A combined multiwavelength VLA/ALMA/Chandra study unveils the complex magnetosphere of the B-type star HR5907

Abstract

We present new radio/millimeter measurements of the hot magnetic star HR 5907 obtained with the VLA and ALMA interferometers. We find that HR 5907 is the most radio luminous early type star in the cm-mm band among those presently known. Its multi-wavelength radio light curves are strongly variable with an amplitude that increases with radio frequency. The radio emission can be explained by the populations of the non-thermal electrons accelerated in the current sheets on the outer border of the magnetosphere of this fast rotating magnetic star. We classify HR 5907 as another member of the growing class of strongly magnetic fast rotating hot stars where the gyro-synchrotron emission mechanism efficiently operates in their magnetospheres. The new radio observations of HR 5907 are combined with archival X-ray data to study the physical condition of its magnetosphere. The X-ray spectra of HR 5907 show tentative evidence for the presence of non-thermal spectral component. We suggest that non-thermal X-rays originate a stellar X-ray aurora due to streams of non-thermal electrons impacting on the stellar surface. Taking advantage of the relation between the spectral indices of the X-ray power-law spectrum and the non-thermal electron energy distributions, we perform 3-D modeling of the radio emission for HR 5907. The wavelength-dependent radio light-curves probe magnetospheric layers at different heights above the stellar surface. A detailed comparison between simulated and observed radio light-curves leads us to conclude that the stellar magnetic field of HR 5907 is likely non-dipolar, providing further indirect evidence of the complex magnetic field topology of HR 5907.

Evidence for radio and X-ray auroral emissions from the magnetic B-type star ρ Oph A

“…The radio spectral luminosity of ρ Oph A is about one order of magnitude smaller then the radio spectral luminosities of three similar hot magnetic stars: HD 37479 (Linsky et al 1992), HR 7355 (Leto et al 2017a), and HR 5907 (Leto et al 2018), which have radio spectral luminosities ∼ 10 18 erg s −1 Hz −1 and polar field strengths ∼ 10 4 G. Comparing with CU Vir (L ν,rad ≈ 3 × 10 16 erg s −1 Hz −1 , Leto et al 2006), a cooler magnetic star (A0Vp) with a polar field strength of 3800 G (Kochukhov et al 2014), the radio spectral luminosity of ρ Oph A is instead an order of magnitude higher.…”

“…At the stellar surface, the average magnetic field vector orientation is related to the B z value. The correlation between the effective magnetic field curve and the circular polarization fraction of the incoherent radio emission has been clearly observed in many cases Bailey et al 2012;Leto et al 2006Leto et al , 2012Leto et al , 2017aLeto et al , 2018Leto et al , 2019. Stellar orientations characterized by magnetic field lines mainly oriented toward the observer (northern magnetic hemisphere dominant) are related to gyrosynchrotron radio emission of mainly right-handed circular polarization (Stokes V positive).…”

“…The incoherent multi-wavelength radio light curves of the well studied early-type magnetic stars, CU Vir, HD 37479, HR 7355, and HR 5907 (Leto et al 2006(Leto et al , 2012(Leto et al , 2017a(Leto et al , 2018, were modeled by using a 3D model for the gyro-synchrotron emission from a dipole shaped stellar magnetosphere ). In the framework of the MCWS model, the magnetically confined wind leads to accumulation of thermal matter within the magnetospheric regions where the magnetic field lines are closed (the "inner magnetosphere").…”

“…hot magnetic stars well studied at radio wavelengths (Leto et al 2017a(Leto et al , 2018, ρ Oph A's spectrum can be fitted using a power law with a quite flat spectral index (≈ 0.1 within the spectral range 2.1-21.2 GHz). The corresponding average radio spectral luminosity is L ν,rad ≈ 1.8 × 10 17 erg s −1 Hz −1 .…”

“…To roughly estimate the radiative energy lost by ρ Oph A via incoherent gyro-synchrotron emission, we exploit the spectral behavior of the hot magnetic stars observed at the millimeter wavelength range (ν > 100 GHz). The low level of detection rate (Leone et al 2004), combined with the flux drop at the high frequencies measured by ALMA (Leto et al 2018), makes us confident in constraining the upper limit of the gyro-synchrotron band at ν < 1000 GHz. Conservatively, we assume the upper limit of the frequency band is equal to 1000 GHz.…”