Search reveals eight new sources of black hole echoes

Date:
May 2, 2022
Source:
Massachusetts Institute of Technology
Summary:
Astronomers discovered eight new echoing black hole binaries in
our galaxy, enabling them to piece together a general picture of
how a black hole evolves during an outburst. The findings will
help scientists trace a black hole's evolution as it feeds on
stellar material.



FULL STORY ========================================================================== Scattered across our Milky Way galaxy are tens of millions of black
holes - - immensely strong gravitational wells of spacetime, from which infalling matter, and even light, can never escape. Black holes are
dark by definition, except on the rare occasions when they feed. As a
black hole pulls in gas and dust from an orbiting star, it can give off spectacular bursts of X-ray light that bounce and echo off the inspiraling
gas, briefly illuminating a black hole's extreme surroundings.


==========================================================================
Now MIT astronomers are looking for flashes and echoes from nearby black
hole X-ray binaries -- systems with a star orbiting, and occasionally
being eaten away by, a black hole. They are analyzing the echoes from
such systems to reconstruct a black hole's immediate, extreme vicinity.

In a study appearing today in the Astrophysical Journal, the researchers
report using a new automated search tool, which they've coined the "Reverberation Machine," to comb through satellite data for signs
of black hole echoes. In their search, they have discovered eight new
echoing black hole binaries in our galaxy. Previously, only two such
systems in the Milky Way were known to emit X-ray echoes.

In comparing the echoes across systems, the team has pieced together a
general picture of how a black hole evolves during an outburst. Across
all systems, they observed that a black hole first undergoes a "hard"
state, whipping up a corona of high-energy photons along with a jet of relativistic particles that is launched away at close to the speed of
light. The researchers discovered that at a certain point, the black
hole gives off one final, high-energy flash, before transitioning to a
"soft," low-energy state.

This final flash may be a sign that a black hole's corona, the region of
high- energy plasma just outside a black hole's boundary, briefly expands, ejecting a final burst of high-energy particles before disappearing
entirely. These findings could help to explain how larger, supermassive
black holes at the center of a galaxy can eject particles across vastly
cosmic scales to shape a galaxy's formation.

"The role of black holes in galaxy evolution is an outstanding question
in modern astrophysics," says Erin Kara, assistant professor of physics
at MIT.

"Interestingly, these black hole binaries appear to be 'mini' supermassive black holes, and so by understanding the outbursts in these small, nearby systems, we can understand how similar outbursts in supermassive black
holes affect the galaxies in which they reside." The study's first
author is MIT graduate student Jingyi Wang; other co-authors include
Matteo Lucchini and Ron Remillard at MIT, along with collaborators from
Caltech and other institutions.



========================================================================== X-ray delays Kara and her colleagues are using X-ray echoes to map a
black hole's vicinity, much the way that bats use sound echoes to navigate their surroundings. When a bat emits a call, the sound can bounce off an obstacle and return to the bat as an echo. The time it takes for the echo
to return is relative to the distance between the bat and the obstacle,
giving the animal a mental map of its surroundings.

In similar fashion, the MIT team is looking to map the immediate
vicinity of a black hole using X-ray echoes. The echoes represent time
delays between two types of X-ray light: light emitted directly from
the corona, and light from the corona that bounces off the accretion
disk of inspiraling gas and dust.

The time when a telescope receives light from the corona, compared to when
it receives the X-ray echoes, gives an estimate of the distance between
the corona and the accretion disk. Watching how these time delays change
can reveal how a black hole's corona and disk evolve as the black hole
consumes stellar material.

Echo evolution In their new study, the team developed search algorithm
to comb through data taken by NASA's Neutron star Interior Composition Explorer, or NICER, a high- time-resolution X-ray telescope aboard the International Space Station. The algorithm picked out 26 black hole X-ray binary systems that were previously known to emit X-ray outbursts. Of
these 26, the team found that 10 systems were close and bright enough
that they could discern X-ray echoes amid the outbursts. Eight of the
10 were previously not known to emit echoes.



==========================================================================
"We see new signatures of reverberation in eight sources," Wang says. "The black holes range in mass from five to 15 times the mass of the sun,
and they're all in binary systems with normal, low-mass, sun-like stars."
As a side project, Kara is working with MIT education and music scholars,
Kyle Keane and Ian Condry, to convert the emission from a typical X-ray
echo into audible sound waves.

Video Echos of a Black Hole: https://youtu.be/iIeIag2Ji8k The researchers
then ran the algorithm on the 10 black hole binaries and divided the
data into groups with similar "spectral timing features," that is,
similar delays between high-energy X-rays and reprocessed echoes. This
helped to quickly track the change in X-ray echoes at every stage during
a black hole's outburst.

The team identified a common evolution across all systems. In the initial "hard" state, in which a corona and jet of high-energy particles dominates
the black hole's energy, they detected time lags that were short and
fast, on the order of milliseconds. This hard state lasts for several
weeks. Then, a transition occurs over several days, in which the corona
and jet sputter and die out, and a soft state takes over, dominated by lower-energy X-rays from the black hole's accretion disk.

During this hard-to-soft transition state, the team discovered that time
lags grew momentarily longer in all 10 systems, implying the distance
between the corona and disk also grew larger. One explanation is that
the corona may briefly expand outward and upward, in a last high-energy
burst before the black hole finishes the bulk of its stellar meal and
goes quiet.

"We're at the beginnings of being able to use these light echoes to
reconstruct the environments closest to the black hole," Kara says. "Now
we've shown these echoes are commonly observed, and we're able to probe connections between a black hole's disk, jet, and corona in a new way."
This research was supported, in part, by NASA.


========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Jingyi Wang, Erin Kara, Matteo Lucchini, Adam Ingram, Michiel
van der
Klis, Guglielmo Mastroserio, Javier A. Garci'a, Thomas Dauser,
Riley Connors, Andrew C. Fabian, James F. Steiner, Ron A. Remillard,
Edward M.

Cackett, Phil Uttley, Diego Altamirano. The NICER "Reverberation
Machine": A Systematic Study of Time Lags in Black Hole X-Ray
Binaries.

The Astrophysical Journal, 2022; 930 (1): 18 DOI: 10.3847/1538-4357/
ac6262 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/05/220502120519.htm

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