2004ApJ...614..358S

We report on Chandra ACIS observations of the ultracompact AM CVn binary ES Cet. This object has a 10.3 minute binary period and is the most compact of the confirmed AM CVn systems. We have, for the first time, unambiguously detected the X-ray counterpart to ES Cet. In a 20 ks ACIS-S image a point-like X-ray source is found within 1" of the cataloged optical position. The mean count rate in ACIS-S is 0.013/s, and there is no strong evidence for variability. We folded the X-ray data using the optical ephemeris of Warner & Woudt, but did not detect any significant modulation. If a ∼100% modulation similar to those seen in the ultracompact candidates V407 Vul and RX J0806.3+1527 were present, then we would have detected it. The upper limit (3 σ) to any modulation at the putative orbital period is ∼15% (rms). We extract the first X-ray spectrum from ES Cet, and find that it is not well described by simple continuum models. We find suggestive evidence for discrete spectral components at ∼470 and 890 eV that can be modeled as Gaussian emission lines. In comparison with recent X-ray detections of nitrogen and neon in another AM CVn system (GP Com), it appears possible that these features may represent emission lines from these same elements; however, deeper spectroscopy will be required to confirm this. Our best spectral model includes a blackbody continuum with kT=0.8 keV along with the Gaussian lines. The unabsorbed 0.2-5 keV X-ray flux was ∼7x10^–14 ergs/cm2/s. The observed luminosity in the hard component is a small fraction of the total expected accretion luminosity, most of which should be radiated below 0.1 keV at the expected mass accretion rate for this orbital period. We discuss the implications of our results for the nature of ES Cet.