Face-to-Face IDT Meeting Session 1 Observation Strategies and EGRET Experience S. W. Digel March 19, 2002 SLAC

EGRET Experience Observing constraints (from S. Hunter) x-z plane within 15° of the sun for radiators Moon, Jupiter, Saturn not in FOV of star cameras (45 or 90° off x-z plane) Atomic O was not a constraint (i.e., could look in direction of orbit) Major constraint: OSEE needed two targets Scheduling software (constraint checker) written by R. Kroeger (NRL) Typical viewing period ~2 weeks inertial pointing (with pointing accuracy requirement comparable to GLAST) Observing time was lost during earth occultations (trigger modes below horizon disabled) All-sky survey, but very non-uniform exposure COMPTEL z EGRET OSSE BATSE x

Considerations for LAT Observing Strategies Constraints: 28.5° inclination orbit, SAA perimeter Slewing profiles Maximum angle from zenith Others? Degrees of freedom: Define where GLAST spends its time within this circle: orbital plane ‘rocking’ zenmax ­ 105° - 55° ‘zenith pointed’

Uniformity of Exposure? GLAST Science Req. Doc. (433-SRD-0001) says must have <±20% uniformity of exposure on time scale of 7 days, not including effects of SAA. Goal is <±10%. [Don’t tell anyone: SRD doesn’t specify the energy] What is uniform at one energy might not be as uniform at a different energy. (AO-Era, don’t take literally) What are the scientific drivers for uniformity of exposure? As opposed to, say, uniformity of sensitivity To not miss the bright AGN flares? (How often do they occur and how bright are they?) To avoid exposure biases in likelihood analysis?

Beyond Uniformity of Exposure Idealized simulations presented here Instantaneous step rocking No accounting for azimuth (location of the sun) No consideration for possible reorientation for downlink, ToOs Averaged over an integral number of precession periods (equivalently, a long time), the exposure depends only on declination Precession period ~55 days Old news: The exposure can be made remarkably independent of declination for representative energies The effective PSF, however, cannot. In the analysis, effective PSF may not apply, but still a useful concept to make discussion of observing strategies semi-quantitative

Beyond Uniform Exposure 2 Dist of coverage Corresponding effective PSFs have angular diameters that vary by 10% or more Likewise (or even more so), the sensitivity to point sources cannot be made uniform with a simple observing strategy, because the diffuse interstellar emission is much brighter near the Galactic equator than at high latitudes EGRET (>100 MeV) Inclination

Beyond Uniform Exposure 3 Rule of thumb scaling Step rocking has the appeal of giving at least some exposure everywhere in the sky every other orbit – for LAT to function as an all- sky monitor, but as we can see, it shortchanges the orbital plane (low declinations) and low Galactic latitudes in terms of sensitivity. d = 0° Galactic center and anticenter! Galactic unident. 3EG sources! EGRET (>100 MeV)

Pointed Observations vs. Scanning Except for low-b, low-d regions, it will be difficult to make the case for a pointed observation just to increase the exposure (i.e., increase the sensitivity for steady sources) Good scientific cases do exist for other kinds of pointed observations – e.g., periodicity searches or campaigns for known transient sources Tradeoff for inertially pointed (or ‘smart’ pointed) observations is how much occultation of the FOV by the earth will be tolerated Accumulation of exposure for inertially pointed observation of Vela

Strategies for Earth Avoidance Basic: Around the horizon vs. around the orbital pole One step up: Simulation comparing inertial pointing with a ‘smart’ observing mode that kept the LAT as close as possible to the desired viewing direction with constraints no part of FOV occulted slew rate < 15°/min for illustrative purposes here

Issues for Planned Pointed Observations Likely duration - (guess > weeks) Scheduling to optimize coverage/efficiency with respect to the 55-day precession period multiplexing targets separated by > FOV What to do in between times that the target is observable? slew back to be in position to catch the rise (minimum angular rate) ‘rock’ about this great circle segment slew to (or quite likely just toward) an alternate target slew to a celestial pole? How closely to track the source How far (in zenith angle) to track the source Lost efficiency to earth occulation Albedo gamma rays (must reject onboard)

More Pointed Issues How much time would be requirede to equalize sensitivity with typical high-latitude region? (A lot) Should this be part of the sky survey year? For sky survey planning: Issues are effect of removing idealizations (finite slewing rates, azimuthal dependence, reorientations/interruptions) and deciding whether we really want uniform exposure