Continuum fitting uncertainties are a major source of error in estimates of the temperature-density relation (usually parameterized as a power-law, T ∝ Δγ – 1) of the intergalactic medium through the flux probability distribution function (PDF) of the Lyα forest. Using a simple order-of-magnitude calculation, we show that few percent-level systematic errors in the placement of the quasar continuum due to, e.g., a uniform low-absorption Gunn-Peterson component could lead to errors in γ of the order of unity. This is quantified further using a simple semi-analytic model of the Lyα forest flux PDF. We find that under(over)estimates in the continuum level can lead to a lower (higher) measured value of γ. By fitting models to mock data realizations generated with current observational errors, we find that continuum errors can cause a systematic bias in the estimated temperature-density relation of δ(γ) ≈ –0.1, while the error is increased to σγ ≈ 0.2 compared to σγ ≈ 0.1 in the absence of continuum errors.
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