Behind a thick cocoon of cold gas, scientists have discovered what might be a new class of astronomical objects in our galaxy. Made up of a very massive star and a compact object such as a neutron star or a black hole, the binary system is unique because it remained hidden behind a cloud of obscuring material for so long. A paper describing the discovery will be published in Astronomy and Astrophysics in November.

The European Space Agency launched the Integral gamma-ray space telescope nearly a year ago. In January, it detected a previously undiscovered object, which astronomers dubbed IGR J16318-4848. Early analysis suggested it was similar to the 300 known combinations of compact objects and their companion stars in the Milky Way. IGR J16318-4848 was different, however, because it had eluded detection for so long. Using the XMM-Newton space observatory, the researchers detected a dense cocoon of cold gas surrounding the pair. The researchers posit that material ejected from the companion star, its so-called stellar wind, is accreted by the black hole to form a dense shell around the pair. The diameter of this thick cloud is roughly equivalent to the distance between the earth and the sun.

Only photons with the highest energies could escape from that cocoon, notes lead author Roland Walter of the Integral Science Data Center in Switzerland. IGR J16318-4848 has therefore not been detected by surveys performed at lower energies, nor by previous gamma-ray missions that were much less sensitive than Integral. The team is now looking for other galactic objects that may be obscured by gas shells. So far, they have discovered two other new pairings and future observations are planned.