592 research outputs found
Direct estimation of kinetic parametric images for dynamic PET.
Dynamic positron emission tomography (PET) can monitor spatiotemporal distribution of radiotracer in vivo. The spatiotemporal information can be used to estimate parametric images of radiotracer kinetics that are of physiological and biochemical interests. Direct estimation of parametric images from raw projection data allows accurate noise modeling and has been shown to offer better image quality than conventional indirect methods, which reconstruct a sequence of PET images first and then perform tracer kinetic modeling pixel-by-pixel. Direct reconstruction of parametric images has gained increasing interests with the advances in computing hardware. Many direct reconstruction algorithms have been developed for different kinetic models. In this paper we review the recent progress in the development of direct reconstruction algorithms for parametric image estimation. Algorithms for linear and nonlinear kinetic models are described and their properties are discussed
A Thirty-Four Billion Solar Mass Black Hole in SMSS J2157-3602, the Most Luminous Known Quasar
From near-infrared spectroscopic measurements of the MgII emission line
doublet, we estimate the black hole (BH) mass of the quasar, SMSS
J215728.21-360215.1, as being (3.4 +/- 0.6) x 10^10 M_sun and refine the
redshift of the quasar to be z=4.692. SMSS J2157 is the most luminous known
quasar, with a 3000A luminosity of (4.7 +/- 0.5) x 10^47 erg/s and an estimated
bolometric luminosity of 1.6 x 10^48 erg/s, yet its Eddington ratio is only
~0.4. Thus, the high luminosity of this quasar is a consequence of its
extremely large BH -- one of the most massive BHs at z > 4.Comment: 7 pages, 3 figures. Accepted for publication in MNRA
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
A Survey for High-redshift Gravitationally Lensed Quasars and Close Quasars Pairs. I. the Discoveries of an Intermediately-lensed Quasar and a Kpc-scale Quasar Pair at
We present the first results from a new survey for high-redshift
gravitationally lensed quasars and close quasar pairs. We carry
out candidate selection based on the colors and shapes of objects in public
imaging surveys, then conduct follow-up observations to confirm the nature of
high-priority candidates. In this paper, we report the discoveries of
J0025--0145 () which we identify as an {intermediately-lensed quasar,
and J2329--0522 () which is a kpc-scale close quasar pair. The {\em
Hubble Space Telescope (HST)} image of J0025--0145 shows a foreground lensing
galaxy located 0\farcs6 away from the quasar. However, J0025--0145 does not
exhibit multiple lensed images of the quasar, and we identify J0025--0145 as an
intermediate lensing system (a lensing system that is not multiply imaged but
has a significant magnification). The spectrum of J0025--0145 implies an
extreme Eddington ratio if the quasar luminosity is intrinsic, which could be
explained by a large lensing magnification. The {\em HST} image of J0025--0145
also indicates a tentative detection of the quasar host galaxy in rest-frame
UV, illustrating the power of lensing magnification and distortion in studies
of high-redshift quasar host galaxies. J2329--0522 consists of two resolved
components with significantly different spectral properties, and a lack of
lensing galaxy detection under sub-arcsecond seeing. We identify it as a close
quasar pair, which is the highest confirmed kpc-scale quasar pair to date. We
also report four lensed quasars and quasar pairs at , and discuss
possible improvements to our survey strategy.Comment: 23 pages, 10 figures, 6 tables. Accepted by the Astronomical Journa
Spectroscopy of broad absorption line quasars at -- I: evidence for quasar winds shaping broad/narrow emission line regions
We present an observational study of 22 broad absorption line quasars (BAL
QSOs) at based on optical/near-IR spectroscopy, aiming
to investigate quasar winds and their effects. The near-IR spectroscopy covers
the \hb\ and/or \mgii\ broad emission lines (BELs) for these quasars, allowing
us to estimate their central black hole (BH) masses in a robust way. We found
that our BAL QSOs on average do not have a higher Eddington ratio than that
from non-BAL QSOs matched in redshift and/or luminosity. In a subset consisting
of seven strong BAL QSOs possessing sub-relativistic BAL outflows, we see the
prevalence of large \civ-BEL blueshift (3100 km s) and weak \oiii\
emission (particularly the narrow \oiii5007 component), indicative of
nuclear outflows affecting the narrow emission-line (NEL) regions. In another
subset consisting of thirteen BAL QSOs having simultaneous observations of
\mgii\ and \hb, we found a strong correlation between 3000~\AA\ and 5000~\AA\
monochromatic luminosity, consistent with that from non-BAL QSOs matched in
redshift and luminosity; however, there is no correlation between \mgii\ and
\hb\ in FWHM, likely due to nuclear outflows influencing the BEL regions. Our
spectroscopic investigations offer strong evidence that the presence of nuclear
outflows plays an important role in shaping the BEL/NEL regions of these
quasars and possibly, regulating the growth of central supermassive black holes
(SMBHs). We propose that BEL blueshift and BAL could be different
manifestations of the same outflow system viewed at different sightlines and/or
phases.Comment: 13 pages, 10 figures. Accepted for publication in Ap
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