arXiv subjects

Shiyan Zhong

Publications and source records attributed to Shiyan Zhong.

18 recordsLinked to original sources

arXiv2026

Early Near-Infrared Excess and Rapid Disk-Corona Evolution in the Tidal Disruption Event 2024aepd

We present multi-wavelength observations of the tidal disruption event (TDE) 2024aepd, spanning primarily the first $\sim$300 days after discovery. A prominent near-infrared (NIR) excess is detected as early as $\sim$40 days. Its nearly flat power-law spectrum strongly deviates from the Rayleigh-Jeans tail of the UV-optical blackbody. Although a conventional dust-echo origin cannot be completely ruled out, free-free emission from a reprocessing photospheric envelope provides a more plausible explanation. The spectral break between the UV-optical and NIR components shifts to higher frequencies, while the inferred density-profile index remains nearly constant, suggesting evolving reprocessing conditions within a broadly unchanged density structure. In addition, the X-ray spectrum is initially dominated by a thermal disk component accompanied by a hard excess. From $\sim$178 days onward, the spectrum becomes power-law dominated and subsequently hardens, indicating the rapid emergence and strengthening of a hot corona. These results provide evidence for frequency-dependent reprocessing at early times and for the rapid development of a disk-corona system, placing new constraints on the structure and evolution of the reprocessing layer around supermassive black holes.

astro-ph.HE↗

arXiv2026

Fitting the light curves of tidal disruption events with non-parabolic model

Tidal disruption events (TDEs) are powerful probes of supermassive black hole (SMBH) properties and accretion physics. The existing light curve fitting tools assume that the disrupted stars are on parabolic orbits, which may introduce systematic biases in derived parameters. In this work, we extend the model of Zhong (2025) to construct a non-parabolic TDE model that incorporates orbital energy of the disrupted star as a free parameter ($\tildeε_{\rm orb}$) to modify the debris mass distribution and mass fallback rate. We apply this model to 30 TDEs from the ZTF-I survey and compare the results with those from a standard parabolic model. We find that neglecting orbital energy leads to biased black hole mass estimates: for eccentric (hyperbolic) orbits, parabolic models systematically underestimate (overestimate) the black hole mass. Additionally, we measure orbital eccentricities ($e$) and penetration factors ($β$) of the disrupted stars in this sample, enabling an investigation of their origins via the $e$-$β$ parameter space. Most events (24/30) are consistent with production via two-body relaxation in spherical nuclear star clusters, but six outliers with high $β$ and $e<1$ suggest alternative mechanisms. Our results highlight the importance of accounting for orbital energy in TDE modeling to improve the accuracy of SMBH mass measurements and to better understand the dynamical origin of the disrupted stars.

astro-ph.HE↗

arXiv2026

Rate of Repeating Tidal Disruption Events with 5--19 years interval

Statistics on tidal disruption events (TDEs) may be contaminated by repeating TDEs (rTDEs), which have been extensively discovered recently. However, the origin of rTDEs remains unclear. In addition, no statistical research on rTDEs with time intervals $>5$ years has been made yet. In this work, we searched for rTDEs with time intervals of 5--19 years using CRTS data in a sample of 16 ZTF BTS TDEs at $z<0.05$. We found 2 rTDE candidates, AT 2019azh and AT 2024pvu, with time intervals of 13.2 and 17.1 years, respectively. The peak luminosities of CRTS flares are close to those of ZTF flares. For the CRTS flare of AT 2024pvu, using GALEX UV observations near the peak, we measured a blackbody temperature of $\sim19500$ K, consistent with TDEs and higher than SNe. Moreover, we estimated the expected number of SNe in the sample to be $\lesssim0.08$, and hence the probability that both CRTS flares are SNe is only 0.3\%. Therefore, the possibility that both CRTS flares are SNe can be ruled out, and it is likely that both are TDEs. Using the two rTDEs, we inferred that the TDE rate is 2--3 orders of magnitude higher than the average over 5--19 years prior to TDE detection. Considering another two rTDEs with intervals of $\sim$2 years in the sample and possible rTDEs missed by CRTS, rTDEs with intervals of $<20$ years may account for 25\%--60\% of the TDE sample. We prefer to explain rTDEs as repeating partial TDEs. If so, the high fraction of rTDEs suggests that the observed optical TDE rate has been overestimated. However, the possibility of independent TDEs cannot be ruled out and requires future observational tests.

astro-ph.HE↗

arXiv2025

Ultraviolet Spectral Evidence for Ansky as a Slowly Evolving Featureless Tidal Disruption Event with Quasiperiodic Eruptions

X-ray quasi-periodic eruptions (QPEs) are rare and enigmatic phenomena that increasingly show a connection to tidal disruption events (TDEs). However, the recently discovered QPEs in ZTF19acnskyy ("Ansky") appear to be linked to an active galactic nucleus (AGN) rather than a TDE, as their slow decay and AGN-like variability differ markedly from that of typical TDEs. This finding may imply broader formation channels for QPEs. To further investigate Ansky's nature, we obtained a timely ultraviolet (UV) spectrum, which reveals a featureless, TDE-like spectrum devoid of broad optical or UV emission lines. Additionally, the steep UV continuum, fitted by a power law with an index of -2.6, aligns more closely with TDEs than with AGNs. Compared to other featureless TDEs, Ansky exhibits a significantly lower blackbody luminosity (10^43 erg s^-1) and much longer rise and decay timescales, suggesting a distinct TDE subclass. An offset TDE involving an intermediate-mass black hole is unlikely, given its position consistent with the galactic center with a 3 sigma upper limit of 54 pc. Instead, we propose that Ansky may result from the tidal disruption of a post-main-sequence star by a typical supermassive black hole. Our findings strengthen the growing evidence for TDE-QPE associations, although other formation channels for QPEs remain plausible and await future observational efforts.

astro-ph.HE↗

arXiv2025

SN 2024aecx: A double-peaked rapidly evolving Type IIb supernova at 11 Mpc

We present the results of low-resolution spectroscopic and densely sampled multi-band photometric follow-up of supernova (SN) 2024aecx. The SN was discovered in the spiral galaxy NGC 3521 (distance $\sim$11 Mpc) within a day after the explosion. The early spectra of SN 2024aecx show a weak signature of hydrogen lines, which disappeared in $\sim$30 days after the explosion. Light curves in all bands show a distinct feature of two peaks, and the first peak is likely due to the shock cooling emission. The early phase light curve evolution of SN 2024aecx has similarity with the typical Type IIb events, but the decay rate in different bands (e.g., $\rm Δm_{15}$ = 1.60 $\pm$ 0.05 mag, $g$-band) is significantly faster in the post-peak phase. It attained the secondary maximum in $\sim$19 days ($g$-band) with a peak absolute magnitude of M$_{g}$ = -17.94 $\pm$ 0.10 mag. SN 2024aecx colors trend redder in early epochs ($<$8 days), followed by a duration in which it grows bluer, then later gets redder again $>$20 days after explosion. The analytical model fitting to the light curves reveals an envelope mass and progenitor radii in the range of $\sim$0.03 - 0.24 $M_\odot$ and $\sim$169 - 200 $R_\odot$, respectively. Modeling of the pseudo-bolometric light curve suggests that synthesized $^{56}$Ni in the explosion was $\sim$0.15 M$_{\odot}$ with ejecta mass and kinetic energy of $\sim$0.7 M$_{\odot}$ and $\sim$0.16 $\times$ 10$^{51}$ erg, respectively. The observational properties and modeling indicate that the SN~2024aecx progenitor belongs to the extended progenitor category.

astro-ph.HE↗

arXiv2025

A Star's Death by a Thousand Cuts: The Runaway Periodic Eruptions of AT2023uqm

Stars on bound orbits around a supermassive black hole may undergo repeated partial tidal disruption events (rpTDEs), producing periodic flares. While several candidates have been suggested, definitive confirmation of these events remains elusive. We report the discovery of AT2023uqm, a nuclear transient that has exhibited at least five periodic optical flares, making it only the second confirmed case of periodicity after ASASSN-14ko. Uniquely, the flares from AT2023uqm show a nearly exponential increase in energy--a "runaway" phenomenon signaling the star's progressive destruction. This behavior is consistent with rpTDEs of low-mass, main-sequence stars or evolved giant stars. Multiwavelength observations and spectroscopic analysis of the two most recent flares reinforce its interpretation as an rpTDE. Intriguingly, each flare displays a similar double-peaked structure, potentially originating from a double-peaked mass fallback rate or two discrete collisions per orbit. The extreme ratio of peak separation to orbital period draws attention to the possibility of a giant star being disrupted, which could be distinguished from a low-mass main-sequence star by its future mass-loss evolution. Our analysis demonstrates the power of rpTDEs to probe the properties of disrupted stars and the physical processes of tidal disruption, though it is currently limited by our knowledge of these events. AT2023uqm emerges as the most compelling rpTDE thus far, serving as a crucial framework for modeling and understanding these phenomena.

astro-ph.HE↗

arXiv2025

Modeling the UV/Optical light curve of re-brightening tidal disruption events

In recent years, a new subclass of tidal disruption events (TDEs) was reported from the literature. The light curve of these TDEs show a re-brightening feature in the decline phase after the first peak, which then leads to a second flare. The re-brightening TDEs challenges the existing light curve fitting tools, which are designed to handle single flare. In this work, we present a model, aimed at reproducing of the light curve of the re-brightening TDEs, based on the scenario that the consecutive flares are produced by the same star who experienced two partial disruptions (PTDEs). We also develop a fitting code from this model, and apply it to two re-brightening TDEs: AT 2022dbl and AT 2023adr. The light curve of both TDEs are well fitted. Finally, we forecast the time and peak brightness of the next flare for these two TDEs, so that the observers could get prepared in advance and make an examination on our model.

astro-ph.HE↗

arXiv2024

Gleeok's Fire-breathing: Triple Flares of AT 2021aeuk within Five Years from the Active Galaxy SDSS J161259.83+421940.3

We present a noteworthy transient AT 2021aeuk exhibiting three distinct optical flares between 2018 and 2023. It is hosted in a radio-loud narrow-line Seyfert 1 (NLSy1) galaxy, with an optical image showing a minor tidal morphology and a red mid-infrared color W1-W2=1.1. Flares II and III exhibit rapid rises, and long-term decays (around 1000 days) with recurring after-peak bumps. The g-r color after subtracting the reference magnitude exhibited a rapid drop and recovery during Flare II, followed by the minor after-peak evolution in blue colors. We applied a canonical tidal disruption event (TDE) fitting on the light curves which gives a decay index p of $-2.99_{-0.14}^{+0.13}$ for Flare II and $-1.61_{-0.65}^{+0.34}$ for Flare III. The blackbody fitting shows lower temperatures ($\sim 10^{3.8}$K) with minor after-peak evolution. The blackbody radius ($\gtrsim 10^{16}\ \rm cm$) and luminosity ($\sim 10^{45}\rm erg\ s^{-1}$) are larger than the typical TDE sample's. The time lag (in rest frame) between ZTF g- and r-band ($\rm τ_{g,r}=3.4^{+1.0}_{-0.9}$ days) significantly exceeds the prediction from the standard accretion disk. Pre-burst spectra reveal prominent Bowen fluorescence lines, indicating a vigorous or potentially long-lasting process that enriches the local metallicity. Additionally, we derived black hole masses of $\log\it{M}_{\bullet}\rm=7.09^{+0.18}_{-0.31}\ \it{M}_{\odot}$ and $\log\it{M}_{\bullet}\rm=7.52^{+0.08}_{-0.10}\ \it{M}_{\odot}$ using $\rm Hβ$ and $\rm Hα$ emission lines. The variation and recurring features of AT 2021aeuk are not likely induced by the radio-beaming effect or Type-II superluminous supernova (SLSN-II), however, we cannot rule out the possibility of TDE or enhanced active galactic nuclei (AGN) accretion process. The unusually high occurrence of three flares within five years may also induced by the complex local environment.

astro-ph.HE↗

arXiv2024

Black hole mass and optical radiation mechanism of the tidal disruption event AT 2023clx

We present the optical light curves of the tidal disruption event (TDE) AT 2023clx in the declining phase, observed with Mephisto. Combining our light curve with the ASAS-SN and ATLAS data in the rising phase, and fitting the composite multi-band light curves with MOSFiT, we estimate black hole mass of AT 2023clx is between $10^{5.67}$--$10^{5.82}~M_{\odot}$. This event may be caused by either a full disruption of a $0.1~M_{\odot}$ star, or a partial disruption of a $0.99~M_{\odot}$ star, depending on the data adopted for the rising phase. Based on those fit results and the non-detection of soft X-ray photons in the first 90 days, we propose that the observed optical radiation is powered by stream-stream collision. We speculate that the soft X-ray photons may gradually emerge in 100--600 days after the optical peak, when the debris is fully circularized into a compact accretion disk.

astro-ph.HE↗

arXiv2024

Early-phase simultaneous multiband observations of the Type II supernova SN 2024ggi with Mephisto

We present early-phase good-cadence (hour-to-day) simultaneous multiband ($ugi$ and $vrz$ bands) imaging of the nearby supernova SN~2024ggi, which exploded in the nearby galaxy, NGC 3621. A quick follow-up was conducted within less than a day after the explosion and continued $\sim$23 days. The $uvg$ band light curves display a rapid rise ($\sim$1.4 mag day$^{-1}$) to maximum in $\sim$4 days and absolute magnitude $M_{g}\sim$--17.75 mag. The post-peak decay rate in redder bands is $\sim$0.01 mag day$^{-1}$. Different colors (e.g., $u-g$ and $v-r$) of SN~2024ggi are slightly redder than SN 2023ixf. A significant rise ($\sim$12.5 kK) in black-body temperature (optical) was noticed within $\sim$2 days after the explosion, which successively decreased, indicating shock break out inside a dense circumstellar medium (CSM) surrounding the progenitor. Using semianalytical modeling, the ejecta mass and progenitor radius were estimated as 1.2 $M_\odot$ and $\sim$550 $R_\odot$. The archival deep images ($g,r,i and z$ bands) from the Dark Energy Camera Legacy Survey were examined, and a possible progenitor was detected in each band ($\sim$22--22.5 mag) and had a mass range of 14--17 $M_\odot$.

astro-ph.HE↗

arXiv2023

Exploring the origin of stars on bound and unbound orbits causing tidal disruption events

Tidal disruption events (TDEs) provide a clue to the properties of a central supermassive black hole (SMBH) and an accretion disk around it, and to the stellar density and velocity distributions in the nuclear star cluster surrounding the SMBH. Deviations of TDE light curves from the standard occurring at a parabolic encounter with the SMBH depends on whether the stellar orbit is hyperbolic or eccentric (Hayasaki et al. 2018) and the penetration factor ($β$, tidal disruption radius to orbital pericenter ratio). We study the orbital parameters of bound and unbound stars being tidally disrupted by comparison of direct $N$-body simulation data with an analytical model. Starting from the classical steady-state Fokker-Planck model of Cohn & Kulsrud (1978), we develop an analytical model of the number density distribution of those stars as a function of orbital eccentricity ($e$) and $β$. To do so fittings of the density and velocity distribution of the nuclear star cluster and of the energy distribution of tidally disrupted stars are required and obtained from $N$-body data. We confirm that most of the stars causing TDEs in a spherical nuclear star cluster originate from the full loss-cone region of phase space, derive analytical boundaries in eccentricity-$β$ space, and find them confirmed by $N$-body data. Since our limiting eccentricities are much smaller than critical eccentricities for full accretion or full escape of stellar debris, we conclude that those stars are only very marginally eccentric or hyperbolic, close to parabolic.

astro-ph.HE↗

arXiv2023

Tracing the Evolution of SMBHs and Stellar Objects in Galaxy Mergers: An Multi-mass Direct N-body Model

By using direct N-body numerical simulations, we model the dynamical co-evolution of two supermassive black holes (SMBHs) and the surrounding stars in merging galaxies. In order to investigate how different stellar components evolve during the merger, we generate evolved stellar distributions with an initial mass function. Special schemes have also been developed to deal with some rare but interesting events, such as tidal disruption of main sequence stars, the plunge of low mass stars, white dwarfs, neutron stars and stellar mass black holes, and the partial tidal disruption of red giants or asymptotic giant branch stars. Our results indicate that the formation of a bound supermassive black hole binary (SMBHB) will enhance the capture rates of stellar objects by the SMBHs. Compared to the equal stellar mass model, the multi-mass model tends to result in a higher average mass of disrupted stars. Instead of being tidally disrupted by the SMBH, roughly half of the captured main sequence stars will directly plunge into the SMBH because of their small stellar radius. Giant stars, on the other hand, can be stripped of their envelopes if they are close enough to the SMBH. Though most remnants of the giant stars can survive after the disruption, a small fraction still could plunge into the SMBH quickly or after many orbital periods. Our results also indicate significant mass segregation of compact stars at the beginning of the merger, and then this effect is destroyed as the two SMBHs form a bound binary.

astro-ph.HE↗

arXiv2022

Extreme-mass-ratio burst detection with TianQin

The capture of compact objects by massive black holes in galaxies or dwarf galaxies will generate short gravitational wave signals, called extreme-mass-ratio bursts (EMRBs), before evolving into extreme-mass-ratio inspirals. Their detection will provide an investigation of the black hole properties and shed light on astronomy and astrophysics. In this work, we investigate the detection number of the TianQin observatory on EMRBs. Our result shows that TianQin can detect tens of EMRBs events during its mission lifetime. For those detected events, we use the Fisher information matrix to quantify these uncertainties in the inference of their parameters. We consider the possible network of TianQin+LISA and study how a network can improve parameter estimation. The result shows that, for most sources, the CO mass, the MBH mass, and the MBH spin can be determined with an accuracy of the order $10^{-1}$ and the sky localization can be determined with an accuracy of 10 square degrees. We further explore the gravitational wave background generated by those unsolved EMRBs and conclude that it is about $10^6$ times weaker than TianQin's sensitivity and thus it can be ignored.

gr-qc↗

arXiv2022

Revisit the rate of tidal disruption events: the role of the partial tidal disruption event

Tidal disruption of stars in dense nuclear star clusters containing supermassive central black holes (SMBH) is modeled by high-accuracy direct N-body simulation. Stars getting too close to the SMBH are tidally disrupted and a tidal disruption event (TDE) happens. TDEs probe properties of SMBH, their accretion disks, and the surrounding nuclear stellar cluster. In this paper we compare rates of full tidal disruption events (FTDE) with partial tidal disruption events (PTDE). Since a PTDE does not destroy the star, a leftover object emerges; we use the term 'leftover star' for it; two novel effects occur in the simulation: (1) variation of the leftover star's mass and radius, (2) variation of the leftover star's orbital energy. After switching on these two effects in our simulation, the number of FTDEs is reduced by roughly 28%, and the reduction is mostly due to the ejection of the leftover stars from PTDEs coming originally from relatively large distance. The number of PTDEs is about 75% higher than the simple estimation given by Stone et al. (2020), and the enhancement is mainly due to the multiple PTDEs produced by the leftover stars residing in the diffusive regime. We compute the peak mass fallback rate for the PTDEs and FTDEs recorded in the simulation, and find 58% of the PTDEs have peak mass fallback rate exceeding the Eddington limit, and the number of super-Eddington PTDEs is 2.3 times the number of super-Eddington FTDEs.

astro-ph.GA↗

arXiv2018

Classification of Tidal Disruption Events Based on Stellar Orbital Properties

We study the rates of tidal disruption of stars by intermediate-mass to supermassive black holes on bound to unbound orbits by using high-accuracy direct N-body experiments. The approaching stars from the star cluster to the black hole can take three types of orbit: eccentric, parabolic, and hyperbolic orbits. Since the mass fallback rate shows a different variability depending on these orbital types, we can classify tidal disruption events (TDEs) into three main categories: eccentric, parabolic, and hyperbolic TDEs. Respective TDEs are characterized by two critical values of the orbital eccentricity: the lower critical eccentricity is the one below which the stars on eccentric orbits cause the finite, intense accretion, and the higher critical eccentricity above which the stars on hyperbolic orbits cause no accretion. Moreover, we find that the parabolic TDEs are divided into three subclasses: precisely parabolic, marginally eccentric, and marginally hyperbolic TDEs. We analytically derive that the mass fallback rate of the marginally eccentric TDEs can be flatter and slightly higher than the standard fallback rate proportional to $t^{-5/3}$, whereas it can be flatter and lower for the marginally hyperbolic TDEs. We confirm by N-body experiments that only few eccentric, precisely parabolic, and hyperbolic TDEs can occur in a spherical stellar system with a single intermediate-mass to supermassive black hole. A substantial fraction of the stars approaching to the black hole would cause the marginally eccentric or marginally hyperbolic TDEs.

astro-ph.HE↗

arXiv2015

Supermassive Black Holes in Galactic Nuclei with Tidal Disruption of Stars: Paper II - Axisymmetric Nuclei

Tidal Disruption of stars by supermassive central black holes from dense rotating star clusters is modelled by high-accuracy direct N-body simulation. As in a previous paper on spherical star clusters we study the time evolution of the stellar tidal disruption rate and the origin of tidally disrupted stars, now according to several classes of orbits which only occur in axisymmetric systems (short axis tube and saucer). Compared with that in spherical systems, we found a higher TD rate in axisymmetric systems. The enhancement can be explained by an enlarged loss-cone in phase space which is raised from the fact that total angular momentum $\bf J$ is not conserved. As in the case of spherical systems, the distribution of the last apocenter distance of tidally accreted stars peaks at the classical critical radius. However, the angular distribution of the origin of the accreted stars reveals interesting features. Inside the influence radius of the supermassive black hole the angular distribution of disrupted stars has a conspicuous bimodal structure with a local minimum near the equatorial plane. Outside the influence radius this dependence is weak. We show that the bimodal structure of orbital parameters can be explained by the presence of two families of regular orbits, namely short axis tube and saucer orbits. Also the consequences of our results for the loss cone in axisymmetric galactic nuclei are presented.

astro-ph.GA↗

arXiv2014

Efficient Scheme for Active Particle Selection in N-body Simulations

We propose an efficient method for active particle selection, working with Hermite Individual Time Steps (HITS) scheme in direct N-body simulation code $φ$GRAPE. For a simulation with $N$ particles, this method can reduce the computation complexity of active particle selection, from $O(N\cdot N_{step})$ to $O(\overline{N_{act}}\cdot N_{step})$, where $\overline{N_{act}}$ is the average active particle number in every time step which is much smaller than $N$ and $N_{step}$ is the total time steps integrated during the simulation. Thus can save a lot of time spent on active particle selection part, especially in the case of low $\overline{N_{act}}$.

cs.DS↗

arXiv2014

SMBH in Galactic Nuclei with Tidal Disruption of Stars

Tidal Disruption of stars by super massive central black holes from dense star clusters is modeled by high-accuracy direct $N$-body simulation. The time evolution of the stellar tidal disruption rate, the effect of tidal disruption on the stellar density profile and for the first time the detailed origin of tidally disrupted stars are carefully examined and compared with classic papers in the field. Up to 128k particles are used in simulation to model the star cluster around the super massive black hole, we use the particle number and the tidal radius of black hole as free parameters for a scaling analysis. The transition from full to empty loss-cone is analyzed in our data, the tidal disruption rate scales with the particle number $N$ in the expected way for both cases. For the first time in numerical simulations (under certain conditions) we can support the concept of a critical radius of Frank & Rees (1976), which claims that most stars are tidally accreted on highly eccentric orbits originating from regions far outside the tidal radius. Due to the consumption of stars moving on radial orbits, a velocity anisotropy is founded inside the cluster. Finally we make an estimation for the real galactic center based on our simulation results and the scaling analysis.

astro-ph.GA↗