11 mins read 09 Apr 2021

Right place, right time. Catching an Intermediate-Mass Black Hole

By combing through decades-old data, captured by an orbiting gamma-ray telescope, Australian scientists were able to find an intriguing signal that could point to a new method of discovering the missing link between stellar and supermassive black holes.

Artist rendition of the light from the GRB 950830 event as it passes the intermediate-mass black hole, curving back towards Earth. Credit: Carl Knox/OzGrav.

Sometimes, when we tiny humans look out into the vastness of the cosmos, things just line up perfectly for us, don’t they?

It’s not because of any special reason, but a likely occurrence when we consider the probability of an infinite number of phenomena, objects and occurrences in every single directional line that points out from our humble third rock eventually meeting something.

But if we consider the Earth as a very dynamic system, moving around the Sun, and the Sun moving around the galaxy, then when something special happens to place us (and in particular our observatories) at the right place, at the right time – well, there’s a chance it is going to be something special.

And that’s exactly what happened on 30 August 1995 – when the Compton Gamma Ray Observatory (CGRO), orbiting some 450 km above the Earth, detected a bright, high-energy flash that lasted just a few important moments, before eventually fading away into darkness.

CGRO, and in particular the instrument named ‘Burst And Transient Source Experiment’ or BATSE, was highly successful at detecting these sudden bright explosions of high-frequency energy, which became to be known as Gamma-Ray Bursts (GRB). In fact, BATSE practically found on average one-per-day building a catalogue of approximately 2,700 of these transient events over its lifetime in orbit.

This information helped scientists at the time determine that these bright bursts of energy must originate at great distances from the Earth, in galaxies far away – and must be extremely energetic to have enough power to be so bright when they reach us.

This specific burst, catalogued as GRB 950830, at the time had nothing special or peculiar about it – and so, along with the thousands of other GRBs, it found its way into a few papers before sitting in the archives.

The Compton Gamma-Ray Observatory in orbit. Credit: NASA.

Now, after the passing of several decades, Australian astronomers have re-analysed the GRB data captured by BATSE all those years ago, and found something rather remarkable about GRB 950830, with their results recently published in the journal, Nature Astronomy.

By looking at the light curve of this particular GRB, an interesting feature emerged. The GRB had an echo, a few milliseconds after the initial flash that was detected, and even more intriguing, the echo follows the same shape as the original flash.

This wasn’t the result of two flashes or separated events that caused the GRB, this was the tell-tale sign that something was exactly in the pathway of the incoming GRB, and it was something big enough to cause a gravitational lensing effect. Something very big.

There we were. Right place. Right time.

By measuring the time delay between the first burst and the echo, the ratio of the fluxes and lens redshift, the team of astronomers were able to calculate the mass of the intervening object to be 55,000 times the mass of our Sun.

There are only three astrophysical objects that contain this range of mass that we are aware of: Globular Clusters (GCs), dark matter halos and black holes. The GCs were ruled out, and dark matter halos are yet to be confirmed directly, which according to the scientists involved with the study left us with the final option – the lensed object has to be a black hole.

Now, black holes have been directly observed using gravitational-wave interferometers for a number of years now, but the thing that made this black hole special was its mass. It was far greater than the mass of the stellar-mass black holes that are detected by the LIGO/Virgo Scientific Collaboration but far smaller than the monstrous supermassive black holes that reside in the centre of galaxies.

It is classified as an intermediate-mass black hole (IMBH), a category in which not much evidence currently exists in the scientific literature, though there are a number of strong leading candidates that could potentially be considered proof. Effectively, IMBHs are the missing link between the two already confirmed populations of black holes that we know about.

The discovery was made by researchers from Monash University and the University of Melbourne, with the method of discovery having the potential to further analyse existing and future data of this nature to hopefully find more examples of IMBH using gravitational lensing methods.

Lead author and University of Melbourne Ph.D. student, James Paynter, said the latest discovery sheds new light on how supermassive black holes form. “While we know that these supermassive black holes lurk in the cores of most, if not all galaxies, we don’t understand how these behemoths are able to grow so large within the age of the Universe,” he said.

In order to determine that the echo was in fact not a secondary, stand-alone burst or event, James and his team had to adapt software that is normally utilised for the detection of gravitational waves generated by stellar-mass black holes through the large-scale interferometers.

“This newly discovered black hole could be an ancient relic - a primordial black hole - created in the early Universe before the first stars and galaxies formed,” said study co-author, Professor Eric Thrane from the Monash University School of Physics and Astronomy and Chief Investigator for the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).

“These early black holes may be the seeds of the supermassive black holes that live in the hearts of galaxies today.”

Paper co-author, gravitational lensing pioneer, Professor Rachel Webster from the University of Melbourne said the findings have the potential to help scientists make even greater strides.

“Using this new black hole candidate, we can estimate the total number of these objects in the Universe. We predicted that this might be possible 30 years ago, and it is exciting to have discovered a strong example.”

Science Check: Intermediate-Mass Black Holes

Artist rendition of the merging black holes that formed the intermediate-mas black hole from the GW190521 event. Credit: LIGO/Caltech/MIT/R. Hurt (IPAC).

Ever since black holes became an outcome of the predictions of Einstein’s General Relativity, they have continually fascinated scientists and the general public, due to their absolute mystery and mind-boggling physics, where our understanding of the laws of the Universe seem to be suspended (time appears to stop, and even light cannot escape).

These objects are formed when any mass is squeezed into a small enough radius, creating a volume of infinite density that has an enormous (yet localised) gravitational field. Anything can be a black hole if we can squeeze it down small enough – for example, if we could crush the Sun into a sphere about 3 km in diameter, it would turn into a black hole.

We know, through observational evidence, that black holes must come in a number of different classes, and whilst we have some evidence as to how some of these classes form – the rest are still very active areas of research for astrophysicists to this day.

Artist rendition of the anatomy of a black hole. Credit: AFP/NASA/JPL-Caltech/EHT.

The first class are stellar-mass black holes, which usually weigh in under 100 solar masses (and even that is pushing it). These black holes are formed by the collapse of massive stars and can accumulate in size through merger events. These are the types of black holes that are measured by instruments like the LIGO/Virgo interferometers.

On the opposite end of the scale, there are supermassive black holes (SMBHs) – whose mass ranges from the hundreds of thousands, through to billions of times that of our Sun. We know they exist because we can see their effects on the stars that orbit near them, surrounding central regions of nearly every galaxy. What we don’t know is how supermassive black holes came to be – did they form through the accumulation of smaller black holes, or did they emerge from the collapse of extremely large gas clouds when galaxies were forming?

The missing link is right in the middle – the intermediate-mass black holes. These monsters have a mass range from about 100 times the mass of the Sun, to about 100,000 times the mass of the Sun.

Observed mass ranges of different types of compact, massive objects relative to the Sun’s mass. Credit: NASA.

So far, there have been about 300 IMBH candidates discovered, which include a few low-luminosity active galactic nuclei and some ultra-luminous x-ray sources. The recent gravitational wave event GW190521 (which was the merger of two very large progenitor stellar black holes with 85 and 65 solar masses each) resulted in a 142 solar mass black hole post-merger, after radiating 9 solar masses away as gravitational waves. This new object is considered to be an IMBH.

There are a number of theories into how IMBHs form, including chain reaction collisions of stars in a cluster, the continual growth of stellar-mass black holes into the IMBH range, and lastly, some scientists believe that they could just be primordial black holes that formed in the Big Bang, though this last theory is likely the weakest of them all. 

In all cases, we know that IMBH cannot form from stellar masses, because they are way too big, and they cannot reside in the centre of galaxies because they are too small and weak to explain the observed orbital velocity of nearby objects.

Scientists hope to find more evidence of IMBHs to hopefully be able to draw the connection between stellar-mass black holes, growing through accretion into IMBHs and eventually becoming the SMBHs we see in the hearts of galaxies today. This theory does have its own problems though – in particular, the time it would have taken to build some of the biggest black holes we observe today, would actually be longer than the current age of the Universe.

So, the mystery continues.

The Bright Light that Twinkled Twice

The light curve of the GRB 950830 event, in four different energy channels: red: 20 – 60 keV; yellow: 60 – 100 keV; green: 110 – 320 keV; and blue: 320 – 2,000 keV. Note the echo at roughly 0.4 seconds after the original burst. Credit: Paynter et al. 2021/Nature Astronomy.

Gamma-ray Bursts (GRBs) were first detected by accident, when satellites used to detect the high-energy signature of nuclear detonations in space by covert actors started finding these bursts of radiation coming from astrophysical sources, in all directions across the sky. Not much was known about them for several decades, until eventually new satellites, observations and data were able to confirm that these events were originating in very distant galaxies.

GRBs are some of the brightest and most powerful events known in the Universe, with bursts that can last from a few milliseconds through to several hours. They’re thought to be caused by violent supernovae or when a high mass star collapses to form a neutron star or black hole.

Two categories of GRBs have been documented so far – the first are called short GRBs (SGRBs) and are the result of merging neutron star events, exhibiting a burst window of under two seconds. GW170817 is a great example of an SGRB.

And the other type is known as long GRBs (LGRBs), lasting longer than two seconds and being the product of a core-collapse supernova event. The majority of observed GRBs are of this longer nature.

The BATSE instrument on the CGRO recorded GRB 950830 across four different energy channels (20 – 60 keV; 60 – 100 keV; 110 – 320 keV; 320 – 2,000 keV) with each channel exhibiting the original burst at time 0, and the echo at approximately 0.4 seconds after the trigger.

To find GRB 950830, James and his team searched across the entire database of 2,700 events and looked for any GRB which had a burst echo roughly equal to or less than 240 seconds of the original burst. From this data, they were then able to extrapolate the mass of the IMBH.

An example of a gravitational lens event occurring in this graphic using the Hubble Space Telescope (HST). A background quasar’s light is lensed by an intervening galaxy. As the outgoing light-rays curve back in due to the galaxy's mass, HST picks up multiple versions of the images. Credit: Stanford University.

As the light from GRB 950830 traversed across towards the Earth, it passed locally to the region of space-time surrounding the IMBH. General Relativity states that massive objects, like this 55,000 solar-mass black hole, curve the regions of space-time around them, so any light rays passing through this region will experience a delay in their travel time because they need to traverse across a larger distance (the curvature of space-time itself).

By studying the brightness, delay and data received from multiple signals of the same event (in this case the original burst and the echo), a determination of the central mass that caused the curvature could be made, and then the mass of said object inferred.

But it all came down to our little planet, with an orbiting high-energy observatory, thankfully being in the right place. At the right time.