'Seeing' black holes with the naked eye

A report from the scientific communities..... A black holes seen in sky..



An international research team reports that
the activity of black holes can be observed as
visible light during outbursts, and that
flickering light emerging from gases
surrounding black holes is a direct indicator of
this. The team's results, published in Nature,
indicate that optical rays and not just X-rays
provide reliable observational data for black
hole activity.
Credit: Eiri Ono/Kyoto University
All you need is a 20 cm telescope to observe a
nearby, active black hole.
An international research team reports that the
activity of such phenomena can be observed by
visible light during outbursts, and that flickering
light emerging from gases surrounding black
holes is a direct indicator of this. The team's
results, published in Nature, indicate that optical
rays and not just X-rays provide reliable
observational data for black hole activity.
"We now know that we can make observations
based on optical rays -- visible light, in other
words -- and that black holes can be observed
without high-spec X-ray or gamma-ray
telescopes," explains lead author Mariko Kimura,
a master's student at Kyoto University.
Once in several decades, some black hole binaries
undergo "outbursts," in which enormous amounts
of energy -- including X-rays -- are emitted from
substances that fall into the black hole. Black
holes are commonly surrounded by an accretion
disk, in which gas from a companion star is
slowly drawn to the hole in a spiral pattern.
Activities of black holes are typically observed
through X-rays, generated in the inner portions of
accretion disks where temperatures reach 10
million degrees Kelvin or more.
V404 Cygni, one of the black hole binaries
thought to be nearest to Earth, "woke up" after 26
years of dormancy on 15 June 2015 as it
underwent such an outburst.
Led by astronomers from Kyoto University, the
team succeeded in obtaining unprecedented
amounts of data from V404 Cygni, detecting
repetitive patterns having timescales of several
minutes to a few hours. The optical fluctuation
patterns, the team found, were correlated with
those of X-rays.
Based on analyses of optical and X-ray
observational data, Kyoto astronomers and their
collaborators at national space agency JAXA,
national laboratory RIKEN, and Hiroshima
University showed that the light originates from
X-rays emerging from the innermost region of the
accretion disk around a black hole. These X-rays
irradiate and heat the outer region of the disk,
making it emit optical rays and thus becoming
visible to the human eye.
The outburst observation, the researchers say,
was the fruit of international collaboration across
countries in different time zones.
"Stars can only be observed after dark, and there
are only so many hours each night, but by
making observations from different locations
around the globe we're able to take more
comprehensive data," says co-author Daisuke
Nogami. "We're very pleased that our
international observation network was able to
come together to document this rare event."
The study also revealed that these repetitive
variations occur at mass accretion rates lower
than one tenth of that previously thought. This
indicates that the quantity of mass accretion rate
isn't the main factor triggering repetitive activity
around black holes, but rather the length of
orbital periods.