A deep look at the Milky Way in hard X-rays

An improved image analysis technique, developed by scientists at the Max Planck Institute for Astrophysics (MPA), led to the most sensitive wide-angle, hard X-ray survey of the Milky Way ever obtained. The team combined images from the IBIS imager on board the international gamma-ray observatory INTEGRAL taken over the course of seven years to achieve a maximum exposure time of more than 230 days in the direction of the Galactic Centre.

Fig. 1: This map shows a region of the Galactic plane, obtained with the IBIS/ISGRI-camera on board INTEGRAL, covering the 17-60 keV energy band. In the galactic centre, the central area of the map, the total exposure time is about 20 million seconds. The top image shows the results when using a previous ("unimproved") sky reconstruction method. The bottom image demonstrates how the sky image is enhanced when using the improved method.

Fig. 2: Enlarged image of the central area (galactic bulge region) in the INTEGRAL/IBIS 7-year map. The overlaid brightness contours of the 4.9 micrometre COBE/DIRBE-image trace the Galaxy's old stellar population and reveal the disk/bulge structure.

Recently, there has been great progress in surveying the sky at high energies. Among the main gamma-ray missions currently in orbit are the Burst Alert Telescope (BAT) of the NASA Swift observatory and the IBIS telescope on board the ESA INTEGRAL spacecraft, which are both covering large angles on the sky. The large sample of active galactic nuclei (AGN) detected by the nearly uniform Swift/BAT all-sky survey shows that this telescope is very valuable for extragalactic studies. In contrast, the INTEGRAL observatory has non-uniform sky coverage and provides a survey that is more sensitive in the Galactic plane. This makes the Swift/BAT and INTEGRAL/IBIS surveys complementary.

A copious amount of exposure time for the Milky Way galaxy, however, represented a challenge for the scientists, as the growing exposure time does not automatically mean a corresponding increase in sensitivity. The scientists faced two main problems: The coded-mask technique of the IBIS telescope — one of the few methods of hard X-ray imaging — leads to a number of systematic effects, which plague the observations and limit the sensitivity of the instruments. This affects especially images along the Galactic Plane and the crowded field of Galactic Centre. The second difficulty comes from a strong, diffuse X-ray radiation emitted in the Galactic Plane, the so-called Galactic Ridge emission (see linkPfeil.gifResearch Highlight March 2006 , linkPfeil.gifResearch Highlight April 2006 , linkPfeil.gifResearch Highlight August 2006), which affects the sky reconstruction that is based on complex background modelling.

Scientists from the MPA have now provided a novel image analysis algorithm, which was developed to improve the sensitivity of the IBIS telescope. First, the new background model now takes the X-ray background of the Galaxy into account. Second, the reconstructed sky images were cleaned from systematic noise by using a special algorithm based on discrete wavelets.

”By combining the sky images of about 50,000 individual observations, carried out from 2002 to 2009, our group conducted a survey of the Milky Way with unprecedented sensitivity in hard X-rays,“ says Roman Krivonos, the MPA researcher who led the study. ”The final sky mosaic allows for the first time to study the population of faint accreting X-ray binaries in the Galaxy.“

These kinds of objects could not be seen in existing soft X-ray surveys of nearby galaxies performed with the Chandra and XMM-Newton space observatories. The new sample of compact, galactic sources, detected in this INTEGRAL survey, now represents a prototype for future compact source samples that will be detected by upcoming hard X-ray missions in nearby galaxies.


Roman Krivonos, Sergey Tsygankov, Mikhail Revnivtsev, Eugene Churazov, Rashid Sunyaev


Related Links

linkPfeil.gifMPA sky reconstruction algorithm
linkPfeil.gifInteractive INTEGRAL map of the galactic bulge
linkPfeilExtern.gifESA Press Release: INTEGRAL completes the deepest all-sky survey in hard X-rays


Related Publications

Krivonos, R.; Revnivtsev, M.; Tsygankov, S.; Sazonov, S.; Vikhlinin, A.; Pavlinsky, M.; Churazov, E.; Sunyaev, R., "INTEGRAL/IBIS 7-year All-Sky Hard X-Ray Survey — Part I: Image Reconstruction", 2010, A&A, Volume 519, id.A107
linkPfeilExtern.gifhttp://dx.doi.org/10.1051/0004-6361/200913814

Krivonos, R.; Tsygankov, S.; Revnivtsev, M.; Grebenev, S.; Churazov, E.; Sunyaev, R., "INTEGRAL/IBIS 7-year All-Sky Hard X-Ray Survey. Part II: Catalog of Sources", 2010, A&A accepted
linkPfeilExtern.gifhttp://arxiv.org/abs/1006.4437