APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

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APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by APOD Robot » Sun Oct 17, 2021 4:05 am

Image The Einstein Cross Gravitational Lens

Explanation: Most galaxies have a single nucleus -- does this galaxy have four? The strange answer leads astronomers to conclude that the nucleus of the surrounding galaxy is not even visible in this image. The central cloverleaf is rather light emitted from a background quasar. The gravitational field of the visible foreground galaxy breaks light from this distant quasar into four distinct images. The quasar must be properly aligned behind the center of a massive galaxy for a mirage like this to be evident. The general effect is known as gravitational lensing, and this specific case is known as the Einstein Cross. Stranger still, the images of the Einstein Cross vary in relative brightness, enhanced occasionally by the additional gravitational microlensing effect of specific stars in the foreground galaxy.

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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by Ann » Sun Oct 17, 2021 6:49 am


Right, today's Einstein Cross is nice, but will someone help me make sense of all the lensed stuff and the Einstein Cross in connection with the Refsdal supernova?

Refsdal supernova galaxy being mulitply lensed by foreground elliptical galaxy.png
A spiral galaxy (in red circles) being multiply lensed by a foreground elliptical galaxy.
NASA, ESA, S. Rodney and the FrontierSN team; T. Treu, P. Kelly and the GLASS team;
J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team;
and Z. Levay (STScI)

All right. A distant spiral galaxy has been lensed into multiple images by a foreground galaxy cluster. A supernova was spotted in 2014 in one of the lensed images of the spiral galaxy. Astronomers calculated that this same supernova must have shown up in another of the lensed images of the galaxy back in 1995, but it wasn't observed back then. However, the astronomers also calculated that the supernova would soon show up in a third of the lensed images, and it did so right on time. The reason why the supernova showed up at different times in the different lensed images is that the paths that light takes around the foreground lensing galaxy cluster are of different lengths, and the image that took the shortest path will show its supernova first, the middle-length image will be the next one to show its supernova, and the one that took the longest path will be the slowest one to show its supernova.

All right. But what the heck is this???

Refsdal supernova Einstein Cross.png

Guess what? It is the same supernova, the Refsdal supernova, showing up in a flour-leaf clover pattern around an elliptical galaxy in the lensing foreground galaxy cluster! And this Einstein Cross version of the supernova could be seen at the same time as a lensed version of the same supernova was seen in one of the "detached" versions of the spiral galaxy!!

How amazing is that???

(Oh, and... as a Color Commentator, I must say something about the fact that the Refsdal supernova is much yellower than the spiral arms of the galaxy in which it exploded. A likely explanation for the yellow color is that the supernova exploded in a dusty environment, so that its color was reddened by dust. But I also find it likely that the supernova was a core-collapse one, because it seems to me that such supernovas are very often yellow in color (think of SN 1987A in the Large Magellanic Cloud, for example), whereas supernovas type Ia are typically blue.)

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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by orin stepanek » Sun Oct 17, 2021 12:05 pm

qso2237_wiyn_1024.jpg
Am I actually seeing a center dot of light?
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Got a Group of kitties today1 :lol2:
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by Astronymus » Sun Oct 17, 2021 12:47 pm

Hm, why isn't it a ring? I forgot.
orin stepanek wrote:
Sun Oct 17, 2021 12:05 pm
Am I actually seeing a center dot of light?
There is a galaxy core in the center.
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by Ann » Sun Oct 17, 2021 1:04 pm

Astronymus wrote:
Sun Oct 17, 2021 12:47 pm
Hm, why isn't it a ring? I forgot.
orin stepanek wrote:
Sun Oct 17, 2021 12:05 pm
Am I actually seeing a center dot of light?
There is a galaxy core in the center.
I think that point sources are likely to be lensed as multiple point sources.

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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by neufer » Sun Oct 17, 2021 2:12 pm

Astronymus wrote:
Sun Oct 17, 2021 12:47 pm

Hm, why isn't it a ring? I forgot.
  • It is not centered properly.
https://en.wikipedia.org/wiki/Einstein_Cross wrote: <<The Einstein Cross (Q2237+030 or QSO 2237+0305) is a gravitationally lensed quasar found in Pegasus that sits directly behind ZW 2237+030, Huchra's Lens. Four images of the same distant quasar (plus one in the centre, too dim to see) appear around a foreground galaxy due to strong gravitational lensing. This system was discovered by John Huchra and coworkers in 1985. While gravitationally lensed light sources are often shaped into an Einstein ring, due to the elongated shape of the lensing galaxy and the quasar being off-centre, the images form a peculiar cross-shape instead.

The quasar's redshift indicates that it is located about 8 billion light years from Earth, while the lensing galaxy is at a distance of 400 million light years. The apparent dimensions of the entire foreground galaxy are 0.87 × 0.34 arcminutes, while the apparent dimension of the cross in its centre accounts for only 1.6 × 1.6 arcseconds. Amateur astronomers are able to see some of the cross using telescopes; however, it requires extremely dark skies and telescope mirrors with diameters of 18 inches (46 cm) or greater.>>
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by johnnydeep » Sun Oct 17, 2021 6:57 pm

First, Ann's post about the Refsdal supernova's gravitational lensing is truly amazing. And as Ann might be, I'm not too clear about what's really going on there.

Second, the "this image" link in the APOD desc has this to say:
This picture of the gravitationally lensed quasar Q2237+0305 and the associated lensing spiral galaxy was taken by the 3.5-meter WIYN telescope, on the night of October 4, 1999. This system is also known as Huchra's Lens, after its discoverer, and the Einstein Cross, because it is such an excellent example of the phenomenon of gravitational lensing, postulated by Einstein as soon as he realised that gravity would be able to bend light and thus could have lens-like effects. The four separate appearances of the same redshift 1.7 quasar are created by the redshift 0.04 galaxy whose nucleus is nicely bracketed by the quasar images. It might seem surprising that such a close alignment exists, with a galaxy exactly along the line of sight from Earth to a distant quasar, but one should remember that the Universe is large enough that unlikely things happen really quite often. This is an especially important example of a gravitational lens, because the close alignment of the galaxy nucleus and the quasar mean that the four images undergo color and brightness variations with a time scale of only a day or so. These changes can be modelled theoretically and easily monitored observationally. This is a two-color picture combining red and green images, using careful processing both to reveal the strongly blue nature of the quasar, as compared to the galaxy, and to show simultaneously the very bright quasar images and the very faint structure of the lensing galaxy.
But I don't understand the bold blue statement. Why is there such a short brightness variation cycle? Surely it can't be due to the changing positions of the quasar and the foreground galaxy over a brief 24 hours - can it?

Third, like Orin said, I think I can see the foreground galaxy's core despite the text saying it's not visible. But maybe that's an illusion caused by the four lensed images overlapping in the center.
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by bystander » Sun Oct 17, 2021 10:24 pm

johnnydeep wrote:
Sun Oct 17, 2021 6:57 pm

Third, like Orin said, I think I can see the foreground galaxy's core despite the text saying it's not visible. But maybe that's an illusion caused by the four lensed images overlapping in the center.
APOD Robot wrote:
Sun Oct 17, 2021 4:05 am
...
The gravitational field of the visible foreground galaxy ...
But it does say the foreground galaxy is visible ...
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by alter-ego » Sun Oct 17, 2021 11:13 pm

johnnydeep wrote:
Sun Oct 17, 2021 6:57 pm
First, Ann's post about the Refsdal supernova's gravitational lensing is truly amazing. And as Ann might be, I'm not too clear about what's really going on there.

Second, the "this image" link in the APOD desc has this to say:
This picture of the gravitationally lensed quasar Q2237+0305 and the associated lensing spiral galaxy was taken by the 3.5-meter WIYN telescope, on the night of October 4, 1999. This system is also known as Huchra's Lens, after its discoverer, and the Einstein Cross, because it is such an excellent example of the phenomenon of gravitational lensing, postulated by Einstein as soon as he realised that gravity would be able to bend light and thus could have lens-like effects. The four separate appearances of the same redshift 1.7 quasar are created by the redshift 0.04 galaxy whose nucleus is nicely bracketed by the quasar images. It might seem surprising that such a close alignment exists, with a galaxy exactly along the line of sight from Earth to a distant quasar, but one should remember that the Universe is large enough that unlikely things happen really quite often. This is an especially important example of a gravitational lens, because the close alignment of the galaxy nucleus and the quasar mean that the four images undergo color and brightness variations with a time scale of only a day or so. These changes can be modelled theoretically and easily monitored observationally. This is a two-color picture combining red and green images, using careful processing both to reveal the strongly blue nature of the quasar, as compared to the galaxy, and to show simultaneously the very bright quasar images and the very faint structure of the lensing galaxy.
But I don't understand the bold blue statement. Why is there such a short brightness variation cycle? Surely it can't be due to the changing positions of the quasar and the foreground galaxy over a brief 24 hours - can it?
...
Since the bold blue statement refers to the 4 images created by the gravitational lens, the path lengths for each image is different. When the quasar changes brightness (for whatever reason), each image will show that change after different propagation delays, which includes spectral changes too due to GR. This day-delay has nothing to do with relative changes between the quasar and foreground galaxy.
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by neufer » Mon Oct 18, 2021 2:56 am

alter-ego wrote:
Sun Oct 17, 2021 11:13 pm
johnnydeep wrote:
Sun Oct 17, 2021 6:57 pm

But I don't understand the bold blue statement. Why is there such a short brightness variation cycle? Surely it can't be due to the changing positions of the quasar and the foreground galaxy over a brief 24 hours - can it?
Since the bold blue statement refers to the 4 images created by the gravitational lens, the path lengths for each image is different. When the quasar changes brightness (for whatever reason), each image will show that change after different propagation delays, which includes spectral changes too due to GR. This day-delay has nothing to do with relative changes between the quasar and foreground galaxy.
https://en.wikipedia.org/wiki/Quasar wrote: <<Quasars' luminosities are variable, with time scales that range from months to hours. This means that quasars generate and emit their energy from a very small region, since each part of the quasar would have to be in contact with other parts on such a time scale as to allow the coordination of the luminosity variations. This would mean that a quasar varying on a time scale of a few weeks cannot be larger than a few light-weeks across. The emission of large amounts of power from a small region requires a power source far more efficient than the nuclear fusion that powers stars. The conversion of gravitational potential energy to radiation by infalling to a black hole converts between 6% and 32% of the mass to energy, compared to 0.7% for the conversion of mass to energy in a star like our Sun. It is the only process known that can produce such high power over a very long term. (Stellar explosions such as supernovas and gamma-ray bursts, and direct matter–antimatter annihilation, can also produce very high power output, but supernovae only last for days, and the universe does not appear to have had large amounts of antimatter at the relevant times.)>>
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by tomatoherd » Mon Oct 18, 2021 1:34 pm

Pardon my ignorance, but here's an image of an Einstein cross, but where are the images of UNlensed quasars? Or quasars lensed in the more typical arc/semi-circle figures? Or, if a point source would not create an arc, then just a single lensed quasar image?

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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by neufer » Mon Oct 18, 2021 1:46 pm


tomatoherd wrote:
Mon Oct 18, 2021 1:34 pm

Pardon my ignorance, but here's an image of an Einstein cross, but where are the images of UNlensed quasars? Or quasars lensed in the more typical arc/semi-circle figures? Or, if a point source would not create an arc, then just a single lensed quasar image?
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by tomatoherd » Mon Oct 18, 2021 1:59 pm

Thanks, Art.
But i meant where are they on APOD. Seems like we never have a day for quasars.
We have nebulae (and dust lanes) without end. Yes, they are more artistic. But to me (and it's my problem), their disorder causes me no awe. Just dust-bunnies of a different magnitude (yawn).

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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by johnnydeep » Mon Oct 18, 2021 2:14 pm

bystander wrote:
Sun Oct 17, 2021 10:24 pm
johnnydeep wrote:
Sun Oct 17, 2021 6:57 pm

Third, like Orin said, I think I can see the foreground galaxy's core despite the text saying it's not visible. But maybe that's an illusion caused by the four lensed images overlapping in the center.
APOD Robot wrote:
Sun Oct 17, 2021 4:05 am
...
The gravitational field of the visible foreground galaxy ...
But it does say the foreground galaxy is visible ...
Huh. I could have sworn that it said it was invisible when I read it!
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by johnnydeep » Mon Oct 18, 2021 2:25 pm

alter-ego wrote:
Sun Oct 17, 2021 11:13 pm
johnnydeep wrote:
Sun Oct 17, 2021 6:57 pm
First, Ann's post about the Refsdal supernova's gravitational lensing is truly amazing. And as Ann might be, I'm not too clear about what's really going on there.

Second, the "this image" link in the APOD desc has this to say:
This picture of the gravitationally lensed quasar Q2237+0305 and the associated lensing spiral galaxy was taken by the 3.5-meter WIYN telescope, on the night of October 4, 1999. This system is also known as Huchra's Lens, after its discoverer, and the Einstein Cross, because it is such an excellent example of the phenomenon of gravitational lensing, postulated by Einstein as soon as he realised that gravity would be able to bend light and thus could have lens-like effects. The four separate appearances of the same redshift 1.7 quasar are created by the redshift 0.04 galaxy whose nucleus is nicely bracketed by the quasar images. It might seem surprising that such a close alignment exists, with a galaxy exactly along the line of sight from Earth to a distant quasar, but one should remember that the Universe is large enough that unlikely things happen really quite often. This is an especially important example of a gravitational lens, because the close alignment of the galaxy nucleus and the quasar mean that the four images undergo color and brightness variations with a time scale of only a day or so. These changes can be modelled theoretically and easily monitored observationally. This is a two-color picture combining red and green images, using careful processing both to reveal the strongly blue nature of the quasar, as compared to the galaxy, and to show simultaneously the very bright quasar images and the very faint structure of the lensing galaxy.
But I don't understand the bold blue statement. Why is there such a short brightness variation cycle? Surely it can't be due to the changing positions of the quasar and the foreground galaxy over a brief 24 hours - can it?
...
Since the bold blue statement refers to the 4 images created by the gravitational lens, the path lengths for each image is different. When the quasar changes brightness (for whatever reason), each image will show that change after different propagation delays, which includes spectral changes too due to GR. This day-delay has nothing to do with relative changes between the quasar and foreground galaxy.
Ok. I didn't realize quasars change brightness so quickly (per neufer's post). But the positions of the 4 images would still be the same, right? So the path lengths would still be the same as before. So why would the color/spectra change due to general relativity?
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by neufer » Mon Oct 18, 2021 3:36 pm

johnnydeep wrote:
Mon Oct 18, 2021 2:25 pm

Ok. I didn't realize quasars change brightness so quickly (per neufer's post). But the positions of the 4 images would still be the same, right? So the path lengths would still be the same as before. So why would the color/spectra change due to general relativity?
  • 1) the positions would still be the same
    2) the path lengths would still be the same
    3) the color/spectra (due to the intergalactic medium) would still be the same
    3) the intensity variations would also still be the same but with different time delays.
https://en.wikipedia.org/wiki/Quasar#Spectral_lines,_reionization,_and_the_early_universe wrote:
<<Quasars provide clues as to the end of the Big Bang's reionization. The oldest known quasars display a Gunn–Peterson trough and have absorption regions in front of them indicating that the intergalactic medium at that time was neutral gas. More recent quasars show no absorption region, but rather their spectra contain a spiky area known as the Lyman-alpha forest; this indicates that the intergalactic medium has undergone reionization into plasma, and that neutral gas exists only in small clouds.>>
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by neufer » Mon Oct 18, 2021 5:11 pm

neufer wrote:
Sun Oct 17, 2021 2:12 pm
Astronymus wrote:
Sun Oct 17, 2021 12:47 pm

Hm, why isn't it a ring? I forgot.
  • 1) The elongated shape of the lensing galaxy brakes the ring into a quadrupole.
    2) The quasar being off-center turns a symmetric quadrupole into a Latin cross.
https://en.wikipedia.org/wiki/Einstein_Cross wrote: <<The Einstein Cross (Q2237+030 or QSO 2237+0305) is a gravitationally lensed quasar found in Pegasus that sits directly behind ZW 2237+030, Huchra's Lens. Four images of the same distant quasar (plus one in the centre, too dim to see) appear around a foreground galaxy due to strong gravitational lensing. This system was discovered by John Huchra and coworkers in 1985. While gravitationally lensed light sources are often shaped into an Einstein ring, due to the elongated shape of the lensing galaxy and the quasar being off-centre, the images form a peculiar cross-shape instead.

The quasar's redshift indicates that it is located about 8 billion light years from Earth, while the lensing galaxy is at a distance of 400 million light years. The apparent dimensions of the entire foreground galaxy are 0.87 × 0.34 arcminutes, while the apparent dimension of the cross in its centre accounts for only 1.6 × 1.6 arcseconds. Amateur astronomers are able to see some of the cross using telescopes; however, it requires extremely dark skies and telescope mirrors with diameters of 18 inches (46 cm) or greater.>>
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by alter-ego » Tue Oct 19, 2021 1:43 am

johnnydeep wrote:
Mon Oct 18, 2021 2:25 pm
Ok. I didn't realize quasars change brightness so quickly (per neufer's post). But the positions of the 4 images would still be the same, right? So the path lengths would still be the same as before. So why would the color/spectra change due to general relativity?
Yes, quasars can change brightness relatively quickly, but I did not consider that attribute to be the point in the APOD description. Quoting the image link, I interpreted the APOD description referred to the relative variations between the 4 images:
This picture of the gravitationally lensed quasar Q2237+0305 and the associated lensing spiral galaxy was taken by the 3.5-meter WIYN telescope, on the night of October 4, 1999. This system is also known as Huchra's Lens, after its discoverer, and the Einstein Cross, because it is such an excellent example of the phenomenon of gravitational lensing, postulated by Einstein as soon as he realised that gravity would be able to bend light and thus could have lens-like effects. The four separate appearances of the same redshift 1.7 quasar are created by the redshift 0.04 galaxy whose nucleus is nicely bracketed by the quasar images. It might seem surprising that such a close alignment exists, with a galaxy exactly along the line of sight from Earth to a distant quasar, but one should remember that the Universe is large enough that unlikely things happen really quite often. This is an especially important example of a gravitational lens, because the close alignment of the galaxy nucleus and the quasar mean that the four images undergo color and brightness variations with a time scale of only a day or so. These changes can be modelled theoretically and easily monitored observationally. This is a two-color picture combining red and green images, using careful processing both to reveal the strongly blue nature of the quasar, as compared to the galaxy, and to show simultaneously the very bright quasar images and the very faint structure of the lensing galaxy.
Note: The reference to theoretical modeling only makes sense for variations due to lensing physics, not the intrinsic quasar behaviors which aren't predictable. I therefore included color variations in my post, but I admit, knowing what I know now, I would've left color out. The simple answer to your question is GR can't introduce wavelength dispersion, and the origin of spectral variations isn't obvious. I can only reference a 2009 paper showing a spectral difference for one (image "A") of the 4 quasar images.

DETECTION OF CHROMATIC MICROLENSING IN Q 2237 + 0305 A
ABSTRACT
We present narrowband images of the gravitational lens system Q 2237 + 0305 made with the Nordic Optical Telescope in eight different filters covering the wavelength interval 3510–8130 Å. Using point-spread function photometry fitting we have derived the difference in magnitude versus wavelength between the four images of Q 2237 + 0305. At λ = 4110 Å, the wavelength range covered by the Strömgren-v filter coincides with the position and width of the C iv emission line. This allows us to determine the existence of microlensing in the continuum and not in the emission lines for two images of the quasar. Moreover, the brightness of image A shows a significant variation with wavelength which can only be explained as a consequence of chromatic microlensing. To perform a complete analysis of this chromatic event, our observations were used together with Optical Gravitational Lensing Experiment light curves. Both data sets cannot be reproduced by the simple phenomenology described under the caustic crossing approximation; using more realistic representations of microlensing at high optical depth, we found solutions consistent with simple thin disk models (rs ∝ λ4/3); however, other accretion disk size–wavelength relationships also lead to good solutions. New chromatic events from the ongoing narrowband photometric monitoring of Q 2237 + 0305 are needed to accurately constrain the physical properties of the accretion disk for this system.
Ok, what's chromatic microlensing??
Well, the wavelength dispersion is not directly due to GR (I did question this after remembering gravity is a conservative force) The paper presents the basis of chromatic microlensing in the introduction. It depends on the angular size of the quasar's accretion disk, and the temperature distribution across the disk. These parameters lead to different magnifications at different wavelengths, which in turn lead to spectral brightness variations :shock: .

Introduction Summary: 1)GR does not introduce dispersion on it's own (first sentence in intro), 2)Color variations of many lensed quasars have been observed, 3) Differential extinction and chromatic lensing can lead to observed chromatic variations. 4) The authors conclude the chromatic microlensing component is resolved.

Note:
I didn't pursue the details, but knowing the different path-length delays permits the chromatic microlensing to be extracted independent of extinction. The rapid source quasar variations allow for more accurate delay determination, and subsequently better data for theoretical modeling. Albeit, more complicated and more assumptions for spectral variation analysis IMO.
1. INTRODUCTION
Gravitational lensing is independent of wavelength (Schneider et al. 1992). However, in many gravitationally lensed quasars, differences in color between the images are observed. These chromatic variations could be produced by two effects: differential extinction in the lens galaxy and chromatic microlensing. When each image's light crosses the interstellar medium of the lens galaxy it may be affected in different amounts by patchily distributed dust. This results in differential extinction between pairs of images, and makes possible the determination of the extinction law of the lens galaxy (Nadeau et al. 1991; Falco et al. 1999; Motta et al. 2002; Muñoz et al. 2004; Mediavilla et al. 2005; Elíasdóttir et al. 2006). The other chromatic phenomenon arises when stars or compact objects in the lens galaxy are nearly aligned with the line of sight between the quasar image and the observer. Due to the relative motion between the quasar, the lens and the observer, the quasar image undergoes a magnification or demagnification known as microlensing (Schneider et al. 2006 and references therein). Therefore, fluctuations in the brightness of a quasar image will be a combination of the intrinsic quasar variability and the microlensing from the stars or compact objects in the lens galaxy. The intrinsic variability of the lensed quasar will appear in all images with certain time delay due to the different light travel times. Once this delay is determined, the light curves of the different images can be shifted, and then subtracted. The remaining fluctuations can be assumed to be caused only by microlensing. The microlensing magnification depends on the angular size of the source, in this case on the accretion disk of the quasar. Because the accretion disk is hotter closer to the black hole, and because the emission of the accretion disk depends on temperature, different magnifications may be observed at different wavelengths. This effect is known as chromatic microlensing, and it offers unprecedented perspectives into the physical properties of accretion disks (Wambsganss & Paczynski 1991). Its detection in a lens system will lead to accurate constraints in the size–wavelength scaling.
Diffential Mag v Wavelength.gif
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Re: APOD: The Einstein Cross Gravitational Lens (2021 Oct 17)

Post by Astronymus » Sun Oct 24, 2021 8:16 pm

neufer wrote:
Sun Oct 17, 2021 2:12 pm
Astronymus wrote:
Sun Oct 17, 2021 12:47 pm

Hm, why isn't it a ring? I forgot.
  • It is not centered properly.
https://en.wikipedia.org/wiki/Einstein_Cross wrote: <<The Einstein Cross (Q2237+030 or QSO 2237+0305) is a gravitationally lensed quasar found in Pegasus that sits directly behind ZW 2237+030, Huchra's Lens. Four images of the same distant quasar (plus one in the centre, too dim to see) appear around a foreground galaxy due to strong gravitational lensing. This system was discovered by John Huchra and coworkers in 1985. While gravitationally lensed light sources are often shaped into an Einstein ring, due to the elongated shape of the lensing galaxy and the quasar being off-centre, the images form a peculiar cross-shape instead.

The quasar's redshift indicates that it is located about 8 billion light years from Earth, while the lensing galaxy is at a distance of 400 million light years. The apparent dimensions of the entire foreground galaxy are 0.87 × 0.34 arcminutes, while the apparent dimension of the cross in its centre accounts for only 1.6 × 1.6 arcseconds. Amateur astronomers are able to see some of the cross using telescopes; however, it requires extremely dark skies and telescope mirrors with diameters of 18 inches (46 cm) or greater.>>
Ah, right. Thank you. Old Astro is getting old.
»Only a dead Earth is a good Earth.«