arXiv subjects

G. D. Schmidt

Publications and source records attributed to G. D. Schmidt.

At least 19 recordsLinked to original sources

arXiv2011

The Optical Properties of PKS 1222+216 During the Fermi Mission

The optical properties of the z = 0.435 quasar PKS 1222+216 (4C+21.35) are summarized since the discovery of impressive gamma-ray activity in this source by Fermi/LAT. Unlike several other gamma-ray-bright blazars, there appears to be little connection between optical and gamma-ray activity. Spectropolarimetry shows this object to be a composite system with optical emission from both a polarized, variable synchrotron power-law and unpolarized light from a stable blue continuum source (+broad emission-line region) contributing to the observed spectrum. Spectrophotometry over a period of about two years does not detect significant variability in the strong, broad emission lines, despite large optical continuum variations. This suggests that the relativistic jet has little influence on the output of the broad emission-line region, possibly either because the highly beamed continuum ionizes only a small portion of the line-emitting gas, or the observed non-thermal continuum originates parsecs downstream from the base of the jet, further away from the central engine than the broad emission-line region.

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arXiv2009

Coordinated Fermi/Optical Monitoring of Blazars and the Great 2009 September Gamma-ray Flare of 3C 454.3

We describe the optical spectropolarimetric monitoring program at Steward Observatory centered around gamma-ray-bright blazars and the LAT Monitored Source List planned for Fermi Cycles 2-4. The large number of measurements made during Cycle 1 of the Fermi mission are available to the research community and the data products are summarized (see http://james.as.arizona.edu/~psmith/Fermi). The optical data include spectropolarimetry at a resolution of ~20 A, broad-band polarization and flux measurements, and flux-calibrated spectra spanning 4000-7600 A. These data provide a comprehensive view of the optical variability of an important sample of objects during the Fermi Era. In addition to broad-band flux and linear polarization monitoring, the spectra allow for the tracking of changes to the spectral index of the synchrotron continuum, importance of non-synchrotron emission features, and how and when the polarization varies with wavelength, an important clue as to the structure of the emission region or the identification of multiple nonthermal components. As an illustration, we present observations of 3C 454.3 obtained in 2009 September during an exceptionally bright gamma-ray flare. The blazar was optically bright during the flare, but except for a few short periods, it showed surprisingly low polarization (P < 5%). Opportunities exist within the Fermi research community to coordinate with our long-term optical monitoring program toward the goal of maximum scientific value to both the Fermi and associated radio VLBI monitoring of blazars.

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arXiv2008

High Accuracy Near-infrared Imaging Polarimetry with NICMOS

The findings of a nine orbit calibration plan carried out during HST Cycle 15, to fully determine the NICMOS camera 2 (2.0 micron) polarization calibration to high accuracy, are reported. Recently Ueta et al. and Batcheldor et al. have suggested that NICMOS possesses a residual instrumental polarization at a level of 1.2-1.5%. This would completely inhibit the data reduction in a number of GO programs, and hamper the ability of the instrument to perform high accuracy polarimetry. We obtained polarimetric calibration observations of three polarimetric standards at three spacecraft roll angles separated by ~60deg. Combined with archival data, these observations were used to characterize the residual instrumental polarization in order for NICMOS to reach its full potential of accurate imaging polarimetry at p~1%. Using these data, we place an 0.6% upper limit on the instrumental polarization and calculate values of the parallel transmission coefficients that reproduce the ground-based results for the polarimetric standards. The uncertainties associated with the parallel transmission coefficients, a result of the photometric repeatability of the observations, are seen to dominate the accuracy of p and theta. However, the updated coefficients do allow imaging polarimetry of targets with p~1.0% at an accuracy of +/-0.6% and +/-15deg. This work enables a new caliber of science with HST.

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arXiv2008

High Accuracy Imaging Polarimetry with NICMOS

The ability of NICMOS to perform high accuracy polarimetry is currently hampered by an uncalibrated residual instrumental polarization at a level of 1.2-1.5%. To better quantify and characterize this residual we obtained observations of three polarimetric standard stars at three separate space-craft roll angles. Combined with archival data, these observations were used to characterize the residual instrumental polarization to enable NICMOS to reach its full polarimetric potential. Using these data, we calculate values of the parallel transmission coefficients that reproduce the ground-based results for the polarimetric standards. The uncertainties associated with the parallel transmission coefficients, a result of the photometric repeatability of the observations, dominate the accuracy of p and theta. However, the new coefficients now enable imaging polarimetry of targets with p~1.0% at an accuracy of +/-0.6% and +/-15 degrees.

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arXiv2008

Hot DQ White Dwarfs: Something Different

We present a detailed analysis of all the known Hot DQ white dwarfs in the Fourth Data Release of the Sloan Digital Sky Survey (SDSS) recently found to have carbon dominated atmospheres. Our spectroscopic and photometric analysis reveals that these objects all have effective temperatures between ~18,000 and 24,000 K. The surface composition is found to be completely dominated by carbon, as revealed by the absence of Hbeta and HeI 4471 lines (or determination of trace amount in a few cases). We find that the surface gravity of all objects but one seems to be ''normal'' and around log g = 8.0 while one is likely near log g = 9.0. The presence of a weak magnetic field is directly detected by spectropolarimetry in one object and is suspected in two others. We propose that these strange stars could be cooled down versions of the weird PG1159 star H1504+65 and form a new family of hydrogen and helium deficient objects following the post-AGB phase. Finally, we present the results of full nonadiabatic calculations dedicated specifically to each of the Hot DQ that show that only SDSS J142625.70+575218.4 is expected to exhibit luminosity variations. This result is in excellent agreement with recent observations by Montgomery et al. who find that J142625.70+575218.4 is the only pulsator among 6 Hot DQ white dwarfs surveyed in February 2008.

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arXiv2007

Discovery of a bright eclipsing cataclysmic variable

We report on the discovery of J0644+3344, a bright deeply eclipsing cataclysmic variable (CV) binary. Spectral signatures of both binary components and an accretion disk can be seen at optical wavelengths. The optical spectrum shows broad H I, He I, and He II accretion disk emission lines with deep narrow absorption components from H I, He I, Mg II and Ca II. The absorption lines are seen throughout the orbital period, disappearing only during primary eclipse. These absorption lines are either the the result of an optically-thick inner accretion disk or from the photosphere of the primary star. Radial velocity measurements show that the H I, He I, and Mg II absorption lines phase with the the primary star, while weak absorption features in the continuum phase with the secondary star. Radial velocity solutions give a 150+/-4 km/s semi-amplitude for the primary star and 192.8+/-5.6 km/s for the secondary. The individual stellar masses are 0.63-0.69 Mdot for the primary and 0.49-0.54 Mdot for the secondary. The bright eclipsing nature of this binary has helped provide masses for both components with an accuracy rarely achieved for CVs. This binary most closely resembles a nova-like UX UMa or SW Sex type of CV. J0644+3344, however, has a longer orbital period than most UX UMa or SW Sex stars. Assuming an evolution toward shorter orbital periods, J0644+3344 is therefore likely to be a young interacting binary. The secondary star is consistent with the size and spectral type of a K8 star, but has an M0 mass.

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arXiv2006

Spectropolarimetric Survey of Hydrogen-rich White Dwarf Stars

We have conducted a survey of 61 southern white dwarfs searching for magnetic fields using Zeeman spectropolarimetry. Our objective is to obtain a magnetic field distribution for these objects and, in particular, to find white dwarfs with weak fields. We found one possible candidate (WD 0310-688) that may have a weak magnetic field of -6.1+/-2.2 kG. Next, we determine the fraction and distribution of magnetic white dwarfs in the Solar neighborhood, and investigate the probability of finding more of these objects based on the current incidence of magnetism in white dwarfs within 20 pc of the Sun. We have also analyzed the spectra of the white dwarfs to obtain effective temperatures and surface gravities.

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arXiv2006

Model Atmosphere Analysis of the Weakly Magnetic DZ White Dwarf G165-7

A reanalysis of the strongly metal-blanketed DZ white dwarf G165-7 is presented. An improved grid of model atmospheres and synthetic spectra is used to analyze BVRI, JHK, and ugriz photometric observations as well as a high quality Sloan Digital Sky Survey spectrum covering the energy distribution from 3600 to 9000 A. The detection of splitting in several lines of Ca, Na, and Fe, suggesting a magnetic field of Bs~650 kG, is confirmed by spectropolarimetric observations that reveal as much as +/- 7.5% circular polarization in many of the absorption lines, most notably Na, Mg, and Fe. Our combined photometric and spectroscopic fit yields Teff = 6440 K, log g=7.99, log H/He=-3.0 and log Ca/He=-8.1. The other heavy elements have solar ratios with respect to calcium, with the exception of Na and Cr that had to be reduced by a factor of two and three, respectively. A crude polarization model based upon the observed local spectral flux gradient yields a longitudinal field of 165 kG, consistent with the mean surface field inferred from the Zeeman splitting. The inclusion of this weak magnetic field in our synthetic spectrum calculations, even in an approximate fashion, is shown to improve our fit significantly.

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arXiv2005

XMM-NEWTON Observations of Red AGN

XMM-Newton spectra of five red, 2MASS AGN, selected from a sample observed by Chandra to be relatively X-ray bright and to cover a range of hardness ratios, confirm the presence of substantial absorbing material in three sources with optical classifications ranging from Type 1 to Type 2. A flat (hard), power law continuum is observed in the other two. The combination of X-ray absorption and broad optical emission lines suggests either a small (nuclear) absorber or a favored viewing angle so as to cover the X-ray source but not the broad emission line region (BELR). A soft excess is detected in all three Type 1 sources. We speculate that this may arise in an extended region of ionised gas, perhaps linked with the polarised (scattered) optical light present in these sources. The spectral complexity revealed by XMM-Newton emphasizes the limitations of the low S/N \chandra data. The new results strengthen our earlier conclusions that the observed X-ray continua of red AGN are unusually hard at energies >2 keV. Their observed spectra are consistent with contributing significantly to the missing hard/absorbed population of the Cosmic X-ray Background (CXRB) although their intrinsic power law slopes are typical of broad-line (Type 1) AGN (Gamma ~1.7-1.9). This suggests that the missing X-ray-absorbed CXRB population may include Type 1 AGN/QSOs in addition to the Type 2 AGN generally assumed.

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arXiv2005

Discovery of a Magnetic White Dwarf/Probable Brown Dwarf Short-Period Binary

The magnetic white dwarf SDSS J121209.31+013627.7 exhibits a weak, narrow Halpha emission line whose radial velocity and strength are modulated on a period of ~90 minutes. Though indicative of irradiation on a nearby companion, no cool continuum component is evident in the optical spectrum, and IR photometry limits the absolute magnitude of the companion to M_J > 13.37. This is equivalent to an isolated L5 dwarf, with T_eff < 1700 K. Consideration of possible evolutionary histories suggests that, until ~0.6 Gyr ago, the brown dwarf orbited a ~1.5 M_sun main seqeunce star with P ~ 1 yr, a ~ 1 AU, thus resembling many of the gaseous superplanets being found in extrasolar planet searches. Common envelope evolution when the massive star left the main sequence reduced the period to only a few hours, and ensuing angular momentum loss has further degraded the orbit. The binary is ripe for additional observations aimed at better studying brown dwarfs and the effects of irradiation on their structure.

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arXiv2005

New Low Accretion-Rate Magnetic Binary Systems and their Significance for the Evolution of Cataclysmic Variables

Discoveries of two new white dwarf plus M star binaries with striking optical cyclotron emission features from the Sloan Digital Sky Survey (SDSS) brings to six the total number of X-ray faint, magnetic accretion binaries that accrete at rates < 10^{-13} Msun/yr, or <1% of the values normally encountered in cataclysmic variables. This fact, coupled with donor stars that underfill their Roche lobes and very cool white dwarfs, brand the binaries as post common-envelope systems whose orbits have not yet decayed to the point of Roche-lobe contact. They are pre-magnetic CVs, or pre-Polars. The systems exhibit spin/orbit synchronism and apparently accrete by efficient capture of the stellar wind from the secondary star, a process that has been dubbed a ``magnetic siphon''. Because of this, period evolution of the binaries will occur solely by gravitational radiation, which is very slow for periods >3 hr. Optical surveys for the cyclotron harmonics appear to be the only means of discovery, so the space density of pre-Polars could rival that of Polars, and the binaries provide an important channel of progenitors (in addition to the asynchronous Intermediate Polars). Both physical and SDSS observational selection effects are identified that may help to explain the clumping of all six systems in a narrow range of magnetic field strength around 60 MG.

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arXiv2004

Discovery and Evolution of an Unusual Luminous Variable Star in NGC 3432 (Supernova 2000ch)

We present photometric and spectroscopic observations of SN 2000ch, an unusual and extremely luminous variable star located in the galaxy NGC 3432. The object was discovered on 2000 May 3.2 during the course of the Lick Observatory Supernova Search, at an unfiltered magnitude of about 17.4. Pre-discovery images obtained in 1997, 1998, and 2000 April show the object with R = 19.2-19.5 mag. Optical spectra obtained beginning on 2000 May 6 show a smooth, flat continuum and strong, broad hydrogen Balmer emission lines at wavelengths consistent with the catalogued redshift of NGC 3432, strengthening the association of the variable with the galaxy. Photometric monitoring reveals a complex and erratic light curve over a time span of ~10 days. Subsequent optical spectra over the next ~3 months continued to show strong Balmer emission lines with a mean full-width at half-maximum intensity ~1550 km/s and a distinct red asymmetry. A spectrum obtained 9 months after the outburst is similar to the previous spectra, but the integrated flux in H-alpha is nearly half that observed during the outburst. The object's photometric behavior, spectrum, and luminosity suggest that it is a very massive and luminous variable star and might be related to some luminous blue variable stars such as eta Carinae and SN 1997bs in NGC 3627. The brightest apparent magnitude implies an absolute magnitude of approximately -12.7 at the distance of NGC 3432, a value which is comparable to eta Carinae during its outburst in the mid-nineteenth century.

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arXiv2002

An Extraordinary Scattered Broad Emission Line in a Type 2 QSO

An infrared-selected, narrow-line QSO has been found to exhibit an extraordinarily broad Halpha emission line in polarized light. Both the extreme width (35,000 km/sec full-width at zero intensity) and 3,000 km/sec redshift of the line centroid with respect to the systemic velocity suggest emission in a deep gravitational potential. An extremely red polarized continuum and partial scattering of the narrow lines at a position angle common to the broad-line emission imply extensive obscuration, with few unimpeded lines of sight to the nucleus.

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arXiv2002

Observations of the Magnetic Cataclysmic Variable VV Puppis with the Far Ultraviolet Spectroscopic Explorer

We present the first far-ultraviolet (FUV) observations of the magnetic cataclysmic variable VV Puppis, obtained with the Far Ultraviolet Spectroscopic Explorer satellite. In addition, we have obtained simultaneous ground-based optical photometric observations of VV Pup during part of the FUV observation. The shapes of the FUV and optical light curves are consistent with each other and with those of past observations at optical, extreme-ultraviolet, and X-ray wavelengths. Time-resolved FUV spectra during the portion of VV Pup's orbit when the accreting magnetic pole of the white dwarf can be seen show an increasing continuum level as the accretion spot becomes more directly visible. The most prominent features in the spectrum are the O VI 1031.9A, 1037.6A emission lines. We interpret the shape and velocity shift of these lines in the context of an origin in the accretion funnel near the white dwarf surface. A blackbody function with T > 90,000 K provides an adequate fit to the FUV spectral energy distribution of VV Pup.

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arXiv2001

The Optical Polarization of Near-Infrared Selected QSOs

Optical broad-band polarimetry is presented for near-infrared color-selected AGN classified QSOs based on their K-band luminosity. More than 10% of a sample of 70 QSOs discovered in the Two Micron All Sky Survey (2MASS) with J-K > 2 and M_K < -23 show high broad-band linear polarization (P > 3%), and values range to a maximum of P = 11%. High polarization tends to be associated with the most luminous objects at K, and with QSOs having the highest near-IR-to-optical flux ratios. The 2MASS QSO sample includes objects possessing a wide range of optical spectral types. High polarization is seen in two of 22 broad emission-line (Type 1) objects, but 1/4 of the QSOs of intermediate spectral type (Type 1.5-1.9) are highly polarized. None of the nine QSOs classified as Type 2 exhibit P > 3%. It is likely that the unavoidable inclusion of unpolarized starlight from the host galaxy within the observation aperture results in reduced polarization for the narrow emission-line objects. The high polarization of 2MASS-discovered QSOs supports the conclusion inferred from their near-IR and optical colors, that the nuclei of many of these objects are obscured to some degree by dust. The 2MASS QSO sample is compared to other, predominantly radio-quiet, QSOs and is found to be consistent with the idea that the orientation of AGN to the line of sight plays a major role in determining their observed properties.

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arXiv2001

HST Ultraviolet and Ground-based Optical Spectropolarimetry of IRAS QSOs: Dusty Scattering in Luminous AGN

We present UV and optical spectropolarimetry of two highly polarized IRAS-selected QSOs, IRAS 13349+2438 and the BALQSO IRAS 14026+4341. The polarization in both objects rises rapidly toward the blue, peaks near 3000 A in the rest frame and remains nearly constant for shorter wavelengths. The rest frame optical polarized flux density spectra also increase rapidly towards the blue, but then decrease dramatically below 3000 A. This distinctive wavelength dependence of polarized flux shows that the polarization is produced by dust scattering. As for many Seyfert, radio and Hyperluminous Infrared Galaxies (HIGs), the lower polarization of the weak [O III] 4959,5007 lines in IRAS 13349+2438 suggests that the scattering grains lie interior to, or mixed with the narrow line gas. We construct full radiative transfer models of these systems consisting of a dusty sphere of modest optical depth illuminated axisymmetrically from within by a powerlaw QSO spectrum. We show that this simple model successfully reproduces the qualitative polarization properties of the objects. Despite similarities to other IRAS-selected BALQSOs, our FOS spectropolarimetry of IRAS 13349+2438 does not reveal broad absorption lines. IRAS 14026+4341 has an Al III BAL in both scattered and total flux density. We discuss these two objects in terms of both orientation and evolutionary unified schemes for QSOs, BALQSOs and HIGs.

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arXiv2001

Studies of Magnetic and Suspected-Magnetic Southern White Dwarfs

Optical spectrophotometry and circular spectropolarimetry are presented for several candidate magnetic white dwarfs that were identified during the Hamburg/ESO survey for bright QSOs. HE 1211-1707 and HE 1043-0502 are rare examples of white dwarfs that show neutral helium lines in a high magnetic field, in these cases ~50 MG and ~800 MG, respectively. The former is also found to be rotating with a period of ~2 hr. HE 1045-0908 is a hydrogen-line star at ~20 MG, spinning with a period in the range ~2-4 hr. Attempts at modeling the limited amount of phase-resolved data that is available suggest that the field structure departs substantially from a simple centered dipolar geometry. Two rather cool white dwarfs with unidentified broad absorption spectral features, HE 0236-2656 and HE 0330-0002, are confirmed to be magnetic. Line identifications for these stars are not yet possible, but the atmospheres are probably helium-rich, with spectral features formed by trace compounds of hydrogen, carbon, and perhaps other metals. HE 0003-5701 and HE 0338-3853 were proposed by Reimers et al. (1996) alongside two similar objects to be magnetic DB white dwarfs, with magnetic fields all near 20 MG. However, our observations show that the first two, and by extension all four, are nonmagnetic white dwarf + cool dwarf pairs, deserving of study in their own right as possible close binaries. Finally, a lack of circular polarisation suggests that HE 0000-3430 and HE 0127-3110 are also nonmagnetic. HE 0000-3430 appears to be a featureless DC white dwarf over the spectral range observed here, while the sole absorption line near 5890A in HE 0127-3110 could be either He I 5876A or the Na I D doublet, but there are difficulties with either interpretation.

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arXiv2001

Phase-resolved HST/STIS spectroscopy of the exposed white dwarf in the high-field polar AR UMa

Phase-resolved HST/STIS ultraviolet spectroscopy of the high-field polar AR UMa confirms that the WD photospheric Ly alpha Zeeman features are formed in a magnetic field of ~200 MG. In addition to the Ly alpha pi and sigma+ components, we detect the forbidden hydrogen 1s0->2s0 transition, which becomes ``enabled'' in the presence of both strong magnetic and electric fields. Our attempt in fitting the overall optical+UV low state spectrum with single temperature magnetic WD models remains rather unsatisfactory, indicating either a shortcoming in the present models or a new physical process acting in AR UMa. As a result, our estimate of the WD temperature remains somewhat uncertain, Twd=20000+-5000K. We detect a broad emission bump centered at ~1445A and present throughout the entire binary orbit, and a second bump near ~1650A, which appears only near the inferior conjunction of the secondary star. These are suggestive of low harmonic cyclotron emission produced by low-level (M-dot~1e-13 Msun/yr) accretion onto both magnetic poles. However, there is no evidence in the power spectrum of light variations for accretion in gas blobs. The observed Ly alpha emission line shows a strong phase dependence with maximum flux and redshift near orbital phase phi~0.3, strongly indicating an origin on the trailing hemisphere of the secondary star. An additional Ly alpha absorption feature with similar phasing as the Ly alpha emission, but a \~700km/s blueshift could tentatively be ascribed to absorption of WD emission in a moderately fast wind. We derive a column density of neutral hydrogen of NH=(1.1+-1.0)1e18 cm**-2, the lowest of any known polar.

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