March 2, 2017

High-mass X-ray binaries trace the Milky Way's spiral arms

Observations of high-energy phenomena, in particular with the Integral spacecraft, have discovered numerous massive X binary sources in the Milky Way. While more than 200 are catalogued, until now their locations have not been fixed very precisely. By measuring exactly the distance of a sample of 50 such sources, two researchers from the Service d'Astrophysique-Laboratoire AIM at CEA-Irfu in Saclay, supported by CNES, showed that these sources are in a cluster of 1,000 light years located in the star formation complexes in the spiral arms of the Milky Way. Their work, published in The Astrophysical Journal, provides new insight into the environment where these systems were born and evolved.

Massive X-ray binaries in the Milky Way

High-mass X-ray binaries (HMXBs) contain stars that consume their hydrogen and helium fuel so quickly that they explode as supernovas within a few tens of millions of years - the blink of an eye in the history of the Universe.

These short-lived stellar systems comprise an extremely dense, compact object (a neutron star or a black hole) which is pulling in matter ejected from a massive companion - a process known as accretion. The stellar companion is usually either a main sequence Be star or an evolved supergiant nearing the end of its life.
Be stars are rapidly rotating objects surrounded by a disc of gas that is ejected by the stars themselves. When the neutron star passes periodically through this disc, gas is strongly heated during accretion onto its surface, creating a blast of X-rays. In the case of supergiant X-ray binaries, the accreted material is derived from the massive companions ejecting large amounts of material in their stellar winds.

Dedicated X-ray observations of the sky, particularly with the INTEGRAL spacecraft, have increased fivefold the known population of high-mass supergiant X-ray binaries in the Galaxy. Some 35 supergiant HMXBs are currently catalogued, out of a total of more than 200 HMXBs.
Until now, their locations within the Milky Way have not been fixed very precisely. However, most of the sources are seen to lie in the Galactic Plane and observations made with INTEGRAL over the last decade show that HMXBs seem to be associated with the spiral structure of the Galaxy.

Taking advantage of the much larger number of detected HMXBs, two French researchers decided to carry out a statistical analysis of their distribution in the Milky Way. The first step was to determine the distances of a sample of HMXBs. This was done by comparing their apparent magnitudes (brightness) with their light spectrum - a signature of their likely temperature and energy output. The spectrum of each HMXB was compared with theoretical models to compute the distance of each source.

"By using this novel technique, we were able to find a strong correlation between the positions of HMXBs and star-forming complexes in the Milky Way" said Alexis Coleiro, a PhD student at Université Paris Diderot, France, and lead author of the paper in the Astrophysical Journal.

Distribution of high-mass X-ray binary stars and star-forming complexes.
From A. Coleiro and S. Chaty, ApJ 764:185, 2013. Copyright 2013 IOP Publishing

Very near but not exactly at the same place

The separation of the HMXBs and the complexes where they formed are largely due to the momentum supplied when the more massive star in the binary exploded as a supernova. Since such explosions tend to be asymmetrical, the dying star receives a kick in a particular direction. As long as the supernova's initial velocity is not too high, the binary stars are held together by their mutual gravitational attraction.

The scientists also compared the positions of current star forming regions in the Galaxy's spiral arms and a sample of 13 HMXBs. This enabled them to constrain the ages and migration distances of the HMXBs as a result of the velocity boost from the supernovas.

"We know that HMXBs are born in star-forming complexes, which are usually located in the Galaxy's spiral arms," said Sylvain Chaty, a professor at Université Paris Diderot, and Alexis Coleiro's research supervisor. "Star formation in the spiral arms is triggered by density waves, regions of enhanced density where interstellar gas and dust are slowed down and compressed."

"We also know that HMXBs are short-lived, surviving for only a few tens of millions of years, so it ought to be possible to link them to their birthplaces. As a result, we decided to investigate the relationship between the binary systems and the spiral structure of the Milky Way."

By assuming that the density waves rotate at a different speed to the matter (stars, dust etc.) in the Galaxy, it was possible to calculate the expected HMXB locations relative to the positions of the spiral arms at certain times in the past. The researchers then compared these positions with the current locations of 13 HMXBs in order to determine the effects of the kicks they received from the supernova explosions.

Their calculations showed that the mean age of four supernova HMXBs was 45 million years, with a mean migration distance of about 325 light years. The mean age of nine Be-class HMXBs was 51 million years, with a mean migration distance of about 360 light years.

"For the first time, we have accurately derived the distances and distribution of a large sample of high-mass X-ray binaries in our Galaxy, bringing new constraints on their formation and evolution," said Coleiro.

"These new methods will allow us to assess the influence of the environment on these high-energy objects with unprecedented reliability," said Chris Winkler, ESA's Integral project scientist.

"This study was made possible because Integral is the only observatory with the sensitivity to observe numerous HMXBs in the Milky Way."

Reference: