More on high cadence LAT

December 11, 2020 - Reijo Keskitalo, Julian Borrill and Sara Simon

Introduction

This post continues the development of the high cadence scanning strategy. Earlier posts on the topic:

• Modulated_scan_high_cadence_LAT
• High_cadence_LAT_from_Chile

Here, we explore the area of the sky that is continuously observed within some time window, driven by the requirements of the transient science goals. In particular we need to be able to follow the evolution of GRB light curves in order to be confident that we can reject false positives, which entails observing the same sky on several consecutive days.

Specifications

We use the azimuth-modulated high cadence strategy with the following parameters:

• 140 degree throw
• 30 degree avoidance radius around the Sun and the Moon
• 40 degree observing elevation
• observe with the full CHLAT : FOV = 8.4 degrees

Results

Top of the plot shows daily hit maps for first four weeks of January. We took all 365 hit maps and evaluated the common sky area in various windows. The sky fraction is plotted in the bottom of the plot.

We find that the common sky area depends strongly on the width of the window and day of the year. The dips in the common sky area occur when the Sun and the Moon obstruct disjoint sets of pixels. Wider windows allow for more Lunar motion leading to reduced common sky area.

Here we collect some numerical information in table form:

Common sky area

Window [days]	Minimum fsky	Min. fsky (best 90%)	Median fsky
1	0.380	0.423	0.468
2	0.327	0.389	0.439
5	0.244	0.303	0.360
7	0.200	0.254	0.315
10	0.117	0.193	0.257
14	0.003	0.095	0.199