Samuel Whitebook

We present a comprehensive multiwavelength analysis of the long-duration gamma-ray burst (GRB) 250424A. Our dataset spans from the prompt gamma-ray emission to late-time optical monitoring, including spectra obtained with the Keck 10 m telescope. We find that the afterglow light curves display a prominent, simultaneous shallow decay phase in both X-ray and optical bands, followed by an achromatic transition to a standard decay regime. The broadband spectral energy distributions are well modeled by a single power-law function, indicating a common synchrotron origin for the emission across frequencies. We interpret the afterglow evolution within the framework of a relativistic forward shock refreshed by continuous energy injection. This scenario successfully reproduces the observed temporal and spectral behavior, yielding an isotropic equivalent kinetic energy of E_K,iso ≈ 5.5 × 10⁵² erg and an injection index of q ≈ 0.34 in a constant-density circumburst environment. The shallow decay phase is consistent with sustained energy injection lasting ∼9 ks. Despite the relatively low redshift, late-time optical observations reveal no distinct supernova component; however, our derived upper limits do not strictly rule out the presence of a typical GRB-associated supernova.

Several dozen binary ultracool and brown dwarf (BD) systems have been identified to date. These systems represent valuable probes of star and planet formation at the lowest mass scales. To date, the study of these ultracool binaries has been constrained to the noninteracting case. In this letter, we investigate the dynamics, stability, and evolution of mass transferring ultracool binaries using numerical simulations with accepted equations of state for BDs. We find that there exists a donor mass inversion, above which the donor dwarf is more massive than the accretor, but below which the accretor is more massive than the donor. Below the hydrogen-burning limit, objects with mass ratios of q ∼ 1 are unstable, but slight deviations from this mass ratio are stable at the onset of mass transfer and remain stable throughout extended periods. We compute theoretical mass transfer rates using several angular momentum loss prescriptions and predict lifespans of approximately 0.1–1 Gyr. We predict that all mass-transferring ultracool binaries are tidally locked and possess orbital periods ranging from just under 1 to 3.5 hr. We find that mass transfer proceeds via direct impact onto the accretor, forming a UV or optically bright hotspot on the surface of the accretor.

Mass transfer in stellar binaries has been well studied in most stellar mass ranges, with the notable exception of ultracool stars and substellar brown dwarfs. We report the discovery of ZTF J1239+8347 with the Zwicky Transient Facility (ZTF), a brown dwarf binary currently undergoing stable mass transfer with an orbital period of 57.41 minutes. Optical time-series photometry reveals an extremely high amplitude (>2 magnitude peak-to-trough) variability at short wavelengths, indicative of an orbiting hot spot slightly buried inside the atmosphere of the accretor. We use parallax measurements from Gaia along with optical and near-infrared spectra to infer an accretion temperature of T _eff = 8904 ± 54 K, an atmospheric temperature of the accretor of T _atmo ≈ 1500 K, and a slightly inflated accretor radius of R acc = 1.2 0 − 0.11 + 0.15 R Jup . ZTF J1239+8347 is a direct impact accretor, typically only seen in double degenerate white dwarf binaries, which are approximately a million times denser than the components in ZTF J1239+8347. The existence of an accreting brown dwarf binary suggests that angular momentum loss can be strong enough to make ultracool binaries interact in a Hubble time. The observed faintness (∼20 mag) and relative proximity (≈300 pc) of ZTF J1239+8347 suggest that many similar systems are likely to be found by the upcoming Rubin Observatory Legacy Survey of Space and Time.

Accreting white dwarfs (WDs) in close binary systems, commonly known as cataclysmic variables (CVs), with orbital periods below the canonical period minimum (≈80 minutes) are rare. Such short periods can only be reached if the donor star in the CV is either significantly evolved before initiating mass transfer to the WD or is metal-poor. We present optical photometry and spectroscopy of Gaia19bxc, a high-amplitude variable identified as a polar CV with an exceptionally short orbital period of 64.42 minutes—well below the canonical CV period minimum. High-speed photometry confirms persistent double-peaked variability consistent with cyclotron beaming, thus indicating the presence of a magnetic WD. Phase-resolved Keck/Low-Resolution Imaging Spectrometer (LRIS) spectroscopy reveals strong hydrogen and helium emission lines but no donor features, indicating the accretor is a magnetic WD and the donor is hydrogen-rich, but cold and faint. The absence of a detectable donor and the low inferred temperature (≲3500 K) disfavor an evolved donor scenario. Instead, the short period and the system’s halo-like kinematics suggest Gaia19bxc may be the first known metal-poor polar. Because metal-poor donors are more compact than solar-metallicity donors of the same mass, they can reach shorter minimum periods. Gaia19bxc is one of only a handful of known metal-poor CVs below the canonical period minimum and has the shortest period of any such magnetic system discovered to date.

We discuss our transient search for directed energy systems in local galaxies, with calculations indicating the ability of modest searches to detect optical Search for Extraterrestrial Intelligence sources in the closest galaxies. Our analysis follows P. Lubin, where a messenger civilization follows a beacon strategy we call “intelligent targeting.” We plot the required laser time to achieve a signal-to-noise ratio of 10 and find the time for a blind transmission to target all stars in the Milky Way to be achievable for local galactic civilizations. As high cadence and sky coverage is the pathway to enable such a detection, we operate the Local Galactic Transient Survey (LGTS) targeting M31 (the Andromeda galaxy), the Large Magellanic Cloud, and the Small Magellanic Cloud via Las Cumbres Observatory’s network of 0.4 m telescopes. We explore the ability of modest searches like the LGTS to detect directed pulses in optical and near-infrared wavelengths from extraterrestrial intelligence at these distances, and conclude that a civilization utilizing less powerful laser technology than we can construct in this century is readily detectable with the LGTS’s observational capabilities. Data processing of 30,000 LGTS images spanning 5 yr is in progress with the TRansient Image Processing Pipeline.

We present the TRansient Image Processing Pipeline (TRIPP), a transient and variable source detection pipeline that employs both difference imaging and light curve analysis techniques for astronomical data. Additionally, we demonstrate TRIPP's rapid analysis capability by detecting transient candidates in near-real time. TRIPP was tested using image data of the supernova SN2023ixf and from the Local Galactic Transient Survey (LGTS, Thomas et al. (2025)) collected by the Las Cumbres Observatory's (LCO) network of 0.4 m telescopes. To verify the methods employed by TRIPP, we compare our results to published findings on the photometry of SN2023ixf. Additionally, we report the ability of TRIPP to detect transient signals from optical Search for Extra Terrestrial Intelligence (SETI) sources.

We present two epochs of radial velocities of the first imaged T dwarf Gliese 229 B obtained with Keck/NIRSPEC. The two radial velocities are discrepant with one another and with the radial velocity of the host star, at ≈11σ significance. This points to the existence of a previously postulated, but as-yet undetected, massive companion to Gl 229 B; we denote the two components as Gl 229 Ba and Gl 229 Bb. We compute the joint likelihood of the radial velocities to constrain the period and mass of the secondary companion. Our radial velocities are consistent with an orbital period between a few days and ≈60 days, and a secondary mass of at least ≈15 M _Jup and up to nearly half the total system mass of Gl 229 B. With a significant fraction of the system mass in a faint companion, the strong tension between Gl 229 B’s dynamical mass and the predictions of evolutionary models is resolved.