arXiv subjects

Shan-Shan Zhao

Publications and source records attributed to Shan-Shan Zhao.

8 recordsLinked to original sources

arXiv2026

Spatially resolved spectral properties of M87* on event horizon scales

The supermassive black hole at the center of the nearby radio galaxy M87 (M87*) is a prime target for studying black hole physics. Spatially resolved spectral measurements on event-horizon scales can reveal the origin of the emission and probe the plasma and gravitational environment in the immediate vicinity of the black hole. Here, we present an analysis of spectral properties based on nearly simultaneous high-resolution images at 3.5 mm (86 GHz) and 1.3 mm (230 GHz), obtained in 2018 with the Global Millimeter VLBI Array (GMVA) including ALMA and the Greenland Telescope, and the Event Horizon Telescope (EHT). We obtain the first spatially resolved spectral-index map ($S_ν\propto ν^α$) within the compact region ($\leq 100\,μ$as). We further detect a robust radial gradient with a modest rise in the inner $\lesssim 20~μ$as (slightly inside the 1.3 mm ring), followed by a systematic decline at larger radii. The spectral index transitions from positive to negative values near $\sim 30~μ$as, close to the 3.5 mm ring radius, consistent with frequency-dependent synchrotron opacity in the innermost accretion flow. These results provide new observational constraints that can help discriminate between models of the horizon-scale emission and the launching of relativistic jets in M87*.

astro-ph.HE↗

arXiv2026

Beyond Sgr A* and M87*: Sub-Microarcsecond Black Hole Shadow Detection via Lunar-based Extremely Long Baseline Interferometry

The 1.3 mm ground-based very long baseline interferometry (VLBI) array Event Horizon Telescope (EHT), is limited by Earth's diameter, restricting horizon-scale imaging to only M87* and Sgr A*. Extending baselines to the Moon would achieve ~0.7 microarcsecond angular resolution at 230 GHz, enabling shadow detection for many more supermassive black holes (SMBHs). The concept is motivated by space VLBI missions and lunar exploration, including the ongoing Lunar Orbit VLBI EXperiment (LOVEX) aboard QueQiao-2 (Chang'E-7) and the planned International Lunar Research Station (ILRS). We assess shadow detectability for 31 SMBHs with predicted large angular sizes, assuming optically thin emission at 230 GHz, exploring different telescope locations and antenna sizes. Assuming a telescope at the lunar antipode, we simulate the Moon-Earth (u,v) coverage and show that sources near the Moon's orbital plane yield projected baselines spanning a wide range, enabling sampling of the first visibility null - a key shadow signature. Using a geometric ring model, we identify six shadow-detectable candidates: M104, NGC 5077, and NGC 1052 are detectable with a 5 m lunar-based telescope; PGC 049940 with 10 m; NGC 524 with 20 m; and NGC 5252 with 40 m. If additional space telescopes fill the baseline coverage gaps between Moon and Earth, 14 candidates are detectable for the n=1 photon-ring region with a lunar-based telescope up to 40 m. These results provide a clear scientific and technical motivation for lunar-based telescopes in future black hole shadow studies.

astro-ph.GA↗

arXiv2023

How many supermassive black hole binaries are detectable through tracking relative motions by sub/millimeter VLBI

The sub/millimeter wavelengths (86-690 GHz) very long baseline interferometry (VLBI) will provide $\sim5-40\ μ$as angular resolution, $\sim10$ mJy baseline sensitivity, and $\sim 1\ μ$as/yr proper motion precision, which can directly detect supermassive black hole binary (SMBHB) systems by imaging the two visible sources and tracking their relative motions. Such a way exhibits an advantage compared to indirect detect methods of observing periodic signals in motion and light curves, which are difficult to confirm from competing models. Moreover, tracking relative motion at sub/millimeter wavelengths is more reliable, as there is a negligible offset between the emission region and the black hole center. In this way, it is unnecessary to correct the black hole location by a prior of jet morphology as it would be required at longer wavelengths. We extend the formalism developed in D'Orazio & Loeb (2018) to link the observations with the orbital evolution of SMBHBs from the $\lesssim$10 kpc dynamical friction stages to the $\lesssim 0.01$ pc gravitational radiation stages, and estimate the detectable numbers of SMBHBs. By assuming 5\% of AGNs holding SMBHBs, we find that the number of detectable SMBHBs with redshift $z\le 0.5$ and mass $M\leq 10^{11}M_\odot$ is about 20. Such detection relies heavily on proper motion precision and sensitivity. Furthermore, we propose that the simultaneous multi-frequency technique plays a key role in meeting the observational requirements.

astro-ph.GA↗

arXiv2022

Applications of the source-frequency phase-referencing technique for ngEHT observations

The source-frequency phase-referencing (SFPR) technique has been demonstrated to have great advantages for mm-VLBI observations. By implementing simultaneous multi-frequency receiving systems on the next generation Event Horizon Telescope (ngEHT) antennas, it is feasible to carry out a frequency phase transfer (FPT) which could calibrate the non-dispersive propagation errors and significantly increase the phase coherence in the visibility data. Such increase offers an efficient approach for weak source or structure detection. SFPR also makes it possible for high precision astrometry, including the core-shift measurements up to sub-mm wavelengths for Sgr A* and M87* etc. We also briefly discuss the technical and scheduling considerations for future SFPR observations with the ngEHT.

astro-ph.IM↗

arXiv2022

Impact of the nonthermal electron radiation effects on the horizon scale image structure of Sagittarius A*

The Event Horizon Telescope (EHT), with $\sim$20 $μ$as high angular resolution, recently resolved the millimeter image of the suppermassive black hole in the Galaxy, Sagittarius A*. This opens a new window to study the plasma on horizon scales. The accreting disk probably contains a small fraction of nonthermal electrons and their emissions should contribute to the observed image. We study if such contributions are sufficient to cause structural differences detectable by current and future observational capabilities. We introduce nonthermal electrons in a semi-analytical accretion disk, which considers viscosity-leading heating processes, and adopt a continued hybrid electron energy distribution of thermal distribution and power-law tail. We generate the black hole images and extract the structural features as crescent parameters. We find the existence of nonthermal electron radiation makes the crescent much brighter, slightly larger, moderately thicker, and much more symmetric. When the nonthermal connecting Lorentz factor $γ_c=65$, which is equivalent to the nonthermal electrons accounting for $\sim1.5$% of the totals, nonthermal effects cause $\sim2$% size difference at 230 GHz. Comparing with the structural changes caused by other physical factors, including inclination between the system and the observer, black hole spin, and interstellar medium scattering effects, we find that although nonthermal electron radiation takes the most unimportant role at 230 GHz, it becomes more significant at 345 GHz.

astro-ph.HE↗

arXiv2022

The Photon Ring in M87*

We report measurements of the gravitationally lensed secondary image -- the first in an infinite series of so-called "photon rings" -- around the supermassive black hole M87* via simultaneous modeling and imaging of the 2017 Event Horizon Telescope (EHT) observations. The inferred ring size remains constant across the seven days of the 2017 EHT observing campaign and is consistent with theoretical expectations, providing clear evidence that such measurements probe spacetime and a striking confirmation of the models underlying the first set of EHT results. The residual diffuse emission evolves on timescales comparable to one week. We are able to detect with high significance a southwestern extension consistent with that expected from the base of a jet that is rapidly rotating in the clockwise direction. This result adds further support to the identification of the jet in M87* with a black hole spin-driven outflow, launched via the Blandford-Znajek process. We present three revised estimates for the mass of M87* based on identifying the modeled thin ring component with the bright ringlike features seen in simulated images, one of which is only weakly sensitive to the astrophysics of the emission region. All three estimates agree with each other and previously reported values. Our strongest mass constraint combines information from both the ring and the diffuse emission region, which together imply a mass-to-distance ratio of $4.20^{+0.12}_{-0.06}~μ{\rm as}$ and a corresponding black hole mass of $(7.13\pm0.39)\times10^9M_\odot$, where the error on the latter is now dominated by the systematic uncertainty arising from the uncertain distance to M87*.

astro-ph.HE↗

arXiv2017

Strong deflection gravitational lensing by a modified Hayward black hole

A modified Hayward black hole is a nonsingular black hole. It is proposed to form when the pressure generated by quantum gravity can stop matter's collapse as the matter reaches Planck density. Strong deflection gravitational lensing happening nearby its event horizon might provide some clues of these quantum effects in its central core. We investigate observables of the strong deflection lensing, including angular separations, brightness differences and time delays between its relativistic images, and estimate their values for the supermassive black hole in the Galactic center. We find that it is possible to distinguish the modified Hayward black hole from a Schwarzschild one, but it demands very high resolution beyond current stage.

gr-qc↗

arXiv2016

Strong field gravitational lensing by a charged Galileon black hole

Strong field gravitational lensings are dramatically disparate from those in the weak field by representing relativistic images due to light winds one to infinity loops around a lens before escaping. We study such a lensing caused by a charged Galileon black hole, which is expected to have possibility to evade no-hair theorem. We calculate the angular separations and time delays between different relativistic images of the charged Galileon black hole. All these observables can potentially be used to discriminate a charged Galileon black hole from others. We estimate the magnitudes of these observables for the closest supermassive black hole Sgr A*. The strong field lensing observables of the charged Galileon black hole can be close to those of a tidal Reissner-Nordström black hole or those of a Reissner-Nordström black hole. It will be helpful to distinguish these black holes if we can separate the outermost relativistic images and determine their angular separation, brightness difference and time delay, although it requires techniques beyond the current limit.

gr-qc↗