Seeking Non-GR Signatures in GW Bursts

Transcription

1 ????????????? Seeking Non-GR Signatures in GW Bursts Peter Shawhan (U. of Maryland) AltGrav workshop Montana State Univ. April 7, 2013 LIGO-G v2 GOES-8 image produced by M. Jentoft-Nilsen, F. Hasler, D. Chesters (NASA/Goddard) and T. Nielsen (Univ. of Hawaii)

2 We can detect unmodeled GW signals, aka bursts 2

3 ?? Types of GW Burst Searches Empirical definition: transients other than fully-modeled CBCs Modeled burst search Targets: Black hole ringdown Neutron star ringdown Cosmic string cusp Parabolic encounter Use matched filtering Issues generally similar to CBC searches Generic burst search Targets: High-mass binary black hole merger Stellar core collapse Other loosely modeled sources Possible signals deviating from model expectations, or from unanticipated sources Use robust detection methods that do not rely on having a model of the signal Can do both all-sky and externally triggered searches 3

4 ?? Excess Power Burst Search Methods Frequency Decompose data stream into time-frequency pixels Fourier components, wavelets, Q transform, etc. Several implementations of this type of search Normalize relative to noise as a function of frequency Look for hot pixels or clusters of pixels Can use various (Dt,Df ) pixel resolutions Time 4

5 ?? Advanced GW Detector Network GEO-HF KAGRA Advanced LIGO 4 km 4 km 4 km 600 m 3 km 3 km Advanced LIGO Advanced VIRGO LIGO India 5

6 ?? Burst Signal Consistency Tests Coincidence Crucial, since an instrumental glitch may look just like a GW burst in a single detector! Require signals in different detectors to have compatible times, frequencies, time-frequency pixels, maybe amplitudes Cross-correlation Look for common signal buried in two data streams H1 L1 Time Looks for consistent waveform shape, regardless of relative amplitude Best to integrate over a time interval comparable to the target signal 6

7 ?? Coherent Burst Analysis Assuming GR Each detector measures a linear combination of h + (t) & h (t) with antenna response factors and relative time delay depending on direction of arrival x 1 x 2 x N = F + 1 F 1 F + 2 F 2 F + N F N h + h + n 1 n 2 n N data = response signal + noise Data from 2 sites could uniquely determine h + (t) and h (t) for an arbitrary signal, if there were no noise and if the arrival direction is known Data from 3 or more sites over-determines h + (t) and h (t) if the arrival direction is known, providing consistency checks 7

8 ?? Geometric View of Coherent Analysis N 2 dimensional null space Null sum detector data Coherent sum: Find linear combination of detector data that maximizes signal to noise ratio Coherent sum 2 dimensional signal space Treat this as a maximum likelihood problem Find most likely h + (t) & h (t), maximizing over arrival directions Null sum: Linear combination of detector data that has no GW signal provides consistency test Can use a regulator to penalize physically unlikely signal hypotheses Workhorse implementations: Coherent WaveBurst and X-Pipeline [ Klimenko et al., CQG 25, ; Sutton et al., NJPhys 12, ] 8

9 ?? Burst Searches Work! Detectability is pretty insensitive to exact form of signal Depends mainly on central frequency and total energy in signal Energy giving 50% detection efficiency for standard-candle sources at 10 kpc Figure from Abadie et al., PRD 85,

10 What if GR is not correct? What signatures could we look for? 10

11 ?? Wave Propagation Speed In some alternative theories, GW speed may be different from speed of light [ See, for instance, sec. 6.4 of Cliff Will s Living Review ] Lack of gravitational Cerenkov radiation implies that gravity can t be very much slower [ Moore & Nelson, JHEP 9, 23 (2001), hep-ph/ ] But gravity could also be faster than light! Most sensitive test: compare relative timing of GW signal and EM or neutrino signal from a GRB or supernova For an event at 100 Mpc, a speed difference of 1 part in yields an arrival time difference of ~100 seconds Choose search time window to allow for range of speed and emission times If difference is more than ~1 part in 10 8, won t see counterpart signal arrive! Without an EM trigger time, could still check consistency of arrival times at different GW detectors Typical burst time resolution ~0.5 ms test speed difference to ~5%, but harder if position of source is also being inferred from arrival times 11

12 ?? Burst Dispersion Frequency If gravitons have mass, then speed depends on energy Broadband burst gets dispersed in transit v 2 g = 1 m gc 2 2 c E Given observational limits on graviton mass, hf m g c 2 for f~100 Hz, so v g c 1 1 m g c 2 2 hf 2 Initial short burst becomes an inverse chirp in transit Assumes we know it was emitted as a short burst Note that a highly dispersed signal may be hard to detect Arrival time 12

13 ?? Additional Polarization Mode(s) In addition to the familiar transverse tensor modes, many alternative theories allow one or more additional modes For instance, transverse scalar breathing mode Can be sourced by monopole, dipole, Other modes in some theories: Longitudinal scalar Long.-transverse skew vector Einstein-aether scissor and trace (long. scalar amplitude = 2 transverse scalar) [Jacobson, private communication] From Cliff Will s Living Review 13

14 ?? Infer Distance from Mode Arrival Time Differences?? Like what is done with seismic waves Different modes travel at different speeds in some alt. theories If more than one mode is detectable, and speeds are known, arrival time difference provides an absolute distance measure! However, maybe that is asking for a lot 14

15 ?? Non-GR Source Dynamics Source dynamics, revealed by the GW signal, may differ from what is expected from GR Need a reliable source model to identify a quantitative change Not available for most burst sources (?) Another possible signature: qualitative change in GW burst signal e.g. if the source does something that it wouldn t do under GR Like, emit a GW burst Example: spontaneous scalarization of a neutron star In tensor-scalar theory with suitable nonlinear coupling of scalar field to metric Transition from unstable weak-scalar-field state to strong-scalar-field equilibrium state [ Damour & Esposito-Farèse, PRD 54, 1474, etc. ; Novak, PRD 58, ] 15

16 Could we really detect a non-gr polarization mode? 16

17 ?? What Does it Take? Simple detector-counting argument: Untangling N modes requires a network of (at least) N detectors Assumes that arrival direction is known, and that there is no noise However, that is too simplistic In practice: Need at least one additional detector to suppress noise / glitches Provide consistency check among detectors Especially important if arrival direction isn t known, since then have to consider all possible directions Anywhere in the sky that antenna response is low for one or more modes, lose ability to untangle them all 17

18 ?? Difficulty Separating Modes Network alignment factor α f / f + for LHO-LLO-Virgo [ Klimenko et al., PRD 83, ] Can measure only one polarization mode well (in appropriate basis) over a large fraction of the sky (Antenna response matrix is not reliably invertible) 18

19 ?? Difficulty Separating Modes Network alignment factor α for LHO-LLO-Virgo-Kagra [ Klimenko et al., PRD 83, ] Smaller bad regions, but this is for using four detectors to separate just two modes (+, ) 19

20 ?? GR Null Stream Search? As Neil noted yesterday, with 3 or more misaligned detectors, you can construct a null stream for the GR tensor modes Need to know the arrival direction to use correct relative time shifts In principle this offers a simple way to test for generic deviations from GR Check for non-gr admixture in GW burst found using standard GR search Or maybe even search for a non-gr burst without tensor-mode content? Caveats: other things that can produce a signal in the null stream Non-stationary detector noise (glitches) note that we often use a null stream as a veto in our searches! Incorrect sky position, particularly in all-sky search Detector calibration errors Maybe could do better with careful use of two GR null streams? 20

21 ?? SNR Issues If the non-gr mode(s) are weaker than the GR tensor modes, then it can easily be ambiguous Not significantly different from pure-gr signal, given detector noise Very challenging to detect a small admixture of extra mode(s) If the non-gr mode(s) are strong, are we sure we d even identify the signal as a candidate? Remember, our current best search algorithms assume GR is correct! To be (more) sure of catching such signals, should we: Do a modified coherent search tuned for an alternative theory or class of theories? Do a search based on simple coincidence? Or just live with degraded sensitivity for non-gr signals? 21

22 Start with a manageable case: Search for a scalar GW burst 22

23 ?? Adding a Scalar Polarization Mode Existence of a transverse scalar mode is common denominator in many alternative theories Assume for now that the scalar mode, like the others, propagates at (approximately) the speed of light Each detector measures linear combination of h + (t), h (t) & h s (t) with antenna response factors and relative time delay depending on direction of arrival x 1 x 2 x N = F + 1 F 1 s F 1 F + 2 F 2 s F 2 F + N F N s F N h + h h s + Could do a 3-mode coherent search, but subject to issues above n 1 n 2 n N data = response signal + noise 23

24 ?? Focus on Just the Scalar Mode Suppose that the scalar mode dominates in the received signal! Motivating picture: spherical (or nearly spherical) collapse events Stellar core collapse to neutron star or black hole Neutron star to black hole Spontaneous scalarization of neutron star Geometric coupling is totally scalar Coupling in the theory determines what wave amplitude propagates away If not quite spherical, tensor component may be present but sub-dominant Could we detect (scalar) GWs from core-collapse supernovae out to farther distances than we ve come to expect from GR?? 24

25 ?? Scalar Burst Sources Source dynamics and GW emission have been modeled (to some degree) in tensor-scalar theories. Examples: Shibata et al, PRD 50, 7304 Dust collapse in Brans-Dicke Novak & Ibáñez, ApJ 533, 392 Stellar core collapse in tensor-scalar Novak, PRD 58, Spontaneous scalarization of neutron star Novak, PRD 57, 4789 NS collapse to BH in tensor-scalar with various nonlinear couplings 25

26 ?? Scalar Burst Search Considerations Single-mode burst search is simple! More constrained than standard GR tensor-mode search Just requires using the correct direction-dependent antenna response Scalar + Assuming long-wavelength limit F s = sin 2 θ cos(2φ) Antenna patterns for other scalar modes are similar, e.g. longitudinal scalar and Einstein-aether [ Maggiore & Nicolis, PRD 62, ] 26

27 ?? Scalar Search Implementation Modified Coherent WaveBurst [Sergey Klimenko, Gabriele Vedovato] Single polarization mode with transverse scalar antenna pattern All-sky search (free to mis-reconstruct source position) Studying detectability of simulated signals [G.Vedovato, Scott Sullivan] Representative waveforms from tensor-scalar collapse models, as well as some ad-hoc burst waveforms Expect other scalar polarization modes to be similar (No actual search of LIGO-Virgo data so far) Parallel study in progress: Detectability of scalar bursts from core-collapse supernovae Modified X-Pipeline [Scott Coughlin] Single polarization mode with transverse scalar antenna pattern Known sky position of source fixes antenna response 27

28 ?? H-L-V Network Response for Scalar Wave [ Gabriele Vedovato ] 28

29 ?? What Have We Learned? Background (false coincidences) are pretty much the same Scalar-CWB successfully detects scalar signals Even ones which look more-or-less like step functions Standard tensor search does almost as well! Novak, PRD 58, Scalar signal Scalar search Scalar signal Tensor search 50% detectable amplitude: % detectable amplitude: 3.50 [Scott Sullivan] 29

30 ?? (Mis-)Reconstruction Scalar search Tensor search [Scott Sullivan] True Reconstructed 30

31 ?? (Mis-)Reconstruction Scalar search Tensor search [Scott Sullivan] True Reconstructed 31

32 ?? What Signal Strength Could We Expect? Consider, for instance, well-known class of tensor-scalar theories Damour & Esposito-Farèse parametrization of coupling function (actually the logarithmic derivative of the coupling): α φ = α 0 + β 0 (φ φ 0 ) α 0 describes the weak-field limit Subject to tight solar system observational constraints Jordan-Fierz-Brans-Dicke coupling parameter is directly related: ω BD 1/2α 0 2 β 0 is nonlinear term, and affects strength of GWs from collapse 32

33 ?? Signal Strength Dependence on β 0 Novak [ PRD 57, 4789 ] NS collapse to BH Get large signals for negative values of β 0 beyond 4 or so Connected to spontaneous scalarization 33

34 ?? Constraints from Binary Pulsars Friere et al., MNRAS 423, 3328 No measurable excess change in orbital period from dipole radiation Tightest constraints are from J NS-WD binary in 8.5-hour orbit Seems to pretty much rule out the spontaneous scalarization regime Implies scalar signals from core-collapse supernovae probably would only be detectable within our galaxy 34

35 Summary There are ways to test for deviations from GR without relying on accurate modeling of the signal Made a complete (??) list of types of signatures we could look for: Wave propagation speed Burst dispersion Additional polarization mode(s) Non-GR source dynamics We re currently studying detectability of (mostly) scalar GW bursts Simple to modify search algorithms to detect such signals Standard tensor search works well too! (Except for position ) Supernova-triggered search might benefit more

36 ??? Questions??? Are there other useful signatures? (Besides propagation speed, dispersion, polarization modes, non-gr dynamics) Are there any more specific alternative theories that we should focus on with GW burst searches? What s your favorite? What other modeling has been / will be done for burst-like GW signals in tensor-scalar or other alternative theories? Are there interesting theories which do NOT feature a scalar mode? Is it worth embarking on a dedicated search(es)? At what point will a 3-mode search be practical and worthwhile? What speed differences are plausible? Should we do non-gr searches for modeled bursts?