scholarly journals 10.5. Short-term variability of Sagittarius A∗ at millimeter wavelengths

1998 ◽  
Vol 184 ◽  
pp. 439-440
Author(s):  
T. Tsutsumi ◽  
T. Kawabata ◽  
A. Miyazaki ◽  
M. Tsuboi
Keyword(s):  
Monitoring Program ◽  
Time Variability ◽  
Very Large Array ◽  
Sagittarius A ◽  
Compact Component ◽  
Millimeter Wavelengths ◽  
Submillimeter Wavelengths ◽  
Extended Component ◽  
Sgr A ◽  
Millimeter And Submillimeter Wavelengths

Time variability of Sgr A∗ at cm wavelengths is well known phenomenon and has been studied by several authors (e.g. Lo 1989; Zhao et al. 1992). On the other hand, variability at millimeter wavelengths is not well established. Wright & Backer (1993) reported significant flux variations at λ = 3.4mm in a month during the decay of a flare observed by the Very Large Array in 1990. However others found no variability at millimeter and submillimeter wavelengths (Gwinn et al. 1991; Zylka et al. 1995). Emission from extended component surrounding Sgr A∗ is significant even at millimeter wavelengths (e.g. Tsuboi et al. 1988), thus it is important to observe with a narrower beam to isolate the compact component for accurate measurement of variability of Sgr A∗. Here we present some results from an on-going millimeter monitoring program of Sgr A∗ with the Nobeyama Millimeter Array (NMA).

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 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 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 Near infrared variability of Sgr A* - spectral index measurements

2013 ◽  
Vol 9 (S303) ◽  
pp. 274-282 ◽  
Author(s):  
G. Witzel ◽  
M. Morris ◽  
A. Ghez ◽  
L. Meyer ◽  
E. Becklin ◽  
...  
Keyword(s):  
Spectral Index ◽  
Near Infrared ◽  
Time Lag ◽  
Time Variable ◽  
Time Variability ◽  
Data Set ◽  
Sagittarius A ◽  
State Process ◽  
Sgr A ◽  
High Cadence

AbstractWe discuss observations of Sagittarius A* with NACO@VLT in K-band and recent synchronous observations with NIRC2@Keck II and OSIRIS@Keck I in L′-band and H-band, respectively. The variability of Sagittarius A* in the near infrared is a continuous one-state process that can be described by a pure red-noise process having a timescale of a few hours. We describe this process and its properties in detail. Our newest observations with the Keck telescopes represent the first truly synchronous high cadence data set to test for time variability of the spectral index within the near infrared. We discovered a time-variable spectral index that might be interpreted as a time lag of the L′-band with respect to the H-band.

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 Monitoring observations of the interaction between Sgr A* and G2 with the Karl G. Jansky Very Large Array

2013 ◽  
Vol 9 (S303) ◽  
pp. 327-329 ◽  
Author(s):  
Loránt O. Sjouwerman ◽  
Claire J. Chandler
Keyword(s):  
Community Service ◽  
Monitoring Program ◽  
Monthly Interval ◽  
Data Archive ◽  
Large Array ◽  
Web Page ◽  
Very Large Array ◽  
Monitoring Effort ◽  
Sgr A ◽  
Science Service

AbstractWe report on an ongoing community service observing program to follow the expected encounter of the G2 cloud with the black hole Sgr A* in 2013. The NRAO Karl G. Jansky Very Large Array (VLA) has been observing the Sgr A region since 2012 October on roughly a bi-monthly interval, each for two hours, cycling through eight observing bands at their default continuum frequencies, using 2 GHz of bandwidth. The data from the monitoring program are publicly available through the NRAO data archive immediately after observing has completed, and the flux densities are published by NRAO staff as soon as the data are reduced. The cumulative results of the monitoring effort are posted on the service observing web page https://science.nrao.edu/science/service-observing and so far do not indicate a significant brightening of the emission from the direction of Sgr A* over the period 2012 October to 2013 September, within the calibration uncertainties.

scholarly journals Profiled Horn Antenna with Wideband Capability Targeting Sub-THz Applications

Electronics ◽  
2021 ◽  
Vol 10 (4) ◽  
pp. 412
Author(s):  
Jay Gupta ◽  
Dhaval Pujara ◽  
Jorge Teniente
Keyword(s):  
Hermite Polynomial ◽  
Polynomial Interpolation ◽  
Horn Antenna ◽  
Submillimeter Wavelengths ◽  
Thz Applications ◽  
Millimeter And Submillimeter Wavelengths ◽  
Better Than

This paper proposes a wideband profiled horn antenna designed using the piecewise biarc Hermite polynomial interpolation and validated experimentally at 55 GHz. The proposed design proves S11 and directivity better than −22 dB and 25.5 dB across the entire band and only needs 3 node points if compared with the well-known spline profiled horn antenna. Our design makes use of an increasing radius and hence does not present non-accessible regions from the aperture, allowing its fabrication with electro erosion techniques especially suitable for millimeter and submillimeter wavelengths.

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