39,334 research outputs found
Recommended from our members
The ADS All-Sky Survey
The ADS All-Sky Survey (ADSASS) is an ongoing effort aimed at turning the NASA Astrophysics Data System (ADS), widely known for its unrivaled value as a literature resource for astronomers, into a data resource. The ADS is not a data repository per se, but it implicitly contains valuable holdings of astronomical data, in the form of images, tables and object references contained within articles. The objective of the ADSASS effort is to extract these data and make them discoverable and available through existing data viewers. The resulting ADSASS data layer promises to greatly enhance workflows and enable new research by tying astronomical literature and data assets into one resource.Astronom
Blindly detecting orbital modulations of jets from merging supermassive black holes
In the last few years before merger, supermassive black hole binaries will
rapidly inspiral and precess in a magnetic field imposed by a surrounding
circumbinary disk. Multiple simulations suggest this relative motion will
convert some of the local energy to a Poynting-dominated outflow, with a
luminosity 10^{43} erg/s * (B/10^4 G)^2(M/10^8 Msun)^2 (v/0.4 c)^2, some of
which may emerge as synchrotron emission at frequencies near 1 GHz where
current and planned wide-field radio surveys will operate. On top of a secular
increase in power on the gravitational wave inspiral timescale, orbital motion
will produce significant, detectable modulations, both on orbital periods and
(if black hole spins are not aligned with the binary's total angular momenta)
spin-orbit precession timescales. Because the gravitational wave merger time
increases rapidly with separation, we find vast numbers of these transients are
ubiquitously predicted, unless explicitly ruled out (by low efficiency
) or obscured (by accretion geometry f_{geo}). If the fraction of
Poynting flux converted to radio emission times the fraction of lines of sight
accessible is sufficiently large (f_{geo} \epsilon > 2\times 10^{-4}
for a 1 year orbital period), at least one event is accessible to future blind
surveys at a nominal 10^4 {deg}^2 with 0.5 mJy sensitivity. Our procedure
generalizes to other flux-limited surveys designed to investigate EM signatures
associated with many modulations produced by merging SMBH binaries.Comment: Submitted to ApJ. v1 original submission; v2 minor changes in
response to refere
Photometry of supernovae in an image series : methods and application to the Supernova Legacy Survey (SNLS)
We present a technique to measure lightcurves of time-variable point sources
on a spatially structured background from imaging data. The technique was
developed to measure light curves of SNLS supernovae in order to infer their
distances. This photometry technique performs simultaneous PSF photometry at
the same sky position on an image series. We describe two implementations of
the method: one that resamples images before measuring fluxes, and one which
does not. In both instances, we sketch the key algorithms involved and present
the validation using semi-artificial sources introduced in real images in order
to assess the accuracy of the supernova flux measurements relative to that of
surrounding stars. We describe the methods required to anchor these PSF fluxes
to calibrated aperture catalogs, in order to derive SN magnitudes. We find a
marginally significant bias of 2 mmag of the after-resampling method, and no
bias at the mmag accuracy for the non-resampling method. Given surrounding star
magnitudes, we determine the systematic uncertainty of SN magnitudes to be less
than 1.5 mmag, which represents about one third of the current photometric
calibration uncertainty affecting SN measurements. The SN photometry delivers
several by-products: bright star PSF flux mea- surements which have a
repeatability of about 0.6%, as for aperture measurements; we measure relative
astrometric positions with a noise floor of 2.4 mas for a single-image bright
star measurement; we show that in all bands of the MegaCam instrument, stars
exhibit a profile linearly broadening with flux by about 0.5% over the whole
brightness range.Comment: Accepted for publication in A&A. 20 page
An Over-Massive Black Hole in the Compact Lenticular Galaxy NGC1277
All massive galaxies likely have supermassive black holes at their centers,
and the masses of the black holes are known to correlate with properties of the
host galaxy bulge component. Several explanations have been proposed for the
existence of these locally-established empirical relationships; they include
the non-causal, statistical process of galaxy-galaxy merging, direct feedback
between the black hole and its host galaxy, or galaxy-galaxy merging and the
subsequent violent relaxation and dissipation. The empirical scaling relations
are thus important for distinguishing between various theoretical models of
galaxy evolution, and they further form the basis for all black hole mass
measurements at large distances. In particular, observations have shown that
the mass of the black hole is typically 0.1% of the stellar bulge mass of the
galaxy. The small galaxy NGC4486B currently has the largest published fraction
of its mass in a black hole at 11%. Here we report observations of the stellar
kinematics of NGC 1277, which is a compact, disky galaxy with a mass of 1.2 x
10^11 Msun. From the data, we determine that the mass of the central black hole
is 1.7 x 10^10 Msun, or 59% its bulge mass. Five other compact galaxies have
properties similar to NGC 1277 and therefore may also contain over-sized black
holes. It is not yet known if these galaxies represent a tail of a
distribution, or if disk-dominated galaxies fail to follow the normal black
hole mass scaling relations.Comment: 7 pages. 6 figures. Nature. Animation at
http://www.mpia.de/~bosch/blackholes.htm
The Hubble Constant
I review the current state of determinations of the Hubble constant, which
gives the length scale of the Universe by relating the expansion velocity of
objects to their distance. There are two broad categories of measurements. The
first uses individual astrophysical objects which have some property that
allows their intrinsic luminosity or size to be determined, or allows the
determination of their distance by geometric means. The second category
comprises the use of all-sky cosmic microwave background, or correlations
between large samples of galaxies, to determine information about the geometry
of the Universe and hence the Hubble constant, typically in a combination with
other cosmological parameters. Many, but not all, object-based measurements
give values of around 72-74km/s/Mpc , with typical errors of 2-3km/s/Mpc.
This is in mild discrepancy with CMB-based measurements, in particular those
from the Planck satellite, which give values of 67-68km/s/Mpc and typical
errors of 1-2km/s/Mpc. The size of the remaining systematics indicate that
accuracy rather than precision is the remaining problem in a good determination
of the Hubble constant. Whether a discrepancy exists, and whether new physics
is needed to resolve it, depends on details of the systematics of the
object-based methods, and also on the assumptions about other cosmological
parameters and which datasets are combined in the case of the all-sky methods.Comment: Extensively revised and updated since the 2007 version: accepted by
Living Reviews in Relativity as a major (2014) update of LRR 10, 4, 200
- …