Betelgeuse's Buddy: X-Ray Constraints on the Nature of α Ori B

The ~2100$d Long Secondary Period of Betelgeuse's optical lightcurve and radial velocity motivated the prediction of a low-mass stellar companion, expected to be at maximal apparent separation from Betelgeuse around December 2024. We carried out Director's Discretionary Time observations with the Chandra X-ray Observatory to identify any X-ray emission from the companion and constrain its nature as either a compact object or young stellar object (YSO). Past X-ray observations occurred at the wrong phase of the companion's orbit for optimal detection prospects and/or lacked the deep exposure required to constrain the typical X-ray luminosities of YSOs. In our 41.85 ks exposure with Chandra, we do not detect an X-ray source at the position of Betelgeuse. For an estimated hydrogen column density NH=6×10²² cm⁻², we place a limit on the X-ray luminosity of LX2×10³⁰ erg s⁻¹ (4.7×10⁻⁴L_$) in $0.5$$-$$8$ keV for a 10 MK plasma temperature spectral model, or $L_X5×10²⁹$ erg s$⁻¹$ ($1.2×10⁻⁴L_) for an absorbed power law with photon index Γ=$$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 (~10 Myr) display a range of X-ray luminosities (10²⁸⁻³² 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.