2016MNRAS.460.3298J

We present a new method for probing the thermal electron content of the Galaxy by spectral analysis of background point sources in the absorption-only limit to the radiative transfer equation. In this limit, calculating the spectral index, α, of these sources using a natural logarithm results in an additive factor, which we denote α_EM, resulting from the absorption of radiation due to the Galactic thermal electron population. We find that this effect is important at very low frequencies (ν <= 200 MHz), and that the frequency spacing is critical. We model this effect by calculating the emission measure across the sky. A (smooth) thermal electron model for the Galaxy does not fit the observed emission measure distribution, but a simple, cloud-based model to represent the clumpy nature of the warm interstellar medium does. This model statistically reproduces the Galactic emission measure distribution as obtained independently from Hα data well. We find that at the lowest frequencies (∼10-50 MHz), the observed spectral index for a large segment of the Galaxy below Galactic latitudes of <=15deg could be changed significantly (i.e. α_EM\gtrsim 0.1). This method therefore provides a correction to low-frequency spectral index measurements of extragalactic sources, and provides a sensitive probe of the thermal electron distribution of the Galaxy using current and next-generation low-frequency radio telescopes. We show that this effect should be robustly detectable individually in the strongest sources, and statistically in source samples at a level of α_EM\gtrsim 0.18,0.06, and 0.02 for source densities of 10, 100, and 1000 sources per square degree.