SS433-W50: When Microquasar Jets & Supernova Remnants Collide
As one of the most exotic objects in out galaxy, the microquasar SS433 has become widely talked about ever since its discovery in 1979. An object which has been the subject of an extensive blog post here previously (A Ghost In The Shell: The Mystery Of SS433). This excitement comes from many aspects of its observed nature, none more so since its discovery as the first stellar source of relativistic jets in our Galaxy (Margon et al. 1979; Fabian & Rees 1979). However, even after all this talking, it’s exact nature remained elusive for the best part of thirty years.
After decades of intense research the system has been identified as a high mass X-ray binary (HMXB) (Begelman et al. 2006). The masses of the constituent bodies have been estimated using an extensive variety of techniques (e.g. Crampton & Hutchings 1981; D’Odorico et al.1991; Collins & Scher 2002; Fuchs et al. 2006; Blundell et al. 2008) yielding some varied, but important, results. It is, however, agreed upon that the spectrum of the secondary companion star suggests that it is a late A-type star ( Hillwig et al. 2004) with the compact constituent more than likely being a black hole (Cherepashchuk 2002).
The full kinematic model as it is now is, in my mind, one of the greatest triumphs of human intellectual understanding. According to this model, rotation of the beam axis provides the observed radial-velocity variations, and the observed emission is produced when matter reaches a critical distance from the central object and cools sufficiently to recombine. Specific predictions are also made regarding the behavior of the variable velocity components during the as-yet unobserved portions of the 164-day period.
However, the observations seem to favour the scenario of a black hole in orbit around a more massive companion star (Fabrika 2004). Recently, the presence of a circumbinary ring, observed via optical spectroscopy (Blundell et al. 2008) and also near infra-red spectroscopy (Perez M. & Blundell 2009) has been noted. It is from these observations that the astronomical community has accepted the mass internal to the ring to be 40, of which 16 is attributable to the compact object and its accompanying accretion disc. Thus, it is perhaps within a respectable paradigm that SS433 hosts a black hole (BH) rather than a neutron star, as neutron stars generally have a mass of around the 1.4 Chandrasekhar limit, with an absolute maximum upper limit of 3.
The BH’s accretion disc is presumably fed by accreted gases from an (extremely!) strong stellar wind or Roche-lobe overflow from the companion star with a suggested rate of 7×10-6 4×10-4 year-1(King et al. 2000), accompanied by two oppositely directed jets which have been measured to eject material at a rate of 10-6 year-1 (Begelman et al. 1980) into the surrounding interstellar medium (ISM). These relativistic jets, as stated in a previous post, are embedded deep within the supernova remnant W50 (Dubner et al. 1998), hence, whether by coincidence or connection, these two objects are intrinsically linked. It is this fact that remains important in understanding the full evolutionary history of both SS433 and W50.
The remarkable phenomenology of W50 spans an impressive 208 pc along its major axis, featuring a circular region which is believed to be the remains of an expanding supernova remnant (SNR) shell, with two elongated lobes to our east and west of the circular shell (Figure 1). It doesn’t take a lot to begin to connect all the dots…the jetted outbursts from the microquasar are smashing into the circular supernova remnant of W50 and elongating it at either side. It also doesn’t take a lot to begin to understand it’s transverse inclination to the observer, as the forces applied by the jets would, as an assumption, be equal in both directions. It may therefore be possible to infer that the remnant is tilted anti-clockwise, respective to our position.
It is this understanding of the kinematics of SS433’s and W50’s joint evolution that has become the focus of a Oxford University based team’s research (Blundell et al. 2011; Goodall et al. 2011; Hirst et al. 2011). Their joint efforts focused on showing that the peculiar morphology of the entire nebula can be reproduced to a good approximation, due to the combined effects of the evolution of the SNR shell from the free-expansion phase through the Sedov-Taylor blast wave and the subsequent interactions, by the effects of the precessing jets of SS433 on W50’s supernova remnant.
Although this has been, and quite rightly, felt throughout the astronomical community for a very long time, their work has provided for the first time an intricate model outlining via hydrodynamical computer simulations, perhaps one of the most sophisticated ever performed. The team’s simulations revealed that the current jet precession characteristics do not simply extrapolate back to produce the lobes of W50, but a history of episodic jet activity having at least three different outbursts with different precession characteristics would be sufficient to produce the W50 nebula.
It is in this reproduction of the W50 that is an extremely valuable step in not only understanding relativistic jets, but also properties of the interstellar medium in and around W50 and the understanding of supernovae remnant evolution. Something which is difficult to say the least, with only the remnant left in the wake SN1987A providing a suitable stepping stone to test current theoretical models, which is severely limited to early-stage SNR evolution. With these latest steps, the mystery of SS433 is slowly, and surely, becoming much much clearer.
- Goodall, P.T.; Blundell, K.M.; Bell-Burnell, S. J. (2011) Probing The History Of SS 433’s Jet Kinematics: Decade-Resolution Radio Observations Of W50. Monthly Notices Royal Astronomical Society, 414 (4) pp. 2828-2837.
- Goodall, P.T.; Alouani-Bibi, F; Blundell, K.M. (2011) When Microquasar Jets & Supernova Collide: Hydrodynamically Simulating The SS433-W50 Interaction. Monthly Notices Royal Astronomical Society, 414 (4) pp. 2838-2859.
- Blundell, K.M. & Hirst, P. (2011) Jet Propulsion Of Wind Ejecta From A Major Flare In The Black Hole Microquasar SS433. The Astrophysical Journal, 735(1) Article I.D.: L7.
- Goodall, P.T. Alouani-Bibi, F.; Blundell, K.M. (2011) When Jets & Supernova Collide: Hydrodynamical Simulation Movies. Astronomy & Astrophysics Department, University Of Oxford.