186 research outputs found
Occultation of the Quiescent Emission from Sgr A* by IR Flares
We have investigated the nature of flare emission from Sgr A* during
multi-wavelength observations of this source that took place in 2004, 2005 and
2006. We present evidence for dimming of submm and radio flux during the peak
of near-IR flares. This suggests that the variability of Sgr A* across its
wavelength spectrum is phenomenologically related. The model explaining this
new behavior of flare activity could be consistent with adiabatically cooling
plasma blobs that are expanding but also partially eclipsing the background
quiescent emission from Sgr A*. When a flare is launched, the plasma blob is
most compact and is brightest in the optically thin regime whereas the emission
in radio/submm wavelengths has a higher opacity. Absorption in the observed
light curve of Sgr A* at radio/submm flux is due to the combined effects of
lower brightness temperature of plasma blobs with respect to the quiescent
brightness temperature and high opacity of plasma blobs. This implies that
plasma blobs are mainly placed in the magnetosphere of a disk-like flow or
further out in the flow. The depth of the absorption being larger in submm than
in radio wavelengths implies that the intrinsic size of the quiescent emission
increases with increasing wavelength which is consistent with previous size
measurements of Sgr A*. Lastly, we believe that occultation of the quiescent
emission of Sgr A* at radio/submm by IR flares can be used as a powerful tool
to identify flare activity at its earliest phase of its evolution.Comment: 11 pages, 5 figures, accepted by ApJ
An Inverse Compton Scattering Origin of X-ray Flares from Sgr A*
The X-ray and near-IR emission from Sgr A* is dominated by flaring, while a
quiescent component dominates the emission at radio and sub-mm wavelengths. The
spectral energy distribution of the quiescent emission from Sgr A* peaks at
sub-mm wavelengths and is modeled as synchrotron radiation from a thermal
population of electrons in the accretion flow, with electron temperatures
ranging up to \,MeV. Here we investigate the mechanism by which
X-ray flare emission is produced through the interaction of the quiescent and
flaring components of Sgr A*. The X-ray flare emission has been interpreted as
inverse Compton, self-synchrotron-Compton, or synchrotron emission. We present
results of simultaneous X-ray and near-IR observations and show evidence that
X-ray peak flare emission lags behind near-IR flare emission with a time delay
ranging from a few to tens of minutes. Our Inverse Compton scattering modeling
places constraints on the electron density and temperature distributions of the
accretion flow and on the locations where flares are produced. In the context
of this model, the strong X-ray counterparts to near-IR flares arising from the
inner disk should show no significant time delay, whereas near-IR flares in the
outer disk should show a broadened and delayed X-ray flare.Comment: 22 pages, 6 figures, 2 tables, AJ (in press
Synchrotron Blob Model of Infrared and X-ray Flares from Sagittarius A
Sagittarius A in the Galactic center harbors a supermassive black hole
and exhibits various active phenomena. Besides quiescent emission in radio and
submillimeter radiation, flares in the near infrared (NIR) and X-ray bands are
observed to occur frequently. We study a time-dependent model of the flares,
assuming that the emission is from a blob ejected from the central object.
Electrons obeying a power law with the exponential cutoff are assumed to be
injected in the blob for a limited time interval. The flare data of 2007 April
4 were used to determine the values of model parameters. The spectral energy
distribution of flare emission is explained by nonthermal synchrotron radiation
in the NIR and X-ray bands. The model light curves suggest that electron
acceleration is still underway during the rising phase of the flares. GeV
gamma-rays are also emitted by synchrotron self-Compton scattering, although
its luminosity is not strictly constrained by the current model. If the GeV
emission is faint, the plasma blob is dominated by the magnetic energy density
over the electron kinetic energy density. Observations in the GeV band will
clarify the origin of the blob.Comment: 11 pages, 8 figures, 1 table. The title is slightly changed and
references are updated. Minor corrections are also include
A Leptonic Model of Steady High-Energy Gamma-Ray Emission from Sgr A
Recent observations of Sgr A by Fermi and HESS have detected steady
gamma-ray emission in the GeV and TeV bands. We present a new model to explain
the GeV gamma-ray emission by inverse Compton scattering by nonthermal
electrons supplied by the NIR/X-ray flares of Sgr A. The escaping electrons
from the flare regions accumulate in a region with a size of cm
and magnetic fields of G. Those electrons produce gamma-rays
by inverse Compton scattering off soft photons emitted by stars and dust around
the central black hole. By fitting the GeV spectrum, we find constraints on the
magnetic field and the energy density of optical-UV radiation in the central 1
pc region around the supermassive black hole. While the GeV spectrum is well
fitted by our model, the TeV -rays, whose spectral index is different
from that of the GeV emission, may be from different sources such as pulsar
wind nebulae.Comment: 11 pages, 5 figures, 2 tables. Accepted for publication in the
Astrophysical Journa
The two states of Sgr A* in the near-infrared: bright episodic flares on top of low-level continuous variability
In this paper we examine properties of the variable source Sgr A* in the
near-infrared (NIR) using a very extensive Ks-band data set from NACO/VLT
observations taken 2004 to 2009. We investigate the variability of Sgr A* with
two different photometric methods and analyze its flux distribution. We find
Sgr A* is continuously emitting and continuously variable in the near-infrared,
with some variability occurring on timescales as long as weeks. The flux
distribution can be described by a lognormal distribution at low intrinsic
fluxes (<~5 mJy, dereddened with A_{Ks}=2.5). The lognormal distribution has a
median flux of approximately 1.1 mJy, but above 5 mJy the flux distribution is
significantly flatter (high flux events are more common) than expected for the
extrapolation of the lognormal distribution to high fluxes. We make a general
identification of the low level emission above 5 mJy as flaring emission and of
the low level emission as the quiescent state. We also report here the
brightest Ks-band flare ever observed (from August 5th, 2008) which reached an
intrinsic Ks-band flux of 27.5 mJy (m_{Ks}=13.5). This flare was a factor 27
increase over the median flux of Sgr A*, close to double the brightness of the
star S2, and 40% brighter than the next brightest flare ever observed from
Sgr~A*.Comment: 14 pages, 6 figures, accepted for publication in Ap
The power of monitoring stellar orbits
The center of the Milky Way hosts a massive black hole. The observational
evidence for its existence is overwhelming. The compact radio source Sgr A* has
been associated with a black hole since its discovery. In the last decade,
high-resolution, near-infrared measurements of individual stellar orbits in the
innermost region of the Galactic Center have shown that at the position of Sgr
A* a highly concentrated mass of 4 x 10^6 M_sun is located. Assuming that
general relativity is correct, the conclusion that Sgr A* is a massive black
hole is inevitable. Without doubt this is the most important application of
stellar orbits in the Galactic Center. Here, we discuss the possibilities going
beyond the mass measurement offered by monitoring these orbits. They are an
extremely useful tool for many scientific questions, such as a geometric
distance estimate to the Galactic Center or the puzzle, how these stars reached
their current orbits. Future improvements in the instrumentation will open up
the route to testing relativistic effects in the gravitational potential of the
black hole, allowing to take full advantage of this unique laboratory for
celestial mechanics.Comment: Proceedings of the Galactic Center Workshop 2009, Shangha
Multi-Wavelength Study of Sgr A*: The Short Time Scale Variability
To understand the correlation and the radiation mechanism of flare emission
in different wavelength bands, we have coordinated a number of telescopes to
observe SgrA* simultaneously. We focus only on one aspect of the preliminary
results of our multi-wavelength observing campaigns, namely, the short time
scale variability of emission from SgrA* in near-IR, X-ray and radio
wavelengths. The structure function analysis indicate most of the power
spectral density is detected on hourly time scales in all wavelength bands. We
also report minute time scale variability at 7 and 13mm placing a strong
constraint on the nature of the variable emission. The hourly time scale
variability can be explained in the context of a model in which the peak
frequency of emission shifts toward lower frequencies as a self-absorbed
synchrotron source expands adiabatically near the acceleration site. The short
time scale variability, on the other hand, places a strong constraint on the
size of the emitting region. Assuming that rapid minute time scale fluctuations
of the emission is optically thick in radio wavelength, light travel arguments
requires relativistic particle energy, thus suggesting the presence of outflow
from SgrA*.Comment: 9 pages, 4 figures, The Galactic Center: A Window on the Nuclear
Environment of Disk Galaxies ASP Conference Series, 2010 eds: M. Morris, D.
Q. Wang and F. Yua
Flares and variability from Sagittarius A*: five nights of simultaneous multi-wavelength observations
Aims. We report on simultaneous observations and modeling of mid-infrared
(MIR), near-infrared (NIR), and submillimeter (submm) emission of the source
Sgr A* associated with the supermassive black hole at the center of our Galaxy.
Our goal was to monitor the activity of Sgr A* at different wavelengths in
order to constrain the emitting processes and gain insight into the nature of
the close environment of Sgr A*. Methods. We used the MIR instrument VISIR in
the BURST imaging mode, the adaptive optics assisted NIR camera NACO, and the
sub-mm antenna APEX to monitor Sgr A* over several nights in July 2007.
Results. The observations reveal remarkable variability in the NIR and sub-mm
during the five nights of observation. No source was detected in the MIR, but
we derived the lowest upper limit for a flare at 8.59 microns (22.4 mJy with
A_8.59mu = 1.6+/- 0.5). This observational constraint makes us discard the
observed NIR emission as coming from a thermal component emitting at sub-mm
frequencies. Moreover, comparison of the sub-mm and NIR variability shows that
the highest NIR fluxes (flares) are coincident with the lowest sub-mm levels of
our five-night campaign involving three flares. We explain this behavior by a
loss of electrons to the system and/or by a decrease in the magnetic field, as
might conceivably occur in scenarios involving fast outflows and/or magnetic
reconnection.Comment: 10 pages, 7 figures, published in A&
Electron heating and acceleration by magnetic reconnection in hot accretion flows
Both analytical and numerical works show that magnetic reconnection must
occur in hot accretion flows. This process will effectively heat and accelerate
electrons. In this paper we use the numerical hybrid simulation of magnetic
reconnection plus test-electron method to investigate the electron acceleration
and heating due to magnetic reconnection in hot accretion flows. We consider
fiducial values of density, temperature, and magnetic parameter
(defined as the ratio of the electron pressure to the magnetic pressure) of the
accretion flow as , , and . We find that electrons are heated to a higher
temperature K, and a fraction of electrons
are accelerated into a broken power-law distribution, , with and 4 below and above MeV,
respectively. We also investigate the effect of varying and . We
find that when is smaller or is larger, i.e, the magnetic field
is stronger, , , and all become larger.Comment: 6 pages, 6 figure; accepted by Ap
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