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.
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
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
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.
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.
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.
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