Plots

I have been doing some X-ray astronomy. In optical astronomy, spectroscopy is a very powerful tool: by looking for emission and absorption lines you can identify the elements present in a gas (helium was discovered this way, for example); the shapes of the lines can tell you about temperatures and velocities in the object, and the shape of the broadband spectrum can also tell you about the temperature and conditions in the emission region. In X-rays, things are more difficult, for a number of reasons. Unfortunately, lines are much rarer (at least when looking at neutron stars), telescope time is very scarce (since the telescopes must be in space), and there's always a shortage of photons. But X-ray spectroscopy still has the potential to tell you about temperatures, sizes, and compositions of neutron stars (for example). So that's what I've been working on.

The standard tool for X-ray spectroscopy is xspec, one of those pieces of scientific software that's had a great deal of cleverness built into it, very little of which has gone into making it easy to use. It could be worse - at least its interface is not stuck in the FORTRAN era, in fact it has a tcl interpreter built in (yack) - but its plotting in particular is pretty rudimentary, tending to produce monstrosities like this:



The worst part of this graph, apart from the fact that it's practically unreadable even for those with normal color vision, is that it's quite deceptive. It looks as if there's clear evidence for a bend in the spectrum just below 2 keV. But look at the plot below the jump for comparison.





The data points on this plot are identical, but drawing a single power-law through the whole thing makes it look like the data's completely straight. Ordinarily one would choose between the models based on the statistics, but we have so few photons (about 8000) that both models are perfectly adequate fits to the data. I suppose Occam's Razor tells me I should pick the simpler model, though whether this should be the simple but not particularly physical power-law or the physically plausible but more complicated power-law plus neutron-star atmosphere model isn't entirely clear to me.

I'll keep thinking about how to improve the graphics, but the problem is I have five data sets in which each data point has its own vertical and horizontal error bars, and the model gives slightly different predictions for each data set (since they use different instruments with slightly different responses). The plotting tools provided by xspec are also not very flexible (and I haven't found a good way to export the relevant data so I can use my own plotting tools).

What really bothered me, though, was how strongly each graph suggests its own interpretation. It would have been easy to look at one and assume that it told the whole story.

2 comments:

jimmy said...

what is the model for the 1st plot? (you said that the power law was for the 2nd plot.)

and what are the 2 greyish lines on the 1st plot?

(sorry, i an generally pretty clueless. but i know even less about this stuff.)

Anne M. Archibald said...

I didn't specify, did I? It's a two-component model, with one component a power law, and the other a thermal neutron star atmosphere model (not too far from a blackbody). The two greyish lines are the contributions of the two model components to the total signal. They demonstrate that the thermal model falls off above about 1 keV, but is never the dominant component anywhere.

That certainly wasn't obvious from the plot or the description I gave, so you certainly shouldn't apologize for not getting it!

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