arXiv subjects

Walter W. Golay

Publications and source records attributed to Walter W. Golay.

13 recordsLinked to original sources

arXiv2026

An Off-Nuclear Tidal Disruption Event Discovered At Late Times: The Case Of TDE 2023mfm

We present the discovery and analysis of the optically selected tidal disruption event (TDE) 2023mfm, which originates from a wandering massive black hole (MBH) in a massive ($\sim 10^{11} \,$M$_{\odot}$) galaxy hosting a central low-luminosity active galactic nucleus (AGN). Our analysis of the ZTF, Lick, Keck, Swift, Chandra, XMM-Newton, HST, and VLA observations reveals that TDE 2023mfm is a TDE-H from a $10^{6.2 \pm 0.5}\,$M$_{\odot}$ black hole which is offset by $0.66 \pm 0.02$" from the center of its host galaxy, corresponding to a projected distance of $1.08 \pm 0.04$ kpc. TDE 2023mfm displays all the traits of optically selected TDEs. The emission remains hot, $T_{\rm bb} \sim 22,000 \pm 1,000$ K, for more than a month after peak, and the $g$-band light curve peaks at $\left( 2.24^{+0.10} _{-0.11} \right) \times 10^{43} \, \rm erg \, s^{-1}$ and stays above half-maximum luminosity for $47.8^{+3.7}_{-3.5}$ days. The late-time optical-to-UV emission observed with the HST suggests the presence of an unresolved stellar population with a stellar mass of $10^7-10^8\,$M$_{\odot}$ at the position of the TDE, possibly being a low-mass dwarf galaxy or a stripped galaxy from a previous minor merger. The radio emission from TDE 2023mfm emerges at least a year after optical discovery, and we find it likely that it originates from a delayed outflow, however, further observations are needed to determine its true origin.

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arXiv2026

A Multiwavelength View of $ρ$ Oph II: Disentangling the Variability of the Multi-Star Component C

We present a multiwavelength analysis of the star $ρ$ Oph C, covering X-ray, optical, near-infrared, and radio observations to test for the presence of interferometrically-detected cool star companions. The X-ray observations from Chandra, XMM-Newton, and NuSTAR show flare-like events lasting minutes or days in time and spectral properties consistent with those of cooler stars instead of the primary magnetic B star. The near-infrared data is also consistent with the proposed trinary system properties due to the presence of CO-band heads. After removing the primary B star's rotation, the optical data from TESS shows a residual signal at the level of 0.8 mmags, matching the variations expected from cool stars in orbit around a bright B star. Finally, multi-epoch radio data reveals that $ρ$ Oph C exhibits significant large scale flux variations that are atypical for a magnetic massive star. Taken as a whole, the multi-epoch and -wavelength data presents a consistent picture of $ρ$\,Oph\,C as a rare trinary system composed of a hot star and two cool stars.

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arXiv2026

A Multiwavelength View of $ρ$ Oph I: Resolving the X-ray Source Between A and B

We present a multiwavelength analysis of the central stellar pair of $ρ$ Oph, components A and B. Using recent high-resolution \textit{Chandra X-ray Observatory} observations, we demonstrate with high confidence that the dominant X-ray source is $ρ$ Oph B, while $ρ$ Oph A is comparatively X-ray faint. This result contrasts with earlier \textit{XMM-Newton} observations, which, due to limited spatial resolutions, attributed the X-ray emission to $ρ$ Oph A. An analysis of $ρ$ Oph B's X-ray light curves and spectra reveals properties more consistent with a cool star than a hot star. We therefore propose that $ρ$ Oph B is an Algol-like binary system, consisting of a B-type primary and an active, X-ray-emitting GK-type companion.

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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

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

The pair-instability origin of supernova 2023vbw

Stars in the initial and carbon-oxygen core mass ranges of $\sim140-260$ and $50-130$ M$_\odot$, respectively, with low metallicity are predicted to experience copious electron-positron pair production in their cores, leading to a runaway thermonuclear explosion that obliterates the entire star in a luminous and long-duration pair-instability supernova explosion. Some previous supernovae have been interpreted in this context but lack the full range of predicted properties. Here, we report detailed observations and modeling of the hydrogen-rich supernova 2023vbw, which exploded in a low-metallicity ($\sim0.1$ Z$_\odot$) environment in a dwarf star-forming galaxy at a redshift of $0.088$. Its light curve exhibits a luminous ($1.6\times10^{43}$ erg s$^{-1}$) and long-duration ($190$ days) main peak, resulting in a total radiated energy of $3\times10^{50}$ erg, more than an order of magnitude greater than canonical core-collapse supernovae. Semi-analytical light-curve modeling yields a blue supergiant-like progenitor with an ejecta mass of $170-350$ M$_\odot$, radioactive nickel mass of $1.2-1.6$ M$_\odot$, and explosion energy of $(6-13)\times10^{52}$ erg, well matched by pair-instability models. The early and late-phase light curve and spectra also show evidence for interaction of the supernova ejecta with an aspherical circumstellar medium. Discoveries of numerous such events with the upcoming Rubin Observatory and Roman Space Telescope will shed light on the deaths of the most massive stars in the Universe.

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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

Radio Emission from the Infrared Tidal Disruption Event WTP14adeqka: The First Directly Resolved Delayed Outflow from a TDE

We present detailed radio observations of the mid-infrared (MIR) tidal disruption event (TDE) WTP14adeqka. We detect rising radio emission starting $\approx 4$ years after the discovery of the MIR emission (and about 2 years after its peak), peaking at $\approx 6.5$ years and declining thereafter, reminiscent of the delayed radio emission recently identified in optically discovered TDEs. The peak radio luminosity, $νL_ν\approx 2\times 10^{39}$ erg s$^{-1}$, is comparable to the brightest radio emission in optical TDEs. Multi-frequency radio observations at 8.9 and 9.7 years reveal a non-relativistic outflow with a mean expansion velocity of $\approx 0.021c$ (for an assumed launch at the time of disruption) and an energy of $\approx 10^{50.7}$ erg, about an order of magnitude larger than in typical optical TDEs. More importantly, Very Long Baseline Array (VLBA) observations at the same epochs directly resolve the radio source and reveal an increase in size from approximately 0.11 pc to 0.13 pc (with no apparent astrometric shift), corresponding to an expansion velocity of $\approx 0.05c$, and a likely delayed launch by about 2 years. The VLBA size measurements rule out an off-axis jet launched at the time of disruption, which would have an expected size of $\gtrsim {\rm pc}$ on these timescales; the possibility of a delayed jet can be evaluated with future VLBA observations. We conclude that MIR TDEs can launch energetic, delayed outflows. Ongoing radio observations of the full MIR TDE sample will reveal whether this behavior is ubiquitous.

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arXiv2025

The First Radio-Bright Off-Nuclear TDE 2024tvd Reveals the Fastest-Evolving Double-Peaked Radio Emission

We present the first multi-epoch broadband radio and millimeter monitoring of an off-nuclear TDE using the VLA, ALMA, ATA, AMI-LA, and the SMA. The off-nuclear TDE 2024tvd exhibits double-peaked radio light curves and the fastest evolving radio emission observed from a TDE to date. With respect to the optical discovery date, the first radio flare rises faster than $F_{\rm ν} \sim t^{9}$ at $Δt = 88-131$ days, and then decays as fast as $F_{\rm ν} \sim t^{-6}$. The emergence of a second radio flare is observed at $Δt \approx 194$ days with an initial fast rise of $F_{\rm ν} \sim t^{18}$, and an optically thin decline of $F_{\rm ν} \sim t ^{-12}$. We interpret these observations in the context of a self-absorbed and free-free absorbed synchrotron spectrum, while accounting for both synchrotron and external inverse-Compton cooling. We find that a single prompt outflow cannot easily explain these observations and it is likely that either there is only one outflow that was launched at $Δt \sim 80$ days, or two distinct outflows, with the second launched at $Δt \sim 170-190$ days. The nature of these outflows, whether sub-, mildly-, or ultra-relativistic, is still unclear, and we explore these different scenarios. Finally, we find a temporal coincidence between the launch time of the first radio-emitting outflow and the onset of a power-law component in the X-ray spectrum, attributed to inverse-Compton scattering of thermal photons.

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arXiv2025

James Webb Space Telescope Observations of the Nearby and Precisely-Localized FRB 20250316A: A Potential Near-IR Counterpart and Implications for the Progenitors of Fast Radio Bursts

We present deep James Webb Space Telescope near-infrared imaging to search for a quiescent or transient counterpart to FRB 20250316A, which was precisely localized with the CHIME/FRB Outriggers array to an area of $11\times13$ pc in the outer regions of NGC 4141 at $d\approx40$ Mpc. Our F150W2 image reveals a faint source near the center of the FRB localization region ("NIR-1"; $M_{\rm F150W2}\approx-2.5$ mag; probability of chance coincidence $\approx0.36$), the only source within $\approx2.7σ$. We find that it is too faint to be a globular cluster, young star cluster, red supergiant star, or a giant star near the tip of the red giant branch (RGB). It is instead consistent with a red giant near the RGB "clump" or a massive ($\gtrsim20$ M$_{\odot}$) main sequence star, although the latter explanation is less likely. The source is too bright to be a supernova remnant, Crab-like pulsar wind nebula, or isolated magnetar. Alternatively, NIR-1 may represent transient emission, namely a dust echo from an energetic outburst associated with the FRB, in which case we would expect it to fade in future observations. We explore the stellar population near the FRB and find that it is composed of a mix of young massive stars ($\sim10-100$ Myr) in a nearby HII region that extends to the location of FRB 20250316A, and old evolved stars ($\gtrsim$ Gyr). The overlap with a young stellar population, containing stars of up to $\approx20$ M$_\odot$, may implicate a neutron star / magnetar produced in the core collapse of a massive star as the source of FRB 20250316A.

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arXiv2025

Methods to Test the Source of the Extreme Gas Motions in WS 35

We present theoretical arguments toward the plausibility of a stellar wind to explain the 16000 km s$^{-1}$ line broadening in the optical spectra of WS 35, the central star in the Pa 30 nebula. The wind model is discussed in the context of super-Eddington flows. We argue that WS 35 potentially occupies a new regime of wind driving theory as the first metal-only wind. While this framework provides a promising avenue for explaining the high speed flow, questions remain about the source's true nature. We further describe how future radio observations can provide an independent test of the spherical wind scenario. A magnetically channeled wind would likely produce a relatively flat and bright radio spectral energy distributions. By contrast a spherical wind should result in a thermal radio spectrum with a canonical continuum slope of $ν^{0.6}$, and a brightness level consistent with the currently predicted mass-loss rate.

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arXiv2024

Time-lapse Very Long Baseline Interferometry Imaging of the Close Active Binary HR 1099

We report multiepoch astrometric very long baseline interferometry observations of the chromospherically active binary HR 1099 (V711 Tau, HD 22468) at six epochs over 63 days using the Very Long Baseline Array at 22.2 GHz. We determined hourly radio positions at each epoch with a positional uncertainty significantly smaller than the component separation. The aggregate radio positions at all epochs define an ellipse in the comoving reference frame with an inclination = $39.5^{+3.6}_{-3.5}°$ and longitude of ascending node $Ω= 212°\pm 22°$. The ellipse center is offset from the Third Gaia Celestial Reference Frame position by $Δα= -0.81^{+0.37}_{-0.25}$, $Δδ= 0.45^{+0.25}_{-0.23}$ mas. All radio centroids are well displaced from the binary center of mass at all epochs, ruling out emission from the interbinary region. We examined the motion of the radio centroids within each epoch by comparing hourly positions over several hours. The measured speeds were not statistically significant for five of the six epochs, with $2σ$ upper limits in the range 200-1000 km/sec. However, for one flaring epoch, there was a $3σ$ detection $v_{\perp} = 228 \pm 85$ km/sec. This speed is comparable to the mean speed of observed coronal mass ejections on the Sun.

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arXiv2024

A search for thermal gyro-synchrotron emission from hot stellar coronae

We searched for thermal gyro-synchrotron radio emission from a sample of five radio-loud stars whose X-ray coronae contain a hot ($T_e>10^7$ K) thermal component. We used the JVLA to measure Stokes I and V/I spectral energy distributions (SEDs) over the frequency range 15--45 GHz, determining the best-fitting model parameters using power-law and thermal gyro-synchrotron emission models. The SEDs of the three chromospherically active binaries (Algol, UX Arietis, HR 1099) were well-fit by a power-law gyro-synchrotron model, with no evidence for a thermal component. However, the SEDs of the two weak-lined T Tauri stars (V410 Tau, HD 283572) had a circularly polarized enhancement above 30 GHz that was inconsistent with a pure power-law distribution. These spectra were well-fit by summing the emission from an extended coronal volume of power-law gyro-synchrotron emission and a smaller region with thermal plasma and a much stronger magnetic field emitting thermal gyro-synchrotron radiation. We used Bayesian inference to estimate the physical plasma parameters of the emission regions (characteristic size, electron density, temperature, power-law index, and magnetic field strength and direction) using independently measured radio sizes, X-ray luminosities, and magnetic field strengths as priors, where available. The derived parameters were well-constrained but somewhat degenerate. The power-law and thermal volumes in the pre-main-sequence stars are probably not co-spatial, and we speculate they may arise from two distinct regions: a tangled-field magnetosphere where reconnection occurs and a recently discovered axisymmetric toroidal magnetic field, respectively.

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