An Ultra-long Wavelength Sky Model with Absorption Effect

TL;DR

This paper introduces ULSA, a comprehensive sky model for frequencies down to 1 MHz, accounting for absorption effects, aiding future ultra-long wavelength radio astronomy observations.

Contribution

The paper develops the first detailed ultra-long wavelength sky model including free-free absorption effects in the Milky Way.

Findings

The sky model reveals a darker Galactic plane at ultra-long wavelengths.

Identification of large-scale shadows of spiral arms in the sky map.

Prediction of a spectral turnover around 3 MHz due to absorption.

Abstract

The radio sky at frequencies below $\sim10$ MHz is still largely unknown, this remains the last unexplored part of the electromagnetic spectrum in astronomy. The upcoming space experiments aiming at such low frequencies (ultra-long wavelength or ultra-low frequency) would benefit from reasonable expectations of the sky brightness distribution at relevant frequencies. In this work, we develop a radio sky model that is valid down to $\sim1$ MHz. In addition to the discrete HII objects, we take into account the free-free absorption by thermal electrons in the Milky Way's warm ionized medium (WIM). This absorption effect becomes obvious at $\lesssim10$ MHz, and could make the global radio spectrum turn over at $\sim3$ MHz. Our sky map shows unique features at the ultra-long wavelengths, including a darker Galactic plane in contrast to the sky at higher frequencies, and the huge shadows of the spiral arms on the sky map. It would be a useful guidance for designing the future ultra-long wavelength observations. Our Ultralong-wavelength Sky Model with Absorption (ULSA) model could be downloaded at https://doi.org/10.5281/zenodo.4454153.