Testing GR usingExternally Triggered Searches:

Astrophysical challenges

R. O’ShaughnessyUWM CGC 2010-05-26

Workshop on Gravitational wave tests ofalternative gravity

Outline

Context• Testing modified GR (example: mergers)• External trigger advantages (pure GR)• Challenges of a new parameter: Spin

– Astrophysical issues with spin

Astrophysical issues vs external triggers• Example 1: Short GRBs [detailed GW model, but mod GR competes with spin, geometry, EM-GW connection]

• Example 2: SN [qualitative timing ok; details hard: physics of explosion uncertain and encoded in wf]

• Example 3: SGR flares

Discussion slide

Challenge: Astrophysics of Source?Example: Dark matter test (Emre)• Host mass unknown

generic problemhigh precision astrophysics rare

(1) Astro, (2) mod GR effects often correlateCan we distinguish (2) given (1)?

OrientationMergers/short GRB as “canonical example”

Review: Testing modified GR (no EM/ν)• Phenomenologize cbc waveform

– Ex: PN phases -> independent parameters [Mishra, 1005.0304]

PPE [Y&P, 0912.2742]

• “Astrophysical” obstacles– Most sources near threshold (little signal power)

• Not much information• distributed b/t source parameters; limited available for additional tests?

– Many physical parameters w/ weak effect (i.e., spin)• Sometimes qualitative changes (I.e., spin precession)• Difficult to constrain all simultaneously @ low SNR [Bayesian posterior large]

– Matter coupling: possibly content-dependent @ (very) high PN order

Spin = helpful analogy - new parameters - small changes (weak spin, aligned) - large, qualitative changes (precession)

OrientationReview: External trigger advantages (pure GR)Lots of information from EM, neutrino “triggers”• Tracking signal [cf. Emre, propagation-based tests]

• Sky location, approximate timing (~ ubiquitous)

• Precise transient-event timing– Neutrino trigger for SN [Pagaroli 0903.1191; H&R 0908.2317]– Short GRB (mod opening angle)– SGR flare

• Phase information [phase accuracy limited by EM emission model & source stability/secular effects]

– SGR oscillations– Pulsar mountains (CW)

• Distance– Short GRB host galaxies (mod identification)– Galactic pulsars, SGRs, LMXBs

• Polarization constraints– Fix angle/content [Crab PSR nebula]

– Supernovae bounce mode (linear)• Tightly modeled sources: Proposed source model [e.g., short GRBs-> mergers; r-modes; NS osciillations]

– Candidate parameters in model [e.g., one component NS; circular polarization]• “Unmodeled”/poorly-understood sources

– Frequency content• directly in trigger (e.g., SGR oscillations)• Plausible frequencies from model (e.g., NS eigenmodes; LMXB r-modes)

– Energy scale– Event timescale (directly or via models)

OrientationReview: External trigger advantages (pure GR)What does it get us?

• Lower detection threshold– Here: possibly stronger GR tests? (or more events)

• Constraints on model <-> reduce model dimension– Here: enable constraints at all; allow tighter, if already possible

• Tracer signal on same geodesic (~same weak lensing, etc)– Here: enable relative timing constraints [cf. dispersive modifications of GR]

OrientationSpin: Example of new parameter• Coupling parameter (a)• Transition vs SNR: localize parameters with loud sources, not otherwise

ExamplevdS et al 0905.1323

OrientationSpin: Example of new parameter• Coupling parameter (a)• Transition vs SNR: localize parameters• Projection bias if not included

OrientationSpin: Example of new parameter• Coupling parameter (a)• Transition vs SNR: localize parameters• Projection bias if not included• Higher parameter dimension• New information provided

– Some astrophysical (via precession)-- diminishes amount for constraints

– Neither observations nor astrophysicsconstrain new parameters tightly

• Qualitative changes possible in– Orbit/dynamics (precession)– EM emission (e.g., tidal disruption): Rantsiou 2008 ApJ 680 1326 & in prep; Ettienne 2009 PRD 79 4024

• Residual disk mass, disruption geometry: spin, m2/m1 dependent• Can allow large ejection fraction (35%!)

OrientationSpin: Example of new parameter• Coupling parameter (a)• Transition vs SNR: localize parameters• Projection bias if not included• Higher parameter dimension• New information provided

– Some astrophysical (via precession) -- diminishes amount for constraints– Neither observations nor astrophysics constrain new parameters tightly

• Qualitative changes possible in– Orbit/dynamics (precession)– EM emission (e.g., tidal disruption): Rantsiou 2008 ApJ 680 1326 & in prep; Ettienne 2009 PRD 79 4024

• Residual disk mass, disruption geometry: spin, m2/m1 dependent• Can allow large ejection fraction (35%!)

• Ambiguity with astrophysics (e.g., tidal disruption point: EOS or a?) Ferrari 2010 PRD 81 4026

• “Composition-dependent” (trivially: not all bodies have same spin)

Spin distributedAlignment = encodes astrophysics

Star forming gasInteracting clusters’ stellar mass binaries

Random spin alignment

Isolated binaries~ aligned spins

References include•Belczynski, Kalogera, Bulik 2002; Belczynski•O’Shaughnessy et al. in prep + astro-ph/0610076; 0609465; 0504479

References include• Sadowski et al 2008•O’Shaughnessy et al PRD 76 061504 O’Leary et al astro-ph/0508224

Short GRBs and modified gravityIssue: Modified gravity vs spin:• Spin mimics proposed parametric phase changes (cf. Mishra et al)• Tight, nonzero spin alignment:

– Empirically likely (binary evolution)– Difficult to constrain independently from PN phenomenology particularly at low SNRs expected

• Question: What if mod GR breaks spin symmetries?– Example:

• Numerical simulations: amplitude depend ~ on• (Parity-violating) mod GR models might allow more times coupling parameter

• Question: Must resort to population statement (common effect to all)?

Issue 2: Modified gravity and composition:• Distinguishing NS vs low-mass BH companion?• Nuclear EOS very uncertain :

– Question: limits ability to constrain composition-dependent phasing changes, if tied to massdistribution (first-order correction?)

Short GRBs and modified gravityMeasuring modified gravity with mergers/short GRBs?• Coupling parameter, to be determined• Transition vs SNR: localize parameters

– Question: How loud do we have to be to localize coupling parameter? Does presence of trigger help us pass localization threshold?

• Higher parameter dimension– Question: Hierarchical search effective?

ExtTrig allows higher search dimension? How much larger?What types of mods accessible?

• Ambiguity with astrophysics– Question: Do we measure EOS properties (e.g., tidal disruption time for EM trigger), composition-

dependent properties * coupling parameter of mod GR?– Question: How is Fisher matrix b/t coupling, physical parameters structured?

Does ExtTrig source model help separate (e.g., face-on)

• Qualitative changes possible in– Question: Is ExtTrig-motivated GW, EM model preserved in new physics (e.g., aligned merger)

• Dramatic change in orbit?• Dramatic change in EM emission, orientations?

• Projection bias if not included– Question: Bias against detection if not included (ethinca/chisquared) & large?

SN and modified gravityIssue: Waveforms encode physics; not tightly modeled Example: Ott et al

Bounce (& core rotn rate; EOS)Convection/SASINeutrinosNS pulsations; bar modes; long-lived r-modesBH formation (collapse)Fragmentation

Conclusion:Timing is most robust part of waveBest coincident timing from neutrinos, not EMNeutrino detection rate low (small reach)GW range likely also small

SGR flares and modified gravityGood timing

Issues: GW, EM details not modeledM, EOS ambiguityEM-GW connection? [phase information?]

DiscussionAstro uncertainties contaminate:

Astro modifies amplitude & phase (spins), lag (for relative timing tests) Suggestion: Preferably tie constraints to robust, ~ model-independent properties:

arrival time (if using neutrinos, or high-energy carrier w/ no dispersion)amplitude (if calibrated)polarization (if LIGO-South present)

Discussion:– What SNR for measure mod GR? How much does (1) timing; (2) face-on merger model help change effective threshold,increase searchable

dimension?– How significant, if at all, are composition-dependent uncertainties (e.g., BH-NS vs NS-NS merger in scalar-tensor)?

Propagation vs wave generation effects:Wave generation in mod GR couples to binary parametersPropagation does not.Discussion:

Are there separable effects (e.g., birefringence)?

ExtTrig + other detections as model guide: (e.g., short GRBs)

Population (GW or EM) constraint on spins, L.S, opening angleBH, NS masses. NS EOS.Suggestion: Strong prior from other analyses; not in isolation

Constraints conditional on what nature provides. ExtTrig guidance to fine details (e.g., EM->residual NS torus mass->spin, if EOS known?)

“Large” modified-GR effects?What if qualitative changes (not 1st order) : new polarizations; strong birefringence; resonant

propagation; other “precession-like” dynamical changes late in merger?Discussion: ExtTrig permits nonparametric tests (=burst searches), esp for sources @ known z Assess how robust @ identifying large changes? [Discussed earlier today; also w/ mod-GR templates]

Which wins: golden event or population?