1,757 research outputs found
Femtosecond electron-bunch dynamics in laser wakefields and vacuum
Recent advances in laser wakefield acceleration demonstrated the generation of extremely short (with a duration of a few femtoseconds) relativistic electron bunches with relatively low (of the order of couple of percent) energy spread. In this article we study the dynamics of such bunches in drift space (vacuum) and in channel-guided laser wakefields. Analytical solutions were found for the transverse coordinate of an electron and for the bunch envelope in the wakefield in the case of arbitrary change in the energy. Our results show strong bunch dynamics already on a millimeter scale propagation distance both in plasma and in vacuum. When the bunch propagates in vacuum, its transverse sizes grow considerably; the same is observed for the normalized bunch emittance that worsens the focusability of the bunch. A scheme of two-stage laser wakefield accelerator with small drift space between the stages is proposed. It is found that fast longitudinal betatron phase mixing occurs in a femtosecond bunch when it propagates along the wakefield axis. When bunch propagates off axis, strong bunch decoherence and fast emittance degradation due to the finite bunch length was observed
Investigating ionized disc models of the variable narrow-line Seyfert 1 PG 1404+226
We investigate the use of relativistically blurred photoionized disc models
on an XMM-Newton observation of the Narrow Line Seyfert 1 galaxy PG 1404+226.
The model is designed to reproduce the radiation from the inner accretion disc
around a Kerr black hole, and is more successful at fitting the spectrum than
models based on a thermal soft excess. The source varies strongly over the
course of the observation, and the disc model works over all observed flux
states. We conclude that it is a useful tool in the study of certain quasars.Comment: 6 pages, 3 figures. Accepted for publication in MNRA
Ponderomotive scattering of an electron-bunch before injection into a laser wakefield
For the purpose of laser wakefield acceleration, it turned out that also the
injection of electron bunches longer than a plasma wavelength can generate
accelerated femtosecond bunches with relatively low energy spread. This is of
high interest because such injecting bunches can be provided, e.g., by
state-of-the-art photo cathode RF guns. Here we point out that when an e-bunch
is injected in the wakefield it is important to take into account the
ponderomotive scattering of the injecting bunch by the laser pulse in the
vacuum region located in front of the plasma. At low energies of the injected
bunch this scattering results in a significant drop of the collection
efficiency. Larger collection efficiency can by reached with lower intensity
laser pulses and relatively high injection energies. We also estimate the
minimum trapping energy for the injected electrons and the length of the
trapped bunch.Comment: 12 pages, 5 figures, submitted to Phys. Rev.
Surface acoustic waves for acousto-optic modulation in buried silicon nitride waveguides
We theoretically investigate the use of Rayleigh surface acoustic waves
(SAWs) for refractive index modulation in optical waveguides consisting of
amorphous dielectrics. Considering low-loss SiN waveguides with a
standard core cross section of 4.40.03 m size, buried 8 m
deep in a SiO cladding we compare surface acoustic wave generation in
various different geometries via a piezo-active, lead zirconate titanate film
placed on top of the surface and driven via an interdigitized transducer (IDT).
Using numerical solutions of the acoustic and optical wave equations, we
determine the strain distribution of the SAW under resonant excitation. From
the overlap of the acoustic strain field with the optical mode field we
calculate and maximize the attainable amplitude of index modulation in the
waveguide. For the example of a near-infrared wavelength of 840 nm, a maximum
shift in relative effective refractive index of 0.7x10 was obtained for
TE polarized light, using an IDT period of 30 - 35 m, a film thickness of
2.5 - 3.5 m, and an IDT voltage of 10 V. For these parameters, the
resonant frequency is in the range 70 - 85 MHz. The maximum shift increases to
1.2x10, with a corresponding resonant frequency of 87 MHz, when the
height of the cladding above the core is reduced to 3 m. The relative
index change is about 300-times higher than in previous work based on
non-resonant proximity piezo-actuation, and the modulation frequency is about
200-times higher. Exploiting the maximum relative index change of
1.210 in a low-loss balanced Mach-Zehnder modulator should allow
full-contrast modulation in devices as short as 120 m (half-wave voltage
length product = 0.24 Vcm).Comment: 19 pages, 8 figure
ROSAT PSPC detection of soft X-ray absorption in GB 1428+4217: The most distant matter yet probed with X-ray spectroscopy
We report on a ROSAT PSPC observation of the highly-luminous z = 4.72
radio-loud quasar GB 1428+4217 obtained between 1998 December 11 and 17, the
final days of the ROSAT satellite. The low-energy sensitivity of the PSPC
detector was employed to constrain the intrinsic X-ray absorption of the
currently most distant X-ray detected object. Here we present the detection of
significant soft X-ray absorption towards GB 1428+4217, making the absorbing
material the most distant matter yet probed with X-ray spectroscopy. X-ray
variability by 25+-8 per cent is detected on a timescale of 6500 s in the rest
frame. The X-ray variation requires an unusually high radiative efficiency of
at least 4.2, further supporting the blazar nature of the source.Comment: 6 pages incl. 6 figures, accepted for publication in Monthly Notice
HST STIS Ultraviolet Spectral Evidence for Outflow in Extreme Narrow-line Seyfert 1 Galaxies: I. Data and Analysis
We present HST STIS observations of two extreme NLS1s, IRAS 13224-3809 and 1H
0707-495. The spectra are characterized by very blue continua, broad, strongly
blueshifted high-ionization lines (including \ion{C}{4} and \ion{N}{5}), and
narrow, symmetric intermediate- (including \ion{C}{3}], \ion{Si}{3}],
\ion{Al}{3}) and low-ionization (e.g., \ion{Mg}{2}) lines centered at their
rest wavelengths. The emission-line profiles suggest that the high-ionization
lines are produced in a wind, and the intermediate- and low-ionization lines
are produced in low-velocity gas associated with the accretion disk or base of
the wind. In this paper, we present the analysis of the spectra from these two
objects; in a companion paper we present photoionization analysis and a toy
dynamical model for the wind. The highly asymmetric profile of \ion{C}{4}
suggests that it is dominated by emission from the wind, so we develop a
template for the wind from the \ion{C}{4} line. We model the bright emission
lines in the spectra using a combination of this template, and a narrow,
symmetric line centered at the rest wavelength. We also analyzed a comparison
sample of HST spectra from 14 additional NLS1s, and construct a correlation
matrix of emission line and continuum properties. A number of strong
correlations were observed, including several involving the asymmetry of the
\ion{C}{4} line.Comment: 26 pages, 10 figures, accepted for publication in ApJ with no change
On the Nature of Soft X-ray Weak Quasi-Stellar Objects
Recent studies of QSOs with ROSAT suggest the existence of a significant
population of Soft X-ray Weak QSOs (SXW QSOs) where the soft X-ray flux is ~
10-30 times smaller than in typical QSOs. As a first step in a systematic study
of these objects, we establish a well-defined sample of SXW QSOs which includes
all alpha_ox<=-2 QSOs from the Boroson & Green (1992) sample of 87 BQS QSOs.
SXW QSOs comprise about 11% of this optically selected QSO sample. From an
analysis of CIV absorption in the 55 BG92 QSOs with available CIV data, we find
a remarkably strong correlation between alpha_ox and the CIV absorption
equivalent width. This correlation suggests that absorption is the primary
cause of soft X-ray weakness in QSOs, and it reveals a continuum of absorption
properties connecting unabsorbed QSOs, X-ray warm absorber QSOs, SXW QSOs and
BAL QSOs. From a practical point of view, our correlation demonstrates that
selection by soft X-ray weakness is an effective (>=80% successful) and
observationally inexpensive way to find low-redshift QSOs with strong and
interesting ultraviolet absorption. We have also identified several notable
differences between the optical emission-line properties of SXW QSOs and those
of the other BG92 QSOs. SXW QSOs show systematically low [O III] luminosities
as well as distinctive H-beta profiles. They tend to lie toward the weak-[O
III] end of BG92 eigenvector 1, as do many low-ionization BAL QSOs. Unabsorbed
Seyferts and QSOs with similar values of eigenvector 1 have been suggested to
have extreme values of a primary physical parameter, perhaps mass accretion
rate relative to the Eddington rate (M-dot/M-dot_{Edd}). If these suggestions
are correct, it is likely that SXW QSOs also tend to have generally high values
of (M-dot/M-dot_{Edd}). (Abridged)Comment: 34 pages, ApJ accepted, also available from
http://www.astro.psu.edu/users/niel/papers/papers.htm
HST STIS Ultraviolet Spectral Evidence of Outflow in Extreme Narrow-line Seyfert 1 Galaxies: II. Modeling and Interpretation
We present modeling to explore the conditions of the broad-line emitting gas
in two extreme Narrow-line Seyfert 1 galaxies, using the observational results
described in the first paper of this series. Photoionization modeling using
Cloudy was conducted for the broad, blueshifted wind lines and the narrow,
symmetric, rest-wavelength-centered disk lines separately. A broad range of
physical conditions were explored for the wind component, and a figure of merit
was used to quantitatively evaluate the simulation results. Of the three minima
in the figure-of-merit parameter space, we favor the solution characterized by
an X-ray weak continuum, elevated abundances, a small column density
(log(N_H)\approx 21.4), relatively high ionization parameter (log(U)\approx
-1.2 - -0.2), a wide range of densities (log(n)\approx 7 - 11), and a covering
fraction of ~0.15. The presence of low-ionization emission lines implies the
disk component is optically thick to the continuum, and the SiIII]/CIII] ratio
implies a density of 10^10 - 10^10.25 cm^-3. A low ionization parameter
(log(U)=-3) is inferred for the intermediate-ionization lines, unless the
continuum is ``filtered'' through the wind before illuminating the
intermediate-line emitting gas, in which case log(U)=-2.1. The location of the
emission regions was inferred from the photoionization modeling and a simple
``toy'' dynamical model. A large black hole mass (1.3 x 10^8 M_\odot) radiating
at 11% of the Eddington luminosity is consistent with the kinematics of both
the disk and wind lines, and an emission radius of ~10^4 R_S is inferred for
both. We compare these results with previous work and discuss implications.Comment: 45 pages, 15 figures (4 color), accepted for publication in ApJ,
abstract shortene
X-ray Spectral Variability and Rapid Variability of the Soft X-ray Spectrum Seyfert 1 Galaxies Ark 564 and Ton S180
The bright, soft X-ray spectrum Seyfert 1 galaxies Ark 564 and Ton S180 were
monitored for 35 days and 12 days with ASCA and RXTE (and EUVE for Ton S180).
The short time scale (hours-days) variability patterns were very similar across
energy bands, with no evidence of lags between any of the energy bands studied.
The fractional variability amplitude was almost independent of energy band. It
is difficult to simultaneously explain soft Seyferts stronger variability,
softer spectra, and weaker energy-dependence of the variability relative to
hard Seyferts. The soft and hard band light curves diverged on the longest time
scales probed, consistent with the fluctuation power density spectra that
showed relatively greater power on long time scales in the softest bands. The
simplest explanation is that a relatively hard, rapidly-variable component
dominates the total X-ray spectrum and a slowly-variable soft excess is present
in the lowest energy channels of ASCA. Although it would be natural to identify
the latter with an accretion disk and the former with a corona surrounding it,
a standard thin disk could not get hot enough to radiate significantly in the
ASCA band, and the observed variability time scales are much too short. The
hard component may have a more complex shape than a pure power-law. The most
rapid factor of 2 flares and dips occurred within ~1000 sec in Ark 564 and a
bit more slowly in Ton S180. The speed of the luminosity changes rules out
viscous or thermal processes and limits the size of the individual emission
regions to <~15 Schwarzschild radii (and probably much less), that is, to
either the inner disk or small regions in a corona
Spectropolarimetry of the Luminous Narrow-Line Seyfert Galaxies IRAS 20181-2244 and IRAS 13224-3809
We observed the narrow-line Seyfert 1 galaxies IRAS 20181-2244 and IRAS
13324-3809 with a new spectropolarimeter on the RC spectrograph at the CTIO 4m
telescope. Previously it had been suggested that IRAS 20181-2244 was a Type 2
QSO and thus might contain an obscured broad-line region which could be
detected by the presence of broad Balmer lines in the polarized flux. We found
the object to be polarized at about 2%, and constant with wavelength, (unlike
most narrow-line Seyfert 1s), but with no evidence of broad Balmer lines in
polarized flux. The spectropolarimetry indicates that the scattering material
is inside the BLR. IRAS 13224-3809, notable for its high variability in X-ray
and UV wavelengths, has a low polarization consistent with a Galactic
interstellar origin.Comment: 19 pages using (AASTEX) aaspp4.sty and 5 postscript figures To be
published in the Astrophysical Journa
- …