scholarly journals First Space-VLBI Observations of Sagittarius A*

2021 ◽  
Vol 922 (2) ◽  
pp. L28
Author(s):  
Michael D. Johnson ◽  
Yuri Y. Kovalev ◽  
Mikhail M. Lisakov ◽  
Petr A. Voitsik ◽  
Carl R. Gwinn ◽  
...  
Keyword(s):  
Angular Resolution ◽  
Galactic Center ◽  
Global Network ◽  
Sagittarius A ◽  
Vlbi Observations ◽  
Submillimeter Wavelengths ◽  
Short Baselines ◽  
Sgr A ◽  
Strong Scattering ◽  
Long Baselines

Abstract We report results from the first Earth-space VLBI observations of the Galactic Center supermassive black hole, Sgr A*. These observations used the space telescope Spektr-R of the RadioAstron project together with a global network of 20 ground telescopes, observing at a wavelength of 1.35 cm. Spektr-R provided baselines up to 3.9 times the diameter of the Earth, corresponding to an angular resolution of approximately 55 μas and a spatial resolution of 5.5R Sch at the source, where R Sch ≡ 2GM/c 2 is the Schwarzschild radius of Sgr A*. Our short ground baseline measurements ( ≲ 80 Mλ) are consistent with an anisotropic Gaussian image, while our intermediate ground baseline measurements (100–250 Mλ) confirm the presence of persistent image substructure in Sgr A*. Both features are consistent with theoretical expectations for strong scattering in the ionized interstellar medium, which produces Gaussian scatter-broadening on short baselines and refractive substructure on long baselines. We do not detect interferometric fringes on any of the longer ground baselines or on any ground–space baselines. While space-VLBI offers a promising pathway to sharper angular resolution and the measurement of key gravitational signatures in black holes, such as their photon rings, our results demonstrate that space-VLBI studies of Sgr A* will require sensitive observations at submillimeter wavelengths.

scholarly journals mm-VLBI: Jets in the Vicinity of Galaxy-Cores

1994 ◽  
Vol 159 ◽  
pp. 187-188 ◽  
Author(s):  
T.P. Krichbaum ◽  
K.J. Standke ◽  
D.A. Graham ◽  
A. Witzel ◽  
C.J. Schalinski ◽  
...  
Keyword(s):  
Angular Resolution ◽  
Galactic Center ◽  
Large Fraction ◽  
Apparent Velocity ◽  
Vlbi Observations ◽  
Curved Trajectories ◽  
Superluminal Motion ◽  
Increased Sensitivity ◽  
Galaxy Cores ◽  
Sgr A

Millimeter-VLBI provides an angular resolution of up to a few tens of microarcseconds and allows imaging of compact radio sources, self-absorbed at longer wavelengths, with unsurpassed angular resolution. At 43 GHz the participation of the VLBA and the 30 m-MRT at Pico Veleta (e.g. Krichbaum et al., 1993 a&b), and at 86 GHz the addition of the 100 m-RT at Effelsberg and the 30 m-MRT (Schalinski et al., 1993, and this volume) have improved the imaging capabilities of mm-VLBI observations.Results: The increased sensitivity of mm-VLBI observations allows the investigation of fainter objects, previously not accessible. As one example we show in Fig.1 the first detection of the compact radio source Sgr A∗ in the Galactic Center with VLBI at 43 GHz in May 1992 (Krichbaum et al., 1993d) and at 86 GHz in April 1993 (Krichbaum et al., 1994). In both observations the size of Sgr A∗ appeared to be larger than its expected scattering size, indicative of intrinsic source structure showing up at mm-wavelengths. Future monitoring with mm-VLBI is necessary to search for (not unexpected) structural variability.Monitoring of AGN with mm-VLBI reveals in all cases observed in sufficient detail jet curvatures of increasing amplitude towards the self-absorbed VLBI-cores (e.g. in 18034-784: Krichbaum, 1990, OJ 287: Krichbaum et al., 1993c), and sub- or superluminal motion along ‘quasi-helically’ bent trajectories (e.g. 3C 84: Krichbaum et al., 1993b; 3C 273: Krichbaum et al., 1993c), which differ sometimes for adjacent jet components (e.g. 3C 345: Krichbaum & Witzel, 1992, Krichbaum et al., 1992&1993a). In 3C 84, 3C 273 and 3C 345 the apparent velocity of jet components varies systematically along the jet axis, in 4C 39.25 (Alberdi et al., 1993) a moving component decelerates and brightens, all of this suggesting differential Doppler boosting and motion along three-dimensionally curved trajectories. In 3C 345 the complex kinematics of C4 and C5 (Zensus, this volume) has been geometrically modeled by motion along a helical path on the surface of a conical jet (Qian et al., 1992, Steffen et al., 1993, and this volume; see also Camenzind, this volume). As a new example, the oscillations of the inner jet and its velocity variations βapp(r) are shown for the BL Lac object 1803+784 in Fig. 2 (see the maps in: Krichbaum et al., 1993b). The frequent occurence of ‘quasi-sinusoidal’ bends in the inner jets of very different classes of AGN (QSO's, BL Lac's, Seyfert's) suggests that this effect is common in a large fraction of AGN and that the underlying jet-physical process may be fundamental for the understanding of the creation of jets.

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 Detailed distributions of the CO J = (2 − 1)/J = (1 − 0) intensity ratios toward a large area of the central molecular zone

2013 ◽  
Vol 9 (S303) ◽  
pp. 106-108
Author(s):  
Kazufumi Torii ◽  
Rei Enokiya ◽  
Yasuo Fukui ◽  
Hiroaki Yamamoto ◽  
Akiko Kawamura ◽  
...  
Keyword(s):  
Angular Resolution ◽  
Galactic Center ◽  
Galactic Plane ◽  
Area Coverage ◽  
High Angular Resolution ◽  
Molecular Gas ◽  
Large Area ◽  
First Results ◽  
Intensity Ratios ◽  
Sgr A

AbstractWe present the first results of the new CO J = (2 − 1) observations toward the central molecular zone (CMZ) using the NANTEN2 telescope at an angular resolution of 100″. Large area coverage of 4° × 2° in l and b and a high angular resolution of 100″ enable us to investigate detailed structures of the molecular gas in the CMZ including peculiar molecular filaments perpendicularly to the Galactic plane to b > |0.5°|. The major components of the CMZ, e.g., Sgr A, Sgr B and Sgr C cloud complexes, show high CO J = (2 − 1)/J = (1 − 0) ratios around 0.9, indicating highly excited conditions of the molecular gas, while the local foreground components show less than 0.4. The molecular filaments show the typical ratios of 0.6–0.7 indicate that they are indeed located in the Galactic center.

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.

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 Quantum gravity effects around Sagittarius A*

2016 ◽  
Vol 25 (12) ◽  
pp. 1644021 ◽  
Author(s):  
Hal M. Haggard ◽  
Carlo Rovelli
Keyword(s):  
Black Hole ◽  
Quantum Gravity ◽  
Event Horizon ◽  
Sagittarius A ◽  
Vlbi Observations ◽  
Gravity Effects ◽  
Sgr A ◽  
Schwarzschild Radius ◽  
Number Of Authors

Recent VLBI observations have resolved Sagittarius A* at horizon scales. The event horizon telescope is expected to provide increasingly good images of the region around the Schwarzschild radius [Formula: see text] of Sgr A* soon. A number of authors have recently pointed out the possibility that nonperturbative quantum gravitational phenomena could affect the space surrounding a black hole. Here, we point out that the existence of a region around [Formula: see text], where these effects should be maximal.

scholarly journals Micro-arcsecond structure of Sagittarius A∗ revealed by high-sensitivity 86 GHz VLBI observations

2019 ◽  
Vol 621 ◽  
pp. A119 ◽  
Author(s):  
Christiaan D. Brinkerink ◽  
Cornelia Müller ◽  
Heino D. Falcke ◽  
Sara Issaoun ◽  
Kazunori Akiyama ◽  
...  
Keyword(s):  
High Sensitivity ◽  
Galactic Centre ◽  
Model Parameters ◽  
Sagittarius A ◽  
Long Baseline ◽  
Vlbi Observations ◽  
Size Frequency ◽  
Frequency Relation ◽  
Sgr A ◽  
Source Geometry

Context. The compact radio source Sagittarius A∗ (Sgr A∗) in the Galactic centre is the primary supermassive black hole candidate. General relativistic magnetohydrodynamical (GRMHD) simulations of the accretion flow around Sgr A∗ predict the presence of sub-structure at observing wavelengths of ∼3 mm and below (frequencies of 86 GHz and above). For very long baseline interferometry (VLBI) observations of Sgr A∗ at this frequency the blurring effect of interstellar scattering becomes sub-dominant, and arrays such as the high sensitivity array (HSA) and the global mm-VLBI array (GMVA) are now capable of resolving potential sub-structure in the source. Such investigations help to improve our understanding of the emission geometry of the mm-wave emission of Sgr A∗, which is crucial for constraining theoretical models and for providing a background to interpret 1 mm VLBI data from the Event Horizon Telescope (EHT). Aims. Following the closure phase analysis in our first paper, which indicates asymmetry in the 3 mm emission of Sgr A∗, here we have used the full visibility information to check for possible sub-structure. We extracted source size information from closure amplitude analysis, and investigate how this constrains a combined fit of the size-frequency relation and the scattering law for Sgr A∗. Methods. We performed high-sensitivity VLBI observations of Sgr A∗ at 3 mm using the Very Long Baseline Array (VLBA) and the Large Millimeter Telescope (LMT) in Mexico on two consecutive days in May 2015, with the second epoch including the Greenbank Telescope (GBT). Results. We confirm the asymmetry for the experiment including GBT. Modelling the emission with an elliptical Gaussian results in significant residual flux of ∼10 mJy in south-eastern direction. The analysis of closure amplitudes allows us to precisely constrain the major and minor axis size of the main emission component. We discuss systematic effects which need to be taken into account. We consider our results in the context of the existing body of size measurements over a range of observing frequencies and investigate how well-constrained the size-frequency relation is by performing a simultaneous fit to the scattering law and the size-frequency relation. Conclusions. We find an overall source geometry that matches previous findings very closely, showing a deviation in fitted model parameters less than 3% over a time scale of weeks and suggesting a highly stable global source geometry over time. The reported sub-structure in the 3 mm emission of Sgr A∗ is consistent with theoretical expectations of refractive noise on long baselines. However, comparing our findings with recent results from 1 mm and 7 mm VLBI observations, which also show evidence for east-west asymmetry, we cannot exclude an intrinsic origin. Confirmation of persistent intrinsic substructure will require further VLBI observations spread out over multiple epochs.

scholarly journals OH in the Environment of Sgr A

1989 ◽  
Vol 136 ◽  
pp. 421-422
Author(s):  
Aa. Sandqvist ◽  
R. Karlsson ◽  
J. B. Whiteoak
Keyword(s):  
Radial Velocity ◽  
Shell Structure ◽  
Spiral Structure ◽  
Angular Resolution ◽  
Galactic Center ◽  
Absorption Lines ◽  
Molecular Gas ◽  
Center Region ◽  
Circumnuclear Disk ◽  
Sgr A

The 18-cm distribution of OH in the Galactic Center region near Sgr A has been mapped in all four of the 1612, 1665, 1667 and 1720 MHz OH absorption lines using the VLA with 4 arcsec angular resolution and 9 kms-1 velocity resolution. The OH gas at +50 and +20 kms-l is seen clearly in absorption against the shell structure of Sgr A East but not against the spiral structure of Sgr A West, possibly implying that this molecular gas lies between the two continuum components - behind Sgr A West and in front of Sgr A East. Inside the Circumnuclear Disk, there is a new neutral streamer which sweeps from the disk in towards Sgr A∗ as the observed radial velocity decreases from +78 to +16 kms-1. The streamer may have a negative-velocity counterpart on the opposite side of Sgr A∗.

scholarly journals The reflection of two past outbursts of Sagittarius A* observed by Chandra during the last decade

2013 ◽  
Vol 9 (S303) ◽  
pp. 344-348
Author(s):  
M. Clavel ◽  
R. Terrier ◽  
A. Goldwurm ◽  
M. R. Morris ◽  
G. Ponti ◽  
...  
Keyword(s):  
Black Hole ◽  
Galactic Center ◽  
Line Of Sight ◽  
High Resolution Data ◽  
The Past ◽  
Sagittarius A ◽  
Cloud Parameters ◽  
Angular Scale ◽  
Sgr A ◽  
First Time

AbstractThe supermassive black hole at the Galactic center, Sagittarius A* has experienced periods of higher activity in the past. The reflection of these past outbursts is observed in the molecular material surrounding the black hole but reconstructing its precise lightcurve is difficult since the distribution of the clouds along the line of sight is poorly constrained.Using Chandra high-resolution data collected from 1999 to 2011 we studied both the 6.4 keV and the 4–8 keV emission of the region located between Sgr A* and the Radio arc, characterizing its variations down to 15″ angular scale and 1-year time scale. The emission from the molecular clouds in the region varies significantly, showing either a 2-year peaked emission or 10-year linear variations. This is the first time that such fast variations are measured. Based on the cloud parameters, we conclude that these two behaviors are likely due to two distinct past outbursts of Sgr A* during which its luminosity rose to at least 1039 erg s−1.

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