Spectral Slope Variations Research Articles

Variations in Spectral Slope in Lake Taihu, a Large Subtropical Shallow Lake in China

Spectral slope (S), describing the exponential decrease of the absorption spectrum over a given wavelength range, is an important parameter in the study of of chromophoric dissolved organic matter (CDOM) dynamics, and also an essential input parameter in remote sensing models. Furthermore, S is often used as a proxy for CDOM composition, including the ratio of fulvic to humic acids and molecular weight. The relative broad range in S values reported in the literature can be explained by the different spectral ranges and fitting methods used. A single exponential model is used to fit the S values for 17 investigations involving 458 samples in Lake Taihu from January to October in 2004. The average S value was 15.18 ± 1.39 μm −1 for the range of 280–500 nm, which fell within the range reported in the literature. The frequency distribution of S value basically obeyed a normal distribution. Significant differences in S values between summer and other seasons showed that phytoplankton degradation was one of the important sources of CDOM in summer, whereas CDOM mainly came from the river input in other seasons. Furthermore, the estimated S value decreased with increasing wavelength range used in regression. The maximum and minimum values derived from the regression were 17.89 ± 1.25 μm −1 and 13.62 ± 2.11 μm −1 for the wavelength ranges of 280–380 nm and 400–500 nm, respectively, a decrease of 23.9%. S values significantly decreased with the increase of CDOM absorption coefficients. CDOM absorption coefficients could be more appropriately estimated from exponential model introducing the variation of S with absorption coefficients, making them useful for a remote sensing bio-optical model of Lake Taihu. DOC-specific absorption coefficient a*(λ) and the parameter M describing molecular size of the humic molecules could also be used as a proxy for the sources and types of CDOM. A general relationship was found between S and a*(λ), and M values. S increased with the decrease of DOC-specific absorption coefficient and the increase of M corresponding to the decrease of molecular weight.

Fourier‐Resolved Spectroscopy of Active Galactic Nuclei UsingXMM‐NewtonData. I. The 3–10 keV Band Results

We present the results from the Fourier Resolved Spectroscopy of archival XMM-Newton data of five AGN, namely, Mrk 766, NGC 3516, NGC 3783, NGC 4051 and Ark 564. This work supplements the earlier study of MCG-6-30-15 as well as those of several Galactic Black Hole Candidate sources. Our results exhibit much larger diversity than those of Galactic sources, a fact we attribute to the diversity of their masses. When we take into account this effect and combine our results with those from Cyg X-1, it seems reasonable to conclude that, at high frequencies, the slope of the Fourier-resolved spectra in accreting black hole systems decreases with increasing frequency as proportional to f^{-0.25}, irrespective of whether the system is in its High or Low state. This result implies that the flux variations in AGN are accompanied by complex spectral slope variations as well. We also find that the Fe Ka line in Mrk 766, NGC 3783 and NGC 4051 is variable on time scales ~day - 1 hour. The iron fluorescence line is absent in the spectra of the highest frequencies,and there is an indication that, just like in Cyg X-1, the equivalent width of the line in the Fourier-resolved of AGN decreases with increasing frequency.

Colored dissolved organic matter in Tampa Bay, Florida

Absorption and fluorescence of colored dissolved organic matter (CDOM) and concentrations of dissolved organic carbon (DOC), chlorophyll and total suspended solids in Tampa Bay and its adjacent rivers were examined in June and October of 2004. Except in Old Tampa Bay (OTB), the spatial distribution of CDOM showed a conservative relationship with salinity in June, 2004 ( a CDOM(400) = − 0.19 × salinity + 6.78, R 2 = 0.98, n = 17, salinity range = 1.1–32.5) with little variations in absorption spectral slope and fluorescence efficiency. This indicates that CDOM distribution was dominated by mixing. In October, 2004, CDOM distribution was nonconservative with an average absorption coefficient ( a CDOM(400), ∼ 7.76 m − 1 ) about seven times higher than that in June (∼ 1.11 m − 1 ). The nonconservative behavior was caused largely by CDOM removal at intermediate salinities (e.g., a CDOM(400) removal > 15% at salinity ∼ 13.0), which likely resulted from photobleaching due to stronger stratification. The spatial and seasonal distributions of CDOM in Tampa Bay showed that the two largest rivers, the Alafia River (AR) and Hillsborough River (HR) were dominant CDOM sources to most of the bay. In OTB, however, CDOM showed distinctive differences: lower absorption coefficient, higher absorption spectral slopes, and lower ratios of CDOM absorption to DOC and higher fluorescence efficiency. These differences may have stemmed from (1) changes in CDOM composition by more intensive photobleaching due to the longer residence time of water mass in OTB; (2) other sources of CDOM than the HR/AR inputs, such as local creeks, streams, groundwater, and/or bottom re-suspension. Average CDOM absorption in Tampa Bay at 443 nm, a CDOM(443), was about five times higher in June and about ten times higher in October than phytoplankton pigment absorption, a ph(443), indicating that blue light attenuation in the water column was dominated by CDOM rather than by phytoplankton absorption throughout the year.

The relation between the optical spectral slope and the luminosity for 17 Palomar-Green quasi-stellar objects

Using 7.5-yr spectroscopic monitoring data of a sample of 17 Palomar-Green quasi-stellar objects (PG QSOs) (z = 0.061-0.371), we obtain the optical spectral slope for each object at all epochs by a power-law fit to the spectra in continuum bands. All of these 17 PG QSOs exhibit obvious spectral-slope variability. Most of the 17 objects show anticorrelation between the spectral slope and the rest-frame 5100-angstrom continuum flux, while five of them exhibit strong anticorrelation (correlation coefficient R larger than 0.5). For the ensemble of these 17 PG QSOs, a strong anticorrelation between the average spectral slope and the average rest-frame 5100-angstrom luminosity is found, while no correlation is found between the spectral slope and the Eddington ratio. A median anticorrelation between spectral-slope changes and continuum-flux variations is also found which indicates a hardening of the spectrum during bright phases. Accretion disc (jet) instability models with other mechanisms associated with changes in the accretion processes are promising.

The optical spectral slope variability of 17 blazars

Many quasi-simultaneous optical observations of 17 blazars are obtained from previous papers published over the last 19 years in order to investigate the spectral slope variability and understand the radiation mechanism of blazars. The long-period dereddened optical spectral slopes are calculated. We analyse the average spectral slope distribution, which suggests that the spectra of flat spectrum radio quasars (FSRQs) and high energy peaked BL Lac objects (HBLs) are probably deformed by other emission components. The average spectral slopes of low energy peaked BL Lac objects(LBLs), which scatter around 1.5, show a good accordance with the synchrotron self-Compton (SSC) loss-dominated model. We present and discuss the variability between the spectral slope and optical luminosity. The spectra of all HBLs and LBLs get flatter when they turn brighter, while for FSRQs this trend does not exist or may even be reversed. This phenomenon may imply that there is a thermal contribution to the optical spectrum for FSRQs. For the FSRQ 1156+295, there is a hint that the slope gets flatter at both the brightest and faintest states. Our result shows that three subclasses locate in different regions in the pattern of slope variability indicator versus average spectral slope. The relativistic jet mechanism is supported by the significant correlation between the optical Doppler factor and the average spectral slope.

The Optical Spectral Slope Variability for 17 Palomar-Green QSOs

Variability phenomenon in active galactic nuclei (AGNs) provides a powerful constrain on their central engine. For a long time the long-term multi-wavelength photometric monitoring plays an important role in obtaining the optical variability of QSOs. However, it just monitors the flux variability in a few points in the wavelength which usually consists of many components including continuum and strong emission line (although the photometric data are accurate). So it is necessary to study the optical spectral variability using spectroscopic monitoring data.

Effects of spectral slope on perceived breathiness in vowels

Previous studies have indicated that breathiness is correlated with changes in spectral slope as well as aspiration noise [Klatt & Klatt, ‘‘Analysis, synthesis, and perception of voice quality variations among female and male talkers,’’ J. Acoust. Soc. Am. 87, 820–857 (1990)]. In a recent study, Shrivastav and Pinero [Shrivastav & Pinero ‘‘Effect of aspiration noise and spectral slope on perceived breathiness in vowels,’’ J. Acoust. Soc. Am. 117, 2622 (2005)] observed that varying spectral slope resulted in minimal changes on the perceived breathiness for synthetic vowels. However, the stimuli tested in this experiment included a relatively narrow range of spectral slope variation. The goal of the present experiment was to verify the role of spectral slope on the perception of breathiness by testing stimuli that had a wider range of variation in spectral slope. Ten voices (5 male and 5 female) were synthesized using a Klatt synthesizer. Each of these was manipulated to generate two continua varying in their spectral slope from −3 to −30 dB/octave. One continuum had a constant cutoff frequency of 500 Hz, while the other continuum had a cutoff frequency between the second harmonic (H2) and the third harmonic (H3). Ten listeners judged the degree of breathiness using a 7-point rating scale. Preliminary results suggest that spectral slope has minimal effect on the perception of breathiness in synthetic vowels. [Research supported by NIH/R21 DC006690.]

The X-ray monitoring of quasar 3C273

XMM-Newton has observed the quasar 3C273 several times between 2000 June and 2005 July. In addition data of NASA's Rossi X-ray Timing Explorer is used. We present the results of this monitoring campaign.The X-ray spectrum of 3C273, in the energy regime of 0.2–100 keV, is believed to be composed of three components: a power law component above about 3 keV, a soft excess below about 2 keV, and an Fe-line. The Fe-line, detected in only a few observations of 3C273, is not detected significantly in individual XMM-observations. Only the 2000 June observations show evidence for a presence of a broad line feature. The high energy component above 3 keV is well described by a single power law model with varying indices and fluxes. The steeper spectra show significantly higher flux levels. The soft excess component below 2 keV also shows variations in flux and spectral slope. Different models are tested to identify the soft excess to be thermal or non-thermal emission.

Long‐Term Spectral Variability of Seyfert Galaxies fromRossi X‐Ray Timing ExplorerColor‐Flux Diagrams

We present results from Rossi X-Ray Timing Explorer data obtained during a systematic monitoring program of four Seyfert galaxies (NGC 5548, NGC 5506, MCG -6-30-15, and NGC 4051). We studied the variability of three hardness ratios derived from the light curves in four energy bands (HR1, which describes the continuum variations, and HR2 and HR3, which are sensitive to the iron line and reflection component variations with respect to the continuum, respectively). All the objects show similar spectral variations in all ratios. In order to interpret the results we computed the hardness ratios corresponding to a simple spectral model of a power law plus iron line plus reflection component. In order to derive the model HR2 and HR3 colors, we considered two possibilities: (1) variations with constant line equivalent width and reflection parameter R (the case of a reflecting/reprocessing material that responds with a short delay to the continuum variability) and (2) variations with constant line and reflection flux (the case of a reprocessor that does not respond to the fast, intrinsic variations). The overall, mean observed trends can be explained by spectral slope variations (ΔΓ 0.2-0.3, and 1 for NGC 4051) and a constant-flux Fe line and reflection component, although the existence of a line component that is variable on short timescales cannot be excluded. Finally, we find that the data are not consistent with an increase of R with flux for individual sources, indicating that, as a single source varies, softer spectra do not correspond to larger R values.

Mineralogical interpretation of reflectance spectra of Eros from NEAR near‐infrared spectrometer low phase flyby

Abstract— High signal‐to‐noise near‐infrared spectrometer (NIS) spectra acquired during the low phase flyby of the near‐Earth asteroid rendezvous (NEAR) mission to 433 Eros are analyzed to determine mineral chemistry and proportions of mafic silicates across the asteroid's surface at 2.68 × 5.50 km spatial resolution. Spectral band parameters are derived, and compared with those of laboratory samples of known mineral composition, grain size distribution and terrestrial, meteoritic and lunar pyroxene spectral properties. The NIS derived band parameters are consistent with ordinary chondrite meteorites. We invoke the presence of a clinopyroxene component in the spectra, which is consistent with ordinary chondrite mineralogy and/or some degree of partial melting of ordinary chondritic material. Spectra measured across the surface of Eros can reveal small but real spectral variations. Most relative spectra are uniform to within 1–2%. Some areas suggest compositional variations of a few percent. Spectral slope variations of a few percent are seen indicating a non‐uniform distribution of materials affecting the slope parameter but with no resolved absorption bands. We find no correlation of slope with viewing geometry or compositional variation. The band parameter values do not consistently indicate a specific ordinary chondrite class but Eros is definitely undifferentiated with possible compositional variations of no more than 1–2%.

Evidence for Rapid Iron Kα Line Flux Variability in MCG −6‐30‐15

This paper employs direct spectral fitting of individual orbital data in order to measure rapid X-ray iron Kα line and continuum spectral slope variations in Seyfert 1 galaxies with unprecedented temporal resolution. Application of this technique to a long RXTE observation of MCG -6-30-15 indicates that the line flux does vary on short (1 day) timescales, but that these variations are not correlated with changes in the continuum flux or slope. These rapid variations indicate that the line does indeed originate close to the black hole, confirming predictions based on its very broad profile. However, the lack of a correlation with the continuum presents problems for models in which the line variations are driven by those in the continuum, modified only by light-travel time effects. Instead, it may be that the line responds according to a physical process with a different timescale, such as ionization instabilities in the disk, or perhaps that the geometry and physical picture is more complex than implied by the simplest disk-corona models. These data also indicate that the slope of the underlying power-law continuum (Γ) shows strong variability and is tightly correlated with the continuum flux in the sense that the spectrum steepens as the source brightens. All of these results have been checked with extensive simulations, which also indicated that a spurious correlation between Γ and Compton reflection fraction (R) will result if these quantities are measured from the same spectra. This casts serious doubts on previous claims of such a Γ-R correlation.

Turbulent behavior of a fine mesh (1/12°) numerical simulation of the North Atlantic

The geostrophic turbulent behavior of a fine mesh (1/12°) numerical simulation of the North Atlantic is discussed and compared to observations and coarser numerical simulations. The Miami Isopycnic Coordinate Ocean Model (MICOM) was configured for the North and Equatorial Atlantic (28°S–65°N) with a horizontal resolution of 1/12° (average mesh size on the order of 6 km) and 16 layers in the vertical. Such a set-up demands the latest in high performance computing. The unique aspects of this simulation are (a) a correct Gulf Stream separation from the North American coast and (b) high eddy activity. These results support the view that an inertial boundary layer (which results from the fine resolution) is an important factor in the separation process and that resolution of the first Rossby radius of deformation is necessary for a correct representation of baroclinic instabilities. Sea surface height variability spectra are computed from the model results and compared to observations and previous models, within the framework of the geostrophic turbulence theory. Length scales representative of the simulated eddy field compare well with observations based on altimeter data. Despite higher eddy activity, there is no significant geographic variation in spectral slope in the inertial ranges consistent with previous simulations with coarser resolution. Observations, on the other hand, show a flattening in the spectra derived from altimeter data in weakly energetic areas when compared to spectra from highly energetic areas of the Atlantic. The possible impact of mechanisms not represented in the model, such as high frequency and high wave number wind forcing, is then discussed.

Measurements of the Effects of Gravity Waves in the Middle Atmosphere Using Parametric Models of Density Fluctuations. Part I: Vertical Wavenumber and Temporal Spectra

Parametric models of spectral analysis offer several distinct advantages over statistical methods such as the correlogram analysis. These advantages include higher spectral resolution and the ability, in principle, to separate correlated (i.e., wave) behavior from noise-driven (i.e., turbulent) behavior in the measurements. Here parametric models are used to highlight the spatial and temporal intermittency of the gravity wave spectrum. In Part II of this series the spatial and temporal spectrum are used to calculate energy dissipation and the eddy diffusion coefficient. The spectra are computed from measurements of density fluctuations obtained using a large power-aperture product lidar during a 6-h period on 30 August 1994. It is shown that parametric models provide an excellent representation of the temporal and spatial data series. One difficulty of parametric models is selecting the model order, an analogous situation to determining the proper lag in the correlogram procedure or the window length in the periodogram method. Extensive experimentation has shown that the ratio of the data matrix eigenvalues to the photon noise eigenvalues is an excellent indicator for the selection of the model order. The underlying spectral form found using the parametric models is similar to the standard correlogram method, that is, nominal underlying spatial and temporal spectral slopes between −2 and −4 and −1.25 and −2, respectively, with variability outside this range. The spatial–temporal behavior of the spectra is highly variable with numerous intermittent and intense features rising well above the photon noise floor. The vertical wavenumber spectra on this night may show a variation of spectral slope with height; however, the slope is both extremely sensitive to the noise level of the data, steepening as the signal-to-noise level increases, and highly variable in time. The temporal spectra also show considerable variation with height, both in magnitude and slope.

X-ray spectral variability of NGC 4051

We present results from RXTE and EUVE observations of the bright, highly variable Seyfert 1 galaxy, NGC 4051. Spectral analysis of RXTE data obtained from monitoring observations and a ‘long look’ (∼ 75 ksec) shows: a) strong spectral slope variability in the medium energy X-ray band, b) low energy absorption which increases with decreasing source flux, c) an ∼ 6 keV iron line which remains effectively constant despite continuum flux changes in excess of factor 10. Simultaneous RXTE and EUVE observations confirm the slope variability and also imply the presence of a constant ‘soft-excess’. We discuss the implications of these results for models of iron line and soft-excess production in NGC 4051.

Multiwavelength monitoring of the BL Lacertae object PKS 2155-304. 3: Ground-based observations in 1991 November

We present ground-based observations of the BL Lac object PKS 2155-304 during 1991 November. These data were obtained as part of a large international campaign of observations spanning the electro-magnetic spectrum from the radio waves to the X-rays. The data presented here include radio and UBVRI fluxes, as well as optical polarimetry. The U to I data show the same behavior in all bands and that only upper limits to any lag can be deduced from the cross-correlation of the light curves. The spectral slope in the U-I domain remained constant on all epochs but 2. There is no correlation between changes in the spectral slope and large variations in the total or polarized flux. The radio flux variations did not follow the same pattern of variability as the optical and infrared fluxes. The polarized flux varied by a larger factor than the total flux. The variations of the polarized flux are poorly correlated with those of the total flux in the optical (and hence UV domain; see the accompanying paper by Edelson et al.) nor with those of the soft X-rays. We conclude that the variability of PKS 2155-304 in the optical and near-infrared spectral domains are easier to understand in the context of variable geometry or bulk Lorentz factor than of variable electron acceleration and cooling rates.

Weathered and unweathered surface spectra of rocks from cold deserts: Identification of weathering processes and remote sensing implications

Abstract Comparisons of the reflectance spectra of weathered and unweathered surfaces of rocks from the Canadian Shield reveal that significant spectral changes can be effected by even the low weathering rates prevalent in cold desert regions. These changes include variations in overall spectral slope, the appearance/disappearance of absorption bands, shifts in absorption band minima wavelength positions, and changes in band shape. The most important alteration processes involve the oxidation of ferrous to ferric iron and the formation of hydrated mineral species. Ferric iron absorption bands are most intense for the most iron-rich lithologies. This may have important implications for the remote sensing detection of iron-rich lithologies such as iron formations and ultramafics. Aluminium- and magnesium-rich lithologies give rise to A1-OH and Mg-OH absorption bands in weathered surface spectra. Dramatic spectral variations betwen interior and exterior spectra occur even when weathered surfaces are thin eno...

Discrimination using spectral slopes at frequencies up to 50 hz

Abstract The spectral characteristics of regional Pn waves recorded at the High Frequency Seismic Element (HFSE) at the NORESS array in southern Norway are investigated to determine if spectral slope can be used to discriminate between earthquakes and chemical explosions. Displacement spectra are calculated for 134 events in southern Norway that include 36 known earthquakes and 36 known chemical explosions. Events range in distance from 230 to 620 km, and most have signal above the noise at frequencies over 30 Hz. The average frequency at the peak signal-to-noise ratio (SNR) is 15.5 Hz. Spectral slopes are measured on a displacement spectrum calculated with a 2-sec window. The slopes range between ω−1.8 and ω−5.1, and there is no strong evidence that the slope steepens with distance for this data set. According to one theory, earthquakes should have steeper slopes than explosions, but when this data set is separated into earthquakes and explosions we do not see separation between sources with respect to the slope of the spectrum. Both source types have events with the same range of spectral slopes, and the mean slopes and standard deviations are similar. Spectra of events at the same locations are often similar. However, explosions from the Blåsjø Mine in western Norway show variability in the spectral slope. Since the path and distance to HFSE are the same for these events, the variability must be due to differences in the source mechanism. For a set of small earthquakes, however, the observed variability in spectral slopes appears to be more related to distance and path effects. Therefore, we observe as much variation in spectral slope for one source type as there is between source types. The time window over which the spectrum is calculated is varied from 1 to 8 sec to determine if a time window consisting of a pure Pn wave was required to discriminate between source types. Spectral slope usually decreases as the window extends deeper into the coda and the energy content of the signal changes. The slopes also become more stable and more reliable as longer windows are used. However, source discrimination is not possible with any time window.

Observations and interpretations of the pulsed emission from the Crab pulsar

HEAO 1 observations of the 18-200 keV pulsed emission from the Crab pulsar show that the systematic variation of spectral slope as a function of pulse shape rises continuously from 4-60 keV to at least 150 keV. This strengthens the claims for the existence of both the emission from the two main pulse peaks, and a separate, interpulse emission connecting the two peaks which contributes 22% of the total pulsed flux in the 15-200 keV range and has a harder spectrum than that of the two pulse peaks. These observations are combined with others from IR to gamma-ray energies. It is concluded that the phase-averaged pulsed spectrum, dominated by the emission from the two pulse peaks, calls for at least three power law components, and that the interpulse emission has a spectrum which is better fitted by either a thin, thermal bremsstrahlung model or a Comptonization model.

X-ray and radio observations of GX 17+2 and GX 13+1

X-ray data on the sources 3U 1813-14 (GX 17+2) and 3U 1811-17 (GX 13+1), obtained by Copernicus between 1975 and 1977, are examined. These reveal correlated intensity and spectral-slope variations in GX 17+2 similar to those seen in Sco X-1 and four other galactic sources, 3U 0614+09, 3U 1702-36, 3U 1728-16, and 3U 1758-25. The data on GX 17+2 have been unsuccessfully searched for further evidence of a shallow 31.9-minute modulation. In the case of GX 13+1 no significant variability was seen in either intensity or spectrum. A simultaneous search made from the NRAO simultaneously with some of the X-ray observations revealed no significant radio emission from either source.

Spectral Association of the 7 August 1972 Solar Radio Burst with Particle Acceleration

(Solar Phys.). Microwave burst data from the August 7, 1972 event recorded on nine discrete frequencies between 245 and 35000 MHz at the Sagamore Hill Radio Observatory (Figure 1) provide a basis for correlation studies (especially timing information) of the associated white light flare, the high energy particle emission, type II bursts, and many other phenomena. This is perhaps the first time that sufficient radio coverage (i.e., data above 10000 MHz) was available to obtain the spectral slope information (related to electron-energy distribution) which is inherent in this part of the spectrum. Heretofore, timing was related to burst flux density profile variations. Improved correlations resulted from shorter centimeter wavelength data which supplied more accurate timing information than that derived from Hα observations. The shape and intensity of the burst peak flux density spectrum has also been used for qualitative analysis of energetic particle and white light events. The ultimate good may possibly come from spectral analysis of the minute by minute variation of the burst microwave radiation spectral slope α (above fmax) in the area between 15000 and 35000 MHz. This may be used alone or in relation to the position of fmax, where fmax is the frequency of burst maximum emission at a given time. This is basically our present investigation.