3,652 research outputs found
Can Self-Organizing Maps accurately predict photometric redshifts?
We present an unsupervised machine learning approach that can be employed for
estimating photometric redshifts. The proposed method is based on a vector
quantization approach called Self--Organizing Mapping (SOM). A variety of
photometrically derived input values were utilized from the Sloan Digital Sky
Survey's Main Galaxy Sample, Luminous Red Galaxy, and Quasar samples along with
the PHAT0 data set from the PHoto-z Accuracy Testing project. Regression
results obtained with this new approach were evaluated in terms of root mean
square error (RMSE) to estimate the accuracy of the photometric redshift
estimates. The results demonstrate competitive RMSE and outlier percentages
when compared with several other popular approaches such as Artificial Neural
Networks and Gaussian Process Regression. SOM RMSE--results (using
z=z--z) for the Main Galaxy Sample are 0.023, for the
Luminous Red Galaxy sample 0.027, Quasars are 0.418, and PHAT0 synthetic data
are 0.022. The results demonstrate that there are non--unique solutions for
estimating SOM RMSEs. Further research is needed in order to find more robust
estimation techniques using SOMs, but the results herein are a positive
indication of their capabilities when compared with other well-known methods.Comment: 5 pages, 3 figures, submitted to PAS
On Dust Extinction of Gamma-ray Burst Host Galaxies
Although it is well recognized that gamma-ray burst (GRB) afterglows are
obscured and reddened by dust in their host galaxies, the wavelength-dependence
and quantity of dust extinction are still poorly known. Current studies on this
mostly rely on fitting the afterglow spectral energy distributions (SEDs) with
template extinction models. The inferred extinction (both quantity and
wavelength-dependence) and dust-to-gas ratios are often in disagreement with
that obtained from dust depletion and X-ray spectroscopy studies. We argue that
this discrepancy could result from the prior assumption of a template
extinction law. We propose an analytical formula to approximate the GRB host
extinction law. With the template extinction laws self-contained, and the
capability of revealing extinction laws differing from the conventional ones,
it is shown that this is a powerful approach in modeling the afterglow SEDs to
derive GRB host extinction.Comment: 9 pages, 4 figures; The Astrophysical Journal, in press (2008 Oct 1
issue
Strain-induced magnetic phase transition in SrCoO thin films
It has been well established that both in bulk at ambient pressure and for
films under modest strains, cubic SrCoO () is a
ferromagnetic metal. Recent theoretical work, however, indicates that a
magnetic phase transition to an antiferromagnetic structure could occur under
large strain accompanied by a metal-insulator transition. We have observed a
strain-induced ferromagnetic to antiferromagnetic phase transition in
SrCoO films grown on DyScO substrates, which provide a large
tensile epitaxial strain, as compared to ferromagnetic films under lower
tensile strain on SrTiO substrates. Magnetometry results demonstrate the
existence of antiferromagnetic spin correlations and neutron diffraction
experiments provide a direct evidence for a G-type antiferromagnetic structure
with Ne\'el temperatures between and depending on the oxygen content of the samples. Therefore, our
data experimentally confirm the predicted strain-induced magnetic phase
transition to an antiferromagnetic state for SrCoO thin films
under large epitaxial strain.Comment: 6 pages, 4 figure
Mid-infrared observations of the SGR 1900+14 error box
We report on mid-infrared observations of the compact stellar cluster located
in the proximity of SGR 1900+14, and the radio/X-ray position of this
soft-gamma repeater. Observations were performed in May and June of 2001 when
the bursting source was in an active state. At the known radio and X-ray
position of the SGR we did not detect transient mid-IR activity, although the
observations were performed only hours before and after an outburst in the
high-energy band.Comment: 4 pages, 3 figures, to appear in "Gamma-Ray Burst and Afterglow
Astronomy 2001", Woods Hole; 5-9 Nov, 200
The environment of the SN-less GRB 111005A at z = 0.0133
The collapsar model has proved highly successful in explaining the properties
of long gamma-ray bursts (GRBs), with the most direct confirmation being the
detection of a supernova (SN) coincident with the majority of nearby long GRBs.
Within this model, a long GRB is produced by the core-collapse of a metal-poor,
rapidly rotating, massive star. The detection of some long GRBs in metal-rich
environments, and more fundamentally the three examples of long GRBs (GRB
060505, GRB 060614 and GRB 111005A) with no coincident SN detection down to
very deep limits is in strong contention with theoretical expectations. In this
paper we present MUSE observations of the host galaxy of GRB 111005A, which is
the most recent and compelling example yet of a SN-less, long GRB. At
z=0.01326, GRB 111005A is the third closest GRB ever detected, and second
closest long duration GRB, enabling the nearby environment to be studied at a
resolution of 270 pc. From the analysis of the MUSE data cube, we find GRB
111005A to have occurred within a metal-rich environment with little signs of
ongoing star formation. Spectral analysis at the position of the GRB indicates
the presence of an old stellar population (tau > 10 Myr), which limits the mass
of the GRB progenitor to M_ZAMS<15 Msolar, in direct conflict with the
collapsar model. Our deep limits on the presence of any SN emission combined
with the environmental conditions at the position of GRB 111005A necessitate
the exploration of a novel long GRB formation mechanism that is unrelated to
massive stars.Comment: Now accepted by A&A. Manuscript replaced to match accepted version.
Some additional discussion added, and velocity map of the host galaxy now
include
GRB afterglow light curves in the pre-Swift era - a statistical study
We present the results of a systematic analysis of the world sample of
optical/near-infrared afterglow light curves observed in the pre-Swift era by
the end of 2004. After selecting the best observed 16 afterglows with
well-sampled light curves that can be described by a Beuermann equation, we
explore the parameter space of the light curve parameters and physical
quantities related to them. In addition, we search for correlations between
these parameters and the corresponding gamma-ray data, and we use our data set
to look for a fine structure in the light curves.Comment: accepted for publication in ApJ; Version 2: minor changes, one figure
adde
Isotope shifts and hyperfine structure of the Fe I 372 nm resonance line
We report measurements of the isotope shifts of the Fe I resonance line at 372 nm between all four stable
isotopes Fe, Fe, Fe, and Fe, as well as the
complete hyperfine structure of that line for Fe, the only stable
isotope having a non-zero nuclear spin. The field and specific mass shift
coefficients of the transition have been derived from the data, as well as the
experimental value for the hyperfine structure magnetic dipole coupling
constant of the excited state of the transition in Fe: MHz. The measurements were done by means of Doppler-free
laser saturated-absorption spectroscopy in a Fe-Ar hollow cathode using both
natural and enriched iron samples. The measured isotope shifts and hyperfine
constants are reported with uncertainties at the percent level.Comment: 5 pages, 5 figure
Another short-burst host galaxy with an optically obscured high star formation rate: The case of GRB 071227
We report on radio continuum observations of the host galaxy of the short
gamma-ray burst 071227 (z=0.381) with the Australia Telescope Compact Array
(ATCA). We detect the galaxy in the 5.5 GHz band with an integrated flux
density of Fnu = 43 +/- 11 microJy, corresponding to an unobscured
star-formation rate (SFR) of about 24 Msun/yr, forty times higher than what was
found from optical emission lines. Among the ~30 well-identified and studied
host galaxies of short bursts this is the third case where the host is found to
undergo an episode of intense star formation. This suggests that a fraction of
all short-burst progenitors hosted in star-forming galaxies could be physically
related to recent star formation activity, implying a relatively short merger
time scale.Comment: 6 pages, ApJ, accepted for publicatio
Reduced sigma-model on AdS_5 x S^5: one-loop scattering amplitudes
We compute one-loop S-matrix in the reduced sigma-model which describes AdS_5
x S^5 string theory in the near-flat-space limit. The result agrees with the
corresponding limit of the S-matrix in the full sigma-model, which demonstrates
the consistency of the reduction at the quantum level.Comment: 9 pages, 1 figure; v2: reference added; v3: misprint in (3.6)
corrected; v4: typo in (3.4) corrected; v5: new form of the actio
Comparison of inter-trial recovery times for the determination of critical power and W' in cycling.
Critical Power (CP) and W' are often determined using multi-day testing protocols. To investigate this cumbersome testing method, the purpose of this study was to compare the differences between the conventional use of a 24-h inter-trial recovery time with those of 3 h and 30 min for the determination of CP and W'.
METHODS:
9 moderately trained cyclists performed an incremental test to exhaustion to establish the power output associated with the maximum oxygen uptake (p[Formula: see text]max), and 3 protocols requiring time-to-exhaustion trials at a constant work-rate performed at 80%, 100% and 105% of p[Formula: see text]max. Design: Protocol A utilised 24-h inter-trial recovery (CP24/W'24), protocol B utilised 3-h inter-trial recovery (CP3/W'3), and protocol C used 30-min inter-trial recovery period (CP0.5/W'0.5). CP and W' were calculated using the inverse time (1/t) versus power (P) relation (P = W'(1/t) + CP).
RESULTS:
95% Limits of Agreement between protocol A and B were -9 to 15 W; -7.4 to 7.8 kJ (CP/W') and between protocol A and protocol C they were -27 to 22 W; -7.2 to 15.1 kJ (CP/W'). Compared to criterion protocol A, the average prediction error of protocol B was 2.5% (CP) and 25.6% (W'), whilst for protocol C it was 3.7% (CP) and 32.9% (W').
CONCLUSION:
3-h and 30-min inter-trial recovery time protocols provide valid methods of determining CP but not W' in cycling
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