Towards improved synchrotron self-absorption energy estimates: accounting for inhomogeneous and non-spherical emitting regions

ABSTRACT Synchrotron self-absorption (SSA) is seen across a variety of astrophysical sources, and observation of an SSA peak in the spectrum is a powerful tool for estimating the physical conditions and the minimum energy of the emitting region. We begin with the (re)derivation of the usual SSA parameter estimates, carefully considering dependencies and assumptions, obtaining the most accurate traditional SSA minimum energy equations currently available. Traditional methods rely on the assumption that the emitting region is quasi-spherical and homogeneous. However, many observations of SSA show that the spectral index at frequencies below the peak is less than the expected $+2.5$ (non-thermal) or $+2$ (thermal). We argue that an inhomogeneous emitting region is the most likely explanation in many cases. Power-law inhomogeneous cylindrical slab and broken power-law inhomogeneous sphere models are used to investigate how the presence of inhomogeneity affects parameter estimates using traditional SSA methods. We find that in some cases inhomogeneity can lead to traditional SSA methods underestimating the minimum energy and the size of the emitting region by over an order of magnitude. Quantitative correction factors are found which can be applied to traditional estimates to correct for inhomogeneity, depending on the value of the observed flattened spectral index and the range in frequency over which this value is observed. Furthermore, we derive simple correction factors for non-spherical homogeneous emitting regions. Finally, we explore the effects of inhomogeneity on measurements of polarization around the spectral peak, and on light curves for expanding emitting regions.