Collapsing Wolf-Rayet Stars and Inverse Compton Scattering of Stellar Photons

A Wolf-Rayet star WR124 with surrounding nebula M1-67 (NASA)

Wolf-Rayet stars are a violent and self-destructive phase of a massive star’s lifetime. This is the point at which they begin to die as a prelude to a supernova and black hole formation. Often, large nebulae can be found around these bright stellar objects (pictured), emitting strong ultraviolet radiation. As Wolf-Rayet (WR) stars continue to lose huge amounts of mass and deplete all their fuel, they become even more unstable, resulting in a huge supernova. Exploding WR stars have been linked with powerful gamma ray (γ-ray) bursts; in fact the largest, most distant GRB was observed on March 19th in the constellation of Boötes by NASA’s Swift Observatory and the Polish “Pie of the Sky” GRB detector. There is some evidence that this GRB was the result of a WR star/neutron star binary pair, but what would happen if a WR star is sitting close to an O-type star just as it explodes?

As the WR star collapses, a shock wave (containing hot, relativistic electrons) sent hurtling toward the O-type star may cause inverse Compton scattering of the stellar photon field, generating powerful, long period emissions of γ-ray radiation. New research suggests that this mechanism may explain the 1-100 GeV γ-rays observed minutes or hours after the main GRB…

We usually understand Compton scattering as a physical effect on high energy electromagnetic waves. As high frequency waves (such as X-rays or γ-rays) interact with electrons, energy is exchanged to the electrons, reducing the electromagnetic wave frequency (and therefore energy; E=hγ). With inverse Compton scattering, the opposite is true. As lower frequency photons such as ultraviolet emissions interact with high energy electrons (i.e. relativistic), energy can be transferred from electron to photon, boosting the frequency (energy) of the photon. Inverse Compton scattering is very important when considering high-energy astrophysical environments. You don’t get much more energetic than a GRB.

According to a paper submitted to the journal Astronomy & Astrophysics in the beginning of last month, Dimitrios Giannios (working at the Max Planck Institute for Astrophysics) thinks it could be inverse Compton scattering that is responsible for the γ-ray emissions observed after a GRB event.

ESO Optical Image of Westerlund 1. This is a dense cluster containing many O-type stars (ESO)

To set the scene, binary pairs are very common in stellar evolution. Our Solar System is a rarity as the Sun doesn’t have a binary partner (and no, Planet X is not a possible partner!), but many observed Wolf-Rayet stars are expected to have a stellar buddy. In a previous Astroengine post, I reviewed a paper that studied the influence a neutron star binary partner had on a WR star and resultant supernova. Giannios’ research focuses on a Wolf-Rayet star that doesn’t necessarily have a binary partner, but it is sitting within a cluster of O-type stars. O-type stars are very common in high-mass clusters and this is often the location of a high proportion (~30%) of observed GRBs. In this situation, it seems reasonable that a GRB may interact with a neighbouring O-type star.

Giannios calculates that for any given WR star within a dense cluster of O-type stars, the WR star will be around 6,700 – 20,000 AU (1−3×1017 cm) from the nearest O-type star. Should a WR star collapse and explode as a supernova, a GRB will be generated. As matter from the explosion passes from the event and into interstellar space (i.e. the space between the WR star and nearest O-type star), a shock front forms which consists of heated near-relativistic electrons. As the ultraviolet photons from the nearest O-type star try to cross this shock front, upscattering occurs. Inverse Compton scattering therefore injects energy into the UV photons, ramping up their frequency, generating γ-rays.

By understanding the interaction of shock waves generated by a GRB, a better idea about the origin of post-GRB γ-ray emissions can be arrived at. Also, by observing these emissions, astronomers should be able to probe the interstellar environment inside dense clusters.

This is a very interesting area of research and all the details can be found in the pre-print on arXiv entitled: arXiv:0805.0258v1 [astro-ph] – Powerful GeV emission from a gamma-ray-burst shock wave scattering stellar photons

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