X-ray Brightening and UV Fading of Tidal Disruption Event ASASSN-15oi

We present late-time observations by Swift and XMM-Newton of the tidal disruption event (TDE) ASASSN-15oi that reveal that the source brightened in the X-rays by a factor of $\sim10$ one year after its discovery, while it faded in the UV/optical by a factor of $\sim 100$. The XMM-Newton observations measure a soft X-ray blackbody component with $kT_{\rm bb} \sim 45$ eV, corresponding to radiation from several gravitational radii of a central $\sim 10^6 M_\odot$ black hole. The last Swift epoch taken almost 600 days after discovery shows that the X-ray source has faded back to its levels during the UV/optical peak. The timescale of the X-ray brightening suggests that the X-ray emission could be coming from delayed accretion through a newly forming debris disk, and that the prompt UV/optical emission is from the prior circularization of the disk through stream-stream collisions. The lack of spectral evolution during the X-ray brightening disfavors ionization breakout of a TDE "veiled" by obscuring material. This is the first time a TDE has been shown to have a delayed peak in soft X-rays relative to the UV/optical peak, which may be the first clear signature of the real-time assembly of a nascent accretion disk, and provides strong evidence for the origin of the UV/optical emission from circularization, as opposed to reprocessed emission of accretion radiation.Comment: Accepted for publication in ApJ Letter

Miradas matemáticas y pensamiento numérico

Este artículo propone incluir dentro del pensamiento numérico el desarrollo de la “mirada matemática”, es decir cultivar la sensibilidad a toda información numérica que encontramos en el mundo que nos rodea y que en general se da por sobreentendida y no se le presta demasiada atención. Presentamos y analizamos ejemplos de situaciones numéricas que sirven de base para plantear diversos tipos de interrogantes y/o mini-investigaciones que revelan información implícita y en general desconocida, y que puede enriquecer nuestro entendimiento de los usos, a veces sofisticados, de los números. Exhortamos a incluir este tipo de actividades en el currículo y en el análisis didáctico de los contenidos a enseñar

The Post-Starburst Evolution of Tidal Disruption Event Host Galaxies

We constrain the recent star formation histories of the host galaxies of eight optical/UV-detected tidal disruption events (TDEs). Six hosts had quick starbursts of <200 Myr duration that ended 10 to 1000 Myr ago, indicating that TDEs arise at different times in their host's post-starburst evolution. If the disrupted star formed in the burst or before, the post-burst age constrains its mass, generally excluding O, most B, and highly massive A stars. If the starburst arose from a galaxy merger, the time since the starburst began limits the coalescence timescale and thus the merger mass ratio to more equal than 12:1 in most hosts. This uncommon ratio, if also that of the central supermassive black hole (SMBH) binary, disfavors the scenario in which the TDE rate is boosted by the binary but is insensitive to its mass ratio. The stellar mass fraction created in the burst is 0.5-10% for most hosts, not enough to explain the observed 30-200x boost in TDE rates, suggesting that the host's core stellar concentration is more important. TDE hosts have stellar masses 10^9.4 - 10^10.3 Msun, consistent with the SDSS volume-corrected, quiescent Balmer-strong comparison sample and implying SMBH masses of 10^5.5 - 10^7.5 Msun. Subtracting the host absorption line spectrum, we uncover emission lines; at least five hosts have ionization sources inconsistent with star formation that instead may be related to circumnuclear gas, merger shocks, or post-AGB stars.Comment: ApJ, 835, 176 (2017

Promoviendo conversaciones entre docentes acerca de clases filmadas de matemáticas

El proyecto VIDEO-LM aspira a desarrollar la capacidad de reflexión acerca de las prácticas de aula en base a observaciones y análisis de videos de clase, a tal efecto la herramienta principal es un marco de análisis que desarrollamos para analizar las clases filmadas. Este marco consiste en seis componentes: ideas matemáticas y meta-matemáticas de la lección; objetivos explícitos e implícitos atribuibles al docente; tareas asignadas y su desarrollo en la clase; interacciones profesor-alumno; dilemas docentes durante la clase y su resolución; creencias del profesor acerca de las matemáticas, su enseñanza y su aprendizaje inferibles de la lección. Se presentan ejemplos y algunos resultados de la investigación que acompaña a este proyecto

Numerically Modeling the First Peak of the Type IIb SN 2016gkg

Many Type IIb supernovae (SNe) show a prominent additional early peak in their light curves, which is generally thought to be due to the shock cooling of extended hydrogen-rich material surrounding the helium core of the exploding star. The recent SN 2016gkg was a nearby Type IIb SN discovered shortly after explosion, which makes it an excellent candidate for studying this first peak. We numerically explode a large grid of extended envelope models and compare these to SN 2016gkg to investigate what constraints can be derived from its light curve. This includes exploring density profiles for both a convective envelope and an optically thick steady-state wind, the latter of which has not typically been considered for Type IIb SNe models. We find that roughly $\sim0.02\,M_\odot$ of extended material with a radius of $\approx180-260\,R_\odot$ reproduces the photometric light curve data, consistent with pre-explosion imaging. These values are independent of the assumed density profile of this material, although a convective profile provides a somewhat better fit. We infer from our modeling that the explosion must have occurred within $\approx2-3\,{\rm hrs}$ of the first observed data point, demonstrating that this event was caught very close to the moment of explosion. Nevertheless, our best-fitting one-dimensional models overpredict the earliest velocity measurements, which suggests that the hydrogen-rich material is not distributed in a spherically symmetric manner. We compare this to the asymmetries seen in the SN IIb remnant Cas A, and we discuss the implications of this for Type IIb SN progenitors and explosion models.Comment: 8 pages, 8 figures, updated version accepted for publication in The Astrophysical Journa

How can a setting influence one’s reflection?

The scientific literature acknowledges the significance and benefits of reflection to teachers’ practice and offers a variety of tools and environments for reflection-based professional development. In this paper, we analyze mathematics teachers' reflection in three different settings, using six categories of reflection we previously developed. We examine the unique opportunities for reflection that each setting offers and how it may cater for teachers’ different needs.This study was supported by the Israel Science Foundation, Grant #1539/15 and by the Weizmann Institute of Science