What can gravitational What can gravitational waves tell uswaves tell us

about neutron stars?about neutron stars?Ben Owen

TeV 2006 @ UW Madison August 30, 2006

Ben Owen What can gravitational waves tell us about neutron stars? 2

ContextContext

• GW frequencies 10-1000Hz mean terrestrial detectors - LIGO & VIRGO, not LISA

• Let’s look 10 years ahead (“advanced” configurations)

OutlineOutline

• Background on gravitational waves and neutron stars

• Periodic signals from rotating neutron stars• Chirp signals from NS/NS or BH/NS mergers• Burst signals from supernova core collapse & other blows

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Gravitational waves:Gravitational waves:How are they different?How are they different?

Gravitational wavesGravitational waves• Couple to mass 4-current• Produced by coherent motions

of high density or curvature• Wavelengths > source size,

like sound waves (no pictures)

• Propagate through everything, so you see dense centers

Electromagnetic wavesElectromagnetic waves• Couple to electric 4-

current• Incoherent superposition of

many microscopic emitters• Wavelengths source size,

can make pictures• Stopped by matter, so

“beauty is skin deep”

NeutrinosNeutrinos• More like EM waves than GW in most respects, except…• Propagate through most things like GW, so you can see dense centers• But neutron stars don’t generate so many after first few minutes

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Neutron stars:Neutron stars:Composition and structureComposition and structure

• Mainly equation of state, equivalent to gross structure

• Indirect evidence for “chemical” composition & phase

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Periodic signalsPeriodic signals

• Astrophysical populations targeted:– Known pulsars (if P < 200ms & high Pdot)– Neutron stars w/o pulsations (if isolated, O(“) position resolution)

– Rapidly accreting neutron stars (low-mass x-ray binaries)– Neutron stars that haven’t been detected at all w/photons

• Interaction w/photon astronomy even at detection stage:– GW detectors “aim” in software, compensating for Doppler shifts– O(“) resolution with few months’ data, huge computational cost to search substantial sky areas or less sensitivity for fixed cost

– Can only take full advantage of GW data if photons help “aim”– Any new NS detections can be localized for photon followups

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Periodic signals:Periodic signals:Pulsar emission mechanismPulsar emission mechanism

• Pulse profiles in different EM bands illuminate mechanism

• Profiles show (phase) timing noise, mostly in young pulsars

• GW won’t show interesting pulse profiles (only lowest harmonic detectable)

• Will be able to test if GW signal has timing noise or not

• Tells us how magnetosphere is coupled to dense interior (Does B-field structure go all the way in? Just crust? …)

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Periodic signals:Periodic signals:How solid is a neutron How solid is a neutron

star?star?• NS definitely have (thin)

solid crust (known from pulsar glitches)

• Normal nuclear crusts can only produce ellipticity < few 10-7

• If “?” is solid quark matter, whole star could be solid, < few 10-4

• If “?” is quark-baryon mixture or meson condensate, half of core could be solid, < 10-5

• High ellipticity measurement means exotic state of matter

• Low ellipticity is inconclusive: strain, buried B-field…

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Periodic signals:Periodic signals:Accreting binariesAccreting binaries

• Low-mass x-ray binaries are best bet– Rapidly accreting (up to Eddington limit)– Rapidly spinning (up to 600Hz) … but why not faster?

– Spin mystery could be nicely solved by GW

• Emission mechanisms:– Elastic mountains

– Magnetic mountains

– R-mode oscillations

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Periodic signals:Periodic signals:Accreting binariesAccreting binaries

• Detecting GW at all confirms that– LMXB spins are regulated by GW emission (not B-fields)

– A particular binary contains NS (not BH) if no pulsations

• Ratio of GW frequency to spin frequency (from x-rays) tells us emission mechanism: 2 is mountain, 4/3 is r-mode

• If it’s r-modes, we learn much more…– Star has to contain some strange matter (else thermal runaway)

– Ratio is really 4/3 minus few % which tells us M/R

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Binary mergersBinary mergers

• Early inspiral tells us masses, spins (matched filtering w/1000s of cycles) and location (multiple detectors)

• BH/NS mergers easier to observe & calculate than NS/NS

• Last bits (less well modeled) tell about NS tidal disruption

• Correlation w/ short GRBs (same timescale) helps search

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Binary mergers:Binary mergers:Tidal disruptionTidal disruption

• Uniform density star: tidal disruption frequency is unique function of NS radius, BH mass (if 50msun or less) & spin

• Dust disk “star”: Plunge spreads spectrum around final BH mode ringdown frequency, separation of peaks proportional to NS radius

• Can confirm NS mergers as engines of short GRBs

• Compare arrival times for info on how long baryons hang around

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Burst signals:Burst signals:Supernova core collapseSupernova core collapse

• Burst from collapse and bounce

• Poorly modeled: different groups predict different waveforms, agree that there is no supernova explosion….

• Long GRBs: knowing time & location helps GW searches

• GRB/GW/neutrino relative delays could shed light on explosion mechanism

• If GW & signals are both short, result is a black hole

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Burst signals:Burst signals:Other hammer blowsOther hammer blows

• Sinusoidal signals ringing down, possibly frequency drift

• Just after supernova (proto-neutron star formation)– Triaxial instabilities, possible fragmentation instability– Many kinds of oscillation modes (r-modes possibly unstable) tell us about structure (possibly changes as PNS shrinks)

• Later in NS life cycle– Pulsar glitches: something “snaps” in solid part, must excite various modes at some level & emit GW (no direct evidence yet)

– SGR superflare: some evidence of crust t-modes (torsional) ringing in x-ray signal after Dec. 27, 2004; frequencies sensitive to crust composition, structure & B-field; GW help break degeneracy

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RecapRecap

• GW in concert with other messengers can tell us a lot:– Several ways of getting mass & radius, thus EOS

– Coupling of magnetosphere to dense core– Composition of dense core (direct evidence for strange particles of some sort)

– Solid/liquid fraction of neutron star– Baryonic environment in short & long GRB engines

• Future looks bright for GW in multi-messenger astronomy!