Here we will perform a spectral fit and imaging of the Crab using the 2016 COSI balloon data. The analyses will mostly follow the notebooks used for the simulated data. For more details on the steps of the spectral analysis, see the main spectral-fit README (here), and likewise for the imaging analysis README (here). This README summarizes the key differences that are needed for analyzing the balloon data.
COSI launched from Wanaka, New Zealand on NASA's Super Pressure Balloon on May 16th, 2016. After a 46 day flight, COSI landed in Southern Peru. The trajectory of the mission is shown in the image below. After one full circumnavigation, COSI floated north and an anomaly in the balloon resulted in the zagged nature of the path for the second half of the flight, indicated with the red curve. For more details of the COSI Balloon instrument, see (Kierans et al. 2017).
The data has been extracted using a
Additionally, we do not consider the first 21 days of the flight, when the instrument background rate was relatively high and variable. Note that the Crab was mostly outside the field of view during these first 3 weeks. We also cut the last day of the mission because of high background rates. This can be seen in the plots below, which show the elevation of the Crab above COSI's horizon. The left plot is for the simulated data, using the entire flight time, and the right plot is for the balloon data with the time cuts applied. As can be seen, the Crab was in COSI's field of view mainly during the latter part of the mission.
Otherwise, the data selection is identical to that of the simulated data. For more details on the 2016 COSI balloon background rates see Siegert et al. 2020.
For the data challenge, the simulated background is constant in time. With the actual balloon data, however, the background rate varies significantly, mostly driven by changes in the latitude (i.e. geomagnetic cutoff) and the altitude (i.e. atmospheric depth) of the balloon. Additionally, the application of the pointing cut produces drastic variation in the light curve as the Crab moves in and out of COSI's field of view. To accomodate these variations, in the fit we apply a so-called "tracer". The tracer is a factor that is applied to the background response and effectively normalizes the background rate to the actual data in each time bin. More specifically, for each time bin, the tracer is calculated by summing the counts over the energy and Compton data space dimensions, and then normalizing by the mean of this sum over all time bins. The difference between the background rates and models with and without a tracer can be seen below, for the balloon data analysis (right) and simulated data analysis (left), respectively.
Results for the Crab fit are shown below. Upper limits are shown for bins with SNR < 3. The fit will output a .dat file, where you can examine the numerical results, including the SNR of each bin. We have verified that bins 4, 5, and 6 are consistent with the physical values (based on our simulated Crab spectrum). The other bins (and most notably, bin 3) are within roughly a factor of ~2 of expectations. It is important to note, however, that the extracted spectrum has a significant dependence on the background model being used. Here we are using a highly simplified constant Ling model (with tracer). Additionally, we haven't accounted for activation backgrounds, nor other astrophysical sources (i.e. 511,
IMPORTANT NOTE: The color scale is in arbitrary units. The COSI team is currently working on determining the level of the flux.
The primary outputs of the imaging, after 100 iterations, are shown below. Note that there is an artifact in the southern pole, where bright emission can be seen. This feature should be surpressed with more iterations. The cyan star in the map shows the known position of the Crab, which overlaps with the brightest pixel in the Richardson-Lucy image (and the dimmer surrounding pixels).
