Ultra-Wideband, Multi-epoch Radio Study of the First Discovered `Main sequence Radio Pulse emitter' CU Vir

Abstract

Presence of large-scale surface magnetic field in early-type stars leads to several unique electromagnetic phenomena producing radiation over X-ray to radio bands. Among them, the rarest type of emission is electron cyclotron maser emission (ECME) observed as periodic, circularly polarized radio pulses. The phenomenon was first discovered in the hot magnetic star CU Vir. Past observations of this star led to the consensus that the star produces only right circularly polarized ECME, suggesting that only one magnetic hemisphere takes part in the phenomenon. Here we present the first ultra-wideband (0.4-4 GHz) study of this star using the upgraded Giant Metrewave Radio telescope and the Karl G. Jansky Very Large Array, which led to the surprising discovery of ECME of both circular polarizations up to around 1.5 GHz. The GHz observations also allowed us to infer that the upper ECME cut-off frequency is at 5 GHz. The sub-GHz observation led to the unexpected observation of more than two pairs of ECME pulses per rotation cycle. In addition, we report the discovery of a 'giant pulse', and transient enhancements, which are potentially the first observational evidence of 'centrifugal breakout' of plasma from the innermost part of the stellar magnetosphere. The stark contrast between the star's behavior at GHz and sub-GHz frequencies could either be due to propagation effects, a manifestation of varying magnetic field topology as a function of height, or a signature of an additional 'ECME engine'.

A scaling relationship for non-thermal radio emission from ordered magnetospheres: from the top of the Main Sequence to planets

“…the Jovian broadband kilometric radiation) (Zarka 1998(Zarka , 2004. The Jupiterlike scenario was recently supported to be scalable also in the case of magnetic early type stars by the frequency dependent behavior of the wide band coherent emission from HD 124224 (CU Vir), the prototype of the class of early-type magnetic stars showing auroral radio emission, where several auroral radio emission components seems to be related to different magnetic field lines of the stellar magnetosphere (Das & Chandra 2021).…”

Discovery of eight 'Main-sequence Radio Pulse emitters' using the GMRT: clues to the onset of coherent radio emission in hot magnetic stars

“…Our central frequency of observation (0.7 GHz) corresponds to a field strength of 250 G for ECME at the fundamental harmonic (using ν B ≈ 2.8B, where ν B is the electron gyrofrequency in MHz and B is the local magnetic field strength in gauss). Until now, ECME upper cut-off frequencies have been reported for only two MRPs: HD 133880 (Das et al 2020a) and CU Vir (Das & Chandra 2021). In both stars, the upper cut-off frequencies are significantly smaller than the electron gyrofrequency corresponding to the maximum magnetic field strength.…”

“…For HD 133880, the lowest height corresponding to the upper cutoff frequency is ≈ 0.6 R * (Das et al 2020a). In case of CU Vir, it was not possible to estimate the height due to the complex lightcurves (Das & Chandra 2021). Assuming 0.6 R * as the minimum height for ECME production, we find that the surface polar strength of the star should be > 1 kG (for emission at the fundamental harmonic).…”

“…Nevertheless, only seven magnetic early-type stars have been observed to produce ECME: CU Vir (Trigilio et al 2000), HD 133880 (Chandra et al 2015;Das et al 2018), HD 142990 (Lenc et al 2018;Das et al 2019a), HD 142301 (Leto et al 2019), HD 35298 (Das et al 2019b), ρ OphA (Leto et al 2020a) and ρ OphC (Leto et al 2020b). These ECMEproducing magnetic early-type stars will be referred as 'Main-sequence Radio Pulse emitters' (MRPs, Das & Chandra 2021) as the emission is observed as period radio pulses.…”

“…CU Vir and HD 133880, Kochukhov et al 2014Kochukhov et al , 2017. Most recently, Das & Chandra (2021) suggested that a complex magnetic field topology might affect the ECME beaming patterns so that even if a star produces ECME, and exhibits 'magnetic null phases' (see §2), the radiation may not be visible to an observer over certain frequency ranges. The role of magnetic field topology on suppressing ECME is thus vague, and it therefore remains an open question why some stars become MRPs and others apparently do not.…”