82 research outputs found
Discovery of 21 New Changing-look AGNs in Northern Sky
The rare case of changing-look (CL) AGNs, with the appearance or
disappearance of broad Balmer emission lines within a few years, challenges our
understanding of the AGN unified model. We present a sample of 21 new CL AGNs
at , which doubles the number of such objects known to date. These
new CL AGNs were discovered by several ways, from (1) repeat spectra in the
SDSS, (2) repeat spectra in the Large Sky Area Multi-Object Fiber Spectroscopic
Telescope (LAMOST) and SDSS, and (3) photometric variability and new
spectroscopic observations. We use the photometric data from surveys, including
the SDSS imaging survey, the Pan-STARRS1, the DESI Legacy imaging survey, the
Wide-field Infrared Survey Explorer (WISE), the Catalina Real-time Transient
Survey, and the Palomar Transient Factory. The estimated upper limits of
transition timescale of the CL AGNs in this sample spans from 0.9 to 13 years
in the rest frame. The continuum flux in the optical and mid-infrared becomes
brighter when the CL AGNs turn on, or vice versa. Variations of more than 0.2
mag in band were detected in 15 CL AGNs during the transition. The optical
and mid-infrared variability is not consistent with the scenario of variable
obscuration in 10 CL AGNs at more than confidence level. We confirm a
bluer-when-brighter trend in the optical. However, the mid-infrared WISE colors
become redder when the objects become brighter in the band,
possibly due to a stronger hot dust contribution in the band when the AGN
activity becomes stronger. The physical mechanism of type transition is
important for understanding the evolution of AGNs.Comment: Accepted for publication in Ap
Functionally distinct and selectively phosphorylated GPCR subpopulations co-exist in a single cell.
G protein-coupled receptors (GPCRs) transduce pleiotropic intracellular signals in a broad range of physiological responses and disease states. Activated GPCRs can undergo agonist-induced phosphorylation by G protein receptor kinases (GRKs) and second messenger-dependent protein kinases such as protein kinase A (PKA). Here, we characterize spatially segregated subpopulations of β2-adrenergic receptor (β2AR) undergoing selective phosphorylation by GRKs or PKA in a single cell. GRKs primarily label monomeric β2ARs that undergo endocytosis, whereas PKA modifies dimeric β2ARs that remain at the cell surface. In hippocampal neurons, PKA-phosphorylated β2ARs are enriched in dendrites, whereas GRK-phosphorylated β2ARs accumulate in soma, being excluded from dendrites in a neuron maturation-dependent manner. Moreover, we show that PKA-phosphorylated β2ARs are necessary to augment the activity of L-type calcium channel. Collectively, these findings provide evidence that functionally distinct subpopulations of this prototypical GPCR exist in a single cell
Quasar Photometric Redshifts and Candidate Selection: A New Algorithm Based on Optical and Mid-Infrared Photometric Data
We present a new algorithm to estimate quasar photometric redshifts
(photo-s), by considering the asymmetries in the relative flux distributions
of quasars. The relative flux models are built with multivariate Skew-t
distributions in the multi-dimensional space of relative fluxes as a function
of redshift and magnitude. For 151,392 quasars in the SDSS, we achieve a
photo- accuracy, defined as the fraction of quasars with the difference
between the photo- and the spectroscopic redshift , within 0.1, of 74%. Combining the WISE W1 and W2 infrared
data with the SDSS data, the photo- accuracy is enhanced to 87%. Using the
Pan-STARRS1 or DECaLS photometry with WISE W1 and W2 data, the photo-
accuracies are 79% and 72%, respectively. The prior probabilities as a function
of magnitude for quasars, stars and galaxies are calculated respectively based
on (1) the quasar luminosity function; (2) the Milky Way synthetic simulation
with the Besan\c{c}on model; (3) the Bayesian Galaxy Photometric Redshift
estimation. The relative fluxes of stars are obtained with the Padova
isochrones, and the relative fluxes of galaxies are modeled through galaxy
templates. We test our classification method to select quasars using the DECaLS
, , , and WISE W1 and W2 photometry. The quasar selection completeness
is higher than 70% for a wide redshift range , and a wide magnitude
range mag. Our photo- regression and classification method has
the potential to extend to future surveys. The photo- code will be publicly
available.Comment: 22 pages, 17 figure, accepted by AJ. The code is available at
https://doi.org/10.5281/zenodo.101440
An ultra-luminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30
So far, roughly 40 quasars with redshifts greater than z=6 have been
discovered. Each quasar contains a black hole with a mass of about one billion
solar masses (). The existence of such black holes when the
Universe was less than 1 billion years old presents substantial challenges to
theories of the formation and growth of black holes and the coevolution of
black holes and galaxies. Here we report the discovery of an ultra-luminous
quasar, SDSS J010013.02+280225.8, at redshift z=6.30. It has an optical and
near-infrared luminosity a few times greater than those of previously known z>6
quasars. On the basis of the deep absorption trough on the blue side of the Ly
emission line in the spectrum, we estimate the proper size of the
ionized proximity zone associated with the quasar to be 26 million light years,
larger than found with other z>6.1 quasars with lower luminosities. We estimate
(on the basis of a near-infrared spectrum) that the black hole has a mass of
, which is consistent with the derived by assuming an Eddington-limited accretion rate.Comment: 24 pages, 4 figures plus 4 extended data figures, published in Nature
on 26 February 201
An ultra-luminous quasar at z=5.363 with a ten billion solar mass black hole and a Metal-Rich DLA at z~5
We report the discovery of an ultra-luminous quasar J030642.51+185315.8
(hereafter J0306+1853) at redshift 5.363, which hosts a super-massive black
hole (SMBH) with . With an
absolute magnitude and bolometric luminosity
, J0306+1853 is one of the most luminous
objects in the early Universe. It is not likely to be a beamed source based on
its small flux variability, low radio loudness and normal broad emission lines.
In addition, a Damped Ly system (DLA) with , among the most metal rich DLAs at , is detected
in the absorption spectrum of this quasar. This ultra-luminous quasar puts
strong constraint on the bright-end of quasar luminosity function and
massive-end of black hole mass function. It will provide a unique laboratory to
the study of BH growth and the co-evolution between BH and host galaxy with
multi-wavelength follow-up observations. The future high resolution spectra
will give more insights to the DLA and other absorption systems along the
line-of-sight of J0306+1853.Comment: 5 pages, 3 figures, accepted for publication in ApJ
An Unusual Mid-infrared Flare in a Type 2 AGN: An Obscured Turning-on AGN or Tidal Disruption Event?
We report the discovery of an exceptional MIR flare in a Type 2 AGN, SDSS J165726.81+234528.1, at z = 0.059. This object brightened by 3 mag in the Wide-field Infrared Survey Explorer (WISE) W1 and W2 bands between 2015 and 2017 (and has been fading since 2018), without significant changes (≾0.2 mag) in the optical over the same period of time. Based on the WISE light curves and near-IR imaging, the flare is more significant at longer wavelengths, suggesting an origin of hot dust emission. The estimated black hole mass (~10^(6.5) M⊙) from different methods places its peak bolometric luminosity around the Eddington limit. The high luminosity of the MIR flare and its multiyear timescale suggest that it most likely originated from reprocessed dust radiation in an extended torus surrounding the AGN, instead of from stellar explosions. The MIR color variability is consistent with known changing-look AGN and tidal disruption events (TDEs), but inconsistent with normal supernovae. We suggest that it is a turning-on Type 2 AGN or TDE, where the optical variability is obscured by the dust torus during the transition. This MIR flare event reveals a population of dramatic nuclear transients that are missed in the optical
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