|
XMM-Newton
'spare-time' provides impressive sky survey
3 May 2006
|
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
|
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
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
|
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.
Source
/ Credit: ESA
|
