Jun. 22nd, 2010

bram: (Default)
The work day is all candy and ice cream when I'm working on my own research projects! Just have to find a new grant or teaching position.

The Sco X-1 tomography is going very well. Even though there are hundreds of absorption lines eating up the spectrum, I can reconstruct what's going on with some confidence, although I want to make some more tests.

This is a double star system in which one star is a neutron star. The brightest X-ray source in the sky. But unlike many neutron stars, this one isn't a pulsar. A pulsar would help us figure out its orbit with great accuracy, by timing the pulses. Also there are no eclipses. Eclipses would help us figure out what's going on too: we'd see each little piece of the star system (gas funneling onto the neutron star, etc.) gradually eclipsed and could figure out what the light from each piece alone looks like. But we don't have that. So some aspects of this star system are not so well known.

That it's not a pulsar my follow from the neutron star having a low magnetic field.

Anyway, here's the Doppler tomogram without taking into account my model for all the absorption lines:

And here is the version with my correction for interstellar absorption lines:

Here are several Doppler tomograms of different emission lines from optical light from 2002 by Steeghs and his colleague, published in The Astrophysical Journal:

So my corrected tomogram looks a lot like the tomogram of He II (ionized Helium) seen in optical light--which is not surprising as the quintuply ionized Oxygen I'm looking at in far ultraviolet is similar to ionized Helium.

Anyway, the signature of an accretion disk in a Doppler tomogram is a ring centered on the lower plus sign (which is where the neutron star would be), and the teardrop shape shows the expected location of the normal star (distorted from spherical shape by rotation and gravity of the neutron star).

So there's evidence for BOTH an accretion disk and for the normal star being lit up by X-rays causing emission from ionized gas.

I'm gonna use this to learn about the accretion disk. Also, I can use the model of the emission to refine my model for the absorption, but I'm going to have to test against other emission lines too--one has to be careful using models to correct the data! However, one fact limits the models of the absorption so that I won't get too carried away and over-fit the data: the absorption has to be constant in time, constant throughout the 15 different times we observed the star system. That's because the star system is orbiting once every 0.8 days or so, and its light is changing, but the gas between us and the star is mostly constant.


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