ar X iv : a st ro - p h / 04 05 20 3 v 1 1 1 M ay 2 00 4 CHANDRA Detection of the AM CVn binary ES Cet ( KUV 01584 - 0939

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 ksec ACIS-S image a point-like X-ray source is found within 1 " of the catalogued optical position. The mean countrate in ACIS-S is 0.013 s −1 , 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 an ≈ 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 (3σ) to any modulation at the putative orbital period is ≈ 40% (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 modelled 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 black body continuum with kT = 0.8 keV along with the gaussian lines. The 0.2-5 keV X-ray flux was ≈ 7 × 10 −14 ergs cm −2 s −1. The luminosity implied by this flux for any reasonable distance is much smaller than that expected for a mass accretion rate as high as ˙ m = 10 −8 M ⊙ yr −1 , suggesting that the bulk of the accretion luminosity is below 100 eV and not seen with Chandra. We discuss the implications of our results for the nature of ES Cet.