The multiple constraints of addressed questions in whole-class interaction: Responses from unaddressed pupils

This article explores pupils' responses to addressed questions in two third-year primary school classes, organized as plenary interaction and based on the next-speaker selection. In this context, unaddressed pupils often produce responses of various kinds spontaneously, showing that the next-speaker selection per se does not exclude unaddressed pupils from participating. Analysis of the design and position of these responses show their orderly nature as mainly depending on the following dimensions: the position of the address term in the question and who has primary access to answers. Pupils' responses display a high level of awareness of the next-speaker selection rule operating in this setting, and more globally, of the turn-taking system. This competence enables pupils to understand and navigate the other-selection rule, often gaining their right to speakership. In line with prior studies on multiparty interactions, the article shows that teachers' questions pose multiple constraints on responses

An ultraviolet excess in the superluminous supernova Gaia16apd reveals a powerful central engine

Since the discovery of superluminous supernovae (SLSNe) in the last decade, it has been known that these events exhibit bluer spectral energy distributions than other supernova subtypes, with significant output in the ultraviolet. However, the event Gaia16apd seems to outshine even the other SLSNe at rest-frame wavelengths below $\sim 3000$ \AA. Yan et al (2016) have recently presented HST UV spectra and attributed the UV flux to low metallicity and hence reduced line blanketing. Here we present UV and optical light curves over a longer baseline in time, revealing a rapid decline at UV wavelengths despite a typical optical evolution. Combining the published UV spectra with our own optical data, we demonstrate that Gaia16apd has a much hotter continuum than virtually any SLSN at maximum light, but it cools rapidly thereafter and is indistinguishable from the others by $\sim 10$-15 days after peak. Comparing the equivalent widths of UV absorption lines with those of other events, we show that the excess UV continuum is a result of a more powerful central power source, rather than a lack of UV absorption relative to other SLSNe or an additional component from interaction with the surrounding medium. These findings strongly support the central-engine hypothesis for hydrogen-poor SLSNe. An explosion ejecting $M_{\rm ej} = 4 (0.2/\kappa)$ M$_\odot$, where $\kappa$ is the opacity in cm$^2$g$^{-1}$, and forming a magnetar with spin period $P=2$ ms, and $B=2\times10^{14}$ G (lower than other SLSNe with comparable rise-times) can consistently explain the light curve evolution and high temperature at peak. The host metallicity, $Z=0.18$ Z$_\odot$, is comparable to other SLSNe.Comment: Updated to match accepted version (ApJL

A Hydrogen-Poor Superluminous Supernova with Enhanced Iron-Group Absorption: A New Link Between SLSNe and Broad-Lined Type Ic SNe

We present optical observations of the Type I superluminous supernova (SLSN-I) SN2017dwh at $z\!\approx\!0.13$, which reached $M_{i}\!\approx\!-21$ mag at peak. Spectra taken a few days after peak show an unusual and strong absorption line centered near 3200\AA\ that we identify with Co II, suggesting a high fraction of synthesized $^{56}$Ni in the ejecta. By $\sim\!1$ month after peak, SN2017dwh became much redder than other SLSNe-I, instead strongly resembling broad-lined Type Ic supernovae (Ic-BL SNe) with clear suppression of the flux redward of $\sim\!5000$ \AA, providing further evidence for a large mass of Fe-group elements. Late-time upper limits indicate a $^{56}$Ni mass of $\lesssim 0.6$ M$_\odot$, leaving open the possibility that SN2017dwh produced a $^{56}$Ni mass comparable to SN1998bw ($\approx\!0.4$ M$_\odot$). Fitting the light curve with a combined magnetar and $^{56}$Ni model using ${\tt MOSFiT}$, we find that the light curve can easily accommodate such masses without affecting the inferred magnetar parameters. We also find that SN2017dwh occurred in the least-luminous detected host galaxy to date for a SLSN-I, with $M_{B} = -13.5$ mag and an implied metallicity of $Z\!\sim\!0.08$ $Z_\odot$. The spectral properties of SN2017dwh provide new evidence linking SLSNe-I with Type Ic-BL SNe, and in particular the high Fe-group abundance may be due to enhanced $^{56}$Ni production or mixing due to asphericity. Finally, we find that SN2017dwh represents the most extreme end of a correlation between continuum shape and Co II absorption strength in the near-peak spectra of SLSNe-I, indicating that Fe-group abundance likely accounts for some of the variation in their spectral shapes.Comment: 16 pages, 7 figures, Submitted to Ap

PS16dtm: A Tidal Disruption Event in a Narrow-line Seyfert 1 Galaxy

[Abridged] We present observations of PS16dtm, a luminous transient that occurred at the nucleus of a known Narrow-line Seyfert 1 galaxy hosting a 10$^6$ M$_\odot$ black hole. The transient was previously claimed to be a Type IIn SLSN due to its luminosity and hydrogen emission lines. The light curve shows that PS16dtm brightened by about two magnitudes in ~50 days relative to the archival host brightness and then exhibited a plateau phase for about 100 days followed by the onset of fading in the UV. During the plateau PS16dtm showed no color evolution, maintained a blackbody temperature of 1.7 x 10$^4$ K, and radiated at approximately $L_{Edd}$ of the SMBH. The spectra exhibit multi-component hydrogen emission lines and strong FeII emission, show little evolution with time, and closely resemble the spectra of NLS1s while being distinct from those of Type IIn SNe. Moreover, PS16dtm is undetected in the X-rays to a limit an order of magnitude below an archival X-ray detection of its host galaxy. These observations strongly link PS16dtm to activity associated with the SMBH and are difficult to reconcile with a SN origin or any known form of AGN variability, and therefore we argue that it is a TDE in which the accretion of the stellar debris powers the rise in the continuum and excitation of the pre-existing broad line region, while providing material that obscures the X-ray emitting region of the pre-existing AGN accretion disk. A detailed TDE model fit to the light curve indicates that PS16dtm will remain bright for several years; we further predict that the X-ray emission will reappear on a similar timescale as the accretion rate declines. Finally, we place PS16dtm in the context of other TDEs and find that TDEs in AGN galaxies are an order of magnitude more efficient and reach Eddington luminosities, likely due to interaction of the stellar debris with the pre-existing accretion disk.Comment: 19 pages, 17 figures, Submitted to Ap

Spitzer Space Telescope Infrared Observations of the Binary Neutron Star Merger GW170817

We present Spitzer Space Telescope 3.6 and 4.5 micron observations of the binary neutron star merger GW170817 at 43, 74, and 264 days post-merger. Using the final observation as a template, we uncover a source at the position of GW170817 at 4.5 micron with a brightness of 22.9+/-0.3 AB mag at 43 days and 23.8+/-0.3 AB mag at 74 days (the uncertainty is dominated by systematics from the image subtraction); no obvious source is detected at 3.6 micron to a 3-sigma limit of >23.3 AB mag in both epochs. The measured brightness is dimmer by a factor of about 2-3 times compared to our previously published kilonova model, which is based on UV, optical, and near-IR data at <30 days. However, the observed fading rate and color (m_{3.6}-m_{4.5}> 0 AB mag) are consistent with our model. We suggest that the discrepancy is likely due to a transition to the nebular phase, or a reduced thermalization efficiency at such late time. Using the Spitzer data as a guide, we briefly discuss the prospects of observing future binary neutron star mergers with Spitzer (in LIGO/Virgo Observing Run 3) and the James Webb Space Telescope (in LIGO/Virgo Observing Run 4 and beyond).Comment: 6 pages, 2 figures, submitted to ApJ

The Unusually Luminous Extragalactic Nova SN 2010U

We present observations of the unusual optical transient SN 2010U, including spectra taken 1.03 days to 15.3 days after maximum light that identify it as a fast and luminous Fe II type nova. Our multi-band light curve traces the fast decline (t_2 = 3.5 days) from maximum light (M_V = -10.2 mag), placing SN 2010U in the top 0.5% of the most luminous novae ever observed. We find typical ejecta velocities of approximately 1100 km/s and that SN 2010U shares many spectral and photometric characteristics with two other fast and luminous Fe II type novae, including Nova LMC 1991 and M31N-2007-11d. For the extreme luminosity of this nova, the maximum magnitude vs. rate of decline relationship indicates a massive white dwarf progenitor with a low pre-outburst accretion rate. However, this prediction is in conflict with emerging theories of nova populations, which predict that luminous novae from massive white dwarfs should preferentially exhibit an alternate spectral type (He/N) near maximum light.Comment: 16 pages, 16 figures. Submitted to the Astrophysical Journa