Readers of this blog are probably aware that I'm interested in launching an X-ray (or gamma-ray) telescope (XRT) on a high altitude balloon (HAB) in order to detect astronomical sources.The brightest source at gamma-ray (and certainly is one of the brightest at X-ray - perhaps about 50% as bright as GX5-1 in soft (2-30keV) X-rays) is the Crab nebula/pulsar. Below is the combined 'best fit' spectra to a number of XRT missions:
This plot shows the Crab's characteristic 'power-law' spectrum and shows the typical photon flux observed for the 4 XRT missions which were used. From the graph it is clear that the place to be to get the most photons is in the soft x-ray region. (Note: I've only calculated the spectrum down to around 2keV to avoid absorption by interstellar Hydrogen which will start to significantly absorb lower energy photons). The graph is plotted on a log-log plot but perhaps I should have taken this advice - but I'd need a much bigger screen!
The flux calculated in the above plot is for the top of atmosphere, and a balloon will typically only get to around 30-40km in altitude - so how much will that change the flux levels? If I combine the above spectrum with the X-ray absorption data I've mentioned previously, I get the following plot:
I've highlighted the altitudes to which you could hope to get a balloon. As you can see there is a very strong absorption of soft x-rays in the atmosphere. So what does this all mean? If we could get a balloon up to, say, 30km, with a 4cm^2 detector and could detect between 2keV and 1Mev photons with a 100% efficient detector, we would get a count rate of 0.06 counts/s. This is pretty low - we'd either have to stare for a very long time (which would be challenging for a number of reasons) or we need a much bigger detector (again, challenging). There is also the fact that we would be effectively looking through an optically think medium to observe the astronomical object which makes me uncomfortable. Another issue to consider is what noise level (background counts) should be expected.
So in summary some further work to determine background count levels, detector efficiency, detector weight, etc in order to roughly model the mass and performance of the XRT. For example, shielding can be used to reduce the background noise level, but what is the optimum amount or material. However, realistically it isn't looking too hopeful due to the low flux rate from the source and the (relatively!) low altitude of a balloon.
