What can gravitational waves tell us about neutron stars? Ben Owen TeV UW MadisonAugust 30, 2006

Ben OwenWhat can gravitational waves tell us about neutron stars?2 Context GW frequencies Hz mean terrestrial detectors - LIGO & VIRGO, not LISA Let’s look 10 years ahead (“advanced” configurations)Outline 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

Ben OwenWhat can gravitational waves tell us about neutron stars?3 Gravitational waves: How are they different? Gravitational 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 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” Neutrinos 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

Ben OwenWhat can gravitational waves tell us about neutron stars?4 Neutron stars: Composition and structure Mainly equation of state, equivalent to gross structure Indirect evidence for “chemical” composition & phase

Ben OwenWhat can gravitational waves tell us about neutron stars?5 Periodic 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

Ben OwenWhat can gravitational waves tell us about neutron stars?6 Periodic signals: Pulsar 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? …)

Ben OwenWhat can gravitational waves tell us about neutron stars?7 Periodic signals: How solid is a neutron star? NS definitely have (thin) solid crust (known from pulsar glitches) Normal nuclear crusts can only produce ellipticity  < few  If “?” is solid quark matter, whole star could be solid,  < few  If “?” is quark-baryon mixture or meson condensate, half of core could be solid,  < High ellipticity measurement means exotic state of matter Low ellipticity is inconclusive: strain, buried B-field…

Ben OwenWhat can gravitational waves tell us about neutron stars?8 Periodic signals: Accreting 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

Ben OwenWhat can gravitational waves tell us about neutron stars?9 Periodic signals: Accreting 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

Ben OwenWhat can gravitational waves tell us about neutron stars?10 Binary 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

Ben OwenWhat can gravitational waves tell us about neutron stars?11 Binary mergers: Tidal 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

Ben OwenWhat can gravitational waves tell us about neutron stars?12 Burst signals: Supernova 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

Ben OwenWhat can gravitational waves tell us about neutron stars?13 Burst signals: Other 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

Ben OwenWhat can gravitational waves tell us about neutron stars?14 Recap 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!