4,174 research outputs found
The faint-galaxy hosts of gamma-ray bursts
The observed redshifts and magnitudes of the host galaxies of gamma-ray
bursts (GRBs) are compared with the predictions of three basic GRB models, in
which the comoving rate density of GRBs is (1) proportional to the cosmic star
formation rate density, (2) proportional to the total integrated stellar
density and (3) constant. All three models make the assumption that at every
epoch the probability of a GRB occuring in a galaxy is proportional to that
galaxy's broad-band luminosity. No assumption is made that GRBs are standard
candles or even that their luminosity function is narrow. All three rate
density models are consistent with the observed GRB host galaxies to date,
although model (2) is slightly disfavored relative to the others. Models (1)
and (3) make very similar predictions for host galaxy magnitude and redshift
distributions; these models will be probably not be distinguished without
measurements of host-galaxy star-formation rates. The fraction of host galaxies
fainter than 28 mag may constrain the faint end of the galaxy luminosity
function at high redshift, or, if the fraction is observed to be low, may
suggest that the bursters are expelled from low-luminosity hosts. In all
models, the probability of finding a z<0.008 GRB among a sample of 11 GRBs is
less than 10^(-4), strongly suggesting that GRB 980425, if associated with
supernova 1998bw, represents a distinct class of GRBs.Comment: 7 pages, ApJ in press, revised to incorporate yet more new and
revised observational result
A systematic search for massive black hole binaries in SDSS spectroscopic sample
We present the results of a systematic search for massive black hole binaries
in the Sloan Digital Sky Survey spectroscopic database. We focus on bound
binaries, under the assumption that one of the black holes is active. In this
framework, the broad lines associated to the accreting black hole are expected
to show systematic velocity shifts with respect to the narrow lines, which
trace the rest-frame of the galaxy. For a sample of 54586 quasars and 3929
galaxies at redshifts 0.1<z<1.5 we brute-force model each spectrum as a mixture
of two quasars at two different redshifts. The spectral model is a data-driven
dimensionality reduction of the SDSS quasar spectra based on a matrix
factorization. We identified 32 objects with peculiar spectra. Nine of them can
be interpreted as black hole binaries. This doubles the number of known black
hole binary candidates. We also report on the discovery of a new class of
extreme double-peaked emitters with exceptionally broad and faint Balmer lines.
For all the interesting sources, we present detailed analysis of the spectra,
and discuss possible interpretations.Comment: 10 pages, 2 figures, accepted for publication in Ap
A maximum-likelihood method for improving faint source flux and color estimates
Flux estimates for faint sources or transients are systematically biased high
because there are far more truly faint sources than bright. Corrections which
account for this effect are presented as a function of signal-to-noise ratio
and the (true) slope of the faint-source number-flux relation. The corrections
depend on the source being originally identified in the image in which it is
being photometered. If a source has been identified in other data, the
corrections are different; a prescription for calculating the corrections is
presented. Implications of these corrections for analyses of surveys are
discussed; the most important is that sources identified at signal-to-noise
ratios of four or less are practically useless.Comment: 9 pp., accepted for publication in PAS
Commentaries: Amendment and Patriation
In the following commentaries, Peter W. Hogg and W. R. Lederman discuss different aspects of Geoffrey Marshall\u27s presentation on amendment and patriation. Professor Hogg\u27s topic is more specifically the role of the United Kingdom Parliament, while Professor Lederman comments upon the positions of the Supreme Court of Canada and the British Government and Parliament
Commentaries: Amendment and Patriation
In the following commentaries, Peter W. Hogg and W. R. Lederman discuss different aspects of Geoffrey Marshall\u27s presentation on amendment and patriation. Professor Hogg\u27s topic is more specifically the role of the United Kingdom Parliament, while Professor Lederman comments upon the positions of the Supreme Court of Canada and the British Government and Parliament
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