Continuum Fitting Research Articles

Measuring the spins of accreting black holes

A typical galaxy is thought to contain tens of millions of stellar-mass black holes, the collapsed remnants of once massive stars, and a single nuclear supermassive black hole. Both classes of black holes accrete gas from their environments. The accreting gas forms a flattened orbiting structure known as an accretion disk. During the past several years, it has become possible to obtain measurements of the spins of the two classes of black holes by modeling the x-ray emission from their accretion disks. Two methods are employed, both of which depend upon identifying the inner radius of the accretion disk with the innermost stable circular orbit, whose radius depends only on the mass and spin of the black hole. In the Fe Kα method, which applies to both classes of black holes, one models the profile of the relativistically broadened iron line with a special focus on the gravitationally redshifted red wing of the line. In the continuum-fitting (CF) method, which has so far only been applied to stellar-mass black holes, one models the thermal x-ray continuum spectrum of the accretion disk. We discuss both methods, with a strong emphasis on the CF method and its application to stellar-mass black holes. Spin results for eight stellar-mass black holes are summarized. These data are used to argue that the high spins of at least some of these black holes are natal, and that the presence or absence of relativistic jets in accreting black holes is not entirely determined by the spin of the black hole.

Measuring black hole spin by the continuum-fitting method: effect of deviations from the Novikov-Thorne disc model

The X-ray spectra of accretion discs of eight stellar-mass black holes have been analyzed to date using the thermal continuum fitting method, and the spectral fits have been used to estimate the spin parameters of the black holes. However, the underlying model used in this method of estimating spin is the general relativistic thin-disc model of Novikov & Thorne, which is only valid for razor-thin discs. We therefore expect errors in the measured values of spin due to inadequacies in the theoretical model. We investigate this issue by computing spectra of numerically calculated models of thin accretion discs around black holes, obtained via three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations. We apply the continuum fitting method to these computed spectra to estimate the black hole spins and check how closely the values match the actual spin used in the GRMHD simulations. We find that the error in the dimensionless spin parameter is up to about 0.2 for a non-spinning black hole, depending on the inclination. For black holes with spins of 0.7, 0.9 and 0.98, the errors are up to about 0.1, 0.03 and 0.01 respectively. These errors are comparable to or smaller than those arising from current levels of observational uncertainty. Furthermore, we estimate that the GRMHD simulated discs from which these error estimates are obtained correspond to effective disc luminosities of about 0.4-0.7 Eddington, and that the errors will be smaller for discs with luminosities of 0.3 Eddington or less, which are used in the continuum-fitting method. We thus conclude that use of the Novikov-Thorne thin-disc model does not presently limit the accuracy of the continuum-fitting method of measuring black hole spin.

Determination of the mean Hi absorption of the intergalactic medium

In recent years, the Lyman-α forest in quasar spectra has been used, together with N-body simulations, to determine the underlying matter distribution in the intergalactic medium (IGM). One of the key parameters to be known in order to compare observations and numerical simulations is the mean HI absorption in the IGM. To derive the latter, one has first to fit the quasar continuum. We have observed 20 high redshift and highly luminous QSOs (m V ≤ 17.5 and 2.40 ≤ z em ≤ 3.91) at intermediate spectral resolution, with either EMMI (ESO Multi-Mode Instrument) on the ESO-NTT telescope or CARELEC at the OHP (Observatoire de Haute-Provence), and applied different methods of determining the QSO continuum to this QSO sample. We have measured the amount of absorption, known as the flux decrement, DA, in the Lyman-α forest for these different methods and compared the results. In addition, we have compared DA values measured along the same lines of sight observed at high and intermediate spectral resolutions. We discuss the systematics resulting from the use of automatic continuum fitting methods.

Disk‐dominated States of 4U 1957+11:Chandra,XMM‐Newton, andRXTEObservations of Ostensibly the Most Rapidly Spinning Galactic Black Hole

We present simultaneous Chandra High-Energy Transmission Gratings (HETG) and Rossi X-ray Timing Explorer (RXTE) observations of a soft state of the black hole candidate 4U 1957+11. These spectra, having limited hard X-ray excess, are an excellent test of disk atmosphere models that include effects of black hole spin. The HETG data show, by modeling the broadband continuum and direct fitting of absorption edges, that the disk spectrum is only very mildly absorbed, with NH = (1–2) × 1021 cm−2. These data additionally reveal λλ13.449 Ne IX absorption consistent with the warm/hot phase of the interstellar medium. The fitted disk model implies an inclined disk around a low-mass black hole rotating with normalized spin a* ≈ 1. We show, however, that pure Schwarzschild models describe the data extremely well, albeit with large disk atmosphere ``color-correction'' factors. Standard correction factors can be attained if one incorporates mild Comptonization. We find that the Chandra observations do not uniquely determine spin, even with this otherwise extremely well-measured, nearly pure disk spectrum. XMM-Newton RXTE observations, taken only six weeks later, are equally unconstraining. This lack of constraint is partly driven by the unknown mass and distance of 4U 1957+11; however, it is also driven by the limited Chandra and XMM-Newton bandpasses. We therefore present a series of 48 RXTE observations taken at different brightness/hardness levels. These data prefer a spin of a* ≈ 1, even when including a mild Comptonization component; however, they also show evolution of the color-correction factors. If the rapid-spin models with standard correction factors are to be believed, then the RXTE observations predict that 4U 1957+11 can range from a 3 M☉ black hole at 10 kpc with a* ≈ 0.83 to a 16 M☉ black hole at 22 kpc with a* ≈ 1.

An improved measurement of the flux distribution of the Ly forest in QSO absorption spectra: the effect of continuum fitting, metal contamination and noise properties

We have performed an extensive Voigt profile analysis of the neutral hydrogen (H i) and metal absorption present in a sample of 18 high-resolution, high signal-to-noise ratio quasi-stellar object (QSO) spectra observed with the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph. We use this analysis to separate the metal contribution from the H absorption and present an improved measurement of the flux probability distribution function (PDF) due to H i absorption alone at (z) = 2.07,2.52 and 2.94. The flux PDF is sensitive to the continuum fit in the normalized flux range 0.8 < F < 1.0 and to metal absorption at 0.2 < F < 0.8. Our new measurements of the flux PDF due to H i absorption alone are systematically lower at 0.2 < F < 0.8 by up to 30 per cent compared to the widely used measurement of McDonald et al., based on a significantly smaller sample of Keck High Resolution Echelle Spectrometer data. This discrepancy is probably due to a combination of our improved removal of the metal absorption and cosmic variance, since variations in the flux PDF between different lines-of-sight are large. The H effective optical depth τ eff Ht at 1.7 < z < 4 is best fit with a single power law, τ eff HI = (0.0023 ± 0.0007)(1 + z) 365±0.21 , in good agreement with previous measurements from comparable data. As also found previously, the effect of noise on the flux distribution is not significant in high-resolution, high signal-to-noise ratio data.

Effects of magnetic coupling on radiation from accretion disc around a Kerr black hole

The effects of magnetic coupling (MC) process on the inner edge of the disc are discussed in detail. It is shown that the inner edge can deviate from the innermost stable circular orbit (ISCO) due to the magnetic transfer of energy and angular momentum between a Kerr black hole (BH) and its surrounding accretion disc. It turns out that the inner edge could move inward and outward for the BH spin $a_{*}$ being greater and less than 0.3594, respectively. The MC effects on disc radiation are discussed based on the displaced inner edge. A very steep emissivity can be provided by the MC process, which is consistent with the observation of MCG-6-30-15. In addition, the BH spins of GRO J1655-40 and GRS 1915+105 are detected by X-ray continuum fitting based on this model.

The probability distribution function of the Lyman transmitted flux from a sample of Sloan Digital Sky Survey quasars

We present a measurement of the probability distribution function (PDF) of the transmitted flux in the Lyman a (Lya) forest from a sample of 3492 quasars included in the Sloan Digital Sky Survey data release 3 (SDSS DR3). Our intention is to investigate the sensitivity of the Lya flux PDF as measured from low-resolution and low signal-to-noise ratio data to a number of systematic errors such as uncertainties in the mean flux, continuum and noise estimate. . The quasar continuum is described by the superposition of a power law and emission lines. We perform a power-law continuum fitting on a spectrum-by-spectrum basis, and obtain an average continuum slope of α v = 0.59 ± 0.36 in the redshift range 2.5 < z < 3.5. We take into account that the variation in the continuum indices increases the mean flux by 3 and 7 per cent at z = 3 and 2.4, respectively, as compared to the values inferred with a single (mean) continuum slope. We compare our measurements to the PDF obtained with mock lognormal spectra, whose statistical properties have been constrained to match the observed Lya flux PDF and power spectrum of high-resolution data. Using our power-law continuum fitting and the SDSS pipeline noise estimate yields a poor agreement between the observed and mock PDFs. Allowing for a break in the continuum slope and, more importantly, for residual scatter in the continuum level substantially improves the agreement. A decrease of ∼10-15 per cent in the mean quasar continuum with a typical rms variance at the 20 per cent level can account for the data, provided that the noise excess correction is no larger than ≤10 per cent.

Thermal emission spectroscopy (5.2–38 μm) of three Trojan asteroids with the Spitzer Space Telescope: Detection of fine-grained silicates

We present thermal emission spectra (5.2–38 μm) of the Trojan asteroids 624 Hektor, 911 Agamemnon, and 1172 Aneas. The observations used the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. Emissivity spectra are created by dividing the measured Spectral Energy Distribution (SED) by a model of the thermal continuum. We employ the Standard Thermal Model (STM), allowing physical parameters (e.g., radius and albedo) to vary in order to find the best thermal continuum fit to the SED. The best-fit effective radius ( R) and visible geometric albedo ( p v ) for Hektor ( R = 110.0 ± 7.3 , p v = 0.038 −0.017 +0.028 ) and Aneas ( R = 69.1 ± 5.1 , p v = 0.044 −0.014 +0.020 ) agree very well with previous estimates, and for Agamemnon ( R = 71.5 ± 5.2 , p v = 0.062 −0.019 +0.024 ) we find slightly a smaller size and higher albedo than previously derived. Other thermal models (e.g., thermophysical) result in estimates of R and p v that vary a few percent from the STM, but the resulting emissivity spectra are identical. The emissivity spectra of all three asteroids display an emissivity plateau near 10-μm and another broader rise from ∼18 to 28 μm. We interpret these as indications of fine-grained silicates on the surfaces of these asteroids. The emissivity spectra more closely resemble emission spectra from cometary comae than those from solid surfaces and measured in the laboratory for powdered meteorites and regolith analogs. We hypothesize that the coma-like emission from the solid surfaces of trojans may be due to small silicate grains being imbedded in a relatively transparent matrix, or to a very under-dense (fairy-castle) surface structure. These hypotheses need to be tested by further laboratory and theoretical scattering work as well as continued thermal emission observations of asteroids.

The Properties of Intergalactic Civand SiivAbsorption. I. Optimal Analysis of an Extremely High Signal-to-Noise Quasar Sample

We have analyzed the properties of metals in the high-redshift intergalactic medium using a novel objective pixel optical depth technique on a sample of extremely high signal-to-noise ratio Keck HIRES and ESI spectra of 26 quasars between redshifts 2.1 and 6.4. The technique relies on using the doublet nature of the common ions C IV and Si IV, which are the principal metal tracers in the intergalactic medium outside of the Lyα forest. Optical depths are statistically corrected for contamination by other lines, telluric absorption, bad pixels, continuum fitting, etc. and for incompleteness, and we achieve in this way an increased sensitivity of approximately 0.5 dex over previous analyses. As with existing pixel optical depth analyses, the method is completely objective and avoids subjective cloud selection and Voigt profile fitting, but, unlike existing techniques, we do not compare the ion optical depths with H I optical depths to determine the ion optical depth distributions; we therefore avoid problems arising from different velocity widths in the ion and H I. We have shown how the conventional analysis can be reproduced using a percolation method to generate pseudoclouds from ion optical depths. Using this set of pseudoclouds, we have generated C IV column density distributions and have confirmed that the shape of the C IV column density distribution remains essentially invariant, with slope -1.44, from z = 1.5 to 5.5. This in turn confirms the lack of redshift evolution of Ω(C IV) for z = 2–5, both for all absorbers with column density log N = 12–15 and for stronger absorbers with log N = 13–14. The generation of pseudoclouds from the optical depth vectors also gives information on the column density environment of a given optical depth. We find that for the higher resolution HIRES data there is a tight relation, τ ∼ N0.7, between the peak optical depth and the column density. We have then analyzed the ion redshift evolution directly and model-independently from the optical depth vectors themselves and show that there is little evolution in the total amount of C IV from z = 2 to 5, although there is a turndown of at least a factor of 2 in Ω(C IV) above z = 5. We do, however, see substantial evolution in the ratio Si IV/C IV. In two subsequent papers in this series, we will use this technique to investigate what fraction of the absorbers lie in galactic wind outflows (Paper II) and what metallicity is associated with regions of τ(Lyα) < 1 (Paper III).

The Far Ultraviolet Spectroscopic Explorer Survey of O vi Absorption in and near the Galaxy

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the O VI λλ1031.926, 1037.617 absorption lines associated with gas in and near the Milky Way, as detected in the spectra of a sample of 100 extragalactic targets and two distant halo stars. We combine data from several FUSE Science Team programs with guest observer data that were public before 2002 May 1. The sight lines cover most of the sky above Galactic latitude |b| > 25°—at lower latitude the ultraviolet extinction is usually too large for extragalactic observations. We describe the details of the calibration, alignment in velocity, continuum fitting, and manner in which several contaminants were removed—Galactic H2, absorption intrinsic to the background target and intergalactic Lyβ lines. This decontamination was done very carefully, and in several sight lines very subtle problems were found. We searched for O VI absorption in the velocity range -1200 to 1200 km s-1. With a few exceptions, we only find O VI in the velocity range -400 to 400 km s-1; the exceptions may be intergalactic O VI. In this paper we analyze the O VI associated with the Milky Way (and possibly with the Local Group). We discuss the separation of the observed O VI absorption into components associated with the Milky Way halo and components at high velocity, which are probably located in the neighborhood of the Milky Way. We describe the measurements of equivalent width and column density, and we analyze the different contributions to the errors. We conclude that low-velocity Galactic O VI absorption occurs along all sight lines—the few nondetections only occur in noisy spectra. We further show that high-velocity O VI is very common, having equivalent width >65 mA in 50% of the sight lines and equivalent width >30 mA in 70% of the high-quality sight lines. The central velocities of high-velocity O VI components range from |vLSR| = 100 to 330 km s-1; there is no correlation between velocity and absorption strength. We discuss the possibilities for studying O VI absorption associated with Local Group galaxies and conclude that O VI is probably detected in M31 and M33. We limit the extent of an O VI halo around M33 to be 200 km s-1 occurs along all sight lines in the region l = 180°-300°, b > 20°.

Pattern‐Recognition Methods in Automatic Techniques for Astronomical Spectral Analysis

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) is developing rapidly in China, and it will yield a very large number of fiber spectra in the near future. To cope with the anticipated flood of spectrographic data, automated methods for reducing the spectra, measuring spectral features, and classifying them are being investigated. First, this dissertation describes the present status of automated methods for analyzing astronomical spectra in other spectral surveys, including principal component analysis (PCA), artificial neural network (ANN), wavelets, and other methods. For example, B. C. Bromley et al. (1998, ApJ, 505, 25) applied PCA for spectra from the Las Campanas Redshift Survey, S. R. Folkes et al. (1999, MNRAS, 308, 459) and S. R. Folkes, O. Lahav, & S. J. Maddox (1998, ApJ, 492, 98) classified spectra from the 2dF Galaxy Redshift Survey using PCA and ANN, and F. J. Castander et al. (2001, AJ, 121, 2331) applied PCA and wavelets in Coma data obtained by the Sloan Digital Sky Survey. One of the most difficult problems for LAMOST is that automated spectral processing must carry out both classification and redshift determinations, so simple independent pipelines will not work. For this thesis, the author has built a pattern-recognition–based automated frame for processing spectral data. According to the widely used pattern-recognition process, the work involves three aspects: preprocessing, feature extraction, and classification determination. The preprocessing step is to fit the spectral continuum. Feature extraction is the key step in the pattern-recognition system, including measuring and identifying spectral features such as lines and the 4000 A break and computing redshifts. Finally, automated classification should make use of the information from spectral lines. A.-L. Luo & Y.-H. Zhao (2000, Acta Astrophys. Sinica, 20, 427; 2001, Chinese J. Astron. Astrophys., 1, 563) described a continuum-fitting method that has been used in this dissertation. Instead of using a fixed wavelet scale (J.-L. Starck, R. Siebenmorgen, & R. Gredel 1997, ApJ, 482, 1011), the author uses a wavelet filter bank to subtract the continuum in a spectrum. A. Luo & Y. Zhao (2001, Spectrosc. Spectral Anal., 21, 19) suggested steps for measuring spectral features and computing redshifts. A multiresolution feature-extraction method is introduced in this dissertation. Generally, automated classification needs blueshifted spectra, classifying them in a space such as PCA space. K. Glazebrook, A. R. Offer, & K. Deeley (1998, ApJ, 492, 98) developed an automated redshift determination method called PCAZ. The author has corrected some errors inherent in this method and has presented an improved approach. The author has also applied statistical learning theory based on the Support Vector Machine to accomplish the classification and search for strange objects. When the signal-to-noise ratio (S/N) is too low, noise will cause decreasing accuracy in the classification. The noise is not Gaussian and varies with wavelength. The results of continuum fitting, feature extraction, and classification depend heavily on estimating the noise. In this dissertation, the author has applied the wavelet-based hidden Marcov model (HMM) to estimate the noise distribution in low S/N spectra (A. Luo & Y. Zhao 2001, Nuovo Cimento B, 116, 879) and has improved M. S. Crouse et al.’s (1998, IEEE Trans. Signal Process., 46, 886) tying method to increase the training data to make the HMM more robust and to estimate the noise more precisely.

Comparison of all atom, continuum, and linear fitting empirical models for charge screening effect of aqueous medium surrounding a protein molecule

To investigate the role hydration plays in the electrostatic interactions of proteins, the time-averaged electrostatic potential of the B1 domain of protein G in an aqueous solution was calculated with full atomic molecular dynamics simulations that explicitly considers every atom (i.e., an all atom model). This all atom calculated potential was compared with the potential obtained from an electrostatic continuum model calculation. In both cases, the charge-screening effect was fairly well formulated with an effective relative dielectric constant which increased linearly with increasing charge–charge distance. This simulated linear dependence agrees with the experimentally determined linear relation proposed by Pickersgill. Cut-off approximations for Coulomb interactions failed to reproduce this linear relation. Correlation between the all atom model and the continuum models was found to be better than the respective correlation calculated for linear fitting to the two models. This confirms that the continuum model is better at treating the complicated shapes of protein conformations than the simple linear fitting empirical model. We have tried a sigmoid fitting empirical model in addition to the linear one. When weights of all data were treated equally, the sigmoid model, which requires two fitting parameters, fits results of both the all atom and the continuum models less accurately than the linear model which requires only one fitting parameter. When potential values are chosen as weighting factors, the fitting error of the sigmoid model became smaller, and the slope of both linear fitting curves became smaller. This suggests the screening effect of an aqueous medium within a short range, where potential values are relatively large, is smaller than that expected from the linear fitting curve whose slope is almost 4. To investigate the linear increase of the effective relative dielectric constant, the Poisson equation of a low-dielectric sphere in a high-dielectric medium was solved and charges distributed near the molecular surface were indicated as leading to the apparent linearity.

Probing the intergalactic medium with the Lyα forest along multiple lines of sight to distant QSOs

We present an effective implementation of analytical calculations of the Ly� opacity distribution of the Intergalactic Medium (IGM) along multiple lines of sight (LOS) to distant quasars in a cosmological setting. The method assumes that the distribution of neutral hydrogen follows that of an underlying dark matter density field and that the density distribution is a (local) lognormal distribution. It fully accounts for the expected correlations between LOS and the cosmic variance in the large-scale modes of the dark matter distribution. Strong correlations extending up to � 300 kpc (proper) and more are found at redshift z � 2 ! 3, in agreement with observations. These correlations are investigated using the cross-correlation coefficient and the cross-power spectrum of the flux distribution along different LOS and by identifying coincident absorption features as fitted with a Voigt profile fitting routine. The cross-correlation coefficient between the LOS can be used to constrain the shape-parameter of the power spect rum if the temperature and the temperature density relation of the IGM can be determined indepedently. We also propose a new technique to recover the 3D linear dark matter power spectrum by integrating over 1D flux cross-spectra which is complementary to the usual ‘differentiation’ of 1D auto-spectra. The cross-power spectrum suffers much less from errors uncorrelated in different LOS, like those introduced by continuum fitting. Investigations of the flux correlations in adjacent LOS should thus allow to extend studies of the

Steps Towards a Fully Automated Classification and Redshift-measurement Pipeline for LAMOST Spectra. I. Continuum level and wavelength estimation for galaxies

The Large Sky-Area Multi-Object Spectroscopic Telescope (LAMOST) under construction by the National Astronomical Observatories will yield up to four thousand multi-fiber spectra of stars and galaxies per field. The present series of papers describes the automated data-reduction pipeline currently being designed in order to cope with the anticipated flood of spectrographic data. In this preliminary paper, we present an automated method for estimating the continuum level, the positions of strong lines and the 4000Å break in galaxy spectra. In order to obtain detailed information on the continuum, we use a wavelet filter bank. After continuum fitting, our software searches for a 4000Å break and distinguishes between emission-line galaxies (ELGs)and non-ELGs according to whether the break is small or large. It then searches for strong lines and measures the intensities of emission lines and the equivalent widths of absorption lines. For non-ELGs, the absorption lines are identified automatically yielding redshift measurements.

On the Normalization of the QSO Lyα Forest Power Spectrum

The calculation of the transmission power spectrum of QSO Lyα absorption requires two parameters for the normalization: the continuum Fc and mean transmission -τ. Traditionally, the continuum is obtained by a polynomial fitting truncating it at a lower order, and the mean transmission is calculated over the entire wavelength range considered. The flux F is then normalized by Fc-τ. However, the fluctuations in the transmitted flux are significantly correlated with the local background flux on scales for which the field is intermittent. As a consequence, the normalization of the entire power spectrum by an overall mean transmission -τ will overlook the effect of the fluctuation-background correlation upon the powers. In this paper we develop a self-normalization algorithm of the transmission power spectrum based on a multiresolution analysis. This self-normalized power spectrum estimator needs neither a continuum fitting nor a predetermining of the mean transmission. With simulated samples, we show that the self-normalization algorithm can perfectly recover the transmission power spectrum from the flux regardless of how the continuum varies with wavelength. We also show that the self-normalized power spectrum is also properly normalized by the mean transmission. Moreover, this power spectrum estimator is sensitive to the nonlinear behavior of the field. That is, the self-normalized power spectrum estimator can distinguish between fields with or without the fluctuation-background correlation. This cannot be accomplished by the power spectrum with the normalization by an overall mean transmission. Applying this analysis to a real data set of Q1700+642 Lyα forest, we demonstrate that the proposed power spectrum estimator can perform correct normalization and effectively reveal the correlation between the fluctuations and background of the transmitted flux on small scales. Therefore, the self-normalized power spectrum would be useful for the discrimination among models without the uncertainties caused by free (or fitting) parameters.

On Estimating the QSO Transmission Power Spectrum

The Lyα forest has become an important tool for measuring the mass power spectrum at high redshifts (z = 2-4). A crucial intermediate step is the measurement of the transmission power spectrum. We present new methods to minimize the systematic and random errors for such a measurement and discuss their implications for observing strategies. Sources of systematic errors explored include metal line contamination and continuum fitting. We advocate the technique of trend removal in place of traditional continuum fitting: here a spectrum is normalized by its (smoothly varying) mean rather than its continuum; this method is easily automated and removes biases introduced by continuum fitting. Moreover, trend removal can be easily applied to spectra where continuum fitting is difficult, such as when the resolution or signal-to-noise ratio (S/N) is low, or for spectra at high redshifts. We further show that a measurement of the continuum power spectrum (plus a related quantity) using trend removal, from either low-redshift quasar spectra or the red side of Lyα, can be used to constrain the amount of spurious large-scale power introduced by the uncertain continuum and in principle allows the removal of such contamination and thereby expanding scales probed to larger ones. We also derive expressions for the shot noise bias and variance of the power spectrum estimate, taking into account the non-Poissonian nature of the shot noise and the non-Gaussianity of the cosmic fluctuations. An appropriate minimum variance weighting of the data is given. Finally, we give practical suggestions on observing strategy: the desired resolution and S/N for different purposes and instruments, as well as how to distribute one's finite observing time among quasar targets. Also discussed is the quasar spectroscopic study of the Sloan Digital Sky Survey (SDSS), which has the potential to measure the power spectrum accurate to better than 1% per mode (Δk ~ 10^(-4) s km^(-1)). The techniques presented here will be useful for tackling the anticipated issues of shot noise and continuum contamination.

ITAL]ROSAT[/ITAL] HRI and [ITAL]ASCA[/ITAL] Observations of the Spiral Galaxy NGC 6946 and Its Northeast Complex of Luminous Supernova Remnants

Analysis of 80 ks ASCA (Advanced Satellite for Cosmology and Astrophysics) and 60 ks ROSAT HRI (High Resolution Image) observations of the face-on spiral galaxy NGC 6946 are presented. The ASCA image is the first observation of this galaxy above approximately 2 keV. Diffuse emission may be present in the inner approximately 4' extending to energies above approximately 2-3 keV. In the HRI data, 14 pointlike sources are detected, the brightest two being a source very close to the nucleus and a source to the northeast that corresponds to a luminous complex of interacting supernova remnants (SNRs). We detect a point source that lies approximately 30 west of the SNR complex but with a luminosity -1115 of the SNR complex. None of the point sources show evidence of strong variability; weak variability would escape our detection. The ASCA spectrum of the SNR complex shows evidence for an emission line at approximately 0.9 keV that could be either Ne IX at approximately 0.915 keV or a blend of ion stages of Fe L-shell emission if the continuum is fitted with a power law. However, a two-component, Raymond-Smith thermal spectrum with no lines gives an equally valid continuum fit and may be more physically plausible given the observed spectrum below 3 keV. Adopting this latter model, we derive a density for the SNR complex of 10-35 cm(exp -3), consistent with estimates inferred from optical emission-line ratios. The complex's extraordinary X-ray luminosity may be related more to the high density of the surrounding medium than to a small but intense interaction region where two of the complex's SNRs are apparently colliding.

Multiwavelength Observations of GX 339−4 in 1996. I. Daily Light Curves and X‐Ray and Gamma‐Ray Spectroscopy

As part of our multiwavelength campaign of GX 339-4 observations in 1996 we present our radio, X-ray, and gamma-ray observations made in July, when the source was in a hard state (= soft X-ray low state). The radio observations were made at the time when there was a possible radio jet. We show that the radio spectrum was flat and significantly variable, and that the radio spectral shape and amplitude at this time were not anomalous for this source. Daily light curves from our pointed observation July 9-23 using OSSE, from BATSE, and from the ASM on RXTE also show that there was no significant change in the X- and gamma-ray flux or hardness during the time the possible radio jet-like feature was seen. The higher energy portion of our pointed RXTE observation made July 26 can be equally well fit using simple power law times exponential (PLE) and Sunyaev-Titarchuk (ST) functions. An additional soft component is required, as well as a broad emission feature centered on 6.4 keV. This may be an iron line that is broadened by orbital Doppler motions and/or scattering off a hot medium. Its equivalent width is 600 eV. Our simplistic continuum fitting does not require an extra reflection component. Both a PLE and a ST model also fit our OSSE spectrum on its own. Although the observations are not quite simultaneous, combining the RXTE and CGRO spectra we find that the PLE model easily fits the joint spectrum. However, the ST model drops off too rapidly with increasing energies to give an acceptable joint fit.

Searching for Wolf‐Rayet Stars in I Zw 18: the Origin of HeiiEmission

I Zw 18 is the most metal-poor star-forming galaxy known and is an ideal laboratory to probe stellar evolution theory at low metallicities. Using archival Hubble Space Telescope WFPC2 imaging and Faint Object Spectrograph spectroscopy, we were able to improve previous studies. We constructed a continuum-free He II λ4686 map, which was used to identify Wolf-Rayet (W-R) stars recently found by ground-based spectroscopy and to locate diffuse nebular emission. Most of the He II λ4686 emission is associated with the northwest stellar cluster clearly displaced from the surrounding shell-like [O III] and Hα emission. We found evidence for He II sources, compatible with five to nine WNL stars and/or compact nebular He II λ4686 emission, as well as residual diffuse emission. Only one of them is outside the northwest cluster. We have done an extensive comparison between our results and the recent ground-based data used by Izotov et al. and Legrand et al. to identify WN and WC stars in I Zw 18. The differences between the various data may be understood in terms of varying slit locations, continuum fits, and contamination by nebular lines. We have calculated evolutionary tracks for massive stars and synthesis models at the appropriate metallicity (Z ≈ 0.02 Z☉). These single-star models predict a mass limit MWR ≈ 90 M☉ for W-R stars to become WN and WC/WO. For an instantaneous burst model with a Salpeter initial mass function extending up to Mup ≈ 120-150 M☉, our model predictions are in reasonable agreement with the observed equivalent widths. Our model is also able to fully reproduce the observed equivalent widths of nebular He II λ4686 emission due to the presence of WC/WO stars. This quantitative agreement and the spatial correlation of nebular He II λ4686 with the stellar cluster and the position of W-R stars shown from the ground-based spectra further supports the hypothesis that W-R stars are responsible for nebular He II emission in extragalactic H II regions.

Gamma-ray spectral analysis with the COSPAJ continuum fitting routine

We present a method to fit γ-ray full-energy peaks. Instead of performing a local fit to a limited number of channels around full-energy peaks simultaneously to the peak fit, it uses a spectral continuum fitting program, COSPAJ, to determine the peak base-line independently to the peak fitting. Full-energy peaks are described by the sum of two exponential distributions convoluted with a Gaussian. The method produces consistent results, accurate determinations of peak areas and it induces relative area uncertainties down to 0.02%.