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Tile or Stare? Optimal Search Strategies for GRBs and Afterglows

Tile or Stare? Optimal Search Strategies for GRBs and Afterglows. Robert J. Nemiroff Michigan Technological University. Tile or Stare? Abstract.

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Tile or Stare? Optimal Search Strategies for GRBs and Afterglows

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  1. Tile or Stare?Optimal Search Strategies for GRBs and Afterglows Robert J. Nemiroff Michigan Technological University

  2. Tile or Stare?Abstract Abstract: To maximize the number of GRB-related transients discovered on the sky, should sky-monitoring projects stare at one location or continually jump from location to location, tiling the sky? If tiling is preferred, what cadence maximizes the discovery rate? General principles behind finding optimal strategies and candences are discussed for initial discovery searches for both GRBs in the gamma-ray and orphan afterglows in the optical.

  3. Tile or Stare?General Transients: Assumptions Generic case considered here: • Transients are discovered on one time-contiguous series of exposures • Sky is isotropic • Effective apparent brightness distribution of transients N(l) is already known • Once discovered, transients are handed off to a separate follow-up telescope “Tile or Stare” & tiling cadence determination important for: • microlensing, GRBs, GRB OTs, supernovae, planet detection, binary star eclipses, stellar flares, blazar flares, QSO flares, Near Earth Objects, comets, meteors & more ...

  4. Tile or Stare?The Two Key Power Indices: , • Variables: • N: effective apparent cumulative brightness distribution of transients • ldim: apparent luminosity at obs. limit • te: exposure time • At the observation limit, quantify: • N  ldim(low background:   -1) • ldim te(high background :   -1/2) • N  te

  5. Tile or Stare?Read the General TvS AJ Paper! • Plenty of preprints should be available below. • Please don’t take this display copy! • Out of preprints? Send email to: nemiroff@mtu.edu

  6. Tile or Stare?A Mathematical Optimization • Find N(l) from existing observations (l: apparent brightness) • Find l(te) from detector, noise, and backgrounds (te: exposure time) • Compute N(te) -- might be conveniently parameterized in terms of power-law indices  &  • Estimate total time of campaign: tc (exact value usually not important) • Find grand total expected transients during campaign: Ng • Write Ng is terms of treturn, the time it takes for a survey to return to a given field (i.e. cadence). Read, down and slew times enter here. • Compute dNg/dtreturn, find solutions to dNg/dtreturn=0. • Find treturn that best maximizes Ng.

  7. Tile or Stare?Decision Summary • If, during exposure, the rate that transients come over the limiting magnitude horizon is increasing fast enough (  > 1), then stare should be preferred. • If, on the other hand, the rate that transients come over the limiting magnitude horizon is not increasing fast enough (  < 1), then tile should be preferred. • Usually the best tiling cadence is the duration of the transient, since a faster tiling cadence will waste effort on transients that have been previously discovered, while a slower tiling cadence will miss transients occurring in other fields. • If, however, the duration of the transient is comparable to the cumulative read-out and/or slew times during a sky-tiling, then a mathematical maximization as described in the preprint will find the most productive cadence.

  8. Tile or Stare?Finding GRBs in the gamma-ray • Objective: maximize GRB transients discovered in gamma-rays • GRB monitors usually well above background:  ~ -1 • For very bright GRBs: • N(l) ~l-1.5 so  ~ -1.5 •   ~ 1.5 • since  > 1 stare is preferred • most GRB monitors stare so no change is suggested • For dim (most) GRBs: • N(l)~l-0.8 so  ~-0.8 •  ~ -0.8 • since  < 1 tile is preferred • most GRB monitors stare so they are likely missing dim GRBs

  9. Tile or Stare?Finding GRBs in the gamma-ray • A GRB monitor that tiles the sky ? • Conceptually similarities to optical sky monitoring but faster time scale. • In other words: given detector area A, what is the optimal search strategy for GRB detection? • A Comparison: “Stare Detector”: spherical detector of area A that stares at the entire sky vs. • Tile Detector: A square detector of area A shielded to only see Omega of the sky at any time, rotating once per time t_rot. • GRBs with t_dur below t_rot: • Stare detector: will only tigger if P> Pmin (t_rot/t_dur) • Tile detector: will only trigger if in field, (t_rot/t_dur) of the time.

  10. Attributes of a GRB “Tile” DetectorScout vs. Cavalry • As most GRB monitors are “stare” monitors, they act as both a “scout”, finding GRBs, and the “cavalry”, taking all the data needed for scientific analysis in s. • A GRB “tile” monitor can act exclusively as a “scout”, more efficiently finding GRBs for hand-off to other “cavalry” instruments, nearby, far away, or at any energy, that can concentrate on the data taking.

  11. Tile or Stare?Finding Orphan Afterglows in the Optical • Assume N(orphan)  N(GRB) • So NOA~l-0.8 • Therefore  < 1 so “tile” is optimal • Return rate to a field should be on the order of the transient duration of interest. One such duration might be jet break. • practical bottleneck is the pipeline processing power needed to detect the afterglow given the bright and variable optical sky

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