3,912 research outputs found
The Jet in the Galactic Center: An Ideal Laboratory for Magnetohydrodynamics and General Relativity
In this paper we review and discuss some of the intriguing properties of the
Galactic Center supermassive black hole candidate Sgr A*. Of all possible black
hole sources, the event horizon of Sgr A*, subtends the largest angular scale
on the sky. It is therefore a prime candidate to study and image plasma
processes in strong gravity and it even allows imaging of the shadow cast by
the event horizon. Recent mm-wave VLBI and radio timing observations as well as
numerical GRMHD simulations now have provided several breakthroughs that put
Sgr A* back into the focus. Firstly, VLBI observations have now measured the
intrinsic size of Sgr A* at multiple frequencies, where the highest frequency
measurements have approached the scale of the black hole shadow. Moreover,
measurements of the radio variability show a clear time lag between 22 GHz and
43 GHz. The combination of size and timing measurements, allows one to actually
measure the flow speed and direction of magnetized plasma at some tens of
Schwarzschild radii. This data strongly support a moderately relativistic
outflow, consistent with an accelerating jet model. This is compared to recent
GRMHD simulation that show the presence of a moderately relativistic outflow
coupled to an accretion flow Sgr A*. Further VLBI and timing observations
coupled to simulations have the potential to map out the velocity profile from
5-40 Schwarzschild radii and to provide a first glimpse at the appearance of a
jet-disk system near the event horizon. Future submm-VLBI experiments would
even be able to directly image those processes in strong gravity and directly
confirm the presence of an event horizon.Comment: invited talk to appear in "Jets on All Scales", IAU Symposium 275,
G.E. Romero, R.A. Sunyaev & T. Belloni, eds., Cambridge University Press, 9
pages, LaTex, 4 figure
Imaging black holes: past, present and future
This paper briefly reviews past, current, and future efforts to image black
holes in the radio regime. Black holes seem like mystical objects, but they are
an integral part of current astrophysics and are at the center of attempts to
unify quantum physics and general relativity. Yet, nobody has ever seen a black
hole. What do they look like? Initially, this question seemed more of an
academic nature. However, this has changed over the past two decades.
Observations and theoretical considerations suggest that the supermassive black
hole, Sgr A*, in the center of our Milky Way is surrounded by a compact, foggy
emission region radiating at and above 230 GHz. It has been predicted that the
event horizon of Sgr A* should cast its shadow onto that emission region, which
could be detectable with a global VLBI array of radio telescopes. In contrast
to earlier pictures of black holes, that dark feature is not supposed to be due
to a hole in the accretion flow, but would represent a true negative image of
the event horizon. Currently, the global Event Horizon Telescope consortium is
attempting to make such an image. In the future those images could be improved
by adding more telescopes to the array, in particular at high sites in Africa.
Ultimately, a space array at THz frequencies, the Event Horizon Imager, could
produce much more detailed images of black holes. In combination with numerical
simulations and precise measurements of the orbits of stars - ideally also of
pulsars - these images will allow us to study black holes with unprecedented
precision.Comment: 10 pages, 3 figures, invited review,
http://iopscience.iop.org/article/10.1088/1742-6596/942/1/01200
A jet model for the broadband spectrum of XTE J1118+480: Synchrotron emission from radio to X-rays in the Low/Hard spectral state
Observations have revealed strong evidence for powerful jets in the Low/Hard
states of black hole candidate X-ray binaries. Correlations, both temporal and
spectral, between the radio -- infrared and X-ray bands suggest that jet
synchrotron as well as inverse Compton emission could also be significantly
contributing at higher frequencies. We show here that, for reasonable
assumptions about the jet physical parameters, the broadband spectrum from
radio through X-rays can be almost entirely fit by synchrotron emission. We
explore a relatively simple model for a relativistic, adiabatically expanding
jet combined with a truncated thermal disk conjoined by an ADAF, in the context
of the recently discovered black hole binary XTE J1118+480. In particular, the
X-ray power-law emission can be explained as optically thin synchrotron
emission from a shock acceleration region in the innermost part of the jet,
with a cutoff determined by cooling losses. For synchrotron cooling-limited
particle acceleration, the spectral cutoff is a function only of dimensionless
plasma parameters and thus should be around a ``canonical'' value for sources
with similar plasma properties. It is therefore possible that non-thermal jet
emission is important for XTE J1118+480 and possibly other X-ray binaries in
the Low/Hard state.Comment: 4 pages, 1 figure, accepted for A&A Letters, reformatted and
shortened to fit page limit, discusses "canonical 100 keV cutoff" and some
minor changes, also available at
http://www.mpifr-bonn.mpg.de/staff/hfalcke/publications.html#j111
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