Eta Carinae in Hard X-rays
20 February 2008Eta Carinae is a complex binary system, at a distance of around 8000 light years, with a large nebulosity surrounding the central objects. Observations with INTEGRAL have confirmed the nature of hard X-ray emission from the region to be from inverse compton scattered photons accelerated in a region where two stellar winds collide.
In a recent paper in Astronomy & Astrophysics, J.-C. Leyder et al. presented analysis of hard X-ray observations from the η Carinae region. By utilising all public observation data within 10° of the target they were able to use an effective total integration time on source of 1.1 million seconds. This allowed the narrowing of error circles to reveal that the emission was coming from η Carinae rather than other known nearby targets.
η Carinae is a binary system with two high mass stars. The primary star, responsible for the optical nebulosity, is thought to be a large blue variable (LBV) star. Relatively little is known about LBVs except that they appear to be stars undergoing a transition between different states. Part of the process involves the loss of around 2.5x10-4 solar masses of material each year. The material is ejected by a stellar wind with velocities in the region of 500 kms-1.
The secondary star is thought to be a massive late-type nitrogen rich O or WR type star. This star is also undergoing mass loss, but at a lower rate than the primary - although the velocity is higher in the region 1500 kms-1.
In the region where these two winds collide, high-energy electrons from the secondary wind are accelerated in the local magnetic field and collide with surrounding photons and accelerate them to higher energies through the process of Inverse Compton scattering. The resulting emission is seen as hard X-rays with energies around 100 keV. Because the electrons scatter off photons of all energies, it is likely that there is higher energy emission extending into the MeV and GeV energy range. This falls outside the observing range of INTEGRAL and will require next generation instrument to perform accurate observations of the region.