arXiv subjects

Kate D. Alexander

Publications and source records attributed to Kate D. Alexander.

At least 19 recordsLinked to original sources

arXiv2026

Aarmed with Data: Bumps, Outflows, and Disk-like Emission in TDE 2025aarm

The origin of the optical emission in tidal disruption events (TDEs) remains one of the major outstanding questions in the field, in part due to the limited number of nearby events with high-cadence monitoring to track their evolving photometric and spectroscopic properties. We present multi-wavelength observations of the nearby ($z=0.01368$) TDE\,2025aarm, including near-daily spectroscopic coverage prior to the optical peak. Its proximity makes it one of the brightest TDEs discovered, reaching a peak magnitude of $m_r\sim15.5$ ($M_r\sim-18$). The light curve deviates from a smooth evolution, exhibiting multiple rebrightening episodes visible in both the individual filter light curves and the bolometric luminosity. Blackbody modelling reveals that these rebrightenings are associated with an increase in temperature of $> 5,000-10,000$\,K, while the inferred photospheric radius remains approximately constant. Simultaneously, the H$\alpha$ line not only increases in blueshift but also broadens, suggesting a link between the continuum rebrightenings to changes in the kinematics of the line-forming gas. We identify a persistent absorption component at $\sim-3900$\,km\,s$^{-1}$ in multiple Balmer lines, providing further evidence for outflowing material. The H$\alpha$ profile also exhibits excess flux compared to a Gaussian on both sides of the line, inconsistent with simple scattering-dominated outflow models. Disk-profile modelling provides evidence for the emergence of a disk-like component least $\sim20$ days after peak, with substantial changes in the disk properties between $\sim50$ and 60 days. These observations highlight the complexity of TDE emission processes and demonstrate how dense multi-wavelength monitoring can disentangle the roles of accretion, reprocessing, and outflows in shaping TDE emission.

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arXiv2026

Revealing the Structure and Magnetization of GRB Jets with ALMA Polarization Observations

We present a systematic study of the currently available ALMA millimeter polarimetric sample of gamma-ray burst (GRB) afterglows. Our sample comprises 24 observations (20 new) of 11 long-duration GRBs spanning $\approx0.1$-87 days after the burst. We detect significant linear polarization in 8 observations across 6 events, with polarization degrees ranging from $\Pi_L\approx0.6\%$ to $2.4\%$. For the remaining observations, we place deep upper limits (median $\Pi_L\lesssim1\%$). Multi-epoch observations reveal diverse polarization evolution. GRB 190114C yields the best-sampled $\Pi_L$ for a radio afterglow to date, with early evolution favoring patchy magnetic fields in the reverse shock (RS) and later polarization broadly consistent with forward-shock (FS) models. GRB 220921A exhibits a rapid rise in polarization from $\Pi_L\lesssim0.4\%$ to $2.4\%$ over 1.9-6.8 days, inconsistent with toroidal RS magnetic-field models but broadly consistent with several FS random-field models. GRB 221009A yields the highest-significance polarization detections in the sample ($\Pi_L\approx1.4$-$1.6\%$), yet neither existing FS nor RS polarization models reproduce both the observed polarization evolution and the viewing geometry inferred from broadband afterglow modeling, with the exception of patchy fields in the RS. Deep upper limits for GRB 171205A rule out RS toroidal-field models for the published off-axis geometry, while a strong, single-epoch detection of GRB 190829A, if RS-dominated, requires a nearly on-axis geometry for toroidal-field configurations. Interpreting the observations within a patchy-field framework implies magnetic-field coherence scales of order $\theta_B\sim10^{-3}$-$10^{-2}$ rad. These observations demonstrate the diagnostic power of radio/mm polarimetry for probing the magnetic-field structure, emission region, and viewing geometry of relativistic GRB jets.

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arXiv2026

The Radio Properties of Extreme Coronal Line Emitters: Constraints on the Sub-parsec Environment

A tiny fraction ($\ll1\%$) of galaxies display luminous, high-ionization metal emission lines, which may be persistent or variable. These extreme coronal lines (ECLs) are produced when soft X-ray photons intercept dense gas ($n\gtrsim10^{6-7}~{\rm cm^{-3}}$). The high X-ray flux required implicates intense nuclear activity, likely originating from tidal disruption events (TDEs) and active galactic nuclei (AGN). As ECLs are rarely seen even within these classes, their production may also require specific environmental conditions, but the details remain unclear (e.g., the geometry and volume filling factor of the ECL-producing gas). Here, we present the radio properties of a population of $27$ low-redshift ($z<0.3$) ECL emitting galaxies (ECLEs), providing a unique and previously unexplored probe of the properties of the circumnuclear medium (CNM; $\lesssim1$ pc from the black hole) in these systems. We find that $\sim 50\%$ of ECLEs produce radio synchrotron emission with luminosity and evolution consistent with TDEs and/or AGN. Radio spectral modeling of four ECLEs reveals that the ECL-producing region is (1) clumpy with a low volume filling factor ($10^{-5}\lesssim f_{V}\lesssim10^{-2}$) and (2) likely distinct from the radio emitting region (implying, e.g., a clumpy toroidal geometry). For time-variable ECLEs, these are some of the first observational constraints on the CNM geometry in formerly quiescent galactic nuclei. The unique nature of ECLEs makes them an excellent high-energy laboratory to connect the physics of accretion, photoionization, and feedback in galactic nuclei, thus motivating continued multi-wavelength monitoring.

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arXiv2026

A Self-Consistent Framework for Synchrotron Equipartition Analysis

Determining the energy, size, and velocity of synchrotron-emitting outflows is essential for testing models of their formation and evolution, but these quantities are often poorly constrained by observations alone. Equipartition analysis, therefore, provides a widely used framework for estimating these properties. Prior works have developed refinements to account for additional physical effects and other sources of energy (e.g., self-absorption, hot protons, and deviations from strict equipartition); however, these corrections are typically applied independently of one another, resulting in internal inconsistencies. In this work, we derive a self-consistent equipartition framework that accounts for the interdependence of various correction factors for Newtonian outflows and on- and off-axis relativistic jets. We implement our framework in an easy-to-use, publicly available code and apply it to study the tidal disruption events ASASSN-19bt and AT2019dsg, fast X-ray transient EP240414a, and active galactic nucleus J0231-0433. The interdependence of the corrections can increase energy estimates by a factor of ~5, suggesting that the energies of other synchrotron sources may be similarly underestimated in the literature. These results indicate that simultaneously incorporating these correction factors is essential for determining accurate outflow properties and constraining launch mechanisms.

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arXiv2026

Radio Observations of the Unusual Tidal Disruption Event AT 2022wtn: a Fast and Highly Energetic Outflow

We present multi-epoch, multi-frequency radio observations of the tidal disruption event (TDE) AT 2022wtn, obtained with the Karl G. Jansky Very Large Array (VLA) and Giant Metrewave Radio Telescope (GMRT), spanning 97-866 days after optical detection. The peak radio flux density increases until 300 days post optical discovery, flattens out for several hundred days, then begins to decrease at 534 days. Utilizing an updated equipartition analysis framework, we estimate several physical parameters of the event and the surrounding medium. We model AT 2022wtn with two different geometries: a spherical and a conical emitting region. The spherical outflow model gives an expansion velocity of $v\approx0.21c$ and a kinetic energy of $\sim3.8\times10^{49}$ erg, and the conical outflow model yields a higher energy ($\sim1.8\times10^{50}$) and velocity ($v\approx0.41c$) than the spherical case. After ruling out the possibility of a relativistic jet, we consider several potential origins for sub-relativistic outflow regions in TDEs including unbound debris streams, collisionally-induced outflows, an accretion-driven wind, and an outflow from an accretion disk state transition, and find only an accretion disk state transition outflow to be consistent with the high energy and velocity found in our equipartition results. AT 2022wtn is a uniquely powerful non-relativistic radio-emitting TDE, and joins a growing population that display a diverse range of outflow properties.

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arXiv2026

JWST observations of SN 2024abup: First Detection of CO in a broad-lined Type Ic Supernova and Constraints on r-process Nucleosynthesis

SN 2024abup is a nearby broad-lined Type Ic supernova (SN Ic-bl) in NGC 0681 at a distance of 23.3 \pm 1.6 Mpc. As energetic explosions of massive stars, SNe Ic-bl are considered a plausible site for rapid-neutron capture nucleosynthesis (r-process) and chemical enrichment from short-lived progenitors. They may also contribute to dust production in the early Universe. We present JWST near- to mid-infrared (NIR+MIR) observations (1-14 micron) of SN Ic-bl 2024abup at +41 days after the V band maximum (+54 days after explosion), the first-ever JWST+MIR observation of a SN Ic-bl along with radio and optical data. Using the spectral synthesis code SUMO, we identify the observed broad IR line features in SN 2024abup and find significant contributions from C, O, Mg, and carbon monoxide (CO) -- the earliest detection of molecules in a core-collapse SN so far. The spectrum shows continuum emission at wavelengths greater than 1.5 micron, which could be explained by dust -- preexisting, newly formed, or a combination-heated by the SN. We do not find compelling evidence for infrared signatures of r-process elements, though our search is hampered by the presence of many broad and blended features from the non-r-process elements. These new observations indicate that SNe Ic-bl could be a contributor to early-universe dust production, and suggest that if r-process elements are produced, revealing their presence from spectra requires very high-quality data and models to disentangle blends.

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arXiv2026

Late-time JWST/NIRCam Observations of the Extremely Long-duration GRB 250702B/EP 250702a and Its Host Galaxy

We present JWST/NIRCam observations of the extremely long-duration gamma-ray burst (GRB) 250702B taken at ~ 95 days post-GRB (observer frame). The observations of the host galaxy reveal a single galaxy with a prominent dust lane observed nearly edge-on. Prospector modeling of the host galaxy photometry finds a high stellar mass (log(M_*/M_Sun) = 11.0 +0.2/-0.3) and large dust column (A_V = 2.8 +/- 0.3 mag), in agreement with previous results. If GRB 250702B is a collapsar-driven GRB, the host galaxy is the brightest (in rest-frame r and rest-frame H) and most massive compared to GRB hosts at similar redshifts. The transient localization is near the dust lane, and while we find no evidence for transient emission in F277W, F356W, and F444W, forced photometry in F150W and F200W reveals possible ~ 3 sigma detections of the transient at m_{F150W} ~ 27.9 AB mag and m_{F200W} ~ 27.4 AB mag. If these are secure detections, they are indicative of a late-time light curve flattening. This behavior is consistent with that of jetted tidal disruption events (TDEs); however, it is also consistent with a supernova plus GRB afterglow model. Alternatively, if these are upper limits, they are consistent with, but do not further constrain, the extrapolated power-law decline of the afterglow. The ambiguity of the possible detection of the transient in F150W and F200W highlights the need for late-time template observations with JWST/NIRCam.

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arXiv2026

VLA Observations Confirm AT 2023mfm as an Off-nuclear Tidal Disruption Event

We report new radio observations of the tidal disruption event (TDE) AT 2023mfm, which we identified as a high-confidence candidate in a systematic search for off-nuclear TDEs. High-resolution NSF Karl G. Jansky Very Large Array C-band (6 GHz) imaging resolves two radio sources: one consistent with the host-galaxy nucleus and one offset by $0.651\pm0.036^{\prime\prime}$ ($1.06\pm0.06$ kpc), consistent with the Zwicky Transient Facility and Pan-STARRS1 positions of AT 2023mfm. These observations confirm the off-nuclear nature of AT 2023mfm, demonstrating the power of high-resolution radio imaging to validate off-nuclear TDE candidates and reveal hidden off-nuclear massive black holes.

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arXiv2026

A 14-year-old Mystery: The Peculiar Case of the Engine-driven SN 2012ap

We present late-time ($\delta t > 3000$ d) optical (Keck), X-ray (Chandra and NuSTAR), and radio (VLA, ALMA, and the uGMRT) observations of the Type Ic-BL SN 2012ap. Previous studies of this SN suggested that it stands out as a key example of a weak engine-driven explosion due to the lack of gamma-ray burst detection and a mildly relativistic ejecta. Recently, radio sky surveys revealed the rebrightening of the radio emission from this SN, highlighting the possibilities of a density enhancement at large radii or the existence of an off-axis relativistic jet. While the late-time optical spectra does not exhibit the broad emission lines seen in other interacting SNe, our analysis of the broadband radio and X-ray emission implies that both scenarios are plausible. If a density enhancement is responsible for the radio rebrightening, it has to result from a change in the mass-loss rate and/or wind velocity, possibly due to the transition of the progenitor from a red supergiant to a Wolf-Rayet star. If the late-time radio component is a result of an off-axis relativistic jet, we find that an energetic narrow jet viewed at $\theta_{\rm obs} \geq 80^{\circ}$ is needed. In this scenario, SN 2012ap is not a result of a weak engine-driven explosion, and, instead, it is similar to other GRBs. However, radio rebrightenings of Type Ic-BL SNe are not enough on their own to determine the existence of off-axis jets and our planned VLBA observation will help reveal the true nature of this SN.

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arXiv2026

First Detection of Faraday Rotation in a Gamma-Ray Burst Afterglow: Low Polarization and High Rotation Measure in GRB 260310A Reveal Jet Magnetic Structure and Environment

We report the detection of linear polarization in the radio afterglow of GRB 260310A, representing the first centimeter-wavelength polarization detection of a gamma-ray burst (GRB) afterglow and the first measurement of Faraday rotation in a GRB environment. We detect linearly polarized emission across $11-25$ GHz, with a polarization fraction decreasing monotonically from $(3.18 \pm 0.18)\%$ at 25 GHz to $(0.69 \pm 0.22)\%$ at 11 GHz. Interpreting the radio data as emission from a reverse shock in a structured, relativistic jet, the observed depolarization toward lower frequencies is consistent with suppression by synchrotron self-absorption, while the low observed polarization at high frequencies relative to the theoretical maximum suggests a patchy magnetic field in the jet with a coherence scale, $\theta_{\rm B}\approx10^{-2}$ rad. We identify a frequency-dependent rotation of the polarization angle consistent with Faraday rotation, with a rotation measure of ${\rm RM} = -(8300 \pm 90)~\rm{rad/m^2}$ at the GRB redshift. The magnitude of the rotation measure is consistent with propagation through a dense, magnetized environment, such as a progenitor HII region. These findings demonstrate that GRB afterglows exhibit measurable linear polarization at centimeter wavelengths, and that their polarimetric properties probe both intrinsic jet magnetization and the surrounding medium. Future multi-frequency polarimetric monitoring over timescales of days to weeks will enable detailed studies of the evolution of magnetic field structure and provide new constraints on the role of magnetic fields in GRB afterglows.

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arXiv2026

Rapid-response 1.3 mm Observations of GRB 260127A with the Submillimeter Array

We present the results from rapid-response 1.3 mm observations of GRB 260127A using the Submillimeter Array (SMA). SMA arrived on-source 12.6 minutes after the initial detection by the Neil Gehrels Swift Observatory, representing the earliest millimeter/submillimeter observations of a GRB to date. From these observations, we find a source with flux density $6.9\pm1.7$ mJy, consistent with the X-ray afterglow position but slightly offset from the optical afterglow position (2.7'' offset, with the SMA detection having a 90% confidence radial position uncertainty of 0.9''). Subsequent observations 1.9 days later show no sources of emission, with a $3\sigma$ upper limit of 0.70 mJy. If the SMA detection is associated with GRB 260127A, we infer that the 1.3 mm light curve for GRB 260127A declined at least as fast as $t^{-0.5}$, suggesting that peak brightness of the event at this wavelength was reached in under a day. We discuss how these findings may be consistent with both forward shock and reverse shock afterglow scenarios, and implications for future millimeter/submillimeter observations of GRBs on these timescales.

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arXiv2026

Search For a Counterpart to the Subsolar Mass Gravitational Wave Candidate S251112cm

The recent candidate gravitational-wave (GW) alert from a compact object merger involving at least one subsolar mass (SSM) object has prompted questions about their origins. S251112cm is reported by LIGO/Virgo with a false alarm rate of 1 per 6.2 years, nearby luminosity distance $93 \pm 27$ Mpc, and probability of containing a SSM object of 100%. Such a system, if astrophysical, likely did not involve the supersolar neutron stars or black holes invoked to explain kilonovae. One must then also invoke hitherto unobserved and speculative models to produce SSM mergers which may have electromagnetic (EM) counterparts. We introduce a framework which vets and scores candidate counterparts to SSM GW events to inform follow-up in search of any among the zoo of potential EM transients: kilonovae, kilonovae-within-supernovae, super-kilonovae, or AGN flares from binary black hole mergers. We use a suite of telescopes to perform tiling, galaxy-targeted observations, and photometric/spectroscopic follow-up of promising candidates. In near-real time, we ingest candidates reported by the community, including some of the first observations reported by the Vera C. Rubin Observatory. We vet and score a total of 456 candidates, including 67 from Rubin, but find no likely counterpart. We nonetheless highlight candidates which demonstrate the ability of our framework to distinguish between different transient types and describe strategies to maximize the chances of detecting a counterpart to the next SSM event. Our framework will be implemented in the forthcoming Multimessenger Tool for Rapid Object Vetting and Examination (TROVE).

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arXiv2026

AT2024lhc and AT2024kmq in the landscape of featureless tidal disruption events

We study AT2024kmq and AT2024lhc, two tidal disruption events (TDEs) with blue featureless spectra associated with high-mass black holes ($M_{\rm BH}\sim 10^8\,M_\odot$). Both events show optical precursors consistent with shock dissipation from stream self-intersection. Their X-ray emission is luminous ($L_{\rm X}\sim 10^{44}\,{\rm erg\,s^{-1}}$), highly variable (with minimum observed variability timescales of 1.3\,hr and 4.8\,hr for factor of $\sim3$ flux changes), long-lasting ($>1\,\rm yr$), emerging no later than the optical peak, and well characterized by power-laws with $1.7<\Gamma<3$ (where $f_\nu \propto \nu^{1-\Gamma}$). The X-ray properties and radio non-detections support a compact corona ($\lesssim 10 r_{\rm g}$) producing Comptonized X-ray emission. Using all published featureless TDEs, we find statistically significant bimodality in the distribution of their peak UV/optical blackbody luminosities and radii. We assemble a comparison TDE sample with early-time X-ray observations with eROSITA, in which we find different $M_{\rm BH}$ distributions in TDEs with different X-ray spectral evolution properties: low-mass black holes ($M_{\rm BH} \sim 10^6 M_\odot$) remain soft ($\Gamma>4$) within $t\lesssim 2$\,yr, intermediate masses ($\sim 10^7 M_\odot$) transition from soft to hard at $\sim$1 yr, while high masses ($\sim 10^8 M_\odot$) are hard ($1.5<\Gamma\lesssim 3$) from the outset. We interpret this result as evidence that the soft-to-hard state transition in TDEs occurs at the critical threshold of $\dot{M}_{\rm acc} \sim 0.03 \dot M_{\rm Edd}$ (similar to X-ray binaries), using the fact that the transition timescale predicted by simple disk theory scales with black hole mass as $t_{\rm tr}\propto M_{\rm BH}^{-3/4}$.

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arXiv2026

SN 2023zcu: A Type IIP SN with Early Flash Features

We present a detailed photometric and spectroscopic analysis of the Type IIP supernova SN~2023zcu, which exploded in the galaxy NGC~2139 (redshift $z$ = 0.006). SN~2023zcu exhibits a well-sampled light curve covering the rise, plateau, and nebular phases. It has an optically thick phase of $100.6 \pm 0.6$ d with a magnitude drop of $\sim$1.7 mag in the {\em V} band during the transition between the plateau and the nebular phases. Weak emission features in the early-time spectra indicate a low-level interaction between circumstellar material (CSM) and the SN ejecta. The spectral evolution is well sampled and exhibits a prominent P-Cygni profile of H$\alpha$, a defining characteristic of Type IIP SNe. Signatures of metal-line formation (e.g., \ion{Fe}{2}, \ion{Ca}{2} near-infrared triplet) are also evident in the spectra as the SN evolves. Spectral modeling with the radiative-transfer code \texttt{TARDIS} during the early photospheric phase (8.7--35.5 d since explosion) yields photospheric temperatures decreasing from $\sim$9,000 to $\sim$6,000 K and expansion velocities declining from $\sim$10,000 to $\sim$5,400 km s$^{-1}$. A tailored expanding photosphere method (EPM) fit based on the \texttt{TARDIS} models provides a distance estimate of $27.8 \pm 2.0$ Mpc. Nebular-phase spectra and bolometric light-curve modeling suggest a progenitor mass in the range 12--15 M$_\odot$. This thorough analysis helps to constrain progenitor properties and explosion parameters, thereby strengthening our understanding of Type IIP SNe.

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arXiv2026

SN 2024abfl: A Flat-Plateau, Low-Luminosity Type IIP Supernova with Early CSM Interaction

We present photometric and spectroscopic observations of SN 2024abfl, a low-luminosity Type IIP supernova (LLSN) discovered shortly after explosion. The transient reached a peak absolute magnitude of $M_V = -14.9$ and exhibited an extended, flat plateau lasting $\sim$125 days. From the late-time bolometric light curve, we estimate a $^{56}$Ni mass of $\sim0.01~M_\odot$, consistent with other LLSNe. Analytical shock-cooling models fail to reproduce the rapid early rise, indicating that circumstellar material (CSM) interaction contributed to the initial emission. The spectroscopic evolution is typical of LLSNe, with relatively narrow metal lines and low expansion velocities ($\lesssim 3000$ km s$^{-1}$) that decline slowly over time. We detect a broad ``ledge'' feature around 4600 \AA within three days of explosion, which we interpret as a blend of high-ionization shock-accelerated CSM lines. Multi-peaked H$\alpha$ profiles develop during the plateau phase, consistent with complex ejecta-CSM interactions. As one of the best-observed examples of LLSNe, SN 2024abfl exhibits a weak explosion and signatures of nearby CSM, offering new insights into progenitor properties, pre-explosion mass loss, and the diversity of LLSNe.

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arXiv2025

SN 2024iss: A Double-peaked Type IIb Supernova with Evidence of Circumstellar Interaction

We present optical, ultraviolet, and X-ray observations of supernova (SN) 2024iss, a Type IIb SN that shows a prominent double-peaked light curve. We modeled the first peak with a semianalytical shock-cooling model and the X-ray emission with a free-free model. We compare the envelope radius and mass-loss rate with other Type IIb SNe to explore the relationships between the progenitor envelope and the circumstellar material (CSM). The shock-cooling peak in the $V$-band light curve reached $M_V = -17.33\pm 0.26$mag, while the $^{56}$Ni-powered second peak attained $M_V = -17.43\pm 0.26$mag. Early spectra show an photospheric velocity of $\sim19,400\,km\,s^{-1}$ at 3.82days from the H$\alpha$ P~Cygni profile. The Balmer lines persist at least +87 days after the explosion, characterizing hydrogen-rich ejecta. Modeling the first light-curve peak suggests an extended envelope with a mass of $0.11\pm0.04\,M_{\odot}$ and a radius of $244\pm43~R_{\odot}$. Fitting the second light-curve peak with an Arnett-like model indicates a typical $^{56}$Ni mass of $ 0.117\pm0.013~M_{\odot}$ and a relatively low ejecta mass of $1.272\pm0.343\,M_{\odot}$. X-ray observations reveal bright thermal bremsstrahlung emission and indicate a mass-loss rate of $1.6\times10^{-5}\ M_{\odot} \ \rm{yr}^{-1}$. SN 2024iss occupies a transitional position between the two subclasses of extended (eIIb) and compact (cIIb) Type IIb SNe. Its envelope radius and pre-explosion mass-loss rate appear to be correlated as theoretically predicted. The observational properties of SN 2024iss are compatible with a binary interaction scenario being the dominant mechanism for envelope stripping. Furthermore, the low column density of neutral hydrogen suggests a compact CSM with an outer radius of $\lesssim1.3\times10^{14}$ cm, indicating that the progenitor star experienced eruptive mass loss within $\sim4\,yr$ of its terminal explosion.

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arXiv2025

Optimizing Kilonova Searches: A Case Study of the Type IIb SN 2025ulz in the Localization Volume of the Low-Significance Gravitational Wave Event S250818k

Kilonovae, the ultraviolet/optical/infrared counterparts to binary neutron star mergers, are an exceptionally rare class of transients. Optical follow-up campaigns are plagued by contaminating transients, which may mimic kilonovae, but do not receive sufficient observations to measure the full photometric evolution. In this work, we present an analysis of the multi-wavelength dataset of supernova (SN) 2025ulz, a proposed kilonova candidate following the low-significance detection of gravitational waves originating from the potential binary neutron star merger S250818k. Despite an early rapid decline in brightness, our multi-wavelength observations of SN 2025ulz reveal that it is a type IIb supernova. As part of this analysis, we demonstrate the capabilities of a novel quantitative scoring algorithm to determine the likelihood that a transient candidate is a kilonova, based primarily on its 3D location and light curve evolution. We also apply our scoring algorithm to other transient candidates in the localization volume of S250818k and find that, at all times after the discovery of SN 2025ulz, there are $\geq 4$ candidates with a score comparable to SN 2025ulz, indicating that the kilonova search may have benefited from the additional follow-up of other candidates. During future kilonova searches, this type of scoring algorithm will be useful to rule out contaminating transients in real time, optimizing the use of valuable telescope resources.

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arXiv2025

No Sign of a Magnetar Remnant Following the Kilonova-Producing Long GRB 211211A $\sim 1.7~$Years Later

In addition to a $\gamma$-ray burst (GRB), the merger of two neutron stars may produce a temporarily or indefinitely stable neutron star remnant with a strong magnetic field (a "magnetar"). As this magnetar remnant spins down, it can deposit its rotational energy into the surrounding kilonova ejecta, producing synchrotron emission that peaks in the radio bands $\sim$months-years after the merger ("boosted kilonova"). The nearby ($z=0.0763$) long-duration GRB 211211A, which has an apparent kilonova counterpart and likely neutron star merger progenitor, may have produced such a remnant. We observed the location of GRB 211211A at 6 GHz with the NSF's Karl G. Jansky Very Large Array (VLA) spanning $\approx 0.54$-$1.7~$years after the burst. We do not detect any radio emission, placing strong limits on the energy deposited into the ejecta by any remnant to $\lesssim 4.4 \times 10^{52}~{\rm erg}$. Due to the proximity of the event, we are also able to place limits on a kilonova afterglow that did not receive any additional energy deposition, though it is possible such emission will be suppressed until $\sim 4~{\rm years}$ after the burst, when the kilonova is expected to overtake the forward shock of the GRB. Future observations with the VLA and next-generation radio facilities will be able to further constrain the magnetar-boosted kilonova and kilonova afterglow scenarios, as well as directly constrain models in the scenario that GRB 211211A was instead produced by a collapsar.

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