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Astrophysicists Detect Hot, Dense Outflowing Wind Close to a Black Hole

A very hot and dense outflowing wind has been detected near a black hole, which is at least 25,000 light-years from planet Earth. The study was carried out by an international team of astrophysicists from Oxford, Southampton, and South Africa.

Illustration of the black hole system studied by the team (Image credit John Paice)

Professor Phil Charles, a lead researcher from the University of Southampton, elaborated that the gas (ionized hydrogen and helium) was produced in bursts and this pattern was repeated every 8 minutes. This kind of behavior was observed around a black hole for the first time.

The results of the study have been reported in the journal, Monthly Notices of the Royal Astronomical Society.

Swift J1357.2-0933 was the object studied by Professor Charles’ team. It was initially identified as an X-ray transient in 2011. X-ray transient is a system that shows intense outbursts. These transients include a compact object, which can be a black hole, neutron star, or white dwarf, and a low-mass star, akin to the Sun. Swift J1357.2-0933, in this case, consists of a black hole compact object that is at least six times the mass of the Sun.

The compact object pulls the material from the standard star into a disc in between the two. Huge outbursts take place when the material in the disc becomes unstable and hot, causing it to discharge large amounts of energy.

What was particularly unusual about this system was that ground-based telescopes had revealed that its optical brightness displayed periodic dips in its output and that the period of these dips slowly changed from around 2 mins to about 10 mins as the outburst evolved. Such strange behaviour has never been seen in any other object.

Phil Charles, Lead Researcher and Professor, University of Southampton

Professor Charles continued, “the cause of these remarkable, fast dips has been a hot topic of scientific debate ever since their discovery. So it was with great excitement that astronomers greeted the second outburst of this object in mid-2017, presenting an opportunity to study this strange behaviour in greater detail.”

He and his research team eventually discovered that the key to obtaining the answer was to achieve optical spectra several times at the time of each dip cycle, essentially examining how their color varied with time.

However, a massive telescope had to be used for this, because the object was approximately 10,000 times fainter than the faintest star seen with the naked eye, and also the dip period was only about 8 minutes. Hence, the researchers used Southern African Large Telescope (SALT)—the largest optical telescope in the southern hemisphere.

The University of Southampton is one among the founding UK partners in SALT, and along with its South African collaborators, are part of a multi-partner Large Science Programme to analyze all types of transients.

SALT has the required large collecting area (it has a 10 m-diameter mirror) and is used in a 100% queue-scheduled way by resident staff astronomers. This means it can readily react to unexpected transient events. This proved to be ideal for Swift J1357.2-0933, and SALT acquired over an hour of spectra, with one taken at an interval of every 100 seconds.

Our timely observations of this fascinating system demonstrates how the quick response of SALT, through its flexible queue-scheduled operation, makes it an ideal facility for follow-up studies of transient objects.

Dr David Buckley, Principal Investigator, SALT Transient Programme

The SALT transient programme is based at the South African Astronomical Observatory.

With the instantaneous availability of a number of different instruments on SALT, we can also dynamically modify our observing plans to suit the science goals and react to results, almost in real-time,” added Buckley.

Professor Charles added, “The results from these spectra were stunning. They showed ionised helium in absorption, which had never been seen in such systems before. This indicated that it must be both dense and hot—around 40,000 degrees. More remarkably, the spectral features were blue-shifted (due to the Doppler effect), indicating that they were blowing towards us at about 600km/s.

He continued, “but what really astonished us was the discovery that these spectral features were visible only during the optical dips in the light-curve. We have interpreted this quite unique property as due to a warp or ripple in the inner accretion disc that orbits the black hole on the dipping timescale. This warp is very close to the black hole at just 1/10 the radius of the disc.”

It is not known what exactly drive this matter away from the black hole. Most certainly, it is the radiation pressure of the powerful X-rays produced close to the black hole. However, it has to be relatively brighter than what humans see directly, indicating that the material impinging on to the black hole conceals it from direct view, similar to clouds concealing the Sun.

This takes place because humans happen to visualize the binary system from a vantage point, where the disc seems edge-on, as shown in the schematic illustration, and moreover, rotating blobs in the disc conceal one’s view of the central black hole.

Fascinatingly, contrary to the expectation, eclipses by the companion star are not observed in the optical or X-ray. This is because it is extremely small, and constantly remains in the disc shadow. This interpretation arises from comprehensive hypothetical modeling of winds being blown off the accretion discs that was handled by one of the researchers, James Matthews at the University of Oxford, with the help of supercomputer calculations.

An object like that has incredible characteristics amongst an already fascinating set of objects that have much more to teach individuals about the formation of compact object and the end-points of stellar evolution.

A couple of dozen black hole binary systems are already known to exist in the Milky Way galaxy, with solar mass ranging between 5 and 15. The solitary black hole at the Galactic Centre is about four million solar masses. All these objects grow by the accretion of matter that has been observed so dramatically in this object. It is also known that a considerable portion of the accreting material is being blown away. When that occurs from the giant black holes at the middle of galaxies, those strong jets and winds can have a major effect on the remaining galaxy.

The South African National Research Foundation, the Leverhulme Trust, STFC, and a UGC-UKIERI Thematic Partnership supported the study.

These short-period binary versions are a perfect way to study this physics in action,” concluded Professor Charles.


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