A microquasar is made up of a compact central object such as a black hole, which is surrounded by an accretion disc of in-falling matter and a pair of bright radio jets. While normal quasars consist of supermassive black holes that are millions of solar masses, microquasars involve a "stellar mass" black hole that is usually about three to ten times the mass of the Sun. Another common feature of microquasars is that their accretion discs tend to be very luminous in the optical and X-ray regions.

Jet setting

In the new work, Roberto Soria at Curtin University, Australia, together with other colleagues in Australia, the US and the Netherlands monitored the outflow of the black hole MQ1 for over a year. “We observed a bubble of hot gas, with characteristic optical and infrared lines from hydrogen, sulphur, oxygen and iron. This suggested that something was heating or shocking this gas,” says Soria. The bubble has two lobes sticking out of it, suggesting that there is a pair of jets. “We detected very strong radio emission from that same bubble…that is usually a signature of a powerful jet slamming onto dense gas and producing energetic electrons,” he says.

Indeed, when they looked at the X-rays coming from the object they saw a point-like source in the centre of the bubble. This type of emission is exactly what is expected from the vicinity of a black hole that is sucking in a lot of mass, according to Soria. They then used the spectral information to estimate the power of the jet. Combined with the temperature and luminosity of the X-ray emissions from the accretion disc, they estimate that the black hole is about 100 km across. From that, the researchers can tell it is definitely less than 100 solar masses. “In fact, I think probably much less…about 10–50 solar masses, but we cannot be sure because that depends on black hole spin and viewing angle, and we do not have any information,” explains Soria.

Going with the flow

What is surprising about the system is that the black hole appears to be emitting more energy than expected for its mass. As mass accretes around a black hole, it gets heated and ionized and the black hole releases energy in the form of radiation (X-rays) and an outflow of particles referred to as mechanical power. The radiation flowing outward cannot exceed the “Eddington limit”, which is related to the black hole’s mass. The Eddington luminosity is the maximum luminosity a stellar body can achieve when there is a balance between the outward force of radiation and the inward gravitational force. While the radiation is strictly governed by this limit, it has not been clear if the mechanical power – in the form of particle jets and winds – is constrained by the same limit.

If you have a few of those beasts in a small galaxy, they can easily heat and blow away all the gas in the galaxy Roberto Soria, Curtin University

“The jet is not limited by the Eddington luminosity, because it's very thin…very collimated. It pierces a hole straight through the gas, like shooting bullets at a cloud,” explains Soria. But astronomers use the Eddington limit as a unit of measurement for “absolute power” of a black hole. Soria explains that the luminosity itself can go three or four times past the limit, which can be thought of as the point at which a further increase in mass has a negligible effect on the power. This occurs because it takes an exponential increase of the mass falling in to cause a linear increase in power.

In the past, it was thought that the mechanical power produced by a stellar-mass black hole was always less than the radiation, but the team’s latest work shows that it can be as high or higher. Soria tells physicsworld.com that over the last few years, astronomers have discovered about a dozen stellar-mass black holes with powerful jets in nearby galaxies and that MQ1 is one of the most powerful. Also, MQ1 is the first object whose mass is constrained, allowing the team to confirm that stellar-mass black holes can reach this mechanical power of a few million times the current output of the Sun. Intriguingly, another class of powerful black holes found in nearby galaxies that reach characteristic luminosities of a few million times the Sun, but with radiation rather than mechanical energy, have been detected. Such “ultraluminous X-ray sources” (ULXs) are also thought to be powered by stellar-mass black holes.

Early ionization

Soria says that because of recent work on microquasars and ULX research, astronomers are starting to form a coherent view of stellar-mass black holes in the local universe. He adds that this knowledge could lead to a better understanding of quasars in the early universe, which were then accreting at the maximum rate.

Mitch Begelman, a theoretical astrophysicist from the University of Colorado, Boulder in the US, who was not involved in the work, says that while the result is not surprising “it does provide a new, intriguing piece of evidence along the lines of what many of us have long suspected. I don't think it requires a new view of black hole physics, but might help to settle some lingering questions about the nature of ULXs and outbursts from microquasars”.

In the long run, the new work could help researchers understand how stellar-mass black holes may have had a significant role in ionizing and heating ambient gas in the early universe. “Today such powerful sources are rare, but we think they were much more common at the time. Star formation was more active at that time, so more of these microquasars were formed. If you have a few of those beasts in a small galaxy, they can easily heat and blow away all the gas in the galaxy,” says Soria.

The research is published in Science.