About

Cholik Chan is a Professor of Aerospace and Mechanical Engineering at the University of Arizona and a member of the Graduate Faculty. His research focuses on various aspects of thermal and fluid dynamics, including laser powder interaction during direct metal deposition, electrothermal flow, microfluidic biosensors, natural convection, and thermal energy storage systems. He has contributed to the understanding of energy transfer processes, phase change materials, and the development of simulation methods such as lattice Boltzmann and boundary element techniques. Professor Chan has been recognized for his expertise in thermal energy storage and heat transfer, with numerous publications advancing the field of energy systems and manufacturing processes.

Research topics

• Astrophysics
• Physics
• Astronomy
• Computer Science
• Optics
• Remote sensing
• Geology
• Computational physics
• Quantum mechanics

Selected publications

• Ring Asymmetry and Spin in M87*

  ArXiv.org · 2026-01-01

  articleOpen access

  Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetric ring of emission. General relativistic magnetohydrodynamic (GRMHD) models of M87* and its accretion disk predict that the amplitude and location of the ring's peak brightness asymmetry should fluctuate due to turbulence in the source plasma. We compare the observed distribution of brightness asymmetry amplitudes to the simulated distribution in GRMHD models, across varying black hole spin $a_{*}$. We show that, for strongly magnetized (MAD) models, three epochs of EHT data marginally disfavor $|a_{*}| \lesssim 0.2$. This is consistent with the Blandford-Znajek model for M87's jet, which predicts that M87* should have nonzero spin. We show quantitatively how future observations could improve spin constraints, and discuss how improved spin constraints could distinguish between differing jet-launching mechanisms and black hole growth scenarios.

  PublisherOA PDF

• Full-polarization millimeter wavelength variability of Sagittarius A* during the 2018 EHT campaign

  arXiv (Cornell University) · 2026-04-11

  preprintOpen access

  Sagittarius A* (Srg A*), the supermassive black hole at the center of the Milky Way, provides a unique laboratory to study accretion dynamics and plasma processes near the event horizon. We investigated the variability and polarization properties of Srg A* using ALMA observations during the 2018 Event Horizon Telescope campaign. We analyzed high-cadence full-polarization light curves from ALMA at millimeter wavelengths, performed time-series analysis, and investigated the temporal behavior during an X-ray flare observed by Chandra on 2018 April 24. The variability characteristics are compared with expectations from standard accretion flow models. We find low variability in total intensity ($σ/μ< 10\%$), but significantly higher variability in linear and circular polarization (~ 30% and ~ 50%, respectively). A time-series analysis reveals red-noise variability, with power spectral densities between -2 and -3 across all Stokes parameters. Polarized intensity shows stable intra-day timescales, while total intensity exhibits more variable timescales, suggesting distinct emission regions, with polarization likely arising from a coherent structure. On April 24, a statistically significant inter-band delay in polarized intensity coincides with a near-simultaneous X-ray and millimeter peak that deviates from the typical delayed flare scenario. This event also features enhanced millimeter variability and coherent polarization loop evolution. The observed simultaneity challenges standard models of transient synchrotron emission with cooling delays, favoring instead a scenario of continuous energy injection in an optically thin region. Our results offer new constraints on the physical mechanisms driving variability in Srg A*, and provide key observational input for refining theoretical models of accretion and plasma behavior in the vicinity of supermassive black holes.

  PublisherDOI

• Ring Asymmetry and Spin in M87*

  arXiv (Cornell University) · 2026-01-01

  preprintOpen access

  Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetric ring of emission. General relativistic magnetohydrodynamic (GRMHD) models of M87* and its accretion disk predict that the amplitude and location of the ring's peak brightness asymmetry should fluctuate due to turbulence in the source plasma. We compare the observed distribution of brightness asymmetry amplitudes to the simulated distribution in GRMHD models, across varying black hole spin $a_{*}$. We show that, for strongly magnetized (MAD) models, three epochs of EHT data marginally disfavor $|a_{*}| \lesssim 0.2$. This is consistent with the Blandford-Znajek model for M87's jet, which predicts that M87* should have nonzero spin. We show quantitatively how future observations could improve spin constraints, and discuss how improved spin constraints could distinguish between differing jet-launching mechanisms and black hole growth scenarios.

  PublisherDOI

• Spatially resolved polarization swings in the supermassive binary black hole candidate OJ 287 with first Event Horizon Telescope observations

  Springer Link (Chiba Institute of Technology) · 2026-01-08

  article

  We present the first Event Horizon Telescope 1.3 mm observations of the supermassive binary black hole candidate OJ 287. The observations achieved an unprecedented angular resolution of 18 μas and reveal significant structural and polarization variability over just five days, marking the shortest timescale on which such changes have been directly imaged in this source. The inner jet exhibits a twisted ridgeline structure, with features displaying apparent superluminal motions up to about 22 c. The linear polarization maps reveal three main polarized features whose electric-vector position angles (EVPAs) change substantially over the time span of our observations, including a component with a radial polarization consistent with being produced by a recollimation shock. Most notably, we directly resolved two innermost jet components whose EVPAs rotate in opposite directions. The faster component, moving at 2.4 ± 0.9 μas/day (17.4 ± 6.5 c), exhibits counterclockwise EVPA swings of roughly 3.7° per day, while the slower component, with a proper motion of 1.4 ± 0.3 μas/day (10.2 ± 2.2 c), rotates clockwise at approximately 2.5° per day. Previous studies inferred helical magnetic fields in AGN jets from time-resolved or integrated polarization variability but lacked the angular resolution to directly image this effect. Our results provide spatially resolved evidence that a helical magnetic field threads the jet’s collimation and acceleration zone, ruling out models based on the superposition of unresolved components. Our analysis suggests that propagating shocks interact with a Kelvin–Helmholtz plasma instability, illuminating different phases of the helical magnetic field and producing the observed polarization spatial and temporal variability. Moreover, our model naturally accounts for the more rapid polarization rotation observed in the faster moving component. Our model predicts even more rapid swings in polarization, which could be tested with future observations featuring a more densely sampled time coverage.

  DOI

• Dynamical Inference from Polarized Light Curves of Sagittarius A^*

  The Astrophysical Journal · 2025-07-04 · 2 citations

  articleOpen accessSenior author

  Abstract Polarimetric light curves of Sagittarius A* (Sgr A*) sometimes exhibit loops in the Stokes Q and U plane over time, often interpreted as orbiting hotspot motion. In this work, we apply the differential geometry of planar curves to develop a new technique for estimating polarimetric rotation rates. Applying this technique to 230 GHz light curves of Sgr A*, we find evidence of clockwise motion not only during a postflare period on 2017 April 11, as previously discovered, but also during the quiescent days imaged by the Event Horizon Telescope (EHT). The data exhibit a clockwise fraction of 0.65 ± 0.09 and an overall Q − U rotation rate of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>−</mml:mo> <mml:mn>2.6</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.6</mml:mn> <mml:mspace width="1em"/> <mml:mi>deg</mml:mi> <mml:mspace width="0.25em"/> <mml:msubsup> <mml:mi>t</mml:mi> <mml:mi>g</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . We analyze a library of general relativistic magnetohydrodynamic simulations and find that face-on, clockwise-rotating models with strong magnetic fields are most likely to be consistent with the observations. These results are consistent with EHT and GRAVITY Collaboration studies and indirectly support an interpretation in which the polarized image of Sgr A* has been rotated by an external Faraday screen. This technique offers a novel probe of event-horizon-scale dynamics that complements dynamical reconstructions.

  PublisherDOI

• Deep learning inference with the Event Horizon Telescope

  Astronomy and Astrophysics · 2025-06-01 · 5 citations

  articleOpen access

  Context . In this second paper in our publication series, we present the open-source Z INGULARITY framework for parameter inference with deep Bayesian artificial neural networks. We carried out supervised learning with synthetic millimeter very long baseline interferometry observations of the Event Horizon Telescope (EHT). Our ground-truth models are based on general relativistic magnetohydrodynamic simulations of Sgr A * and M87 * on horizon scales. The models predict the synchrotron emission produced by these accreting supermassive black hole systems. Aims . We investigated how well Z INGULARITY neural networks are able to infer key model parameters from EHT observations, such as the black hole spin and the magnetic state of the accretion disk, when uncertainties in the data are accurately taken into account. Methods . Z INGULARITY makes use of the T ENSORFLOW P ROBABILITY library and is able to handle large amounts of data with a combination of the efficient TFRecord data format plus the H OROVOD framework for distributed deep learning. Our approach is the first analysis of EHT data with Bayesian neural networks, where an unprecedented training data size, under consideration of a closely modeled EHT signal path, and the full information content of the observational data are used. Z INGULARITY infers parameters based on salient features in the data and is containerized for scientific reproducibility. Results . Through parameter surveys and dedicated validation tests, we identified neural network architectures, that are robust against internal stochastic processes and unaffected by noise in the observational and model data. We give examples of how different data properties affect the network training. We show how the Bayesian nature of our networks gives trustworthy uncertainties and uncovers failure modes for uncharacterizable data. Conclusions . It is easy to achieve low validation errors during training on synthetic data with neural networks, particularly when the forward modeling is too simplified. Through careful studies, we demonstrate that our trained networks can generalize well so that reliable results can be obtained from observational data.

  PublisherOA PDFDOI

• Nuclear Neural Networks: Emulating Late Burning Stages in Core Collapse Supernova Progenitors

  ArXiv.org · 2025-02-28

  preprintOpen access

  One of the main challenges in modeling massive stars to the onset of core collapse is the computational bottleneck of nucleosynthesis during advanced burning stages. The number of isotopes formed requires solving a large set of fully-coupled stiff ordinary differential equations (ODEs), making the simulations computationally intensive and prone to numerical instability. To overcome this barrier, we design a nuclear neural network (NNN) framework with multiple hidden layers to emulate nucleosynthesis calculations and conduct a proof-of-concept to evaluate its performance. The NNN takes the temperature, density and composition of a burning region as input and predicts the resulting isotopic abundances along with the energy generation and loss rates. We generate training sets for initial conditions corresponding to oxygen core depletion and beyond using large nuclear reaction networks, and compare the predictions of the NNNs to results from a commonly used small net. We find that the NNNs improve the accuracy of the electron fraction by $280-660\:\%$, the average atomic and mass numbers by $150-360 \%$ and the nuclear energy generation by $250-750\:\%$, consistently outperforming the small network across all timesteps. They also achieve significantly better predictions of neutrino losses on relatively short timescales, with improvements ranging from $100-10^{6}\:\%$. While further work is needed to enhance their accuracy and applicability to different stellar conditions, integrating NNN trained models into stellar evolution codes is promising for facilitating large-scale generation of core-collapse supernova (CCSN) progenitors with higher physical fidelity.

  PublisherOA PDFDOI

• High-Order Photon Rings around Kerr Naked Singularities

  ArXiv.org · 2025-11-25

  preprintOpen accessSenior author

  We present a detailed study of higher-order photon rings of an accreting Kerr naked singularity (KNS) with dimensionless spin parameter $a=1.01$; i.e., a horizonless, overly spinning compact object. Motivated by horizon-scale very-long-baseline interferometry (VLBI) including Event Horizon Telescope (EHT) and future missions such as the Black Hole Explorer (BHEX), we analyze image morphology and interferometric visibilities to identify observational signatures that differentiate KNS from Kerr black holes. We find that higher-order photon rings are tightly concentrated within the nominal ``shadow'' region and that the shadow develops a pronounced gap at sufficiently large observer inclination. These morphological differences produce measurable deviations in the complex visibilities relative to Kerr black hole predictions. Our results indicate that photon-ring structure and visibility-domain diagnostics at horizon-resolving baselines can provide a direct observational test of the presence (or absence) of an event horizon and thus offer a concrete avenue to test general relativity with future horizon-scale observations.

  PublisherOA PDFDOI

• <tt>Mahakala</tt>: A <tt>Python</tt>-based Modular Ray-tracing and Radiative Transfer Algorithm for Curved Spacetimes

  The Astrophysical Journal · 2025-05-13 · 3 citations

  articleOpen access

  Abstract We introduce Mahakala , a Python -based, modular, radiative ray-tracing code for curved spacetimes. We employ Google’s JAX framework for accelerated automatic differentiation, which can efficiently compute Christoffel symbols directly from the metric, allowing the user to easily and quickly simulate photon trajectories through non-Kerr spacetimes. JAX also enables Mahakala to run in parallel on both CPUs and GPUs. Mahakala natively uses the Cartesian Kerr–Schild coordinate system, which avoids numerical issues caused by the pole in spherical coordinate systems. We demonstrate Mahakala ’s capabilities by simulating 1.3 mm wavelength images (the wavelength of Event Horizon Telescope observations) of general relativistic magnetohydrodynamic simulations of low-accretion rate supermassive black holes. The modular nature of Mahakala allows us to quantitatively explore how different regions of the flow influence different image features. We show that most of the emission seen in 1.3 mm images originates close to the black hole and peaks near the photon orbit. We also quantify the relative contribution of the disk, forward jet, and counterjet to 1.3 mm images.

  PublisherDOI

• Horizon-scale variability of M87* from 2017–2021 EHT observations

  Astronomy and Astrophysics · 2025-09-15 · 9 citations

  articleOpen access

  We report three epochs of polarized images of M87* at 230 GHz using data from the Event Horizon Telescope (EHT) taken in 2017, 2018, and 2021. The baseline coverage of the 2021 observations is significantly improved through the addition of two new EHT stations: the 12 m Kitt Peak Telescope and the Northern Extended Millimetre Array (NOEMA). All observations result in images dominated by a bright, asymmetric ring with a persistent diameter of 43.9 ± 0.6 μas, consistent with expectations for lensed synchrotron emission encircling the apparent shadow of a supermassive black hole. We find that the total intensity and linear polarization of M87* vary significantly across the three epochs. Specifically, the azimuthal brightness distribution of the total intensity images varies from year to year, as expected for a stochastic accretion flow. However, despite a gamma-ray flare erupting in M87 quasi-contemporaneously to the 2018 observations, the 2018 and 2021 images look remarkably similar. The resolved linear polarization fractions in 2018 and 2021 peak at ∼5%, compared to ∼15% in 2017. The spiral polarization pattern on the ring also varies from year to year, including a change in the electric vector position angle helicity in 2021 that could reflect changes in the magnetized accretion flow or an external Faraday screen. The improved 2021 coverage also provides the first EHT constraints on jet emission outside the ring, on scales of ≲1 mas. Overall, these observations provide strong proof of the reliability of the EHT images and probe the dynamic properties of the horizon-scale accretion flow surrounding M87*.

  PublisherOA PDFDOI

Recent grants

• PIRE: Black Hole Astrophysics in the Era of Distributed Resources and Expertise

  NSF · $5.7M · 2017–2023

Frequent coauthors

• G. Desvignes

  367 shared

• Jonathan Weintroub

  Center for Astrophysics Harvard & Smithsonian

  302 shared

• Ue‐Li Pen

  289 shared

• Shiro Ikeda

  The University of Tokyo

  282 shared

• Kazunori Akiyama

  268 shared

• R. P. J. Tilanus

  Dutch Research Council

  232 shared

• H. J. van Langevelde

  214 shared

• Jordy Davelaar

  212 shared

Awards & honors

• Arizona Engineering Education Fellow