Astrophysics 102: Gamma-Ray Binaries and Young Windy Pulsars
(This is the first in a continuing series of posts on various aspects of galactic high-energy astrophysics. I’ll also be covering some of the recent papers published in this area each week or so for Research Blogging)
Sometimes its hard to grasp just how profound the rate of scientific progress is in some fields, especially in my own area of study, galactic high-energy astrophysics. Until a few decades ago, for example, we didn’t know that X-ray binaries (XRBs) in our own galaxy could emit relativistic jets. Jets were thought to be the domain of farflung and exotic extragalactic objects such as Active Galactic Nuclei or Quasars, indeed the first such jet had been discovered as far back as 1918!
It wasn’t until 1979 that the presence of relativistic jets in our galaxy was finally confirmed. The unusual elongated radio source SS433 turned out to be a relativistic jet being emitted from a galactic binary system, and thus was what we now know to be the first galactic microquasar (Image source here):
It was long thought to be a unique object, but with the discoveries of other galactic microquasars in the early nineties by, amongst others, Felix Mirabel and Luis Felipe Rodriguez (see here an early but comprehensive review by Mirabel) it was soon realised that a good sized-proportion of the 700 or so XRBs in our galaxy possessed jets and were local analogues of quasars (the current best estimate for the total size of the galactic microquasar population, according to a recent review by Josep Paredes and Victor Zabalza of the University of Barcelona is ~65.
These jets usually emit most of their energy in the radio. but a small number of them have recently been discovered to emit also at more energetic frequences, in gamma-rays, much like their larger cousins do, via the mechanism of Inverse Compton Scattering where a low energy photon (such as one emitted from a star) bounces off a lepton (e.g. an electron) travelling at relativistic speeds (such as one in a relativistic jet) and energy is transferred from the lepton to the photon causing the photon to ‘upscatter’ and reach extremely high energy levels (original image here):
Up until now, only a few of these so-called “gamma-ray binaries” have been identified (yes, astronomers love to categorise and pigeonhole things to an extent that would make even Carl Linneus blush). But just to complicate matters, it turns out that some of them don’t even have relativistic jets, but instead are an entirely different type of binary called a Pulsar Wind Binary. The diagram (source: F. Mirabel) below summarises the differences between the two types of gamma-ray binary:
Unlike how they are generated in a microquasar, the gamma-rays in a young windy pulsar are emitted due to interactions between the fast (i.e. relativistic) stellar wind from the pulsar and a gaseous circumstellar disk that orbits the main star in the binary. But the end emission is to all intents and purposes identical in both scenarios, and that gives astrophysicists a problem: how can you figure out what type of object a particular gamma-ray binary is?
I’ll be covering the methods used in some upcoming posts.
W. Bednarek & R. Protheroe (1997) Interactions of Stars with AGN Jets: Gamma Ray Production in Relativistic Jets in AGNs, Proceedings of the International Conference, p.318-323 (ADS)
G.Dubus (2006) A&A 456 801 (ADS)
F. Mirabel & L. Rodriguez (1998) Nature 392 673 (ADS)
F. Mirabel (2010) Microquasars: Summary and Outlook in The Jet Paradigm, Lecture Notes in Physics, Volume 794. Springer-Verlag Berlin Heidelberg, 2010 (ADS)
J. M. Paredes & V. Zabalza, V (2010) Microquasars in the GeV-TeV era Invited review at the “7th Agile Meeting and The Bright Gamma-Ray Sky”, held in Frascati, Italy, 29 September to 1 October 2009 (ADS)
R. Spencer (1979) Nature 282 483 (ADS)