33 research outputs found
A new bow-shock source with bipolar morphology in the vicinity of Sgr A*
Here we present a new bowshock blue-shifted source in the close vicinity of
Sgr A* that we name X8. We use data-sets that are based on SINFONI observations
with the Very Large Telescope. We can trace the source between 2006 and 2016 in
the blue-shifted line maps and it shows not only positional similarities to X7
but also the same spectral footprint. The symmetry axis of both extended
sources points towards Sgr A* and exhibits [Fe III] emission lines that arise
due to wind-wind shocks. In particular, the source X8 has a bipolar morphology,
which makes it the closest bipolar source in the vicinity of Sgr A*. In
addition, we can trace a K-band continuum counterpart of X8. This points
towards a stellar counterpart to the line-map emission. Overall, the source X8
can be interpreted as either a Young Stellar Object or a young planetary
nebula, which makes this source unique among so-far detected main-sequence OB
stars in this region.Comment: Accepted for publication by A&A, 15 pages, 13 figure
Nuclear Activity and the Conditions of Star-formation at the Galactic Center
The Galactic Center is the closest galactic nucleus that can be studied with
unprecedented angular resolution and sensitivity. We summarize recent basic
observational results on Sagittarius A* and the conditions for star formation
in the central stellar cluster. We cover results from the radio, infrared, and
X-ray domain and include results from simulation as well. From (sub-)mm and
near-infrared variability and near-infrared polarization data we find that the
SgrA* system (supermassive black hole spin, a potential temporary accretion
disk and/or outflow) is well ordered in its geometrical orientation and in its
emission process that we assume to reflect the accretion process onto the
supermassive black hole (SMBH).Comment: 11 pages, 4 figures, 1 table; published in PoS-SISSA Proceedings of
the: Frontier Research in Astrophysics - II, 23-28 May 2016, Mondello
(Palermo), Ital
Experimental Indicators of Accretion Processes in Active Galactic Nuclei
Bright Active Galactic Nuclei are powered by accretion of mass onto the super
massive black holes at the centers of the host galaxies. For fainter objects
star formation may significantly contribute to the luminosity. We summarize
experimental indicators of the accretion processes in Active Galactic Nuclei
(AGN), i.e., observable activity indicators that allow us to conclude on the
nature of accretion. The Galactic Center is the closest galactic nucleus that
can be studied with unprecedented angular resolution and sensitivity.
Therefore, here we also include the presentation of recent observational
results on Sagittarius A* and the conditions for star formation in the central
stellar cluster. We cover results across the electromagnetic spectrum and find
that the Sagittarius A* (SgrA*) system is well ordered with respect to its
geometrical orientation and its emission processes of which we assume to
reflect the accretion process onto the super massive black hole.Comment: 16 pages, 4 figures, conference proceeding: Accretion Processes in
Cosmic Sources - APCS2016 - 5-10 September 2016, Saint Petersburg, Russi
OJ287: deciphering the 'Rosetta stone of blazars
OJ287 is the best candidate active galactic nucleus (AGN) for hosting a supermassive binary black hole (SMBBH) at very close separation. We present 120 Very Long Baseline Array (VLBA) observations (at 15 GHz) covering the time between April 1995 and April 2017. We find that the OJ287 radio jet is precessing on a time-scale of similar to 22 yr. In addition, our data are consistent with a jet-axis rotation on a yearly time-scale. We model the precession (24 +/- 2 yr) and combined motion of jet precession and jet-axis rotation. The jet motion explains the variability of the total radio flux-density via viewing angle changes and Doppler beaming. Half of the jet-precession time-scale is of the order of the dominant optical periodicity time-scale. We suggest that the optical emission is synchrotron emission and related to the jet radiation. The jet dynamics and flux-density light curves can be understood in terms of geometrical effects. Disturbances of an accretion disc caused by a plunging BH do not seem necessary to explain the observed variability. Although the SMBBH model does not seem necessary to explain the observed variability, an SMBBH or Lense-Thirring precession (disc around single BH) seem to be required to explain the time-scale of the precessing motion. Besides jet rotation also nutation of the jet axis could explain the observed motion of the jet axis. We find a strikingly similar scaling for the time-scales for precession and nutation as indicated for SS433 with a factor of roughly 50 times longer in OJ287
Monitoring the Dusty S-Cluster Object (DSO/G2) on its Orbit towards the Galactic Center Black Hole
We analyse and report in detail new near-infrared (1.45 - 2.45 microns)
observations of the Dusty S-cluster Object (DSO/G2) during its approach to the
black hole at the center of the Galaxy that were carried out with ESO
VLT/SINFONI between February and September 2014. Before May 2014 we detect
spatially compact Br-gamma and Pa-alpha line emission from the DSO at about
40mas east of SgrA*. The velocity of the source, measured from the red-shifted
emission, is 2700+-60 km/s. No blue-shifted emission above the noise level is
detected at the position of SgrA* or upstream the presumed orbit. After May we
find spatially compact Br-gamma blue-shifted line emission from the DSO at
about 30mas west of SgrA* at a velocity of -3320+-60 km/s and no indication for
significant red-shifted emission. We do not detect any significant extension of
velocity gradient across the source. We find a Br-gamma-line full width at half
maximum of 50+-10 Angstroem before and 15+-10 Angstroem after the peribothron
transit, i.e. no significant line broadening with respect to last year is
observed. Br-gamma line maps show that the bulk of the line emission originates
from a region of less than 20mas diameter. This is consistent with a very
compact source on an elliptical orbit with a peribothron time passage in
2014.39+-0.14. For the moment, the flaring activity of the black hole in the
near-infrared regime has not shown any statistically significant increment.
Increased accretion activity of SgrA* may still be upcoming. We discuss details
of a source model according to which the DSO is rather a young accreting star
than a coreless gas and dust cloud.Comment: 32 pages - 3 tables - 17 figure - accepted by Ap
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