We find that the selection has little effect on the shape of the curve or the location of its maximum and minimum. Next, I decided to explore whether the apparent sinusoidal variation might disappear if I discarded outlying values, which might conceivably vary differently in time than those which make up the bulk of the database. I pruned the bell curve at about one standard deviation, then used the remaining data to prepare the plot at the left. As you can see, the case for a sinusoidal variation is eroded somewhat, but the general shape, magnitude, and location of extrema is conserved. Finally, I decided to plot the average radiation flux against local sidereal time.
Sidereal time tracks the position of the distant stars as viewed from a given point on the Earth.
Correlation with the sidereal period is powerful evidence for a distant source for a given effect. For example, it was such correlation which provided early radio astronomers evidence the centre of the galaxy and Crab Nebula were celestial sources of the noise they were monitoring. Here's a plot of average background radiation flux by sidereal time. Any inference based on sidereal time is highly suspect if based on a database covering less than a year, and these data cover only three months, so the apparent bimodal curve may simply be the result of limited data collection time. It will be fun to see how the peaks in the local time and sidereal time evolve as the database grows over the next year.