October 20, 2007 LSC-VIRGO / NS meeting 1

Interactions between gravitational waves and photon astronomy

(periodic signals)

Ben Owen

October 20, 2007 LSC-VIRGO / NS meeting 2

Intro

• We can look for things better if we know more about them from photon astronomy (we think of 4 NS populations)

• Photon astronomy sets indirect upper limits on GW - milestones for sensitivities of our searches

• GW emission mechanisms influence where we look

• Our interpretation of our results depends on emission mechanisms and previous indirect upper limits

• Some review in Abbott et al gr-qc/0605028

October 20, 2007 LSC-VIRGO / NS meeting 3

GW emission mechanisms

• Non-accreting stars (indirect limits beatable now!)– Free precession (looks pretty weak, I’ll skip)

– Elastically supported “mountains” - internal too

– Magnetically supported mountains (Melatos talk)

• Accreting stars (indirect limits beatable with advLIGO…?)– Accretion provides natural mountain building mechanism

– R-mode oscillations build themselves (CFS instability)

– More likely to radiate at indirect limits

• All mechanisms: how high is max & how to drive it there?– Put strength in terms of ellipticity ~ quadrupole, propto h

October 20, 2007 LSC-VIRGO / NS meeting 4

Elastic mountains• How big can they be? (Owen PRL 2005)

– Depends on structure, shear modulus (increases with density)

• Standard neutron star – Bildsten ApJL 1998, Ushomirsky et al MNRAS 2000– Thin crust, < 1/2 nuclear density: < few10-7

• Mixed phase star (quark/baryon or meson/baryon hybrid)– Glendenning PRD 1992 … Phys Rept 2001– Solid core up to 1/2 star, several nuclear density: < 10-5

• Quark star (ad hoc model or color superconductor)– Xu ApJL 2003 …, Mannarelli et al hep-ph/0702021– Whole star solid, high density: < few10-4

• Also Lin PRD 2007, Haskell et al arXiv:0708.2984

October 20, 2007 LSC-VIRGO / NS meeting 5

Elastic mountains in accreting stars

• How to build high mountains?

• Non-uniform accretion flow hot & cold spots on crust

• Hot spot at fixed density faster electron capture layer of denser nuclei moves upward (non-barotropic EOS)

• If GW balance accretion, is determined by x-ray flux

• Best (Sco X-1) is few10-7, same as predicted max for normal neutron star crust

Bildsten ApJL 1998, Ushomirsky et al MNRAS 2000

October 20, 2007 LSC-VIRGO / NS meeting 6

R-modes in accreting stars

• Complicated phenomenology (Stergioulas Living Review)

• 2-stream instability (CFS)

• Viscosity stabilizes modes

• Accretion keeps star balanced at critical frequency … if strange particles are in core

• Max perturbation v/v ~ 10-5 from coupling to other modes

• GW frequency = 4/3 spin freq. minus few % (depends on EOS)

October 20, 2007 LSC-VIRGO / NS meeting 7

Four types of neutron stars

• Known pulsars (e.g. Crab)– Position & frequency evolution known (including derivatives,

timing noise, glitches, orbit) Computationally inexpensive

• Unseen neutron stars (e.g. ???)– Nothing known, search over position, frequency & its derivatives

Could use infinite computing power, must do sub-optimally

• Accreting neutron stars (e.g. Sco X-1)– Position known, search over orbit & frequency (+ random walk)– Emission mechanisms different indirect limits

• Non-pulsing neutron stars (“directed searches” e.g. Cas A)– Position known, search over frequency & derivatives

(P>50ms is off our radar)

October 20, 2007 LSC-VIRGO / NS meeting 8

Indirect upper limits

• Assume quadrupole GW emission • Use predicted M, R, I (could be off by 2)• Assume energy conservation & all df/dt from GW• Known pulsars - “spin-down limit”

– Best is Crab at 1.410-24

• Non-pulsing NS - substitute age = f/(-4df/dt)

– Best is Cas A at 1.210-24

October 20, 2007 LSC-VIRGO / NS meeting 9

Indirect upper limits• Accreting stars - energy conservation violated

– Assume accretion spin-up = GW spin-down (Wagoner ApJL 1984)

– Infer accretion rate from x-ray flux

– Best is Sco X-1 at 210-26

• Unknown neutron stars - ???– Assume simple population model

– Plug in supernova rate in galaxy

– Most optimistic estimate is 410-24 (Abbott et al gr-qc/0605028)

October 20, 2007 LSC-VIRGO / NS meeting 10

Known pulsars

• What we’ve published:– Limits on 1 pulsar in S1: Abbott et al PRD 2004

– Limits on 28 pulsars in S2: Abbott et al PRD 2005

– Limits on 78 pulsars in S3 & S4: Abbott et al PRD 2007

– Note Kramer & Lyne in “et al”: timing data was crucial!

– Best limit was 310-25 for PSR J1603-7202

• When it gets interesting:– Last year (S5) for the Crab! (Pitkin talk)

October 20, 2007 LSC-VIRGO / NS meeting 11

Known pulsars

Crab, IL = 710-4

J0537-6910, IL = 910-5

J1952+3252, IL = 110-4

95% confidence thresholdby end of S5

October 20, 2007 LSC-VIRGO / NS meeting 12

Known pulsars

• What we’ve published:– Limits on 1 pulsar in S1: Abbott et al PRD 2004– Limits on 28 pulsars in S2: Abbott et al PRD 2005– Limits on 78 pulsars in S3 & S4: Abbott et al PRD 2007– Note Kramer & Lyne in “et al”: timing data was crucial!– Best limit was 310-25 for PSR J1603-7202

• When it’s interesting:– Last year (S5) for the Crab! (Pitkin talk)

• Where we’re going:– Now 97 of 160+ pulsars in our band … but want more! Timing!– Further down the road: SKA would provide us with many more

October 20, 2007 LSC-VIRGO / NS meeting 13

Unseen neutron stars

• What we’ve published:– S2 10 hours coherent search (Abbott et al gr-qc/0605028)

– S2 few weeks semi-coherent search (Abbott et al 2005)

– S4 few weeks semi-coherent searches (Abbott et al arXiv:0708.3818)

– Best strain upper limit is 210-24 (sky & polarization combo)

• When it’s interesting:– Already comparable to supernova limit, though that’s fuzzy

October 20, 2007 LSC-VIRGO / NS meeting 14

Unseen neutron stars

October 20, 2007 LSC-VIRGO / NS meeting 15

Unseen neutron stars

• What we’ve published:– S2 10 hours coherent search (Abbott et al gr-qc/0605028)

– S2 few weeks semi-coherent search (Abbott et al 2005)

– S4 few weeks semi-coherent searches (Abbott et al arXiv:0708.3818)

– Best strain upper limit is 210-24 (sky & polarization combo)

• When it’s interesting:– Already comparable to supernova limit, though that’s fuzzy

• Where we’re going:– S4 & S5 longer datasets (longest coherent integration 25 hours)

– Einstein@Home now on S5 - like SETI@Home but LIGO data, download from http://einstein.phys.uwm.edu

October 20, 2007 LSC-VIRGO / NS meeting 16

Directed searches

• What we’re doing:– Cas A (youngest known neutron star?) ~10 days S5

– Galactic center (innermost parsec, good place for unknowns)

• When it’s interesting:– Cas A and any ~100yr old star in center have hIL ~ 110-24

– Doable with present sensitivity!

– Anything detectable now would require solid quark matter

October 20, 2007 LSC-VIRGO / NS meeting 17

Directed searches

IL = 10-5

IL = 10-4

October 20, 2007 LSC-VIRGO / NS meeting 18

Directed searches

• What we’re doing:– Cas A (youngest known neutron star?) ~10 days S5

– Galactic center (innermost parsec, good place for unknowns)

• When it’s interesting:– Cas A and any ~100yr old star in center have hIL ~ 110-24

– Doable with present sensitivity!

– Anything detectable now would require solid quark matter

• How photon astronomers can help:– Narrow positions on suspected neutron stars (e.g. HESSChandra):

arcminute is OK, arcsecond is better

– Find more young isolated neutron stars, small PWNe and SNRs

October 20, 2007 LSC-VIRGO / NS meeting 19

Accreting neutron stars in LMXBs

• What we’ve published (Sco X-1):– S2 6 hours coherent integration (Abbott et al gr-qc/0605028)

– S4 20 days incoherent “radiometer” (Abbott et al astro-ph/0703234)

– Best strain upper limit is 310-24 at 200Hz

• When it’s interesting:– 100 lower than that (Watts talk)

– What kills our sensitivity? Not knowing frequency (orbit too)

• What we’re doing:– Trying to come up with better methods (Krishnan talk)

– Other sources? (Chakrabarty talk, Galloway talk)

October 20, 2007 LSC-VIRGO / NS meeting 20

Observational interactions

• Timing data for known pulsars– Jodrell Bank, several others have agreed to more timing– RXTE: J0537-6910 (Marshall et al)

• Timing data for LMXBs– Keeping RXTE alive would be a good thing…– Make friends in India: AstroSat?

• New discoveries (& proposed discoveries)– When you hunt new PSR/CCO/etc, think of indirect GW limits

• Old discoveries– Several NS positions poorly known (ROSAT/XMM), firming up

with Chandra or Hubble would help our searches

October 20, 2007 LSC-VIRGO / NS meeting 21

Theory(-ish) interactions

• Interpretation of upper limits– Beating indirect limits on h is more exciting

– How fuzzy are indirect limits? Distances, braking indices…

– Can’t rule out equations of state (stars could just be flat) unless we know mountain building, so what builds mountains?

• Interpretation of detections (let’s hope!)– Frequency confirms emission mechanism (LMXBs)

– R-mode signal means strange particles in core

– High ellipticity means funny equation of state

– Somewhat high means EOS or high internal B field: what max?

October 20, 2007 LSC-VIRGO / NS meeting 22

Wrap

• Starting to get interesting sooner than we thought

• More interesting faster w/help from photon astronomy

• Lots of theory stuff to think about too, even if we don’t see anything until advanced LIGO

• Download Einstein@Home!