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

The ∼2100 daylong 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 2024 December. 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 × 10 22 cm −2 , we place a limit on the X-ray luminosity L X ≲ 2 × 10 30 erg s −1 (≲4.7 × 10 −4 L ⊙ ) in 0.5–8 keV for a 10 MK plasma temperature spectral model, or L X ≲ 5 × 10 29 erg s −1 (≲1.2 × 10 −4 L ⊙ ) 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 (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.