Non-detection of Betelgeuse in X-rays

ATel #13501; Vinay L. Kashyap, Jeremy J. Drake, Daniel Patnaude (Center for Astrophysics | Harvard & Smithsonian)
on 19 Feb 2020; 03:53 UT
Credential Certification: Jeremy J. Drake (jdrake@cfa.harvard.edu)

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The red supergiant Betelgeuse (alpha Ori; M1-M2Ia-Iab) has been undergoing a significant dimming in the optical (see e.g., ATel #13439). The large apparent change in the radius of >10% within a free-fall timescale led to a search for possible high-energy processes with the Chandra X-ray telescope.

The star was observed with the HRC-I instrument from 17 February 2020 19:17 UT for 5.1 ksec. The HRC-I is well-suited for this purpose as its red leak contamination is minuscule, and its soft response extends down to 0.06 keV. The star was not detected. At the putative proper-motion corrected position of Betelgeuse (05:55:10.3426, +07:24:25.6579), 1 count was registered within a circle of radius 2.5 arcsec, with a scaled background of $2.78\pm 0.08\; counts.\; At\; least\; 9\; counts\; are\; required\; to\; rule\; out\; a\; background\; fluctuation\; at\; the\; 3-sigma\; (p=0.003)\; level.$

A source with an intrinsic brightness of 5.9 counts would have been detected at this level with a probability of 1/2, so we place an upper limit on the count rate of <1.15 counts/ksec. This corresponds to an upper limit on the absorbed X-ray luminosity that is strongly dependent on the intervening column density and the plasma temperature. Assuming an ISM absorption of 4e20 cm^-2 based on Stilism (https://stilism.obspm.fr) yields an upper limit of <~4-10e28 ergs/s (corresponding to an Lx/Lbol of <~1e-10). The local absorption component is likely higher, and assuming a column of 1e23 cm^-2, the upper limits range from <~4e28 ergs/s for low plasma temperatures (kT~0.02 keV) to <~1e30 ergs/s for high plasma temperatures (kT~5 keV).