268 research outputs found
PPM demodulation: On approaching fundamental limits of optical communications
We consider the problem of demodulating M-ary optical PPM (pulse-position
modulation) waveforms, and propose a structured receiver whose mean probability
of symbol error is smaller than all known receivers, and approaches the quantum
limit. The receiver uses photodetection coupled with optimized phase-coherent
optical feedback control and a phase-sensitive parametric amplifier. We present
a general framework of optical receivers known as the conditional pulse nulling
receiver, and present new results on ultimate limits and achievable regions of
spectral versus photon efficiency tradeoffs for the single-spatial-mode
pure-loss optical communication channel.Comment: 5 pages, 6 figures, IEEE ISIT, Austin, TX (2010
Testing Hydrodynamic Models of LMC X-4 with UV and X-ray Spectra
We compare the predictions of hydrodynamic models of the LMC X-4 X-ray binary
system with observations of UV P Cygni lines with the GHRS and STIS
spectrographs on the Hubble Space Telescope. The hydrodynamic model determines
density and velocity fields of the stellar wind, wind-compressed disk,
accretion stream, Keplerian accretion disk, and accretion disk wind. We use a
Monte Carlo code to determine the UV P Cygni line profiles by simulating the
radiative transfer of UV photons that originate on the star and are scattered
in the wind. The qualitative orbital variation predicted is similar to that
observed, although the model fails to reproduce the strong orbital asymmetry
(the observed absorption is strongest for phi>0.5). The model predicts a
mid-eclipse X-ray spectrum, due almost entirely to Compton scattering, with a
factor 4 less flux than observed with ASCA. We discuss how the model may need
to be altered to explain the spectral variability of the system.Comment: 11 figures, accepted by Ap
X-Ray Spectral Study of AGN Sources Content in Some Deep Extragalactic XMM-Newton Fields
We undertake a spectral study of a sample of bright X-ray sources taken from
six XMM-Newton fields at high galactic latitudes, where AGN are the most
populous class. These six fields were chosen such that the observation had an
exposure time more than 60 ksec, had data from the EPIC-pn detector in the
full-Frame mode and lying at high galactic latitude . The analysis
started by fitting the spectra of all sources with an absorbed power-law model,
and then we fitted all the spectra with an absorbed power-law with a low energy
black-body component model.The sources for which we added a black body gave an
F-test probability of 0.01 or less (i.e. at 99% confidence level), were
recognized as sources that display soft excess. We perform a comparative
analysis of soft excess spectral parameters with respect to the underlying
power-law one for sources that satisfy this criterion. Those sources, that do
not show evidence for a soft excess, based on the F-test probability at a 99%
confidence level, were also fitted with the absorbed power-law with a low
energy black-body component model with the black-body temperature fixed at 0.1
and 0.2 keV. We establish upper limits on the soft excess flux for those
sources at these two temperatures. Finally we have made use of Aladdin
interactive sky atlas and matching with NASA/IPAC Extragalactic Database (NED)
to identify the X-ray sources in our sample. For those sources which are
identified in the NED catalogue, we make a comparative study of the soft excess
phenomenon for different types of systems
On-axis spectroscopy of the z=0.144 radio-loud quasar HE 1434-1600: an elliptical host with a highly ionized ISM
VLT on-axis optical spectroscopy of the z=0.144 radio-loud quasar HE
1434-1600 is presented. The spatially resolved spectra of the host galaxy are
deconvolved and separated from those of the central quasar in order to study
the dynamics of the stars and gas as well as the physical conditions of the
ISM. We find that the host of HE 1434-1600 is an elliptical galaxy that resides
in a group of at least 5 member galaxies, and that most likely experienced a
recent collision with its nearest companion. Compared with other quasar host
galaxies, HE 1434-1600 has a highly ionized ISM. The ionization state
corresponds to that of typical Seyferts, but the ionized regions are not
distributed in a homogeneous way around the QSO, and are located preferentially
several kiloparsecs away from it. While the stellar absorption lines do not
show any significant velocity field, the gas emission lines do. The observed
gas velocity field is hard to reconcile with dynamical models involving
rotating disk, modified Hubble laws or power laws, that all require extreme
central masses (M > 10^9 M_sun) to provide only poor fit to the data. Power law
models, which best fit the data, provide a total mass of M(< 10 kpc)=9.2 10^10
M_sun. We conclude that the recent interaction between HE 1434-1600 and its
closest companion has strongly affected the gas velocity and ionization state,
from the center of the galaxy to its most external parts.Comment: 11 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
The Intrinsic Absorber in QSO 2359-1241: Keck and HST Observations
We present detailed analyses of the absorption spectrum seen in QSO 2359-1241
(NVSS J235953-124148). Keck HIRES data reveal absorption from twenty
transitions arising from: He I, Mg I, Mg II, Ca II, and Fe II. HST data show
broad absorption lines (BALs) from Al III 1857, C IV 1549, Si IV 1397, and N V
1240. Absorption from excited Fe II states constrains the temperature of the
absorber to 2000K < T < 10,000K and puts a lower limit of 10^5 cm^{-3} on the
electron number density. Saturation diagnostics show that the real column
densities of He I and Fe II can be determined, allowing to derive meaningful
constraints on the ionization equilibrium and abundances in the flow. The
ionization parameter is constrained by the iron, helium and magnesium data to
-3.0 < log(U) < -2.5 and the observed column densities can be reproduced
without assuming departure from solar abundances. From comparison of the He I
and Fe II absorption features we infer that the outflow seen in QSO 2359-1241
is not shielded by a hydrogen ionization front and therefore that the existence
of low-ionization species in the outflow (e.g., Mg II, Al III, Fe II) does not
necessitate the existence of such a front. We find that the velocity width of
the absorption systematically increases as a function of ionization and to a
lesser extent with abundance. Complementary analyses of the radio and
polarization properties of the object are discussed in a companion paper
(Brotherton et al. 2000).Comment: 30 pages, 9 figures, in press with the Ap
On the nature of Seyfert galaxies with high [OIII]5007 blueshifts
We have studied the properties of Seyfert galaxies with high [OIII]5007
blueshifts (`blue outliers'), originally identified because of their strong
deviation from the M_BH - sigma relation of normal, narrow-line Seyfert 1
(NLS1) and broad-line Seyfert 1 (BLS1) galaxies. These blue outliers turn out
to be important test-beds for models of the narrow-line region (NLR), for
mechanisms of driving large-scale outflows, for links between NLS1 galaxies and
radio galaxies, and for orientation-dependent NLS1 models. We report the
detection of a strong correlation of line blueshift with ionization potential
in each galaxy, including the measurement of coronal lines with radial
velocities up to 500--1000 km/s. All [OIII] blue outliers have narrow widths of
their broad Balmer lines and high Eddington ratios. While the presence of
non-shifted low-ionization lines signifies the presence of a classical outer
quiescent NLR in blue outliers, we also report the absence of any second,
non-blueshifted [OIII] component from a classical inner NLR. These results
place tight constraints on NLR models. We favor a scenario in which the NLR
clouds are entrained in a decelerating wind which explains the strong
stratification and the absence of a zero-blueshift inner NLR of blue outliers.
The origin of the wind remains speculative at this time (collimated radio
plasma, thermal winds, radiatively accelerated clouds). It is perhaps linked to
the high Eddington ratios of blue outliers. Similar, less powerful winds could
be present in all Seyfert galaxies, but would generally only affect the coronal
line region (CLR), or level off even before reaching the CLR. Similarities
between blue outliers in NLS1 galaxies and (compact) radio sources are briefly
discussed.Comment: ApJ in press (scheduled for June 20 issue); incl. 4 colour figures.
This, and related paper showing that NLS1 galaxies follow the M-sigma
relation based on [SII], is also available at
http://www.xray.mpe.mpg.de/~skomossa
The rest-frame ultraviolet properties of radio-loud broad absorption line quasars
We recently presented radio observations of a large sample of radio-loud
broad absorption line (BAL) quasars from the SDSS and FIRST surveys, as well as
a well matched sample of unabsorbed quasars, primarily to measure their radio
spectral indices and estimate ensemble orientations. Here, we analyze the SDSS
spectra of these samples and compare the rest-frame ultraviolet properties of
radio-loud BAL and non-BAL quasars. Ultraviolet properties include the
continuum shape, emission-line measurements of C IV, Al III, C III], Fe II, and
Mg II, and BAL properties including the balnicity index (BI), absorption index
(AI), and minimum and maximum outflow velocities. We find that radio-loud BAL
quasars have similar ultraviolet properties compared to radio-loud non-BAL
sources, though they do appear to have redder continua and stronger Fe II
emission, which is consistent with what is found for radio-quiet BAL sources.
No correlations exist between outflow properties and orientation (radio
spectral index), suggesting that BAL winds along any line of sight are driven
by the same mechanisms. There are also few correlations between spectral index
and other properties. We conclude that BAL outflows occur along all lines of
sight with similar strengths and velocities.Comment: Accepted for publication in MNRAS. Tables 1 and 2 will be published
in full with the final online publicatio
An Interstellar Conduction Front Within a Wolf-Rayet Ring Nebula Observed with the GHRS
With the High Resolution Spectrograph aboard the Hubble Space Telescope we
obtained high signal-to-noise (S/N > 200 - 600 per 17 km/s resolution element)
spectra of narrow absorption lines toward the Wolf-Rayet star HD 50896. The
ring nebula S308 that surrounds this star is thought to be caused by a
pressure-driven bubble bounded by circumstellar gas (most likely from a red
supergiant or luminous blue variable progenitor) pushed aside by a strong
stellar wind. Our observation has shown for the first time that blueshifted
(approximately 70 km/s relative to the star) absorption components of C IV and
N V arise in a conduction front between the hot interior of the bubble and the
cold shell of swept-up material. These lines set limits on models of the
conduction front. Nitrogen in the shell appears to be overabundant by a factor
~10. The P Cygni profiles of N V and C IV are variable, possibly due to a
suspected binary companion to HD 50896.Comment: 32 pages, Latex, to appear in the Astrophysical Journal, April, 199
Evidence of Compton cooling during an X-ray flare supports a neutron star nature of the compact object in 4U1700-37
Based on new Chandra X-ray telescope data, we present empirical evidence of
plasma Compton cooling during a flare in the non pulsating massive X-ray binary
4U1700-37. This behaviour might be explained by quasispherical accretion onto a
slowly rotating magnetised neutron star. In quiescence, the neutron star in
4U1700-37 is surrounded by a hot radiatively cooling shell. Its presence is
supported by the detection of mHz quasi periodic oscillations likely produced
by its convection cells. The high plasma temperature and the relatively low
X-ray luminosity observed during the quiescence, point to a small emitting area
about 1 km, compatible with a hot spot on a NS surface. The sudden transition
from a radiative to a significantly more efficient Compton cooling regime
triggers an episode of enhanced accretion resulting in a flare. During the
flare, the plasma temperature drops quickly. The predicted luminosity for such
transitions, Lx = 3 x 10^35 erg s-1, is very close to the luminosity of
4U1700-37 during quiescence. The transition may be caused by the accretion of a
clump in the stellar wind of the donor star. Thus, a magnetised NS nature of
the compact object is strongly favoured.Comment: Accepted for publication in MNRA
- …