This first discovery of polarized gamma rays from the vicinity of a black hole opens a new observational window on these enigmatic objects. The results of this study are published online in Science Express.

Black holes that originate from the collapse of massive stars are very compact objects, enclosing up to a few tens of solar masses within a radius of only a few kilometres. Also referred to as 'stellar black holes', they are so dense that nothing, not even light, can escape their gravity.

In theory, these black holes should therefore be impossible to observe, however, when a black hole is part of a binary stellar system, interesting effects arise that can uncover the surroundings of these otherwise 'invisible' objects. It is this phenomenon that has enabled astronomers using INTEGRAL to probe the environment of the black hole in the Cygnus X-1 binary system.

In a binary system, a black hole and a normal star are gravitationally bound and orbit each other around their common centre of mass. With its intense gravitational field, the black hole draws matter from its companion, and the stripped material spirals around the black hole, forming an accretion disc. As friction in the disc heats the material up to millions of degrees, making it shine in X-rays, these objects are known as X-Ray Binaries (XRB).

First observed in the 1960s, soon after observations at these wavelengths became possible, these objects raised immediate interest as black hole candidates; furthermore, in the following decades, data gathered in the radio band revealed jets of relativistic particles emanating from several XRBs, suggesting that a link exists between accretion and ejection of matter in the proximity of such stellar black holes.

Astronomers call these objects 'microquasars' because they appear as miniature versions of quasars - the nuclei of active galaxies that harbour, in their cores, supermassive black holes that are millions of times more massive than their stellar counterparts.

In spite of their very different sizes, black holes in both types of systems exhibit very similar dynamics, vigorously accreting matter from their surroundings via a disc and funnelling part of it, by means of the disc rotation, into highly collimated, bipolar jets of particles that are released at relativistic speeds.

"The connection between accretion and ejection of material in microquasars is well established by now, but many questions about the connection between the accretion disc and jets remain open," explains Philippe Laurent from CEA, France, who led the team that discovered highly polarized gamma-ray emission coming from Cygnus X-1, one of the best studied microquasars in the Milky Way.

"Our study finally starts to answer some of these questions," he adds.

In 2006, observations performed with INTEGRAL revealed gamma rays from Cygnus X-1, but their origin was unclear. "Some models predict that microquasars may produce jets so powerful they emit even up to gamma rays: in that case, the gamma-ray signal is expected to be highly polarized," notes co-author Jerome Rodriguez, also from CEA.

Polarization is, in fact, a signature of the non-thermal radiation processes that take place in the jets, such as synchrotron or inverse Compton emission. "The discovery was thus a pleasant surprise, allowing us to identify the jets as the source of the gamma-ray emission," he continues.

This is the first time that polarized gamma-ray emission has been observed in a black hole binary system, and only the second time ever: in 2008, INTEGRAL detected polarized emission from the Crab Nebula.

"INTEGRAL is the only instrument currently capable of performing polarimetry measurements in the gamma-ray band on astronomical sources other than the Sun," comments Chris Winkler, INTEGRAL project scientist at ESA. This is due to its particular configuration based on two separated detector planes, where incoming photons are initially scattered, in the first plane, and then absorbed in the second one.

"By analysing the pattern described by the photons detected in the second plane, it is possible to infer whether the radiation is polarized or not," explains Winkler.

Since the polarized component is only a fraction, albeit large, of the total radiation, the signal is rather weak and it is thus necessary to stack a substantial number of observations of the same source in order to reveal polarization at a reasonable significance - even in this case, the detection is only possible for the brightest sources in the high energy sky. For the current study, the team has used all archival data for Cygnus X-1 collected by INTEGRAL over a period of almost 8 years.

"The discovery of polarized gamma-ray emission offers a brand new perspective to look at microquasars, since it also carries important information on the configuration of the source's magnetic field," continues Laurent. Together with further observations of the jets at radio wavelengths, this will allow a better characterisation of the mechanism producing gamma rays in these objects.

"Future studies might reveal polarization also in other, less bright black hole binaries, thus shedding light on the overall accretion and ejection scenario in microquasars," he concludes.

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