Serendipitous X-Ray Sources in the Chandra HRC Field around Alpha Centauri

Abstract

Abstract For more than a decade, Alpha Centauri AB (G2 V+K1 V) has been observed by Chandra, in a long-term program to follow coronal (T ∼ 106 K) activity cycles of the two sunlike stars. Over 2008.4–2017.8, 19 HRC-I exposures were taken, each about 10 ks in duration, and spaced about six months apart. Beyond monitoring the AB X-ray luminosities, the HRC-I sequence represents a unique decadal record of the dozen, or so, serendipitous X-ray sources in the α Cen field, which is at low Galactic latitude and thus dominated by nearby stars. For the present study, the 10 brightest candidates were considered. Only a handful of these were persistent; most were variable, some highly so, flaring in a few epochs, weak or absent in the others. All 10 X-ray sources have Gaia objects within about 2″; mostly late-type dwarfs, but a few giants. However, two of the proposed optical counterparts have statistically significant offsets, and possible conflicts between the X-ray and optical properties. Another of the candidates brightened by a factor of 100 in X-rays during a single exposure, briefly attaining super-flare status. The Gaia counterpart is anomalously blue for its absolute G-magnitude and likely is a WD+dM pair. To the extent that the low Galactic latitude field is representative, the Chandra time-domain view emphasizes that the high-energy stellar sky is biased toward transient sources, so any snapshot survey surely will miss many of the most interesting objects.