scholarly journals Daily monitor of Sagittarius A* at 22 GHz with the Japanese VLBI Network

2013 ◽  
Vol 9 (S303) ◽  
pp. 382-384
Author(s):  
M. Tsuboi ◽  
Y. Asaki ◽  
Y. Yonekura ◽  
Y. Miyamoto ◽  
H. Kaneko ◽  
...  
Keyword(s):  
Flux Density ◽  
Accretion Disk ◽  
Sagittarius A ◽  
Sgr A ◽  
Large Flare

AbstractWe have been monitoring the flux density of Sagittarius A* (Sgr A*) at 22 GHz since DOY=42 (11 Feb. 2013) with a sub-array of the Japanese VLBI Network in order to search the increase of 22-GHz emission from Sgr A* induced by the interaction of the G2 cloud with the accretion disk. The flux densities observed until DOY=322 (18 Nov. 2013) are consistent with the previously observed values before the approaching of the cloud. We have detected no large flare during this period.

The 3-mm flux density monitoring of Sagittarius A* with the ATCA

2007 ◽  
Vol 3 (S248) ◽  
pp. 204-205
Author(s):  
J. Li ◽  
Z. Q. Shen ◽  
A. Miyazaki ◽  
M. Miyoshi ◽  
T. Tsutsumi ◽  
...  
Keyword(s):  
Radio Source ◽  
Flux Density ◽  
Galactic Center ◽  
Sagittarius A ◽  
Compact Radio Source ◽  
Sgr A

AbstractWe have performed monitoring observations of the 3-mm flux density toward the Galactic center compact radio source Sgr A* with the ATCA since 2005 October. It has been found that during several observing epochs Sgr A* was quite active, showing significant intraday variation. Here we report the detection of an IDV in Sgr A* on 2006 August 13, which exhibits a 27% fractional variation in about 2 hrs.

scholarly journals Flux monitoring observations of Sgr A* at 8 GHz and 2 GHz with the NICT Kashima–Koganei VLBI System

2013 ◽  
Vol 9 (S303) ◽  
pp. 330-332
Author(s):  
S. Takekawa ◽  
T. Oka ◽  
M. Sekido
Keyword(s):  
Flux Density ◽  
Accretion Disk ◽  
Primary Objective ◽  
Flux Variation ◽  
Flux Monitoring ◽  
Significant Change ◽  
Sgr A ◽  
Continue Monitoring

AbstractWe have been conducting flux monitoring observations of Sgr A* at 8 GHz and 2 GHz using the NICT Kashima-Koganei VLBI system (109 km baseline) since mid-February 2013. The primary objective of the monitoring is a search for flux variation which is expected to be caused by the interaction between the G2 cloud and the accretion disk. Until 2013 September 22, we observed Sgr A* for 39 days, five hours on each day. Four quasars (NRAO 530, PKS 1622–253, PKS 1622–297, PKS 1921–293) were also observed as flux calibrators every 6 minutes. No significant change nor variation has been detected in the 8 GHz flux density of Sgr A*. The 8 GHz flux density was 0.81 ± 0.07 Jy (preliminary), while no significant 2 GHz emission was detected by our system. We will continue monitoring as often as possible until at least 2014 May.

scholarly journals Short-Term Variability of Sagittarius A* at Millimeter Wavelengths

1998 ◽  
Vol 164 ◽  
pp. 207-208
Author(s):  
T. Tsutsumi ◽  
T. Kawabata ◽  
A. Miyazaki ◽  
M. Tsuboi
Keyword(s):  
Flux Density ◽  
Spatial Resolution ◽  
Galactic Center ◽  
High Spatial Resolution ◽  
Short Term ◽  
Compact Source ◽  
Sagittarius A ◽  
Millimeter Wavelengths ◽  
Sgr A ◽  
Short Term Variability

AbstractWe present preliminary results from the observations of the Galactic Center compact source, Sgr A* at 3 and 2 millimeter wavelengths using the Nobeyama Millimeter Array to monitor flux density variations on timescales shorter than a month. Such high spatial resolution observations at millimeter wavelengths are important to shed more light on the origin of the variability and the nature of this compact source. Our observations indicate the flux density varies at least by ~30% in one to two weeks at 3 mm.

scholarly journals Class I methanol masers in the Galactic center

2013 ◽  
Vol 9 (S303) ◽  
pp. 147-149
Author(s):  
L. O. Sjouwerman ◽  
Y. M. Pihlström
Keyword(s):  
Galactic Center ◽  
Class I ◽  
Large Array ◽  
Maser Emission ◽  
Very Large Array ◽  
Sagittarius A ◽  
Current Location ◽  
Theoretical Predictions ◽  
Methanol Masers ◽  
Sgr A

AbstractWe report on the detection of 36 and 44 GHz Class I methanol (CH3OH) maser emission in the Sagittarius A (Sgr A) complex with the Karl G. Jansky Very Large Array (VLA). These VLA observations show that the Sgr A complex harbors at least three different maser tracers of shocked regions in the radio regime. The 44 GHz masers correlate with the positions and velocities of previously detected 36 GHz CH3OH masers, but less with 1720 MHz OH masers. Our detections agree with theoretical predictions that the densities and temperatures conducive for 1720 MHz OH masers may also produce 36 and 44 GHz CH3OH maser emission. However, many 44 GHz masers do not overlap with 36 GHz methanol masers, suggesting that 44 GHz masers also arise in regions too hot and too dense for 36 GHz masers to form. This agrees with the non-detection of 1720 MHz OH masers in the same area, which are thought to be excited under even cooler and less dense conditions. We speculate that the geometry of the 36 GHz masers outlines the current location of a shock front.

scholarly journals How far actually is the Galactic Center IRS 13E3 from Sagittarius A*?

2020 ◽  
Vol 72 (3) ◽  
Author(s):  
Masato Tsuboi ◽  
Yoshimi Kitamura ◽  
Takahiro Tsutsumi ◽  
Ryosuke Miyawaki ◽  
Makoto Miyoshi ◽  
...  
Keyword(s):  
Galactic Center ◽  
Three Dimensional ◽  
Line Of Sight ◽  
Recombination Line ◽  
Intermediate Mass ◽  
Ionized Gas ◽  
Sagittarius A ◽  
Projection Distance ◽  
Spectroscopic Information ◽  
Sgr A

Abstract The Galactic Center IRS 13E cluster is a very intriguing infrared object located at ${\sim } 0.13$ pc from Sagittarius A$^\ast$ (Sgr A$^\ast$) in projection distance. There are arguments both for and against the hypothesis that a dark mass like an intermediate mass black hole (IMBH) exists in the cluster. We recently detected the rotating ionized gas ring around IRS 13E3, which belongs to the cluster, in the H30$\alpha$ recombination line using ALMA. The enclosed mass is derived to be $M_{\mathrm{encl.}}\simeq 2\times 10^{4}\, M_\odot$, which agrees with an IMBH and is barely less than the astrometric upper limit mass of an IMBH around Sgr A$^\ast$. Because the limit mass depends on the true three-dimensional (3D) distance from Sgr A$^\ast$, it is very important to determine it observationally. However, the 3D distance is indefinite because it is hard to determine the line-of-sight (LOS) distance by usual methods. We attempt here to estimate the LOS distance from spectroscopic information. The CH$_3$OH molecule is easily destroyed by the cosmic rays around Sgr A$^{\ast }$. However, we detected a highly excited CH$_3$OH emission line in the ionized gas stream associated with IRS 13E3. This indicates that IRS 13E3 is located at $r\gtrsim 0.4$ pc from Sgr A$^{\ast }$.

scholarly journals Time Variations in the Flux Density of Sgr A* at 230 GHz Detected with ALMA

2020 ◽  
Vol 892 (2) ◽  
pp. L30 ◽  
Author(s):  
Yuhei Iwata ◽  
Tomoharu Oka ◽  
Masato Tsuboi ◽  
Makoto Miyoshi ◽  
Shunya Takekawa
Keyword(s):  
Flux Density ◽  
Time Variations ◽  
Sgr A

scholarly journals Long-Term Variations of the Compact Radio Source Sgr A∗ at the Galactic Center

1989 ◽  
Vol 136 ◽  
pp. 535-541 ◽  
Author(s):  
Jun-Hui Zhao ◽  
R. D. Ekers ◽  
W. M. Goss ◽  
K. Y. Lo ◽  
Ramesh Narayan
Keyword(s):  
Radio Source ◽  
Flux Density ◽  
High Velocity ◽  
Galactic Center ◽  
Light Curves ◽  
Radio Flux ◽  
Compact Radio Source ◽  
Sgr A ◽  
Long Term Variations

We investigate the long-term flux density variations of the compact radio source Sgr A∗ at the galactic center by combining recent VLA observations with previous Green Bank interferometer data. We present radio flux density light-curves for Sgr A∗ at 20, 11, 6 and 3.7 cm from 1974 to 1987. Long-term variability with a timescale of at least 5 years is seen at 20 cm and there is evidence for more rapid variations at the shorter wavelengths. The variability timescales at 20, 11 and 6 cm fit the λ2 scaling predicted by the theory of refractive scintillation suggesting that the variability could be due to this cause. However, the timescales are relatively short, implying an unusually high velocity in the scattering screen. The modulation index of the variability is large and relatively independent of wavelength.

scholarly journals The surprisingly small impact of magnetic fields on the inner accretion flow of Sagittarius A* fueled by stellar winds

2019 ◽  
Vol 492 (3) ◽  
pp. 3272-3293 ◽  
Author(s):  
S M Ressler ◽  
E Quataert ◽  
J M Stone
Keyword(s):  
Angular Momentum ◽  
Magnetic Fields ◽  
Event Horizon ◽  
Initial Conditions ◽  
Rotational Instability ◽  
Stellar Winds ◽  
Accretion Flow ◽  
Sagittarius A ◽  
Mhd Simulations ◽  
Sgr A

ABSTRACT We study the flow structure in 3D magnetohydrodynamic (MHD) simulations of accretion on to Sagittarius A* via the magnetized winds of the orbiting Wolf–Rayet stars. These simulations cover over 3 orders of magnitude in radius to reach ≈300 gravitational radii, with only one poorly constrained parameter (the magnetic field in the stellar winds). Even for winds with relatively weak magnetic fields (e.g. plasma β ∼ 106), flux freezing/compression in the inflowing gas amplifies the field to β ∼ few well before it reaches the event horizon. Overall, the dynamics, accretion rate, and spherically averaged flow profiles (e.g. density, velocity) in our MHD simulations are remarkably similar to analogous hydrodynamic simulations. We attribute this to the broad distribution of angular momentum provided by the stellar winds, which sources accretion even absent much angular momentum transport. We find that the magneto-rotational instability is not important because of (i) strong magnetic fields that are amplified by flux freezing/compression, and (ii) the rapid inflow/outflow times of the gas and inefficient radiative cooling preclude circularization. The primary effect of magnetic fields is that they drive a polar outflow that is absent in hydrodynamics. The dynamical state of the accretion flow found in our simulations is unlike the rotationally supported tori used as initial conditions in horizon scale simulations, which could have implications for models being used to interpret Event Horizon Telescope and GRAVITY observations of Sgr A*.

scholarly journals Class I Methanol Masers in the Galactic Center

2012 ◽  
Vol 8 (S287) ◽  
pp. 449-454
Author(s):  
Loránt O. Sjouwerman ◽  
Ylva M. Pihlström
Keyword(s):  
Galactic Center ◽  
Class I ◽  
Large Array ◽  
Maser Emission ◽  
Very Large Array ◽  
Sagittarius A ◽  
Wave Radio ◽  
Theoretical Predictions ◽  
Methanol Masers ◽  
Sgr A

AbstractWe report on 36 and 44 GHz Class I methanol (CH3OH) maser emission in the Sagittarius A (Sgr A) region with the Expanded Very Large Array (EVLA). At least three different maser transitions tracing shocked regions in the cm-wave radio regime can be found in Sgr A. 44 GHz masers correlate with the positions and velocities of 36 GHz CH3OH masers, but the methanol masers correlate less with 1720 MHz OH masers. Our results agree with theoretical predictions that the densities and temperatures conducive for 1720 MHz OH masers may also produce 36 and 44 GHz CH3OH maser emission. However, many 44 GHz masers do not overlap with 36 GHz methanol masers, suggesting that 44 GHz masers also arise in regions too hot and too dense for 36 GHz masers to form. This agrees with the non-detection of 1720 MHz OH masers in the same area, which are thought to be excited under cooler or denser conditions. We speculate that the geometry of the bright 36 GHz masers in Sgr A East outlines the location of a SNR shock front.

scholarly journals Exploring plasma evolution during Sagittarius A* flares

2013 ◽  
Vol 9 (S303) ◽  
pp. 293-297
Author(s):  
S. Dibi ◽  
S. Markoff ◽  
R. Belmont ◽  
J. Malzac ◽  
N. M. Barrière ◽  
...  
Keyword(s):  
Particle Distribution ◽  
Magnetized Plasma ◽  
Flare Event ◽  
X Ray ◽  
Sagittarius A ◽  
Self Consistent ◽  
Infra Red ◽  
Sgr A ◽  
The Magnetic Field ◽  
Specific Particle

AbstractWe present a new way of describing the flares occurring from Sgr A* within a single zone with a self-consistent calculation of the particle distribution. The results allow us to give an interpretation to the flaring events generated very close to the supermassive black hole (SMBH) without assuming a specific particle distribution. We conclude that the flare data are more likely generated by a weakly magnetized plasma in which the particles flow in and out as expected from an accretion flow. Such a plasma, with prescription for non-thermal acceleration, injection, escape, and cooling losses, gives a spectrum with a break between the infra-red and the X-ray, allowing a better simultaneous match in the different wavelengths. The parameters favor the non-thermal synchrotron spectrum, and a decrease/increase of the magnetic field and plasma density are not favored for producing the flare event, but particle acceleration must be happening by other means. We show that under certain conditions, the real particle distribution can differ significantly from the standard distributions used in such studies.

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