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Fast Variations in X-rays from Neutron Stars

Fast Variations in X-rays from Neutron Stars

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[Astronomy Picture of the Day: Spirit on Mars]

I will have to make more sense of this preprint (preprint 1) from last Thursday. It's about a new neutron star system that's similar to a bunch of other mysterious systems--but it changes its brightness a lot more. So there's hope that by watching it go through its changes we'll learn more about the other systems.

And then there's preprint 2, this paper on a couple of accreting millisecond pulsars.

What's so great about accreting millisecond pulsars?

People had known that radio pulsars can spin fast--as fast as once a millisecond or so--even if they are very old. The brightness of radio pulsars is thought to be powered by their rotation so they should slow down over time. The theoretical solution? Pulsars are sped up if they go through a phase in their lives in which they are in a binary system and accrete spinning gas. This was just theory until 1998, when Deepto Chakrabarty found the missing link,
the first accreting millisecond pulsar.

Both papers make use of the venerable Rossi X-ray Timing Explorer, an X-ray telescope optimized to detect very fast time variability in X-ray sources. Optimized how? It has great sensitivity ("effective area") so it can see down to very low X-ray levels. Also when it reports its data back to Earth there's a lot of flexibility in how it does that--it can bin up the data according to a known period or bin the time of arrival into about a microsecond. It's not so great though when it comes to spectral resolution to spatial resolution: just time resolution!

(It also has an All Sky Monitor on board--it's awesome! Go to xte.mit.edu and have a look. The All Sky Monitor sweeps the sky a few times a day to determine how all the really bright X-ray stars and galaxies are changing their brightness over months... So RXTE really can see X-rays change on all time scales from microseconds to years (one year is about 3.15x107 seconds--so that's from microseconds to hundreds of megaseconds! A factor of something like 100 trillion!)

Jean Swank was one of the pioneers of RXTE, and her page says she's now working on detectors to see X-ray polarization.

Anyway, what about these two papers and fast variations in X-rays from neutron stars?

I think I will have to go into more detail later. The co-authors are all very experienced in this area. van der Klis was one of the pioneers in finding fast "quasi-periodic" oscillations from neutron stars. Walter Lewin was one of the pioneers in studying X-ray bursts, which are thermonuclear explosions on the surfaces of neutron stars. He's a real wild character--I was on a committee with him once. When I get broadband I'll download some of his MIT lectures from his page.

I will only point out that some of the explanations in Preprint 1 look a little "kludgey" but that I'm reluctant to discount them because of the prestige of the authors. Namely, they suggest that the changing amount of matter falling onto the neutron star is responsible for two kinds of systematic variability in the system--that struck me like saying, "Hm, rotation of the Earth explains the day/night cycle pretty well, maybe it can explain the seasons too?"

You can make something like an X-ray "HR diagram" for these mysterious neutron star systems, and you often find something like a "Z" in one star as it changes its X-rays over time. So they say that the motion along the Z is caused by changing amounts of matter falling onto the neutron star. But also the track of the Z itself changes in time and is different from star to star (5 stars are shown below in different colors, from this paper:

  • I don't know very much astronomy, but I enjoy reading your postings. They fascinate me.

    Thank you for posting.
    • Thanks!

      Oops, I listed the number of seconds in a year in this entry as 3.15x106 instead of 3.15x107!

      Anyway, I'm writing this blog mainly to teach myself about current science that interests me, but I'm also looking for feedback and participation... If it gets way too technical I can cut down on the jargon, etc.
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