The effect of interstellar scattering on coherent radio emission from stars: the case of CU Vir

TL;DR

This study investigates how interstellar scintillation affects the observed radio pulses from magnetic stars like CU Vir, highlighting the importance of considering extrinsic effects in magnetospheric studies.

Contribution

It demonstrates that interstellar scintillation can significantly influence stellar radio emission observations, emphasizing the need to account for extrinsic effects in data interpretation.

Findings

Interstellar scintillation can produce spectral features similar to intrinsic stellar phenomena.

A turbulence level along the line of sight can explain observed spectral evolution at 400 MHz.

Caution is needed to distinguish between intrinsic and extrinsic variations in low-frequency stellar radio observations.

Abstract

A subset of magnetic stars exhibit periodic radio pulses produced by the coherent electron cyclotron maser mechanism. These pulses are known to exhibit both temporal and spectral variations, which have been attributed to phenomena intrinsic to the stellar magnetosphere. However, in order to fully characterise the radio pulses and use them as magnetospheric probes (as suggested by past studies), it is also important to consider the effects of phenomena extrinsic to the magnetosphere. In this paper, we investigate whether interstellar scintillation could be a relevant mechanism for explaining spectral and temporal variations observed for coherent stellar radio emission. For that, we consider the case of the well-characterised magnetic hot star CU Vir. At 400 MHz, coherent radio emission from the star was reported to exhibit a peculiar spectral evolution that remains unexplained. We show that a plausible level of turbulence along the line of sight can produce the observed phenomenon of spectral features. Our analysis shows that diffractive interstellar scintillation can have a strong effect on the observed dynamic spectrum of radio emission from stars, for an assumed size of the emitting region of $0.01r_\odot$, and that caution should therefore be taken in separating intrinsic and extrinsic features, particularly at low frequencies. These results are preliminary and further work is required to fully model the scintillation of ECME from stars (in particular the change in source location with frequency), and to explore the full range of plausible scintillation parameters. We suggest how further observations may be used to test the interstellar scintillation hypothesis.