969 research outputs found
A generalized noise variance analysis model and its application to the characterization of 1/f noise
We present a novel generalized model for the analysis of noise with a known spectral density. This model is particularly appropriate for the analysis of noise with a 1/f^a distribution in a homodyne interferometer. The noise model reveals that, for α>1, 1/f^a noise significantly impacts the homodyne signal-to-noise ratio (SNR) for integration times that near a characteristic time, beyond which the SNR will no longer significantly improve with increasing integration time. We experimentally verify our theoretical findings with a set of experiments employing a quadrature homodyne optical coherence tomography (OCT) system, finding good agreement. The characteristic integration time is measured to be approximately 2 ms for our system. Additionally, we find that the 1/f noise characteristics, including the exponent, α, as well as the characteristic integration time, are system and photodetector dependent
SNR enhancement through phase dependent signal reconstruction algorithms for phase separated interferometric signals
We report several signal reconstruction algorithms for processing phase separated homodyne interferometric signals. Methods that take advantage of the phase of the signal are experimentally shown to achieve a signal-to-noise ratio (SNR) improvement of up to 5 dB over commonly used algorithms. To begin, we present a derivation of the SNR resulting from five image reconstruction algorithms in the context of a 3x3 fiber-coupler based homodyne optical coherence tomography (OCT) system, and clearly show the improvement in SNR associated with phase-based algorithms. Finally, we experimentally verify this improvement and demonstrate the enhancement in contrast and improved image quality afforded by these algorithms through homodyne OCT imaging of a Xenopus laevis tadpole. These algorithms can be generally applied in signal extraction processing where multiple phase separated measurements are available
Chandra High Energy Grating Observations of the Fe Ka Line Core in Type 2 Seyfert Galaxies: A Comparison with Type 1 Nuclei
We present a study of the core of the Fe Ka emission line at ~6.4 keV in a
sample of type II Seyfert galaxies observed by the Chandra High Energy Grating
(HEG). The sample consists of 29 observations of 10 unique sources. We present
measurements of the Fe Ka line parameters with the highest spectral resolution
currently available. In particular, we derive the most robust intrinsic line
widths for some of the sources in the sample to date. We obtained a weighted
mean FWHM of 2000 \pm 160 km/s for 8 out of 10 sources (the remaining sources
had insufficient signal-to-noise). From a comparison with the optical
emission-line widths obtained from spectropolarimetric observations, we found
that the location of Fe Ka line-emitting material is a factor of ~0.7-11 times
the size of the optical BLR. Furthermore, compared to 13 type I AGNs for which
the best Fe Ka line FWHM constraints were obtained, we found no difference in
the FWHM distribution or the mean FWHM, and this conclusion is independent of
the central black hole mass. This result suggests that the bulk of the Fe Ka
line emission may originate from a universal region at the same radius with
respect to the gravitational radius, ~30,000 Rg on average. By examining the
correlation between the Fe Ka luminosity and the [O IV] line luminosity, we
found a marginal difference in the Fe K line flux between type I and type II
AGNs, but the spread in the ratio of L(Fe) to L([O IV]) is about two orders of
magnitude. Our results confirm the theoretical expectation that the Fe Ka
emission-line luminosity cannot trivially be used as a proxy of the intrinsic
AGN luminosity, unless a detailed comparison of the data with proper models is
applied.Comment: 16 pages, 6 figures, and 2 tables, accepted for publication in Ap
A Highly Doppler Blueshifted Fe-K Emission Line in the High-Redshift QSO PKS 2149-306
We report the results from an \asca observation of the QSO PKS 2149-306
(z=2.345). We detect an emission line centered at keV in the quasar
frame. Line emission at this energy has not been observed in any other active
galaxy or quasar to date. We present evidence rejecting the possibility that
this line is the result of instrumental artifacts, or a serendipitous source.
The most likely explanation is blueshifted Fe-K emission (the EW is 300+/-200
eV, QSO frame). Bulk velocities of the order of 0.75c are implied by the data.
We show that Fe-K line photons originating in an accretion disk and
Compton-scattering off a leptonic can account for the emission line. Curiously,
if the emission-line feature recently discovered in another quasar PKS
0637752, , is blueshifted Ovii, the Doppler factor is the same
(~2.7) for both.Comment: 15 pages plus 3 figures. Latex with separate .ps files (Accepted by
Astrophysical Journal Letters
NGC 2992 in an X-ray high state observed by XMM: Response of the Relativistic Fe K Line to the Continuum
We present the analysis of an XMM observation of the Seyfert galaxy NGC 2992.
The source was found in its highest level of X-ray activity yet detected, a
factor higher in 2--10 keV flux than the historical minimum. NGC
2992 is known to exhibit X-ray flaring activity on timescales of days to weeks,
and the XMM data provide at least factor of better spectral resolution
in the Fe K band than any previously measured flaring X-ray state. We find that
there is a broad feature in the \sim 5-7 keV band which could be interpreted as
a relativistic Fe K emission line. Its flux appears to have increased
in tandem with the 2--10 keV continuum when compared to a previous Suzaku
observation when the continuum was a factor of lower than that during
the XMM observation. The XMM data are consistent with the general picture that
increased X-ray activity and corresponding changes in the Fe K line
emission occur in the innermost regions of the putative accretion disk. This
behavior contrasts with the behavior of other AGN in which the Fe K
line does not respond to variability in the X-ray.Comment: 30 pages, 6 figures, Accepted to Ap
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