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

TL;DR

This study uses deep Chandra X-ray observations timed at the predicted maximum separation of Betelgeuse’s proposed low-mass companion to test whether α Ori B is a compact object or a young stellar object. By converting non-detections into robust upper limits on the unabsorbed X-ray luminosity across plausible wind-absorption scenarios, the authors rule out accreting white dwarfs and neutron stars, while remaining consistent with a low-mass YSO as the companion. The analysis integrates wind absorption estimates (N_H) from Betelgeuse’s circumstellar environment, archival X-ray data, and theoretical expectations for YSOs, WD symbiotics, and NS systems, with complementary UV constraints from a companion HST study. The results imply a sub-solar mass, likely young stellar companion, and place Betelgeuse’s surface X-ray flux among the quietest supergiants, reinforcing the interpretation of α Ori B as a YSO rather than a compact remnant. A contemporaneous direct-imaging result ( Howell2025 ) further supports a stellar-mass companion in the $1.4$–$2.0$ M$_\odot$ range, consistent with the X-ray constraints presented here.

Abstract

The $\sim$$2100$d Long Secondary Period of Betelgeuse's optical lightcurve and radial velocity motivated the prediction of a low-mass stellar companion, expected to be at maximal apparent separation from Betelgeuse around December 2024. We carried out Director's Discretionary Time observations with the Chandra X-ray Observatory to identify any X-ray emission from the companion and constrain its nature as either a compact object or young stellar object (YSO). Past X-ray observations occurred at the wrong phase of the companion's orbit for optimal detection prospects and/or lacked the deep exposure required to constrain the typical X-ray luminosities of YSOs. In our 41.85 ks exposure with Chandra, we do not detect an X-ray source at the position of Betelgeuse. For an estimated hydrogen column density $N_H$$=$$6\times10^{22}$ cm$^{-2}$, we place a limit on the X-ray luminosity of $L_X$$\lesssim$$2\times10^{30}$ erg s$^{-1}$ ($\lesssim$$4.7\times10^{-4}L_\odot$) in $0.5$$-$$8$ keV for a 10 MK plasma temperature spectral model, or $L_X$$\lesssim$$5\times10^{29}$ erg s$^{-1}$ ($\lesssim$$1.2\times10^{-4}L_\odot$) for an absorbed power law with photon index $Γ$$=$$2$. These limits robustly exclude an accreting compact object (white dwarf or neutron star) as the companion. Solar mass YSOs with an age similar to Betelgeuse ($\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.

Figures (4)

• Figure 1: Separation of the putative companion from Betelgeuse as a function of time using the Goldberg2024 ephemeris, where the sign represents the companion moving away from (+) or towards (-) the observer. Vertical lines indicate the relative timing of past on-axis Chandra observations (Posson-Brown-2006Kashyap-2020, black dashed and dot-dashed lines), and our latest observations (red line), which occurred at a previously unprobed phase of the LSP with maximal separation from Betelgeuse. The gray shaded bar represents the 43mas radial extent of Betelgeuse (dark gray) and the $\approx$1.5$R_*$ dust shell inferred by Haubois2019Haubois2023. Purple shaded regions indicate epochs when the companion is fully obscured by Betelgeuse and the circumstellar shell (dark/light purple). Note that the recent Kashyap-2020 observations, which took place during the Great Dimming of 2020, occurred while $\alpha$ Ori B was fully obscured by Betelgeuse, and the Posson-Brown-2006 observations were strongly off-axis (see text).
• Figure 2: Finding chart showing our observations of Betelgeuse. The top panel displays the merged 41.85 ks image in the full $0.1$$-$$10$ keV energy range. The bottom panels display individual exposures, and the bottom right shows the merged 41.85 ks image filtered to $0.5$$-$$8$ keV. The solid magenta circle in all images corresponds to a circular source region with radius 1.5$^{\prime\prime}$, corresponding to the 93% encircled energy fraction of the PSF. The image is shown in the native 0.13$^{\prime\prime}$ pixel scale of HRC-I. In all images the orientation is such that North is up and East is to the left.
• Figure 3: Upper limits on the unabsorbed X-ray luminosity ($0.5$$-$$8$ keV) of Betelgeuse and its companion as a function of the Hydrogen column density $N_H$ for a variety of model parameters. Solid lines refer to an absorbed powerlaw with photon index $\Gamma$$=$$\{1.5,2,3\}$, and dashed lines refer to a thermal APEC model with temperature $kT$$=$$\{1,5,10,60\}$ MK. The downward arrows are added to a single line to clarify it is an upper bound for that assumed model. All other lines are also upper bounds for the specific model, but arrows are not similarly shown to avoid crowding.
• Figure 4: Unabsorbed X-ray luminosity ($0.5$$-$$8$ keV) versus stellar mass (left panel) and age (right panel). YSOs from Getman2005COUP0 and Getman2022 are plotted as gold and blue circles, respectively. In both panels, darker shading indicates a higher density of sources for the Getman2022 YSOs, and in the right panel Getman2005COUP0 YSOs with masses outside the estimated range of the companion are plotted as grey squares to aid in viewing. Our lower limits on the unabsorbed X-ray luminosity of the system, assuming an APEC thermal plasma model of $kT$$=$$10$ MK are indicated with dashed red lines, of increasing thickness and darkness corresponding to our estimate of the neutral Hydrogen column density of increasing denseness. Dotted black lines indicate the mass estimate for the companion ($0.45\leq\frac{\mathrm{M}}{\mathrm{M}_\odot}\leq2.2$) and the conservative age estimate for Betelgeuse ($5$$-$$15$ Myr).