Save the Sky: Adventures in Sky Monitoring

Robert J. Nemiroff

Who am I?Most cited science papers:

GRBs: time dilation, cosmology, lens searchesMicrolensing: finite source size effects, AGN BLR probe

Favorite science papers:On the Probability of Detection of a Single Gravitational Lens (1989)Visual Distortions Near a Black Hole and Neutron Star (1993)Toward a Continuous Record of the Sky (1999)Tile or Stare? Cadence and Sky-monitoring Observing Strategies That Maximize the Number of Discovered Transients (2003)

Who am I?(Know your visitors)

Web:Black hole movies at: http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.htmlGR correct! (Could make another IAS talk)

Astronomy Picture of the Day at:http://apod.nasa.gov/NASA’s top-ranked site!

Save the Sky

What happened in the sky last night?Supernova? Nova? Eta Carina flare? GRB afterglow? Undocumented flash? Flurry of sporadic meteors?Clouds obscure your remote observing?Cirrus affect data on Jan 22 at KPNO?Are clouds rolling in just now?Is last night’s sky gone forever?

Save the SkyPopular Name: The Night Sky Live Project

Web address: http://concam.net

Deploys CONtinuous CAMeras (CONCAMs)

CONCAM: ObjectivesPrimary Science

Unprecedented temporal monitoring for GRB OTs, meteors, variable stars, comets, novae, supernovae

Support ScienceUnprecedented ability to act as instantaneous cloud monitors, archival cloud monitors, generate all-sky transparency maps, all-sky emissivity maps

Education / OutreachUnprecedented ability to show your class last night’s (real) sky, archival skies, monitor meteor showers in real time, show educational sky movies, run educational modules

CONCAM Locations

Save the Sky: 4 CONCAM locationsKitt Peak Mt. Wilson

Mauna Kea Wise Obs.

CONCAM: HardwareCONCAMs are essentially fisheye lenses attached to CCDs run by a PC computer and connected to the internet. CONCAMs do not move - they are completely passive.

Most simply put: light comes in the top, electricity comes in the bottom, and data flow out the bottom.

In building CONCAMs, we have three montras:

“If it moves, it breaks.” “The lens IS the dome.” “Don’t spend 90% of your time trying to get 10% more images.”

CONCAM: DataAll recent images are available through http://concam.net All data are free and public domain. All FITS and JPG data are archived to DVDs (previously CDs). Each CONCAM node generates about 500Mb of raw image data per night. Higher level data products (e.g. photometry) are now being generated in real time for some CONCAMs.

CONCAM Scientific MilestonesFirst CCD device to image the position of a gamma-ray burst during the time of the gamma-ray burst trigger (#1: GRB 001005)Most complete, global, and uniform coverage of a meteor storm: the 2001 Leonids Most complete light curves for hundreds of bright variable stars starting from May 2000, when the first CONCAM was deployed on Kitt Peak.First devices to give real-time optical ground truth for the whole sky in support of major astronomical telescopes, including Gemini North, Keck, Subaru, IRTF, SpaceWatch, Wise, ING 4-m, Mayall 4-M, SARA, and WIYN.In May 2003, fisheye night sky webcams now image most of the night sky, most of the time. For example, were SN 1987A to go off tomorrow, there would be a good chance that a CONCAM saw it.

Tile or Stare?A sky monitor’s classic conundrum

Sky monitoring increasingCurrent Projects (see BP webpage: abridged, expanded)CONCAM R. J. NemiroffKAIT A. FilippenkoLINEAR LINEAR teamLONEOS T. BowellLOTIS H. S. ParkMEGA A. CrottsNEAT E. HelinRAPTOR W. T. VestrandROTSE C. AckerloffSpacewatch R. S. McMillanSTARE T. M. BrownSuperMACHO C. StubbsTAOS C. AlcockYSTAR Y. I. Byun

Tile or Stare?Likely future sky monitoring projects include (much abridged):

Pan-STARRS N. KaiserLSST A. TysonGLAST P. F. Michelson

Tile or Stare?: AssumptionsGeneric case considered here:

Transients are discovered and confirmed on a time-contiguous series of exposuresSky is isotropicEffective apparent brightness distribution of transients N(l) is already knownOnce discovered, transients are handed off to a separate follow-up telescope

“Tile or Stare” & tiling cadence determination important for:microlensing, GRB OTs, supernovae, planet detection, binary star eclipses, stellar flares, blazar flares, QSO flares, Near Earth Objects, comets, meteors & more ...

Tile or Stare?The Two Key Power Indices: ,

Variables: N: effective apparent cumulative brightness distribution of transientsldim: 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

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.

Save the Sky: Cadence

Tile or Stare: Cadence

Tile or Stare: Cadence

Tile or Stare? Decision SummaryIf, 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.

Tile or Stare? SuperMACHOObjective: maximize microlensing transients discoveredLMC N(l) has < 1: tile beats stare for identical fields

what cadence?LMC not isotropic: fields with highest N(ldim) preferred

N(l) may change with seeing or be better determined with timeTherefore, choosing the next field to observe is very complicated -- not unlike a chess game. Optimization might involve real-time Monte-Carlo simulations.

Field return rate still attracted toward transient “duration of interest”

faster cadence inefficiently re-discovers known microlenses (competes with field richness at ldim)“duration of interest” may be the microlens rise time: ~ two weeks, although microlens rise times have wide variety of durations

Tile or Stare?: LSSTObjective (example): maximizing Type IA supernovae discoveredSky essentially isotropic (out of Galactic plane)N(l): > 1 for I < 24: stare preferred

effectively creates a minimum observation time per fieldN(l): < 1 for I < 24: tile preferred

what cadence?Return time (cadence) optimized at the “duration of interest”

faster cadence inefficiently re-discovers known supernovaeslower cadence inefficiently misses supernovae in neglected fields“duration of interest” could be rise time of SNe: ~ 15 days (1+z)

Different cadences will optimize discovery rates for different transients

might have Guest Investigators (GIs) program where GIs change filters and cadence to optimize discovery rate of GI-preferred transients

Tile or Stare?: GLASTObjective: maximize blazars (quiescent phase) discoveredGLAST’s survey mode constrains it to point away from the Earth, but rock at some cadence between the N&S Celestial Poles.N(l) away from Galactic Plane: > 1: stare

stare = GLAST Deep Field (GDF); should maximize detectionsstare only possible at NCP, SCP or during pointing modeGDF exposures should end if/when faint blazars saturate ( drops below unity)

N(1) in Galactic Plane: < 1: tileGDF strategy inefficient in Galactic Planequiescent nature allows co-adding at any time, cadence unimportant

, , GDF existence, GDF location are energy dependant.

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