Most of their time is spent frozen in the outer reaches of the solar
system. But when these balls of ice and dust, which we know as comets,
decide to make an appearance, the spectacle is often grandiose. This is
mainly caused by their warming up as they approach the Sun. Astronomers
then have a chance to investigate comets closely, including at X-ray
wavelengths, as XMM-Newton did at the end of January 2001.

The primary objective of the XMM-Newton mission is the study of the
X-ray-hot, violent universe. However since the ROSAT observatory’s
discovery in 1996 that “dirty snowballs” also shine quite unexpectedly
in X-rays, it had been envisaged that XMM-Newton would be asked to
contribute and help astronomers trying to understand this X-ray emission.

The comet McNaught-Hartley was revealed in October 1999. During most of
last year it was visible in the Southern hemisphere steadily becoming
brighter and by mid-December, now in view of the Northern hemisphere, it
had reached 8th magnitude. The coma diameter was estimated to be between
3 and 4 arcminutes. Brightness is important because for most comets the
X-ray luminosity tends to follow the optical luminosity.

McNaught-Hartley was the opportunity several X-ray astronomers had been
waiting for. At the Max Planck Institute for Extraterrestrial Physics
in Garching, Konrad Dennerl — who had already been involved in the
initial ROSAT discovery — and XMM-Newton Telescope scientist Bernd
Aschenbach proposed to carry out an observation.

“XMM-Newton’s field of view is very small for these type of observations,
only 30 arcminutes and catching a comet is not easy” explains Bernd
Aschenbach. “The satellite’s normal operating mode is to have a fixed
attitude. The observatory currently has no scan mode and can not
continuously track a moving object. So once we had Project Scientist
Fred Jansen’s green light to make the attempt, as part of my guaranteed
time, we had to find the right strategy.”

Early in December, Konrad Dennerl and Pedro Rodriguez, a mission planner
from XMM-Newton’s Science Operations Centre at VILSPA, started calculating
whether the speed at which McNaught-Hartley was travelling was slow enough
to get enough exposure as it passed over the telescope’s field of view.

They were lucky! In three separate 10 thousand second duration
observations on 29 and 30 January, XMM-Newton was able to catch its
first comet. McNaught-Hartley, which is a dynamically new comet coming
perpendicularly to the ecliptic plane, was then at a distance of some
192 million kilometres (1,29 AU).

Comets may be bright at optical wavelengths, but their X-ray flux is
generally some ten thousand times weaker. With its large collecting area
for soft X-rays, where comets are expected to radiate predominantly,
XMM-Newton is an excellent tool. The data obtained by the EPIC cameras
is still being analysed but already Konrad Dennerl and Bernd Aschenbach
are very excited.

Settling for one of five possible explanations

The study of X-ray emission in comets is still very much virgin territory
and a field where there is intense competition. Five theories, from most
to least probable according to current thinking, have been advanced to
explain the phenomenon:

* interaction of highly ionised elements in the solar wind (such as
carbon, nitrogen and oxygen) with the gas being released by the comet.
This would be a ‘charge exchange’ process, where for instance, oxygen
atoms with six or seven lost electrons, would be recharged by the
cometary gas.

* a “Bremsstrahlung”effect when fast moving electrons in the solar wind
hit, and are slowed down, by the nuclei of cometary atoms.

* X-rays from the Sun itself may be scattered, and re-emitted by dust
particles of the comet.

* Interaction between the ionised plasma in the solar wind and the plasma
in the vicinity of the comet, with a heating process in both.

* Collision and “sputtering” of solar system dust particles and cometary
dust particles at relative velocities high enough to produce X-rays.

XMM-Newton obtained sufficient X-ray counts with the EPIC-pn camera.
EPIC-MOS camera data will also be examined because of their high spectral
resolution at low energies. In addition, the investigation will benefit
from the image obtained simultaneously in the ultra-violet by the Optical

“The EPIC-pn camera image I have produced, at a very early state of
analysis, is proof that XMM-Newton is able to observe cometary X-ray
emission, which is a challenge in itself” says Konrad Dennerl. “The
image looks quite similar to what ROSAT observations showed, with an
X-ray halo which is elongated perpendicularly to the solar direction.”

“But the really exciting part is the spectroscopy, as a first glimpse of
the data shows. With possible contamination of the soft X-ray signal by
optical light, it is one of the most challenging tasks of data analysis
and this will certainly take some time.”

Konrad Dennerl and Bernd Aschenbach are sure that the scientific clues
lie in the excellent spectra they have obtained from McNaught-Hartley,
and that from the proposed theories, they can probably settle for just
one. It could be the fullest explanation ever of cometary X-ray emission.

Acknowledgements to Konrad Dennerl and Bernd Aschenbach of the Max Planck
Institute for Extraterrestrial Physics, Garching, Germany.


* Max Planck Institute (MPE): X-ray astronomy home page

* XMM-Newton home page

* About Comets

* Other Comets

* A night to remember: the Giotto flyby of Halley’s comet

* Observatori Astr=F2nomici de Mallorca


[Image 1:]

XMM-Newton EPIC-pn image of comet McNaught-Hartley.

[Image 2:]
Comet McNaught-Hartley in the ultraviolet passing through XMM-Newton’s
Optical Monitor field of view.

[Image 3:]
Ground-based CCD image of comet McNaught-Hartley. Taken by Juan Rodr=EDgu=
at the Observatori Astr=F2nomic de Mallorca.

[Image 4:]
Picture of the nucleus of Halley’s comet taken by ESA’s Giotto probe.