So the culture around here includes dining with neighbors. Most of the tables are set for four, while the households are one or two people (about 40/60 respectively). So unless you make prior arrangements, the host-person makes more or less random associations.
The day in question I was seated with D and his wife. I had met D (and his wife? I forget) at a new club/organization recently organized by a friend and fellow retired astrophysicist called "Scientists and Friends" with the idea that retired science types could give talks about what they did before they retired. At the organizational meeting we passed a microphone around the room (lots of folks are hard of hearing) and introduced ourselves briefly. D is a microbiologist or virologist or something such. I, as you know, am an astrophysicist.
Side note: I wrote about some of this stuff nearly ten years ago now, here.
So D wanted me to talk briefly about something I personally had discovered. After drawing a blank, due in part to the fact that most of my career has been collaborative, I started talking. I thought you-all might enjoy a summary of that conversation.
---
So if you look at the night sky in x-rays instead of visible light, you see an assortment of stars and galaxies and quasars and such. The ones that are bright in x-rays may or may not be the same ones that are bright in the optical, but you kind of expected that. But another feature becomes apparent: the space in between the sources isn't nearly as dark in x-rays as it is in the visible band. There's a diffuse x-ray background.
So it had been known since the early 70s at least that at high energies the shape of the energy distribution (i.e. spectrum) broadly conformed to a thermal spectrum at a very high temperature: kT = 40 keV means a temperature of about four hundred million K. Gas that hot wouldn't be bound to the Galaxy, so it was thought the x-rays must come from very far away. People tried, mostly in vain, to come up with some way of adding up the spectra of known quasars and fainter counterparts to explain it as a very large number of unresolved point sources. The instruments of the time had poor angular resolution, so the idea of a cloud of unresolved quasars and active galactic nuclei was plausible on the surface.
At lower energies, there's a problem, and that is that the amount of stuff the x-ray signal has to propagate through in our Galaxy to get to us is enough to absorb it. So William Kraushaar had the clever idea of building a sounding rocket payload to map the sky in a variety of energies, going to low enough energy x-rays that he could, in effect, x-ray the interstellar medium of the Galaxy. X-rays tend to interact only with the inner-shell electrons, so they hardly care about chemical bonding, whether the atoms in question are in solids or gases, etc. So you'd get the total amount of stuff along the line of sight that's in gas or dust. Planets and stars would be opaque, of course, like the bones on your medical x-ray films.
So they did that, and they discovered a problem. Yes, there are non-zero levels of x-rays, even to very low energies, in all directions in the sky. No, the ratios of those energies didn't look like absorption. Lower energy x-rays should be more strongly absorbed than higher energy x-rays, but the lowest few bands in their detectors didn't do that. The ratios were constant. Besides all that, there's enough stuff in the Galaxy near the plane to absorb all the low-energy x-rays coming from beyond.
There must be a local-ish source of x-rays. There was an argument from the process of elimination (e.g. if it's synchrotron radiation, there should also be bright radio emission coming from the same electrons, and there isn't) that the "soft" x-ray (i.e. low energy, below about 1 keV or longer wavelengths than about 12 angstroms) diffuse background had to be coming from a thermal source, i.e. hot gas, somewhere within a few hundred parsecs of the solar system (i.e. maybe 1000 light years tops).
Two efforts were launched as a result. One was theoretical, involving my advisor and several of his grad students, including me. He pointed out that there's a dearth of interstellar gas seen at other wavelengths out to that magic one or two hundred parsecs, and if you filled it with million Kelvin gas, there's just about one supernova worth of energy in it. Details to be supplied.
So I made a model of the diffuse soft x-ray background as a supernova blast wave seen from inside. I used the best available atomic physics data to simulate the spectrum vs. time and various input parameters for a bunch of models. In a time when running a fancy computer code was expensive enough that I wrote a thesis based on about 40 runs of the code.
So the picture is that somewhere nearby, a supernova exploded a few hundred thousand years ago, the blast wave swept over us and onward, and we're looking out through it wherever we look. The interior pressure of such a model was uncomfortably high; maybe 10 times the estimate for interstellar pressures from cooler gas. So Don started tilting at that windmill, insisting on a stronger magnetic field than people thought plausible, which brings the pressure up a factor of a few. It might just fit together.
Meanwhile the hardware group set out to build a spectrometer, that would be able to resolve the spectrum of the diffuse soft x-ray background, and see whether the details matched models like the ones I ran. There should be spectral lines (i.e. favorite wavelengths for radiating) representing stuff on the 3rd row of the periodic table, like Ne, Mg, Si, S, Ar, down through iron, minus the valence electrons, so with only 6 or 8 electrons left in most cases (a few more for iron).
After a spin in Virginia being a post-doc, I came back to Wisconsin to work on the Diffuse X-ray Spectrometer. It had flown once on a sounding rocket, and in 5 minutes of observing, they thought they probably saw spectral lines. The idea was to adapt it to fly on the Space Shuttle, where we'd get most of a week of observing time. So we did that, and it flew on STS-54 in January 1993, in the final week of the Geo HW Bush administration.
I remember coming off the overnight shift in the control room (at Goddard Space Flight Center in Greenbelt, MD), driving sleepily back to the hotel with the radio on, hearing an announcement of some inaugural event on the Mall on "Saturday afternoon." After thinking fairly hard about it, I came to the realization that I had no idea what either word in that phrase meant.
Anyway, analysis of the data proved difficult. If you follow the link above you can get some of the ideas, and the actual spectrum itself, that we pursued. Yes, there were clearly lines in the spectrum. Everybody expected that, but nobody had seen them before. Hooray. No, the spectrum didn't fit any of the thermal models I could run. Maybe that was because of approximations made in compiling the data for the spectral synthesis codes, or in calculating the atomic spectra in the first place (very few of them had been actually measured). In the paper, I fit the spectrum to a bunch of very narrow Gaussian lines broadened by the instrument response, and listed wavelengths and intensities, but not identifications of the lines. So it's an astronomical observation, yes, but not yet astrophysics.
Object lesson for x-ray astronomers: If you want to learn something about the cosmos by doing spectroscopy, you should give money to the atomic physics people who can measure or calculate the spectra of the various interesting ions on the ground. I'm still not convinced that's being done enough.
Oh, and? Arguing from a process of elimination, you'd better be damn sure that everything is on the table...
There was this alternative idea to hot gas out there someplace, namely that neutral atoms from the local interstellar cloud (it has a temperature of about 10,000 K, a density of about 0.1 nuclei per cubic centimeter, and it's partially ionized by the light of a bunch of hot stars in the vicinity, notably epsilon Canis Majoris). Anyway, neutral hydrogen and helium atoms can fly right through the Magneto Hydrodynamical (MHD) bow shock of the heliosphere, and eventually exchange an electron with an ion in the solar wind. The electron tends to end up in an excited state in the target ion, with a binding energy similar to what it had in the H or He atom, and then it cascades down, emitting photons, some of which are x-rays.
This mechanism was boosted by a surprise: a target-of-opportunity observation of a comet by the Rosat German x-ray astronomy mission showed that comets are bright in x-rays. The charge exchange mechanism was the only thing people could think of to explain that. And oh by the way, it should be going on between interstellar neutrals and the solar wind everywhere in the heliosphere...
Great. What spectra do you expect, then? Well, the same batch of lines you'd get from hot gas, but in different patterns.
The one feature in the DXS spectrum that was consistent with an unblended, solitary spectral line was at a wavelength (as I had noted in the DXS data paper) that matches a 5->2 transition in single-electron oxygen atoms. It turns out the n=5 state in such atoms is one that should be populated by charge exchange...
All neatly suggestive, but until we had decent models for stuff like Mg+9 and the like, there wasn't much to be done.
In due course an effort to revisit, recompile, and actually create a modern database of x-ray spectral data was being run by a friend of mine, a fellow graduate of the same advisor back in Wisconsin. And R came up with a way of approximating the x-ray emission of a bunch of ions due to charge exchange. Well, several different ways, depending on one parameter he couldn't calculate, but as it happens it didn't much impact the output spectra.
So we set out to try to fit the DXS data again, with a sum of a hot gas spectrum (new and improved with data from his database) and charge exchange (function of that single parameter). And, boy howdy, is that fit ever better than anything I could do with hot gas alone (either static or in a blast wave model). Something like 3/4 of the x-rays in this fit are coming from charge exchange in the heliosphere. And oh, by the way, the required pressure in the hot gas supplying the other 1/4 is now consistent with that of the rest of the interstellar gas in the Galaxy. This research is described in a paper on arXiv.org.
So the bottom line is, I managed to bookend my career by [a] coming up with a plausible model for the diffuse soft x-ray background and then [b] disproving it.
I think I've written way more words than I actually used explaining to my dinner companion...
The day in question I was seated with D and his wife. I had met D (and his wife? I forget) at a new club/organization recently organized by a friend and fellow retired astrophysicist called "Scientists and Friends" with the idea that retired science types could give talks about what they did before they retired. At the organizational meeting we passed a microphone around the room (lots of folks are hard of hearing) and introduced ourselves briefly. D is a microbiologist or virologist or something such. I, as you know, am an astrophysicist.
Side note: I wrote about some of this stuff nearly ten years ago now, here.
So D wanted me to talk briefly about something I personally had discovered. After drawing a blank, due in part to the fact that most of my career has been collaborative, I started talking. I thought you-all might enjoy a summary of that conversation.
---
So if you look at the night sky in x-rays instead of visible light, you see an assortment of stars and galaxies and quasars and such. The ones that are bright in x-rays may or may not be the same ones that are bright in the optical, but you kind of expected that. But another feature becomes apparent: the space in between the sources isn't nearly as dark in x-rays as it is in the visible band. There's a diffuse x-ray background.
So it had been known since the early 70s at least that at high energies the shape of the energy distribution (i.e. spectrum) broadly conformed to a thermal spectrum at a very high temperature: kT = 40 keV means a temperature of about four hundred million K. Gas that hot wouldn't be bound to the Galaxy, so it was thought the x-rays must come from very far away. People tried, mostly in vain, to come up with some way of adding up the spectra of known quasars and fainter counterparts to explain it as a very large number of unresolved point sources. The instruments of the time had poor angular resolution, so the idea of a cloud of unresolved quasars and active galactic nuclei was plausible on the surface.
At lower energies, there's a problem, and that is that the amount of stuff the x-ray signal has to propagate through in our Galaxy to get to us is enough to absorb it. So William Kraushaar had the clever idea of building a sounding rocket payload to map the sky in a variety of energies, going to low enough energy x-rays that he could, in effect, x-ray the interstellar medium of the Galaxy. X-rays tend to interact only with the inner-shell electrons, so they hardly care about chemical bonding, whether the atoms in question are in solids or gases, etc. So you'd get the total amount of stuff along the line of sight that's in gas or dust. Planets and stars would be opaque, of course, like the bones on your medical x-ray films.
So they did that, and they discovered a problem. Yes, there are non-zero levels of x-rays, even to very low energies, in all directions in the sky. No, the ratios of those energies didn't look like absorption. Lower energy x-rays should be more strongly absorbed than higher energy x-rays, but the lowest few bands in their detectors didn't do that. The ratios were constant. Besides all that, there's enough stuff in the Galaxy near the plane to absorb all the low-energy x-rays coming from beyond.
There must be a local-ish source of x-rays. There was an argument from the process of elimination (e.g. if it's synchrotron radiation, there should also be bright radio emission coming from the same electrons, and there isn't) that the "soft" x-ray (i.e. low energy, below about 1 keV or longer wavelengths than about 12 angstroms) diffuse background had to be coming from a thermal source, i.e. hot gas, somewhere within a few hundred parsecs of the solar system (i.e. maybe 1000 light years tops).
Two efforts were launched as a result. One was theoretical, involving my advisor and several of his grad students, including me. He pointed out that there's a dearth of interstellar gas seen at other wavelengths out to that magic one or two hundred parsecs, and if you filled it with million Kelvin gas, there's just about one supernova worth of energy in it. Details to be supplied.
So I made a model of the diffuse soft x-ray background as a supernova blast wave seen from inside. I used the best available atomic physics data to simulate the spectrum vs. time and various input parameters for a bunch of models. In a time when running a fancy computer code was expensive enough that I wrote a thesis based on about 40 runs of the code.
So the picture is that somewhere nearby, a supernova exploded a few hundred thousand years ago, the blast wave swept over us and onward, and we're looking out through it wherever we look. The interior pressure of such a model was uncomfortably high; maybe 10 times the estimate for interstellar pressures from cooler gas. So Don started tilting at that windmill, insisting on a stronger magnetic field than people thought plausible, which brings the pressure up a factor of a few. It might just fit together.
Meanwhile the hardware group set out to build a spectrometer, that would be able to resolve the spectrum of the diffuse soft x-ray background, and see whether the details matched models like the ones I ran. There should be spectral lines (i.e. favorite wavelengths for radiating) representing stuff on the 3rd row of the periodic table, like Ne, Mg, Si, S, Ar, down through iron, minus the valence electrons, so with only 6 or 8 electrons left in most cases (a few more for iron).
After a spin in Virginia being a post-doc, I came back to Wisconsin to work on the Diffuse X-ray Spectrometer. It had flown once on a sounding rocket, and in 5 minutes of observing, they thought they probably saw spectral lines. The idea was to adapt it to fly on the Space Shuttle, where we'd get most of a week of observing time. So we did that, and it flew on STS-54 in January 1993, in the final week of the Geo HW Bush administration.
I remember coming off the overnight shift in the control room (at Goddard Space Flight Center in Greenbelt, MD), driving sleepily back to the hotel with the radio on, hearing an announcement of some inaugural event on the Mall on "Saturday afternoon." After thinking fairly hard about it, I came to the realization that I had no idea what either word in that phrase meant.
Anyway, analysis of the data proved difficult. If you follow the link above you can get some of the ideas, and the actual spectrum itself, that we pursued. Yes, there were clearly lines in the spectrum. Everybody expected that, but nobody had seen them before. Hooray. No, the spectrum didn't fit any of the thermal models I could run. Maybe that was because of approximations made in compiling the data for the spectral synthesis codes, or in calculating the atomic spectra in the first place (very few of them had been actually measured). In the paper, I fit the spectrum to a bunch of very narrow Gaussian lines broadened by the instrument response, and listed wavelengths and intensities, but not identifications of the lines. So it's an astronomical observation, yes, but not yet astrophysics.
Object lesson for x-ray astronomers: If you want to learn something about the cosmos by doing spectroscopy, you should give money to the atomic physics people who can measure or calculate the spectra of the various interesting ions on the ground. I'm still not convinced that's being done enough.
Oh, and? Arguing from a process of elimination, you'd better be damn sure that everything is on the table...
There was this alternative idea to hot gas out there someplace, namely that neutral atoms from the local interstellar cloud (it has a temperature of about 10,000 K, a density of about 0.1 nuclei per cubic centimeter, and it's partially ionized by the light of a bunch of hot stars in the vicinity, notably epsilon Canis Majoris). Anyway, neutral hydrogen and helium atoms can fly right through the Magneto Hydrodynamical (MHD) bow shock of the heliosphere, and eventually exchange an electron with an ion in the solar wind. The electron tends to end up in an excited state in the target ion, with a binding energy similar to what it had in the H or He atom, and then it cascades down, emitting photons, some of which are x-rays.
This mechanism was boosted by a surprise: a target-of-opportunity observation of a comet by the Rosat German x-ray astronomy mission showed that comets are bright in x-rays. The charge exchange mechanism was the only thing people could think of to explain that. And oh by the way, it should be going on between interstellar neutrals and the solar wind everywhere in the heliosphere...
Great. What spectra do you expect, then? Well, the same batch of lines you'd get from hot gas, but in different patterns.
The one feature in the DXS spectrum that was consistent with an unblended, solitary spectral line was at a wavelength (as I had noted in the DXS data paper) that matches a 5->2 transition in single-electron oxygen atoms. It turns out the n=5 state in such atoms is one that should be populated by charge exchange...
All neatly suggestive, but until we had decent models for stuff like Mg+9 and the like, there wasn't much to be done.
In due course an effort to revisit, recompile, and actually create a modern database of x-ray spectral data was being run by a friend of mine, a fellow graduate of the same advisor back in Wisconsin. And R came up with a way of approximating the x-ray emission of a bunch of ions due to charge exchange. Well, several different ways, depending on one parameter he couldn't calculate, but as it happens it didn't much impact the output spectra.
So we set out to try to fit the DXS data again, with a sum of a hot gas spectrum (new and improved with data from his database) and charge exchange (function of that single parameter). And, boy howdy, is that fit ever better than anything I could do with hot gas alone (either static or in a blast wave model). Something like 3/4 of the x-rays in this fit are coming from charge exchange in the heliosphere. And oh, by the way, the required pressure in the hot gas supplying the other 1/4 is now consistent with that of the rest of the interstellar gas in the Galaxy. This research is described in a paper on arXiv.org.
So the bottom line is, I managed to bookend my career by [a] coming up with a plausible model for the diffuse soft x-ray background and then [b] disproving it.
I think I've written way more words than I actually used explaining to my dinner companion...
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