peter r. saulson syracuse university

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How will theory, observation, and instrument development interact?

Test case: If we find “unknown” GW bursts,

how will we figure out what they are?

Peter R. Saulson

Syracuse University

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Inspiration

Prediction is hard, especially about the future.

Niels Bohr

or Yogi Berra?

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Outline

• Role of angular resolution in understanding a new astronomical phenomenon

• Cautionary tale: the history of gamma ray bursts• Will we be luckier?

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How astrophysical phenomena get understood

Understanding a new astronomical phenomenon:

Needed: energy scaleHence, needed: distance

Could use purely geometric method, i.e. parallaxBut that is hard, and hasn’t worked for any of the “new astronomies”

Needs very accurate position measurement

Still only works very nearby

Key step: identification with objects already knownWhose distances are known via the distance ladder

Thus linked to stellar parallax

Historically, this is usually done by matching sky positions.

How good do positions need to be for this to work?

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Angular resolution

GW detectors have a very broad beam pattern.“Almost” isotropic

Position information from time delays between signal reception at different detectors in a global network.

Precision:

Historically, when has much finer angular resolution been required for identification?

When is coarse angular resolution sufficient?

timing.msec 0.1 baseline, globalfor degree, a~sin

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When fine resolutionwas required

Famous optical identifications:» Cygnus A (Baade and Minkowski 1954)

Identified with peculiar galaxy inside a radio error box of ½ arcmin diam.

» 3C273 (Schmidt 1963)Identified with 13th mag star with jet,

using position with errors of a few arcsec

» Sco X-1 (Gursky et al. 1966)Identified with 13th mag UV-excess star

inside error boxes of area 4 arcmin2

Identification requires:» Good position

» Counterpart that is both visible and unusual

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Precise position allowed the discovery of quasars

Time series from three different lunar occultations.

Precise locations, 2 components “Star” + jet, aligned with radio sources

It took arcsec precision to spot the odd starlike object (although with a jet!) coincident with 3C273.

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When coarse resolutionis enough

When enough objects are found that sky map reveals a distinctive pattern,

then, we can apply the time-tested method that dates to Harlow Shapley and H. D. Curtis.Great Debate of 1920 on the Galaxy Question.

This method was recalled in the 75th Anniversary Great Debate on the nature of gamma ray bursts. esp., paper by Paczynski

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Nearby stars

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Planetary nebulae

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Supernova remnants

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Globular clusters

Shapley found the shape and size of the Galaxy from this map.

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Nearby galaxies

Curtis used this map to argue that spiral nebulae are outside the Galaxy.

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Extragalactic radio sources

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Low mass X-ray binaries

Luck smiled in this case. Distance scale was obviously of order 10 kpc; energy scale linked LMXB phenomenon to neutron stars.

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The long sad storyof gamma ray bursts

Very crude positions (tens of sq. deg., from time delays) were enough to establish from the outset (Klebesadel et al. 1973) that GRBs didn’t come from very nearby sources.» Earth (i.e., nuclear explosions)» or any other place in the Solar System

There followed a long slow process of accumulating catalogs of events with only somewhat better positions, with awkwardly shaped error boxes.

Ten years after discovery, confusion still reigned.It took about twenty years for the story to become clear.

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What sky maps look like when error boxes are annuli

Could one recognize with any confidence that these maps represent an isotropic distribution?

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Gamma ray burstsfrom BATSE

GRO launched 1991, first results announced September ’91.

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Proper interpretationof log N – log S rel’n

Scale-free map came along with a number vs. luminosity relation that clearly had an outer scale.

Expected an outer boundary to the distribution in two cases:» If the distribution were galactic, but then expected the sky map to

be anisotropic.

» If the distribution were cosmological, with the outer boundary representing redshift effects.

This tipped the scales in favor of cosmological distances, but many were still not convinced.

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Breakthrough with BeppoSAX

1997 observations with multi-wavelength BeppoSAX satellite allowed observations of X-ray transients linked to GRBs. Error circles with radius of ~ 3 to 5 arcmin.

GRB970508 was then identified with an optical transient. Optical spectra showed an absorption-line spectrum with redshift z = 0.83.

In this case, several stages of finer angular resolution measurements used the transient character of the source to allow identification.

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Will we be luckier?

• Will we only find GW counterparts of known phenomena?That would be the most boring outcome.

• Will high SNR detections yield good positions, thus identifications?

• Will burst sources always have E/M counterparts?

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Will our sourcestell their own stories?

• Can the waveform of a gravitational wave burst be linked uniquely to identity of object, e.g. through the modal structure of BH or NS?

• Binary systems will have distinct polarization and waveforms, and endpoints will reveal mass scale.

• Collapse events have both a mass scale and a non-sphericity parameter – they will need distances to untangle.

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Can we localize a source better than this?

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Using theory, observation, and instrument development to make our own luck

• Push noise levels down.• Learn to use a global network.

Not just event timing, but detailed waveform determination will be key to getting positions, and to gaining other clues.

• Operate a network with many elements, good duty cycle, and with best possible performance.Need error boxes that are boxes, not annuli.

• Become confident enough to issue prompt alerts.• Run long enough to get lots of events.

Spatial patterns don’t emerge unless maps are full.

• Learn to sort events into classes, if necessary.• Be prepared to understand waveform information.

Numerical relativity, heuristic astrophysics are valuable.(But we don’t need more theories than events. )