X-RAY EMISSION FROM ETA CARINAE NEAR PERIASTRON IN 2009. I. A TWO-STATE SOLUTION

Abstract

X-ray emission from the supermassive binary system Eta Carinae declines\nsharply around periastron. This X-ray minimum has two distinct phases - the\nlowest flux phase in the first ~3 weeks and a brighter phase thereafter. In\n2009, the Chandra X-ray Observatory monitored the first phase five times and\nfound the lowest observed flux at ~1.9e-12 ergs cm-2 s-1 (3-8 keV). The\nspectral shape changed such that the hard band above ~4 keV dropped quickly at\nthe beginning and the soft band flux gradually decreased to its lowest observed\nvalue in ~2 weeks. The hard band spectrum had begun to recover by that time.\nThis spectral variation suggests that the shocked gas producing the hottest\nX-ray gas near the apex of the wind-wind collision (WWC) is blocked behind the\ndense inner wind of the primary star, which later occults slightly cooler gas\ndownstream. Shocked gas previously produced by the system at earlier orbital\nphases is suggested to produce the faint residual X-ray emission seen when the\nemission near the apex is completely blocked by the primary wind. The brighter\nphase is probably caused by the re-appearance of the WWC plasma, whose\nemissivity significantly declined during the occultation. We interpret this to\nmean that the X-ray minimum is produced by a hybrid mechanism of an occultation\nand a decline in emissivity of the WWC shock.\n