XMM-Newton 'spare-time' provides impressive sky survey (Forwarded)

ESA News
http://www.esa.int

3 May 2006

XMM-Newton 'spare-time' provides impressive sky survey

For the past four years, while ESA's XMM-Newton X-ray observatory has been
slewing between different targets ready for the next observation, it has
kept its cameras open and used this spare time to quietly look at the
heavens. The result is a 'free-of-charge' mission spin-off -- a survey that
has now covered an impressive 25 percent of the sky.

The rapid slewing of the satellite across the sky means that a star or a
galaxy passes in the field of view of the telescope for ten seconds only.
However, the great collecting area of the XMM-Newton mirrors, coupled with
the efficiency of its image sensors, is allowing thousands of sources to be
detected.

Furthermore, XMM-Newton can pinpoint the position of X-rays coming from the
sky with a resolution far superior to that available for most previous
all-sky surveys. This is sufficient to allow the source of these X-rays to
be found in many cases.

By comparing XMM-Newton survey's data with those obtained over a decade ago
by the international ROSAT mission, which also performed an all-sky survey,
scientists can now check the long-term stability, or the evolution, of about
two thousand objects in the sky.

An initial look shows that some sources have changed their brightness level
by an incredible amount. The most extreme of these are variable stars and
more surprisingly galaxies, whose unusual volatility may be due to large
quantities of matter being consumed by an otherwise dormant central black
hole.

The slew survey is particularly sensitive to active galactic nuclei (AGN) --
galaxies with an unusually bright nucleus -- which can be traced out to a
distance of ten thousand million light years.

While most stars and galaxies look like points in the sky, about 15 percent
of the sources catalogued by XMM-Newton have an extended X-ray emission.
Most of these are clusters of galaxies -- gigantic conglomerations of
galaxies which trap hot gas that emit X-rays over scales of a million light
years.

Eighty-one of these clusters are already famous from earlier work but many
other clusters, previously unknown, appear in this new XMM-Newton sky
catalogue.

Scientists hope that the newly detected sources of this kind also include
very distant clusters which are highly luminous in X-rays, as these objects
are invaluable for investigating the evolution of the Universe. Follow-up
observations by large optical telescopes are now needed to determine the
distances of the individual galaxies in the newly discovered clusters.

Using traditional pointed observations, it takes huge amounts of
telescope-time to image very large sky features, such as old supernova
remnants, in their entirety. The slewing mechanism provides a very efficient
method of mapping these objects, and several have been imaged including the
20 000 year-old Vela supernova remnant, which occupies a sky area 150 times
larger than the full moon.

Extraordinarily bright, low-mass X-ray binary systems of stars (called
'LMXB') -- either powered by matter pulled from a normal star, or exploding
onto the surface of a neutron star, or being consumed by a black hole -- are
observed with sufficient sensitivity to record their detailed light
spectrum. Passes across these intense X-ray sources can help astronomers to
understand the long-term physics of the interaction between the two stars of
the binary system.

Many areas of astronomy are expected to be influenced by the XMM-Newton sky
survey. Today, 3 May 2006, the XMM-Newton scientist have released a part of
the catalogue resulting from the initial processing of the highest quality
data obtained so far.

Such data correspond to a sky coverage of about 15 percent, and include more
than 2700 very bright sources and a further 2000 sources of lower
significance. Currently, about 55 percent of the catalogue entries have been
identified with known stars, galaxies, quasars and clusters of galaxies.

A faster turn-around of slew-data processing is now planned to catch
interesting transient (or temporary) targets in the act, before they have a
chance to fade. This will give access to rare, energetic events, which only
a sensitive wide-angle survey such as XMM-Newton's can achieve.

It is planned to continually update the catalogue as XMM-Newton charts its
way through the stars. This will cover at least 80 percent of the sky,
leaving a tremendous legacy for the future.

For more information:

Matteo Guainazzi, ESA XMM-Newton Science Operations
Email: matteo.guainazzi @ sciops.esa.int

Richard Saxton, ESA XMM-Newton Science Operations
Email: richard.saxton @ sciops.esa.int

Norbert Schartel, ESA XMM-Newton Project Scientist
Email: norbert.schartel @ sciops.esa.int

More about ...

* XMM-Newton overview
http://www.esa.int/esaSC/120385_index_0_m.html

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IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/esaCP/SEMA7OOFGLE_index_1.html]
In its on-going slew survey of the sky, XMM-Newton is able to map with high
efficiency very large sky features. Among these, is the 20 000 year-old Vela
supernova remnant (right) -- occupying a sky area 150 times larger than the
full moon. This object is compared here with an image previously taken by
the former ROSAT mission (left).

Credits: ESA/ROSAT

[Image 2:
http://www.esa.int/esaCP/SEMA7OOFGLE...html#subhead2]
Eighty-one galaxy clusters -- objects that make extended X-ray emissions --
are know from former sky surveys (see image). Previously unknown sources of
these kind are now being catalogued thanks to XMM-Newton's slew sky survey.

Studying these objects, especially when highly luminous, is very important
to investigate the evolution of the Universe.

Credits: ESA and the XMM-Newton EPIC consortium