907 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
The Nature of Nonthermal X-ray Filaments Near the Galactic Center
Recent Chandra and XMM-{\it Newton} observations reported evidence of two
X-ray filaments G359.88-0.08 (SgrA-E) and G359.54+0.18 (the ripple filament)
near the Galactic center. The X-ray emission from these filaments has a
nonthermal spectrum and coincides with synchrotron emitting radio sources.
Here, we report the detection of a new X-ray feature coincident with a radio
filament G359.90-0.06 (SgrA-F) and show more detailed VLA, Chandra and BIMA
observations of the radio and X-ray filaments. In particular, we show that
radio emission from the nonthermal filaments G359.90-0.06 (SgrA-F) and
G359.54+0.18 (the ripple) has a steep spectrum whereas G359.88-0.08 (SgrA-E)
has a flat spectrum. The X-ray emission from both these sources could be due to
synchrotron radiation. However, given that the 20 \kms molecular cloud, with
its intense 1.2mm dust emission, lies in the vicinity of SgrA-F, it is possible
that the X-rays could be produced by inverse Compton scattering of far-infrared
photons from dust by the relativistic electrons responsible for the radio
synchrotron emission. The production of X-ray emission from ICS allows an
estimate of the magnetic field strength of ~0.08 mG within the nonthermal
filament. This should be an important parameter for any models of the Galactic
center nonthermal filaments.Comment: 14 pages, 9 figures, in Cospar 2004 session E1.4; editors: Cara
Rakowski and Shami Chatterjee; "Young Neutron Stars and Supernova Remnants",
publication: Advances in Space Research (in press
Discovery of New Interacting Supernova Remnants in the Inner Galaxy
OH(1720 MHz) masers are excellent signposts of interaction between supernova
remnants(SNRs) and molecular clouds. Using the GBT and VLA we have surveyed 75
SNRs and six candidates for maser emission. Four new interacting SNRs are
detected with OH masers: G5.4-1.2, G5.7-0.0, G8.7-0.1 and G9.7-0.0. The newly
detected interacting SNRs G5.7-0.0 and G8.7-0.1 have TeV gamma-ray counterparts
which may indicate a local cosmic ray enhancement. It has been noted that
maser-emitting SNRs are preferentially distributed in the Molecular Ring and
Nuclear Disk. We use the present and existing surveys to demonstrate that
masers are strongly confined to within 50 degrees Galactic longitude at a rate
of 15 percent of the total SNR population. All new detections are within 10
degrees Galactic longitude emphasizing this trend. Additionally, a substantial
number of SNR masers have peak fluxes at or below the detection threshold of
existing surveys. This calls into question whether maser surveys of Galactic
SNRs can be considered complete and how many maser-emitting remnants remain to
be detected in the Galaxy.Comment: Accepted to ApJ Letters, with 2 figures and 2 table
The Underluminous Nature of Sgr A*
In the last several years, a number of observing campaigns of the massive
black hole Sgr A* has been carried out in order to address two important
issues: one concerns the underluminous nature of Sgr A* with its bolometric
luminosity being several orders of magnitude less than those of its more
massive counterparts. It turns out that the angular momentum of the ionized
stellar winds from orbiting stars in one or two disks orbiting Sgr A* could be
a critical factor in estimating accurately the accretion rate unto Sgr A*. A
net angular momentum of ionized gas feeding Sgr A* could lower the Bondi rate.
Furthermore, the recent time delay picture of the peak flare emission can be
understood in the context of adiabatic expansion of hot plasma. The expansion
speed of the plasma is estimated to be sub-relativistic. However, relativistic
bulk motion of the plasma could lead to outflow from Sgr A*. Significant
outflow from Sgr A* could then act as a feedback which could then reduce Bondi
accretion rate. These uncertain factors can in part explain the underluminous
nature of Sgr A*. The other issue is related to the emission mechanism and the
cause of flare activity in different wavelength bands. Modeling of X-ray and
near-IR flares suggests that inverse Compton scattering (ICS) of IR flare
photons by the energetic electrons responsible for the submm emission can
account for the X-ray flares. A time delay of minutes to tens of minutes is
predicted between the peak flaring in the near-IR and X-rays, NOT due to
adiabatic expansion of optically thick hot plasma, but to the time taken for IR
flare photons to cross the accretion flow before being upscattered.Comment: 4 pages, To appear in Proceedings of "X-ray Astronomy 2009: Present
Status, Multi-Wavelength Approach and Future Perspectives", Bologna, Italy,
September 7-11, 2009, AIP, eds. A. Comastri, M. Cappi, and L. Angelin
On the origin of the central 1" hole in the stellar disk of Sgr A* and the Fermi gamma-ray bubbles
The supermassive black hole Sgr A* at the center of the Galaxy is surrounded
by two misaligned disks of young, massive stars extending from ~0.04 to 0.4 pc.
The stellar surface density increases as ~ r^-2 towards Sgr A* but is truncated
within 1" (0.04pc). We explore the origin of this annulus using a model in
which star formation occurs in a disk of gas created through the partial
capture of a gas cloud as it sweeps through the inner few parsecs of the galaxy
and temporarily engulfs Sgr A*. We identify the locations within which star
formation and/or accretion onto Sgr A* take place. Within 0.04 pc the disk is
magnetically active and the associated heating and enhanced pressure prevents
the disk from becoming self gravitating. Instead, it forms a magneto-turbulent
disk that drains onto Sgr A* within 3 Myr. Meanwhile, fragmentation of the gas
beyond the central 0.04 pc hole creates the observed young stellar disk. The
two large scale bubbles of gamma-ray emission extending perpendicular to the
Galactic plane may be created by a burst of accretion of ~10^5 Msun of gas
lying between 0.01 and 0.03 pc. The observed stellar ages imply that this
capture event occurred ~10^6.5 yr ago, thus such events occurring over the life
time of the Galaxy could have significantly contributed to the current mass of
Sgr A* and to the inner few parsec of the nuclear star cluster. We suggest that
these events also occur in extragalactic systems.Comment: ApJL accepted; eq 1 for surface density profile generalised from
previous version plus minor change
The Origin of Keplerian Megamaser Disks
Several examples of thin, Keplerian, sub-parsec megamaser disks have been
discovered in the nuclei of active galaxies and used to precisely determine the
mass of their host black holes. We show that there is an empirical linear
correlation between the disk radius and black hole mass and that such disks are
naturally formed as molecular clouds pass through the galactic nucleus and
temporarily engulf the central supermassive black hole. For initial cloud
column densities below about 10^{23.5} cm^{-2} the disk is non-self
gravitating, but for higher cloud columns the disk would fragment and produce a
compact stellar disk similar to that observed around Sgr A* at the galactic
centre.Comment: 2 pages, to appear in Proc IAU Symp 287, Cosmic Masers: from OH to
H_0, R. Booth, E. Humphries & W. Vlemmings, ed
The Origin of Parsec-Scale Gaseous and Stellar Disks in the Galactic Center and AGNs
The Galactic center stellar disk and the circumnuclear ring provide a unique
opportunity to study in detail the dynamics and physical conditions of distant
molecular disks in the nuclei of galaxies. One of the key questions is how
these disks form so close to their host black holes and under what condition
they form stars in a tidally stressed environment. We argue that disk formation
around a massive black hole is due to partial accretion of extended molecular
clouds that temporarily pass through the central region of the Galaxy. The
cancellation of angular momentum of the gravitationally focused gas naturally
creates a compact gaseous disk. The disk can potentially become gravitationally
unstable and form stars. We apply these ideas to explain the origin of
sub-parsec megamaser disks found in the nuclei of Seyfert 2 galaxies. We show
that an empirical scaling relation between the mass of the black hole and the
size of the disk can be understood in the context of the cloud capture
scenario. We conclude that the stellar and gas disks found in our Galactic
center act as a bridge to further our understanding of more distant mega-maser
disks in the nuclei of Seyfert 2 galaxies.Comment: 6 pages, 2 figures, to appear in "The Central Kiloparsec in Galactic
Nuclei: Astronomy at High Angular Resolution 2011", open access Journal of
Physics: Conference Series (JPCS), published by IOP Publishin
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