Using the Subaru Telescope, researchers at the Special Astrophysical
Observatory in Russia and Kyoto University in Japan have found evidence
that enigmatic objects in nearby galaxies – called ultra-luminous X-ray
sources (ULXs) – exhibit strong outflows that are created as matter
falls onto their black holes at unexpectedly high rates. The strong
outflows suggest that the black holes in these ULXs must be much smaller
than expected. Curiously, these objects appear to be "cousins" of SS
433, one of the most exotic objects in our own Milky Way Galaxy. The
team's observations help shed light on the nature of ULXs, and impact
our understanding of how supermassive black holes in galactic centers
are formed and how matter rapidly falls onto those black holes (Figure 1).
Figure 1: Multi-color optical image around the ULX
"X-1" (indicated by the arrow) in the dwarf galaxy Holmberg II, located
in the direction of the constellation Ursa Major, at a distance of 11
million light-years. The image size corresponds to 1,100 × 900
light-years at the galaxy. The red color represents spectral line
emission from hydrogen atoms. (Credit: Special Astrophysical
Observatory/Hubble Space Telescope)
X-ray observations of nearby galaxies have revealed these
exceptionally luminous sources at off-nuclear positions that radiate
about million times higher power than the Sun. The origins of ULXs have
been a subject of heated debate for a long time. The basic idea is that a
ULX is a close binary system consisting of a black hole and a star. As
matter from the star falls onto the black hole, an accretion disk forms
around the black hole. As the gravitational energy of the material is
released, the innermost part of the disk is heated up to a temperature
higher than 10 million degrees, which causes it to emit strong X-rays.
The unsolved key question about these objects asks: what is the mass
of the black hole in these bright objects? ULXs are typically more than a
hundred times more luminous than known black hole binaries in the Milky
Way, whose black hole masses are at most 20 times the mass of the Sun.
There are two different black hole scenarios proposed to explain these objects: (1)
they contain very "big" black holes that could be more than a thousand times more massive than the Sun (Note 1),
or (2) they are relatively small black holes, "little monsters" with
masses no more than a hundred times that of the Sun, that shine at
luminosities exceeding theoretical limits for standard accretion (called
"supercritical (or super-Eddington) accretion," Note 2). Such supercritical accretion is expected to produce powerful outflow in a form of a dense disk wind.
To understand which scenario explains the observed ULXs researchers
observed four objects: Holmberg II X-1, Holmberg IX X-1, NGC 4559 X-7,
NGC 5204 X-1, and took high-quality spectra with the FOCAS instrument on
Subaru Telescope for four nights. Figure 1 shows an optical multi-color
image toward Holmberg II X-1 as observed with Hubble Space Telescope.
The object X-1, indicated by the arrow, is surrounded by a nebula
(colored in red), which is most likely the gas heated by strong
radiation from the ULX.
The team discovered a prominent feature in the optical spectra of all the ULXs observed (
Figure 2).
It is a broad emission line from helium ions, which indicates the
presence of gas heated to temperatures of several tens of thousands of
degrees in the system. In addition, they found that the width of the
hydrogen line, which is emitted from cooler gas (with a temperature of
about 10,000 K), is broader than the helium line. The width of a
spectral line reflects velocity dispersion of the gas and shows up due
to the Doppler effect caused by a distribution of the velocities of gas
molecules.
These findings suggest that the gas must be accelerated outward as a
wind from either the disk or the companion star and that it is cooling
down as it escapes.
Figure 2: Optical spectra of the four ULXs observed
with the Subaru Telescope (from upper to lower, Holmberg II X-1,
Holmberg IX X-1, NGC 4559 X-7, NGC 5204 X-1). He II and Hα denote the
spectral lines from helium ions and from hydrogen atoms, respectively.
(Credit: Kyoto University)
Distant ULXs and a Similar Mysterious Object in the Milky Way
The activity of these ULXs in distant galaxies is very similar to a
mysterious object in our own Milky Way. The team noticed that the same
line features are also observed at SS 433, a close binary consisting of
an A-type star and most probably a black hole with a mass less than 10
times that of the Sun. SS 433 is famous for its persistent jets with a
velocity of 0.26 times the speed of light. It is the only confirmed
system that shows supercritical accretion (that is, an excessive amount
of accretion that results in a very powerful outflow). By contrast, such
features
have not been observed from "normal" black hole X-ray binaries in the
Milky Way where sub-critical accretion takes place.
After carefully examining several possibilities, the team concluded
that huge amounts of gas are rapidly falling onto "little monster" black
holes in each of these ULXs, which produces a dense disk wind flowing
away from the supercritical accretion disk. They suggest that
"bona-fide" ULXs with luminosities of about million times that of the
Sun must belong to a homogeneous class of objects, and SS 433 is an
extreme case of the same population. In these, even though the black
hole is small, very luminous X-ray radiation is emitted as the
surrounding gas falls onto the disk at a huge rate.
Figure 3
is a schematic view of the ULXs (upper side) and SS 433 (lower side).
If the system is observed from a vertical direction, it's clear that the
central part of the accretion disk emits intense X-rays. If SS 433 were
observed in the same direction, it would be recognized as the brightest
X-ray source in the Milky Way. In reality, since we are looking at SS
433 almost along the disk plane, our line-of-sight view towards the
inner disk is blocked by the outer disk. The accretion rate is inferred
to be much larger in SS 433 than in the ULXs, which could explain the
presence of persistent jets in SS 433.
Figure 3: Schematic view of ULXs (looking from upper
side) and SS 433 (looking from left side). Strong X-rays are emitted
from the inner region of the supercritical accretion disk. Powerful
winds are launched from the disk, which eventually emit spectral lines
of helium ions and hydrogen atoms. (Credit: Kyoto University)
Such "supercritical accretion" is thought to be a possible mechanism
in the formation of supermassive black holes at galactic centers in very
short time periods (which are observed very early in cosmic time). The
discovery of these phenomena in the nearby universe has significant
impacts on our understanding of how supermassive black holes are formed
and how matter rapidly falls onto them.
There are still some remaining questions: What are the typical mass
ranges of the black holes in ULXs? In what conditions can steady
baryonic jets as observed in SS 433 be produced? Dr. Yoshihiro Ueda, a
core member of the team, expresses his enthusiasm for future research in
this area. "We would like to tackle these unresolved problems by using
the new X-ray observations by ASTRO-H, planned to be launched early next
year, and by more sensitive future X-ray satellites, together with
multi-wavelength observations of ULXs and SS 433," he said.
This work has been published online in
Nature Physics on 2015 June 1
(Fabrika et al. 2015, "Supercritical Accretion Discs in Ultraluminous
X-ray Sources and SS 433",
10.1038/nphys3348). The research was supported by the Japan Society for
the Promotion of Science's KAKENHI Grant number 26400228.
Authors:
- Sergei Fabrika (Special Astrophysical Observatory, Russia; Kazan Federal University, Russia)
- Yoshihiro Ueda (Department of Astronomy, Kyoto University, Japan)
- Alexander Vinokurov (Special Astrophysical Observatory, Russia)
- Olga Sholukhova (Special Astrophysical Observatory, Russia)
- Megumi Shidatsu (Department of Astronomy, Kyoto University, Japan)
Notes:
- Generally, black holes with masses between about 100 and about
100,000 times that of the Sun are called "intermediate-mass black
holes," although there is no strict definition for the mass range.
- In a spherically symmetric case, matter cannot fall onto a central
object when the radiation pressure exceeds the gravity. This luminosity
is called the Eddington limit, which is proportional to the mass of the
central object. When matter is accreted at rates higher than that
corresponding to the Eddington limit, it is called "supercritical (or
super-Eddington) accretion." In the case of non-spherical geometry, such
as disk accretion, supercritical accretion may happen.
