Integral looks at Earth to seek source of cosmic radiation (Forwarded)

ESA News
http://www.esa.int

10 February 2006

Integral looks at Earth to seek source of cosmic radiation

Cosmic space is filled with continuous, diffuse high-energy radiation. To
find out how this energy is produced, the scientists behind ESA's Integral
gamma-ray observatory have tried an unusual method: observing Earth from
space.

During a four-phase observation campaign started on 24 January this year,
continued until 9 February, Integral has been looking at Earth. Needing
complex control operations from the ground, the satellite has been kept in
a fixed orientation in space, while waiting for Earth to drift through its
field of view.

Unusually, the main objective of these observations is not Earth itself,
but what can be seen in the background when Earth moves in front of the
satellite. This is the origin of the diffuse high-energy radiation known
as the 'cosmic X-ray background'.

Until now with Integral, this was never studied simultaneously with such a
broad band of energy coverage since the 1970s, and certainly not with such
advanced instruments.

Astronomers believe that the 'cosmic X-ray background' is produced by
numerous supermassive and accreting black holes, distributed throughout
deep space. These powerful monsters attract matter, which is then hugely
accelerated and so emit high energy in the form of gamma- and X-rays.

X-ray observatories such as ESA's XMM-Newton and NASA's Chandra have been
able to identify and directly count a large number of individual sources
-- likely black holes -- that already account for more than 80 percent of
the measured cosmic diffuse X-ray background.

However, very little is known about the origin of the highest energy band
of this cosmic radiation, above the range of these two satellites. This is
spread out in the form of high-energy X- and gamma-rays, within the reach
of Integral.

It is believed that most of the gamma-ray background emission is produced
by individual supermassive black holes too, but scientists need to couple
this emission with clearly identified sources to make a definitive
statement. In fact, other sources such as far-away galaxies or close weak
sources could be also be responsible.

Identifying the individual sources in the gamma-ray range that make up the
diffuse cosmic background is much more difficult than counting the
individual X-ray sources. In fact, the powerful gamma-rays cannot be
focused with lenses or mirrors, because they simply pass straight through.

So to produce a gamma-ray image of a source, Integral uses a 'mask'
technique -- an indirect imaging method that consists of detecting the
shadow of a mask placed on top of the telescope, as projected by a
gamma-ray source.

During the observations, the scientists used Earth's disk as an 'extra
mask'. Earth naturally blocks, or shades, the highest energy flux from
millions of distant black holes.

Their combined flux can be accurately measured in an indirect way, that is
by measuring the amplitude and the energy spectrum of the energy drop when
Earth passes through Integral's field of view. Once this is known,
scientists can eventually try to connect the radiation to individual
sources.

All the observations were very successful, as all the gamma-ray and X-ray
instruments on board Integral (IBIS, SPI and JEM-X) recorded clear and
unambiguous signals in line with expectations.

The Integral scientists are already proceeding with the analysis of the
data. The aim is to ultimately understand the origin of the highest energy
background radiation and, possibly, provide new clues on the history of
growth of super-massive black holes since the early epochs of the
Universe.

Notes to editors:

The Earth observation campaign is a successful joint effort of the full
Integral Science Working Team (composed of the instrument and data centre
Principal Investigators, mission scientists, project scientist and
representatives of the Russian and US scientific communities under the
leadership of the Russian science working team members who proposed these
unique 'non-routine' observations).

The campaign required a particularly complex planning and execution, which
greatly involved ESA's European Space Operations Centre (ESOC) and the
Integral Science Operations Centre (ISOC).

The Earth campaign is also providing an excellent opportunity to calibrate
the Integral instruments. Through a precise characterisation of the cosmic
X-ray background, scientists can identify and separate it from the
instruments' normal background signal, for easier data analysis in the
future.

As secondary objectives, the Earth observations will also allow
astronomers to study the high-energy emission from Earth's atmosphere.
This is coming from the reflection of the X-ray background, from the
interaction with cosmic rays and even from aurorae, as preliminary data
from these Integral observations have already hinted.

For more information:

Chris Winkler, ESA Integral Project Scientist
E-mail: christoph.winkler @ rssd.esa.int

Arvind Parmar, ESA Integral Mission Manager
E-mail: arvind.parmar @ esa.int

Eugene Churazov, Max Planck Institute for Astrophysics, Garching, Germany
E-mail: churazov @ mpa-garching.mpg.de

More about...

* ESA's gamma-ray astronomy mission
http://www.esa.int/SPECIALS/Integral/index.html
* Integral factsheet
http://www.esa.int/esaSC/SEMD9G1A6BD_index_0.html

Related articles

* Integral identifies supernova rate for Milky Way
http://www.esa.int/esaSC/SEMACK0VRHE_index_0.html
* Integral: three years of insight into the violent cosmos
http://www.esa.int/esaSC/SEMY146Y3EE_index_0.html
* Star eats companion
http://www.esa.int/esaSC/SEMWSAA5QCE_index_0.html
* Three satellites needed to bring out 'shy star'
http://www.esa.int/esaSC/SEMSOI6DIAE_index_0.html
* Integral rolls back history of Milky Way's super-massive black hole
http://www.esa.int/SPECIALS/Integral/SEMSKPO3E4E_0.html
* ESA's Integral detects closest cosmic gamma-ray burst
http://www.esa.int/SPECIALS/Integral/SEMV9P0XDYD_0.html
* ESA's Integral solves thirty-year old gamma-ray mystery
http://www.esa.int/esaSC/SEM24EX5WRD_extreme_0.html
* Integral -- tracking extreme radiation across the Universe
http://www.esa.int/esaCP/ESAI0BTHN6D_Expanding_0.html
* Observations: Seeing in the gamma-ray wavelengths
http://www.esa.int/esaSC/SEM3A2T1VED_index_0.html

Related links

* Integral Science Operations Centre
http://integral.esac.esa.int
* Max Planck Institut
http://www.mpe.mpg.de/gamma/instrume.../integral.html

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/esaSC/SEM8EVLVGJE_index_1.html]
Artist's impression of Integral observing Earth

ESA's Integral gamma-ray observatory has been observing Earth in a
four-slot campaign, during a period spanning from 24 January to 9 February
2006. The main purpose of the observations has been studying the
high-energy diffuse background radiation known as 'cosmic X-ray
background' (CXB), by analysis of the drop of its isotropic flux (not
varying with distance or direction) while Earth passes in front of
Integral's field of view. High-energy emission from the atmosphere, due to
reflections of the CXB, interaction with cosmic rays and aurorae, have
also been observed.

Credits: ESA

[Image 2:
http://www.esa.int/esaSC/SEM8EVLVGJE...html#subhead1]
Earth against the cosmic X-ray background

This preliminary gamma-ray image has been taken by the SPI spectrometer on
board Integral, during the Earth observation campaign (January-February
2006). The dark central part represents the Earth disc (about 5-7 degrees
in diameter) transiting in the instrument's field of view (2.5 deg), that
explains the blurred Earth rim in the image. The cosmic X-ray background
radiation in the vicinity of Earth is characterised by a strong emission
of high-energy (hard) X-rays.

Credits: (c) CESR and SPI team

[Image 3:
http://www.esa.int/esaSC/SEM8EVLVGJE...html#subhead2]
Diffuse high-energy flux drops during Earth transition

This simulation shows the passage of Earth through the field of view of
Integral's IBIS/ISGRI gamma-ray imager, during the observations performed
on 24 and 25 January 2006. The North Pole is indicated by a cross on
Earth's disc, and the time on the top right corner is given in hours and
minutes UTC. Earth is occulting the diffuse cosmic X-ray background (CXB),
but its atmosphere is also emitting X-rays both through reflection of the
CXB and through interaction with cosmic rays.

The occultation of the CXB resulted in one of the 'lightcurves' recorded
by IBIS/ ISGRI (around 30-40 keV energy range), and here indicated by the
dotted line. The asymmetry of the curve is due to the decreasing size of
Earth as seen by Integral moving away from the planet on its orbit, and to
enhanced cosmic-ray induced emission closer to the North Pole.

Credits: (c) ISDC and IBIS team

[Image 4:
http://www.esa.int/esaSC/SEM8EVLVGJE...html#subhead3]
X-ray images of Earth and cosmic X-ray background

These preliminary X-ray images were taken by Integral's JEM-X instrument
in two separate slots of the Earth observation campaign, in late January
(top row) and early February 2006 (bottom row).

The four images in each row correspond to different energy intervals. The
plot shows the image of Earth against the diffuse cosmic X-ray background,
as the JEM-X field of view sweeps across it (fixed stars would make
stripes in this representation). The most remarkable thing to note is the
bright spot visible at the two bottom left images, presumably indicating a
northern auroral X-ray emission.

Credits: (c) DSRI and JEM-X team