Pointing mode status

Toby Burnett
14 November 2006

Introduction

This is a scheme to interleave background events when we are in pointed mode. The DC2 approach had to be modified since the Earth albedo changes dramatically as the Earth’s limb comes into view.

Design

The idea is based on the fact that the tuple has entries which can be used to predict both the trigger and downlink rates for the given source.

• Use the tuple name (or names?) as a key in an XML database.
• The database would have information to map values of the key(s) to:
• The trigger rate, for purpose of reducing the live time
• The downlink rate, used by the associated source to request events
• A file of events, or range of event numbers within a single file for the interleave, or perhaps organized by TTree within a single TFile
• The simulation mechanism would have to set the value for the tuple name and generate enough events for each value to properly sample the distribution

This is all set up in the package Interleave, see especially the class BackgroundSelection, and the methods triggerRate(), downlinkRate(), and selectEvent(), all of which need to be finished.

Earth albedo gammas

For this, the important orientation is the angle between the SC z-axis and the zenith. This is set by, e.g. FluxAlg.zenithTheta=90; I think that 25 equal intervals in the cosine of this angle is sufficient. The x-axis orientation should be the same for the pointed run and the simulations. Otherwise, we have to add it as a second variable to track for selecting events at least. The downlink rates have been measured, and are shown in Fig. 1

Figure 1

The increase at the left (nadir-pointing) is because the albedo gammas have entered the field of view. Here is a plot showing the directions of reconstructed gammas:

Figure 2 (“thetaGlast” is the same as “thetazenith” above.)

Charged cosmic rays

Fig. 3 is a plot showing that the downlink rate for primary protons vs. GLAST zenith orientation:

Figure 3 Each point represents 4.25 orbit-seconds.

Since events from this source seem not to depend much on the orientation with respect to the zenith, so we will factor this off, and generate these events separately. That is, all sources except the Earth albedo. This should be binned into ranges of the magnetic latitude for selection. Note that while this is a signed quantity, the sign is not relevant, so that plus and minus bins should return the same rates and map to the same events.