Removable Matter-power-spectrum Covariance from Bias Fluctuations

We find a simple, accurate model for the covariance matrix of the real-space cosmological matter power spectrum on slightly nonlinear scales (k ~ 0.1-0.8 h Mpc^-1 at z = 0), where off-diagonal matrix elements become substantial. The model includes a multiplicative, scale-independent modulation of the power spectrum. It has only one parameter, the variance (among realizations) of the variance of the nonlinear density field in cells, with little dependence on the cell size between 2 and 8 h ^-1 Mpc. Furthermore, we find that this extra covariance can be modeled out by instead measuring the power spectrum of δ/σ_cell, i.e., the ratio of the overdensity to its dispersion in grid cells. Dividing δ by σ_cell essentially removes the non-Gaussian part of the covariance matrix, nearly diagonalizing it.