SS 433 – A Magnificent Microquasar

This post was co-written by PeterC & HannahH

SS 433 is a very interesting system with an equally enthralling history behind it. It is located well within our own galaxy at a distance of around 18,000 light years (roughly three times the distance to the Crab Nebula) in the direction of the constellation Aquila – The Eagle. It is visible using a large (8 inch or larger) amateur telescope, though as it is a 14th magnitude object (in visible wavelengths) you would need to improve your eyesight  by a factor of 10,000 to have a chance of seeing it yourself from an urban back garden. It sits just outside the galactic plane as the image below shows rather nicely.

RGB calibration of SS 433 and galactic plane. R is at 60 microns G is at 25 microns B is at 12 microns Data: IRAS Calibration: Hannah Hutchins

SS 433 is a binary star system; that is two stars orbiting around a point of common mass known as a barycentre.^[1] No two binary systems are exactly the same, tiny differences in the mass ratio and age of both stars as well as the precise orbital dynamics; produce a vast range of possible systems. In the case of SS 433 the system has undergone extreme stresses in its lifetime, approximately ten thousand years ago the larger mass star in the system ran out of the nuclear fuel required for the fusion reactions in its core and detonated as a supernova. This created the supernova remnant nebula W50 in which the system is located – and as can be expected near the centre.
The resulting collapsar remnant (the dense object left by the supernova rather than the gas and dust thrown out by the explosion to form a nebula) is believed to be either a neutron star or a stellar mass black hole.

The secondary, lower mass star, has been identified as a spectral class A7Ib star. For those of you unfamiliar with stellar spectral classification, such a designation identifies the star as a relatively low luminosity blue-white supergiant with a surface temperature of around 8200 kelvin. Research into the mass of the secondary star have provided estimates of anywhere between 3 and 30 , though a middle value of around 13 is the standard mass of a member of said spectral class so who knows!^[2]
As the two orbit the barycentre, they periodically occlude one another from our point of view – the fancy way of saying one passes over and blocks out some of the other’s light. This can be represented in general terms like so;

The graph along the bottom of the image shows apparent luminosity, which falls to its minimum when the dimmer star passes in front of the brighter.

SS 433 is a considerably more complicated and interesting system than this primitive illustration. It falls into the class of X-ray binary stars; these systems are unusually bright at X-ray wavelengths. This can only be explained by the system containing exceedingly hot material as X-rays are only emitted (in any large quantities) at such high temperatures. Given that the system contains a collapsar remnant (more likely a black hole than neutron star as we shall see later), and a large supergiant star we can account for the production of said X-rays via frictional heating.

The likely situation involves the original system being in the form of a detached binary; essentially each of the stars evolves unaffected by its partner. This situation would have remained unchanged even after the higher mass star went supernova, as evidence suggests, that in most cases at least, supernova detonations do not drastically affect the nature of binary star systems. However after the lower mass star aged it expanded, as it did so it filled its Roche Lobe – the area of space where its gravity exceeds the gravity of its partner.

After it did so material began to be sucked off its surface like water flowing from a drain pipe. This is pulled by the companion star into a flat disk around its equator forming an accretion disk. A simplistic way of describing this would be a CD-ROM. The gap in the middle contains the neutron star or black hole (and thus in reality there would not be a gap), material enters the disk at the outer edge and then spirals inwards on decaying orbits until it is sucked onto the central object’s equator from the inner edge.

This explains where the material comes from though does not explain how the material heats up enough to produce the X-rays. For that, we must look into the conservation of energy.
Material on the edge of the disk has to ‘fall down’ the gravitation potential well that the mass of the central object creates.
This is formed by the concentration of several  into a small region of space, as such high concentrations of matter significantly distorts space-time forming a deep ‘well’ (indeed for a black hole, which has an infinite density, the walls of such wells are infinitely steep). A representation of such a potential well is below.

Credit: AllenMcC.

As material moves closer to the central object, it must lose the corresponding amount of gravitational energy. As energy cannot be created or destroyed (as every pupil in a secondary school science class will tell you) this must be converted to another form, in this case it is converted to kinetic energy, meaning that material accelerates as it approaches the inner edge (more properly explained as the conservation of angular momentum).

As the material is spiralling towards its doom, there are many internal collisions between the constituent molecules of the disk. Such collisions generate the heat that is required to generate the X-rays. However, it is only the inner regions of the disk that are at the millions of kelvin required to produce the detected level of X-rays. The intensity of thermal energy within the disk increases moving towards the inner collapsar; this is directly related to the number and ferocity of the collisions in this region of the disk.

As you move through the disk, particle concentration increases as equal numbers of particles are compacted into smaller regions of space. This combined with the increased velocity of each particle, creates a larger number of violent collisions, that through friction, generate the extreme temperatures required.
There is another area of the accretion disk where temperatures reach such ludicrous levels. All the material flowing from the secondary star enters the disk via a small region; this drastically increases friction within that area producing a hot spot.
A large amount of the X-ray emission of the whole object comes from this tiny region at the edge of the accretion disk.

The SS 433 system could (in a highly simplistic view of course) look something like the diagram below.

I should also note the colour of the secondary star is a little too blue as that would be more representative of a B class star compared to the true blue-white colour of A class stars.

The ESO has also produced a diagram of the system, that is slightly more ‘professional’ than my attempt:

Credit: ESO

I had previously mentioned that the dense body in SS 433 could be either a neutron star or black hole; though evidence suggests that it is indeed a black hole. I shall now have a brief look at this evidence.
Neutron stars occupy a tight mass band between an upper and lower limit. They can’t have less than around 1.4 as that would be a mass below the Chandrasekhar limit and thus the star would have formed a White dwarf, as it had too little mass to overcome the resistance provided by electron degeneracy. However, they can’t exceed any more than (though the exact value is much less certain) 3 as this would be greater than the Tolman–Oppenheimer–Volkoff limit and so neutron degeneracy would be overcome and they would collapse into a black hole. So it follows that if SS 433′s dense object is a neutron star it should weigh in less than 3  any heavier and it is more than likely that it’s a black hole. Up until recent times we had, no real way of determining what is the case though recent research may hold the answer.
Optical and Near I-R spectroscopy has identified an inner ring of material around the system that contains around 40. Of this 16 can be linked directly to the collapsar and its accretion disk. So either it’s a black hole with a reasonable mass and accretion disk or a neutron star with more than four times its own mass in orbit around it, I think we can see which is more likely.

Update: We can confirm that the mass of the collapsar is 16 and so it is indeed a black hole

The Discovery of SS 433

SS 433 has been documented extensively since its discovery, with well over a hundred papers documenting all aspects of the system on record.

A study in 1978 identified an X-ray source – A1909+04 – within the confines of the supernova remnant W50. It was within the margin of error for the location of a radio source that had been detected within the nebula. The location of both sources was not exact as at that time radio and X-ray astronomy were still unable to precisely locate objects. As such, the X-ray and radio source could not be directly paired with a visible source. Eventually a spectroscopic study identified a visible ‘star’ with what, at the time appeared to be a standard spectrum of an X-ray source.

That star had been previously catalogued by Bruce Stephenson and Nicholas Sanduleak as number 433 in their list of H-Alpha emission sources – hence the name SS 433.

Detailed, longer term spectroscopic observation of SS 433 revealed that this was no ordinary object. The spectra showed that along with the normal spectral lines of hydrogen and helium there were other lines at seemingly random wavelengths. To make matters even more maddening these lines changed in wavelength between the observations; an exceedingly rare phenomenon. The explanation for these moving lines comes from the Doppler shifts experienced by rapidly moving material that is constantly changing direction.
The seemingly random lines are actually composed of two sets of lines. Each set moves in opposite directions before crossing over. After each set reaches a fixed, extreme wavelength, they reverse direction and move the other way, the total time for one complete cycle (with two crossings) is 164 days. This defies common sense though the explanation is deceptively simple.

These seemingly random lines are actually Doppler shifted hydrogen lines. One set is blueshifted and the other redshifted, which can be explained if we are seeing the jets of the Microquasar with one pointed towards us and the other away. The lines move as the black hole moves around in its orbit within the system and so the orientations of the jets alters relative to us over a period of time. We see the un-shifted spectrum as the central star isn’t moving with any great speed compared to us so the Doppler effect is minimal. Taken from the Doppler shifts of the jets we can conclude that jet material is moving at very high speeds, a good fraction the speed of light.

The Jets of SS 433
As mentioned SS 433 is a microquasar. As you can see they are quite similar to AGN in that they have an accretion disk and a pair of jets, but instead of a super massive black hole powering it, we have stellar mass black hole (though neutron stars are also possible power sources). Due to their similarity, they are very useful as a ‘close-by’ lab to see how the dynamics of AGN in far distant galaxies work, since microquasars follow the same principles.

The material that flows into the disk that isn’t sucked into the grips of the black hole is siphoned out of the system altogether as bipolar jets. As the gas spins round it, and the radiation produced as a result can escape through the gap between the black hole and the disk, since there’s less material in here for it to get caught up and swept round once again.

The jets themselves are forced out at 20 degrees near the black hole’s axis of rotation, beaming out at about 80 degrees away from our planet. The jets are moving at velocities of up to 0.26 c, all the while completing one rotation on its axis once every 164 days. To give you an idea as to just how fast 0.26 c is, the jet will cover one kilometre in just 13 microseconds!

The jets also fluctuate in brightness as the radiation they contain slams into material getting in the way, sending electrons flying out of their atoms and creating yet more ionizing radiation.

Another interesting effect is time dilation. As the jets are moving at 26% percent the speed of light, it’s sufficient enough for normally negligible effects to become problematic if you don’t take them into account in your calculations. If you take the redshift of the system at face value, it’s travelling away from us at 12,000 km a second when really this isn’t the case; since if it was it’s just exceeded (by quite a few times) the escape velocity of the Milky Way– 450 km/s – and as such would be rocketing out of the galaxy rather than inhabiting its stable orbit of the galactic core.

The faster the photons (that is the guise light takes when it behaves as a stream of particles) moving in the jet vibrate the more squashed their wavelengths of light they emit will be, conversely the slower the atoms vibrate the more stretched out wavelengths of radiation will be. Now in the SS 433 system you’d imagine that the system, according to its redshift, would be travelling at a velocity that matches other objects orbiting in the Milky Way and not 12,000 km per second, and in fact it is, but time dilation skews this:

Let’s imagine Hannah and Peter each had an atomic clock. Peter stays behind on earth with his, and Hannah leaves earth with hers and reaches a relativistic velocity. Now if Peter could see her clock he’d see time was moving significantly slower than his own as well as Hannah herself behaving more slowly that usual. Though from her point of view, Hannah experiences the ticking of time just as it always has been and would see Peter and his clock behaving in an accelerated way.

In our jet the atoms are vibrating quickly away just as they should, but from our point of view they are moving much slower; just as Peter could see Hannah’s clock ticking by slower than his own. Because the light is stretched out more than it should be the redshift is also higher than it is, causing us to measure, the velocity as much faster than what it is in reality.

Observations of SS 433 and W50

The surrounding nebula (W50) as mentioned before, was produced by the supernova detonation of the primary star 10,000 years ago from our point of view (28,000 years ago taking light lag into account). The nebula has long since faded from visible wavelengths but the dust it released is still visible in the infrared. NASA’s Spitzer Space Telescope has taken several observations of the area at various wavelengths. This first image shows some of the detail within the nebula, with SS 433 being the bright source in the centre. This short wavelength allows us to gaze at the dust within the nebula with a reasonably high resolution, though nowhere near the detail we can observe structures in the optical with instruments like Hubble.

Credit: NASA, Spitzer Calibration: Peter Clark

This second image is at a much longer wavelength (70 microns is equivalent to 700,000 ångströms which is also roughly the same as the thickness of a human hair) which is able to penetrate all but the densest clumps of dust surrounding the system, with a similar shape of clumps being visible in both images. Unfortunately, at this wavelength the fine structure of the nebula has been lost from view.
Note that the scale of the two images is not the same with the second image being more zoomed onto SS 433.

Credit: NASA, Spitzer Calibration: Peter Clark

A colour coded VLA radio map reds and yellows show the highest levels of radio detection with greens and blues showing less powerful areas of emission; SS 433′s radio jets are also clearly visible.

Credit: Blundell & Bowler NRAOAUINSF

A greyscale version of the above image with the calculated path of particles emitted at varying speeds at ten-day intervals. Beads of the same colour have been ejected at the same speed. The main point of this image is to show that the emission from the central object is not constant and varies over short time scales.

Credit: Blundell & Bowler, NRAO, AUI, NSF

This is a compilation of several VLA radio images taken over a consecutive period of time. It shows clearly the uneven concentration of material being emitted by the object, even over a short space of time.
Quasars and radio galaxies are predicted to show similar variation but over much longer time scales, much longer than a human lifetime and so such variation has yet to be observed. Therefore, in a way SS 433 allows us to peer into the heart of Quasars and learn about their structure in a way that isn’t currently possible for the full scale versions.

This two panelled image shows X-ray and radio observations of SS 433 and W50 in two different ways.
The first panel shows in detail the radio emission of the region with both jets visible though the eastern jet is much more pronounced. The overlaid contours map the regions of X-ray emission that more clearly define the jets.
The second panel is the reverse, with radio contours overlaid on to the X-ray data. The jets are strikingly visible with the image also highlighting just how powerful an X-ray source SS 433 is. The images also show the distortion of W50 by the jets of material emitted by SS 433 further showing their power.

Credit: Goodall, P.T.; Alouani-Bibi, F.; Blundell, K.M., Mon.Not. Roy. Ast. Soc. 414 (4) pp. 2838-2859.

Suggestions for further reading: -

The Sky at Einstein’s Feet by Professor William Keel (Chapter 2 contains lots of information on SS 433).