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  • What does it mean in astrophysics for X-rays to be reflected?

    From Thomas Womack@21:1/5 to All on Mon Jan 14 13:14:36 2019
    Various articles about black holes talk about X-rays emitted near the
    event horizon 'reflecting off the accretion disc'.

    What kind of material is it that can *reflect* X-rays? I've worked in
    X-ray crystallography, and we needed grazing incidence off very
    precisely figured monocrystalline silicon to get something that
    reflected X-rays at 12.7keV (selenium K line); astrophysical X-rays
    seem to be more at iron K which is about half that energy, but still generally-occuring materials either absorb or transmit them.

    Is this in fact more like the process around a nuclear detonation,
    where things absorb X-rays and are themselves heated to X-ray-emitting temperatures?

    Tom

    [[Mod. note -- Yes, thermal re-emission is one possibility.
    Compton scattering is another possibility. As you note, coherent
    reflection seems unlikely.
    -- jt]]

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  • From Martin Hardcastle@21:1/5 to twomack@chiark.greenend.org.uk on Tue Jan 15 19:59:51 2019
    In article <i3o*XqXdx@news.chiark.greenend.org.uk>,
    Thomas Womack <twomack@chiark.greenend.org.uk> wrote:
    Is this in fact more like the process around a nuclear detonation,
    where things absorb X-rays and are themselves heated to X-ray-emitting >temperatures?

    The emission lines people talk about are X-ray fluorescence from
    (relatively) cold material.

    Martin
    --
    Martin Hardcastle
    School of Physics, Astronomy and Mathematics, University of Hertfordshire, UK
    Please replace the xxx.xxx.xxx in the header with herts.ac.uk to mail me

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  • From Eric Flesch@21:1/5 to Thomas Womack on Tue Jan 15 22:15:59 2019
    On Mon, 14 Jan 2019, Thomas Womack wrote:
    generally-occuring materials either absorb or transmit them.
    [[Mod. note -- ... As you note, coherent reflection seems unlikely.

    Does not radiation reflection *always* consist of absorption &
    re-transmission? By "coherent reflection", JT seems to imply
    preservation of the original photons. But photons don't "bounce" in
    the rubber-ball sense, right?

    I'm reminded of transferring money in the banking system -- it's not
    the same dollar that moves around, money is "fungible" in the sense
    that a dollar has no individual identity as such. I am suspecting
    that photons are fungible in the same way, but that that element is
    not built into the model of light as we know it.

    [[Mod. note -- My apologies for being unclear. What I was trying to
    get at with the phrase "coherent reflection" (which in hindsight was
    a poor choice of words on my part) was "reflecting like a beam of
    optical light from a mirror, with angle-of-reflection =
    angle-of-incidence".

    As to whether elastic scattering of any sort yields the "same" photon,
    I suspect that you're right and that photons don't have an individual
    identify. In fact, I suspect that "the same photon" isn't even a
    meaningful concept in quantum optics. But my knowledge of quantum
    optics is alas very small, so I can't speak with any authority on
    this....
    -- jt]]

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  • From Steve Willner@21:1/5 to Eric Flesch on Tue Feb 5 10:50:44 2019
    In article <5c3e6e4d.832560953@news.aioe.org>,
    Eric Flesch <eric@flesch.org> writes:
    As to whether elastic scattering of any sort yields the "same" photon,
    I suspect that you're right and that photons don't have an individual identify.

    Doesn't this have to be true? Wouldn't the Planck law have a
    different form if photons were distinguishable? When I studied
    thermodynamics, there were four cases: particles could be
    distinguishable or indistinguishable, and they could or could not
    occupy the same state. (Distinguishable particles that can occupy
    the same state are rare.)

    --
    Help keep our newsgroup healthy; please don't feed the trolls.
    Steve Willner Phone 617-495-7123 swillner@cfa.harvard.edu Cambridge, MA 02138 USA

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