On 19/04/2019 22:01, Martin Brown wrote:
[snip]
I know it is bad form to reply to one's own post but here goes.
Is there any prospect of computing a somewhat larger image zoomed out by
a factor of 3, 10 or 100 from the existing VLBI dataset?
The initial rough image on her facebook page has a tantalising point
source just north of the ring and about one diameter away.
https://www.facebook.com/photo.php?fbid=10213321822457967&set=a.1407432103727&type=3&theater
The final published image was perhaps a bit close cropped.
A couple of things occur to me. M87 spin axis is pretty much pointed
towards us with the best estimate of 17 degrees off line of sight. So we
are in effect looking down into the throat of the jet engine.
(ignoring for the moment the huge GR ray tracing distortions)
Are there any EHT candidate radio galaxies near enough to image with the
spin axis perpendicular to our line of sight? Cygnus A is too far away.
Would the EHT be capable of taking a look at a starburst galaxy like M82
and making sense of the various odd compact objects lurking in there?
I'm guessing most of them would be in the beam of most of the antennae.
Or even closer to home could EHT do M1 the crab nebula and look into a
much smaller accretion disk very much closer to home. I guess temporal variations in the emission might stymie any such attempt.
I presume SgrA* has caused problems because its emissions were varying
during the observations. Perhaps that limits the technique to a mere
handful of super massive black holes in relatively nearby galaxies.
It is an impressive achievement to image the accretion disk/black hole shadow. It looks remarkably like the theoretical model predictions.
][[Mod. note -- Getting either higher resolution, or a wider field of
]view (probably at lower resolution) would be somewhat difficult. The
Getting any higher resolution would be impossible. They have already
pushed the data just about as far as it will go in that direction.
I don't see why they can't map a slightly wider region though. It will obviously look rather scrappy due to the sparse VLBI u-v coverage. The
zone around the phase centre should be OK for a few mas or so.
]problem is that (oversimplifying a bit), the observations measure the
]2-D Fourier transform of the sky brightness, at spatial frequencies
]given by the projections of each antenna-to-antenna baseline onto the
]sky plane. These projections are time-dependent due to the Earth's
]rotation.
I should perhaps declare an interest in that long ago I wrote software
for aperture synthesis and I have followed M87 jet VLBI for a while.
]So, given a small finite set of radio telescopes, and a finite time
]span of observations, one gets measurements along only a finite set
]of "tracks" in the spatial-frequency plane. These tracks are shown
]in Figure 2 of the EHT collaboration's Paper I
](
https://iopscience.iop.org/article/10.3847/2041-8213/ab0ec7 ).
Looking at that u-v coverage it strikes me that a larger image with some horrendous hexagonal artefacts ought to be possible. It may be that the emissions at 1.3mm are just too faint other than in the accretion disk.
Here is the earlier 3mm 100GHz VLBI series of results published in 2016
for a somewhat wider field of view:
https://arxiv.org/pdf/1609.07896.pdf
I was hoping that the new 1.3mm dataset would have been just about
sufficient to image a region approximately one third of that size IOW
the BH and the first hotspot/plateau at the very start of the jets.
Also similar results at 86GHz at 3.5mm:
https://iopscience.iop.org/article/10.3847/0004-637X/817/2/131/pdf
There is also the VLBI movie at twice that wavelength 7mm 43GHz:
http://www.aoc.nrao.edu/~cwalker/M87/M87_movies_only.html
I guess something suddenly gets much tougher at the shortest wavelength
- in some ways it is astonishing that they can make it work at all.
]Since interferometry is only possible with *simultaneous* observation
]from different telescopes, it's restricted to times when the source
]is simultaneously above the horizon for all the telescopes. So making
]the individual spatial-frequency tracks longer by observing for longer ]periods is probably impossible.
]Thus, getting data at other spatial frequencies basically requires
]finding (and getting time on) additional radio telescopes (with
]suitable properties for these observations) in other parts of the
]world, beyond those already used for these observations. That's
]possible, but hard -- there aren't very many big millimeter-wave
]radio telescopes in the world.
]-- jt]]
Agreed. But I am a bit puzzled what the practical differences are
between ETH operations at 1.3mm and the earlier 3mm VLBI work.
The ETH processing has concentrated on absolute maximum resolution of
fine detail in the highest signal to noise region to get that amazing
image of the accretion disk/shadow. But once they have a basic phase
solution why can't they make a crude image of a slightly wider region?
I'm surprised that there hasn't been any further discussion of the the
M87 results here beyond your own moderator's notes (for which thanks)...
--
Regards,
Martin Brown
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