GRAVITATIONAL WAVE DETECTION OF ASTROPHYSICAL

SOURCES

BARRY C. BARISH

California Institute of Technology, Pasadena, CA 91125, USA

E-mail: barish@ligo.caltech.edu

ABSTRACT

The quest for the detection of gravitational waves from astrophysical sources isreviewed; in particular, the use of long baseline suspended mass interferometeron the earths surface with broad-band sensitivity covering a frequency range ofabout 10 to 104 Hz. As a specific example, the sensitivity to spinning pulsars inour galaxy from the LIGO interferometers is shown.

1. Introduction

Gravitational waves are a necessary consequence of Special Relativity with its

finite speed for information transfer. Einstein in 1916 and 1918 1) 2) put forward theformulation of gravitational waves in General Relativity. He showed that time depen-

dent gravitational fields come from the acceleration of masses and propagate awayfrom their sources as a space-time warpage at the speed of light. This propagation is

called gravitational waves.Evidence of these waves resulted from the beautiful observations of Russell Hulse

and Joseph Taylor in their studies of a neutron star binary system PSR1913+16.They discovered this particular compact binary pulsar system in 1974. The pulsar

frequency is about 17/sec. It was identified as being a binary system because they

observed a variation of the frequency with just under an 8 hour period. Subsequentmeasurement accurately determined the characteristics of the overall binary system

with remarkable precision. The most important parameters for our purpose are thatthe two neutron stars are separated by about 106 miles, have masses m1 = 1.4M� and

m2 = 1.36M�, and the ellipticity of the orbit is ε = 0.617. They demonstrated thatthe motion of the pulsar around its companion could not be understood unless the

dissipative reaction force associated with gravitational wave production was included.The system radiates away energy, presumably in the form of gravitational waves, and

the two neutron stars spiral in toward one another speeding up the orbit. In detailthe inspiral is only 3 mm /orbit so it will be more than 106 years before they actually

coalesce.The theoretical motivation for gravitational waves, coupled with the experimen-

tal results of Hulse and Taylor, make a very strong case for the existence of suchwaves. The direct observation of gravitational waves offers the possibility of studying

gravitation in highly relativistic settings, offering tests of Relativistic Gravitation in

the strong field limit, where the effects are not merely a correction to Newtonian

Figure 1: A schematic view of a suspended mass interferometer used for the detection of gravitationalwaves. A gravitational wave causes one arm to stretch and the other to squash slightly, alternatelyat the gravitational wave frequency. This difference in length of the two arms is measured throughprecision interferometry.

Gravitation but produces fundamentally new physics through the strong curvature ofthe space-time geometry. Of course, the waves at Earth are not expected to be other

than weak perturbations on the local flat space, however they provide information onthe conditions at their strong field sources. The detection of gravitational waves will

also allow determination of the wave properties such as their propagation velocityand polarization states.

In terms of astrophysics, the observation of gravitational waves will provide a verydifferent view of the Universe. These waves arise from motions of large aggregates of

matter, rather than from particulate sources that are the source of electromagnetic

waves. For example, the types of known sources from bulk motions that can lead togravitational radiation include gravitational collapse of stars, radiation from binary

systems, and periodic signals from rotating systems. The waves are not scattered intheir propagation from the source and provide information of the dynamics in the

innermost and densest regions of the astrophysical sources. So, gravitational waveswill probe the Universe in a very different way, increasing the likelihood for exciting

surprises and new astrophysics.A new generation of detectors (LIGO, GEO, TAMA and VIRGO) based on sus-

pended mass interferometry promise to attain the sensitivity to observe gravitationalwaves.

The Laser Interferometer Gravitational-wave Observatory (LIGO) is a joint Caltech-

MIT project supported by the National Science Foundation. The LIGO observatoriesare located at Hanford, Washington and Livingston, Louisiana. To unambiguously

make detections of these rare events a time coincidence within ±10ms (transit timeat the speed of light) between detectors separated by 3002 km will be sought. The

two facilities can be seen in the aerial photograph in Figure 2a and 2b.The facilities developed to support the initial interferometers will allow for the

evolution of the detectors to probe the field of gravitational wave astrophysics withmore and more sensitivity over the next two decades. Sensitivity improvements and

special purpose detectors will be needed either to enable detection if strong enoughsources are not found with the initial interferometer, or following detection, to in-

crease the rate in order to enable the detections to begin to become a new tool forastrophysical research. The first stage of incremental improvements using improved

test masses, lasers, suspensions, etc. should bring the sensitivity to h ∼ 10−22, whichshould make detection likely (if they have not been seen with the initial detectors) or

significantly improve the rate, which scales as the improved sensitivity cubed.

The network of detectors in the world offer potential simultaneous detection ofastrophysical sources and there are plans underway to correlate signals from all oper-

ating detectors in a worldwide network. This will enable much greater confidence indetection for early observations and will also both give information on the location

of the source by comparing the relative timing and information on the polarizationof the gravitational wave signal detected by comparing detailed waveforms from the

different detectors, which have different orientations to the incoming gravitationalwave.

2. INITIAL PERFORMANCE

The success of the detector ultimately will depend on how well we are able tocontrol the noise in the measurement of very small strains. Noise is broadly but also

usefully categorized in terms of those phenomena, which limit the ability to sense andregister the small motions (sensing noise limits) and those that perturb the masses by

causing small motions (random force noise). Eventually one reaches a practical butnot fundamental limiting noise, the quantum limit, which combines the sensing noise

with a random force limit. The primary noise sources for the initial LIGO detectorare shown in Figure 3.

a)

b )

Figure 2: Aerial photographs of the 4km long suspended mass interferometers installed at Livingston,Louisiana (top, a) and Hanford, Washington (bottom, b).

The initial LIGO interferometers have are nearing design sensitivity and we nowexpect to enter our first extended, one to two year, data run before the end of 2005.

Much progress has been made in reducing the technical noise as is illustrated in Figure4.

3. ASTROPHYSICAL SIGNALS FROM SPINNING NEUTRON STARS

IN OUR GALAXY

There are a many known astrophysical processes in the Universe that produce

gravitational radiation. Terrestrial interferometers, like LIGO, will search for sig-nals from in the 10Hz - 10KHz frequency band. Characteristic signals from astro-

physical sources will be sought over background noise from continuously recordedtime-frequency series of the strain.

Astrophysics sources include the inspiral of compact objects, like a binary neutronstar system yielding chirp like signals, signals from supernovae or GRBs that yield

burst like signals and stochastic background signals coming from the early universe.We are searching for all of these sources, but in the presentation I will concentrate

only on possible signals from spinning neutron sources in our own galaxy.

Radiation from rotating non-axisymmetric neutron stars will produce periodic sig-nals in the detectors. The emitted gravitational wave frequency is twice the rotation

frequency. For many known pulsars, the frequency falls within the LIGO sensitiv-ity band. Searches for signals from spinning neutron stars will involve tracking the

system for many cycles, taking into account the Doppler shift for the motion of theEarth around the Sun, and including the effects of spin-down of the pulsar. Both

targeted searches for known pulsars and general sky searches are anticipated.The very first scientific data run for both LIGO and GEO-600 was carried out

in September 2002. Data was collected over a period of 17 days. Data collected bythe GEO600 and LIGO interferometric gravitational wave detectors during their first

observational science run were searched for continuous gravitational waves from thepulsar J1939+2134 at twice its rotation frequency. Two independent analysis methods

were used and are demonstrated in this paper3): a frequency domain method and atime domain method. Both achieve consistent null results, placing new upper limits

on the strength of the pulsars gravitational wave emission.

In our second science run, S2, we place direct upper limits on the amplitude ofgravitational waves from 28 isolated radio pulsars by a coherent multi-detector anal-

ysis of the data collected during the second science run of the LIGO interferometricdetectors (Figure 5). These are the first direct upper limits for 26 of the 28 pulsars.

We use coordinated radio observations for the first time to build radio-guided phasetemplates for the expected gravitational wave signals. The unprecedented sensitivity

of the detectors allows us to set strain upper limits as low as as h ∼ few × 10−24.These strain limits translate into limits on the equatorial ellipticities of the pulsars,

Figure 3: The limiting and other noise sources for LIGO are shown. The shaded area indicates thedetector sensitivity.

Figure 4: Sensitivity improvements are shown for LIGO since 2001. The data is for our E7, S1, S2and S3 data as the sensitivity has improved. The analysis discussed in this presentation is from theS2 data set and the solid line represents the design goal we expect to reach by the end of 2005.

Figure 5: Upper curves: h0 amplitudes detectable from a known generic source with a 1% false alarmrate and 10% false dismissal rate for single detector analyses and for a joint detector analysis. Allthe curves use typical S2 sensitivities and observation times. Lower curve: LIGO design sensitivityfor 1 yr of data. Stars: upper limits for 28 known pulsars. Circles: spindown upper limits for thepulsars with negative frequency derivative values if all the measured rotational energy loss were dueto gravitational waves and assuming a moment of inertia of 1045g · cm2.

which are smaller than 10−5 for the four closest pulsars.As expected, none of these upper limits improves on those inferred from simple

arguments based on the gravitational luminosities achievable from the observed lossof pulsar rotational kinetic energy. However, for pulsars in globular clusters such

arguments are complicated by cluster dynamics, which the direct limits presentedhere avoid.

The result for the Crab pulsar (B0531+21) is within a factor of about 30× of

the spindown limit and over an order of magnitude better than the previous directupper limit. The equatorial ellipticities of the four closest pulsars (J0030+0451,

J2124+3358, J1024-0719, and J1744-1134) are constrained to be less than 10−5.These limits are about three orders of magnitude higher than the upper limit

obtained from the pulsars measured spindown rate. However, such a large ellipticitycould in principle be generated by an interior magnetic field of B ∼ 10−16 gauss or

it could probably be sustained in a neutron star with a solid core. Therefore, theseearly limits are already of interest and demonstrate the ability and techniques for

extracting such source signals. Over the next few years the LIGO sensitivity willimprove to where it will begin to confront the expectations from spin down rates for

some selected pulsars.

4. References

1) B.C. Barish, and R. Weiss, Physics Today, October 1999 and LIGO Web site

http://www.ligo.caltech.edu/.2) K.S. Thorne, 300 Years of Gravitation Edited by S.W. Hawking and W. Israel

Cambridge University Press, Cambridge, England 1987 Chapter 9 Gravita-tional radiation.

3) Limits on gravitational wave emission from selected pulsars using LIGO dataLIGO Science Collaboration, gr-qc/04 10007 v2 Jan 2005 submitted to Phy

Rev D, to be published.