7,215 research outputs found
The apparent shape of the "Str\"omgren sphere'' around the highest-redshift QSOs with Gunn-Peterson troughs
Although the highest redshift QSOs (z>6.1) are embedded in a significantly
neutral background universe (mass-averaged neutral hydrogen fraction >1%) as
suggested by the Gunn-Peterson absorption troughs in their spectra, the
intergalactic medium in their vicinity is highly ionized. The highly ionized
region is generally idealized as spherical and called the Str\"omgren sphere.
In this paper, by combining the expected evolution of the Str\"omgren sphere
with the rule that the speed of light is finite, we illustrate the apparent
shape of the ionization fronts around the highest redshift QSOs and its
evolution, which depends on the age, luminosity evolution, and environment of
the QSO (e.g., the hydrogen reionization history). The apparent shape may
systematically deviate from a spherical shape, unless the QSO age is
significantly long compared to the hydrogen recombination process within the
ionization front and the QSO luminosity evolution is significantly slow.
Effects of anisotropy of QSO emission are also discussed. The apparent shape of
the "Str\"omgren sphere'' may be directly mapped by transmitted spectra of
background sources behind or inside the ionized regions or by surveys of the
hyperfine transition (21cm) line emission of neutral hydrogen.Comment: 7 pages, 5 figures; discussion on effects of anisotropy of QSO
emission expanded; ApJ in pres
The luminosity evolution over the EQuiTemporal Surfaces in the prompt emission of Gamma-Ray Bursts
Due to the ultrarelativistic velocity of the expanding "fireshell" (Lorentz
gamma factor \gamma \sim 10^2 - 10^3), photons emitted at the same time from
the fireshell surface do not reach the observer at the same arrival time. In
interpreting Gamma-Ray Bursts (GRBs) it is crucial to determine the properties
of the EQuiTemporal Surfaces (EQTSs): the locus of points which are source of
radiation reaching the observer at the same arrival time. In the current
literature this analysis is performed only in the latest phases of the
afterglow. Here we study the distribution of the GRB bolometric luminosity over
the EQTSs, with special attention to the prompt emission phase. We analyze as
well the temporal evolution of the EQTS apparent size in the sky. We use the
analytic solutions of the equations of motion of the fireshell and the
corresponding analytic expressions of the EQTSs which have been presented in
recent works and which are valid for both the fully radiative and the adiabatic
dynamics. We find the novel result that at the beginning of the prompt emission
the most luminous regions of the EQTSs are the ones closest to the line of
sight. On the contrary, in the late prompt emission and in the early afterglow
phases the most luminous EQTS regions are the ones closest to the boundary of
the visible region. This transition in the emitting region may lead to specific
observational signatures, i.e. an anomalous spectral evolution, in the rising
part or at the peak of the prompt emission. We find as well an expression for
the apparent radius of the EQTS in the sky, valid in both the fully radiative
and the adiabatic regimes. Such considerations are essential for the
theoretical interpretation of the prompt emission phase of GRBs.Comment: 11 pages, 5 figures, in the Proceedings of the 1st Galileo-Xu GuangQi
Meeting, October 26-30, 2009, Shangha
Control of near-infrared supercontinuum bandwidth by adjusting pump pulse duration
We experimentally and numerically investigated the impact of input pump pulse duration on the near-infrared bandwidth of supercontinuum generation in a photonic crystal fiber. We continuously stretched the temporal duration of the input pump laser (centered at 1030 nm) pulses from 500 fs up to 10 ps, while keeping fixed the pump peak power. We observed that the long-wavelength edge of the supercontinuum spectrum is increased by 200 nm as the pump pulse duration grows from 500 fs to 10 ps. We provide a quantitative fit of the experimental results by means of numerical simulations. Moreover, we have explained the observed spectral broadening enhancement induced by pump pulse energy by developing an approximate yet fully analytical model for soliton energy exchange through a series of collisions in the presence of stimulated Raman scattering
Spatiotemporal Characterization of Supercontinuum Extending from the Visible to the Mid-Infrared in Multimode Graded-Index Optical Fiber
We experimentally demonstrate that pumping a graded-index multimode fiber
with sub-ns pulses from a microchip Nd:YAG laser leads to spectrally flat
supercontinuum generation with a uniform bell-shaped spatial beam profile
extending from the visible to the mid-infrared at 2500\,nm. We study the
development of the supercontinuum along the multimode fiber by the cut-back
method, which permits us to analyze the competition between the Kerr-induced
geometric parametric instability and stimulated Raman scattering. We also
performed a spectrally resolved temporal analysis of the supercontinuum
emission.Comment: 5 pages 7 figure
Spatial beam self-cleaning and supercontinuum generation with Yb-doped multimode graded-index fiber taper based on accelerating self-imaging and dissipative landscape
We experimentally demonstrate spatial beam self-cleaning and supercontinuum generation in a tapered Ytterbium-doped multimode optical fiber with parabolic core refractive index profile when 1064 nm pulsed beams propagate from wider (122 µm) into smaller (37 µm) diameter. In the passive mode, increasing the input beam peak power above 20 kW leads to a bell-shaped output beam profile. In the active configuration, gain from the pump laser diode permits to combine beam self-cleaning with supercontinuum generation between 520-2600 nm. By taper cut-back, we observed that the dissipative landscape, i.e., a non-monotonic variation of the average beam power along the MMF, leads to modal transitions of self-cleaned beams along the taper length
Probing Interstellar Dust with Infrared Echoes from the Cas A Supernova
We present the analysis of an IRS 5-38 {\mu}m spectrum and MIPS photometric
measurements of an infrared echo near the Cassiopeia A supernova remnant
observed with the Spitzer Space Telescope. We have modeled the recorded echo
accounting for PAHs, quantum-heated carbon and silicate grains, as well as
thermal carbon and silicate particles. Using the fact that optical light echo
spectroscopy has established that Cas A originated from a type IIb supernova
explosion showing an optical spectrum remarkably similar to the prototypical
type IIb SN 1993J, we use the latter to construct template data input for our
simulations. We are then able to reproduce the recorded infrared echo spectrum
by combining the emission of dust heated by the UV burst produced at the shock
breakout after the core-collapse and dust heated by optical light emitted near
the visual maximum of the supernova light curve, where the UV burst and optical
light curve characteristics are based on SN 1993J. We find a mean density of
\sim680 H cm^{-3} for the echo region, with a size of a few light years across.
We also find evidence of dust processing in the form of a lack of small PAHs
with less than \sim300 carbon atoms, consistent with a scenario of PAHs
destruction by the UV burst via photodissociation at the estimated distance of
the echo region from Cas A. Furthermore, our simulations suggest that the weak
11 {\mu}m features of our recorded infrared echo spectrum are consistent with a
strong dehydrogenated state of the PAHs. This exploratory study highlights the
potential of investigating dust processing in the interstellar medium through
infrared echoes.Comment: 16 pages, 14 figures, accepted for publication in the Astrophysical
Journa
Intra-cavity frequency shifted laser pumps for non-degenerate and partially coherent Bragg-Scattering FWM in nonlinear fiber
International audienceIn this work the authors experimentally study the problem of non-degenerate four-wave-mixing (FWM) by using a pair of partially coherent pumps, and focus our attention on a specific type of FWM, which is generally called "Bragg-Scattering" (BS-FWM). This kind of FWM has attracted a renewed interest because of its intrinsically low-noise nature which makes it potentially applicable for light-by-light manipulation even for very faint signals such as quantum keys
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