SN 2008es is one of the rare cases of a Type II superluminous supernova (SLSN), showing no narrow features in its early-time spectra, and therefore, its powering mechanism is under debate between ...circumstellar interaction (CSI) and magnetar spin-down. Late-time data are required for better constraints. We present optical and near-infrared (NIR) photometry obtained from Gemini, Keck, and Palomar Observatories from 192 to 554 d after explosion. Only broad H α emission is detected in a Gemini spectrum at 288 d. The line profile exhibits red-wing attenuation relative to the early-time spectrum. In addition to the cooling SN photosphere, an NIR excess with blackbody temperature ∼1500 K and radius ∼1016 cm is observed. This evidence supports dust condensation in the cool dense shell being responsible for the spectral evolution and NIR excess. We favour CSI, with ∼2–3 Mꙩ of circumstellar material (CSM) and ∼10–20 Mꙩ of ejecta, as the powering mechanism, which still dominates at our late-time epochs. Both models of uniform density and steady wind fit the data equally well, with an effective CSM radius ∼1015 cm, supporting the efficient conversion of shock energy to radiation by CSI. A low amount (≲0.4 Mꙩ) of 56Ni is possible but cannot be verified yet, since the light curve is dominated by CSI. The magnetar spin-down powering mechanism cannot be ruled out, but is less favoured because it overpredicts the late-time fluxes and may be inconsistent with the presence of dust.
We present the first observations of a Type I superluminous supernova (SLSN) at 1000 days after maximum light. We observed SN 2015bn using the Hubble Space Telescope (HST) Advanced Camera for Surveys ...in the F475W, F625W and F775W filters at 721 days and 1068 days. SN 2015bn is clearly detected and resolved from its compact host, allowing reliable photometry. A galaxy template constructed from these data further enables us to isolate the SLSN flux in deep ground-based imaging. We measure a light curve decline rate at >700 days of 0.19 0.03 mag(100 d)−1, much shallower than the earlier evolution, and slower than previous SLSNe (at any phase) or the decay rate of 56Co. Neither additional radioactive isotopes nor a light echo can consistently account for the slow decline. A spectrum at 1083 days shows the same O i λ6300 and Ca ii λ7300 lines as seen at ∼300-400 days, with no new features to indicate strong circumstellar interaction. Radio limits with the Very Large Array rule out an extended wind for mass-loss rates yr−1 (where v10 is the wind velocity in units of 10 km s−1). The optical light curve is consistent with L ∝ t−4, which we show is expected for magnetar spin-down with inefficient trapping; furthermore, the evolution matches predictions from earlier magnetar model fits. The opacity to magnetar radiation is constrained at ∼0.01 cm2 g−1, consistent with photon-matter pair-production over a broad ∼GeV-TeV range. This suggests that the magnetar spectral energy distribution, and hence the "missing energy" leaking from the ejecta, may peak in this range.
The power sources of some superluminous supernovae (SLSNe), which are at peak 10–100 times brighter than typical SNe, are still unknown. While some hydrogen-rich SLSNe that show narrow Hα emission ...(SLSNe-IIn) might be explained by strong circumstellar interaction (CSI) similar to typical SNe IIn, there are some hydrogen-rich events without the narrow Hα features (SLSNe-II) and hydrogen-poor ones (SLSNe-I) that strong CSI has difficulties to explain. In this dissertation, I investigate the power sources of these two SLSN classes. SN 2015bn (SLSN-I) and SN 2008es (SLSN-II) are the targets in this study. I perform late-time multi-wavelength observations on these objects to determine their power sources. Evidence supports that SN 2008es was powered by strong CSI, while the late-time X-ray non-detection we observed neither supports nor denies magnetar spindown as the most preferred power origin of SN 2015bn. Interestingly, we identify the missing energy problem for SN 2015bn: >97 \% of the total spindown luminosity must be in other forms besides the UV/optical/infrared and 0.3–10 keV X-rays. This dissertation also contains a preliminary study of the UV/optical photometric properties of CSI motivated by SN 2008es. In future studies, I aim to understand the UV excess phase of CSI SNe, and hope to be able to develop a better way to describe the spectral energy distribution (SED) and its evolution. Preliminary systematic study of 15 SNe IIn reveals interesting features, and shows promising results that would lead to interesting implications such as a better description for the SED of CSI SNe during the UV excess.
SN 2015bn is a nearby hydrogen-poor superluminous supernova (SLSN-I) that has been intensively observed in X-rays with the goal to detect the spindown powered emission from a magnetar engine. The ...early-time ultraviolet/optical/infrared (UVOIR) data fit well to the magnetar model, but require leakage of energy at late times of 1043 erg s−1, which is expected to be partially emitted in X-rays. Deep X-ray limits until ∼300 days after explosion revealed no X-ray emission. Here, we present the latest deep 0.3-10 keV X-ray limit at 805 days obtained with XMM-Newton. We find LX < 1041 erg s−1, with no direct evidence for central-engine powered emission. While the late-time optical data still follow the prediction of the magnetar model, the best-fit model to the bolometric light curve predicts that ∼97% of the total input luminosity of the magnetar is escaping outside of the UVOIR bandpass at the time of observation. Our X-ray upper limit is <1.5% of the input luminosity, strongly constraining the high-energy leakage, unless non-radiative losses are important. These deep X-ray observations identify a missing energy problem in SLSNe-I, and we suggest future observations in hard X-rays and γ-rays for better constraints. Also, independent of the optical data, we constrain the parameter spaces of various X-ray emission scenarios, including ionization breakout by magnetar spindown, shock interaction between the ejecta and external circumstellar medium, off-axis γ-ray burst afterglow, and black hole fallback accretion.
This economic experiment initiates in evaluating a model's performance in predicting a decision. The reciprocity model is measured its accuracy rate in prediction and informativeness as aspects of ...the model's performance. Seventy-nine undergraduate students voluntarily joined the experiment. They made decisions contingently in designed situations as the first player in a dictator game and all roles in trust-share games. The study controls effects of choice set (equal split, competitive, and different social welfare choices) and framing effect. The result shows that the model has high performance in both prediction and informative. Furthermore, it shows an existence of the loss aversion behavior, and a significant relationship between decisions in the dictator game and the trustshare games. The study suggests that the more complicated model may not be marginally useful in predicting decision in the positive reciprocity situations.
The majority of long duration (\(>2\) s) gamma-ray bursts (GRBs) are believed to arise from the collapse of massive stars \cite{Hjorth+03}, with a small proportion created from the merger of compact ...objects. Most of these systems are likely formed via standard stellar evolution pathways. However, it has long been thought that a fraction of GRBs may instead be an outcome of dynamical interactions in dense environments, channels which could also contribute significantly to the samples of compact object mergers detected as gravitational wave sources. Here we report the case of GRB 191019A, a long GRB (T_90 = 64.4 +/- 4.5 s) which we pinpoint close (<100 pc projected) to the nucleus of an ancient (>1~Gyr old) host galaxy at z=0.248. The lack of evidence for star formation and deep limits on any supernova emission make a massive star origin difficult to reconcile with observations, while the timescales of the emission rule out a direct interaction with the supermassive black hole in the nucleus of the galaxy, We suggest that the most likely route for progenitor formation is via dynamical interactions in the dense nucleus of the host, consistent with the centres of such galaxies exhibiting interaction rates up to two orders of magnitude larger than typical field galaxies. The burst properties could naturally be explained via compact object mergers involving white dwarfs (WD), neutron stars (NS) or black holes (BH). These may form dynamically in dense stellar clusters, or originate in a gaseous disc around the supermassive black hole. Future electromagnetic and gravitational-wave observations in tandem thus offer a route to probe the dynamical fraction and the details of dynamical interactions in galactic nuclei and other high density stellar systems.
This economic experiment initiates in evaluating a model's performance in predicting a decision. The reciprocity model is measured its accuracy rate in prediction and informativeness as aspects of ...the model's performance. Seventy-nine undergraduate students voluntarily joined the experiment. They made decisions contingently in designed situations as the first player in a dictator game and all roles in trust-share games. The study controls effects of choice set (equal split, competitive, and different social welfare choices) and framing effect. The result shows that the model has high performance in both prediction and informative. Furthermore, it shows an existence of the loss aversion behavior, and a significant relationship between decisions in the dictator game and the trustshare games. The study suggests that the more complicated model may not be marginally useful in predicting decision in the positive reciprocity situations.
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