Understanding the Radio Emission from the $\beta$ Cep star V2187 Cyg

TL;DR

This study investigates the radio emission from the $eta$ Cep star V2187 Cyg, revealing a likely magnetic origin for its radio signals through analysis of archival VLA data, spectral index, and observed radio pulses.

Contribution

It provides the first detailed radio emission analysis of V2187 Cyg, suggesting a magnetic origin over wind-related mechanisms, and proposes future polarization and magnetic field measurements.

Findings

Spectral index of $eta$ Cep V2187 Cyg consistent with ionized wind or synchrotron emission.

Detected a month-long radio pulse modeled as an internal shock in the stellar wind.

Quiescent radio emission likely has a magnetic origin, not explained by wind models.

Abstract

We analyze the radio emission from the $\beta$ Cep star V2187 Cyg using archive data from the Jansky Very Large Array. The observations were made in ten epochs at 1.39 and 4.96 GHz in the highest angular resolution A configuration. We determine a spectral index of of $\alpha = 0.6\pm0.2$ ($S_{\nu} \propto \nu^\alpha$), consistent with an ionized wind or a partially optically-thick synchrotron or gyrosynchrotron source. The emission is spatially unresolved at both frequencies. The 4.96 GHz data shows a radio pulse with a duration of about one month that can be modeled in terms of an internal shock in the stellar wind produced by a sudden increase in the mass-loss rate and the terminal velocity. The quiescent radio emission of V2187 Cyg at 4.96 GHz (with a flux density of $\simeq 150~\mu Jy$), cannot be explained in terms of an internally (by V2187 Cyg) or externally (by a nearby O star) photoionized wind. We conclude that, despite the spectral index suggestive of free-free emission from an ionized wind, the radio emission of V2187 Cyg most likely has a magnetic origin, a possibility that can be tested with a sensitive search for circular polarization in the radio, as expected from gyro-synchrotron radiation, and also by trying to measure the stellar magnetic field, that is expected to be in the range of several kGauss.