The Lazuli Space Observatory is a 3-meter aperture astronomical facility designed for rapid-response observations and precision astrophysics across visible to near-infrared 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: a Wide-field Context Camera (WCC) delivers multi-band imaging over a 35' × 12' footprint with high-cadence photometry; an Integral Field Spectrograph (IFS) provides continuous 400-1700 nm spectroscopy at R ∼ 100-500 for stable spectrophotometry; and an ExtraSolar Coronagraph (ESC) enables high-contrast imaging expected to reach raw contrasts of 10^-8 and post-processed 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 multi-messenger 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 capable of supporting a wide range of astrophysical investigations, 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.

Since their first detection in 2015, gravitational wave observations have enabled a variety of studies, ranging from stellar evolution to fundamental physics. In this chapter, we focus on their use as "standard sirens", describing the different methodologies that can be adopted to measure cosmological parameters with compact object binaries from ground-based gravitational wave detectors. We cover the three main classes of standard siren measurements, showing how the expansion of the Universe can be constrained through Bayesian statistics both with gravitational wave observations alone and with the aid of electromagnetic emission from the electromagnetic counterpart of gravitational wave events and from galaxies. Finally, we summarize the existing measurements and prospects for future constraints on cosmological parameters.

With the arrival of ever higher throughput wide-field surveys and a multitude of multi-messenger and multi-wavelength instruments to complement them, software capable of harnessing these associated data streams is urgently required. To meet these needs, a number of community supported alert brokers have been built, currently focused on processing of Zwicky Transient Facility (ZTF; similar to 105-106 alerts per night) with an eye toward Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST; similar to 2 & times; 107 alerts per night). Building upon the system that successfully ran in production for ZTF's first seven years of operation, we introduce Burst & Outburst Observations Monitor (BOOM), an analysis framework focused on real-time, joint brokering of these alert streams. BOOM harnesses the performance of a Rust-based software stack relying on a non-relational MongoDB database combined with a Valkey in-memory processing queue and a Kafka cluster for message sharing. With this system, we demonstrate feature parity with the existing ZTF system with a throughput similar to 7 & times; higher. We describe the workflow that enables the real-time processing as well as the results with custom filters we have built to demonstrate the system's capabilities. In conclusion, we present the development roadmap for both BOOM and Babamul-the public-facing LSST alert broker built atop BOOM-as we begin the Rubin era.

We present the discovery of SN 2025mkn, a gravitationally lensed Type II supernova. First detected as a blue transient in Zwicky Transient Facility (ZTF), 0 .'' 83 from a z = 0.42 elliptical galaxy, the follow-up SNIFS/UH2.2 m and LRIS/Keck spectra revealed absorption lines at z = 1.371. Later JWST NIRCam imaging shows that the bright transient is a close pair of point sources separated by similar to 0.'' 07 , and a 30 times fainter counterimage opposite the lens, for which NIRSpec reveals strong H alpha emission also at z = 1.371. The lightcurves and spectra are consistent with the Type II supernova source being magnified greater than or similar to 100 times, with similar to 250 required to reconcile its luminosity with that of nearby events such as SN 2023ixf. Lens models are consistent with such high magnifications, and always show that the faint image arrived first (undetected in earlier ZTF imaging), consistent with the later spectral phase of this fainter image. A fourth image is also predicted and possibly detected in the NIRSpec data. Lightcurve-based time-delay measurements are not possible due to the first image being the faintest; however, the resolved NIRSpec spectra offer a future opportunity for time-delay cosmography through supernova phase measurements.

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 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 ML classifier , which classifies new ZTF transients based on their lightcurves. Follow-up observations confirm that TDE 2025abcr is a TDE-H+He, occurring 9.5" (10.3 kpc projected distance) from the nucleus of a massive galaxy (M_⋆ = 10^11.18 ± 0.03M_⊙) with a central black hole mass of 10^8.82 ± 0.65M_⊙. TDE 2025abcr itself was likely disrupted by a much lighter black hole (10^6.09±0.53M_⊙, as estimated with peak luminosity scaling relations). The black hole 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 'orphan' black hole. The rate of highly offset (≳3 kpc) TDEs can be constrained to <10

Galaxy mergers can both trigger star formation and rearrange where stars live, producing long-lived tidal structures and collisionally driven density waves (known as collisional rings) that can extend for tens of kpc from their host galaxy centers. Here we report the discovery of SN 2025adpq, a Type Ia supernova at z=0.1540, found within a collisional ring, which we call Pika's Halo, with circumference ∼70 kpc that was produced by a major merger between two comparable mass galaxies (log(M_*/M_⊙)≈10.5). The supernova lies along the ring at a projected offset of ∼11.4 kpc from the nucleus of the primary galaxy (hereafter G1). Optical spectroscopy obtained with the Southern African Large Telescope (SALT) and Gemini South reveal signatures consistent with merger induced ongoing star formation, while prominent Calcium H and K absorption indicates a substantial old stellar population within the ring. Given the long delay times expected for most SN Ia progenitors, we argue that SN 2025adpq most likely arose from an old progenitor system that was displaced from G1 during the head-on encounter. The progenitor was likely stripped from its parent galaxy by the collisionally induced pressure wave and exploded far from its birthplace. This event highlights collisional rings as a pathway for producing large offset SNe Ia, and it motivates targeted searches for faint, dynamically displaced old populations in seemingly hostless SN Ia environments. We additionally identify other supernovae, including supernova siblings, in the low redshift sample of collisional ring galaxies, and find that SN 2025adpq is one of only a handful of classified supernova identified in the expanding ring of a collisional ring complex.

Gravitational wave (GW) and short Gamma Ray Burst (sGRB) observations provide us with complementary views of compact object mergers. The paucity of binary neutron star merger (BNS) detections in the latest LIGO/Virgo/KAGRA (LVK) observing run raises the question of whether the GW merger rates are sufficient to explain the observed sGRB rate with compact object mergers alone. We investigate this connection using the latest merger rate constraints from the fourth LVK observing run (O4) and published estimates of the local sGRB rate density. For an observed sGRB rate density of ∼ 1-7 Gpc^-3 yr^-1, if >55% of BNS mergers can successfully launch a jet, we find that the current LVK BNS merger rate can be reconciled with a sGRB merger population containing a significant fraction of relatively wide jets with core half-opening angles θ_j ≥ 10^∘. Meanwhile, a narrow jet population (θ_j ∼ 6^∘) can only be matched with the O4 neutron star merger rate estimates for an observed sGRB rate density of ≲ 1 Gpc^-3 yr^-1, which is broadly consistent with several of the latest available estimates. We also find that neutron star-black hole mergers (NSBH) are expected to be a subdominant component of the sGRB population compared to BNS mergers, and they cannot help reconcile some of the highest available sGRB rate (>7 Gpc^-3 yr^-1) with the GW rate estimates. However, they can still substantially contribute to the sGRB population, comprising ∼ 6-16% of it for an observed sGRB rate density of ∼ 1-3 Gpc^-3 yr^-1. Overall, our results indicate that present GW and sGRB observations remain broadly consistent with BNS mergers as the main progenitors of sGRBs.

We present the searches conducted with the Zwicky Transient Facility (ZTF) in response to S250206dm, a bona fide event with an online false alarm rate of one in 25 yr, detected by the International Gravitational Wave Network. Although the event is significant, the nature of the compact objects involved remains unclear, with at least one likely neutron star. ZTF covered 68% of the last refined Bilby localization region, though we did not identify any likely optical counterpart. We describe the ZTF strategy, potential candidates, and the observations that helped rule out candidates, including sources circulated by other collaborations. Similar to Ahumada et al., we perform a frequentist analysis, using simsurvey, as well as Bayesian analysis, using nimbus, to quantify the efficiency of our searches. We find that, given the nominal up-to-date distance to this event of 373 +/- 104 Mpc, our efficiencies are above 10% for KNe brighter than -17.5 absolute magnitude. Assuming the optical counterpart known as kilonova (KN) lies within the ZTF footprint, our limits constrain the brightest end of the KN parameter space. Through dedicated radiative transfer simulations of KNe from binary neutron star (BNS) and black hole-neutron star mergers, we exclude parts of the BNS KN parameter space. Up to 35% of the models with high wind ejecta mass (Mwind approximate to 0.13 M circle dot) are ruled out when viewed face-on ( cos theta obs=1.0 ). Finally, we present a joint analysis using the combined coverage from ZTF and the Gravitational Wave Multimessenger Dark Energy Camera Survey. The joint observations cover 73% of the Bilby localization region, and the combined efficiency has a stronger impact on rising and slowly fading models, allowing us to rule out 55% of the high-mass KN models viewed face-on.

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 non-parametric 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 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.

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 seconds, 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 ≳ 10^50erg 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 ∼ 0.35c and interact with an extended circumstellar medium with radius ∼ 10^13 cm, generating X-ray breakout emission through non-thermal 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 ^56Ni powers the late-time emission. We end by discussing the landscape of XRF-SNe within the context of EP's recent discoveries.

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 similar to 99% of real transients while rejecting similar to 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 similar to 50 s from calibrated image processing to alert generation using a modest allocation of computing resources.

Following the wealth of new results enabled by multimessenger observations of the binary neutron star (BNS) merger GW170817, the next goal is increasing the number of detections of electromagnetic counterparts to gravitational-wave (GW) events. We study the detectability of the prompt emission and afterglows produced by the relativistic jets launched by BNS mergers that will be detected by LIGO-Virgo-KAGRA during their fifth observing run (O5), and by next-generation (XG) GW detectors (Einstein Telescope and Cosmic Explorer). We quantify the impact of various BNS merger and jet afterglow parameters on the likelihood of detection, focusing on the impact of the observer's viewing angle and the jet's core half-opening angle. We explore detectability over a wide range of current state-of-the-art facilities (e.g., the James Webb Space Telescope, Chandra X-ray Observatory) as well as upcoming XG facilities (e.g., AXIS, NewAthena, ngVLA, SKA). We find that a few GW events (similar to 0-4) per year may have a detectable afterglow component in O5, with the largest detection rates expected with SKA in the radio and JWST in the near-infrared. In the XG era, hundreds of multimessenger detections of afterglows per year may be possible with a range of instruments, such as NewAthena in the X-ray and ngVLA in the radio. While zero to a few GW events per year are expected to be accompanied by a detectable prompt emission in O5, dozens per year may be detectable in XG.

On 2025 August 18, the LIGO-Virgo-KAGRA collaboration reported a subthreshold gravitational-wave candidate detection consistent with a subsolar-mass neutron star merger, denoted S250818k. An optical transient, AT2025ulz, was discovered within the localization region. AT2025ulz initially appeared to meet the expected behavior of kilonova emission, the telltale signature of a binary neutron star merger. The transient subsequently rebrightened after similar to 5 days and was classified as a Type IIb supernova. In this work, we analyze the observations of its host galaxy obtained by the Dark Energy Spectroscopic Instrument (DESI). From the DESI spectrum, we obtain a secure redshift of z = 0.084840 +/- 0.000006. If S250818k has an astrophysical origin, this places the transient within 2 sigma of the gravitational-wave distance and results in an integral overlap between the gravitational-wave alert and the transient location of log10I approximate to 3.9-4.2 . Our analysis of the host galaxy's spectral energy distribution reveals a star-forming, dusty galaxy with stellar mass similar to 1010 M circle dot, broadly consistent with the population of both short gamma-ray bursts and core-collapse supernova host galaxies. We also present our follow-up of DESI-selected candidate host galaxies using the Fraunhofer Telescope at the Wendelstein Observatory, and show the promise of DESI for associating or rejecting candidate electromagnetic counterparts to gravitational-wave alerts. These results emphasize the value of DESI's extensive spectroscopic dataset in rapidly characterizing host galaxies, enabling spectroscopic host subtraction, and guiding targeted follow-up.

Modeling the intrinsic alignment (IA) of galaxies poses a challenge to weak lensing analyses. Using the Dark Energy Survey Year 3 shape catalog, we expect less impact from IA when we limit the sample to blue, star-forming galaxies. The cosmological parameter constraints from this BLUE cosmic shear sample are stable to IA model choice, unlike passive galaxies in the full DES Y3 sample, the goodness-of-fit is improved and the Omega(m) and S-8 better agree with the observations from Planck on the cosmic microwave background. Mitigating IA with sample selection in DES, rather than flexible model choices, can reduce uncertainty in S-8 by a factor of 1.5.

We present the first systematic spectroscopic observations of extragalactic transients from the Dark Energy Spectroscopic Instrument (DESI), as part of the DESI Transients Survey program. With 5000 fibers and an similar to 8 deg2 field of view, we exploit DESI as a machine for the discovery and classification of transients. We present transient classifications from archival DESI data in Data Releases 1 and 2, relying on a combination of a secondary target program and serendipitous observations. We also present observations from the first 6 months of the DESI spare fiber program dedicated to transients. The program is run in coordination with a dedicated DECam time-domain survey, serving as a pathfinder for what we will be able to achieve in conjunction with the Rubin Observatory Legacy Survey of Space and Time (LSST). We classify over 250 transients, of which the majority were previously unclassified. The sample comprises thermonuclear and core-collapse supernovae and tidal disruption events (TDEs), including a TDE observed before its discovery in imaging. We demonstrate DESI's ability to classify a population of faint transients down to r similar to 22.5 mag during main survey operations, with negligible impacts on DESI's main observations. With the start of Rubin LSST operations, we expect to classify O(1000) transients per year.

Using 1533 type Ia supernovae (SNe Ia) from the five-year sample of the Dark Energy Survey (DES), we investigate the relationship between the projected galactocentric separation of the SNe and their host galaxies and their light curves and standardization. We show, for the first time, that the difference in SN Ia post-standardization brightnesses between high and low-mass hosts reduces from 0.078±0.011 mag in the full sample to 0.036 ± 0.018 mag for SNe Ia located in the outer regions of their host galaxies, while increasing to 0.100 ± 0.014 mag for SNe in the inner regions. The difference in the size of the mass step between inner and outer regions is 0.064±0.023 mag. In these inner regions, the step can be reduced (but not removed) using a model where the R_V of dust along the line-of-sight to the SN changes as a function of galaxy properties. We investigate the remaining difference using the distributions of the SN Ia stretch parameter to test the inferred age of SN progenitors. Comparing red (older) environments only, outer regions have a higher proportion of high-stretch SNe and a more homogeneous stretch distribution. However, this effect cannot explain the reduction in significance of any Hubble residual step in outer regions. We conclude that the standardized distances of SNe Ia located in the outer regions of galaxies are less affected by their global host galaxy properties than those in the inner regions.

The present state of cosmology is facing a crisis where there is a fundamental disagreement in measurements of the Hubble constant (H-0), with significant tension between the early and late Universe methods. Type Ia supernovae (SNe Ia) are important to measuring H-0 through the astronomical distance ladder. However, there remains potential to better standardize SN Ia light curves by using known dependencies on host galaxy properties after the standard light curve width and colour corrections have been applied to the peak SN Ia luminosities. To explore this, we use the 5-yr photometrically identified SNe Ia sample obtained by the Dark Energy Survey, along with host galaxy spectra obtained by the Australian Dark Energy Survey. Using host galaxy spectroscopy, we find a significant trend with the equivalent width (EW) of the [O II] lambda lambda 3727, 29 doublet, a proxy for specific star formation rate, and Hubble residuals. We find that the correlation with [O II] EW is a powerful alternative to the commonly used mass step after initial light-curve corrections. Applying this [O II] EW correction to 20 SNe Ia in calibrator galaxies observed with WiFeS, we examined the impact on SN Ia absolute magnitudes and H-0. Our [O II] EW corrections result in H-0 values ranging between 73.04 and 73.51 kms(-1)Mpc(-1), with a combined statistical and systematic uncertainty of similar to 1.31kms(-1)Mpc(-1). However, even with this additional correction, the impact of host galaxy properties in standardizing SNe Ia appears limited in reducing the current tension (similar to 5 sigma) with the cosmic microwave background result for H-0.

We measure the current expansion rate of the Universe, Hubble's constant H-0, by calibrating the absolute magnitudes of supernovae to distances measured by baryon acoustic oscillations (BAO). This 'inverse distance ladder' technique provides an alternative to calibrating supernovae using nearby absolute distance measurements, replacing the calibration with a high-redshift anchor. We use the recent release of 1829 supernovae from the Dark Energy Survey spanning 0.01 < z < 1.13 anchored to the recent baryon acoustic oscillation measurements from Dark Energy Spectroscopic Instrument (DESI) spanning 0.30 < z(eff) < 2.33. To trace cosmology to z = 0, we use the third-, fourth-, and fifth-order cosmographic models, which, by design, are agnostic about the energy content and expansion history of the universe. With the inclusion of the higher redshift DESI-BAO data, the third-order model is a poor fit to both data sets, with the fourth-order model being preferred by the Akaike Information Criterion. Using the fourth-order cosmographic model, we find H-0 = 67. 19(-0.64)(+0.66) km s(-1) Mpc(-1), in agreement with the value found by Planck without the need to assume Flat-Lambda CDM. However, the best-fitting expansion history differs from that of Planck, providing continued motivation to investigate these tensions.

The detection of the gravitational wave event GW230529, presumably a neutron star-black hole (NSBH) merger, by the LIGO-Virgo-KAGRA Collaboration marks an exciting discovery for multimessenger astronomy. The black hole (BH) has a high probability of falling within the "mass gap" (mg) between the neutron star and the BH mass distributions. Because of the relatively low primary mass, this system has a higher likelihood of producing an electromagnetic counterpart than previously detected NSBH mergers. We analyze the potential kilonova (KN) emission from GW230529 and find that, if the source was an NSBH merger, there is a similar to 2-28% probability (depending on the assumed equation of state) that it produced a KN peaking at similar to 1 day postmerger with g less than or similar to 23.5 and i <23. Hence, it could have been detected by ground-based telescopes. If instead the event was a binary neutron star merger, the probability of KN production drops to similar to 0-10%. Motivated by these results, we simulate a broader population of mass gap NSBH mergers expected during the fifth LIGO/Virgo/KAGRA observing run (O5) and find a 2%-3% chance of KN production per event. Such KNe would typically be fainter than GW230529, with g less than or similar to 26 and i less than or similar to 25. Based on these findings, DECam-like instruments may be able to detect up to similar to 70% of future mgNSBH KNe, corresponding to 1-2 multimessenger mgNSBH per year in O5.

Context. Fast X-ray transients (FXTs) are a rare and poorly understood phenomenon with a variety of possible progenitors. The launch of the Einstein Probe (EP) mission has facilitated a rapid increase in the real-time discovery and follow-up of FXTs. Aims. We focus on the recent EP discovered transient EP241021a, which shows a peculiar panchromatic behavior, with the aim of understanding its origin. Methods. We obtained optical and near-infrared multiband imaging and spectroscopy with the Fraunhofer Telescope at Wendelstein Observatory, the Hobby-Eberly Telescope, and the Very Large Telescope of the newly discovered EP transient EP241021a over the first 100 days of its evolution. Results. EP241021a was discovered by EP as a soft X-ray trigger, but was not detected at gamma-ray frequencies. The observed soft X-ray prompt emission spectrum is consistent with nonthermal radiation, which requires at least a mildly relativistic outflow with a bulk Lorentz factor Gamma greater than or similar to 4. The optical and near-infrared light curve displays a two-component behavior, where an initially fading component, similar to t(-1), transitions to a rise steeper than similar to t(3) after a few days, before peaking at an absolute magnitude of M-r approximate to -21.8 mag and quickly returning to the initial decay. Standard supernova models are unable to reproduce either the absolute magnitude or the rapid timescale (< 2 d) of the rebrightening. The X-ray, optical and near-infrared spectral energy distributions display a red color, r - J approximate to 0.8 mag, and point to a nonthermal origin (similar to nu(-1)) for the broadband emission. Considering a gamma-ray burst as a plausible scenario, we favor a refreshed shock as the cause of the rebrightening. This is consistent with the inference of an at least mildly relativistic outflow based on the prompt trigger. Conclusions. Our results suggest a link between EP-discovered FXTs and gamma-ray bursts, despite the lack of gamma-ray detections for the majority of EP transients.