There're several pieces of information one needs to understand this.

Although stars more massive than 70 solar masses exist, when they become black holes, they usually lose mass in the process. The exact amount of mass lost depends on the metallicity (which is a technical term that describes how much "metals" - the astronomer's definition of metals is anything heavier than hydrogen & helium - is in the star). The companion star that we see today is at solar metallicity, so it's probable that the original star (the one that became the black hole) was also at solar metallicity. Unfortunately, that means it shouldn't leave a 70-mass black hole remnant. From the paper:

This [70 solar mass black hole] would strongly challenge current stellar evolution models, which only allow for the formation of black holes up to $25 M_{sun}$ at solar metallicity.

Where, then, did this 70-solar mass black hole come from? The paper discusses a few alternatives. The obvious one is that two smaller black holes simply merged to form this one. Problem with that is, you still need two 35-solar mass black holes, and 35 is clearly > 25. (In principle you could also have a 25-solar mass black hole merge with a 45-solar mass black hole, but that still leaves the question of where the 45-solar mass black hole came from in the first place.) Note also that this black hole probably didn't arise from neutron stars merging, since neutron stars have a mass limit of about 2 solar masses. Finally there is three black holes merging into one, but this is unlikely: mergers are already rare events, and having two mergers must be even rarer.

Here're a few more unlikely explanations I can think of:

• Perhaps it's a primordial black hole. Problem with this is that primordial black holes are something of a unicorn - a last resort explanation - because they work whenever you need a missing mass. If you can explain an observation without invoking primordial black holes, that's much preferable. See also the next bullet point.


• Perhaps the black hole formed elsewhere, in an environment that isn't at solar metallicity, then travelled to this place and captured its companion star. Problem with this is that stellar capture is not a likely process. In the same way we can say that the Earth probably formed around the Sun; it didn't form elsewhere and was captured by the Sun later. There's another problem, which is that the star is observed to have an eccentricity of almost zero (this means its orbit is roughly circular). Newton's laws predict an elliptical orbit. There are processes that will drive the orbits towards circular, but the time taken is long. A (rare) stellar capture event that leads to almost exactly a circular orbit is even more unlikely.

The paper discusses a couple of more credible alternatives:

• Perhaps this was a triple system where one star became a black hole, and then this black hole "fell into" one of the other stars and ate it from the inside.


• Perhaps there was a "fallback supernova". This is when a star goes supernova, but the ejected material somehow falls back onto the stellar remnant. This has never been directly observed, and this might be the first.


• Perhaps something is wrong with the measurement, e.g. an unaccounted for systematic effect. This is the most mundane explanation.

In any case, the system is now an attractive target for telescopes.

Update: there are now several articles claiming an error in the analysis, and that there is no 70-solar mass black hole in this system.

Perhaps this was a galactic black hole and is now a remnant from a previously accreted galaxy. Our Milky Way is rather large and could have swallowed up a smaller galaxy.