The detection of variable radio emission from the fast rotating magnetic hot B-star HR 7355 and evidence for its X-ray aurorae

Abstract

In this paper we investigate the multiwavelengths properties of the magnetic early B-type star HR 7355. We present its radio light curves at several frequencies, taken with the Jansky Very Large Array, and X-ray spectra, taken with the XMM-Newton X-ray telescope. Modeling of the radio light curves for the Stokes I and V provides a quantitative analysis of the HR 7355 magnetosphere. A comparison between HR 7355 and a similar analysis for the Ap star CU Vir, allows us to study how the different physical parameters of the two stars affect the structure of the respective magnetospheres where the non-thermal electrons originate. Our model includes a cold thermal plasma component that accumulates at high magnetic latitudes that influences the radio regime, but does not give rise to X-ray emission. Instead, the thermal X-ray emission arises from shocks generated by wind stream collisions close to the magnetic equatorial plane. The analysis of the X-ray spectrum of HR 7355 also suggests the presence of a non-thermal radiation. Comparison between the spectral index of the power-law X-ray energy distribution with the non-thermal electron energy distribution indicates that the non-thermal X-ray component could be the auroral signature of the non-thermal electrons that impact the stellar surface, the same non-thermal electrons that are responsible for the observed radio emission. On the basis of our analysis, we suggest a novel model that simultaneously explains the X-ray and the radio features of HR 7355 and is likely relevant for magnetospheres of other magnetic early type stars.

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“…This model explains the enhancement of the measured radio emission when the stellar rotational phases are close to the maxima of the effective magnetic field curve, as commonly observed in the hot magnetic stars Bailey et al 2012;Leto et al 2006Leto et al , 2012Leto et al , 2017a). …”

“…This was calculated in the frequency range where the source spectrum is almost flat averaging all the radio flux density measurements performed between 6 and 102 GHz. This radio luminosity makes HR5907 about 2 times brighter than either σ Ori E (Linsky, Drake & Bastian 1992) or HR 7355 (Leto et al 2017a). At the present HR 5907 has the highest published luminosity in the cm-mm band for a non-degenerate magnetic massive star.…”

“…5, δ = 2.6 was adopted. The low-energy cutoff of the non-thermal electrons was fixed at 100 keV (corresponding to a Lorentz factor γ = 1.2) like the case of the near-twin star HR 7355, that was the subject of a similar study (Leto et al 2017a).…”

“…In a previous paper, using the same 3D model and the same simulation approach, we successfully simulated the multiwavelength radio light curves of HR 7355 (a twin of HR 5907), simultaneously for the total intensity and the circularly polarized radio emission (Leto et al 2017a). Both stars have similar radio and X-ray luminosities.…”

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“…This coherent radio emission process has a counterpart also at X-ray wavelengths, like the case of the hot magnetic star HR 7355, which shows evidence of X-ray aurorae (Leto et al 2017). These early type main sequence stars are characterised by a kGauss magnetic field with an overall dipolar topology.…”