Engineering topics

Schmidt, Gary D.

Publications and source records attributed to Schmidt, Gary D..

At least 19 records

NASA NTRS2005

X-Ray Absorbed, Broad-Lined, Red AGN and the Cosmic X-ray Background

This award represents the second phase of an X-ray study of QSOs that are heavily obscured in the optical/near-IR. An earlier survey revealed that these active galactic nuclei (AGN) are also typically strongly absorbed at high photon energies, but the enhanced sensitivity of XMM-Newton provided for the first time the opportunity to measure the spectral indices of individual sources and to test the possibility that obscured AGN are responsible for a substantial portion of the cosmic X-ray background (CXRB). The new observations confirm and greatly extend the earlier results. Substantial intervening absorbing material is detected for 3 of the sources, and 2 additional targets show hard power-law continua. In addition, soft X-ray excesses are detected in 3 of sources, indicating the presence of extended regions of ionized gas. This component is-particularly well-defined in the Type 1 source 2MASS2344+1221, allowing analysis and modeling at a level of detail unusual for such distant sources. An important finding that parallels conclusions from optical polarimetry is the lack of dependence of absorption on optical spectral class (Type 1 or 2). The combination of strong polarization and X-ray absorption in sources that show strong, broad emission lines indicates either a very small (nuclear) absorber or a favored viewing angle which allows the X-ray source to be covered but the surrounding broad emission-line region to be largely exposed. The observed spectral also indicate that obscured AGN of both Type 1 and 2 contribute significantly to the CXRB at higher X-ray energies.

Schmidt, Gary D.↗

NASA NTRS1997

Optical Detection of the Hidden Nuclear Engine in NGC 4258

The subparsec masing disk recently found to be orbiting a central mass of approx. 3.6 x 10(exp 7) Ms in the Seyfert/LINER galaxy NGC 4258 (Miyoshi and coworkers) provides the most compelling evidence to date for the existence of a massive black hole in the nucleus of a galaxy. The disk is oriented nearly edge-on, and the X-ray spectrum is heavily absorbed. Therefore, in this galaxy, the optical emission-line spectrum generally exhibited by an active galactic nucleus is perhaps best sought using polarized light: probing for light scattered off material surrounding the central source. New polarimetry of NGC 4258 has uncovered a compact polarized nucleus whose spectrum consists of a faint blue continuum similar to those of unobscured quasars (F(sub upsilon) proportional to upsilon(sup -1.1)), plus broadened (approx. 1000 km/s) emission lines. The lines are strongly linearly polarized (5%-10%) at a position angle (85 deg +/- 2 deg) coincident with the plane of the maser disk. This result provides substantiating evidence for a weakly active central engine in NGC 4258 and for the existence of obscuring, orbiting tori, which impart many of the perceived distinctions between various types of active galaxies.

Wilkes, Belinda J.↗

NASA NTRS1995

On the nature of spectral features in peculiar DQ white dwarfs

Spectropolarimetric measurements are presented for the DQ white dwarfs ESO 439-162, LHS 1126, and G225-68, whose spectroscopic features in the optical have been interpreted in the past as pressure-shifted or magnetically shifted C2 Swan bands. The results convincingly demonstrate that none of these objects is strongly magnetic, with upper limits of 30, 3, and 2 MG respectively. Since Bergeron et al. (B 94) have recently ruled out the pressure-shift interpretation for LHS 1126 as well, we discuss alternative physical mechanisms for displacing the Swan bands. Although possibilities for explaining the observed shifts may exist, a comparison of the optical spectra (and that of a similar DQ star, LP 77-57) indicates that the locations and shapes of the profiles in all four objects are virtually identical. This last result suggests instead that a different molecular species could be responsible. A detailed chemical equilibrium analysis of H/He/C mixtures under the physical conditions encountered in the atmospheres of these peculiar objects reveals that C2H is a molecule preferentially formed in the photospheric regions. The nature and evolution of these objects are discussed.

Schmidt, Gary D.↗

NASA NTRS1995

The polarization and ultraviolet spectrum of Markarian 231

Ultraviolet spectropolarimetry acquired with the Hubble Space Telescope (HST) of the peculiar Seyfert galaxy Mrk 231 is combined with new high-quality ground-based measurements to provide the first, nearly complete, record of its linear polarization from 1575 to 7900 A. The accompanying ultraviolet spectrum portrays the heavily extinguished emission-line spectrum of the active nucleus plus the emergence of a blue continuum shortward of approximately 2400 A. In addition, absorption features due to He I lambda 3188, Mg I lambda 2853, Mg II lambda 2798, and especially several resonance multiplets of Fe II are identified with a well-known optical absorption system blueshifted approximately 4600 km/s with respect to emission lines. The continuum is attributed to approximately 10(exp 5) hot, young stars surrounding the nucleus. This component dilutes the polarized nuclear light, implying that the intrinsic polarization of the active galactic nucleus (AGN) spectrum approaches 20% at 2800 A. The rapid decline in degree of polarization toward longer wavelengths is best explained by the strongly frequency-dependent scattering cross section of dust grains coupled with modest starlight dilution. Peculiar S-shaped inflections in both the degree and position angle of polarization through H alpha and other major emission lines are interpreted as effects of scattering from two regions offset in velocity by several hundred km/s. A third source of (weakly) polarized flux is required to explain a nearly 40 deg rotation in position angle between 3200 and 1800 A. The displaced absorption features, polarimetry, and optical/infrared properties of Mrk 231 all point to its classification as a low-ionization, or Mg II broad absorption line quasar, in which most, if not all, lines of sight to the active nucleus are heavily obscured by dust and low-ionization gas clouds.

Smith, Paul S.↗

NASA NTRS1995

Detection of the hot white dwarf in the magnetic nova V1500 Cygni with the Hubble Space Telescope

A comprehensive investigation of the AM Herculis-like nova system V1500 Cyg is presented, featuring time-resolved spectrophotometry with the Faint Object Spectrograph (FOS) of the Hubble Space Telescope (HST), coeval optical circular spectropolarimetry and spectrophotometry, and new broadband polarimetry. A revised ephemeris for the rotating white dwarf refines our knowledge of the timescale for synchronization with the orbital period -- and for becoming a bona fide AM Herculis system -- to 170 +/- 8 yrs. Three features of the orbital-phase-dependent spectral energy distribution are keys for understanding the relative contributions of several radiation sources in this system: (1) the brightness and shape of the ultraviolet continuum, which rises abruptly at short wavelengths and shows no significant phase dependence for lambda less than 2200 A, (2) the greater than or equal to 1 mag variations in the optical continuum and emission-line fluxes, and (3) a spectral slope in the optical which is reddest when brightest. These data lead us to conclude that the hot, postnova white dwarf is the dominant source of ultraviolet flux and of the unusually large bolometric luminosity of the system, compared with typical active AM Herculis binaries. The flat mid-ultraviolet spectrum requires a radiation component in addition to the white dwarf photosphere. We consider three possible origins: (1) cyclotron emission, (2) relatively broad, optically thick accretion columns with lumps of dense gas, and/or (3) orbiting debris in a disklike geometry. Observational tests are suggested to distinguish among these alternatives.

Schmidt, Gary D.↗

NASA NTRS1994

Eclipse mapping of the accreting magnetic white dwarf in DP Leonis with HST

We present time-resolved ultraviolet spectrophotometry of the AM Her system DP Leo (E1114+182), obtained with the Faint Object Spectrograph on the Hubble Space Telescope (HST) in 1991 October. During this period, the binary was in a low-activity state. Complementary optical spectrophotometry and spectopolarimetry were obtained in 1991 October and December, as well as in 1992 April when the object had entered a high-accretion state. The HST spectrophotometry reveals two UV-emitting components-a hot 'spot' (T approximately 50,000 K, very approximately) near the magnetic pole into which most material is accreted, and the white dwarf photosphere (T approximately 16,000 K), which dominates when the spot is not in view. Both components appear as rapid and gradual components in the ingress and egress of secondary eclipse, the latter indicating a normal white dwarf radius of 0.8 x 10(exp 9) cm. The fractional area of the spot (f approximately 0.006) implied by the eclipse data is consistent with the observed UV flux and a minimum distance of 380 pc is similar to those found in other AM Her systems. The nearly identical eclipse light curves from the HST and earlier optical observations imply that the UV-emitting reprocessing area is not more extended than the cyclotron source of optical radiation in the accretion funnel. The timing of the onset of the bright phase indicated that the spot leads the secondary in orbital azimuth by approximately 3 deg.

Stockman, H. S.↗

NASA NTRS1994

The hidden nuclear spectrum of the luminous IRAS source FSC 10214+4724

Optical spectropolarimetry of the luminous IRAS source FSC 10214+4724 (z=2.286) reveals that the strong (approximately 17%) linear polarization detected by Lawrence et al. is shared by both the narrow ultraviolet (UV) emission lines and the underlying continuum. This observation and the brightness of the source rule out synchrotron emission and dichroic extinction by dust as the polarizing mechanism, leaving scattering as the only plausible cause of the polarized emission. The narrowness of the lines requires that the scatterers be dust grains or cool (below 1.6 x 10(exp 4) K) electrons. We can recover the spectrum that is incident on the scattering medium provided we make some reasonable assumptions regarding the source geometry. The scattered UV spectrum has a power-law index alpha of -1.2 plus or minus 0.6 (F(sub nu) varies as nu(sup alpha)), steeper than would be expected from a young burst of star formation but similar to that of many active galactic nucleus (AGNs).

Jannuzi, Buell T.↗

NASA NTRS1994

The synchrotron continuum of the highly polarized quasar PKS 1546+027

The wavelength dependence of optical linear polarization of the highly polarized quasar (HPQ) PKS 1546+027 is investigated for the first time. Both spectropolarimetry and broad-band filter polarimetry are presented for a period when the object was undergoing a strong ourburst in flux and polarization. The rapid polarization variations observed argues that an optically thin synchrotron-emitting region, similar in polarization properties to BL Lacertae objects and optically violent variable (OVV) quasars, is the major source of polarized flux from this object. The continuum polarization increases dramatically with wavelength and is about twice as high at 8000 A than at 4000 A. Spectropolarimetry reveals that the emission-line flux from PKS 1546+027 dilutes the continuum polarization and is probably unpolarized. There is no evidence for any variation in polarization position angle with wavelength. The polarized flux spectrum is well-represented by a steep power law with a spectral index of -2.2, supporting the view that the polarized flux has a nonthermal origin. The observed polarization properties of PKS 1546+027 are consistent with a model that has been proposed to explain the optical to ultraviolet continua of several OVVs. In this model the optical to near-UV spectral flux distribution is a combination of light from a polarized synchrotron component plus an unpolarized component with spectral properties typical of low-polarization quasars.

Smith, Paul S.↗

NASA NTRS1994

The incidence of magnetism among white dwarfs: The first stars below 100 kilogauss

A survey for magnetic fields among a magnitude-limited sample of DA white dwarfs has identified two stars with weak circular polarization features across the profiles of H(alpha) and H(beta) WD 1350-090 (LP 907-037) was found to have a disk-averaged longitudinal field component B(sub e) = +85 +/- 9 kG at one epoch, while WD 0009+501 (G 217-037) has been measured on several occasions at values between B(sub e) approximately 0 and nearly -100 kG. The latter results imply an oblique rotator with a period between 2 and 20 hr. Magnetism on white dwarfs has now been detected over more than four orders of magnitude in strength. Assuming flux conservation, the new discoveries imply organized field patterns near the end of the main-sequence phase of only approximately 10 G. However, the overall incidence of magnetism among white dwarfs remains low, with more than 90% of stars having fields below approximately 10 kG. There is tentative evidence from line profile analysis that WD 1350-090 is a high-mass object (M greater than 1 solar mass), but an accurate parallax and more thorough spectroscopic study are required.

Schmidt, Gary D.↗

NASA NTRS1993

Optical and Hubble Space Telescope ultraviolet spectropolarimetry of 3C 273 and PG 1114+445

New optical spectropolarimetry and broad-band measurements, made during periods when the linear polarization of the quasar 3C 273 was no greater than 1 pct, are combined with ultraviolet polarization measurements made with the Faint Object Spectrograph of the Hubble Space Telescope. Very weak UV polarization during the time of observation is seen in the HST measurements, which is consistent with the completely unpolarized UV continuum and emission lines. Optical spectropolarimetry, however, reveals polarized light, the source of which is consistent with a nonthermal component, diluted by other unpolarized emission sources, such as the 'big blue bump'. Optical spectropolarimetry of the radio-quiet quasar PG 1114+445 is also presented. The polarization of this QSO is thought to result from scattering by dust external to the narrow-line region. 3C 273 and PG 1114+445 are useful in illustrating the range of polarizing mechanisms which can operate in low-polarization QSOs.

Smith, Paul S.↗

NASA NTRS1992

Discovery of a sub-megagauss magnetic white dwarf through spectropolarimetry

A circular polarization survey for weakly magnetized (about 10 kG-1 MG) white dwarfs has been initiated using a newly developed CCD imaging/spectropolarimeter. Among the first two-dozen stars studied, WD 0637 + 477 revealed strong polarization reversals across both H-alpha and H-beta, indicating a mean longitudinal ('effective') field of Be = +349 +/- 19 kG. Corresponding estimates of the mean surface field and dipolar field values are Bs = 950 +/- 100 kG and Bp = 1.2 +/- 0.2 MG. WD 0637 + 477 portrays the weakest field yet found on a magnetic white dwarf by a factor of 2. Given the small survey sample observed to date, statistics on the incidence of magnetic fields in the range 100 kG-1 MG permit a distribution which either increases smoothly with decreasing field strength, or is bimodal, wherein a small percentage of stars possess fields above about 1 MG and the vast majority are essentially nonmagnetized.

Schmidt, Gary D.↗

NASA NTRS1991

PG0027 + 260 - An example of a class of cataclysmic binaries with mysterious, but consistent, behavior

This paper reports extensive optical observations on the PG0027 + 260 binary, carried out on August 1984 with the 1.3 McGraw-Hill telescope and Mark II spectrometer at Michigan-Dartmouth-MIT Observatory on Kitt Peak. It is shown that this object is an eclipsing novalike variable with an orbital period of 3.51 hr. The PG0027 + 260 displays several unexplained phenomena which are remarkably similar to those of the SW Sex, DW UMa, and V1315 Aql, which are eclipsing novalike stars with periods between 3 and 4 hrs. The eclipse of the PG0027 + 260 is modeled, and it is shown that, while the mean eclipse light curve is easy to match, there is no simple explanation for the variable depth.

Thorstensen, John R.↗

NASA NTRS1991

Synchronization of the magnetic Nova V1500 Cygni

The study addresses a program of polarimetric monitoring of V1500 Cyg and a detection of spin-down of the magnetic primary star. Data on the spectral shape of polarized cyclotron flux is presented, and a lower limit to the field strength on the magnetic primary star is derived. The implications which the field strength and observed rate of period change have on locking scenarios are discussed, including the likelihood of identifying other AM Her systems in the process of formation. It is noted that two-color polarimetry of V1500 Cyg indicates a polar magnetic-field strength consistent with those measured in the synchronized AM Her variables; however, the symmetry and long-term repeatability of the polarized flux curves indicate that accretion occurs onto large regions of the white-dwarf surface.

Schmidt, Gary D.↗

NASA NTRS1989

Time-resolved spectroscopy and photometry of the eclipsing AM Herculis binary EXO 033319 - 2554.2

Time-resolved optical observations of the eclipsing AM Herculis binary EXO 033319 - 2554.2 are presented. High-speed photometry of an eclipse is presented and used to derive a new ephemeris for the system and to estimate the size of the region responsible for the cyclotron emission. Optical spectra that span the orbital cycle are presented, the cyclotron emission in these spectra is discussed, and the flux and radial velocity variations of H-beta, H-gamma, and He II 4686 A are examined. Models of the flux and radial velocity variations of the emission lines indicate that about half the line emission comes from low-velocity material that is about 1.4 x 10 to the 10th cm from the white dwarf. The rest comes from high-velocity material that is about 10 to the 10th cm from the white dwarf and is moving toward it at about 600 km/s.

Allen, Richard G.↗

NASA NTRS1988

V1500 Cygni - Discovery of a magnetic nova

Polarimetric observations of V1500 Cygni, the remnant of Nova Cygni 1975, reveal periodic variations of optical circular polarization with semi-amplitudes of about + or - 1.5 pct. Measured over a six-month baseline, the period of variation, 0.137154 + or - 0.000004 days, is 1.8 percent shorter than the stable photometric period, 0.1396129 days, obtained by Patterson (1979), and refined by Kaluzny and Semeniuk (1987). The circular polarization is interpreted as high-harmonic optical cyclotron emission from an accreting, magnetic white dwarf primary. However, the dominant system light and photometric variability are due to the heated companion star. In this picture, the prenova system was a synchronized AM Herculis-type magnetic variable. During the outburst and subsequent dense-wind phase, coupling between the white dwarf and the expanded envelope and the interaction of the orbiting secondary star were responsible for the asynchronism of the current system and the remarkable photometric variations observed in 1975-1976. Other unusual characteristics of Nova Cygni 1975 are also explained by this model.

Stockman, H. S.↗

NASA NTRS1987

The rotationally modulated Zeeman spectrum at nearly 10 to the 9th Gauss of the white dwarf PG 1031 + 234

Detailed analyses are performed of high-quality, phase-resolved CCD spectroscopy of the absorption-line spectrum throughout its rotation period of the new white dwarf PG 1031 + 234. The spectral variations are discussed and compared with new theoretical calculations of the behavior of hydrogen in strong magnetic fields. This analysis is then extended through a modeling procedure which produces a synthetic magnetically distorted spectrum for a star of arbitrary field strength and structure. The results confirm that PG 1031 + 234 possesses the strongest field yet detected on a white dwarf, with regions on the surface spanning the range of about 200 to nearly 1000 MG. The spectroscopic data reflect a field pattern containing a slightly offset global component of polar field strength of about 500 MG together with a localized magnetic 'spot' whose central field approaches 2000 MG.

Latter, William B.↗

NASA NTRS1987

1E 1048.5 + 5421 - A new 114 minute AM Herculis binary

The discovery of a new AM Herculis binary system, found as a serendipitous Einstein X-ray source, is described. Like the previously discovered mass-transfer binaries involving synchronously rotating magnetic white-dwarf primaries, the system exhibits strong circular polarization, X-ray and optical continuum variations, and optical emission lines, all of which seem to be modulated with these binary periods of 114.5 + or - 0.2 minutes. Although all data are not concurrent, the new system appears to possess the highest ratio of F(x)/F(opt) yet found for an AM Her system. The surprising accumulation of AM Her variables with periods near 114 minute is commented on.

Morris, Simon L.↗

NASA NTRS1987

The magnetic white dwarfs

Observations of isolated magnetic white dwarfs are compared with those of Angel et al. (1981). A list of 21 currently known magnetic white dwarfs is presented, and additions and deletions of stars from the list of Angel et al. are discussed. The spectrum of the hydrogen atom in extremely strong fields, and the field distribution of stars in the 1-700 MG range are calculated. It is observed that the distribution of magnetic stars has remained constant/decade of field strength from about 10 to the 6th to 10 to the 9th G. Consideration is given to the rotation rates and suface compositions of white dwarfs.

Schmidt, Gary D.↗