Refractive Interstellar Scintillation and the Search for Very-Low-Frequency Gravitational Radiation
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**Session 8 -- Pulsars**
*Display presentation, Monday, 30, 1994, 9:20-6:30*

## [8.08] Refractive Interstellar Scintillation and the Search for Very-Low-Frequency Gravitational Radiation

*J. W. Armstrong (JPL)*

Very-low-frequency (Fourier frequencies less than $\sim$10 microhertz)
gravitational wave searches involving pulsar timing use the earth and
pulsar as free test masses. Gravitational waves buffeting earth and
pulsar show up as pulse time-of-arrival (TOA) fluctuations. Observed
TOA fluctuations are the sum of gravitational wave perturbations (at
some level) and "noise". Prior to the discovery of PSR 1937+21, the
leading noise was intrinsic pulsar timing variability. PSR 1937+21,
however, has very low intrinsic timing instability and has been used to
place stringent upper limits on a VLF stochastic background of
gravitational waves. A noise source important in the ultimate
sensitivity of these observations is TOA fluctuations caused by
propagation of the pulsar signal through electron density fluctuations
in the ISM.

\par

In this paper I calculate the contribution of refractive interstellar
scintillation (RISS) noise to gravitational wave searches. Formulas
relating the ISM density power spectrum, the spectrum of TOA and
relative dimensionless velocity, and inferred gravitational wave upper
limits are developed and used to calculate the RISS-noise limit for VLF
gravitational wave bursts, periodic waves, and backgrounds. For a
standard Kolmogorov ISM turbulence model with $C_N^2 = 10^{-3}
m^{-6.67}$ the timing instability caused by RISS, expressed as a
fractional frequency stability for observations at 0.126m wavelength
over an integration time $\tau$,
is $\sim$4 $X 10^{-14} (\tau / 1 year)^{-1/6}$. This is, on time scales
$\sim$years, comparable to the estimated stabilities of the best
atomic clocks, and corresponds to a VLF gravitational wave background
limit at or near those derived from 1937+21 observations.

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