Betelgeuse's Buddy: X-Ray Constraints on the Nature of $\alpha$ Ori B

Abstract

The $\sim$$2100$d Long Secondary Period of Betelgeuse's optical lightcurve and radial velocity motivated the prediction of a low-mass stellar companion, expected to be at maximal apparent separation from Betelgeuse around December 2024. We carried out Director's Discretionary Time observations with the Chandra X-ray Observatory to identify any X-ray emission from the companion and constrain its nature as either a compact object or young stellar object (YSO). Past X-ray observations occurred at the wrong phase of the companion's orbit for optimal detection prospects and/or lacked the deep exposure required to constrain the typical X-ray luminosities of YSOs. In our 41.85 ks exposure with Chandra, we do not detect an X-ray source at the position of Betelgeuse. For an estimated hydrogen column density $N_H$$=$$6\times 10^{22}$ cm${}^{-2}$, we place a limit on the X-ray luminosity of $L_X$$\lesssim$$2\times 10^{30}$ erg s${}^{-1}$ ($\lesssim$$4.7\times 10^{-4}{L}_{\odot }$) in $0.5$$-$$8$ keV for a 10 MK plasma temperature spectral model, or $L_X$$\lesssim$$5\times 10^{29}$ erg s${}^{-1}$ ($\lesssim$$1.2\times 10^{-4}{L}_{\odot }$) for an absorbed power law with photon index $\Gamma$$=$$2$. These limits robustly exclude an accreting compact object (white dwarf or neutron star) as the companion. Solar mass YSOs with an age similar to Betelgeuse ($\sim$10 Myr) display a range of X-ray luminosities ($10^{28-32}$ erg s${}^{-1}$), and we can place upper bounds within this range for most absorbing columns. Based on these considerations, we conclude that the companion to Betelgeuse is likely a low-mass YSO.

arXiv comment15 pages including references and appendices, 4 figures, 3 tables. Accepted by ApJ. See also companion publication, Goldberg et al. 2025

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