Antonella Palmese

Electromagnetic counterparts to gravitational-wave (GW) sources probe the properties, environments, and evolution of compact-object mergers. S251112cm belongs to an emerging class of GW candidates with possible subsolar-mass components. We present an optical counterpart search for S251112cm with the Wide Field Survey Telescope (WFST). The observations began 20.2 hr after the GW trigger and continued for 3 nights, covering approximately 780 deg ² and 51% of the localization probability in the updated skymap. We searched for newly emerging, rapidly evolving, off-nuclear optical transients and identified four candidates whose host-galaxy distances are broadly consistent with the GW distance estimate. However, all four evolve substantially more slowly than AT 2017gfo, ruling out an AT 2017gfo-like origin and disfavoring their association with S251112cm as rapidly evolving kilonova counterparts. Combining WFST and DECam observations increases the covered localization probability to approximately 67%. Conditional on the counterpart lying within this footprint, we constrain a grid of binary-neutron-star kilonova models. For viewing angles θ _obs < 60 ^∘ , with the other parameters fixed to the best-fitting values for AT 2017gfo, models with dynamical ejecta mass M _dyn ≳ 0.01 M _⊙ or wind ejecta mass M _wind ≳ 0.05 M _⊙ are disfavored over most of the covered GW probability. Beyond kilonova models, our phenomenological analysis constrains rapidly evolving optical transients with T _rise,1/2 ≤ 2 days to peak absolute magnitudes fainter than approximately −13 mag. To our knowledge, WFST provides the strongest optical constraints to date in the northern sky on possible rapidly evolving counterparts to this new class of GW candidates involving subsolar-mass compact objects. The comparable depths and complementary sky coverage of WFST and DECam extend these constraints across a larger fraction of the localization region at similar sensitivity.

The Lazuli Space Observatory is a 3 m aperture astronomical facility designed for rapid-response observations and precision astrophysics across visible to near-IR wavelengths (400–1700 nm bandpass). An off-axis, freeform telescope delivers diffraction-limited image quality (Strehl >0.8 at 633 nm) to three instruments across a wide, flat focal plane. The three instruments provide complementary capabilities; the Wide-field Context Camera (WCC) delivers multiband imaging over a 35 ′ × 12 ′ footprint with high-cadence photometry; the Integral-field Spectrograph (IFS) provides continuous 400–1700 nm spectroscopy at R ∼ 100–500 for stable spectrophotometry; and the ExtraSolar Coronagraph (ESC) enables high-contrast imaging expected to reach raw contrasts of 10 ⁻⁸ and postprocessed contrasts approaching 10 ⁻⁹ . Operating from a 3:1 lunar-resonant orbit, Lazuli will respond to targets of opportunity in under four hours—a programmatic requirement designed to enable routine temporal responsiveness that is unprecedented for a space telescope of this size. Lazuli’s technical capabilities are shaped around three broad science areas: (1) time-domain and multimessenger astronomy, (2) stars and planets, and (3) cosmology. These capabilities enable a potent mix of science spanning gravitational-wave counterpart characterization, fast-evolving transients, Type Ia supernova cosmology, high-contrast exoplanet imaging, and spectroscopy of exoplanet atmospheres. While these areas guide the observatory design, Lazuli is conceived as a general-purpose facility with open time for the global community. We describe the observatory architecture and capabilities in the preliminary design phase, with science operations anticipated following a rapid development cycle from concept to launch; quoted performance specifications reflect current design requirements and modeling predictions, pending on-orbit verification.

Advancing time-domain and multi-messenger astronomy requires a multi-wavelength network of observatories capable of rapidly discovering, classifying, and characterizing transient phenomena. A critical gap in current capabilities is the inability to follow up faint, fast-evolving transients with sensitive, wide-band imaging and spectroscopic observations from space on timescales of minutes to hours. We discuss how the Lazuli Space Observatory will address this gap through a large collecting area, optical/NIR photometry and low-resolution integral field spectroscopy, and a rapid-response architecture with a mission requirement of <4 hr from trigger to first photon. Based on a latency analysis, we find a credible path to realizing response times well below this requirement, with best-case scenarios below 90 minutes under favorable conditions. We highlight extragalactic science opportunities in currently un(der)explored parts of parameter space, including gravitational wave follow-up, kilonova characterization, supernova progenitor physics, and a wide variety of fast-evolving transients and high redshift events. We further outline new observational capabilities for Galactic time-domain science, including high frequency variability in accreting systems, precision astrometry of compact objects, and the detection of compact and ultracompact binaries, enabled by high-frequency, diffraction-limited imaging and astrometry. Together, its capabilities—combining flagship sensitivity with response times one to two orders of magnitude faster than existing large space observatories—position Lazuli to make transformative contributions across time-domain and multi-messenger astrophysics.

Gamma-ray bursts (GRBs) have traditionally been classified by their prompt emission duration and spectral hardness, with short GRBs (sGRB; ≲2 s) originating from compact object mergers and long GRBs (LGRB; >2 s) from massive star core collapse. Recent kilonova (KN) associations with long-duration GRBs have challenged this standard picture. We analyze the host galaxies of nine GRBs with associated kilonova candidates at z < 0.6, including five sGRB-KNe and four LGRB-KNe. Using both parametric and nonparametric modeling of the host light distributions, we investigate the progenitor environments of these events and test whether their hosts show evidence for recent galaxy interactions that could favor dynamical formation channels or isolated pathways following merger-driven star formation episodes for neutron star binaries. We find that five of the nine hosts display tidal features that show they have likely undergone recent mergers, suggesting that merger-driven, dynamical formation pathways may contribute in some systems. We find no clear morphological distinction between sGRB-KN and LGRB-KN hosts as both populations span a wide range of morphologies, including ellipticals, spirals, and interacting systems with tidal features. Multi-Sérsic modeling of the host light profiles further shows that host-normalized offsets inferred from single-Sérsic fits can be either under- or overestimated when the transient is associated with a specific subcomponent of a complex host light profile. These results highlight the importance of decomposing host morphology into physically relevant components when interpreting GRB environments and galactocentric offsets.

Dual active galactic nuclei (DAGN) mark the observable stage of massive black hole (MBH) pairing during galaxy mergers and are the progenitors of the MBH binaries that generate low-frequency gravitational waves. Using the large-volume ASTRID cosmological simulation, we construct mock DAGN catalogs tailored to the selection functions of current (COSMOS-Web and DESI) and forthcoming (Nancy Grace Roman Space Telescope (Roman) and Lynx X-ray Observatory (Lynx)) surveys, enabling direct comparisons between simulations and observations. With realistic observational selections, ASTRID reproduces the observed dual fractions, projected separations, and host-galaxy properties across redshifts. We predict a substantial population of small-separation ( 1, as the environments most likely to host imminent MBH binaries. These results establish a unified cosmological framework connecting DAGN demographics, MBH binary evolution, and gravitational-wave sources, while identifying high-priority targets for coordinated electromagnetic and multimessenger observations in the coming decade.

The association of broad-lined Type Ic (Ic-BL) supernovae with long-duration gamma-ray bursts (GRBs) has been known for 28 yr. However, only about 70 GRB supernovae (GRB-SNe) have been identified, of which only one-half have spectroscopic classifications. At z = 0.153, GRB 260310A is the twelfth spectroscopically confirmed GRB-SN discovered within 1 Gpc, offering a critical opportunity to follow one of these rare supernovae in detail. We present optical to near-infrared imaging and spectroscopy of GRB 260310A and SN 2026fgk out to 65 days after discovery. The optical afterglow is among the brightest ever observed from a GRB. Spectra obtained more than 2 weeks after the explosion reveal broad absorption features that securely identify SN 2026fgk as a Type Ic-BL supernova (SN). Modeling of the multiwavelength ( grizJK _s ) lightcurve shows that the SN is approximately one-half the luminosity ( k _98bw = 0.4–0.6) of the canonical GRB-SN 1998bw. We derive a nickel mass of M _Ni = 0.4–0.5 M _⊙ with a total ejected mass of M _ej ≈ 4–6 M _⊙ and kinetic energy E _K = (3–8) × 10 ⁵¹ erg. The GRB exploded at an extremely large offset of 15 kpc from its host galaxy. Long-slit spectra reveal a “bridge” of nebular emission extending along the galaxy’s disk to the GRB location, which has a subsolar metallicity (∼0.4 Z _⊙ ), compared to a near-solar metallicity for the host galaxy. This indicates that the large offset arises from the galaxy’s extended light profile rather than an isolated environment.

Tidal disruption events (TDEs) have traditionally been discovered in optical sky surveys through targeted searches of nuclear transients. However, it is expected that some TDEs will occur outside the galaxy nucleus, arising from wandering black holes (BHs) originating in galaxy mergers. Here, we present observations of TDE 2025abcr, the first optical TDE discovered in the outskirts of a host galaxy. The TDE was identified by a custom “off-nuclear” implementation of the machine learning classifier tdescore , which classifies new ZTF transients based on their lightcurves. Follow-up observations confirm that TDE 2025abcr is a TDE-H+He, occurring 9 . ″ 5 (9.3 kpc projected distance) from the nucleus of a massive galaxy ( M _⋆ = 10 ^11.18±0.03 M _⊙ ) with a central BH mass of 10 ^8.82±0.65 M _⊙ . TDE 2025abcr itself was likely disrupted by a much lighter BH (10 ^6.09±0.53 M _⊙ , as estimated with peak luminosity scaling relations). The BH was either dynamically ejected from the nucleus or lies at the center of a very faint tidally stripped dwarf galaxy undergoing a minor merger. Late-time observations of TDE 2025abcr could confirm the origin of this apparent “wandering” BH. The rate of highly offset (≳3 kpc) TDEs can be constrained to <10% of the nuclear TDE rate, but our discovery implies that many dozens of similar sources will be detected by the Vera C. Rubin Observatory each year with resolvable offsets.

We present the discovery of EP250827b/SN 2025wkm, an X-ray flash (XRF) discovered by the Einstein Probe (EP), accompanied by a broad-line Type Ic supernova (SN Ic-BL) at z = 0.1194. EP250827b possesses a prompt X-ray luminosity of ∼10 ⁴⁵ erg s ⁻¹ , lasts over 1000 s, and has a peak energy E _p < 1.5 keV at 90% confidence. SN 2025wkm possesses a double-peaked optical light curve (LC), though its bolometric luminosity plateaus after its initial peak for ∼20 days, consistent with a central engine injecting additional energy into the explosion. Its spectrum transitions from a blue to red continuum with clear blueshifted broad absorption features consistent with a SN Ic-BL classification. We do not detect any transient radio emission and rule out the existence of an on-axis, energetic jet ≳10 ⁵⁰ erg assuming a typical LGRB circumburst constant density ( n ≈ 10 ⁻³ –10 ⁻¹ cm ⁻³ ) and microphysical parameters ( ϵ _e = 0.1 and ϵ _B = 0.01). In the model we invoke, the collapse gives rise to a long-lived magnetar, potentially surrounded by an accretion disk. Magnetically driven winds from the magnetar and the disk mix together and break out with a velocity ∼0.35 c and interact with an extended circumstellar medium with radius ∼10 ¹³ cm, generating X-ray breakout emission through nonthermal free–free processes. The disk outflows and magnetar winds power blackbody photospheric emission as they cool adiabatically and thermalize, producing the first SN peak. The spin-down luminosity of the magnetar and radioactive decay of ⁵⁶ Ni power the late-time emission. We end by discussing the landscape of XRF-SNe within the context of EP’s recent discoveries.

The explosion of a star is first marked by the shock wave breaking out of the stellar surface, producing a burst of ultraviolet and X-ray radiation. These events are observationally rare, despite likely accompanying the majority of supernovae (SNe). Here, we report on our multiwavelength observing campaign of the closest Einstein Probe fast X-ray transient (FXT) EP260321a at z = 0.0344. The thermal (kT = 130 eV) X-ray emission with peak luminosity of 1.0 × 10⁴⁵ erg s⁻¹ points to a shock breakout origin. We demonstrate that EP260321a is accompanied by a broad-lined Type Ic supernova, SN 2026gzf. The supernova (SN) properties, including its spectral evolution, lightcurve evolution, and expansion velocities, are all typical of the energetic stripped-envelope SNe associated with gamma-ray bursts (GRBs). However, deep X-ray upper limits obtained with the Chandra X-ray Observatory do not detect an X-ray afterglow, and instead exclude the afterglow of known GRBs or FXTs. If the stellar explosion launched a successful relativistic jet, we require that it had both a low Lorentz factor of Γ₀ < 30 and a kinetic energy of E_kin < 10⁴⁹ erg for a stellar wind density of A_* ≳ 1. We propose that EP260321a originated from a mildly relativistic, weak outflow that was choked by the progenitor star. This scenario is capable of naturally explaining its low X-ray luminosity and lack of prompt gamma-ray emission. EP260321a bridges the gap between SN 2008D and low-luminosity GRBs, suggesting a greater diversity in the physical parameters of stripped stars as they undergo terminal collapse.

We present a GPU-accelerated transient detection pipeline developed for time-domain surveys with the Dark Energy Camera (DECam). It enables real-time-capable image processing, incorporating science-driven candidate filtering to support rapid transient identification in time-critical observing programs. The pipeline serves as the core transient discovery engine for multiple long-term DECam programs, including the GW-MMADS gravitational-wave follow-up campaign and the DESIRT survey for intermediate-redshift transients with DESI synergy. The pipeline ingests calibrated imaging products from the DECam Community Pipeline and performs image differencing using SFFT, a GPU-accelerated Fourier-domain image-subtraction method, coupled with convolutional neural network-based real–bogus classification, to produce science-ready transient alerts and light curves that are delivered to community brokers. We validate the pipeline using archival DECam data from the DESIRT survey. Our AI-based real–bogus classifier achieves a completeness of ∼99% of real transients while rejecting ∼96% of subtraction artifacts, and the workflow typically reduces the candidate load to a manageable level for survey operations. With GPU acceleration, the typical processing time per DECam exposure is ∼50 s from calibrated image processing to alert generation using a modest allocation of computing resources.