Chromatic microlensing is a gravitational microlensing event in which the observed amount of magnification changes with wavelength, so the source appears to change color during the event. The important twist is that gravity itself is not acting like a prism. In ordinary gravitational lensing, the deflection of light is essentially independent of wavelength. The chromatic signal usually appears because different wavelengths come from different parts of an extended source, or because additional unresolved light contaminates the observation.
That makes chromatic microlensing scientifically valuable rather than merely inconvenient. By comparing how strongly a source is magnified in several filters, astronomers can sometimes investigate the surface of a distant star, measure stellar limb darkening, identify blended light, or probe the tiny accretion disk around a supermassive black hole in a quasar.
Why Is Ordinary Gravitational Microlensing Achromatic?
Gravitational microlensing occurs when a foreground mass passes sufficiently close to the line of sight toward a more distant luminous object. The foreground object bends the paths of light through the curvature of spacetime, temporarily magnifying the background source. NASA provides a useful overview of this process in its introduction to gravitational microlensing.
For an ideal point-like source viewed through an isolated gravitational lens, the same lens geometry applies to red, visible, ultraviolet, and other wavelengths. In other words, the magnification factor does not intrinsically depend on the wavelength of the photons. NASA’s Ask an Astrophysicist discussion of gravitational lensing describes this wavelength independence as one of the fundamental characteristics of gravitational lensing.
If a star were a perfect point source with no contaminating light nearby, its brightness might rise dramatically during a microlensing event while its color remained essentially unchanged.
Real astronomical sources, however, are not always point-like or spatially uniform. That is where chromatic microlensing enters the picture.
How Can an Achromatic Lens Produce a Chromatic Signal?
The distinction is between the physics of gravitational deflection and the observed brightness distribution of the source. The lens does not need to bend blue photons differently from red photons for the observed amplification to become wavelength-dependent.
Instead, suppose the blue-emitting region of an astronomical object is compact while the red-emitting region is much larger. A tiny gravitational magnification pattern can strongly amplify the compact blue region while averaging over much of the larger red region. The observer then records different magnification factors in the two bands.
The result is a temporary change in measured color even though the underlying gravitational deflection remains achromatic.
Chromatic Microlensing of Stars and Limb Darkening
A particularly clear stellar example involves limb darkening. A star is not uniformly bright across its visible disk. Its center generally appears brighter than its edge, or limb, because an observer sees radiation emerging from different depths and temperatures in the stellar atmosphere.
The strength of limb darkening also depends on wavelength. A star therefore has slightly different surface-brightness profiles when observed through different filters.
Normally, a distant star is far too small for a telescope to resolve this structure directly. But when a microlens passes extremely close to or across the apparent stellar disk, it does not magnify every part of the star equally. Regions closest to the lens can receive much greater magnification than other parts of the disk.
Because those regions have wavelength-dependent brightness, the total magnification can become wavelength-dependent as well. A detailed study published in the Monthly Notices of the Royal Astronomical Society on the chromaticity of microlensing events examined how differential amplification of a limb-darkened stellar source produces measurable color curves.
For a source-transit event, the color pattern can even change direction as the lens moves from the cooler-looking limb toward the brighter central portions of the stellar disk. That detailed pattern contains information about both the source star and the lens trajectory.
Blending Can Also Make a Microlensing Event Look Chromatic
Not every color change reveals the surface structure of the lensed star. Blending is one of the most important complications.
A telescope may record light from the magnified source together with light from neighboring unresolved stars, or sometimes from a luminous foreground lens. The source brightens during microlensing, but the unrelated blended component does not receive the same magnification.
If the blended star has a different color from the lensed source, the relative contribution of the two objects changes as the event brightens and fades. The combined image therefore changes color.
A simplified description of the measured flux in a wavelength band is:
Observed flux = magnification × source flux + blended flux.
The magnified and unmagnified components may have different spectra. Consequently, even an intrinsically achromatic point-source microlensing event can exhibit an apparent chromatic signature.
This is why astronomers cannot automatically interpret every color change as evidence for limb darkening or wavelength-dependent source size. Modeling the blended light is an important part of multiband microlensing analysis.
Chromatic Microlensing of Quasars
One of the most powerful applications of chromatic microlensing involves gravitationally lensed quasars.
A quasar is powered by matter falling toward a supermassive black hole. Much of its ultraviolet and optical radiation is produced by an accretion disk surrounding the black hole. Different parts of this disk have different temperatures.
In the conventional picture, the hotter inner regions contribute more strongly at shorter wavelengths, while progressively larger and cooler regions contribute more strongly at longer wavelengths. The apparent size of the quasar’s continuum-emitting region therefore depends on wavelength.
Now place that quasar behind another galaxy. The galaxy can create several large-scale gravitationally lensed images of the same quasar. Individual stars within the foreground galaxy then act as additional microlenses, producing complicated networks of high- and low-magnification regions.
The small blue-emitting region of the accretion disk can respond strongly to that fine magnification pattern. A larger red-emitting region averages over more of the pattern and may show a weaker response. Research on quasar microlensing and wavelength-dependent source size describes this as a way for an intrinsically achromatic gravitational process to produce observable chromatic microlensing.
In effect, the foreground stars provide astronomers with a natural microscope capable of probing emission regions far too small to resolve directly with conventional imaging.
What Can Chromatic Microlensing Reveal About a Quasar?
Suppose astronomers measure several images of a strongly lensed quasar through multiple filters. If one image is more strongly microlensed in blue light than in red light, models can compare the observed wavelength dependence with simulated microlensing magnification maps.
The analysis can constrain how the apparent size of the accretion disk changes with wavelength. A commonly modeled relationship has the form:
R(λ) ∝ λp
where R represents a characteristic size of the emitting region, λ is wavelength, and p describes how rapidly that size grows toward longer wavelengths.
Measurements of this relationship give astronomers an observational test of accretion-disk models. For example, multiband observations of the Einstein Cross quasar QSO 2237+0305 used chromatic microlensing to investigate the energy profile of its accretion disk.
This is remarkable because the relevant structures around distant quasars are normally much smaller on the sky than conventional telescopes can image directly.
Chromatic Microlensing Is Not the Same as Gravitational Dispersion
The word chromatic can easily create the wrong mental picture. A glass prism separates colors because the material’s refractive index depends on wavelength. Standard gravitational lensing does not work that way.
Chromatic microlensing therefore does not normally mean that gravity bends blue light at one angle and red light at another.
Instead, it generally means that the gravitational magnification pattern is sampling a source whose size, surface brightness, or spatial structure changes with wavelength.
A useful analogy is a magnifying glass moving across a multicolored painting. The glass itself does not need to magnify colors according to different optical rules. If blue paint is concentrated in tiny details while red paint covers a broad area, moving the magnifier across the picture can change the observed ratio of blue to red. In astronomical microlensing, the magnification pattern plays the role of the moving magnifier.
Other Effects Can Mimic Chromatic Microlensing
Color differences require careful interpretation because microlensing is not the only possible cause. Dust extinction can preferentially absorb shorter wavelengths. Quasars can vary intrinsically, while their multiple gravitationally lensed images can reach Earth with different time delays. Broad emission lines, host-galaxy light, unresolved neighboring stars, and wavelength-dependent observational effects can also complicate measurements.
A strong chromatic signal is therefore not, by itself, proof of a particular source structure. Astronomers compare observations across filters, epochs, spectral features, and different lensed images while modeling extinction, blending, intrinsic variability, and the microlensing magnification pattern.
How Do Astronomers Detect Chromatic Microlensing?
The basic strategy is multiband observation. Astronomers monitor the same microlensing system at two or more wavelengths and construct separate light curves or flux ratios.
- Stellar events: researchers compare the changing brightness of the source through filters such as optical red and blue bands, particularly near high-magnification or source-crossing phases.
- Binary or planetary microlensing: caustic crossings can strongly magnify narrow portions of an extended stellar disk, making finite-source and limb-darkening effects easier to detect.
- Lensed quasars: astronomers compare the wavelength-dependent flux ratios among multiple quasar images and often repeat the measurements over time.
- Spectroscopic observations: continuum emission, broad emission lines, and other spectral components can respond differently because they originate from regions with very different physical sizes.
Detailed modeling then asks whether one wavelength behaves as though it originates from a smaller or differently distributed source than another.
Why Does Chromatic Microlensing Matter?
Microlensing is valuable precisely because its effective angular resolution is not limited in the same way as direct telescope imaging. A compact lens can selectively amplify extraordinarily small regions of a distant object.
Chromatic measurements add another dimension to that natural microscope. Instead of measuring only how bright an object becomes, astronomers examine how magnification changes across wavelength.
For stars, that can reveal information about atmospheric intensity profiles and limb darkening. For quasar systems, it can constrain the size and temperature structure of accretion disks surrounding supermassive black holes. At the same time, apparent chromaticity can warn researchers that blending, extinction, or another contaminating effect must be included in the model.
The Key Idea
Chromatic microlensing is wavelength-dependent observed magnification produced when a gravitational microlens interacts with a source whose apparent structure or surrounding light differs with wavelength.
The underlying gravitational lensing remains fundamentally achromatic. The color signal arises because astronomy rarely gives us perfect point sources floating alone against a black background. Stars have atmospheres and limbs, quasars have temperature-structured accretion disks, and crowded fields contain blended sources.
That apparent complication becomes a powerful measurement tool. By watching how the universe’s gravitational magnifying glasses treat different wavelengths, astronomers can study structures that would otherwise remain far below the resolving power of their telescopes.