A new method to probe the thermal electron content of the Galaxy through spectral analysis of background sources
| Autor: | Jones, D. I., Igoshev, A. P., Haverkorn, M. |
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| Rok vydání: | 2016 |
| Předmět: | |
| Druh dokumentu: | Working Paper |
| DOI: | 10.1093/mnras/stw1192 |
| Popis: | 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, $\alpha$, of these sources using a natural logarithm results in an additive factor, which we denote $\alpha_\mathrm{EM}$, resulting from the absorption of radiation due to the Galactic thermal electron population. We find that this effect is important at very low frequencies ($\nu\lesssim200$ 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 $H_\alpha$ data well. We find that at the lowest frequencies ($\sim10-50$ MHz), the observed spectral index for a large segment of the Galaxy below Galactic latitudes of $\lesssim15^\circ$ could be changed significantly (i.e., $\alpha_\mathrm{EM}\gtrsim0.1$). 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 $\alpha_\mathrm{EM}\gtrsim0.18,0.06$, and 0.02 for source densities of 10, 100 and 1,000 sources per square degree. Comment: 8 pages, 10 figures, accepted to MNRAS on 17-05-2016 |
| Databáze: | arXiv |
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