From Curtin University and ICRAR via Science Alert: “Millions of High-Speed Black Holes Could Be Zooming Around The Milky Way”

From Curtin University

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ScienceAlert

Science Alert

30 AUG 2019
MICHELLE STARR

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(StudioM1/iStock)

How are black holes born? Astrophysicists have theories, but we don’t actually know for certain. It could be massive stars quietly imploding with a floompf, or perhaps black holes are born in the explosions of colossal supernovas. New observations now indicate it might indeed be the latter.

In fact, the research suggests that those explosions are so powerful, they can kick the black holes across the galaxy at speeds greater than 70 kilometres per second (43 miles per second).

“This work basically talks about the first observational evidence that you can actually see black holes moving with high velocities in the galaxy and associate it to the kick the black hole system received at birth,” astronomer Pikky Atri of Curtin University and the International Centre for Radio Astronomy Research (ICRAR) told ScienceAlert.

And it means there are potentially millions stellar-mass black holes zooming around the galaxy at high speed. The paper has been accepted into the Monthly Notices of the Royal Astronomical Society.

The study was based on 16 black holes in binary systems. Unless they’re actively feeding, we can’t actually find black holes, since no detectable electromagnetic radiation can escape their insane gravity. But if they’re in a binary pair and actively feeding on the other star, the matter swirling around the black hole gives off powerful X-rays and radio waves.

Once we can see these black hole beacons, we can see how the black hole is behaving. The international team of researchers used this behaviour to try and reconstruct the black hole’s history.

“We tracked how these systems were moving in our galaxy – so, figured out their velocities today, moved back in time, and tried to understand what the velocity was of the system when it was born, individually for each of these 16 systems,” Atri explained.

“Based on the velocities, you can actually find out if they were born with a supernova explosion, or if the stars just directly collapsed onto themselves without a supernova explosion.”

We know that neutron stars can be violently punted out across space at high speeds by their own supernova explosions – this is called a Blaauw kick, or natal kick, and it happens when the supernova explosion is lopsided, resulting in a recoil.

It was unknown if black holes could be kicked in the same way. Hypothetically, they might – and indeed seven black hole x-ray binaries have been previously associated with natal kicks.

The new research has analysed these, as well as nine others, in greater detail, combining measured proper motions, systemic radial velocities, and distances to these systems for the most detailed analysis yet.

The motion of one of these black holes as calculated by the team can be seen in the video below.

The researchers found that 12 of these 16 black hole X-ray binaries did indeed have high velocities and trajectories that indicated a natal kick. That’s 75 percent of the sample. If this scales up to the estimated 10 million black holes in the Milky Way, that might mean around 7.5 million high-speed black holes careening out there. And 10 million is a low estimate.

In line with previous theories, these speeding black holes are slower than kicked neutron stars by a factor of about three or four, due to their higher mass. Interestingly, there seemed to be no correlation between black hole mass and velocity, which means we don’t yet know if there’s a correlation between progenitor star mass and the likelihood of a supernova.

This is a relatively small sample size of black holes, of course. But, according to Atri, it’s a step towards building up a larger sample that can help us to understand how stars evolve and die, and give rise to black holes.

“Eventually, all of this will feed into how many black holes we expect in our galaxy, how many black holes that will actually merge to give those gravitational wave detections that LIGO finds,” she added.

To continue to build on the research, the team will keep watching the sky. These binary systems aren’t always bright – they come and go, transient. So the researchers are hoping to find more of these binary systems to continue building a census of Milky Way black holes, whether speeding or not.

And, in case you’re worried right now abut a black hole cruising right into our Solar System, you don’t really need to panic.

“The closest black hole, we think it’s two kiloparsecs away [6,523 light-years],” Atri said.

“It’s very, very far away. So there’s no chance that we’re getting sucked up by any black hole any time soon.”

See the full article here .


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ICRAR is an equal joint venture between Curtin University and The University of Western Australia with funding support from the State Government of Western Australia. The Centre’s headquarters are located at UWA, with research nodes at both UWA and the Curtin Institute for Radio Astronomy (CIRA).
ICRAR has strong support from the government of Australia and is working closely with industry and the astronomy community, including CSIRO and the Australian Telescope National Facility, <a
ICRAR is:

Playing a key role in the international Square Kilometre Array (SKA) project, the world's biggest ground-based telescope array.

Attracting some of the world’s leading researchers in radio astronomy, who will also contribute to national and international scientific and technical programs for SKA and ASKAP.
Creating a collaborative environment for scientists and engineers to engage and work with industry to produce studies, prototypes and systems linked to the overall scientific success of the SKA, MWA and ASKAP.

Murchison Widefield Array,SKA Murchison Widefield Array, Boolardy station in outback Western Australia, at the Murchison Radio-astronomy Observatory (MRO)

A Small part of the Murchison Widefield Array

Curtin University (formerly known as Curtin University of Technology and Western Australian Institute of Technology) is an Australian public research university based in Bentley and Perth, Western Australia. The university is named after the 14th Prime Minister of Australia, John Curtin, and is the largest university in Western Australia, with over 58,000 students (as of 2016).

Curtin was conferred university status after legislation was passed by the Parliament of Western Australia in 1986. Since then, the university has been expanding its presence and has campuses in Singapore, Malaysia, Dubai and Mauritius. It has ties with 90 exchange universities in 20 countries. The University comprises five main faculties with over 95 specialists centres. The University formerly had a Sydney campus between 2005 & 2016. On 17 September 2015, Curtin University Council made a decision to close its Sydney campus by early 2017.

Curtin University is a member of Australian Technology Network (ATN), and is active in research in a range of academic and practical fields, including Resources and Energy (e.g., petroleum gas), Information and Communication, Health, Ageing and Well-being (Public Health), Communities and Changing Environments, Growth and Prosperity and Creative Writing.

It is the only Western Australian university to produce a PhD recipient of the AINSE gold medal, which is the highest recognition for PhD-level research excellence in Australia and New Zealand.

Curtin has become active in research and partnerships overseas, particularly in mainland China. It is involved in a number of business, management, and research projects, particularly in supercomputing, where the university participates in a tri-continental array with nodes in Perth, Beijing, and Edinburgh. Western Australia has become an important exporter of minerals, petroleum and natural gas. The Chinese Premier Wen Jiabao visited the Woodside-funded hydrocarbon research facility during his visit to Australia in 2005.