153 research outputs found
Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHz
We measured the linear polarization of Sagittarius A* to be 7.2 +/- 0.6 % at
230 GHzusing the BIMA array with a resolution of 3.6 x 0.9 arcsec. This
confirms the previously reported detection with the JCMT 14-m antenna. Our high
resolution observations demonstrate that the polarization does not arise from
dust but from a synchrotron source associated with Sgr A*. We see no change in
the polarization position angle and only a small change in the polarization
fraction in four observations distributed over 60 days. We find a position
angle 139 +/- 4 degrees that differs substantially from what was found in
earlier JCMT observations at the same frequency. Polarized dust emission cannot
account for this discrepancy leaving variability and observational error as the
only explanations. The BIMA observations alone place an upper limit on the
magnitude of the rotation measure of 2 x 10^6 rad m^-2. These new observations
when combined with the JCMT observations at 150, 375 and 400 GHz suggest RM
=-4.3 +/- 0.1 x 10^5 rad m^-2. This RM may be caused by an external Faraday
screen. Barring a special geometry or a high number of field reversals, this RM
rules out accretion rates greater than ~ 10^-7 M_sun y^-1. This measurement is
inconsistent with high accretion rates necessary in standard advection
dominated accretion flow and Bondi-Hoyle models for Sgr A*. It argues for low
accretion rates as a major factor in the overall faintness of Sgr A*.Comment: accepted for publication in ApJ, 18 pages, 4 figure
Rapidly Evolving Circularly Polarized Emission during the 1994 Outburst of GRO J1665-40
We report the detection of circular polarization during the 1994 outburst of
the Galactic microquasar GRO J1655-40. The circular polarization is clearly
detected at 1.4 and 2.4GHz, but not at 4.8 and 8.4GHz, where its magnitude
never exceeds 5 mJy. Both the sign and magnitude of the circular polarization
evolve during the outburst. The time dependence and magnitude of the polarized
emission can be qualitatively explained by a model based on synchrotron
emission from the outbursts, but is most consistent with circular polarization
arising from propagation effects through the relativistic plasma surrounding
the object.Comment: 8 pages, 3 figs., A&A accepte
Impact of Systematic Errors in Sunyaev-Zel'dovich Surveys of Galaxy Clusters
Future high-resolution microwave background measurements hold the promise of
detecting galaxy clusters throughout our Hubble volume through their
Sunyaev-Zel'dovich (SZ) signature, down to a given limiting flux. The number
density of galaxy clusters is highly sensitive to cluster mass through
fluctuations in the matter power spectrum, as well as redshift through the
comoving volume and the growth factor. This sensitivity in principle allows
tight constraints on such quantities as the equation of state of dark energy
and the neutrino mass. We evaluate the ability of future cluster surveys to
measure these quantities simultaneously when combined with PLANCK-like CMB
data. Using a simple effective model for uncertainties in the cluster mass-SZ
flux relation, we evaluate systematic shifts in cosmological constraints from
cluster SZ surveys. We find that a systematic bias of 10% in cluster mass
measurements can give rise to shifts in cosmological parameter estimates at
levels larger than the statistical errors. Systematic errors are
unlikely to be detected from the mass and redshift dependence of cluster number
counts alone; increasing survey size has only a marginal effect. Implications
for upcoming experiments are discussed.Comment: 12 pages, 6 figures; accepted to JCAP; revised to match submitted
versio
X-ray iron line variability for the model of an orbiting flare above a black hole accretion disc
The broad X-ray iron line, detected in many active galactic nuclei, is likely
to be produced by fluorescence from the X-ray illuminated central parts of an
accretion disc close to a supermassive black hole. The time-averaged shape of
the line can be explained most naturally by a combination of special and
general relativistic effects. Such line profiles contain information about the
black hole spin and the accretion disc as well as the geometry of the emitting
region and may help to test general relativity in the strong gravity regime. In
this paper we embark on the computation of the temporal response of the line to
the illuminating flux. Previous studies concentrated on the calculation of
reverberation signatures from static sources illuminating the disc. In this
paper we focus on the more physically justified case of flares located above
the accretion disc and corotating with it. We compute the time dependent iron
line taking into account all general relativistic effects and show that its
shape is of very complex nature, and also present light curves accompanying the
iron line variability. We suggest that future X-ray satellites like XMM or
Constellation-X may be capable of detecting features present in the computed
reverberation maps.Comment: Accepted for publication in MNRAS, 11 pages, 12 figure
HST/ACS Emission Line Imaging of Low Redshift 3CR Radio Galaxies I: The Data
We present 19 nearby (z<0.3) 3CR radio galaxies imaged at low- and
high-excitation as part of a Cycle 15 Hubble Space Telescope snapshot survey
with the Advanced Camera for Surveys. These images consist of exposures of the
H-alpha (6563 \AA, plus [NII] contamination) and [OIII] 5007 \AA emission lines
using narrow-band linear ramp filters adjusted according to the redshift of the
target. To facilitate continuum subtraction, a single-pointing 60 s line-free
exposure was taken with a medium-band filter appropriate for the target's
redshift. We discuss the steps taken to reduce these images independently of
the automated recalibration pipeline so as to use more recent ACS flat-field
data as well as to better reject cosmic rays. We describe the method used to
produce continuum-free (pure line-emission) images, and present these images
along with qualitative descriptions of the narrow-line region morphologies we
observe. We present H-alpha+[NII] and [OIII] line fluxes from aperture
photometry, finding the values to fall expectedly on the redshift-luminosity
trend from a past HST/WFPC2 emission line study of a larger, generally higher
redshift subset of the 3CR. We also find expected trends between emission line
luminosity and total radio power, as well as a positive correlation between the
size of the emission line region and redshift. We discuss the associated
interpretation of these results, and conclude with a summary of future work
enabled by this dataset.Comment: 18 pages, 12 figures, accepted for publication in ApJ
A Chandra X-ray Analysis of Abell 1664: Cooling, Feedback and Star Formation in the Central Cluster Galaxy
The brightest cluster galaxy (BCG) in the Abell 1664 cluster is unusually
blue and is forming stars at a rate of ~ 23 M_{\sun} yr^{-1}. The BCG is
located within 5 kpc of the X-ray peak, where the cooling time of 3.5x10^8 yr
and entropy of 10.4 keV cm^2 are consistent with other star-forming BCGs in
cooling flow clusters. The center of A1664 has an elongated, "bar-like" X-ray
structure whose mass is comparable to the mass of molecular hydrogen, ~ 10^{10}
M_{\sun} in the BCG. We show that this gas is unlikely to have been stripped
from interloping galaxies. The cooling rate in this region is roughly
consistent with the star formation rate, suggesting that the hot gas is
condensing onto the BCG. We use the scaling relations of Birzan et al. 2008 to
show that the AGN is underpowered compared to the central X-ray cooling
luminosity by roughly a factor of three. We suggest that A1664 is experiencing
rapid cooling and star formation during a low-state of an AGN feedback cycle
that regulates the rates of cooling and star formation. Modeling the emission
as a single temperature plasma, we find that the metallicity peaks 100 kpc from
the X-ray center, resulting in a central metallicity dip. However, a
multi-temperature cooling flow model improves the fit to the X-ray emission and
is able to recover the expected, centrally-peaked metallicity profile.Comment: 15 pages, 13 figure
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
Intermediate-Mass Black Holes as LISA Sources
Intermediate-mass black holes (IMBHs), with masses of hundreds to thousands
of solar masses, will be unique sources of gravitational waves for LISA. Here
we discuss their context as well as specific characteristics of IMBH-IMBH and
IMBH-supermassive black hole mergers and how these would allow sensitive tests
of the predictions of general relativity in strong gravity.Comment: Accepted by CQG, LISA 7 Special Issu
The Fermi Bubbles: Supersonic Active Galactic Nucleus Jets with Anisotropic Cosmic-Ray Diffusion
The Fermi Gamma-Ray Space Telescope reveals two large bubbles in the Galaxy, which extend nearly symmetrically ~50° above and below the Galactic center. Using three-dimensional (3D) magnetohydrodynamic simulations that self-consistently include the dynamical interaction between cosmic rays (CRs) and thermal gas and anisotropic CR diffusion along the magnetic field lines, we show that the key characteristics of the observed gamma-ray bubbles and the spatially correlated X-ray features in the ROSAT 1.5 keV map can be successfully reproduced by recent jet activity from the central active galactic nucleus. We find that after taking into account the projection of the 3D bubbles onto the sky the physical heights of the bubbles can be much smaller than previously thought, greatly reducing the formation time of the bubbles to about a Myr. This relatively small bubble age is needed to reconcile the simulations with the upper limit of bubble ages estimated from the cooling time of high-energy electrons. No additional physical mechanisms are required to suppress large-scale hydrodynamic instabilities because the evolution time is too short for them to develop. The simulated CR bubbles are edge-brightened, which is consistent with the observed projected flat surface brightness distribution. Furthermore, we demonstrate that the sharp edges of the observed bubbles can be due to anisotropic CR diffusion along magnetic field lines that drape around the bubbles during their supersonic expansion, with suppressed perpendicular diffusion across the bubble surface. Possible causes of the slight bends of the Fermi bubbles to the west are also discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98587/1/0004-637X_761_2_185.pd
Why Are Outcomes Different for Registry Patients Enrolled Prospectively and Retrospectively? Insights from the Global Anticoagulant Registry in the FIELD-Atrial Fibrillation (GARFIELD-AF).
Background: Retrospective and prospective observational studies are designed to reflect real-world evidence on clinical practice, but can yield conflicting results. The GARFIELD-AF Registry includes both methods of enrolment and allows analysis of differences in patient characteristics and outcomes that may result. Methods and Results: Patients with atrial fibrillation (AF) and ≥1 risk factor for stroke at diagnosis of AF were recruited either retrospectively (n = 5069) or prospectively (n = 5501) from 19 countries and then followed prospectively. The retrospectively enrolled cohort comprised patients with established AF (for a least 6, and up to 24 months before enrolment), who were identified retrospectively (and baseline and partial follow-up data were collected from the emedical records) and then followed prospectively between 0-18 months (such that the total time of follow-up was 24 months; data collection Dec-2009 and Oct-2010). In the prospectively enrolled cohort, patients with newly diagnosed AF (≤6 weeks after diagnosis) were recruited between Mar-2010 and Oct-2011 and were followed for 24 months after enrolment. Differences between the cohorts were observed in clinical characteristics, including type of AF, stroke prevention strategies, and event rates. More patients in the retrospectively identified cohort received vitamin K antagonists (62.1% vs. 53.2%) and fewer received non-vitamin K oral anticoagulants (1.8% vs . 4.2%). All-cause mortality rates per 100 person-years during the prospective follow-up (starting the first study visit up to 1 year) were significantly lower in the retrospective than prospectively identified cohort (3.04 [95% CI 2.51 to 3.67] vs . 4.05 [95% CI 3.53 to 4.63]; p = 0.016). Conclusions: Interpretations of data from registries that aim to evaluate the characteristics and outcomes of patients with AF must take account of differences in registry design and the impact of recall bias and survivorship bias that is incurred with retrospective enrolment. Clinical Trial Registration: - URL: http://www.clinicaltrials.gov . Unique identifier for GARFIELD-AF (NCT01090362)
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