rays have a relatively short distance that they can travel through intergalactic space becuase they have a high cross-section for interacting with CMB photons and particles, and losing energy and direction. It is unlikely that we'll see many of these from more than 100 million lightyears away.

As to dark accelerators, it is thought that they may well be simply normal sources which are concealed from us by dust sufficient to block x-rays, but not high energy gammas. There is no need to invoke little green men, or EU hopes.

Nereid
2005-May-06, 09:45 PM
That was totally unexpected
I guess that depends on who was doing the expecting (or not)! ;)

'Point sources' of TeV gammas have been detected well before HESS got going, e.g. by CANGAROO (http://icrhp9.icrr.u-tokyo.ac.jp/) (yes, it's in Australia); the only 'news' is that the HESS folks are a lot more skilful at marketing than the CANGAROOs ... no, that's not entirely fair; the HESS results are well worth some PR (but so were the CANGAROO ones).

Back to the topic .... what's so exciting about this stuff is that the observatories can be built for such low cost, yet probe a domain that we here with our €€billion LHC (etc) cannot even dream of exploring. Oh, and there's plenty of exciting physics to get into, even for something as 'well known', as SN shock fronts.

Sorry to say VanderL, plenty of plasma physics, but no 'untethered' electric currents.

VanderL
2005-May-06, 10:05 PM
Sorry to say VanderL, plenty of plasma physics, but no 'untethered' electric currents.

"Untethered" would also be impossible in an EU model, but I didn't claim this as evidence for an EU model did I? Just curious when something new is on offer.


As to dark accelerators, it is thought that they may well be simply normal sources which are concealed from us by dust sufficient to block x-rays, but not high energy gammas. There is no need to invoke little green men, or EU hopes.

The authors state:
"At least two sources have no identified counterpart in radio or X-rays, which suggests the exciting possibility of a new class of ‘dark’ nucleonic particle accelerators."

Antoniseb, your suggestion that it is dust blocking everything but the gamma rays, is just one of several options, nothing definitive, so your conclusions on what is needed are premature at least.
And how can dust blocking anything but gamma rays be reconciled with your assertion that local gamma-rays are to be expected because of attenuation?

Cheers.

antoniseb
2005-May-06, 10:20 PM
Originally posted by VanderL@May 6 2005, 10:05 PM
how can dust blocking anything but gamma rays be reconciled with your assertion that local gamma-rays are to be expected because of attenuation?
The attenuation is from interactions with CMB photons which do not increase or decrease in number in areas of dust. Note that this CMB interaction is especially important for gammas above 10^17eV.

Note that Dark GRBs are also thought to be caused by normal GRB phenomena hidden behind dust. A recent paper came out showing that a 2002 Dark GRB has shown some evidence for this. A story about that may appear in on UT sometime soon.

VanderL
2005-May-06, 11:08 PM
The attenuation is from interactions with CMB photons which do not increase or decrease in number in areas of dust. Note that this CMB interaction is especially important for gammas above 10^17eV.

Are you sure about this number, I only saw reference to 10 TeV as the cut-off level for "attenuated" gamma rays, and the HESS detector already detects 100 GeV gamma rays, so a different part of the spectrum.
And wouldn't dust also block some of the gamma rays?


Note that Dark GRBs are also thought to be caused by normal GRB phenomena hidden behind dust. A recent paper came out showing that a 2002 Dark GRB has shown some evidence for this. A story about that may appear in on UT sometime soon.

Nice, but are you suggesting the authors had an exciting figment of the mind when they proposed "dark accelerators"?

Cheers.

VanderL
2005-May-07, 10:51 AM
There is no need to invoke little green men, or EU hopes.

What isn't needed is snide remarks meant to ridicule.

Cheers.

wstevenbrown
2005-May-07, 11:17 AM
QUOTE (VanderL @ May 6 2005, 08:06 PM)
Why would gamma-rays be local?

As Antoniseb has pointed out, a noticeable damping starts at 10 TEV, for distant sources. As energy increases, the probability of extralocal (more than 100MPc) origin goes down steeply, becoming almost a general prohibition around 50 TeV. Thus, any UHECR's in the EeV or ZeV range are assumed to be 'local'.

It's called the Greisen-Zatsepin-Kuz'min effect, or the GZK cutoff. A good description of this in expository prose is given in http://arxiv.org/astro-ph/0309027, around page 4, for which I am indebted to Nereid. The import of it is that, counterintuitively, the more energetic the CR, the less likely it is to have come from far away.

Sorry, mates-- didn't mean to start a brouhaha. S

VanderL
2005-May-07, 12:11 PM
Sorry, mates-- didn't mean to start a brouhaha. S

You didn't.

Thanks for the info, but as I understand it the HESS experiments are also sensitive to the lower energy range of gamma rays, which would make the argument of local versus distant different.

Cheers.

Nereid
2005-May-07, 04:40 PM
Just a note to say let's be careful to say clearly what we're all talking about.

CANGAROO and HESS (and a few others; I think FlyEye was the first) observe high energy gammas (i.e. photons), via Cherenkov emission (which, in this case, are essentially 'photonic booms', somewhat analogous to 'sonic booms' generated by planes, or bullets, travelling faster than the speed of sound). The term 'cosmic ray' is rather woolly - it can mean the high energy protons from solar flares (which, in turn, are only 'high' in energy relative to 'normal' solar wind particles), or the particles from elsewhere, with energies up to 10^20 eV (these are mostly protons, but do include other particles, such as alphas); historically, 'cosmic ray' was also used for high energy photons. Generally, cosmic ray 'observatories' detect 'air showers', which are the cascades of particles resulting from the primary CR particle colliding with the nucleus of an atom in the upper atmosphere (usually, duh!, an O or N nucleus).

CANGAROO and HESS type observatories are particularly interesting because they may be able to 'see' where CRs originate (particles and high energy gammas very likely originate in the same place); as CR protons (etc) are charged, their direction of arrival generally tells us nothing of where they came from (their paths are bent by galactic magnetic fields).

The Stecker paper (glad to hear you enjoyed it Steve) deals with high energy CRs in general, and (IIRC) how much light HESS etc will be able shed on the astrophysical and particle physics challenges de jour isn't directly addressed.

tnetcenter
2005-May-07, 05:21 PM
In the article there is a term referenced w/o an explanation. What is AGN referring to??

Thanks, and pardon my ignorance. :unsure:

Jeff Moore

Nereid
2005-May-08, 12:33 PM
Active galactic nucleus (http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html), or a galaxy which has an active nucleus.

Guest
2005-May-08, 04:46 PM
Thanks for the explanation and the link was very informative

The Near-Sighted Astronomer
2005-May-09, 06:11 PM
Hi Folks,

As the author of the article on HESS and IACTS in general I find this thread very informative - there are definitely some smart astro-dudes out their in UT-land!

Some of the things I learned just reading this thread:

1. That photon scattering robs high energy photons of much of their energy. (Does this mean that we see scattered versions as lower energy gammas and x-rays? Are scattered gammas like a CR noise hitting the atmosphere?)

2. That charged particles can't be tracked back to their sources because of magnetic field deflection as they pass through space. (I note that IACTS can distinguish between particles and photons but am not sure how.)

3. That there may have been an even earlier version of an IACT than CT1.

I just love the fact that Fraser hosts forums like this associated with the articles!

Cheers,

jeff

Fraser
2005-May-09, 06:35 PM
Me too. :-)

Nereid
2005-May-10, 08:55 AM
I note that IACTS can distinguish between particles and photons but am not sure how
Here (http://imagine.gsfc.nasa.gov/docs/science/how_l2/cerenkov.html) is one explanation. Here (http://www.gae.ucm.es/~emma/tesina/tesina.html) is a more detailed set of webpages. There are also references to other IACTs (and a note that the Fly Eye uses a different detection technique, but still observes UHE CRs and gammas).

The Near-Sighted Astronomer
2005-May-11, 01:48 AM
Just looked over the NASA article - very clear.

Particle showers are far more chaotic in Cherenkov signature in the atmosphere than photonic events of comparable energy levels. I wonder if this has more to do with "increased frontal area" in particles or the fact that they are electrically charged...

Answers caught in the act of breeding questions,

jeff