Active galactic nuclei (AGNs) that show strong rest-frame optical/UV variability in their blue continuum and broad line emission are classified as changing-look AGN, or at higher luminosities, ...changing-look quasars (CLQs). These surprisingly large and sometimes rapid transitions challenge accepted models of quasar physics and duty cycles, offer several new avenues for study of quasar host galaxies, and open a wider interpretation of the cause of differences between broad and narrow-line AGN. To better characterize extreme quasar variability, we present follow-up spectroscopy as part of a comprehensive search for CLQs across the full Sloan Digital Sky Survey (SDSS) footprint using spectroscopically confirmed quasars from the SDSS DR7 catalog. Our primary selection requires large-amplitude ( mag, mag) variability over any of the available time baselines probed by the SDSS and Pan-STARRS 1 surveys. We employ photometry from the Catalina Sky Survey to verify variability behavior in CLQ candidates where available, and confirm CLQs using optical spectroscopy from the William Herschel, MMT, Magellan, and Palomar telescopes. For our adopted signal-to-noise ratio threshold on variability of broad Hβ emission, we find 17 new CLQs, yielding a confirmation rate of 20%. These candidates are at lower Eddington ratio relative to the overall quasar population, which supports a disk-wind model for the broad line region. Based on our sample, the CLQ fraction increases from 10% to roughly half as the continuum flux ratio between repeat spectra at 3420 increases from 1.5 to 6. We release a catalog of more than 200 highly variable candidates to facilitate future CLQ searches.
Abstract
SDSS J2232−0806 (the ‘Big Dipper’) has been identified as a ‘slow-blue nuclear hypervariable’: a galaxy with no previously known active nucleus, blue colours, and large-amplitude brightness ...evolution occurring on a time-scale of years. Subsequent observations have shown that this source does indeed contain an active galactic nucleus (AGN). Our optical photometric and spectroscopic monitoring campaign has recorded one major dimming event (and subsequent rise) over a period of around 4 yr; there is also evidence of previous events consistent with this in archival data recorded over the last 20 yr. Here we report an analysis of the 11 optical spectra obtained to date and we assemble a multiwavelength data set including infrared, ultraviolet, and X-ray observations. We find that an intrinsic change in the luminosity is the most favoured explanation of the observations, based on a comparison of continuum and line variability and the apparent lagged response of the hot dust. This source, along with several other recently discovered ‘changing-look’ objects, demonstrate that AGNs can exhibit large-amplitude luminosity changes on time-scales much shorter than those predicted by standard thin accretion disc models.
On the progenitor of the Type IIP SN 2013ej in M74 Fraser, Morgan; Maund, Justyn R; Smartt, Stephen J ...
Monthly notices of the Royal Astronomical Society. Letters,
03/2014, Letnik:
439, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We use natural seeing imaging of SN 2013ej in M74 to identify a progenitor candidate in archival Hubble Space Telescope (HST) + Advanced Camera for Survey images. We find a source coincident with the ...supernova (SN) in the F814W filter within the total 75 mas (∼3 pc astrometric uncertainty; however, the position of the progenitor candidate in contemporaneous F435W and F555W filters is significantly offset. We conclude that the 'progenitor candidate' is in fact two physically unrelated sources; a blue source which is likely unrelated to the SN, and a red source which we suggest exploded as SN 2013ej. Deep images with the same instrument on board HST taken when the SN has faded (in approximately two year's time) will allow us to accurately characterize the unrelated neighbouring source and hence determine the intrinsic flux of the progenitor in three filters. We suggest that the F814W flux is dominated by the progenitor of SN 2013ej, and assuming a bolometric correction appropriate to an M-type supergiant, we estimate that the mass of the progenitor of SN 2013ej was between 8 and 15.5 M.
•High-pressure methane sorption isotherms were measured on Chinese shale samples.•The maximum methane excess sorption and Langmuir sorption capacity were revealed.•TOC is the primary controlling ...factor on methane sorption capacity.•Clay content shows positive effect on TOC-normalized sorption capacity values.•Tvap-GC results show negative effect of weathering on methane sorption capacity.
The marine black shale formations on the Upper Yangtze Platform (UYP), South China are exploration targets for shale gas. Here, we report on the methane sorption capacity of thermally overmature samples from the Lower Silurian and Lower Cambrian black shale intervals in the UYP (UYP-samples). Two immature shale samples from the Middle Cambrian formation in the Georgina Basin, North Australia (AU samples) were also tested to investigate the effect of thermal maturity on sorption isotherms. Excess sorption isotherms were performed over a pressure range of 0–25MPa at 46°C. The effects of TOC content, thermal maturity, clay minerals, moisture content, pore properties, particle size, temperature, and pressure on methane sorption capacity were analysed. In addition, thermovaporisation gas chromatography (Tvap-GC) was used to measure the residual gas that is stored in the samples under atmospheric pressure and temperature conditions.
The results indicate that the maximum methane excess sorption of the Lower Silurian samples is between 0.045 and 0.064mmol/g rock and that of Lower Cambrian samples is between 0.036 and 0.210mmol/g rock. The Langmuir sorption capacity of the Lower Silurian samples ranges from 0.096 to 0.115mmol/g rock, whereas that of the Lower Cambrian shale ranges from 0.077 to 0.310mmol/g rock. These results are close to the sorption capacities of the Barnett (U.S.), Devonian–Mississippian (Western Canada), and Alum (Southern Scandinavia) shale samples. The shape of the sorption isotherms and methane sorption capacity vary from sample to sample. Under the measured pressure range, the isotherms of the selected immature AU Cambrian samples increase monotonously with pressure, whereas the overmature UYP samples exhibit maxima. The methane sorption capacity of the measured samples positively correlates with TOC content and exhibits a distinct linear relation. The TOC-normalised sorption capacity shows a positive correlation with thermal maturity; however, the corresponding pressure of maximum excess sorption and Langmuir pressure decrease substantially with increasing thermal maturity. The clay minerals show a positive effect on the TOC-normalised sorption capacity. The sorption capacity of clay minerals, however, should have been reduced by the moisture content. The two Lower Cambrian samples that have similar maturities were measured for porosity and pore-size distribution. The sample with a high TOC content shows a high total cumulative pore volume, surface area, total porosity and thus a higher sorption capacity than the sample with less TOC. In addition, larger-sized particles show slightly less sorption capacity than smaller-sized particles. The Tvap-GC results show that the residual gas content of core samples is evidently higher than that of the outcrop samples, which implies a remarkably negative effect of the weathering process.
ABSTRACT
We investigate the responsiveness of the 2798 Å Mg ii broad emission line in active galactic nuclei (AGNs) on time-scales of several years. Our study is based on a sample of extremely ...variable AGNs as well as a broad population sample. The observed response of the line in previous studies has been mixed. By focusing on extreme variability (|Δg| > 1), we find that Mg ii clearly does respond to the continuum. However, the degree of responsiveness varies strikingly from one object to another: We see cases of Mg ii changing by as much as the continuum, more than the continuum, or very little at all. In 74 per cent of the highly variable sample, the behaviour of Mg ii corresponds with that of H β, with 30 per cent of the objects showing large variations in both lines. We do not detect any change in the line width that would correspond to broad-line region ‘breathing’, in accordance with results from the literature. Some of the objects in our highly variable sample show a clear asymmetry in the Mg ii profile. This skewness can be both to the blue and the red of the line centre. Results from our broad population sample show that highly variable quasars have lower Eddington ratios. This result holds for the variability of the continuum, but the correlation is significantly reduced for the variability of the Mg ii line. For the first time, we present an overview of the value of the intrinsic Baldwin effect for Mg ii in a large sample.
In this article, we describe the geological features of the Ediacaran (upper Sinian), lower Cambrian and lower Silurian shale intervals in the Upper Yangtze Platform, South China, and report on the ...gas potential of 53 samples from these major marine shale formations. Reflected light microscopy, total organic carbon (TOC) measurement, Rock-Eval, carbon isotope ratio analysis, thermovaporization gas chromatography (Tvap-GC), and open pyrolysis gas chromatography (open py-GC) were used to characterize the organic matter. Measured TOC in this research is normally >2% and averages 5%. TOC contents are roughly positively correlated with increasing geological age, i.e. lower Silurian shales exhibit generally lower TOC contents than lower Cambrian shales, which in turn commonly have lower TOC contents than Ediacaran shales. Kerogen has evolved to the metagenesis stage, which was demonstrated by the abundant pyrobitumen on microphotographs, the high calculated vitrinite reflectance (R
o
= 3%) via bitumen reflectance (R
b
), as well as δ
13
C of gas (methane) inclusions. Pyrolysates from Tvap-GC and open py-GC are quantitatively low and only light hydrocarbons were detected. The lower Silurian shale generally exhibits higher generation of hydrocarbon than the lower Cambrian and Ediacaran shale. Cooles' method and Claypool's equations were used to reconstruct the original TOC and Rock-Eval parameters of these overmature samples. Excellent original hydrocarbon generation was revealed in that the original TOC (TOC
o
) is between 5% and 23%, and original S1+S2 (S1
o
+S2
o
) is ranging from 29 to 215 mg HC/g rock.
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For over a century, the Danish thinker Søren Kierkegaard (1813-55) has been at the center of a number of important discussions, concerning not only philosophy and theology, but also, more recently, ...fields such as social thought, psychology, and contemporary aesthetics, especially literary theory.
Despite his relatively short life, Kierkegaard was an extraordinarily prolific writer, as attested to by the 26-volume Princeton University Press edition of all of his published writings. But Kierkegaard left behind nearly as much unpublished writing, most of which consists of what are called his "journals and notebooks." Kierkegaard has long been recognized as one of history's great journal keepers, but only rather small portions of his journals and notebooks are what we usually understand by the term "diaries." By far the greater part of Kierkegaard's journals and notebooks consists of reflections on a myriad of subjects-philosophical, religious, political, personal. Studying his journals and notebooks takes us into his workshop, where we can see his entire universe of thought. We can witness the genesis of his published works, to be sure-but we can also see whole galaxies of concepts, new insights, and fragments, large and small, of partially (or almost entirely) completed but unpublished works.Kierkegaard's Journals and Notebooksenables us to see the thinker in dialogue with his times and with himself.
Kierkegaard wrote his journals in a two-column format, one for his initial entries and the second for the extensive marginal comments that he added later. This edition of the journals reproduces this format, includes several photographs of original manuscript pages, and contains extensive scholarly commentary on the various entries and on the history of the manuscripts being reproduced.
Volume 9 of this 11-volume series includes five of Kierkegaard's important "NB" journals (Journals NB26 through NB30), which span from June 1852 to August 1854. This period was marked by Kierkegaard's increasing preoccupation with what he saw as an unbridgeable gulf in Christianity-between the absolute ideal of the religion of the New Testament and the official, state-sanctioned culture of "Christendom," which, embodied by the Danish People's Church, Kierkegaard rejected with increasing vehemence. Crucially, Kierkegaard's nemesis, Bishop Jakob Peter Mynster, died during this period and, in the months following, Kierkegaard can be seen moving inexorably toward the famous "attack on Christendom" with which he ended his life.
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