Hole in extraGalactic emission

I. The facts:

When I tried the source detection methods on the checkout 3 (55 days) data, I noticed that the diffuse model did not seem to fit well the data. To push the matter further, I looked at images without the sources (easy to do with the MC_SRC_ID column). It confirmed the problem. To understand whether the problem came from the exposure map or from my reconstitution of the Galactic diffuse emission, I split the diffuse emission into Galactic (MC_SRC_ID=0) and extraGalactic (MC_SRC_ID=3). I then noted a strange hole in the extraGalactic photons. In Galactic coordinates it appeared with a sinuous shape, but when I looked at the same image in Equatorial coordinates (all sky over all energies, 0,0 at center) I saw that:

There exists a clear deficit of photons near the Equatorial anticenter, shaped as a disk with sharp edges and radius 65° or so. There is no such feature in the exposure map, which has a maximum contrast of only 20% between maximum and minimum, not aligned with that feature. To quantify the deficit, I have produced a profile in a band of 40° width along the Equatorial plane:

The count rate in the hole is about constant at one third that elsewhere. I don’t imagine what real effect could cause that, so I imagine there must be a bug in the simulation.

II. The history

I then wondered whether the same feature was already present in the checkout 2 (30 days) data. It is much more difficult to see there because the photons are not tagged with an MC_SRC_ID column. The full image in the same coordinates (and SQRT distributed colors carefully chosen to emphasize what I want to show) is:

When one knows where to look, I think it is clear that the same hole is there as well.

I then looked back at the DC1 data and … it’s not there ! Finally, I looked at the Galactic diffuse emission. No such hole there either.

III. Proposed fix

Rather than redo the entire simulation, it would be easy to excise the extraGalactic diffuse photons (MC_SRC_ID=3) from the simulated data and run a specific simulation of the extraGalactic diffuse emission (in the same conditions) after the bug is fixed. Then all one would have to do is add a fifth (extraGalactic) events list to the four (excised) already existing ones.

For the checkout 2 data, it is impossible to remove the already simulated extraGalactic photons. One would have to simulate only the missing events (in the hole). If this is possible, then one could add a secondary events list in the same way.

IV. Solution

It was found that the bug disappeared between ScienceTools v6r0 (used to simulate the data) and the current v6r0p4.

Because the simulated data has other problems (incorrect timing, missing 3EG sources), it was decided to regenerate the full data set from scratch.