7,304 research outputs found
A Lensed Arc in the Low Redshift Cluster Abell 2124
We report the discovery of an arc-like object 27" from the center of the cD
galaxy in the redshift cluster A2124. Observations with the Keck II
telescope reveal that the object is a background galaxy at ,
apparently lensed into an arc of length \sim 8 \farcs5 and total R magnitude
. The width of the arc is resolved; we estimate it to be
0\farcs6 after correcting for seeing. A lens model of the A2124 core mass
distribution consistent with the cluster galaxy velocity dispersion reproduces
the observed arc geometry and indicates a magnification factor \gta 9. With
this magnification, the strength of the [OII] \lambda 3727 line implies a
star-formation rate of SFR \sim 0.4 h^{-2}\msun yr^{-1}$. A2124 thus appears to
be the lowest redshift cluster known to exhibit strong lensing of a distant
background galaxy.Comment: 6 pages using emulateapj.sty; 4 Postscript figures; Figure 4 uses
color. Accepted for publication, but ApJ Letters' new policy of counting data
images makes the manuscript too long; will appear in main journal. This final
version has minor correction
Does GRS 1915+105 exhibit "canonical" black-hole states?
We have analysed RXTE data of the superluminal source GRS 1915+105 in order
to investigate if, despite its extreme variability, it also exhibits the
canonical source states that characterise other black-hole candidates. The
phenomenology of GRS 1915+105 has been described in terms of three states
(named A, B and C) based on their hardness ratios and position in the
colour-colour diagram. We have investigated the connection between these states
and the canonical behaviour and found that the shape of the power spectral
continuum and the values of the best-fit model parameters to the noise
components in all three states indicate that the source shows properties
similar to the canonical very high state.Comment: 5 pages, 3 figures, accepted for publication in A&
High-frequency Quasi-Periodic Oscillations from GRS 1915+105 in its C state
We report the results of a systematic timing analysis of RXTE observations of
GRS 1915+105 when the source was in its variability class theta, characterized
by alternating soft and hard states on a time scale of a few hundred seconds.
The aim was to examine the high-frequency part of the power spectrum in order
to confirm the hecto-Hertz Quasi-Periodic Oscillations (QPO) previously
reported from observations from mixed variability behaviours. During the hard
intervals (corresponding to state C in the classification of Belloni et al.,
2000, A&A, 35, 271), we find a significant QPO at a frequency of ~170 Hz,
although much broader (Q~2) than previously reported. No other significant peak
is observed at frequencies >30 Hz. A time-resolved spectral analysis of
selected observations shows that the hard intervals from class theta show a
stronger and steeper (Gamma=2.8-3.0) power-law component than hard intervals
from other classes. We discuss these results in the framework of hecto-Hertz
QPOs reported from GRS 1915+105 and other black-hole binaries.Comment: 7 pages, 8 figures, accepted for publication on MNRA
A transient low-frequency QPO from the black hole binary GRS 1915+105
We present the results of the timing analysis of five Rossi X-ray Timing
Explorer observations of the Black Hole Candidate GRS 1915+105 between 1996
September and 1997 December. The aim was to investigate the possible presence
of a type-B quasi-periodic oscillation (QPO). Since in other systems this QPO
is found to appear during spectral transitions from Hard to Soft states, we
analyzed observations characterized by a fast and strong variability, in order
to have a large number of transitions. In GRS 1915+105, transitions occur on
very short time scales (~ sec): to single them out we averaged Power Density
Spectra following the regular path covered by the source on a 3D
Hardness-Hardness-Intensity Diagram. We identified both the type-C and the
type-B quasi-periodic oscillations (QPOs): this is the first detection of a
type-B QPO in GRS 1915+105. As the spectral transitions have been associated to
the emission and collimation of relativistic radio-jets, their presence in the
prototypical galactic jet source strengthens this connection.Comment: 13 pages, 9 figures, accepted for publication on MNRA
Sunflower under conventional and organic farming systems: results from a long term experiment in Central Italy
Sunflower productivity under organic and conventional agricultural systems was studied between 2002 and 2004 in the frame of the MASCOT experiment (Mediterranean Arable Systems COmparison Trial), established in 2001. The aim was to compare organic and conventional management systems for a typical arable crop rotation of Central Italy in the long-term. Sunflower was cultivated as a part of a five-year stockless arable crop rotation (sugar beet-common wheatsunflower-pigeon bean-durum wheat). In the organic system, red clover (Trifolium pratense) is interseeded in common and durum wheat and used as a green manure for sunflower or sugar beet. Grain yield of organically-grown sunflower was lower by 41%, 17% and 44% in 2002, 2003 and 2004 respectively, but no significant differences in percent seed oil content were found between the conventional and organic sunflower in two years out of three
Pivotal estimation in high-dimensional regression via linear programming
We propose a new method of estimation in high-dimensional linear regression
model. It allows for very weak distributional assumptions including
heteroscedasticity, and does not require the knowledge of the variance of
random errors. The method is based on linear programming only, so that its
numerical implementation is faster than for previously known techniques using
conic programs, and it allows one to deal with higher dimensional models. We
provide upper bounds for estimation and prediction errors of the proposed
estimator showing that it achieves the same rate as in the more restrictive
situation of fixed design and i.i.d. Gaussian errors with known variance.
Following Gautier and Tsybakov (2011), we obtain the results under weaker
sensitivity assumptions than the restricted eigenvalue or assimilated
conditions
A transient high-coherence oscillation in 4U 1820-30
We re-analyzed two Rossi X-Ray Timing Explorer archival observations of the
atoll source 4U 1820-30 in order to investigate the detailed time-frequency
properties of the source during the intervals when a ~7 Hz QPO was detected by
Wijnands et al. (1999, ApJ, 512, L39). We find that in both observations, in
addition to a QPO signal lasting a couple of minutes as previously reported,
there is a much narrower transient oscillation with a life time of only a few
seconds. Within this time, the oscillation is consistent with being coherent.
Its integrated fractional rms is around 10% and its frequency 7.3 Hz and 5.7 Hz
in the two observations. We discuss the possible association of this QPO with
other oscillations known both in Neutron-Star and Black-Hole systems,
concentrating on the similarities with the narrow 5-7 Hz oscillations observed
at high flux in Black-Hole Candidates.Comment: 5 pages, 3 figures, accepted for publication in A&A. FIgure 1 is
reduced in resolution, full-resolution version of this text available at
http://www.merate.mi.astro.it/~belloni/ms0335.ps.g
Low Luminosity States of the Black Hole Candidate GX~339--4. II. Timing Analysis
Here we present timing analysis of a set of eight Rossi X-ray Timing Explorer
(RXTE) observations of the black hole candidate GX 339-4 that were taken during
its hard/low state. On long time scales, the RXTE All Sky Monitor data reveal
evidence of a 240 day periodicity, comparable to timescales expected from
warped, precessing accretion disks. On short timescales all observations save
one show evidence of a persistent f approximately equal to 0.3 Hz QPO. The
broad band (10^{-3}-10^2 Hz) power appears to be dominated by two independent
processes that can be modeled as very broad Lorentzians with Q approximately
less than 1. The coherence function between soft and hard photon variability
shows that if these are truly independent processes, then they are individually
coherent, but they are incoherent with one another. This is evidenced by the
fact that the coherence function between the hard and soft variability is near
unity between 0.005-10 Hz but shows evidence of a dip at f approximately equal
to 1 Hz. This is the region of overlap between the broad Lorentzian fits to the
PSD. Similar to Cyg X-1, the coherence also drops dramatically at frequencies
approximately greater than 10 Hz. Also similar to Cyg X-1, the hard photon
variability is seen to lag the soft photon variability with the lag time
increasing with decreasing Fourier frequency. The magnitude of this time lag
appears to be positively correlated with the flux of GX 339-4. We discuss all
of these observations in light of current theoretical models of both black hole
spectra and temporal variability.Comment: To Appear in the AStrophysical Journa
A simple model for the complex lag structure of microquasars
The phase lag structure between the hard and soft X-ray photons observed in
GRS 1915+105 and XTE J1550+564 has been said to be ``complex'' because the
phase of the Quasi-Periodic Oscillation fundamental Fourier mode changes with
time and because the even and odd harmonics signs behave differentely.
From simultaneous X-ray and radio observations this seems to be related to
the presence of a jet (level of radio emission). We propose a simple idea where
a partial absorption of the signal can shift the phases of the Fourier modes
and account for the phase lag reversal. We also briefly discuss a possible
physical mechanism that could lead to such an absorption of the quasi-periodic
oscillation modulation.Comment: accepted by A&A Letter
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