arXiv subjects

S. Yang

Publications and source records attributed to S. Yang.

At least 19 recordsLinked to original sources

arXiv2026

AT 2024qfm: a luminous fast blue optical transient at a redshift of z = 0.2267 identified by Lasair-ZTF

Luminous fast blue optical transients (LFBOTs) emit from x-ray to radio wavelengths, epitomised by the discovery of AT 2018cow in a host galaxy at 65 Mpc. In the following eight years eleven more have been found, at redshifts $0.075 \lesssim z \lesssim0.34$, plus one identified retrospectively from 2016. Here we present the discovery of AT 2024qfm, classified as an LFBOT in a host galaxy at $z = 0.2267 \pm 0.0002$. Its ultraviolet-to-optical luminosity and rapid 13 day fade closely match AT 2018cow. We describe how the transient was identified in the Zwicky Transient Facility alert stream using a custom filter in the Lasair broker that flags flux gradients over time. Another LFBOT candidate was identified with the same methodology (AT 2024kth). The physical origin of LFBOTs remains debated with no firm consensus, and further progress requires more discoveries, host-galaxy characterisation, and multi-wavelength analysis to constrain theory. We discuss this discovery in the context of Rubin Observatory's Legacy Survey of Space and Time (LSST), whose sensitivity will increase the effective LFBOT survey volume tenfold relative to ZTF, out to $z \lesssim 0.6$, and show that our FastFinder filter could recover such events. We highlight the challenge of detecting their fast evolution with sufficiently low latency to trigger multi-wavelength follow-up that can constrain theoretical models.

astro-ph.HE↗

arXiv2026

Probing a new subclass of llGRB-SN transients: Insights from EP250304a and its associated supernova

With the advent of the Einstein Probe (EP) mission, we are entering a new era in the study of gamma-ray bursts (GRBs), enabling the detection of faint, low-luminosity transients that would previously have gone undetected. EP250304a was an event discovered by EP associated with the broad-lined type Ic supernova (SN) SN 2025fhm located at z = 0.2. Despite no gamma-ray emission being detected at the time of the EP trigger, we identify evidence for a relativistic outflow consistent with a GRB-like jet across multiple wavelengths. We present a detailed spectral and photometric analysis of EP250304a/SN 2025fhm, including multi-band light curve modelling performed with the Redback Python package. We find that this event closely resembles low-luminosity GRB-SNe (llGRB-SNe) such as GRB 060218/SN 2006aj, GRB 100316D/SN 2010bh, and GRB 171205A/SN 2017iuk, all of which exhibit early-time emission consistent with a thermal shocked cocoon. These similarities suggest that EP250304A/SN 2025fhm may belong to an emerging subclass of shocked cocoon-dominated llGRB-SNe, representing the low-luminosity end of a broader continuum of engine-driven GRB-SN explosions.

astro-ph.HE↗

arXiv2026

Photometric classification of supernovae detected by the Zwicky Transient Facility using noise augmentation

Modern time-domain surveys, such as the Zwicky Transient Facility (ZTF), detect far more extragalactic transients than can be spectroscopically classified. Photometric classification offers a scalable alternative, enabling the identification of larger, fainter, and higher-redshift supernova samples suitable for applications such as Type Ia supernova (SN Ia) cosmology. We present a feature-based photometric classifier for SNe detected by ZTF, with the primary goal of constructing a photometric SN Ia sample for cosmological analyses. Our approach utilises the autoencoder architecture of ParSNIP (Boone 2021) to capture the intrinsic diversity of SN light curves. We trained the model on a spectroscopically classified ZTF SN sample, incorporating a realistic noise augmentation procedure that simulates the flux uncertainties of fainter sources. Light curve features were used to train a gradient-boosted decision tree classifier, implemented in both binary (SN Ia vs. non-Ia) and multi-class configurations. We validated our classifier on independent, fainter ZTF data with and without noise augmentation. To evaluate real-time performance, we also applied our classifier to live ZTF alerts and conducted a spectroscopic classification survey within the ePESSTO+ collaboration. We found that noise augmentation significantly improves classification performance, particularly for fainter sources. Our binary classifier achieves an SN Ia recall of (98.1 $\pm$ 0.4)%, averaged across five train-test splits. SN Ia recall exceeds 98% for events with a peak apparent magnitude up to 20 and more than 10 detections, and remains above 96% up to magnitude 20.5. Overall, 95% of sources were correctly classified in both binary and multi-class modes. Our classifier performs efficiently on real ZTF data and enables construction of a large photometric SN Ia sample for cosmology.

astro-ph.IM↗

arXiv2026

The nearby He-rich superluminous supernova SN 2021bnw during photospheric phases

Aim. We present and interpret the data of the nearby hydrogen-deficient but helium-rich superluminous supernova SN 2021bnw which reached a magnitude of -20.7 at maximum luminosity in g band. Methods. We discuss the light curves and spectra of SN 2021bnw based on its spectro-photometric follow up exploiting different observational facilities. We reproduce the NIR spectrum of SN 2021bnw with TARDIS to inspect the chemical composition at late photospheric phases and identify helium features. We also use a STELLA model coupling hydrodynamics and radiation transport to constrain the physical parameters of the explosion assmunig a 56Ni+CSM scenario. Results. We suggest that SN 2021bnw was mainly powered by the interaction of the ejecta with a previously lost He-rich circumstellar material, coupled with a central power source. Conclusions. This work expands the data sample of He-rich superluminous supernovae rich (SLSNe Ib) and, assuming a single progenitor scenario, can constrain the masses and the physics of their progenitors.

astro-ph.HE↗

arXiv2025

Competing Lattice and Defect Dynamics Govern Terahertz-Induced Ferroelectricity in Quantum Paraelectric SrTiO$_3$

Intense terahertz (THz) pulses induce transient inversion-symmetry breaking in quantum paraelectric SrTiO$_3$, yet the underlying mechanism remains controversial. Using fields up to $\sim$1.1 MV/cm, we reveal spatially inhomogeneous THz-field-induced second harmonic generation (TFISH) governed by competing lattice and defect dynamics. Short-lived coherent antiferrodistortive (AFD) modes suppress dipole correlations within $\sim$5 ps, while heavily damped soft/AFD modes and a defect-induced low-frequency mode ($\sim$0.1-0.3 THz) jointly prevent long-range ferroelectric coherence in oxygen-vacancy-rich regions. Collective modes manifested by oscillatory TFISH components exhibit softening followed by hardening below a critical temperature $T^*\simeq$28 K, confirming transient ferroelectric order where defects are sparse. These results reconcile conflicting interpretations, establish defect-mediated competition as a central regulator of light-induced ferroelectricity, and open routes to ultrafast control of quantum materials.

cond-mat.mtrl-sci↗

arXiv2025

The radio flare and multi-wavelength afterglow of the short GRB 231117A: energy injection from a violent shell collision

We present the early radio detection and multi-wavelength modeling of the short gamma-ray burst (GRB) 231117A at redshift $z=0.257$. The Australia Telescope Compact Array automatically triggered a 9-hour observation of GRB 231117A at 5.5 and 9 GHz following its detection by the Neil Gehrels Swift Observatory just 1.3 hours post-burst. Splitting this observation into 1-hour time bins, the early radio afterglow exhibited flaring, scintillating and plateau phases. The scintillation allowed us to place the earliest upper limit ($<10$ hours) on the size of a GRB blast wave to date, constraining it to $<1\times10^{16}$ cm. Multi-wavelength modeling of the full afterglow required a period of significant energy injection between $\sim 0.02$ and $1$ day. The energy injection was modeled as a violent collision of two shells: a reverse shock passing through the injection shell explains the early radio plateau, while an X-ray flare is consistent with a shock passing through the leading impulsive shell. Beyond 1 day, the blast wave evolves as a classic decelerating forward shock with an electron distribution index of $p=1.66\pm0.01$. Our model also indicates a jet-break at $\sim2$ days, and a half-opening angle of $\theta_j=16\mathring{.}6 \pm 1\mathring{.}1$. Following the period of injection, the total energy is $\zeta\sim18$ times the initial impulsive energy, with a final collimation-corrected energy of $E_{\mathrm{Kf}}\sim5.7\times10^{49}$ erg. The minimum Lorentz factors this model requires are consistent with constraints from the early radio measurements of $\Gamma>35$ to $\Gamma>5$ between $\sim0.1$ and $1$ day. These results demonstrate the importance of rapid and sensitive radio follow-up of GRBs for exploring their central engines and outflow behaviour.

astro-ph.HE↗

arXiv2025

Spectroscopic orbit and variability of the Be star V1624 Cyg = 28 Cyg

In recent years the idea, first formulated many decades ago, that the Be phenomenon could be causally related to the duplicity of Be stars, has been repeatedly reconsidered from various perspectives. It is important, therefore, to have reliable information on Be stars, which are confirmed members of binary systems. This study is devoted to V1624 Cyg = 28 Cygni, which was recently identified as a binary with a compact secondary. By measuring the radial velocities (RVs) of the wings of the H alpha emission line and using archival data and published RVs from the International Ultraviolet Explorer, we demonstrate that the Be primary moves in the 359.26 d orbit found recently from interferometry. Our preliminary radial-velocity solution leads to binary masses of 5.6, and 0.66 solar masses. Moreover, we documented large and irregular spectral, brightness, and colour changes over a time interval of several decades to show that the object never completely lost its circumstellar matter.

astro-ph.SR↗

arXiv2025

EP 250108a/SN 2025kg: Observations of the most nearby Broad-Line Type Ic Supernova following an Einstein Probe Fast X-ray Transient

With a small sample of fast X-ray transients (FXTs) with multi-wavelength counterparts discovered to date, the progenitors of FXTs and their connections to gamma-ray bursts (GRBs) and supernovae (SNe) remain ambiguous. Here, we present photometric and spectroscopic observations of SN 2025kg, the supernova counterpart to the FXT EP 250108a. At $z=0.17641$, this is the closest known SN discovered following an Einstein Probe (EP) FXT. We show that SN 2025kg's optical spectra reveal the hallmark features of a broad-lined Type Ic SN. Its light curve evolution and expansion velocities are also comparable to those of GRB-SNe, including SN 1998bw, and several past FXT SNe. We present JWST/NIRSpec spectroscopy taken around SN 2025kg's maximum light, and find weak absorption due to He I $\lambda 1.0830, \lambda 2.0581$ $\mu$m and a broad, unidentified feature at $\sim$ 4-4.5 $\mu$m. Further, we observe clear evidence for broadened H$\alpha$ in optical data at 42.5 days that is not detected at other epochs, indicating interaction with hydrogen-rich material. From its light curve, we derive a $^{56}$Ni mass of 0.2 - 0.6 $M_{\odot}$. Together with our companion paper (Eyles-Ferris et al. 2025), our broadband data of EP 250108a/SN 2025kg are consistent with a trapped or low energy ($\lesssim 10^{51}$ ergs) jet-driven explosion from a collapsar with a zero-age main sequence mass of 15-30 $M_{\odot}$. Finally, we show that the sample of EP FXT SNe support past rate estimates that low-luminosity jets seen through FXTs are more common than successful (GRB) jets, and that similar FXT-like signatures are likely present in at least a few percent of the brightest Ic-BL SNe.

astro-ph.HE↗

arXiv2025

Reconstruction of proton relative stopping power with a granular calorimeter detector model

Proton computed tomography (pCT) aims to facilitate precise dose planning for hadron therapy, a promising and effective method for cancer treatment. Hadron therapy utilizes protons and heavy ions to deliver well focused doses of radiation, leveraging the Bragg peak phenomenon to target tumors while sparing healthy tissues. The Bergen pCT Collaboration aims to develop a novel pCT scanner, and accompanying reconstruction algorithms to overcome current limitations. This paper focuses on advancing the track- and image reconstruction algorithms, thereby enhancing the precision of the dose planning and reducing side effects of hadron therapy. A neural network aided track reconstruction method is presented.

physics.comp-ph↗

arXiv2025

New insight into the Rapid Burster by Insight-HXMT

We report the timing and spectral analyses upon of the type II X-ray bursts from the Rapid Burster (MXB 1730--335) observed by Insight-HXMT and Swift/XRT. By stacking the long-duration bursts, we find for the first time that the hard X-rays are lagging than the soft X-rays by 3 seconds. However, such a lag is not visible for the short-duration bursts, probably because of the poor statistics. For all bursts the energy spectrum is found to be non-thermal, thanks to the broad band coverage of Insight-HXMT. These findings put new insights into the type-II bursts and require a temporally showing-up corona for possible interpretation.

astro-ph.HE↗

arXiv2025

Atmospheric dynamics of the hypergiant RW Cep during the Great Dimming

The hypergiant RW Cep is one of the largest stars in our galaxy. The evolution and mass loss of such stars has profound effects on their surrounding regions and the galaxy as a whole. Between 2020 and 2024, RW Cep experienced a historic mass-loss event known as the Great Dimming. This study provides a spectroscopic analysis of RW Cep during the Great Dimming. We examine its atmospheric dynamics and place it in the context of the star's variability behaviour since the early 2000s. We conducted high-cadence spectroscopic observations of RW Cep during the dimming event using the Tartu Observatory 1.5-meter telescope and the Nordic Optical Telescope. We analysed the atmospheric dynamics by measuring the radial velocities and line depths of Fe I and other spectral lines. The radial velocities of the Fe I lines reveal a vertical velocity gradient of 10-20 km/s in the atmosphere, correlating with the strength of the spectral lines. Stronger lines, formed in higher atmospheric layers, have higher radial velocities. We measured the systemic velocity at -50.3 km/s. During the dimming, radial velocities were affected by additional emission from the ejected gas, which was blue-shifted relative to the absorption lines. Post-dimming, we observed large-scale atmospheric motions with amplitude ~25 km/s. Strong resonance lines of Ba II, K I, Na I and Ca I showed stable central emission components at -56 km/s, likely of circumstellar origin.

astro-ph.SR↗

arXiv2024

Identification of the optical counterpart of the fast X-ray transient EP240414a

Fast X-ray transients (FXTs) are extragalactic bursts of X-rays first identified in archival X-ray data, and now routinely discovered by the Einstein Probe in real time, which is continuously surveying the night sky in the soft ($0.5 - 4$ keV) X-ray regime. In this Letter, we report the discovery of the second optical counterpart (AT2024gsa) to an FXT (EP240414a). EP240414a is located at a projected radial separation of 27 kpc from its likely host galaxy at $z = 0.4018 \pm 0.0010$. The optical light curve of AT2024gsa displays three distinct components. The initial decay from our first observation is followed by a re-brightening episode, displaying a rapid rise in luminosity to an absolute magnitude of $M_r \sim -21$ after two rest-frame days. While the early optical luminosity and decline rate is similar to luminous fast blue optical transients, the colour temperature of AT2024gsa is distinctly red and we show that the peak flux is inconsistent with a thermal origin. The third component peaks at $M_i \sim -19$ at $\gtrsim 16$ rest-frame days post-FXT, and is compatible with an emerging supernova. We fit the $riz$-band data with a series of power laws and find that the decaying components are in agreement with gamma-ray burst afterglow models, and that the re-brightening may originate from refreshed shocks. By considering EP240414a in context with all previously reported known-redshift FXT events, we propose that Einstein Probe FXT discoveries may predominantly result from (high-redshift) gamma-ray bursts, and thus appear to be distinct from the previously discovered lower redshift, lower luminosity population of FXTs.

astro-ph.HE↗

arXiv2024

Latency Minimization for IRS-enhanced Wideband MEC Networks with Practical Reflection Model

Intelligent reflecting surface (IRS) has been considered as an efficient way to boost the computation capability of mobile edge computing (MEC) system, especially when the communication links is blocked or the communication signal is weak. However, most existing works are restricted to narrow-band channel and ideal IRS reflection model, which is not practical and may lead to significant performance degradation in realistic systems. To further exploit the benefits of IRS in MEC system, we consider an IRS-enhanced wideband MEC system with practical IRS reflection model. With the aim of minimizing the weighted latency of all devices, the offloading data volume, edge computing resource, BS's receiving vector, and IRS passive beamforming are jointly optimized. Since the formulated problem is non-convex, we employ the block coordinate descent (BCD) technique to decouple it into two subproblems for alternatively optimizing computing and communication settings. The effectiveness and convergence of the proposed algorithm are validate via numerical analyses. In addition, simulation results demonstrate that the proposed algorithm can achieve lower latency compared to that based on the ideal IRS reflection model, which confirms the necessary of considering practical model when designing an IRS-enhanced wideband MEC system.

cs.IT↗

arXiv2024

SN 2023zaw: the low-energy explosion of an ultra-stripped star

Most stripped-envelope supernova progenitors are thought to be formed through binary interaction, losing hydrogen and/or helium from their outer layers. Ultra-stripped supernovae are an emerging class of transient which are expected to be produced through envelope-stripping by a NS companion. However, relatively few examples are known and the outcomes of such systems can be diverse and are poorly understood at present. Here, we present spectroscopic observations and high-cadence, multi-band photometry of SN 2023zaw, a rapidly evolving supernova with a low ejecta mass discovered in a nearby spiral galaxy at D = 39.7 Mpc. It has significant Milky Way extinction, $E(B-V)_{\rm MW} = 0.21$, and significant (but uncertain) host extinction. Bayesian evidence comparison reveals that nickel is not the only power source and an additional energy source is required to explain our observations. Our models suggest an ejecta mass of $M_{\rm ej} \sim 0.07$ $\rm M_{\odot}$ and a synthesized nickel mass of $M_{\rm Ni} \sim 0.007$ $\rm M_{\odot}$ are required to explain the observations. We find that additional heating from a central engine, or interaction with circumstellar material can power the early light curve.

astro-ph.HE↗

arXiv2024

Discovery of the optical and radio counterpart to the fast X-ray transient EP240315a

Fast X-ray Transients (FXTs) are extragalactic bursts of soft X-rays first identified >10 years ago. Since then, nearly 40 events have been discovered, although almost all of these have been recovered from archival Chandra and XMM-Newton data. To date, optical sky surveys and follow-up searches have not revealed any multi-wavelength counterparts. The Einstein Probe, launched in January 2024, has started surveying the sky in the soft X-ray regime (0.5-4 keV) and will rapidly increase the sample of FXTs discovered in real time. Here, we report the first discovery of both an optical and radio counterpart to a distant FXT, the fourth source publicly released by the Einstein Probe. We discovered a fast-fading optical transient within the 3 arcmin localisation radius of EP240315a with the all-sky optical survey ATLAS, and our follow-up Gemini spectrum provides a redshift, z=4.859+/-0.002. Furthermore, we uncovered a radio counterpart in the S-band (3.0 GHz) with the MeerKAT radio interferometer. The optical (rest-frame UV) and radio luminosities indicate the FXT most likely originates from either a long gamma-ray burst or a relativistic tidal disruption event. This may be a fortuitous early mission detection by the Einstein Probe or may signpost a mode of discovery for high-redshift, high-energy transients through soft X-ray surveys, combined with locating multi-wavelength counterparts.

astro-ph.HE↗

arXiv2024

High-finesse nanofiber Fabry-P\'{e}rot resonator in a portable storage container

We present characterization and storage methods for a high-finesse nanofiber Fabry-P\'{e}rot resonator. Reflection spectroscopy from both ends of the resonator allows for evaluation of the mirror transmittances and optical loss inside the resonator. To maintain the quality of the nanofiber resonator after the fabrication, we have developed a portable storage container. By filling the container with dry, clean nitrogen gas, we can prevent contamination of the nanofiber during storage. This approach allows us to minimize the additional optical loss to less than 0.08% over a week. The portable container facilitates both the fabrication and subsequent experimentation with the resonator in different locations. This flexibility expands the range of applications, including quantum optics, communication, and sensing.

physics.ins-det↗

arXiv2023

Performance of the electromagnetic and hadronic prototype segments of the ALICE Forward Calorimeter

We present the performance of a full-length prototype of the ALICE Forward Calorimeter (FoCal). The detector is composed of a silicon-tungsten electromagnetic sampling calorimeter with longitudinal and transverse segmentation (FoCal-E) of about 20$X_0$ and a hadronic copper-scintillating-fiber calorimeter (FoCal-H) of about 5$\lambda_{\rm int}$. The data were taken between 2021 and 2023 at the CERN PS and SPS beam lines with hadron (electron) beams up to energies of 350 (300) GeV. Regarding FoCal-E, we report a comprehensive analysis of its response to minimum ionizing particles across all pad layers. The longitudinal shower profile of electromagnetic showers is measured with a layer-wise segmentation of 1$X_0$. As a projection to the performance of the final detector in electromagnetic showers, we demonstrate linearity in the full energy range, and show that the energy resolution fulfills the requirements for the physics needs. Additionally, the performance to separate two-showers events was studied by quantifying the transverse shower width. Regarding FoCal-H, we report a detailed analysis of the response to hadron beams between 60 and 350 GeV. The results are compared to simulations obtained with a Geant4 model of the test beam setup, which in particular for FoCal-E are in good agreement with the data. The energy resolution of FoCal-E was found to be lower than 3% at energies larger than 100 GeV. The response of FoCal-H to hadron beams was found to be linear, albeit with a significant intercept that is about factor 2 larger than in simulations. Its resolution, which is non-Gaussian and generally larger than in simulations, was quantified using the FWHM, and decreases from about 16% at 100 GeV to about 11% at 350 GeV. The discrepancy to simulations, which is particularly evident at low hadron energies, needs to be further investigated.

physics.ins-det↗

arXiv2023

Evidence of mini-jet emission in a large emission zone from a magnetically-dominated gamma-ray burst jet

The second brightest GRB in history, GRB230307A, provides an ideal laboratory to study the mechanism of GRB prompt emission thanks to its extraordinarily high photon statistics and its single episode activity. Here we demonstrate that the rapidly variable components of its prompt emission compose an overall broad single pulse-like profile. Although these individual rapid components are aligned in time across all energy bands, this overall profile conspires to show a well-defined energy-dependent behavior which is typically seen in single GRB pulses. Such a feature demonstrates that the prompt emission of this burst is from many individual emitting units that are casually linked in a emission site at a large distance from the central engine. Such a scenario is in natural consistency with the internal-collision-induced magnetic reconnection and turbulence framework, which invokes many mini-jets due to local magnetic reconnection that constantly appear and disappear in a global magnetically-dominated jet.

astro-ph.HE↗