The Experts below are selected from a list of 60060 Experts worldwide ranked by ideXlab platform
Yue Shen - One of the best experts on this subject based on the ideXlab platform.
-
spectral Variability of a sample of Extreme Variability quasars and implications for the mg ii broad line region
Monthly Notices of the Royal Astronomical Society, 2020Co-Authors: Qian Yang, Yue Shen, Xin Liu, Yuching Chen, James Annis, S Avila, Emmanuel Bertin, David J Brooks, E Buckleygeer, Aurelio Carnero RosellAbstract:We present new Gemini/GMOS optical spectroscopy of 16 Extreme Variability quasars (EVQs) that dimmed by more than 1.5 mag in the g band between the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) epochs (separated by a few years in the quasar rest frame). The quasar sample covers a redshift range of 0:5 < z < 2:1. Nearly half of these EVQs brightened signi_cantly (by more than 0.5 mag in the g band) in a few years after reaching their previous faintest state, and some EVQs showed rapid (non-blazar) variations of greater than 1-2 mag on timescales of only months. Leveraging on the large dynamic range in continuum Variability between the earlier SDSS and the new GMOS spectra, we explore the associated variations in the broad Mg ii _2798 line, whose Variability properties have not been well studied before. The broad Mgii ux varies in the same direction as the continuum ux, albeit with a smaller amplitude, which indicates at least some portion of Mg ii is reverberating to continuum changes. However, the width (FWHM) of Mgii does not vary accordingly as continuum changes for most objects in the sample, in contrast to the case of the broad Balmer lines. Using the width of broad Mg ii to estimate the black hole mass therefore introduces a luminosity-dependent bias. We present new Gemini/GMOS optical spectroscopy of 16 Extreme Variability quasars(EVQs) that dimmed by more than 1.5 mag in the g band between the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) epochs (separated by a fewyears in the quasar rest frame). The quasar sample covers a redshift range of 0:5
timescales of only months. Leveraging on the large dynamic range in continuum Variability between the earlier SDSS and the new GMOS spectra, we explore the associated variations in the broad Mg ii _2798 line, whose Variability properties have not been well studied before. The broad Mgii ux varies in the same direction as the continuum ux, albeit with a smaller amplitude, which indicates at least some portion of Mg ii is reverberating to continuum changes. However, the width (FWHM) of Mgii does not vary accordingly as continuum changes for most objects in the sample, in contrast to the case of the broad Balmer lines. Using the width of broad Mg ii to estimate the black hole mass therefore introduces a luminosity-dependent bias. -
Extreme Variability quasars from the sloan digital sky survey and the dark energy survey
The Astrophysical Journal, 2018Co-Authors: N Rumbaugh, Yue Shen, Eric Morganson, Xin Liu, M Banerji, R G Mcmahon, F B Abdalla, A BenoitlevyAbstract:We perform a systematic search for long-term Extreme Variability quasars (EVQs) in the overlapping Sloan Digital Sky Survey and 3 Year Dark Energy Survey imaging, which provide light curves spanning more than 15 years. We identified ~1000 EVQs with a maximum change in g-band magnitude of more than 1 mag over this period, about 10% of all quasars searched. The EVQs have L bol ~ 1045–1047 erg s−1 and L/L Edd ~ 0.01–1. Accounting for selection effects, we estimate an intrinsic EVQ fraction of ~30%–50% among all $g\lesssim 22$ quasars over a baseline of ~15 yr. We performed detailed multi-wavelength, spectral, and Variability analyses for the EVQs and compared them to their parent quasar sample. We found that EVQs are distinct from a control sample of quasars matched in redshift and optical luminosity: (1) their UV broad emission lines have larger equivalent widths; (2) their Eddington ratios are systematically lower; and (3) they are more variable on all timescales. The intrinsic difference in quasar properties for EVQs suggests that internal processes associated with accretion are the main driver for the observed Extreme long-term Variability. However, despite their different properties, EVQs seem to be in the tail of a continuous distribution of quasar properties, rather than standing out as a distinct population. We speculate that EVQs are normal quasars accreting at relatively low rates, where the accretion flow is more likely to experience instabilities that drive the changes in flux by a factor of a few on multi-year timescales.
-
Extreme Variability quasars from the sloan digital sky survey and the dark energy survey
arXiv: Astrophysics of Galaxies, 2017Co-Authors: N Rumbaugh, Yue Shen, Eric Morganson, Xin Liu, M Banerji, R G Mcmahon, F B Abdalla, A Benoitlevy, E Bertin, David J BrooksAbstract:We perform a systematic search for long-term Extreme Variability quasars (EVQs) in the overlapping Sloan Digital Sky Survey (SDSS) and 3-Year Dark Energy Survey (DES) imaging, which provide light curves spanning more than 15 years. We identified ~1000 EVQs with a maximum g band magnitude change of more than 1 mag over this period, about 10% of all quasars searched. The EVQs have L_bol~10^45-10^47 erg/s and L/L_Edd~0.01-1. Accounting for selection effects, we estimate an intrinsic EVQ fraction of ~30-50% among all g<~22 quasars over a baseline of ~15 years. These EVQs are good candidates for so-called "changing-look quasars", where a spectral transition between the two types of quasars (broad-line and narrow-line) is observed between the dim and bright states. We performed detailed multi-wavelength, spectral and Variability analyses for the EVQs and compared to their parent quasar sample. We found that EVQs are distinct from a control sample of quasars matched in redshift and optical luminosity: (1) their UV broad emission lines have larger equivalent widths; (2) their Eddington ratios are systematically lower; and (3) they are more variable on all timescales. The intrinsic difference in quasar properties for EVQs suggest that internal processes associated with accretion are the main driver for the observed Extreme long-term Variability. However, despite their different properties, EVQs seem to be in the tail of a continuous distribution of quasar properties, rather than standing out as a distinct population. We speculate that EVQs are normal quasars accreting at relatively low accretion rates, where the accretion flow is more likely to experience instabilities that drive the factor of few changes in flux on multi-year timescales.
L J Gray - One of the best experts on this subject based on the ideXlab platform.
-
a practical method to identify displaced and split stratospheric polar vortex events
Geophysical Research Letters, 2013Co-Authors: William J M Seviour, Dann Mitchell, L J GrayAbstract:[1] Extreme Variability of the stratospheric polar vortex during winter can manifest as a displaced vortex event or a split vortex event. The influence of this vortex disruption can extend downwards and affect surface weather patterns. In particular, vortex splitting events have been associated with a negative Arctic Oscillation pattern. An assessment of the impacts of climate change on the polar vortex is therefore important, and more climate models now include a well‒resolved stratosphere. To aid this analysis, we introduce a practical threshold‒based method to distinguish between displaced and split vortex events. It requires only geopotential height at 10 hPa to measure the geometry of the vortex using two‒dimensional moment diagnostics. It captures Extremes of vortex Variability at least, as well as previous methods when applied to reanalysis data, and has the advantage of being easily employed to analyze climate model simulations.
-
the effect of climate change on the Variability of the northern hemisphere stratospheric polar vortex
Journal of the Atmospheric Sciences, 2012Co-Authors: Dann Mitchell, Andrew Charltonperez, L J Gray, Scott Osprey, Neal Butchart, Steven C Hardiman, Peter A G WatsonAbstract:AbstractWith Extreme Variability of the Arctic polar vortex being a key link for stratosphere–troposphere influences, its evolution into the twenty-first century is important for projections of changing surface climate in response to greenhouse gases. Variability of the stratospheric vortex is examined using a state-of-the-art climate model and a suite of specifically developed vortex diagnostics. The model has a fully coupled ocean and a fully resolved stratosphere. Analysis of the standard stratospheric zonal mean wind diagnostic shows no significant increase over the twenty-first century in the number of major sudden stratospheric warmings (SSWs) from its historical value of 0.7 events per decade, although the monthly distribution of SSWs does vary, with events becoming more evenly dispersed throughout the winter. However, further analyses using geometric-based vortex diagnostics show that the vortex mean state becomes weaker, and the vortex centroid is climatologically more equatorward by up to 2.5°, ...
-
characterizing the Variability and Extremes of the stratospheric polar vortices using 2d moment analysis
Journal of the Atmospheric Sciences, 2011Co-Authors: Dann Mitchell, Andrew Charltonperez, L J GrayAbstract:AbstractThe mean state, Variability, and Extreme Variability of the stratospheric polar vortices, with an emphasis on the Northern Hemisphere (NH) vortex, are examined using two-dimensional moment analysis and Extreme value theory (EVT). The use of moments as an analysis tool gives rise to information about the vortex area, centroid latitude, aspect ratio, and kurtosis. The application of EVT to these moment-derived quantities allows the Extreme Variability of the vortex to be assessed. The data used for this study are 40-yr ECMWF Re-Analysis (ERA-40) potential vorticity fields on interpolated isentropic surfaces that range from 450 to 1450 K.Analyses show that the most Extreme vortex Variability occurs most commonly in late January and early February, consistent with when most planetary wave driving from the troposphere is observed. Composites around sudden stratospheric warming (SSW) events reveal that the moment diagnostics evolve in statistically different ways between vortex splitting events and vort...
A Benoitlevy - One of the best experts on this subject based on the ideXlab platform.
-
Extreme Variability quasars from the sloan digital sky survey and the dark energy survey
The Astrophysical Journal, 2018Co-Authors: N Rumbaugh, Yue Shen, Eric Morganson, Xin Liu, M Banerji, R G Mcmahon, F B Abdalla, A BenoitlevyAbstract:We perform a systematic search for long-term Extreme Variability quasars (EVQs) in the overlapping Sloan Digital Sky Survey and 3 Year Dark Energy Survey imaging, which provide light curves spanning more than 15 years. We identified ~1000 EVQs with a maximum change in g-band magnitude of more than 1 mag over this period, about 10% of all quasars searched. The EVQs have L bol ~ 1045–1047 erg s−1 and L/L Edd ~ 0.01–1. Accounting for selection effects, we estimate an intrinsic EVQ fraction of ~30%–50% among all $g\lesssim 22$ quasars over a baseline of ~15 yr. We performed detailed multi-wavelength, spectral, and Variability analyses for the EVQs and compared them to their parent quasar sample. We found that EVQs are distinct from a control sample of quasars matched in redshift and optical luminosity: (1) their UV broad emission lines have larger equivalent widths; (2) their Eddington ratios are systematically lower; and (3) they are more variable on all timescales. The intrinsic difference in quasar properties for EVQs suggests that internal processes associated with accretion are the main driver for the observed Extreme long-term Variability. However, despite their different properties, EVQs seem to be in the tail of a continuous distribution of quasar properties, rather than standing out as a distinct population. We speculate that EVQs are normal quasars accreting at relatively low rates, where the accretion flow is more likely to experience instabilities that drive the changes in flux by a factor of a few on multi-year timescales.
-
Extreme Variability quasars from the sloan digital sky survey and the dark energy survey
arXiv: Astrophysics of Galaxies, 2017Co-Authors: N Rumbaugh, Yue Shen, Eric Morganson, Xin Liu, M Banerji, R G Mcmahon, F B Abdalla, A Benoitlevy, E Bertin, David J BrooksAbstract:We perform a systematic search for long-term Extreme Variability quasars (EVQs) in the overlapping Sloan Digital Sky Survey (SDSS) and 3-Year Dark Energy Survey (DES) imaging, which provide light curves spanning more than 15 years. We identified ~1000 EVQs with a maximum g band magnitude change of more than 1 mag over this period, about 10% of all quasars searched. The EVQs have L_bol~10^45-10^47 erg/s and L/L_Edd~0.01-1. Accounting for selection effects, we estimate an intrinsic EVQ fraction of ~30-50% among all g<~22 quasars over a baseline of ~15 years. These EVQs are good candidates for so-called "changing-look quasars", where a spectral transition between the two types of quasars (broad-line and narrow-line) is observed between the dim and bright states. We performed detailed multi-wavelength, spectral and Variability analyses for the EVQs and compared to their parent quasar sample. We found that EVQs are distinct from a control sample of quasars matched in redshift and optical luminosity: (1) their UV broad emission lines have larger equivalent widths; (2) their Eddington ratios are systematically lower; and (3) they are more variable on all timescales. The intrinsic difference in quasar properties for EVQs suggest that internal processes associated with accretion are the main driver for the observed Extreme long-term Variability. However, despite their different properties, EVQs seem to be in the tail of a continuous distribution of quasar properties, rather than standing out as a distinct population. We speculate that EVQs are normal quasars accreting at relatively low accretion rates, where the accretion flow is more likely to experience instabilities that drive the factor of few changes in flux on multi-year timescales.
Xin Liu - One of the best experts on this subject based on the ideXlab platform.
-
spectral Variability of a sample of Extreme Variability quasars and implications for the mg ii broad line region
Monthly Notices of the Royal Astronomical Society, 2020Co-Authors: Qian Yang, Yue Shen, Xin Liu, Yuching Chen, James Annis, S Avila, Emmanuel Bertin, David J Brooks, E Buckleygeer, Aurelio Carnero RosellAbstract:We present new Gemini/GMOS optical spectroscopy of 16 Extreme Variability quasars (EVQs) that dimmed by more than 1.5 mag in the g band between the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) epochs (separated by a few years in the quasar rest frame). The quasar sample covers a redshift range of 0:5 < z < 2:1. Nearly half of these EVQs brightened signi_cantly (by more than 0.5 mag in the g band) in a few years after reaching their previous faintest state, and some EVQs showed rapid (non-blazar) variations of greater than 1-2 mag on timescales of only months. Leveraging on the large dynamic range in continuum Variability between the earlier SDSS and the new GMOS spectra, we explore the associated variations in the broad Mg ii _2798 line, whose Variability properties have not been well studied before. The broad Mgii ux varies in the same direction as the continuum ux, albeit with a smaller amplitude, which indicates at least some portion of Mg ii is reverberating to continuum changes. However, the width (FWHM) of Mgii does not vary accordingly as continuum changes for most objects in the sample, in contrast to the case of the broad Balmer lines. Using the width of broad Mg ii to estimate the black hole mass therefore introduces a luminosity-dependent bias. We present new Gemini/GMOS optical spectroscopy of 16 Extreme Variability quasars(EVQs) that dimmed by more than 1.5 mag in the g band between the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) epochs (separated by a fewyears in the quasar rest frame). The quasar sample covers a redshift range of 0:5
timescales of only months. Leveraging on the large dynamic range in continuum Variability between the earlier SDSS and the new GMOS spectra, we explore the associated variations in the broad Mg ii _2798 line, whose Variability properties have not been well studied before. The broad Mgii ux varies in the same direction as the continuum ux, albeit with a smaller amplitude, which indicates at least some portion of Mg ii is reverberating to continuum changes. However, the width (FWHM) of Mgii does not vary accordingly as continuum changes for most objects in the sample, in contrast to the case of the broad Balmer lines. Using the width of broad Mg ii to estimate the black hole mass therefore introduces a luminosity-dependent bias. -
Extreme Variability quasars from the sloan digital sky survey and the dark energy survey
The Astrophysical Journal, 2018Co-Authors: N Rumbaugh, Yue Shen, Eric Morganson, Xin Liu, M Banerji, R G Mcmahon, F B Abdalla, A BenoitlevyAbstract:We perform a systematic search for long-term Extreme Variability quasars (EVQs) in the overlapping Sloan Digital Sky Survey and 3 Year Dark Energy Survey imaging, which provide light curves spanning more than 15 years. We identified ~1000 EVQs with a maximum change in g-band magnitude of more than 1 mag over this period, about 10% of all quasars searched. The EVQs have L bol ~ 1045–1047 erg s−1 and L/L Edd ~ 0.01–1. Accounting for selection effects, we estimate an intrinsic EVQ fraction of ~30%–50% among all $g\lesssim 22$ quasars over a baseline of ~15 yr. We performed detailed multi-wavelength, spectral, and Variability analyses for the EVQs and compared them to their parent quasar sample. We found that EVQs are distinct from a control sample of quasars matched in redshift and optical luminosity: (1) their UV broad emission lines have larger equivalent widths; (2) their Eddington ratios are systematically lower; and (3) they are more variable on all timescales. The intrinsic difference in quasar properties for EVQs suggests that internal processes associated with accretion are the main driver for the observed Extreme long-term Variability. However, despite their different properties, EVQs seem to be in the tail of a continuous distribution of quasar properties, rather than standing out as a distinct population. We speculate that EVQs are normal quasars accreting at relatively low rates, where the accretion flow is more likely to experience instabilities that drive the changes in flux by a factor of a few on multi-year timescales.
-
Extreme Variability quasars from the sloan digital sky survey and the dark energy survey
arXiv: Astrophysics of Galaxies, 2017Co-Authors: N Rumbaugh, Yue Shen, Eric Morganson, Xin Liu, M Banerji, R G Mcmahon, F B Abdalla, A Benoitlevy, E Bertin, David J BrooksAbstract:We perform a systematic search for long-term Extreme Variability quasars (EVQs) in the overlapping Sloan Digital Sky Survey (SDSS) and 3-Year Dark Energy Survey (DES) imaging, which provide light curves spanning more than 15 years. We identified ~1000 EVQs with a maximum g band magnitude change of more than 1 mag over this period, about 10% of all quasars searched. The EVQs have L_bol~10^45-10^47 erg/s and L/L_Edd~0.01-1. Accounting for selection effects, we estimate an intrinsic EVQ fraction of ~30-50% among all g<~22 quasars over a baseline of ~15 years. These EVQs are good candidates for so-called "changing-look quasars", where a spectral transition between the two types of quasars (broad-line and narrow-line) is observed between the dim and bright states. We performed detailed multi-wavelength, spectral and Variability analyses for the EVQs and compared to their parent quasar sample. We found that EVQs are distinct from a control sample of quasars matched in redshift and optical luminosity: (1) their UV broad emission lines have larger equivalent widths; (2) their Eddington ratios are systematically lower; and (3) they are more variable on all timescales. The intrinsic difference in quasar properties for EVQs suggest that internal processes associated with accretion are the main driver for the observed Extreme long-term Variability. However, despite their different properties, EVQs seem to be in the tail of a continuous distribution of quasar properties, rather than standing out as a distinct population. We speculate that EVQs are normal quasars accreting at relatively low accretion rates, where the accretion flow is more likely to experience instabilities that drive the factor of few changes in flux on multi-year timescales.
F B Abdalla - One of the best experts on this subject based on the ideXlab platform.
-
Extreme Variability quasars from the sloan digital sky survey and the dark energy survey
The Astrophysical Journal, 2018Co-Authors: N Rumbaugh, Yue Shen, Eric Morganson, Xin Liu, M Banerji, R G Mcmahon, F B Abdalla, A BenoitlevyAbstract:We perform a systematic search for long-term Extreme Variability quasars (EVQs) in the overlapping Sloan Digital Sky Survey and 3 Year Dark Energy Survey imaging, which provide light curves spanning more than 15 years. We identified ~1000 EVQs with a maximum change in g-band magnitude of more than 1 mag over this period, about 10% of all quasars searched. The EVQs have L bol ~ 1045–1047 erg s−1 and L/L Edd ~ 0.01–1. Accounting for selection effects, we estimate an intrinsic EVQ fraction of ~30%–50% among all $g\lesssim 22$ quasars over a baseline of ~15 yr. We performed detailed multi-wavelength, spectral, and Variability analyses for the EVQs and compared them to their parent quasar sample. We found that EVQs are distinct from a control sample of quasars matched in redshift and optical luminosity: (1) their UV broad emission lines have larger equivalent widths; (2) their Eddington ratios are systematically lower; and (3) they are more variable on all timescales. The intrinsic difference in quasar properties for EVQs suggests that internal processes associated with accretion are the main driver for the observed Extreme long-term Variability. However, despite their different properties, EVQs seem to be in the tail of a continuous distribution of quasar properties, rather than standing out as a distinct population. We speculate that EVQs are normal quasars accreting at relatively low rates, where the accretion flow is more likely to experience instabilities that drive the changes in flux by a factor of a few on multi-year timescales.
-
Extreme Variability quasars from the sloan digital sky survey and the dark energy survey
arXiv: Astrophysics of Galaxies, 2017Co-Authors: N Rumbaugh, Yue Shen, Eric Morganson, Xin Liu, M Banerji, R G Mcmahon, F B Abdalla, A Benoitlevy, E Bertin, David J BrooksAbstract:We perform a systematic search for long-term Extreme Variability quasars (EVQs) in the overlapping Sloan Digital Sky Survey (SDSS) and 3-Year Dark Energy Survey (DES) imaging, which provide light curves spanning more than 15 years. We identified ~1000 EVQs with a maximum g band magnitude change of more than 1 mag over this period, about 10% of all quasars searched. The EVQs have L_bol~10^45-10^47 erg/s and L/L_Edd~0.01-1. Accounting for selection effects, we estimate an intrinsic EVQ fraction of ~30-50% among all g<~22 quasars over a baseline of ~15 years. These EVQs are good candidates for so-called "changing-look quasars", where a spectral transition between the two types of quasars (broad-line and narrow-line) is observed between the dim and bright states. We performed detailed multi-wavelength, spectral and Variability analyses for the EVQs and compared to their parent quasar sample. We found that EVQs are distinct from a control sample of quasars matched in redshift and optical luminosity: (1) their UV broad emission lines have larger equivalent widths; (2) their Eddington ratios are systematically lower; and (3) they are more variable on all timescales. The intrinsic difference in quasar properties for EVQs suggest that internal processes associated with accretion are the main driver for the observed Extreme long-term Variability. However, despite their different properties, EVQs seem to be in the tail of a continuous distribution of quasar properties, rather than standing out as a distinct population. We speculate that EVQs are normal quasars accreting at relatively low accretion rates, where the accretion flow is more likely to experience instabilities that drive the factor of few changes in flux on multi-year timescales.
