900 research outputs found
Do quasar broad-line velocity widths add any information to virial black hole mass estimates?
We examine how much information measured broad-line widths add to virial BH
mass estimates for flux limited samples of quasars. We do this by comparing the
BH mass estimates to those derived by randomly reassigning the quasar
broad-line widths to different objects and re-calculating the BH mass. For 9000
BH masses derived from the H_beta line we find that the distributions of
original and randomized BH masses in the M_BH-redshift plane and the
M_BH-luminosity plane are formally identical. A 2D KS test does not find a
difference at >90% confidence. For the MgII line (32000 quasars) we do find
very significant differences between the randomized and original BH masses, but
the amplitude of the difference is still small. The difference for the CIV line
(14000 quasars) is 2-3sigma and again the amplitude of the difference is small.
Subdividing the data into redshift and luminosity bins we find that the median
absolute difference in BH mass between the original and randomized data is
0.025, 0.01 and 0.04 dex for H_beta, MgII and CIV respectively. The maximum
absolute difference is always <=0.1 dex. We investigate whether our results are
sensitive to corrections to MgII virial masses (e.g. Onken & Kollmeier 2008).
These corrections do not influence our results. Moreover, the mass residual -
Eddington ratio correlation discussed by Onken & Kollmeier is more directly
attributable to the slope of the relation between H_beta and MgII line width.
Our results imply that the measured quasar broad-line velocity widths provide
little extra information, after allowing for the mean velocity width. In this
case virial estimates are equivalent to M_BH L^alpha, with L/L_edd L^(1-alpha)
(with alpha~0.5). This leaves an unanswered question of why the accretion
efficiency changes with luminosity in just the right way to keep the mean
broad-line widths fixed as a function of luminosity. (abridged)Comment: 9 Pages, 9 figures. Accepted by Astrophysical Journa
Reverberation Mapping Results from MDM Observatory
We present results from a multi-month reverberation mapping campaign
undertaken primarily at MDM Observatory with supporting observations from
around the world. We measure broad line region (BLR) radii and black hole
masses for six objects. A velocity-resolved analysis of the H_beta response
shows the presence of diverse kinematic signatures in the BLR.Comment: To appear in the Proceedings of the IAU Symposium No. 267:
Co-Evolution of Central Black Holes and Galaxies, Rio de Janeiro, 200
Synthesizing realistic neural population activity patterns using generative adversarial networks
The ability to synthesize realistic patterns of neural activity is crucial for studying neural information processing. Here we used the Generative Adversarial Networks (GANs) framework to simulate the concerted activity of a population of neurons. We adapted the Wasserstein-GAN variant to facilitate the generation of unconstrained neural population activity patterns while still benefiting from parameter sharing in the temporal domain. We demonstrate that our proposed GAN, which we termed Spike-GAN, generates spike trains that match accurately the first- and second-order statistics of datasets of tens of neurons and also approximates well their higher-order statistics. We applied Spike-GAN to a real dataset recorded from salamander retina and showed that it performs as well as state-of-the-art approaches based on the maximum entropy and the dichotomized Gaussian frameworks. Importantly, Spike-GAN does not require to specify a priori the statistics to be matched by the model, and so constitutes a more flexible method than these alternative approaches. Finally, we show how to exploit a trained Spike-GAN to construct’importance maps’ to detect the most relevant statistical structures present in a spike train. Spike-GAN provides a powerful, easy-to-use technique for generating realistic spiking neural activity and for describing the most relevant features of the large-scale neural population recordings studied in modern systems neuroscience
CalciumGAN: A Generative Adversarial Network Model for Synthesising Realistic Calcium Imaging Data of Neuronal Populations
Calcium imaging has become a powerful and popular technique to monitor the
activity of large populations of neurons in vivo. However, for ethical
considerations and despite recent technical developments, recordings are still
constrained to a limited number of trials and animals. This limits the amount
of data available from individual experiments and hinders the development of
analysis techniques and models for more realistic size of neuronal populations.
The ability to artificially synthesize realistic neuronal calcium signals could
greatly alleviate this problem by scaling up the number of trials. Here we
propose a Generative Adversarial Network (GAN) model to generate realistic
calcium signals as seen in neuronal somata with calcium imaging. To this end,
we adapt the WaveGAN architecture and train it with the Wasserstein distance.
We test the model on artificial data with known ground-truth and show that the
distribution of the generated signals closely resembles the underlying data
distribution. Then, we train the model on real calcium signals recorded from
the primary visual cortex of behaving mice and confirm that the deconvolved
spike trains match the statistics of the recorded data. Together, these results
demonstrate that our model can successfully generate realistic calcium imaging
data, thereby providing the means to augment existing datasets of neuronal
activity for enhanced data exploration and modeling
V1T: large-scale mouse V1 response prediction using a Vision Transformer
Accurate predictive models of the visual cortex neural response to natural
visual stimuli remain a challenge in computational neuroscience. In this work,
we introduce V1T, a novel Vision Transformer based architecture that learns a
shared visual and behavioral representation across animals. We evaluate our
model on two large datasets recorded from mouse primary visual cortex and
outperform previous convolution-based models by more than 12.7% in prediction
performance. Moreover, we show that the self-attention weights learned by the
Transformer correlate with the population receptive fields. Our model thus sets
a new benchmark for neural response prediction and can be used jointly with
behavioral and neural recordings to reveal meaningful characteristic features
of the visual cortex.Comment: updated references and added link to code repository; add analysis on
generalization and visualize aRF
Black Hole Masses in Three Seyfert Galaxies
We analyze published reverberation mapping data for three Seyfert galaxies
(NGC 3227, NGC 3516, and NGC 4593) to refine the mass estimate for the
supermassive black hole in the center of each object. Treatment of the data in
a manner more consistent with other large compilations of such masses allows us
to more securely compare our results to wider samples of data, e.g., in the
investigation of the M_bh-sigma relationship for active and quiescent galaxies.Comment: 14 pages, 4 figures. Accepted for publication in Ap
Metastatic model of HPV+ oropharyngeal squamous cell carcinoma demonstrates heterogeneity in tumor metastasis
Human papillomavirus induced (HPV+) cancer incidence is rapidly rising, comprising 60–80% of oropharyngeal squamous cell carcinomas (OPSCCs); while rare, recurrent/metastatic disease accounts for nearly all related deaths. An in vivo pre-clinical model for these invasive cancers is necessary for testing new therapies. We characterize an immune competent recurrent/metastatic HPV+ murine model of OPSSC which consists of four lung metastatic (MLM) cell lines isolated from an animal with HPV+ OPSCC that failed cisplatin/radiation treatment. These individual metastatic clonal cell lines were tested to verify their origin (parental transgene expression and define their physiological properties: proliferation, metastatic potential, heterogeneity and sensitivity/resistance to cisplatin and radiation. All MLMs retain expression of parental HPV16 E6 and E7 and degrade P53 yet are heterogeneous from one another and from the parental cell line as defined by Illumina expression microarray. Consistent with this, reverse phase protein array defines differences in protein expression/activation between MLMs as well as the parental line. While in vitro growth rates of MLMs are slower than the parental line, in vivo growth of MLM clones is greatly enhanced. Moreover, in vivo resistance to standard therapies is dramatically increased in 3 of the 4 MLMs. Lymphatic and/or lung metastasis occurs 100% of the time in one MLM line. This recurrent/metastatic model of HPV+ OPSCC retains the characteristics evident in refractory human disease (heterogeneity, resistance to therapy, metastasis in lymph nodes/lungs) thus serving as an ideal translational system to test novel therapeutics. Moreover, this system may provide insights into the molecular mechanisms of metastasis
NASA's Ares I and Ares V Launch Vehicles--Effective Space Operations Through Efficient Ground Operations
The United States (U.S.) is charting a renewed course for lunar exploration, with the fielding of a new human-rated space transportation system to replace the venerable Space Shuttle, which will be retired after it completes its missions of building the International Space Station (ISS) and servicing the Hubble Space Telescope. Powering the future of space-based scientific exploration will be the Ares I Crew Launch Vehicle, which will transport the Orion Crew Exploration Vehicle to orbit where it will rendezvous with the Altair Lunar Lander, which will be delivered by the Ares V Cargo Launch Vehicle (fig. 1). This configuration will empower rekindled investigation of Earth's natural satellite in the not too distant future. This new exploration infrastructure, developed by the National Aeronautics and Space Administration (NASA), will allow astronauts to leave low-Earth orbit (LEO) for extended lunar missions and preparation for the first long-distance journeys to Mars. All space-based operations - to LEO and beyond - are controlled from Earth. NASA's philosophy is to deliver safe, reliable, and cost-effective architecture solutions to sustain this multi-billion-dollar program across several decades. Leveraging SO years of lessons learned, NASA is partnering with private industry and academia, while building on proven hardware experience. This paper outlines a few ways that the Engineering Directorate at NASA's Marshall Space Flight Center is working with the Constellation Program and its project offices to streamline ground operations concepts by designing for operability, which reduces lifecycle costs and promotes sustainable space exploration
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