17 research outputs found
Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre
Recent measurements of stellar orbits provide compelling evidence that the
compact radio source Sagittarius A* at the Galactic Centre is a
3.6-million-solar-mass black hole. Sgr A* is remarkably faint in all wavebands
other than the radio region, however, which challenges current theories of
matter accretion and radiation surrounding black holes. The black hole's
rotation rate is not known, and therefore neither is the structure of
space-time around it.Here we report high-resolution infrared observations of
Sgr A* that reveal 'quiescent' emission and several flares. The infrared
emission originates from within a few milliarcseconds of the black hole, and
traces very energetic electrons or moderately hot gas within the innermost
accretion region. Two flares exhibit a 17-minute quasi-periodic variability. If
the periodicity arises from relativistic modulation of orbiting gas, the
emission must come from just outside the event horizon, and the black hole must
be rotating at about half of the maximum possible rate.Comment: 5 pages, 3 figures to appear in the Oct 30 issue of Natur
A gas cloud on its way towards the super-massive black hole in the Galactic Centre
Measurements of stellar orbits provide compelling evidence that the compact
radio source Sagittarius A* at the Galactic Centre is a black hole four million
times the mass of the Sun. With the exception of modest X-ray and infrared
flares, Sgr A* is surprisingly faint, suggesting that the accretion rate and
radiation efficiency near the event horizon are currently very low. Here we
report the presence of a dense gas cloud approximately three times the mass of
Earth that is falling into the accretion zone of Sgr A*. Our observations
tightly constrain the cloud's orbit to be highly eccentric, with an innermost
radius of approach of only ~3,100 times the event horizon that will be reached
in 2013. Over the past three years the cloud has begun to disrupt, probably
mainly through tidal shearing arising from the black hole's gravitational
force. The cloud's dynamic evolution and radiation in the next few years will
probe the properties of the accretion flow and the feeding processes of the
super-massive black hole. The kilo-electronvolt X-ray emission of Sgr A* may
brighten significantly when the cloud reaches pericentre. There may also be a
giant radiation flare several years from now if the cloud breaks up and its
fragments feed gas into the central accretion zone.Comment: in press at Natur
The Galactic Center Black Hole Laboratory
The super-massive 4 million solar mass black hole Sagittarius~A* (SgrA*)
shows flare emission from the millimeter to the X-ray domain. A detailed
analysis of the infrared light curves allows us to address the accretion
phenomenon in a statistical way. The analysis shows that the near-infrared
flare amplitudes are dominated by a single state power law, with the low states
in SgrA* limited by confusion through the unresolved stellar background. There
are several dusty objects in the immediate vicinity of SgrA*. The source G2/DSO
is one of them. Its nature is unclear. It may be comparable to similar stellar
dusty sources in the region or may consist predominantly of gas and dust. In
this case a particularly enhanced accretion activity onto SgrA* may be expected
in the near future. Here the interpretation of recent data and ongoing
observations are discussed.Comment: 30 pages - 7 figures - accepted for publication by Springer's
"Fundamental Theories of Physics" series; summarizing GC contributions of 2
conferences: 'Equations of Motion in Relativistic Gravity' at the
Physikzentrum Bad Honnef, Bad Honnef, Germany, (Feb. 17-23, 2013) and the
COST MP0905 'The Galactic Center Black Hole Laboratory' Granada, Spain (Nov.
19 - 22, 2013
Theory of disk accretion onto supermassive black holes
Accretion onto supermassive black holes produces both the dramatic phenomena
associated with active galactic nuclei and the underwhelming displays seen in
the Galactic Center and most other nearby galaxies. I review selected aspects
of the current theoretical understanding of black hole accretion, emphasizing
the role of magnetohydrodynamic turbulence and gravitational instabilities in
driving the actual accretion and the importance of the efficacy of cooling in
determining the structure and observational appearance of the accretion flow.
Ongoing investigations into the dynamics of the plunging region, the origin of
variability in the accretion process, and the evolution of warped, twisted, or
eccentric disks are summarized.Comment: Mostly introductory review, to appear in "Supermassive black holes in
the distant Universe", ed. A.J. Barger, Kluwer Academic Publishers, in pres
Closest Star Seen Orbiting the Supermassive Black Hole at the Centre of the Milky Way
Measurements of stellar velocities and variable X-ray emission near the
centre of the Milky Way have provided the strongest evidence so far that the
dark mass concentrations seen in many galactic nuclei are likely supermassive
black holes, but have not yet excluded several alternative configurations. Here
we report ten years of high resolution astrometric imaging that allow us to
trace two thirds of the orbit of the star currently closest to the compact
radio source and massive black hole candidate SgrA*. In particular, we have
observed both peri- and apocentre passages. Our observations show that the star
is on a bound, highly elliptical Keplerian orbit around SgrA*, with an orbital
period of 15.2 years and a peri-centre distance of only 17 light hours. The
orbital elements require an enclosed point mass of 3.7+-1.5x10^6 solar masses.
The data exclude with high confidence that the central dark mass consists of a
cluster of astrophysical objects or massive, degenerate fermions, and strongly
constrain the central density structure.Comment: 13 pages, 3 figures, scheduled for publication in Nature on 17 Oct
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The First Decade of Science with Chandra and XMM-Newton
NASA's Chandra X-ray Observatory and ESA's XMM-Newton made their first
observations one decade ago. The unprecedented and complementary capabilities
of these observatories to detect, image, and measure the energy of cosmic
X-rays, achieved less than 50 years after the first detection of an extra-solar
X-ray source, represent an increase in sensitivity comparable in going from
naked-eye observations to the most powerful optical telescopes over the past
400 years! In this review, we highlight some of the many discoveries made by
Chandra and XMM-Newton that have transformed 21st century astronomy and briefly
discuss prospects for future research.Comment: 8 pages, 10 figures, published in Natur
First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
The Event Horizon Telescope (EHT) has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. To this end, we construct a large library of models based on general relativistic magnetohydrodynamic (GRMHD) simulations and synthetic images produced by general relativistic ray tracing. We compare the observed visibilities with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. The ring radius and ring asymmetry depend on black hole mass and spin, respectively, and both are therefore expected to be stable when observed in future EHT campaigns. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87's large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. At the same time, in those models that produce a sufficiently powerful jet, the latter is powered by extraction of black hole spin energy through mechanisms akin to the Blandford-Znajek process. We briefly consider alternatives to a black hole for the central compact object. Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models, as will future EHT campaigns at 230 and 345 GHz
First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by
gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have
assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of
1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center
of the giant elliptical galaxy M87. We have resolved the central compact radio source as an asymmetric bright emission
ring with a diameter of 42 ± 3 μas, which is circular and encompasses a central depression in brightness with a flux
ratio 10:1. The emission ring is recovered using different calibration and imaging schemes, with its diameter and
width remaining stable over four different observations carried out in different days. Overall, the observed image is
consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in
brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to
the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic
magnetohydrodynamic simulations of black holes and derive a central mass of M = (6.5 ± 0.7) × 109 Me. Our radiowave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies
and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme
limit and on a mass scale that was so far not accessible