Photon Diffusion Research Articles

Two-stage deep learning method for sparse-view fluorescence molecular tomography reconstruction.

Fluorescence molecular tomography (FMT) is a preclinical optical tomographic imaging technique that can trace various physiological and pathological processes at the cellular or even molecular level. Reducing the number of FMT projection views can improve the data acquisition speed, which is significant in applications such as dynamic problems. However, a reduction in the number of projection views will dramatically aggravate the ill-posedness of the FMT inverse problem and lead to significant degradation of the reconstructed images. To deal with this problem, we have proposed a deep-learning-based reconstruction method for sparse-view FMT that only uses four perpendicular projection views and divides the image reconstruction into two stages: image restoration and inverse Radon transform. In the first stage, the projection views of the surface fluorescence are restored to eliminate the blur derived from photon diffusion through a fully convolutional neural network. In the second stage, another convolutional neural network is used to implement the inverse Radon transform between the restored projections from the first stage and the reconstructed transverse slices. Numerical simulation and phantom and mouse experiments are carried out. The results show that the proposed method can effectively deal with the image reconstruction problem of sparse-view FMT.

Diffuse photon remission associated with the center-illuminated-area-detection geometry. II. Approach to the time-domain model.

This part proposes a model of time-dependent diffuse photon remission for the center-illuminated-area-detection (CIAD) geometry, by virtue of area integration of the radially resolved time-dependent diffuse photon remission formulated with the master-slave dual-source scheme demonstrated in Part I for steady-state measurements. The time-domain model is assessed against Monte Carlo (MC) simulations limiting to only the Heyney-Greenstein scattering phase function for CIAD of physical scales and medium properties relevant to single-fiber reflectance (SfR) and over a 2ns duration, in compliance with the timespan of the only experimental report of SfR demonstrated with a 50µm gradient index fiber. The time-domain model-MC assessments are carried out for an absorption coefficient ranging three orders of magnitude over [0.001,0.01,0.1,1]m m -1 at a fixed scattering, and a reduced scattering coefficient ranging three orders of magnitude over [0.01,0.1,1,10]m m -1 at a fixed absorption, among others. Photons of shorter and longer propagation times, relative to the diameter of the area of collection, respond differently to the scattering and absorption changes. Limited comparisons of MC between CIAD and a top-hat geometry as the idealization of SfR reveal that the time-domain photon remissions of the two geometries differ appreciably in only the early arriving photons.

Neutrino magnetic moment portal and supernovae: New constraints and multimessenger opportunities

We scrutinize the hypothesis that gauge singlet fermions -- sterile neutrinos -- interact with Standard Model particles through the transition magnetic moment portal. These interactions lead to the production of sterile neutrinos in supernovae followed by their decay into photons and active neutrinos which can be detected at $\gamma$-ray telescopes and neutrino detectors, respectively. We find that the non-observation of active neutrinos and photons from sterile-neutrino decay associated to SN1987A yields the strongest constraints to date on magnetic-moment-coupled sterile neutrinos if their masses are inside a $0.1-100$ MeV window. Assuming a near-future galactic supernova explosion, we estimate the sensitivity of several present and near-future experiments, including Fermi-LAT, e-ASTROGAM, DUNE, and Hyper-Kamiokande, to magnetic-moment-coupled sterile neutrinos. We also study the diffuse photon and neutrino fluxes produced in the decay of magnetic-moment coupled sterile neutrinos produced in all past supernova explosions and find that the absence of these decay daughters yields the strongest constraints to date for sterile neutrino masses inside a $1-100$ keV window.

Shock cooling emission from explosions of red supergiants – I. A numerically calibrated analytic model

ABSTRACT Supernova light curves are dominated at early time, hours to days, by photons escaping from the expanding shock heated envelope. We provide a simple analytic description of the time-dependent luminosity, L, and colour temperature, Tcol, for explosions of red supergiants (with convective polytropic envelopes), valid up to H recombination (T ≈ 0.7 eV). The analytic description interpolates between existing expressions valid at different (planar then spherical) stages of the expansion, and is calibrated against numerical hydrodynamic diffusion calculations for a wide range of progenitor parameters (mass, radius, core/envelope mass and radius ratios, and metalicity), and explosion energies. The numerically derived L and Tcol are described by the analytic expressions with $10{{\ \rm per\ cent}}$ and $5{{\ \rm per\ cent}}$ accuracy, respectively. Tcol is inferred from the hydrodynamic profiles using frequency independent opacity, based on tables we constructed for this purpose (and will be made publicly available) including bound–bound and bound–free contributions. In an accompanying paper (Paper II) we show − using a large set of multigroup photon diffusion calculations − that the spectral energy distribution is well described by a Planck spectrum with T = Tcol, except at ultraviolet (UV) frequencies, where the flux can be significantly suppressed due to strong line absorption. We defer the full discussion of the multigroup results to paper II, but provide here for completeness an analytic description also of the UV suppression. Our analytic results are a useful tool for inferring progenitor properties, explosion velocity, and also relative extinction based on early multiband shock cooling observations of supernovae.

On the contribution of cosmic-ray interactions in the circumgalactic gas to the observed high-energy neutrino flux

Cosmic rays escaping the Milky-Way disk interact with circumgalactic gas which fills the virial volume of our Galaxy. These interactions should produce guaranteed fluxes of energetic diffuse neutrinos and photons observable at the Earth. This neutrino flux would be a plausible contribution to the spectrum measured by the IceCube neutrino observatory: the energy emitted in this way is weakly constrained from cascade gamma rays, since the cascades have no time to develop, but the arrival directions of the neutrinos do not point to the Galactic disk, in agreement with observations. However, previous studies reported very different estimates of the corresponding neutrino flux, so it was unclear if this contribution to the observed spectrum is essential. Here we readdress the calculation of this diffuse neutrino flux component under various assumptions about the cosmic-ray spectrum and propagation in the circumgalactic medium. We find that even with these variations, this contribution to the observed neutrino flux remains subleading provided multimessenger constraints are satisfied.

New biosensors detect light deep inside the brain

In recent years, the field of biosensors has seen significant advances in the development of fluorescent sensors, including quantum dots,1 upconversion nanoparticles,2 and fluorescent proteins,3 to monitor the generation of information in living systems. The fluorescence of these sensors can be observed by shining a laser at them. However, conventional fluorescent sensors are limited in their ability to image signals in deep tissues because most of the light is absorbed or scattered as it penetrates the tissue. To address this challenge, a team led by Jasanoff developed a novel sensor that converts light into a magnetic signal that is unaffected by absorption or scattering. This allows the response of the light detector to be visualized using magnetic resonance imaging (MRI; Figure 1).4 The development of this sensor has significant potential to improve our understanding of information processing in deep tissues. Schematic illustration of mapping light distribution in tissues using MRI-detectable photosensitive liposomes. To fabricate the photosensitive MRI probe, magnetic particles were encapsulated in light-responsive azobenzene-conjugated liposomes (called Light-LisNRs).5 By adjusting the composition and proportion of the lipid bilayer molecules, these liposome nanoparticles can switch from being permeable to being impenetrable, depending on the type of light exposure. This property allowed modulation of the MRI contrast of the Light-LisNRs and facilitated the optimization of the switchable longitudinal relaxation time (T1). Specifically, upon exposure to ultraviolet (UV) light, the Light-LisNRs became more permeable to water, resulting in a strong interaction between the magnetic particles and water and thereby producing strong MRI signals. Conversely, exposure to blue light caused the Light-LisNRs to become impermeable to water, resulting in the lack of a detectable MRI signal. The optimized Light-LisNRs could potentially be used to map light distribution in live animals. When these nanoparticles were injected into the living rat brain, they effectively diffused through the brain by convection, as evidenced by changes in the magnetic resonance signal. The probes exhibited exceptional light sensitivity, which could be demonstrated by changes in magnetic relaxation under blue and UV irradiation. Relative to the initial baseline, the probes showed significant differences in the mean MRI signals in response to UV and blue light, and the temporal characteristics of the light response observed during repeated photoperiods were consistent. The steady performance of Light-LisNRs in the rat brain suggests that they are suitable for the quantitative measurement of the light intensity distribution in tissues. In addition, the researchers used a hybrid model consisting of a beam spreading function combined with a homogeneous photon diffusion term to fit the experimental data and produced a quantitative map of the distribution of light emitted by the optical fiber implanted near the brain's striatum. These results highlight the potential of the optimized Light-LisNRs in mapping the distribution of light in living animals. In summary, this study describes the design of a novel sensor and its application elucidating light propagation in optically opaque environments. The sensor exploits the photomodulation of liposomal permeability to enhance the contrast produced by contrast agent molecules, leading to improved visualization in MRI. The results of this work demonstrate the potential of Light-LisNRs as a versatile tool for photon detection and highlight opportunities for further optimization through adjustments to action spectra, absorption cross sections, and contrast agent packaging parameters. The sensing approach outlined here holds promise for the future development of MRI probes capable of detecting stimuli beyond light, such as neurochemicals or other molecular species in the brain. In addition, the sensor may serve as a valuable tool for monitoring patients undergoing light-based therapies, including photodynamic therapy, which uses lasers to ablate cancer cells. Lei Luo: Conceptualization, Visualization, Writing - Original draft; Dandan Yang: Writing - Reviewing & Editing; Yu Yang: Conceptualization, Funding Acquisition, Writing - Reviewing & Editing. The authors declare no conflicts of interest.

Characterization of heterogeneous dimer and oligomer formation of amyloid-beta using single-molecule FRET.

Protein aggregation is implicated as the cause of pathology in various diseases. Polymorphism in the structure of fibrils formed by aggregation suggests the existence of many different assembly pathways and therefore a heterogeneous ensemble of soluble oligomers. Characterization of this heterogeneity is the key to understanding the aggregation mechanism and toxicity of specific oligomers, but in practice it is extremely difficult to probe individual aggregation pathways in a mixture. Using fluorescence lifetime imaging and deep learning, we have recently visualized the formation of highly diverse fibrils of amyloid-beta (Abeta) in real time (PNAS_2022_e2116736119). Here, we further characterize the heterogeneity in the early stages of aggregation, dimerization and oligomerization of 42-residue Abeta (Abeta42), using single-molecule FRET. We found the dimerization of Abeta is extremely rare. The dimer dissociation constant is unexpectedly high, >5 mM. In addition, the dimerization is highly heterogeneous in terms of the FRET efficiency and kinetics. At least 6 clusters of dimers were needed to fit the entire equilibrium dimerization trajectories. This structural and kinetic diversity suggests that the heterogeneous fibril formation starts from the beginning of the assembly, the dimerization step. Then, we sought the formation of larger stable oligomers of Abeta42 that appear during aggregation. Combination of peptide incubation in a plate reader, single-molecule free-diffusion FRET experiment, and a newly-developed maximum likelihood method based on the rigorous photon statistics and diffusion theory allows for the accurate characterization of stable oligomers appearing at various stages of aggregation. We found that the size of these oligomers ranges from 30- to 100-mer and their overall population is unexpectedly low, less than 1 nM, before and during aggregation of 1-µM Abeta42 peptide. The amounts of small oligomers such as dimers and tetramers are negligible.

Diffuse flux of ultra-high energy photons from cosmic-ray interactions in the disk of the Galaxy and implications for the search for decaying super-heavy dark matter

When propagating to Earth, UHECRs can interact with the gas in our Milky Way and produce secondary particles including photons. This can impact the search for UHE photons as we face a diffuse flux of UHE photons resulting from their propagation. This flux, together with the photon flux expected from the GZK interactions, participates in the diffuse photon flux which is limiting the detection of UHE-gamma sources. We present an estimate of the diffuse flux resulting from the interactions of UHECRs in our galaxy above 1017 eV, using results from the Pierre Auger Collaboration concerning the flux and its composition, and two different interstellar gas density models. We also discuss the impact of the evaluated diffuse flux of UHE photons on SHDM searches, as the former can be considered as a floor below which other signals would be overwhelmed. Similarly, the neutrino flux produced during the same process can be evaluated, by accounting for the neutrino mixing.

Image restoration for blurry optical images caused by photon diffusion with deep learning.

Optical macroscopic imaging techniques have shown great significance in the investigations of biomedical issues by revealing structural or functional information of living bodies through the detection of visible or near-infrared light derived from different mechanisms. However, optical macroscopic imaging techniques suffer from poor spatial resolution due to photon diffusion in biological tissues. This dramatically restricts the application of optical imaging techniques in numerous situations. In this paper, an image restoration method based on deep learning is proposed to eliminate the blur caused by photon diffusion in optical macroscopic imaging. Two blurry images captured at orthogonal angles are used as the additional information to ensure the uniqueness of the solution and restore the small targets at deep locations. Then a fully convolutional neural network is proposed to accomplish the image restoration, which consists of three sectors: V-shaped network for central view, V-shaped network for side views, and synthetical path. The two V-shaped networks are concatenated to the synthetical path with skip connections to generate the output image. Simulations as well as phantom and mouse experiments are implemented. Results indicate the effectiveness of the proposed method.

Expanding fireball in magnetar bursts and fast radio bursts

ABSTRACT A fireball of radiation plasma created near the surface of a neutron star (NS) expands under its own pressure along magnetic field lines, and produces photon emission and relativistic matter outflow. We comprehensively classify the expanding fireball evolution into five cases, and obtain the photospheric luminosity and the kinetic energy of the outflow, taking into account key processes; lateral diffusion of photons escaping from a magnetic flux tube, effects of strong magnetic field, baryon loading from the NS surface, and radiative acceleration via cyclotron resonant scattering, some of which have not been considered in the context of gamma-ray bursts. Applying our model to magnetar bursts with fast radio bursts (FRBs), in particular the X-ray short bursts from SGR 1935+2154 associated with the Galactic FRB 20200428A, we show that the burst radiation can accelerate the outflow to high-Lorentz factor with sufficient energy to power FRBs.

Diffuse x-ray and gamma-ray limits on boson stars that interact with nuclei

Light bosonic dark matter can form gravitationally bound states known as boson stars. In this work, we explore a new signature of these objects interacting with the interstellar medium (ISM). We show how small effective couplings between the bosonic dark matter and the nucleon lead to a potential that accelerates ISM baryons as they transit the boson star, making the ISM within radiate at a high rate and energy. The low ISM density, however, implies the majority of Galactic boson stars will be too faint to be observable through this effect. By contrast, the diffuse photon flux, in hard x-rays and soft gamma-rays, produced by boson stars interacting with the ionized ISM phases can be sizable. We compute this diffuse flux and compare it to existing observations from HEAO-1, INTEGRAL and COMPTEL to infer limits on the fraction of these objects. This novel method places constraints on boson star dark matter while avoiding back-action effects from ambient baryons on the boson star configuration, unlike terrestrial searches where it has been noted that back-action can screen light bosonic fields. In addition, this study could be extended to other couplings and structures formed from light dark matter. For dark matter masses (10-14, 10-8) eV and boson star masses (10-10, 10-1) M ⊙, we find the constraints on the fraction can go down to f* ≲ 10-9 for dark matter in boson stars that is directly coupled to the Standard Model.

Constraining axion-like particles with the diffuse gamma-ray flux measured by the Large High Altitude Air Shower Observatory

The detection of very high-energy neutrinos by IceCube experiment supports the existence of a comparable gamma-ray counterpart from the same cosmic accelerators. Under the likely assumption that the sources of these particles are of extragalactic origin, the emitted photon flux would be significantly absorbed during its propagation over cosmic distances. However, in the presence of photon mixing with ultra-light axion-like-particles (ALPs), this expectation would be strongly modified. Notably, photon-ALP conversions in the host galaxy would produce an ALP flux which propagates unimpeded in the extragalactic space. Then, the back-conversion of ALPs in the Galactic magnetic field leads to a diffuse high-energy photon flux. In this context, the recent detection of the diffuse high-energy photon flux by the Large High Altitude Air Shower Observatory (LHAASO) allows us to exclude at the 95% CL an ALP-photon coupling g_{agamma } > rsim 3.9–7.8 times 10^{-11}~mathrm {GeV^{-1}} for m_{a}lesssim 4times 10^{-7}~textrm{eV}, depending on the assumptions on the magnetic fields and on the original gamma-ray spectrum. This new bound is complementary with other ALP constraints from very-high-energy gamma-ray experiments and sensitivities of future experiments.

Efficient computation of the steady-state and time-domain solutions of the photon diffusion equation in layered turbid media

Accurate and efficient forward models of photon migration in heterogeneous geometries are important for many applications of light in medicine because many biological tissues exhibit a layered structure of independent optical properties and thickness. However, closed form analytical solutions are not readily available for layered tissue-models, and often are modeled using computationally expensive numerical techniques or theoretical approximations that limit accuracy and real-time analysis. Here, we develop an open-source accurate, efficient, and stable numerical routine to solve the diffusion equation in the steady-state and time-domain for a layered cylinder tissue model with an arbitrary number of layers and specified thickness and optical coefficients. We show that the steady-state (< 0.1 ms) and time-domain (< 0.5 ms) fluence (for an 8-layer medium) can be calculated with absolute numerical errors approaching machine precision. The numerical implementation increased computation speed by 3 to 4 orders of magnitude compared to previously reported theoretical solutions in layered media. We verify our solutions asymptotically to homogeneous tissue geometries using closed form analytical solutions to assess convergence and numerical accuracy. Approximate solutions to compute the reflected intensity are presented which can decrease the computation time by an additional 2–3 orders of magnitude. We also compare our solutions for 2, 3, and 5 layered media to gold-standard Monte Carlo simulations in layered tissue models of high interest in biomedical optics (e.g. skin/fat/muscle and brain). The presented routine could enable more robust real-time data analysis tools in heterogeneous tissues that are important in many clinical applications such as functional brain imaging and diffuse optical spectroscopy.

Diffuse photon remission associated with thecenter-illuminated-area-detection geometry: PartI, an approach to the steady-state model.

Diffuse photon remission associated with the center-illuminated-area-detection (CIAD) geometry has been useful for non-contact sensing and may inform single-fiber reflectance (SfR). This series of work advances model approaches that help enrich the understanding and applicability of the photon remission by CIAD. The general approach is to derive the diffuse photon remission by the area integration of the radially resolved diffuse reflectance while limiting the analysis to a medium exhibiting only the Heyney-Greenstein (HG) scattering phase function. Part I assesses the steady-state photon remission in CIAD over a reduced scattering scaled diameter of μ s ' d a r e a ∈[0.5×10-3,103] that covers the range extensively modeled for SfR. The corresponding radially resolved diffuse reflectance is obtained by concatenating an empirical expression for the semi-ballistic region near the point-of-illumination and a formula utilizing a master-slave dual-source scheme over the semi-diffusive to a diffusive regime while being constrained by an extrapolated zero-boundary condition. The terminal algebraic photon remission is examined against Monte Carlo simulations for an absorption coefficient over [0.001,1]m m -1, a reduced scattering coefficient over [0.01,1000]m m -1, a HG scattering anisotropy factor within [0.5,0.95], and a diameter of the area of collection ranging [50,1000]µm. The algebraic model is also applied to phantom data acquired over a ∼2c m non-contact CIAD configuration and with a 200µm SfR probe. The model approach will be extended in a subsequent work towards the time-of-flight characteristics of CIAD.

Steady jet ejections from the innermost region of advection-dominated accretion flow around a black hole

Abstract We study ejection mechanisms for two kinds of steady jets: one observed from black hole binaries in the low/hard state and the other from SS 433. The specific energy of the ejected gas is required to be positive for the jets to get to infinity, while that of the accreted gas is naively considered to be negative at the outermost boundary of the accretion flow. To reconcile the opposite sign of the specific energies, we propose a situation where two layers exist in the accretion flow and one layer receives energy from the other sufficiently for the specific energy to be positive. For the steady jets in the low/hard state, the accretion ring at the outermost end of the accretion flow is considered to yield two-layer flow in which a geometrically thick advection-dominated accretion flow (ADAF) sandwiches a geometrically thin accretion disk and the thin disk is supposed to change to another ADAF on the inner side. The energy transfer is expected to occur through turbulent mixing between the two layers and the upper layer is discussed as having a positive specific energy large enough for the terminal velocity to be ∼0.1 c. For the steady jets from SS 433, it is argued that a slim disk separates into two stratified layers due to the photon diffusion in the direction perpendicular to the equatorial plane under the advection-dominated situation. In this case, the specific energy of the upper layer is expected to be positive such that the terminal velocity exceeds 0.2c. The jet ejection process near the black hole is investigated commonly to both the two-layer cases and predicts that the jet opening angle becomes as small as 2°.

Transport through Amorphous Photonic Materials with Localization and Bandgap Regimes.

We propose a framework that unifies the description of light transmission through three-dimensional amorphous dielectric materials that exhibit both localization and a photonic bandgap. We argue that direct, coherent reflection near and in the bandgap attenuates the generation of diffuse or localized photons. Using the self-consistent theory of localization and considering the density of states of photons, we can quantitatively describe the total transmission of light for all transport regimes: transparency, light diffusion, localization, and bandgap. Comparison with numerical simulations of light transport through hyperuniform networks supports our theoretical approach.

Realization of clinical translation of photoacoustic imaging

Photoacoustic (PA) imaging capitalizes on sound and light to probe tissue optical absorption at depth. Owing to acoustic propagation of absorbed diffuse photons it exceeds imaging depth of most pure optical methods. Moreover, since the detection of PA signals relies on ultrasound transducers, the modality inherently allows for acquisition of co-registered ultrasound images, enabling a structural and functional examination of tissue. Diagnosis of diseases such as atherosclerosis could benefit greatly from uncovering such dual information. Clinical translation of photoacoustic imaging, led by breast cancer imaging application is—pending a couple of hurdles—on the brink of happening. Systematic approaches in assessing contrast through spectroscopy, designing acquisition systems through optimization for both PA/US signal sensitivity and image metrics and finally designing instrumentation to account for tissue access limitations have been researched and show promising results. In this talk, we discuss the final challenges in translating photoacoustic imaging to the clinics with respect to contrast, instrumentation and system development.

The GALPROP Cosmic-ray Propagation and Nonthermal Emissions Framework: Release v57

The past decade has brought impressive advances in the astrophysics of cosmic rays (CRs) and multiwavelength astronomy, thanks to the new instrumentation launched into space and built on the ground. Modern technologies employed by those instruments provide measurements with unmatched precision, enabling searches for subtle signatures of dark matter and new physics. Understanding the astrophysical backgrounds to better precision than the observed data is vital in moving to this new territory. A state-of-the-art CR propagation code, called GalProp, is designed to address exactly this challenge. Having 25 yr of development behind it, the GalProp framework has become a de facto standard in the astrophysics of CRs, diffuse photon emissions (radio to γ-rays), and searches for new physics. GalProp uses information from astronomy, particle physics, and nuclear physics to predict CRs and their associated emissions self-consistently, providing a unifying modeling framework. The range of its physical validity covers 18 orders of magnitude in energy, from sub-keV to PeV energies for particles and from μeV to PeV energies for photons. The framework and the data sets are public and are extensively used by many experimental collaborations and by thousands of individual researchers worldwide for interpretation of their data and for making predictions. This paper details the latest release of the GalProp framework and updated cross sections, further developments of its initially auxiliary data sets for models of the interstellar medium that grew into independent studies of the Galactic structure—distributions of gas, dust, radiation, and magnetic fields—as well as the extension of its modeling capabilities. Example applications included with the distribution illustrating usage of the new features are also described.

Molecular Chemistry for Dark Matter. II. Recombination, Molecule Formation, and Halo Mass Function in Atomic Dark Matter

Dissipative dark matter predicts rich observable phenomena that can be tested with future large-scale structure surveys. As a specific example, we study atomic dark matter, consisting of a heavy particle and a light particle charged under a dark electromagnetism. In particular, we calculate the cosmological evolution of atomic dark matter focusing on dark recombination and dark molecule formation. We have obtained the relevant interaction rate coefficients by rescaling the rates for normal hydrogen, and evolved the abundances for ionized, atomic, and molecular states using a modified version of Recfast++ (which we have released publicly at a a https://github.com/jamesgurian/RecfastJulia ). We also provide an analytical approximation for the final abundances. We then calculate the effects of atomic dark matter on the linear power spectrum, which enter through a dark photon diffusion and dark acoustic oscillations. At formation time, the atomic dark matter model suppresses halo abundances on scales smaller than the diffusion scale, just as warm dark matter models suppress the abundance below the free-streaming scale. The subsequent evolution with radiative cooling, however, will alter the halo mass function further.

The emergence of diffused gamma-ray burst afterglows from the discs of active galactic nuclei

ABSTRACT The discs of active galactic nuclei (AGNs) have emerged as rich environments for the production and capture of stars and the compact objects that they leave behind. These stars produce long gamma-ray bursts (GRBs) at their deaths, while frequent interactions among compact objects form binary neutron stars and neutron star–black hole binaries, leading to short GRBs upon their merger. Predicting the properties of these transients as they emerge from the dense environments of AGN discs is key to their proper identification and to better constrain the star and compact object population in AGN discs. Some of these transients would appear unusual because they take place in much higher densities than the interstellar medium. Others, which are the subject of this paper, would additionally be modified by radiation diffusion, since they are generated within optically thick regions of the accretion discs. Here, we compute the GRB afterglow light curves for diffused GRB sources for a representative variety of central black hole masses and disc locations. We find that the radiation from radio to ultraviolet and soft X-rays can be strongly suppressed by synchrotron self-absorption in the dense medium of the AGN disc. In addition, photon diffusion can significantly delay the emergence of the emission peak, turning a beamed, fast transient into a slow, isotropic, and dimmer one. These would appear as broad-band correlated AGN variability with a dominance at the higher frequencies. Their properties can constrain both the stellar populations within AGN discs and the disc structure.