J/A+A/621/A119 SgrA* and NRAS530 86GHz images (Brinkerink+, 2019)
Micro-arcsecond structure of Sagittarius A* revealed by high-sensitivity 86 GHz
VLBI observations.
Brinkerink C.D., Mueller C., Falcke H.D., Issaoun S., Akiyama K.,
Bower G.C., Krichbaum T.P., Deller A.T., Castillo E., Doeleman S.S.,
Fraga-Encinas R., Goddi C., Hernandez-Gomez A., Hughes D.H., Kramer M.,
Leon-Tavares J., Loinard L., Montana A., Moscibrodzka M., Ortiz-Leon G.N.,
Sanchez-Arguelles D., Tilanus R.P.J., Wilson G.W., Zensus A.
<Astron. Astrophys. 621, A119 (2019)>
=2019A&A...621A.119B 2019A&A...621A.119B (SIMBAD/NED BibCode)
ADC_Keywords: Radio sources ; Accretion ; Interferometry
Keywords: accretion - accretion disks - black hole physics - scattering -
techniques: high angular resolution - techniques: interferometric -
radio continuum: galaxies
Abstract:
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 ∼3mm and below (frequencies of 86GHz 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 1mm VLBI data from the Event Horizon Telescope (EHT).
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*.
We performed high-sensitivity VLBI observations of Sgr A* at 3mm
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).
We confirm the asymmetry for the experiment including GBT. Modelling
the emission with an elliptical Gaussian results in significant
residual flux of ∼10mJy 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.
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 3mm
emission of Sgr A* is consistent with theoretical expectations of
refractive noise on long baselines. However, comparing our findings
with recent results from 1mm and 7mm 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.
Description:
We performed 86 GHz VLBI observations of Sgr A*. Here we present the
analysis of two datasets: one epoch using the VLBA (all 86GHz capable
stations1) together with the LMT (project code: BF114A) on May 22nd,
2015, and one epoch using VLBA, LMT, and GBT on May 23rd, 2015
(project code: BF114B). Both observations were made in left-circular
polarization mode only, at a centre frequency of 86.068 GHz and a
sampling rate of 2 Gbps (512 MHz on-sky bandwidth). For fringe finding
we used the primary calibrators 3C 279 and 3C 454.3 at the start and
end of the track respectively. In between, the scans alternated every
5-min between Sgr A* and the secondary fringe finder NRAO 530 ([HB89]
1730-130 with short regular gaps (every ∼30min) for pointing and
longer GBT-only gaps every ∼4h for focusing.
Objects:
---------------------------------------------------
RA (2000) DE Designation(s)
---------------------------------------------------
17 33 02.71 -13 04 49.5 NRAO 530 = QSO B1730-130
17 45 40.04 -29 00 28.2 Sgr A* = NAME Sgr A*
---------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 157 4 List of fits images
fits/* . 4 Individual fits images
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See also:
J/MNRAS/428/2731 : SgrA* emission at 7mm (Beaklini+, 2013)
J/A+A/618/A35 : ALMA images of the CND and Sgr A* (Goicoechea+, 2018)
J/A+A/601/A80 : LABOCA 345GHz observations of Sgr A* (Subroweit+, 2017)
J/MNRAS/418/2260 : VLBI observations of NRAO530 at 22, 43 and 86GHz (Lu+, 2011)
Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 22 I3 --- Nx Number of pixels along X-axis
24- 26 I3 --- Ny Number of pixels along Y-axis
28- 37 A10 "date" Obs.date Observation date
39- 45 F7.4 GHz Obs.Freq Observed frequency
47- 50 I4 Kibyte size Size of FITS file
52- 70 A19 --- FileName Name of FITS file, in subdirectory fits
72-157 A86 --- Title Title of the FITS file
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Acknowledgements:
References:
(End) Patricia Vannier [CDS] 29-Jan-2019