arXiv subjects

Cheng-Han Lai

Publications and source records attributed to Cheng-Han Lai.

5 recordsLinked to original sources

arXiv2026

A temporally clustered gravitational wave pair and a Type IIb supernova

Compact object mergers are generally expected long after the stellar collapse that forms their remnants. Here we report two marginal gravitational wave (GW) candidates, S190910d and S190910h, separated by 7.1 hr, whose three-dimensional localizations overlap at the position and distance of the Type IIb supernova SN 2019rsq. The GW pair lies in the extreme tail of the joint distribution of temporal separation and localization overlap, with a pairwise null probability of $7.6 \times 10^{-5}$ under our Monte Carlo null test. Independent re-analyses recover both as signal-like candidates. Under a common-origin hypothesis, S190910h remains consistent with a low-mass compact binary with source-frame masses near $2.5 + 1.3\,M_\odot$, whereas S190910d favours a strongly asymmetric $\sim 34 + 3\,M_\odot$ system whose observed amplitude is compatible with the supernova distance. SN 2019rsq is a rapidly evolving stripped-envelope Type IIb explosion whose light curve is consistent with stellar collapse occurring near the GW activity. Although the supernova coincidence alone is not statistically rare, its timing and physical properties provide a stellar collapse anchor for the unusual GW pair. A fragmenting collapsar disk provides a plausible framework that can accommodate the component masses, hour-scale delays and supernova properties, raising the possibility that multiple compact object mergers can accompany a single stellar collapse episode.

astro-ph.HE↗

arXiv2026

SN 2025wiu: A Type Ib Supernova with Post-Peak Optical Rebrightening

We present optical photometric and optical/near-infrared spectroscopic observations of SN 2025wiu, a Type Ib supernova (SN) at z = 0.046927. The r-band light curve exhibits an initial maximum of approximately -18.2 mag, followed by a weaker optical rebrightening reaching approximately -17.8 mag at +38 days in the rest frame relative to the first maximum. However, the reconstructed bolometric light curve does not show a distinct second peak, but instead exhibits a reduced decline rate during the corresponding phase. The spectra consistently show Type Ib features without narrow emission components associated with circumstellar material. The late-time bolometric decline rate of 0.054 +/- 0.008 mag/day is faster than expected for complete trapping of 56Co decay. A radioactive-decay model for the first peak requires nickel mass = 0.47 solar masses, above the range typical of SNe Ib. A combined radioactive-decay and circumstellar-interaction model reproduces the full bolometric light curve with nickel mass ~ 0.38 solar masses, ejecta mass ~ 4.7 solar masses, and an effective circumstellar mass of ~ 0.10 solar masses, with the interaction contributing from the first peak onward. The lack of interaction signatures in the spectra favours an aspherical, disk-like circumstellar structure engulfed by the ejecta, in which the interaction energy is thermalized below the ejecta photosphere; the fitted circumstellar parameters are therefore effective values rather than direct measurements of the circumstellar geometry. SN 2025wiu demonstrates that optical rebrightening does not necessarily imply a genuine bolometric secondary maximum and highlights the importance of multi-band observations for identifying the powering mechanisms of double-peaked stripped-envelope supernovae.

astro-ph.HE↗

arXiv2026

Decadal pre-explosion activity and circumstellar interaction in a supernova

When a massive star explodes as a supernova, crucial information about its immediate environment is lost within hours. Here we report rapid optical observations from Lulin Observatory of the broad-lined Type Ic supernova SN 2026gzf, beginning 1.25 hours after Einstein Probe detected the X-ray transient EP260321a. Our data led to the discovery of the optical counterpart and showed a luminous blue first-day excess that cannot be reproduced by standard radioactive models. We find that interaction between the ejecta and $\approx 0.02$ M$_{\odot}$ of circumstellar material accounts for the early excess. Archival Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) images show variability at the explosion site over the previous $\sim 12$ years, with the source brightening by a factor of $\sim 1.5$ in the final $\sim 3$ years before explosion, providing rare evidence for pre-explosion activity in a stripped-envelope progenitor system. The precursor brightening suggests enhanced eruptive mass loss during late-stage oxygen burning before core collapse, while an additional silicon-burning episode shortly before explosion may have created the compact nearby material responsible for the X-ray shock-breakout signal. SN 2026gzf therefore offers the first view of how a stripped progenitor modifies its immediate environment shortly before death, linking long-term precursor variability, circumstellar interaction and the explosion itself.

astro-ph.HE↗

arXiv2025

Search for the Optical Counterpart of Einstein Probe Discovered Fast X-ray Transients from Lulin Observatory

The launch of the Einstein Probe (EP) mission has revolutionized the detection and follow-up observations of fast X-ray transients (FXTs) by providing prompt and timely access to their precise localizations. In the first year of its operation, the EP-mission reports the discovery of 72 high signal-to-noise FXTs. Subjected to the visibility in the sky and weather conditions, we search for the optical counterparts of 42 EP-discovered FXTs from the Lulin Observatory. We successfully detect the optical counterparts of 12 FXTs, and five of those are first discovered by us from the Lulin Observatory. We find that the optical counterparts are generally faint ($r>20$\,mag) and decline rapidly ($>0.5$\,mag per day). We also find that 12 out of 42 FXTs show direct evidence of their association with Gamma-Ray Bursts (GRBs) through significant temporal and spatial overlapping. Furthermore, the luminosities and redshifts of FXTs with confirmed optical counterparts in our observations are fully consistent with the faintest end of the GRB population. However, the non-detection of any associated optical counterpart with a significant fraction of FXTs suggests that EP FXTs are likely a subset of so-called `dark FXTs', similar to `dark GRBs'. Additionally, the luminosities of {\bf two FXTs with confirmed redshifts} are also consistent with jetted tidal disruption events (TDEs). However, we find that the optical luminosities of FXTs differ significantly from typical supernova shock breakout or kilonova emissions. Thus, we conclude that a significant fraction of EP-discovered FXTs are associated with events having relativistic jets; either a GRB or a jetted TDE.

astro-ph.HE↗

arXiv2024

Discovery and Extensive Follow-Up of SN 2024ggi, a nearby type IIP supernova in NGC 3621

We present the discovery and early observations of the nearby Type II supernova (SN) 2024ggi in NGC 3621 at 6.64 +/- 0.3 Mpc. The SN was caught 5.8 (+1.9 -2.9) hours after its explosion by the ATLAS survey. Early-phase, high-cadence, and multi-band photometric follow-up was performed by the Kinder (Kilonova Finder) project, collecting over 1000 photometric data points within a week. The combined o- and r-band light curves show a rapid rise of 3.3 magnitudes in 13.7 hours, much faster than SN 2023ixf (another recent, nearby, and well-observed SN II). Between 13.8 and 18.8 hours after explosion SN 2024ggi became bluer, with u-g colour dropping from 0.53 to 0.15 mag. The rapid blueward evolution indicates a wind shock breakout (SBO) scenario. No hour-long brightening expected for the SBO from a bare stellar surface was detected during our observations. The classification spectrum, taken 17 hours after the SN explosion, shows flash features of high-ionization species such as Balmer lines, He I, C III, and N III. Detailed light curve modeling reveals critical insights into the properties of the circumstellar material (CSM). Our favoured model has an explosion energy of 2 x 10^51 erg, a mass-loss rate of 10^-3 solar_mass/yr (with an assumed 10 km/s wind), and a confined CSM radius of 6 x 10^14 cm. The corresponding CSM mass is 0.4 solar_mass. Comparisons with SN 2023ixf highlight that SN 2024ggi has a smaller CSM density, resulting in a faster rise and fainter UV flux. The extensive dataset and the involvement of citizen astronomers underscore that a collaborative network is essential for SBO searches, leading to more precise and comprehensive SN characterizations.

astro-ph.HE↗