Determining the upper limit for n_H of the absorber. This example shows the results for trough T2, based on the C iv column density measurements. For each of the 935 simulated light curves (see Sect. 5), we explicitly solve the time-dependent photoionization equations and calculate the expected ratio of C iv column density over a time period equal to the differences between the STIS and COS epochs. Each panel represents the fractional solution distribution of N(C iv)_COS/N(C iv)_STIS for a given log (n_H) (cm^-3), which increases in the panels from left to right and top to bottom. The vertical dashed lines show the positive and negative 1σ errors on the measured value. We determine an n_H upper limit by increasing n_H until more than 99% of the calculated N(C iv)_COS/N(C iv)_STIS are outside the error bars. In this example, we find n_H < 10^3.2 cm^-3.