Figure 1. Artist’s visualization of the environment around M101 ULX-1,
showing a stellar-mass black hole (foreground) with accretion disk. Gas
from the Wolf-Rayet star (background) feeds the black hole’s voracious
appetite. Credit: Gemini Observatory/AURA artwork by Lynette Cook. Full Resolution JPEG (5 MB) | Medium Resolution JPEG | Full Resolution TIFF (24 MB)
Figure 2. ULX-1 is located near a spiral arm of M101. The image for M101
is composed from X-ray (Chandra X-ray Observatory; Purple), Infrared
(Spitzer Satellite; Red), Optical (Hubble Space Telescope; Yellow) and
Ultraviolet (GALEX satellite; Blue). Credit: Chandra X-ray Observatory, Spitzer Satellite, Hubble Space Telescope, and GALEX Satellite. Full Resolution JPEG (2 MB)
Gemini observations support an unexpected discovery in the galaxy
Messier 101. A relatively small black hole (20-30 times the mass of our
Sun) can sustain a hugely voracious appetite while consuming material in
an efficient and tidy manner – something previously thought impossible.
The research also affects the long quest for elusive intermediate-mass
black holes. The findings are published in the November 28, 2013, issue
of the journal Nature.
After embargo, the complete Nature paper can be accessed at: http://dx.doi.org/10.1038/nature12762
Observations of a black hole powering an energetic X-ray source in a
galaxy some 22 million light-years away could change our thinking about
how some black holes consume matter. The findings indicate that this
particular black hole, thought to be the engine behind the X-ray
source’s high-energy light output, is unexpectedly lightweight, and,
despite the generous amount of dust and gas being fed to it by a massive
stellar companion, it swallows this material in a surprisingly orderly
fashion.
“It has elegant manners,” says research team member Stephen Justham, of
the National Astronomical Observatories of China, Chinese Academy of
Sciences. Such lightweights, he explains, must devour matter at close to
their theoretical limits of consumption to sustain the kind of energy
output observed. "We thought that when small black holes were pushed to
these limits, they would not be able to maintain such refined ways of
consuming matter," Justham explains. "We expected them to display more
complicated behavior when eating so quickly. Apparently we were wrong."
A Surprising Twist
X-ray sources give off high- and low-energy X-rays, which astronomers
call hard and soft X-rays, respectively. In what might seem like a
contradiction, larger black holes tend to produce more soft X-rays,
while smaller black holes tend to produce relatively more hard X-rays.
This source, called M101 ULX-1, is dominated by soft X-rays, so
researchers expected to find a larger black hole as its energy source.
In a surprising twist, however, the new observations made at the Gemini
Observatory, and published in the November 28th issue of the journal
Nature, indicate that M101 ULX-1’s black hole is on the small side, and
astrophysicists don’t understand why.
In theoretical models of how matter falls into black holes and radiates
energy, the soft X-rays come primarily from the accretion disk (see
illustration), while hard X-rays are typically generated by a
high-energy “corona” around the disk. The models show that the corona’s
emission strength should increase as the rate of accretion gets closer
to the theoretical limit of consumption. Interactions between the disk
and corona are also expected to become more complex.
Based on the size of the black hole found in this work, the region
around M101-ULX-1 should, theoretically, be dominated by hard X-rays and
appear structurally more complicated. However, that isn’t the case.
“Theories have been suggested which allow such low-mass black holes to
eat this quickly and shine this brightly in X-rays. But those mechanisms
leave signatures in the emitted X-ray spectrum, which this system does
not display,” says lead author Jifeng Liu, of the National Astronomical
Observatories of China, Chinese Academy of Sciences. “Somehow this black
hole, with a mass only 20-30 times the mass of our Sun, is able to eat
at a rate near to its theoretical maximum while remaining relatively
placid. It’s amazing. Theory now needs to somehow explain what’s going
on.”
An Intermediate-mass Black Hole Dilemma
The discovery also delivers a blow to astronomers hoping to find
conclusive evidence for an “intermediate-mass” black hole in M101 ULX-1.
Such black holes would have masses roughly between 100 and 1000 times
the mass of the Sun, placing them between normal stellar-mass black
holes and the monstrous supermassive black holes that reside in the
centers of galaxies. So far these objects have been frustratingly
elusive, with potential candidates but no broadly-accepted detection.
Ultra-luminous X-ray sources (ULXs) have been one of the main proposed
hiding places for intermediate-mass black holes, and M101 ULX-1 was one
of the most promising-looking contenders.
“Astronomers hoping to study these objects will now have to focus on
other locations for which indirect evidence of this class of black holes
has been suggested, either in the even brighter ‘hyper-luminous’ X-ray
sources or inside some dense clusters of stars,” explains research team
member Joel Bregman of the University of Michigan.
“Many scientists thought it was just a matter of time until we had
evidence for an intermediate-mass black hole in M101 ULX-1,” says Liu.
But the new Gemini findings both take away some of that hope to solve an
old puzzle and adds the fresh mystery of how this stellar-mass black
hole can consume matter so calmly.
To determine the mass of the black hole, the researchers used the Gemini
Multi-Object Spectrograph at the Gemini North telescope on Mauna Kea,
Hawai‘i to measure the motion of the companion. This star, which feeds
matter to the black hole, is of the Wolf-Rayet variety. Such stars emit
strong stellar winds, from which the black hole can then draw in
material. This study also revealed that the black hole in M101 ULX-1 can
capture more material from that stellar wind than astronomers had
anticipated.
M101 ULX-1 is ultra-luminous, shining a million times more brightly than
the Sun in both X-rays (from the black hole accretion disk) and in the
ultraviolet (from the companion star). Co-author Paul Crowther from the
University of Sheffield in the United Kingdom adds, "Although this isn't
the first Wolf-Rayet black hole binary ever discovered, at some 22
million light-years away, it does set a new distance record for such a
system. The Wolf-Rayet star will have died in a small fraction of the
time it has taken for light to reach us, so this system is now likely a
double black hole binary."
“Studying objects like M101 ULX-1 in distant galaxies gives us a vastly
larger sampling of the diversity of objects in our universe,” says
Bregman. “It’s absolutely amazing that we have the technology to observe
a star orbiting a black hole in another galaxy this far away.”
Media Contacts:
-
Peter Michaud
Gemini Observatory, Hilo, HI
Email: pmichaud@gemini.edu
Cell: (808) 936-6643
Desk: (808) 974-2510
Science Contacts:
- Ji-Feng Liu
Chinese Academy of Sciences, Beijing, China
Email: jfliu@nao.cas.cn
Desk: +86 010 6488 8713 - Stephen Justham
Chinese Academy of Sciences, Beijing, China
Email: sjustham@bao.ac.cn
Cell: +86 150 1100 3278 - Paul Crowther
University of Sheffield, Sheffield, UK
Email: Paul.crowther@sheffield.ac.uk
Cell: +44 (0) 7946 638474
Desk: +44 (0)114 222 4291 - Joel Bregman
University of Michigan
Email: jbregman@umich.edu
Cell: 734-476-9338
Desk: 734-764-3441
