arXiv subjects

Knox S. Long

Publications and source records attributed to Knox S. Long.

At least 145 records · Page 8Linked to original sources

arXiv2004

Time-Resolved Ultraviolet Spectroscopy of the SW Sex Star DW UMa: Confirmation of a Hidden White Dwarf and the UV Counterpart to Phase 0.5 Absorption Events

We present time-resolved, ultraviolet (UV) spectroscopy of the SW Sex star DW UMa in the high state. We confirm that, shortward of 1500 Å, the high-state, UV continuum level is lower than the white dwarf (WD)-dominated low-state level. We also do not see the WD contact phases in the high state eclipse light curves. These results confirm our earlier finding that the WD in this system is hidden from view in the high state. Based on this, we caution that eclipse mapping of high-inclination SW Sex stars in the high state may yield incorrect or misleading results. In the context of DW UMa, we demonstrate explicitly that distance estimates obtained by recent eclipse mapping studies cannot be reconciled with the WD-dominated low-state spectrum. We also show that the fluxes of the UV emission lines in the high state drop near orbital phase 0.5. This is the first detection of a UV counterpart to the class-defining phase 0.5 absorption seen in the optical emission lines of SW Sex stars.

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arXiv2004

Chandra HETG Spectra of SS Cyg and U Gem in Quiescence and Outburst

Chandra HETG spectra of the prototypical dwarf novae SS Cyg and U Gem in quiescence and outburst are presented and discussed. When SS Cyg goes into outburst, it becomes dimmer in hard X-rays and displays a dramatic shift in its relative line strengths, whereas when U Gem goes into outburst, it becomes brighter in hard X-rays and displays only a minor shift in its relative line strengths. In both systems, the emission lines become significantly broader in outburst, signaling the presence of high velocity gas either in Keplerian orbits around the white dwarf or flowing outward from the system.

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arXiv2004

Far Ultraviolet Observations of the Dwarf Nova VW Hyi in Quiescence

We present a 904-1183 A spectrum of the dwarf nova VW Hydri taken with the Far Ultraviolet Spectroscopic Explorer during quiescence, eleven days after a normal outburst, when the underlying white dwarf accreter is clearly exposed in the far ultraviolet. However, model fitting show that a uniform temperature white dwarf does not reproduce the overall spectrum, especially at the shortest wavelengths. A better approximation to the spectrum is obtained with a model consisting of a white dwarf and a rapidly rotating ``accretion belt''. The white dwarf component accounts for 83% of the total flux, has a temperature of 23,000K, a v sin i = 400 km/s, and a low carbon abundance. The best-fit accretion belt component accounts for 17% of the total flux, has a temperature of about 48,000-50,000K, and a rotation rate Vrot sin i around 3,000-4,000 km/s. The requirement of two components in the modeling of the spectrum of VW Hyi in quiescence helps to resolve some of the differences in interpretation of ultraviolet spectra of VW Hyi in quiescence. However, the physical existence of a second component (and its exact nature) in VW Hyi itself is still relatively uncertain, given the lack of better models for spectra of the inner disk in a quiescent dwarf nova.

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arXiv2003

A Far-Ultraviolet Survey of 47 Tucanae.II The Long-Period Cataclysmic Variable AKO 9

We present time-resolved, far-ultraviolet (FUV) spectroscopy and photometry of the 1.1 day eclipsing binary system AKO 9 in the globular cluster 47 Tucanae. The FUV spectrum of AKO 9 is blue and exhibits prominent C IV and He II emission lines. The spectrum broadly resembles that of long-period, cataclysmic variables in the galactic field. Combining our time-resolved FUV data with archival optical photometry of 47 Tuc, we refine the orbital period of AKO 9 and define an accurate ephemeris for the system. We also place constraints on several other system parameters, using a variety of observational constraints. We find that all of the empirical evidence is consistent with AKO 9 being a long-period dwarf nova in which mass transfer is driven by the nuclear expansion of a sub-giant donor star. We therefore conclude that AKO 9 is the first spectroscopically confirmed cataclysmic variable in 47 Tuc. We also briefly consider AKO 9's likely formation and ultimate evolution. Regarding the former, we find that the system was almost certainly formed dynamically, either via tidal capture or in a 3-body encounter. Regarding the latter, we show that AKO 9 will probably end its CV phase by becoming a detached, double WD system or by exploding in a Type Ia supernova.

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arXiv2003

WZ Sge: HST Spectroscopy of the Cooling of the White Dwarf after the 2001 Outburst

Following an outburst in 2001 July, the UV flux of WZ Sge has declined slowly toward pre-outburst levels. Here, we describe new 1150-1710 Angstrom HST/STIS spectra of WZ Sge obtained between 2002 April and 2003 March to follow the decline. A combined analysis of these and spectra obtained in the fall of 2001 show that if log g=8.5, then the white dwarf temperature decreased from 23,400 K in 2001 October, shortly after the steady decline of the system began, to 15,900 K 17 months later. The UV flux in 2003 March was still 2.4 times higher than measured prior to the outburst and so the system was still recovering from the outburst. During this period, the shape of the spectrum and the flux from the system are consistent with a log g=8.5, 0.9 solar mass white dwarf at the astrometrically-determined distance to WZ Sge. Although the spectrum from 2001 September resembles that of a white dwarf with a temperature of 28,200 K, the implied radius is smaller than in the remainder of the observations. If the entire white dwarf was visible in 2001 September, then either a second component, such as the disk emission, was distorting the spectrum, or more likely, the temperature on the white dwarf photosphere was not uniform then. The metal lines in the spectra of WZ Sge have weakened with time. Model fits that allow for material along the line of sight to the white dwarf photosphere are substantial improvements over models which assume all the lines arise from the photosphere. This obviates the need to explain very unusual abundance ratios in the photosphere and complicates the determination of the rotation rate of the white dwarf.

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arXiv2003

Observations of the SW Sextantis star DW Ursae Majoris with the Far Ultraviolet Spectroscopic Explorer

We present an analysis of the first far-ultraviolet observations of the SW Sextantis-type cataclysmic variable DW Ursae Majoris, obtained in November 2001 with the Far Ultraviolet Spectroscopic Explorer. The time-averaged spectrum of DW UMa shows a rich assortment of emission lines (plus some contamination from interstellar absorption lines including molecular hydrogen). Accretion disk model spectra do not provide an adequate fit to the far-ultraviolet spectrum of DW UMa. We constructed a light curve by summing far-ultraviolet spectra extracted in 60-sec bins; this shows a modulation on the orbital period, with a maximum near photometric phase 0.93 and a minimum half an orbit later. No other periodic variability was found in the light curve data. We also extracted spectra in bins spanning 0.1 in orbital phase; these show substantial variation in the profile shapes and velocity shifts of the emission lines during an orbital cycle of DW UMa. Finally, we discuss possible physical models that can qualitatively account for the observed far-ultraviolet behavior of DW UMa, in the context of recent observational evidence for the presence of a self-occulting disk in DW UMa and the possibility that the SW Sex stars may be the intermediate polars with the highest mass transfer rates and/or weakest magnetic fields.

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arXiv2003

Anomalous ultraviolet line flux ratios in the cataclysmic variables 1RXSJ232953.9+062814, CE315, BZ UMa and EY Cyg observed with HST/STIS

Brief HST/STIS spectroscopic snapshot exposures of the cataclysmic variables 1RXSJ232953.9+062814, CE315, BZ UMa and EY Cyg reveal very large NV/CIV line flux ratios, similar to those observed in AE Aqr. Such anomalous line flux ratios have so far been observed in 10 systems, and presumably reflect a different composition of the accreted material compared to the majority of cataclysmic variables. We discuss the properties of this small sample in the context of the recent proposal by Schenker et al. (2002) that a significant fraction of the present-day population of cataclysmic variables may have passed through a phase of thermal time-scale mass transfer.

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arXiv2003

WZ Sge: FUSE Spectroscopy of the 2001 Outburst

The short-period cataclysmic variable WZ Sge underwent its first dwarf nova outburst in 23 years in 2001 July. Here we describe 905-1187 Ang. spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) during the outburst and within the first few months after the outburst. The first spectrum, obtained about 7 days after the outburst began and 6.4 days past optical maximum, is dominated by a broad (50-60 Ang. EW) feature due to O VI and can be characterized in terms of emission from a highly inclined disk modulated by the effects of a highly ionized wind. WZ Sge is fainter in later observations (starting about 45 days after optical maximum) and increasingly "red" in the far ultraviolet. The spectral shape increasingly resembles that expected from a metal-enriched white dwarf photosphere, although it seems likely that some disk contribution remains at least through the second FUSE observation, when the visible light curve showed the system to be undergoing a rebrightening event. The absorption lines observed on 29 September and in early November, when the visible light curve was in a fairly steady decline toward quiescence, are narrow, with widths (FWHM) of order 0.7-1.3 Ang. (200-400 km/s). The most natural explanation for these spectra is that the continuum arises from a relatively massive, slowly rotating white dwarf, and that the lines are created by a combination of metals in the atmosphere and of absorbing material along the line of sight to the white dwarf. If the white dwarf has log{g}= 8.5, then the white dwarf cooled from ~25,000 K on 29 September to 23,000 K in early November.

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arXiv2002

The SN 1006 Remnant: Optical Proper Motions, Deep Imaging, Distance, and Brightness at Maximum

We report the first measurement of proper motions in the SN1006 remnant (G327.6+14.6) based entirely on digital images. CCD images from three epochs spanning a period of 11 years are used: 1987 from Las Campanas, and 1991 and 1998 from CTIO. Measuring the shift of delicate Balmer filaments along the northwest rim of the remnant, we obtain proper motions of 280 +/- 8 mas/yr along the entire length where the filaments are well defined, with little systematic variation along the filaments. We also report very deep Halpha imaging observations of the entire remnant that clearly show very faint emission surrounding almost the entire shell, as well as some diffuse emission regions in the (projected) interior. Combining the proper motion measurement with a recent measurement of the shock velocity based on spectra of the same filaments by Ghavamian et al. leads to a distance of 2.17 +/- 0.08 kpc to SN1006. Several lines of argument suggest that SN1006 was a Type Ia event, so the improved distance measurement can be combined with the peak luminosity for SNeIa, as determined for events in galaxies with Cepheid-based distances, to calculate the apparent brightness of the spectacular event that drew wide attention in the eleventh century. The result, V_max = -7.5 =/- 0.4, lies squarely in the middle of the wide range of estimates based on the historical observations.

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arXiv2002

Modeling the Spectral Signatures of Accretion Disk Winds: A New Monte Carlo Approach

Bipolar outflows are present in many disk-accreting astrophysical systems. In disk-accreting cataclysmic variables (CVs), these outflows are responsible for most of the strong features in the ultraviolet spectra of these systems. However, there have been few attempts to model these features quantitatively. Here, we describe a new, hybrid Monte Carlo/Sobolev code, which allows us to synthesize the complete spectrum of a disk-dominated, mass-losing CV. The line profiles we calculate for C IV resemble those calculated by previous workers when an identical geometry is assumed. However, our synthetic spectra exhibit not only the well-known resonance lines of O VI, N V, Si IV and C IV, but, with an appropriate choice of mass-loss rate and wind geometry, also many lines originating from excited lower states. Many of these lines have already been seen in the far ultraviolet spectra of CVs obtained with HUT, FUSE, and HST. In order to illustrate the degree to which we are currently able to reproduce observed spectra, we finally present a preliminary fit to the Hopkins Ultraviolet Telescope spectrum of the dwarf nova Z Cam in outburst.

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arXiv2002

A Far-Ultraviolet Survey of 47 Tucanae. I. Imaging

We present results from the imaging portion of a far-ultraviolet (FUV) survey of the core of 47 Tucanae. We have detected 767 FUV sources, 527 of which have optical counterparts in archival HST/WFPC2 images of the same field. Most of our FUV sources are main-sequence (MS) turn-off stars near the detection limit of our survey. However, the FUV/optical color-magnitude diagram (CMD) also reveals 19 blue stragglers (BSs), 17 white dwarfs (WDs) and 16 cataclysmic variable (CV) candidates. The BSs lie on the extended cluster MS, and four of them are variable in the FUV data. The WDs occupy the top of the cluster cooling sequence, down to an effective temperature of T_{eff} \simeq 20,000 K. Our FUV source catalog probably contains many additional, cooler WDs without optical counterparts. Finally, the CV candidates are objects between the WD cooling track and the extended cluster MS. Four of the CV candidates are previously known or suspected cataclysmics. All of these are bright and variable in the FUV. Another CV candidate is associated with the semi-detached binary system V36 that was recently found by Albrow et al. (2001). V36 has an orbital period of 0.4 or 0.8 days, blue optical colors and is located within 1 arcsec of a Chandra x-ray source. A few of the remaining CV candidates may represent chance superpositions or SMC interlopers, but at least half are expected to be real cluster members with peculiar colors. However, only a few of these CV candidates are possible counterparts to Chandra x-ray sources. Thus it is not yet clear which, if any, of them are true CVs, rather than non-interacting MS/WD binaries or Helium WDs.

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arXiv2002

The Optical Spectrum of the SN 1006 Supernova Remnant Revisited

We present the deepest optical spectrum acquired to date of Balmer-dominated shocks in the NW rim of SN 1006. We detect the broad and narrow components of H-alpha, H-beta and H-gamma and report the first detection of the He I 6678 emission line in this supernova remnant. We may have detected, at the 1.5-sigma level, faint He II 4686 emission. We measure a full width half maximum of 2290 +/- 80 km/s in the broad component H-alpha line, with broad-to-narrow flux ratios of 0.84^+(0.03)_(-0.01) and 0.93^(+0.18)_(-0.16) in H-alpha and H-beta, respectively. To match these observations, our nonradiative shock models require a low degree of electron-proton equilibration at the shock front, T_e/T_p <= 0.07, and a shock speed of 2890 +/- 100 km/s. These results agree well with an earlier analysis of ultraviolet lines from SN 1006. The He I/H-alpha and He I/He II flux ratios also indicate low equilibration. Furthermore, our models match the observations for mostly ionized (~ 90%) preshock H and mostly neutral (>~ 70%) preshock He, respectively. We conclude that the high H ionization fraction cannot be explained by either photoionization from the reverse shock or relic ionization from EUV photons released in the 1006 A.D. supernova. The most plausible explanation appears to be photoionization from the Galactic Lyman continuum.

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arXiv2001

HST Spectroscopy of the Nucleus of M33

We have used Hubble Space Telescope to obtain moderate resolution 1150-5700 Ang. spectroscopy of the nucleus of M33 and a blue star ~1 arcsec NNW of the nucleus in an attempt to find the optical counterpart of the nuclear X-ray source and to characterize stellar populations in the nuclear region. The STIS spectra of the nucleus can be modelled in terms of two starbursts, one with a mass of about 9000 solar masses at 40 Myrs and the other with a mass of about 76,000 solar masses at 1 Gyrs. The blue star is a late type O giant, with no obvious spectral anomalies to indicate that it is associated with the luminous X-ray source. The nuclear region is not heavily reddened; 2200 Ang absorption features in the spectra of both the nucleus and the star are weak. The data and the star formation history support the hypothesis that the M33 nuclear source, the brightest persistent source in the Local group, is an ~10 solar mass black hole binary.

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arXiv2001

Modelling the Spectral Signatures of Accretion Disk Winds in Cataclysmic Variables

Bipolar outflows are known to be present in many disk-accreting astrophysical systems. In disk-dominated cataclysmic variables, these outflows are responsible for most of the features in UV and FUV spectra. However, there have been very few attempts to model the features that appear in the spectra of disk-accreting cataclysmic variables quantitatively. The modelling that has been attempted has been concentrated almost entirely on explaining the shape of C IV. Here we describe a new hybrid Monte Carlo/Sobolev code that allows a synthesis of the complete UV spectrum of a disk-dominated cataclysmic variable. A large range of azimuthally-symmetric wind geometries can be modelled. Changes in line shape in eclipsing systems can also be studied. Features in the synthesized spectra include not only well-known resonance lines of O VI, NV, Si IV, and C IV, but, with an appropriate choice of mass loss rate and wind geometry, many of the lines originating from excited lower states that are observed in HUT, FUSE, and ORFEUS spectra. The line profiles of C IV resemble those calculated by previous workers, when an identical geometry is assumed. We compare the synthesized spectra to HUT spectrum of Z Cam, showing that in this case a reasonably good ``fit'' to the spectrum can be obtained.

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arXiv2001

First results from multi-wavelength observations during the 2001 outburst of WZ Sge

WZ Sge has shown superoutbursts in 1913, 1946 and 1978. On 2001 July 23 a new outburst was announced, about 10 years `too early'. Target of opportunity satellite observations with Chandra, FUSE, HST and RXTE were performed throughout the outburst. From the ground WZ Sge was monitored by numerous professional and amateur astronomers, in the optical, IR and radio. We give an account of the first exciting results from these multi-wavelength observations.

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arXiv1999

What We Learn from Quantitative Ultraviolet Spectroscopy of Naked White D warfs in Cataclysmic Variables

Using the Hopkins Ultraviolet Telescope and Hubble Space Telescope, observers have now obtained UV spectra with sufficient signal to noise and resolution to allow quantitative spectroscopic analyses of the WDs in several DNe. In the ``cleanest'' DNe, such as U Gem, the observations are allowing the basic physical parameters of the WD -- temperature, radius, gravity, rotation rate, and surface abundances -- to be established. A second component also exists in these systems, which may either be the disk or may be related to the WD itself. Here I summarize the current state of the observations and our understanding of the data, highlighting some of the uncertainties in the analyses as well the prospects for fundamentally advancing our understanding of DNe and WDs with future observations.

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arXiv1999

The Distance to the Cygnus Loop from Hubble Space Telescope Imaging of the Primary Shock Front

We present a Hubble Space Telescope/WFPC2 narrow-band H-alpha image of a region on the northeastern limb of the Cygnus Loop supernova remnant. This location provides a detailed example of where the primary blast wave first encounters the surrounding interstellar medium. The filament structure is seen in exquisite detail in this image, which was obtained primarily as an EARLY ACQuisition image for a follow-up spectroscopic program. We compare the HST image to a digitized version of the POSS-I red plate to measure the proper motion of this filament. By combining this value for the proper motion with previous measurements of the shock velocity at this position we find that the distance to the Cygnus Loop is 440 (+130, -100) pc, considerably smaller than the canonical value of 770 pc. We briefly discuss the ramifications of this new distance estimate for our understanding of this prototypical supernova remnant.

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arXiv1998

Optical Light Curve of the Type Ia Supernova 1998bu in M96 and the Supernova Calibration of the Hubble Constant

We present the UBVRI light curves of the Type Ia supernova SN 1998bu which appeared in the nearby galaxy M96 (NGC 3368). M96 is a spiral galaxy in the Leo I group which has a Cepheid-based distance. Our photometry allows us to calculate the absolute magnitude and reddening of this supernova. These data, when combined with measurements of the four other well-observed supernovae with Cepheid based distances, allow us to calculate the Hubble constant with respect to the Hubble flow defined by the distant Calan/Tololo Type Ia sample. We find a Hubble constant of 64.0 +/- 2.2(internal) +/- 3.5(external) km/s/Mpc, consistent with most previous estimates based on Type Ia supernovae. We note that the two well-observed Type Ia supernovae in Fornax, if placed at the Cepheid distance to the possible Fornax spiral NGC 1365, are apparently too faint with respect to the Calan/Tololo sample calibrated with the five Type Ia supernovae with Cepheid distances to the host galaxies.

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