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PraedSt
2011-Nov-09, 04:22 PM
Just a short definitional question. What do the words "hard" and "soft" mean when used so:

hard spectra
hard emissions
soft gamma rays

Thanks!

thoth II
2011-Nov-09, 04:54 PM
it should be referring to the energy ranges: for example soft gamma rays border on hard X - rays, but I don't know the Mev or keV position.

Noclevername
2011-Nov-09, 05:16 PM
Hard spectra refers to photons at higher (shorter) frequencies. IIRC the term hard emissions generally refers to ionizing radiation.

Nope, I was wrong. Checked Wikipedia and it refers to hard EM radiation only-- X-rays and gamma rays.

ngc3314
2011-Nov-09, 05:22 PM
In astrophysical contexts, a hard spectrum is one relatively rich in higher-energy photons, so it is a comparative term. It may be associated with an index of a power-law spectral shape, or derived more crudely from the ratio of photons detected in various energy ranges.

PraedSt
2011-Nov-09, 05:39 PM
Thanks for your answers guys.

So would it be correct to say a red-shifted spectrum has "become softer"?

Noclevername
2011-Nov-09, 05:41 PM
Thanks for your answers guys.

So would it be correct to say a red-shifted spectrum has "become softer"?

It would be technically correct-- the best kind of correct!

PraedSt
2011-Nov-09, 06:59 PM
It would be technically correct-- the best kind of correct!Thanks!

Jeff Root
2011-Nov-09, 08:00 PM
I think I recall seeing the term "hard spectrum" before, but
didn't bother to find out what was meant. I've certainly seen
the terms "hard X-rays" and "soft X-rays", the interpretation
of which seems obvious, if vague.



Hard spectra refers to photons at higher (shorter) frequencies.
IIRC the term hard emissions generally refers to ionizing radiation.

Nope, I was wrong. Checked Wikipedia and it refers to hard EM
radiation only-- X-rays and gamma rays.
What gives you the idea you were wrong??? I think you're
wrong: You weren't wrong!!! Seriously. I think that if you
apply the term to ultraviolet, "hard UV" would be ionizing,
while "soft UV" would not. Is the term applied to UV?

-- Jeff, in Minneapolis