What can gravitational waves tell us about neutron stars?

1. What can gravitational waves tell usabout neutron stars? Ben Owen TeV 2006 @ UW Madison August 30, 2006

2. Context • GW frequencies 10-1000Hz 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 What can gravitational waves tell us about neutron stars?

3. 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” Gravitational waves:How are they different? • 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 What 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 What 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 What 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? …) What 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  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… What 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 What 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 What 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 What 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 What 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 What 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 What 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! What can gravitational waves tell us about neutron stars?