# Detection of coherent emission from the Bp star HD 142990 at uGMRT frequencies

## TL;DR

This study reports the detection of coherent electron cyclotron maser emission from the Bp star HD 142990 across a broad frequency range, revealing unique polarization features and suggesting mode transitions in the emission mechanism.

## Contribution

First detection of ECME from HD 142990 across 200 MHz to 1420 MHz, with novel polarization behavior indicating mode transitions, expanding understanding of magnetic star emissions.

## Key findings

- ECME detected across 200-1420 MHz band
- Opposite circular polarizations observed from same magnetic pole
- Suggests mode transition in ECME emission mechanism

## Abstract

HD 142990 is a Bp-type star with a nearly dipolar surface magnetic field of kG strength. Recently Lenc et al. (2018) reported the tentative discovery of Electron Cyclotron Maser Emission (ECME) from this star at 200 MHz. This type of emission has been observed from only three other hot magnetic stars. In this paper, we present our observations of HD 142990 with the upgraded Giant Metrewave Radio telescope (uGMRT) at 550-804 MHz and with the legacy GMRT at 1420 MHz near the rotational phases of the nulls of the longitudinal magnetic field. We found strong enhancements in flux density in both circular polarisations suggesting an ECME bandwith of at least 1.2 GHz (200-1420 MHz). In one of the observation sessions, we observed enhancements with opposite circular polarisations from the same magnetic pole. This has not been reported in any other hot magnetic star known to exhibit ECME. In order to explain this unusual finding, we suggest a scenario that involves a transition of the dominant mode of ECME between the magneto-ionic modes.