X-ray Observations of Eta Carinae
movie displays visually the variation of the X-ray emission from the
central source of Eta Car, based on our RXTE observations from January 1997
through September 1999, using an image of the core from a CHANDRA/ACIS
image obtained in Sep 1999. The false-color image shows regions of bright
X-ray intensity as reddish-white, and regions of fainter intensity as
bluish-black. At the start, the source shows brief periodic flashes of
X-rays. In mid 1997, the brightness of the central source increases
dramatically and the variability of the source becomes much more extreme:
then suddenly, in December 1997, the X-ray emission almost entirely
disappears. It reappears in about 3 months, and regains (almost) its
previous brightness and the X-ray flaring resumes. For comparison see the
movie created by Stephen White and Bob Duncan.
- Detailed Monitoring of Eta Car with the Rossi X-ray
Timing Explorer: X-raying the Beating Heart of the Homunculus
- Long-Term X-ray Variability
- HRI Observations - Spatially resolved Variability
- ASCA Observations of the X-ray Spectrum
- Upcoming X-ray Observations
- For More Information
Extremely massive stars are key astronomical objects, as they play a role
in chemical enrichment and galactic evolution. They mark the end of their
stellar lives as supernovae explosions in which a single supernova can
equal the entire radiant output of a galaxy of a billion stars. Recently
the extreme members of this class have been suggested by Paczynski (1998)
to produce the "hypernovae" which might explain the bursts of gamma
radiation which have been an astronomical mystery for 30 years. The energy
emitted in a "hypernova" is astounding; perhaps the equivalent to the
radiant energy output of an entire universe of galaxies. Such
extraordinary explosions require stellar precursors of unusually large
mass, and so should be relatively rare. Alarmingly, the Milky Way
possesses one possible member of this putative class, the massive,
luminous, and relatively nearby star, Eta Carinae.
Eta Carinae is both an extremely massive star and an extremely unstable one.
It's notorious for erupting in the mid-19th century, and is surrounded by
the ejecta from this eruption. The Hubble Space Telescope has an
impressive image of Eta Car
and this ejecta .
The entire field is beautiful at X-ray energies, as you can see from this
ROSAT High Resolution Imager
image. For comparison, here's the same field in the optical.
Recent observations in the optical and near IR by Augusto Damineli and
co-workers suggest that certain emission lines vary periodically
with a cycle of 5.52 years. The exact cause of this periodicity is not
really known, but the most likely explanation is that the period reflects
the orbital motion of one star around another star: Eta Car may be
two stars, not one!.
If Eta Car really is a binary system, then our understanding of the
evolution of the star and the nature of its instabilities will need to be
radically revised. So this is a crucial question: Is the star really a
binary, or could single-star models be found to explain the
X-ray observations can help resolve this important issue.
Eta Car is a strong X-ray source. X-rays are produced as the ejecta
expands into the circumstellar medium near the star at speeds of 100-1000
km/s. There is also a mysterious point-like source of hard emission
centered on the star itself. Recent X-ray observations with the ROSAT PSPC and HRI
show that the X-ray emission from this point-like source varies by a
factor of 3 on timescales of months (Corcoran et
al., 1995, ApJ, 445, L121, or see the press
The ROSAT PSPC provides
energy-filtered X-ray imaging of Eta Car, too. Notice how the X-ray
emission is extended at lower energies, but more point-like at higher
Detailed Monitoring of Eta Car with the Rossi X-ray
Timing Explorer: X-raying the Beating Heart of the Homunculus.
The ROSAT observations show that the X-ray source varies, but give little
information about the timescale of the variability, since sampling is so
poor. Since early 1996 we (M. Corcoran, J. Swank, K. Ishibashi, K.
Davidson, R. Petre, and others, see Nature 390, 587) have been monitoring
the X-ray flux from Eta Car using RXTE. RXTE provides good time sampling
(roughly one observation every two weeks with limited intervals of daily
observations) of the hard (2-10 keV) X-ray flux from Eta Car, since Eta Car
happily lies in the RXTE continuous viewing zone. This sampling is the key
to understanding how the hard X-ray flux from Eta Car varies, and for
correlating the changes in the X-ray emission with those changes observed
at other energies.
analysis of the 2-10 keV X-ray spectrum (Ishibashi et al., ApJ 1999,
in press; the postscript version of the paper, and figures, and the tabular data is available.) is the first such
detailed analysis for this star (or for any massive star for that matter).
For the first time we are able to document the variety of changes in Eta
Car's X-ray emission properties:
So what's going on? If Eta Car really is a binary, the hard X-ray emission
could naturally arise at the shocked interface created by the collision of
the wind from the primary component with the wind (or surface) of the
secondary. Such "Colliding Wind" emission has been observed in a
number of other massive binary systems. In this "colliding wind" scenario,
the periodic variation in the X-ray flux could be produced by periodic
variations in the strength of the wind-collision shock, and/or by periodic
changes in the amount of absorbing material between the shock and us. Such
changes can easily be accomplished in the type of eccentric binary model
suggested by the emission line variations. However, numerical models
predict that at periastron passage of the system (i.e. the time when the 2
stars are at closest approach) the intrinsic X-ray flux in the 2-10 keV
band should be at a maximum. But our RXTE (and recent ASCA) observations
show that the intrinsic flux is at a minimum. This could suggest
that the binary model is wrong, or that the simplyfying
assumptions that make the numerical modelling tractable are in fact too
simple>. Our best guess is that the assumptions are too simple, and that
the star really is a binary - the X-ray "low state" looks too much like an
- 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
- starting in Jan 1997, the rate of increase in the X-ray flux
- 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 1998 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
- There are also significant changes in the column density during this
time as well.
The other important result from the RXTE data is the mysterious, 85-day
"flaring". We still don't have a firm idea on the cause of this period,
but there are at least 3 likely possible causes:
- It may be the orbital period
of a close binary system. Is Eta Car really a triple system?
- The period could represent the stellar
rotation period. Perhaps streams of fast-moving material from the star
collide with localized slow-moving gas once per rotation period,
similar to Dermot Mullan's ``corotating interaction regions'' scenario. Or
perhaps the second star interacts with the rotating streams to produce
large amounts of shocked gas every 85 days.
- The X-ray period could represent a stellar pulsation period. A photospheric pulsation could
presumably modulate the density in the stellar wind, which could manifest
itself as a periodic variation in the volume emission measure (and
resultant flux) of the X-ray zone.
ROSAT HRI Observations of Eta Car: Variations of the Core and the
Here's a mosaic of a WFPC1 image of Eta Car
with an overlay of smoothed contours from the 1994 ROSAT HRI. Note
how extended the X-ray "shell" is compared to nebulosity visible in the
The ROSAT HRI, while providing no spectral information, does provide the
best spatially-resolved imaging of the X-ray emission from Eta Car
currently available. In addition, we can compare HRI images taken at
different epochs in order to look for X-ray spatial variations. This contour map shows the exposure-corrected,
aspect corrected HRI image of Eta Car in Jun 1992 (red contours) compared
to the exposure and aspect corrected image in Jun 1994. The spatial scale
in the image is 2 arcsec/pixel, or roughly 0.02 pc/pixel at the distance
of Eta Car (roughly 2600 pc). The point source about 1 arcmin to the NE
of Eta Car is the O3 star HDE 303308 which serves as a reference for the
exposure and aspect correction. If we
subtract the earlier image from the latter image we immediately see
the brightening of the core of Eta Car, which is unresolved even at the
spatial resolution of the HRI. We also see evidence of 2 rings of
emission in the outer part of the nebula, one to the SE and one to the NW
of the core. These rings are more apparent if we compare the difference image and a map of the
signal-to-noise of the difference image . The image on the left is
the difference image, while the image on the right is the S/N ratio of the
difference, generated by taking the square root of the square of the
difference map divided by the square of the error of the difference map.
Brighter features in the S/N map indicate features with higher statistical
significance. The brightening of the core is detected with a high
statistical significance, < 99%. Though the "rings" surrounding the core
are detected at a lower significance, ~ 68%, the similarity of the
morphology of the X-ray "rings" to the "bubbles" seen in HST images of the
homunculus suggest that the X-ray rings are in fact real (although the
X-ray "rings" are larger than the "bubbles" in the Hubble images). The
X-ray "rings" seen in the difference maps could represent the expansion of
the X-ray nebula in the interval 1992-1994, though the velocity of the
material would need to be near 10,000 km/s. Such a velocity is higher
than any yet measured for material near Eta Car.
Here's an overlay of the smoothed
difference contours on the WFPC1 mosaic.
We've recently obtained a deep (100 ksec) exposure of Eta Car with the
ASCA SIS and GIS instruments. This is probably the best spectrum of Eta
Car yet obtained. The ASCA observation took place on Jul 29 1996. This
is close in time to the "maximum" of the XTE lightcurve which occurred on
Jul 22 1996.
You can download the submitted version of the paper,
entitled "The ASCA X-ray Spectrum of Eta Car"
as a gzipped postscript file.
Our spectral modeling is used to derive the "unfolded" SIS Eta Car spectrum. Note how
similar the 1-10 keV spectrum is to the X-ray spectrum of WR 140, a colliding wind
system. The WR 140 spectrum shown here is from an ASCA SIS PV
observation, when the system was near periastron passage. Both WR 140 and
Eta Car show highly absorbed, very high-temperature thermal components.
Eta Car also has considerable emission at energies below 1 keV, but this
mainly originates in the extended emission associated with the
The region of the spectrum near the Fe K-alpha line at 6.7 keV is
complex. Here's a plot of the photon
spectrum in the Fe K-alpha region along with the best-fit RS thermal
equilibrium model discussed above.
For an appreciation of the actual resolution of the SIS,
the observed SIS0 count spectrum and RS model
is also shown.
The strongest lines in this region of the spectrum are:
The 7.5 keV RS model (with slightly reduced Fe abundances) does a good job
of reproducing the observed line strengths, and implies that the emitting
plasma is in thermal equilibrium.
- the Fe flourescence line at 6.4 keV;
- the Fe XXV He-alpha line at 6.7 keV;
- the Fe XXVI Ly-alpha line at 6.9 keV;
- and the Fe XXV He-beta line at 7.8 keV.
The campaign of observations (covering radio through UV and X-ray to Gamma-ray observations) of the upcoming X-ray eclipse/"shell event" is available at
For More Information
Page Author: Dr. Mike
Last modified December 1, 1999