APOD: Unexpected X-Rays from Perseus... (2018 Jan 02)

[rstevenson]

[edit]
I just downloaded and installed Stylish, a popular add-on for customizing your view of particular sites. It served to remind me why the APOD page needs some rebuilding, even to make it possible to sensibly customize the style for your own viewing needs. The page has effectively no modern structural elements in it, so it's nearly impossible to re-position or style in other ways any of its content elements independently of any other element. For example, it was easy to shrink the description paragraph's width, but it hugged the left page edge. Applying a margin to push it over inexplicably pushed over and shrank the image above it. And so on.

Try this. Though there are no class definitions, there are still ways to get around it. I have done more complicated things by using the order of elements themselves to select what I want... although that still broke sometimes because the links at the bottom of the page are sometimes there, and sometimes not. Anyway, this, so far, has not broken. First two lines optional.

Code: Select all

p, h2, h3, center { font-family: sans-serif; }
h1{ font-weight: normal; font-family: serif; }
p { max-width: 800px; margin: 15px auto; line-height: 130%; }
center p:nth-child(3) { max-width: none; }

Nice! I learned my HTML skills before HTML 5 and its CSS capabilities became common, so I'd never heard of the nth-child(n) selector. Very handy. I see you can also do things like nth-child(odd) to target every second element. It's a good geek day!

Rob

[neufer]

I think the problem is that such a mechanism doesn't explain the nature of the observed spectrum. While there are well understood processes that result in x-ray emission lines, those lines exist against a continuum, and multiple lines are present. It seems like what we have here is a monochromatic source.

I'm not dismissing your original laser suggestion.

I'm just thinking that it would probably involve the excitation of iron ions by ram-pressure stripping from (abundant) hydrogen gas protons. No dark matter physics (or even black hole physics) is necessary when one has two galaxies colliding at 3000 km/s (as Ann suggested).

It wasn't really a suggestion. Just a comment about the first thing I would normally think of when considering a monochromatic source.

It seems like in this case it's easier to say what we aren't looking at then what we are.

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<<Nebulium was a proposed element found in astronomical observation of a nebula by William Huggins in 1864. The strong green emission lines of the Cat's Eye Nebula, discovered using spectroscopy, led to the postulation that an as yet unknown element was responsible for this emission. In 1927, Ira Sprague Bowen showed that the lines are emitted by doubly ionized oxygen (O²⁺), and no new element was necessary to explain them.

In 1911, John William Nicholson theorized that all known elements consisted of four protoelements, one of which was Nebulium. The development of the periodic table by Dimitri Mendeleev and the determination of the atomic numbers by Henry Moseley in 1913 left nearly no room for a new element. In 1914 French astronomers were able to determine the atomic weight of nebulium. With a measured value of 2.74 for the lines near 372 nm and a slightly lower value for the 500.7 nm line indicating two elements responsible for the spectrum.

Ira Sprague Bowen was working on UV spectroscopy and on the calculation of spectra of the light elements of the periodic table when he became aware of the green lines discovered by Huggins. With this knowledge he was able to suggest that the green lines might be forbidden transitions. They were shown as due to doubly ionized oxygen at extremely low density, rather than the hypothetical nebulium. As Henry Norris Russell put it, "Nebulium has vanished into thin air." Nebulae are typically extremely rarefied, much less dense than the hardest vacuums produced on Earth. In these conditions, lines can form which are suppressed at normal densities. These lines are known as forbidden lines, and are the strongest lines in most nebular spectra.>>

BROAD IRON LINES IN ACTIVE GALACTIC NUCLEI
A.C. Fabian 1, K. Iwasawa 1, C.S. Reynolds 2, 3, A.J. Young 4
Published in PASP, 112, 1145, 2000.

Abstract. An intrinsically narrow line emitted by an accretion disk around a black hole appears broadened and skewed as a result of the Doppler effect and gravitational redshift. The fluorescent iron line in the X-ray band at 6.4 - 6.9 keV is the strongest such line and is seen in the X-ray spectrum of many active galactic nuclei and, in particular, Seyfert galaxies. It is an important diagnostic with which to study the geometry and other properties of the accretion flow very close to the central black hole. The broad iron line indicates the presence of a standard thin accretion disk in those objects, often seen at low inclination. The broad iron line has opened up strong gravitational effects around black holes to observational study with wide-reaching consequences for both astrophysics and physics.

<< 2.1. Line Production

A substantial amount of the power in AGN is thought to be emitted as X-rays from the accretion disk corona in active or flaring regions. Thermal Comptonization (i.e. multiple inverse Compton scattering by hot thermal electrons; Zdziarski et al. 1994) of soft optical/UV disk photons by the corona naturally gives rise to a power-law X-ray spectrum. The flares irradiate the accretion disk which is relatively cold resulting in the formation of a ``reflection'' component within the X-ray spectrum. A similar component is produced in the Solar spectrum by flares on the solar photosphere (Bai & Ramaty 1978), in X-ray binaries by irradiation of the stellar companion (Basko 1978) and in accreting white dwarfs.

The basic physics of X-ray reflection and iron line fluorescence can be understood by considering a hard X-ray (power-law) continuum illuminating a semi-infinite slab of cold gas. When a hard X-ray photon enters the slab, it is subject to a number of possible interactions: Compton scattering by free or bound electrons (5) photoelectric absorption followed by fluorescent line emission, or photoelectric absorption followed by Auger de-excitation. A given incident photon is either destroyed by Auger de-excitation, scattered out of the slab, or reprocessed into a fluorescent line photon which escapes the slab.

[b][color=#0000FF]Figure 1. X-ray reflection from an illuminated slab. Dashed line shows the incident continuum and solid line shows the reflected spectrum (integrated over all angles). Monte Carlo simulation from Reynolds (1996).[/color][/b]

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Figure 1 shows the results of a Monte Carlo calculation which includes all of the above processes (Reynolds 1996; based on similar calculations by George & Fabian 1991). Due to the energy dependence of photoelectric absorption, incident soft X-rays are mostly absorbed, whereas hard photons are rarely absorbed and tend to Compton scatter back out of the slab. The reflected continuum is therefore a factor of about sigmaT / sigmape below the incident one. Above energies of several tens of kilovolts, Compton recoil reduces the backscattered photon flux. These effects give the reflection spectrum a broad hump-like shape. In addition, there is an emission line spectrum resulting primarily from fluorescent Kalpha lines of the most abundant metals. The iron Kalpha line at 6.4 keV is the strongest of these lines. For most geometries relevant to this discussion, the observer will see this reflection component superposed on the direct (power-law) primary continuum. Under such circumstances, the main observables of the reflection are a flattening of the spectrum above approximately 10 keV (as the reflection hump starts to emerge) and an iron line at 6.4 keV.

The fluorescent iron line is produced when one of the 2 K-shell (i.e. n = 1) electrons of an iron atom (or ion) is ejected following photoelectric absorption of an X-ray. The threshold for the absorption by neutral iron is 7.1 keV. Following the photoelectric event, the resulting excited state can decay in one of two ways. An L-shell (n = 2) electron can then drop into the K-shell releasing 6.4 keV of energy either as an emission line photon (34 per cent probability) or an Auger electron (66 per cent probability). (This latter case is equivalent to the photon produced by the n = 2 -> n = 1 transition being internally absorbed by another electron which is consequently ejected from the ion.) In detail there are two components to the Kalpha line, Kalpha1 at 6.404 and Kalpha2 at 6.391 keV, which are not separately distinguished in our discussion here. There is also a Kbeta line at 7.06 keV and a nickel Kalpha line at 7.5 keV is expected.

For ionized iron, the outer electrons are less effective at screening the inner K-shell from the nuclear charge and the energy of both the photoelectric threshold and the Kalpha line are increased. (The line energy is only significantly above 6.4 keV when the M-shell is lost, i.e. FeXVII and higher states.) The fluorescent yield (i.e. the probability that a photoelectric absorption event is followed by fluorescent line emission rather than the Auger effect) is also a weak function of the ionization state from neutral iron (FeI) upto FeXXIII. For Lithium-like iron (FeXXIV) through to Hydrogen-like iron (FeXXVI), the lack of at least 2 electrons in the L-shell means that the Auger effect cannot occur. For He- and H-like iron ions the line is produced by the capture of free electrons, i.e. recombination. The equivalent fluorescent yield is high and depends on the conditions (see Matt, Fabian & Reynolds 1997).

The fluorescent yield for neutral matter varies as the fourth power of atomic number Z4, for example being less than one half per cent for oxygen. Predicted equivalent widths for low Z lines are given in Matt et al (1997). Fluorescent X-ray spectroscopy is a well-known, non-invasive way to determine the surface composition of materials in the laboratory, or even of a planetary surface.

For cosmic abundances the optical depth to bound-free iron absorption is higher than, but close to, the Thomson depth, The iron line production in an X-ray irradiated surface therefore takes place in the outer Thomson depth. This is only a small fraction of the thickness (say 1 to 0.1 per cent) of a typical accretion disk and it is the ionization state of this thin skin which determines the nature of the iron line.

The strength of the iron line is usually measured in terms of its equivalent width with respect to the direct emission. (The equivalent width is the width of the continuum in, say eV, at the position of the line which contains the same flux as the line. Its determination is not entirely straightforward when the line is very broad.) It is a function of the geometry of the accretion disk (primarily the solid angle subtended by the ``reflecting'' matter as seen by the X-ray source), the elemental abundances of the reflecting matter, the inclination angle at which the reflecting surface is viewed, and the ionization state of the surface layers of the disk. We will address the last three of these dependences in turn.>>