124 research outputs found
The Event Horizon of M87
The 6 billion solar mass supermassive black hole at the center of the giant
elliptical galaxy M87 powers a relativistic jet. Observations at millimeter
wavelengths with the Event Horizon Telescope have localized the emission from
the base of this jet to angular scales comparable to the putative black hole
horizon. The jet might be powered directly by an accretion disk or by
electromagnetic extraction of the rotational energy of the black hole. However,
even the latter mechanism requires a confining thick accretion disk to maintain
the required magnetic flux near the black hole. Therefore, regardless of the
jet mechanism, the observed jet power in M87 implies a certain minimum mass
accretion rate. If the central compact object in M87 were not a black hole but
had a surface, this accretion would result in considerable thermal
near-infrared and optical emission from the surface. Current flux limits on the
nucleus of M87 strongly constrain any such surface emission. This rules out the
presence of a surface and thereby provides indirect evidence for an event
horizon.Comment: 9 pages, 2 figures, submitted to Ap
Polarization Observations with the Cosmic Background Imager
We describe polarization observations of the CMBR with the Cosmic Background Imager, a 13 element interferometer which operates in the 26-36 GHz band from Llano de Chajnantour in northern Chile. The array consists of 90-cm Cassegrain antennas mounted on a steerable platform which can be rotated about the optical axis to facilitate polarization observations. The CBI employs single mode circularly polarized receivers which sample multipoles from ℓ~400
to ℓ~4250. The instrumental polarization of the CBI was calibrated with 3C279, a bright polarized point source
which was monitored with the VLA
Relative Astrometry of Compact Flaring Structures in Sgr A* with Polarimetric VLBI
We demonstrate that polarimetric interferometry can be used to extract
precise spatial information about compact polarized flares of Sgr A*. We show
that, for a faint dynamical component, a single interferometric baseline
suffices to determine both its polarization and projected displacement from the
quiescent intensity centroid. A second baseline enables two-dimensional
reconstruction of the displacement, and additional baselines can self-calibrate
using the flare, enhancing synthesis imaging of the quiescent emission. We
apply this technique to simulated 1.3-mm wavelength observations of a "hot
spot" embedded in a radiatively inefficient accretion disk around Sgr A*. Our
results indicate that, even with current sensitivities, polarimetric
interferometry with the Event Horizon Telescope can achieve ~5 microarcsecond
relative astrometry of compact flaring structures near Sgr A* on timescales of
minutes.Comment: 9 Pages, 4 Figures, accepted for publication in Ap
High Resolution Linear Polarimetric Imaging for the Event Horizon Telescope
Images of the linear polarization of synchrotron radiation around Active
Galactic Nuclei (AGN) identify their projected magnetic field lines and provide
key data for understanding the physics of accretion and outflow from
supermassive black holes. The highest resolution polarimetric images of AGN are
produced with Very Long Baseline Interferometry (VLBI). Because VLBI
incompletely samples the Fourier transform of the source image, any image
reconstruction that fills in unmeasured spatial frequencies will not be unique
and reconstruction algorithms are required. In this paper, we explore
extensions of the Maximum Entropy Method (MEM) to linear polarimetric VLBI
imaging. In contrast to previous work, our polarimetric MEM algorithm combines
a Stokes I imager that uses only bispectrum measurements that are immune to
atmospheric phase corruption with a joint Stokes Q and U imager that operates
on robust polarimetric ratios. We demonstrate the effectiveness of our
technique on 7- and 3-mm wavelength quasar observations from the VLBA and
simulated 1.3-mm Event Horizon Telescope observations of Sgr A* and M87.
Consistent with past studies, we find that polarimetric MEM can produce
superior resolution compared to the standard CLEAN algorithm when imaging
smooth and compact source distributions. As an imaging framework, MEM is highly
adaptable, allowing a range of constraints on polarization structure.
Polarimetric MEM is thus an attractive choice for image reconstruction with the
EHT.Comment: 19 pages, 9 figures. Accepted for publication in ApJ. Imaging code
available at https://github.com/achael/eht-imaging
Observing---and Imaging---Active Galactic Nuclei with the Event Horizon Telescope
Originally developed to image the shadow region of the central black hole in
Sagittarius A* and in the nearby galaxy M87, the Event Horizon Telescope (EHT)
provides deep, very high angular resolution data on other AGN sources too. The
challenges of working with EHT data have spurred the development of new image
reconstruction algorithms. This work briefly reviews the status of the EHT and
its utility for observing AGN sources, with emphasis on novel imaging
techniques that offer the promise of better reconstructions at 1.3 mm and other
wavelengths.Comment: 10 pages, proceedings contribution for Blazars through Sharp
Multi-Wavelength Eyes, submitted to Galaxie
The Role of SIRT1 in Skeletal Muscle Function and Repair of Older Mice
Background Sirtuin 1 (SIRT1) is a NAD+ sensitive deacetylase that has been linked to longevity and has been suggested to confer beneficial effects that counter aging-associated deterioration. Muscle repair is dependent upon satellite cell function, which is reported to be reduced with aging; however, it is not known if this is linked to an aging-suppression of SIRT1. This study tested the hypothesis that Sirtuin 1 (SIRT1) overexpression would increase the extent of muscle repair and muscle function in older mice. Methods We examined satellite cell dependent repair in tibialis anterior, gastrocnemius, and soleus muscles of 13 young wild-type mice (20–30 weeks) and 49 older (80+ weeks) mice that were controls (n = 13), overexpressed SIRT1 in skeletal muscle (n = 14), and had a skeletal muscle SIRT1 knockout (n = 12) or a satellite cell SIRT1 knockout (n = 10). Acute muscle injury was induced by injection of cardiotoxin (CTX), and phosphate-buffered saline was used as a vector control. Plantarflexor muscle force and fatigue were evaluated before or 21 days after CTX injection. Satellite cell proliferation and mitochondrial function were also evaluated in undamaged muscles. Results Maximal muscle force was significantly lower in control muscles of older satellite cell knockout SIRT1 mice compared to young adult wild-type (YWT) mice (P \u3c 0.001). Mean contraction force at 40 Hz stimulation was significantly greater after recovery from CTX injury in older mice that overexpressed muscle SIRT1 than age-matched SIRT1 knockout mice (P \u3c 0.05). SIRT1 muscle knockout models (P \u3c 0.05) had greater levels of p53 (P \u3c 0.05 MKO, P \u3c 0.001 OE) in CTX-damaged tissues as compared to YWT CTX mice. SIRT1 overexpression with co-expression of p53 was associated with increased fatigue resistance and increased force potentiation during repeated contractions as compared to wild-type or SIRT1 knockout models (P \u3c 0.001). Muscle structure and mitochondrial function were not different between the groups, but proliferation of satellite cells was significantly greater in older mice with SIRT1 muscle knockout (P \u3c 0.05), but not older SIRT1 satellite cell knockout models, in vitro, although this effect was attenuated in vivo after 21 days of recovery. Conclusions The data suggest skeletal muscle structure, function, and recovery after CTX-induced injury are not significantly influenced by gain or loss of SIRT1 abundance alone in skeletal muscle; however, muscle function is impaired by ablation of SIRT1 in satellite cells. SIRT1 appears to interact with p53 to improve muscle fatigue resistance after repair from muscle injury
Adapting a Cryogenic Sapphire Oscillator for Very Long Baseline Interferometry
Extension of very long baseline interferometry (VLBI) to observing
wavelengths shorter than 1.3mm provides exceptional angular resolution (~20
micro arcsec) and access to new spectral regimes for the study of astrophysical
phenomena. To maintain phase coherence across a global VLBI array at these
wavelengths requires that ultrastable frequency references be used for the
heterodyne receivers at all participating telescopes. Hydrogen masers have
traditionally been used as VLBI references, but atmospheric turbulence
typically limits (sub) millimeter VLBI coherence times to ~1-30 s. Cryogenic
Sapphire Oscillators (CSO) have better stability than Hydrogen masers on these
time scale and are potential alternatives to masers as VLBI references. Here,
We describe the design, implementation and tests of a system to produce a 10
MHz VLBI frequency standard from the microwave (11.2 GHz) output of a CSO. To
improve long-term stability of the new reference, the CSO was locked to the
timing signal from the Global Positioning System satellites and corrected for
the oscillator aging. The long-term performance of the CSO was measured by
comparison against a hydrogen maser in the same laboratory. The superb
short-term performance, along with the improved long-term performance achieved
by conditioning, makes the CSO a suitable reference for VLBI at wavelengths
less than 1.3mm.Comment: 24 pages, 15 figure
A VLBI receiving system for the South Pole Telescope
The Event Horizon Telescope (EHT) is a very-long-baseline interferometry
(VLBI) experiment that aims to observe supermassive black holes with an angular
resolution that is comparable to the event horizon scale. The South Pole
occupies an important position in the array, greatly increasing its north-south
extent and therefore its resolution.
The South Pole Telescope (SPT) is a 10-meter diameter, millimeter-wavelength
telescope equipped for bolometric observations of the cosmic microwave
background. To enable VLBI observations with the SPT we have constructed a
coherent signal chain suitable for the South Pole environment. The
dual-frequency receiver incorporates state-of-the-art SIS mixers and is
installed in the SPT receiver cabin. The VLBI signal chain also includes a
recording system and reference frequency generator tied to a hydrogen maser.
Here we describe the SPT VLBI system design in detail and present both the lab
measurements and on-sky results.Comment: 14 pages, 11 figures, to appear in the Proceedings of the SPIE (SPIE
Astronomical Telescopes + Instrumentation 2018; Millimeter, Submillimeter,
and Far-Infrared Detectors and Instrumentation for Astronomy IX
The Murchison Widefield Array: Design Overview
The Murchison Widefield Array (MWA) is a dipole-based aperture array
synthesis telescope designed to operate in the 80-300 MHz frequency range. It
is capable of a wide range of science investigations, but is initially focused
on three key science projects. These are detection and characterization of
3-dimensional brightness temperature fluctuations in the 21cm line of neutral
hydrogen during the Epoch of Reionization (EoR) at redshifts from 6 to 10,
solar imaging and remote sensing of the inner heliosphere via propagation
effects on signals from distant background sources,and high-sensitivity
exploration of the variable radio sky. The array design features 8192
dual-polarization broad-band active dipoles, arranged into 512 tiles comprising
16 dipoles each. The tiles are quasi-randomly distributed over an aperture
1.5km in diameter, with a small number of outliers extending to 3km. All
tile-tile baselines are correlated in custom FPGA-based hardware, yielding a
Nyquist-sampled instantaneous monochromatic uv coverage and unprecedented point
spread function (PSF) quality. The correlated data are calibrated in real time
using novel position-dependent self-calibration algorithms. The array is
located in the Murchison region of outback Western Australia. This region is
characterized by extremely low population density and a superbly radio-quiet
environment,allowing full exploitation of the instrumental capabilities.Comment: 9 pages, 5 figures, 1 table. Accepted for publication in Proceedings
of the IEE
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