arXiv subjects

Margaret Shepherd

Publications and source records attributed to Margaret Shepherd.

8 recordsLinked to original sources

arXiv2026

The Delay Time Distribution of Quasi-Periodic Eruptions

Quasi-periodic eruptions (QPEs) are quasi-periodic X-ray bursts observed in the nucleus of a galaxy. Multiple pieces of observational evidence link QPEs to tidal disruption events (TDEs), which occur when stars are disrupted after approaching a supermassive black hole too closely. Post-starburst galaxies are overrepresented among the host galaxies of both TDEs and QPEs, though the mechanism causing this overrepresentation is unknown. While their physical origin is unclear, the delay time distribution (DTD) of QPEs, or rate of QPEs as a function of time since a burst of star formation, can constrain what mechanisms influence the QPE rate and possible QPE formation channels. We compile a catalog of 10 QPE host galaxies with optical spectra, model the stellar populations with Bagpipes, and retrieve the age of the most recent burst of star formation to construct the DTD of QPEs. We find that the QPE rate increases with post-burst age to reach a peak at ~1 Gyr relative to a control sample, similar to the observational TDE DTD, though we cannot rule out a flat distribution of burst ages relative to a control sample. However, the fraction of QPE host galaxies with high (>1%) burst mass fractions is larger than the fraction of galaxies with high burst mass fractions in either a sample of TDE host galaxies or a sample of control galaxies. If the preferred QPE formation channel requires extreme mass ratio inspirals (EMRIs), then such EMRIs may be more readily produced by large, ~1-Gyr-old bursts of star formation.

astro-ph.GA↗

arXiv2026

NAPTIME: A Neural-Process Framework for Rubin Alert Classification

The Vera C. Rubin Observatory Legacy Survey of Space and Time will produce a high-volume stream of irregularly sampled multiband alerts for which spectroscopic confirmation will be available only for a small minority of sources. Tidal disruption events are rare phenomena that provide a direct probe of dormant massive black holes, but their light curves can be confused with nuclear variability and other transient subclasses. We present NAPTIME (Neural Astrophysical Photometric Transient Identification and Modeling Engine), a neural-process framework for photometric transient classification under sparse and partial observational context. NAPTIME models irregular multiband light curves directly, combining probabilistic light-curve reconstruction with classification and optional host-galaxy context, as well as photometric-redshift information. We evaluate on two simulated benchmarks: ELAsTiCC2, our primary Rubin-like broad-classification benchmark, and MALLORN, a photometry-only TDE-focused benchmark. On the 15-family ELAsTiCC2 task, the metadata-aware model reaches macro $\mathrm{F1} = 0.903$ and macro $\mathrm{AUROC} = 0.991$, while a matched photometry-only variant reaches 0.874 and 0.986. Viewed as a TDE-versus-rest ranking model, the classifier yields TDE average precision 0.985 with metadata and 0.979 without. Metadata is most valuable in the low-context regime. Using only the earliest 10\% of detected observations, macro F1 is $\sim$0.42 with metadata and $\sim$0.34 without it. On MALLORN, NAPTIME reaches macro $\mathrm{F1} = 0.693$ and macro $\mathrm{AUROC} = 0.958$. These results show that neural processes provide a practical probabilistic framework for Rubin-like transient classification and remain effective for TDE-focused candidate recovery.

astro-ph.IM↗

arXiv2026

Identifying Changing-Look AGN Transitions in Light Curve Data with the Zwicky Transient Facility

Changing-Look AGN (CL-AGN) are AGN which transition between Seyfert types, challenging AGN unification models. Most CL-AGN have been identified via repeat spectroscopy, making it difficult to determine the duration and magnitude of the CL-AGN transition. As such, the physical mechanisms behind this transition are still unknown. We use synthetic photometry in combination with ZTF light curve data to develop a new criterion to identify photometric CL-AGN transitions based on changes in g-band magnitude and g-r color. We find that a CL-AGN criterion of $| \Delta g| > 0.4$ mag and $| \Delta (g-r)| > 0.2$ mag recovers a photometric transition in $9.6^{+4.9}_{-3.4}\%$ of CL-AGN hosts over the six-year ZTF survey, including a candidate repeating changing-look event in SDSS J084957.78+274728.9. Using simulated AGN light curves, we estimate the false positive rate among the simulated Seyferts to be $1.6^{+0.19}_{-0.17}\%$. We find that the rate of similar flares among Type 1 Seyferts is $1.2^{+0.87}_{-0.50}\%$ , and among Type 2 Seyferts is $\leq 0.39\%$ over six years. Photometric CL-AGN transitions last between 21 and 560 days, with a median duration of 360 days, consistent with the thermal or orbital timescales for AGN disks. We do not detect a correlation between black hole mass and transition duration, likely due to the small sample of detected photometric transitions. This method can be applied to the upcoming Legacy Survey of Space and Time to identify CL-AGN candidates and test theories of their origins

astro-ph.GA↗

arXiv2026

The Delay Time Distribution of Tidal Disruption Events

Tidal disruption events (TDEs) can be observed when stars get too close to supermassive black holes and are torn apart and accreted. The delay time distribution of TDEs, or rate of TDEs as a function of time since a burst of star formation, can be used to determine what mechanisms influence the TDE rate. We compile a catalog of 41 TDE host galaxies with optical spectra, model the stellar populations with Bagpipes, and retrieve the age of the most recent burst of star formation to construct the delay time distribution of TDEs. TDEs occur more frequently in post-starburst galaxies than in other types of galaxies, though the mechanism causing this rate enhancement is unknown. We find that the TDE rate increases with post-burst age to reach a peak at ~1 Gyr relative to a control sample. We compare the observational TDE delay time distribution to theoretical models, which propose overdense stellar nuclei, radial anisotropies in stellar orbits, supermassive black hole binaries, and AGN disks as potential mechanisms that may enhance the TDE rate in post-starburst galaxies. Most models predict a TDE rate that declines with post-burst age, in contrast to our observational results, though some models are still feasible at certain ages (e.g., the black hole binary model matches at old burst ages and the stellar overdensity model matches at intermediate burst ages).

astro-ph.GA↗

arXiv2025

FEADME: Fast Elliptical Accretion Disk Modeling Engine

We present FEADME (Fast Elliptical Accretion Disk Modeling Engine), a GPU-accelerated Python framework for modeling broad Balmer-line emission using a relativistic elliptical accretion-disk formalism. Leveraging JAX and NumPyro for differentiable forward modeling and efficient Bayesian inference, FEADME enables large-sample, reproducible analyses of disk-dominated emission-line profiles. We apply the framework to 237 double-peaked emitters (DPEs) from the literature and to five tidal disruption events (TDEs) with disk-like H$\alpha$ emission, fitting three physically motivated model families per spectrum and selecting the preferred model using the widely applicable information criterion (WAIC). After posterior-quality filtering, the disk-bearing active galactic nuclei (AGN) analysis sample contains 165 sources and the TDE sample contains 27 usable epochs. We find that AGN occupy a broad, continuous distribution of disk geometries and kinematics that is usefully summarized by five phenomenological Gaussian-mixture morphology bins. The TDE disk parameters overlap substantially with the AGN population in radial scale, local broadening, and emissivity slope, but TDEs are systematically less eccentric and show broader non-disk Gaussian components. The majority of both AGN and TDEs favor models that include both a disk and an additional broad-line component, suggesting that disk emission commonly coexists with more isotropic or wind-driven gas. These results indicate that once a line-emitting disk forms, its spectroscopic appearance is governed by similar physical processes in both persistent AGN and transient TDE accretion flows, and they demonstrate the utility of FEADME for population-level studies of disk structure in galactic nuclei.

astro-ph.HE↗

arXiv2025

Radio Variability in Recently-Quenched Galaxies: The Impact of TDE or AGN Driven Outflows

Outflows and jets launched from the nuclei of galaxies emit radio synchrotron emission that can be used to study the impact of accretion energy on the host galaxy. The decades-long baseline now enabled by large radio surveys allows us to identify cases where new outflows or jets have been launched. Here, we present the results of a targeted VLA program observing four post-starburst galaxies that have brightened significantly in radio emission over the past ~20 years. We obtain quasi-simultaneous observations in five bands (1-18 GHz) for each source. We find peaked spectral energy distributions, indicative of self-absorbed synchrotron emission. While all four sources have risen significantly over the past ~20 years in the 1-2 GHz band, two also show clear recent flares in the 2-4 GHz band. These sources are less luminous than typical peaked spectrum radio AGN. It remains unclear whether these sources are low luminosity analogs of the peaked radio AGN from accreted gas, or driven by tidal disruption events with missed optical flares. Regardless of the source of the accreted material, these newly-launched outflows contain sufficient energy to drive the molecular gas outflows observed in post-starburst galaxies and to drive turbulence suppressing star formation.

astro-ph.GA↗

arXiv2025

Modeling Star Formation Histories of Changing-Look AGN Host Galaxies with Prospector

Changing-look active galactic nuclei, or CL-AGN, are AGN which appear to transition between Seyfert Type 1 and 2 over periods of months to years. Several mechanisms to trigger these transitions have been proposed, but we have yet to conclusively determine their cause. Recent studies suggest CL-AGN are hosted primarily in galaxies which are shutting down star formation (Dodd et al. 2021; Liu et al. 2021, Wang et al. 2023), which may indicate a link between galaxy quenching and changing look events. We use Prospector stellar population synthesis software (Leja et al. 2017; Johnson & Leja 2017; Johnson et al. 2021) to model non-parametric star formation histories for 39 CL-AGN host galaxies. We find that $43^{+13}_{-12}\%$ of our gold sample CL-AGN at z < 0.15 are star forming, while $29^{+13}_{-10}\%$ fall in the Green Valley of the stellar mass-sSFR diagram. At z > 0.15, $57^{+13}_{-18}\%$ of CL-AGN in the gold sample are star-forming and $29^{+19}_{-14}\%$ are in the Green Valley. CL-AGN hosts have similar star formation properties to the host galaxies of Seyfert 1 and 2 AGN at z < 0.15 and to Seyfert 2 AGN at z > 0.15. We find no statistically significant differences in the star formation properties of turn-on and turn-off CL-AGN. We also find no evidence for rapid quenching in the Green Valley CL-AGN. We conclude that CL-AGN state transitions are not associated with the formation history of CL-AGN host galaxies on large spatial scales, implying CL-AGN state transitions may instead result from nuclear-scale or accretion disk effects.

astro-ph.GA↗

arXiv2024

AT 2020nov: Evidence for Disk Reprocessing in a Rare Tidal Disruption Event

We present a detailed analysis of AT 2020nov, a tidal disruption event (TDE) in the center of its host galaxy, located at a redshift of $z = 0.083$. AT 2020nov exhibits unique features, including double-peaked Balmer emission lines, a broad UV/optical flare, and a peak log luminosity in the extreme ultra-violet (EUV) estimated at $\sim$$45.66^{+0.10}_{-0.33} \; \mathrm{erg} \, \mathrm{s^{-1}}$. A late-time X-ray flare was also observed, reaching an absorbed luminosity of $1.67 \times 10^{43} \; \mathrm{erg} \, \mathrm{s^{-1}}$ approximately 300 days after the UV/optical peak. Multi-wavelength coverage, spanning optical, UV, X-ray, and mid-infrared (MIR) bands, reveals a complex spectral energy distribution (SED) that includes MIR flaring indicative of dust echoes, suggesting a dust covering fraction consistent with typical TDEs. Spectral modeling indicates the presence of an extended, quiescent disk around the central supermassive black hole (SMBH) with a radius of $\sim$$5.06^{+0.59}_{-0.77} \times 10^4 \; \mathrm{R_g}$. The multi-component SED model, which includes a significant EUV component, suggests that the primary emission from the TDE is reprocessed by this extended disk, producing the observed optical and MIR features. The lack of strong AGN signatures in the host galaxy, combined with the quiescent disk structure, highlights AT 2020nov as a rare example of a TDE occurring in a galaxy with a dormant but extended pre-existing accretion structure.

astro-ph.HE↗