ESA: INTEGRAL completes deepest all-sky survey in hard Xray

[bystander]

INTEGRAL completes the deepest all-sky survey in hard X-rays
ESA | Science & Technology | 11 Aug 2010

A newly developed image analysis technique has significantly improved the sensitivity limits reached by the IBIS imager on board INTEGRAL, resulting in the deepest survey ever compiled of the entire sky in the energy range between 17 and 60 keV. Pushing the instrument towards its very limits, the novel method discloses a vast number of previously undetected faint sources, galactic and extragalactic alike.

For more than seven years, the INTEGRAL observatory has been surveying the entire X-ray and gamma-ray sky and has accumulated a copious amount of exposure time, targeting both the crowded regions along the Galactic Plane and the high-latitude portions of the sky, this latter region being dominated by extragalactic sources. Theoretically, a longer exposure time translates into an improvement in sensitivity, but this connection is not always straightforward: a number of systematic effects plague the observations and limit the sensitivity of the instruments despite the increased exposure time. Hence, new techniques are sought, and implemented, in order to overcome these systematic effects and to fully exploit the instrument performance.

A successful example of this synergy is a novel image analysis algorithm, recently developed to improve the sensitivity achieved by IBIS, the Imager on Board the INTEGRAL Satellite.
...
The newly developed method suppresses systematic effects almost completely in extragalactic, high-latitude fields, and yields a significant, albeit not total, removal also in the portion of the sky dominated by the Galaxy. Observations have now a more-or-less uniform sky background, enabling the detection of previously unnoticeable faint sources.

The result is the deepest all-sky survey compiled to date in hard X-rays, covering the energy range between 17 and 60 keV. The sensitivity has reached instrumental limits on extragalactic observations, where the IBIS imager aboard INTEGRAL is working at its maximum efficiency; on galactic fields, observations do not reach, but significantly approach, the instrumental limits, delivering a survey of the Galaxy with the best currently available sensitivity in this energy band.

[neufer]

<<The European Space Agency's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) is an operational Earth satellite, launched in 2002 for detecting some of the most energetic radiation that comes from space. It is the most sensitive gamma ray observatory ever launched. INTEGRAL has had some notable successes, for example in detecting a mysterious 'iron quasar'. It has also had great success in investigating gamma-ray bursters and evidence for black holes.

The INTEGRAL imager, IBIS (Imager on-Board the INTEGRAL Satellite) observes from 15 keV (hard X-rays) to 10 MeV (gamma rays). Mechanical resolution is 12 arcmin, but deconvolution can reduce that to as little as 1 arcmin. A 95 x 95 mask of rectangular tungsten tiles sits 3.2 meters above the detectors. The detector system contains a forward plane of 128 x 128 Cadmium-Telluride tiles (ISGRI- Integral Soft Gamma-Ray Imager), backed by a 64 x 64 plane of Caesium-Iodide tiles (PICsIT- Pixellated Caesium-Iodide Telescope). ISGRI is sensitive up to 500 keV, while PICsIT extends to 10 MeV. Both are surrounded by passive shields of tungsten and lead.>>

<<X-radiation (composed of X-rays) is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma rays. The distinction between X-rays and gamma rays has changed in recent decades. The two types of radiation are now usually distinguished by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.

X-rays from about 0.12 to 12 keV (10 to 0.10 nm) are "soft" X-rays
X-rays from about 12 to 120 keV (0.01 to 0.10 nm) as "hard" X-rays.

Hard X-rays can penetrate solid objects, and their largest use is to take images of the inside of objects in diagnostic radiography and crystallography. As a result, the term X-ray is metonymically used to refer to a radiographic image produced using this method, in addition to the method itself. By contrast, soft X-rays can hardly be said to penetrate matter at all; for instance, the attenuation length of 600 eV (~ 2 nm) x-rays in water is less than 1 micrometer.

Though X-rays are otherwise invisible it is possible to see the ionization of the air molecules if the intensity of the X-ray beam is high enough. And while generally considered invisible to the human eye, in special circumstances X-rays can be visible. Brandes, in an experiment a short time after Röntgen's landmark 1895 paper, reported after dark adaptation and placing his eye close to an X-ray tube, seeing a faint "blue-gray" glow which seemed to originate within the eye itself. Upon hearing this, Röntgen reviewed his record books and found he too had seen the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen had noted the same blue glow, seeming to emanate from the eye itself, but thought his observations to be spurious because he only saw the effect when he used one type of tube. Later he realized that the tube which had created the effect was the only one powerful enough to make the glow plainly visible and the experiment was thereafter readily repeatable. The knowledge that X-rays are actually faintly visible to the dark-adapted naked eye has largely been forgotten today; this is probably due to the desire not to repeat what would now be seen as a recklessly dangerous and potentially harmful experiment with ionizing radiation. It is not known what exact mechanism in the eye produces the visibility: it could be due to conventional detection (excitation of rhodopsin molecules in the retina), direct excitation of retinal nerve cells, or secondary detection via, for instance, X-ray induction of phosphorescence in the eyeball with conventional retinal detection of the secondarily produced visible light.>>