X-ray Telescope on a HAB

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.

ISS/GRAVES, X-ray telescopes & things......

One quick observation I've noticed over the last week or so of measuring the returns from GRAVES via the ISS is that to get a strong signal I need ISS to be at 20 degree elevation angle. The pass yestersay at around 19:20 (local) I only got the following short detection:

In order to investigate why ISS was only detectable in a small amount of the pass I made the following graph:

The areas in blue are outside of the nominal azimuth & elevation pattern of the transmitter at GRAVES. Hence it is quite obvious why only a small part of the pass I managed to detect ISS. There was an earlier pass which had an elevation of 5 degrees to me here, which I didn't detect at all. If I make a plot for that pass then I get the following:

From the graph it is clear that ISS would have only been illuminated briefly and when combined with the extreme range (and local trees/houses/etc) would make it difficult to detect it.

The things I intend to investigate further are:

1. If H-polarisation improves the signal-to-noise (ideally I'd do this using 2 identical sets of antennas and receivers but that would have to wait a while).
2. Create a program which analyses the data collected and detects all the meteors/planes/etc and saves the 'interesting' data to be further analysed.
3. Use the program I've developed to make the graphs above, be able to search for suitable times for me to attempt to detect the moon.

As an aside I've ordered the parts to make the frequency doubler which is necessary to improve the frequency stability of the RTLSDR dongles and hopefully allow me build a 2 (or maybe more) channel coherent receiver.

One of the other projects I'm currently looking at is building an X-ray telescope to be launched on a high altitude balloon. To be honest calling it an X-ray telescope is a bit grandiose, it'll be a 1 pixel detector! Anyway the first stage of the design process is to see what the atmospheric attenuation of X-rays is like. The graph below shows the altitude at which 0.5, 0.1 and 0.01 of the incident radiation is left.

The X-ray absorption data (for O2, N2, Ar) is from "AD-A278 139, NBS Circular 583" and the atmospheric model is from the C implementation of NRLMSISE-00 which returns the number density for O2, N2 & Ar. In order to check that I've not made any blunders, I compared with the only other X-ray atmospheric graph I could find here in fig 2.4, page 46. Although the line for 0.5 agrees well, the other lines do not. In order to double check my answers I tried using the classic 1976 atmospheric model (which just returns the density of air) and the X-ray absorption data for air. Using these new set of data I get a very similar graph to that above (and get very good agreement at sea level which I also check with NIST). I'm unsure how to proceed at the moment with this, but will start to look at some of the other issues, such as energy resolution, collimation, weight, power, etc.

Balloons & things

It has been a bank holiday this weekend - and weirdly hasn't been too wet. Well, actually it poured with rain on Saturday. Anyway,  there were a number of high-altitude balloon launches on Saturday and again today. More information can be found here but I don't know where that page gets archived. Another good link is here. I could pick up all 4 balloons from today when they got reasonably high - and when I switch my yagi to be in V pol - d'oh! (Using one of my RTL_SDR receivers). I'm having difficulty in getting dl-fldigi to decode the output via pulse audio (it works if I save the stream to an 8k samps/s WAV file and then replay that in dl-fldigi) - I'm sure there are better ways to connect 2 programs. I shall explore further and see what I can do. its not helped by my recent transition to GnuRadio 3.7 which seems to have broken a lot.

In other areas I've got my stepper motor working with my beaglebone black - now just have to get the rest of the engineering sorted.