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RXTE Monitoring of Mysterious Massive Star Eta Carinae Raises More Questions RXTE
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HST Image

HST Image Credit: Jon Morse

Eta Carinae is an extremely massive star in our galaxy, and an extremely unstable one. It is notorious for erupting in the mid-19th century, and is presently surrounded by the ejecta from this eruption. Recent optical and near IR observations (Damineli) suggest that certain spectral emission lines vary with a period of 5.52 years, implying that Eta Car may be two stars, not one!

Eta Car emits X-rays from the ejecta as it expands into the circumstellar medium near the star, and from a point-like source centered on the star itself. Since early 1996, a team led by Mike Corcoran have been monitoring the X-ray flux from this region using RXTE. The sampling rate has been roughly once every two weeks, with limited intervals of daily observations. The RXTE light curve in the 2-10 keV band is the first of its kind for Eta Car, or any massive star for that matter. The flux variations confirm the 5.52 year variation proposed by Damineli, and also show noticeable peaks in the emission which seem to recur every 85 days.

If Eta Car is a binary, the X-ray emission could arise at the shocked interface created by the collision of the wind from the primary with the wind or surface of the secondary. The cause of the 85-day flaring is a mystery. Some likely causes are:

  • the orbital period of a close binary system. Is Eta Car really a triple system!?
  • the stellar rotation period.
  • a stellar pulsation period.

    As RXTE continues to provide the first detailed monitoring of the X-ray flux of Eta Car, coordinated observations are encouraged to helps shed some light on these burning questions.

    RXTE light curve of Eta Car (2-10 keV)

    Figure 1: RXTE light curve of Eta Car (2-10 keV).

  • at the start of the observations in Feb 1996 the X-ray flux showed a slow rise (note that the points earlier than 1996.4 in the figure are artificially low due to a change in the gain of RXTE). This slow increase is consistent with the change observed by ROSAT and ASCA observations after Jan 1993;
  • starting in Jan 1997, the rate of increase in the X-ray flux accelerated dramatically;
  • In Nov 1997 the X-ray emission reached a maximum, then quickly plummeted to a minimum value, where it stayed for about 3 months. This confirms that the X-ray emission changes with the same period as determined spectroscopically by Damineli and co-workers.
  • In Mar 1997 the X-ray flux started to rise; the rise was at first rapid (though slower than the descent to minimum) but has slowed somewhat in recent days;
  • In addition, the X-ray flux shows significant variability on a timescale of weeks-months, with noticeable peaks in the emission which seem to recur every 85 days. The peaks of the flares are shown by triangles in the figure.

    Figure, Caption and supporting text provided by M. Corcoran


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