Wrestling the Crab Data

The Crab data is winning, too.

A few weeks ago, before the proposal was in our scopes, Kim had suggested that I use the Crab data as a way to check our results with MCG -6-30-15. This sounded like a reasonable idea that shouldn’t take too much work, since I’m familiar with reducing RXTE data (more than familiar, really, at least for extra-galactic sources). However, I had not factored in the complications of dealing with a galactic source. Why is this a problem? Since the source is so much closer, the telescope gets a much larger signal from it, and then complications of instrument dead-time arise^*.

My first results looked weird, and Kim later pointed out that this was likely because of a dead-time correction. So, I looked up how to correct for dead-time on the RXTE data anlaysis pages, and found an article on correcting spectra. However, there was nothing on how to correct light curves. I decided to see how the results would look using count rates derived from integrating under the spectrum, which I finally finished yesterday. Unfortunately it really doesn’t tell me much. My previous plots had data binned to 512 seconds, which produced a few thousand data points. With these spectrum-derived points, there are only about 20 points. Not a good comparison, really.

Now I’m pondering how to correct the lightcurves directly, so I can get my volume of data points back up. In the mean time, I’m going to start writing scripts to produce flux-selected spectra in the data pipeline. I need to consider how best to do this to balance the need for lots of photons per spectrum while trying not to have the spectrum cross into more than one flux state.

^* Detectors take a finite time to recover from detecting an incomming photon. If more photons come in during this recovery period, they are not fully registered, so the count rate measured is actually a bit less than the actual count rate. For extra-galactic X-ray sources this isn’t really a problem because the photons don’t come in fast enough, in general, to arrive during the dead-time from a previous photon. However, this can be a real problem for galactic sources.