arXiv subjects

K. S. Long

Publications and source records attributed to K. S. Long.

89 records · Page 5Linked to original sources

arXiv2002

Accretion and Outflow in Interacting Binary Systems: FUSE Observations of the Novalike Cataclysmic Variable, UX Ursae Majoris

We present far-ultraviolet (905 -- 1182 A), time-series spectroscopy of the eclipsing, novalike cataclysmic variable, UX UMa, acquired with FUSE. The time-averaged spectrum is complex and is dominated by overlapping spectral features. The most prominent features are emission lines of CIII, NIII}, NIV, and OVI. They are broad (FWHM >= 1800 km/s) and double-peaked with a central absorption at zero velocity. During eclipse, the spectrum is simpler: the emission lines remain bright, but the absorption components of the lines and the weaker features between the emission lines disappear entirely, leaving a flat continuum. This behavior is also evident in GHRS (1149 -- 1660 A) spectra that we retrieved from the HST archive. The FUV spectra show flickering on time scales of several minutes. The flickering is seen primarily in the continuum and/or the weaker lines rather than in the prominent emission lines. The orbital light curve has a dip in the FUV flux between orbital phases 0.45 -- 0.65, similar to a pre-eclipse dip detected in HST observations. The EWs of the line absorption features decrease during the dip. We have detected a systematic wavelength shift of spectral features on the orbital period, but with a phase lag of ~ 20 degrees, a phenomenon that has been reported at optical wavelengths. We discuss the implications of our results in the context of models of an accretion disk with a chromosphere between the disk and the extended wind. Finally, we note that the observed FUV flux is too low to be consistent with the temperature and radius of the WD derived by Baptista et al. (1995), suggesting that their remaining binary parameters, including a mass ratio of 1, ought to be viewed with skepticism.

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arXiv2002

The system parameters of DW Ursae Majoris

We present new constraints on the system parameters of the SW Sextantis star DW Ursae Majoris, based on ultraviolet (UV) eclipse observations with the Hubble Space Telescope. Our data were obtained during a low state of the system, in which the UV light was dominated by the hot white dwarf (WD) primary. Eclipse analysis, using the full Roche lobe geometry, allows us to set firm limits on the masses and radii of the system components and the distance between them: 0.67 \leq M_1/M_sun \leq 1.06, 0.008 \leq R_1/R_sun \leq 0.014, M_2/M_sun > 0.16, R_2/R_sun > 0.28 and a/R_sun > 1.05. For q = M_2/M_1 < 1.5 the inclination must satisfy i > 71 degrees. Using Smith & Dhillon's mass-period relation for CV secondaries, our estimates for the system parameters become M_1/M_sun = 0.77 \pm 0.07, R_1/R_sun = 0.012 \pm 0.001, M_2/M_sun = 0.30 \pm 0.10, R_2/R_sun = 0.34 \pm 0.04, q =0.39 \pm 0.12, i = 82 \pm 4 degrees and a/R_sun = 1.14 \pm 0.06. We have also estimated the spectral type of the secondary, M3.5 \pm 1.0, and distance to the system, d =930 \pm 160 pc, from time-resolved I- and K-band photometry. Finally, we have repeated Knigge et al.'s WD model atmosphere fit to the low-state UV spectrum of DW UMa in order to account for the higher surface gravity indicated by our eclipse analysis. In this way we obtained a second estimate for the distance, d = 590 \pm 100 pc, which allows us to obtain a second estimate for the spectral type of the secondary, M7 \pm 2.0. We conclude that the true value for the distance and spectral type will probably be in between the values obtained by the two methods.

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arXiv2002

QU Car: a very high luminosity nova-like binary with a carbon-enriched companion

QU Car is listed in cataclysmic variable star catalogues as a nova-like variable. This little-studied, yet bright interacting binary is re-appraised here in the light of new high-quality ultraviolet (UV) interstellar line data obtained with STIS on board the Hubble Space Telescope. The detection of a component of interstellar absorption at a mean LSR velocity of $-$14 km s$^{-1}$ indicates that the distance to QU Car may be $\sim$2 kpc or more -- a considerable increase on the previous lower-limiting distance of 500 pc. If so, the bolometric luminosity of QU Car could exceed $10^{37}$ ergs s$^{-1}$. This would place this binary in the luminosity domain occupied by known compact-binary supersoft X-ray sources. Even at a 500 pc, QU Car appears to be the most luminous nova-like variable known. New intermediate dispersion optical spectroscopy of QU Car spanning 3800--7000 Åis presented. These data yield the discovery that C{\sc iv} $λλ$5801,12 is present as an unusually prominent emission line in an otherwise low-contrast line spectrum. Using measurements of this and other lines in a recombination line analysis, it is shown that the C/He abundance as proxied by the n(C$^{4+}$)/n(He$^{2+}$) ratio may be as high as 0.06 (an order of magnitude higher than the solar ratio). Furthermore, the C/O abundance ratio is estimated to be greater than 1. These findings suggest that the companion in QU Car is a carbon star. If so, it would be the first example of a carbon star in such a binary. An early-type R star best matches the required abundance pattern and could escape detection at optical wavelengths provided the distance to QU Car is $\sim$2 kpc or more.

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arXiv2002

Time-resolved ultraviolet spectroscopy of the compact interacting binary QU Car

We present HST/STIS (1160--1700A) echelle spectra of the cataclysmic variable (CV) star, QU Car, observed in time-tag mode at three epochs. In catalogues this binary is classified as a nova-like variable. We find evidence of a high-state non-magnetic CV at low inclination, with unusually high ionisation. We observed narrow absorption lines (few hundred km/s wide) in N V1240, O V1371 and Si IV1398, as well as broader (HWZI ~1000km/s) emission in C III1176, C IV1549 and He II1640, all with a superposed absorption component. High ionisation is indicated by the unusually string He II emission and the relative strength of the O V absorption line. The dereddened UV SED of, on average, -2.3 suggests that disc accretion dominates the spectral energy distribution. In two observations velocity shifting is noted in the absorption lines on a timescale long enough not to repeat within the ~2600-sec exposures. The absorption superposed on the C IV emission line moves coherently with the N V and Si IV absorption, suggesting the same origin for all absorption lines -- most likely to be in the accretion disc atmosphere. Weak blueshifted absorption in NV and C\IV provides evidence of an outflow component and we estimate a maximum outflow velocity of ~2000km/s. This may be linked to a wind launched from further out in the disc than is typically seen in those high-state non-magnetic CV whose wind speeds are observed to reach to >4000km/s. Unusually, three ionisation stages of carbon -- C II, C III and C IV -- are present in emission, with line width increasing with higher ionisation. The presence of C II in emission and the positive line-width/ionisation correlation is most easily reconciled with an origin in a disc chromosphere, beyond the influence of the EUV-emitting inner disc.

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arXiv2001

Testing the line-driven disk wind model: time-resolved UV spectroscopy of IX Vel and V3885 Sgr

To confront the predictions of the most recent line-driven disk wind models with observational evidence, we have obtained HST STIS (1180--1700A) echelle spectra of the nova-like variables IX Vel and V3885 Sgr at three epochs. The targets were observed in timetag mode for ~2000sec on each occasion, allowing us to study the spectral time evolution on timescales down to ~10sec. The mean UV spectra are characterised by the wind signature of broad blueshifted absorption in Lya, NV1240, SiIV1398, CIV1549 and HeII1640. There is very little redshifted emission other than in CIV. Narrow blueshifted absorption dips, superposed on the broad absorption at around -900km/s, accompany periods of well-developed wind activity. The continuum level and mean line profiles vary markedly from observation to observation -- with the wind signatures almost disappearing in one epoch of observation of IX Vel. The strong positive correlation between UV brightness and wind activity predicted by line-driven disk wind models is disobeyed by both binaries. The wind signatures in IX Vel's UV spectrum are revealed to be remarkably steady on timescales ranging from ~10 to ~1000sec. More variability is seen in V3885 Sgr, the binary with the lower opacity outflow. But there is only one epoch in which the line profile changes significantly in ~100sec or less. Narrow absorption dips, when present, show only smooth, small changes in velocity.

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arXiv2001

The Evolution of Disks and Winds in Dwarf Novae Outbursts

Far ultraviolet (FUV) observations are excellent probes of the inner accretion disk, disk outflows, and the mass-accumulating white dwarf in cataclysmic variables. Here we study the contrasting behavior of two canonical dwarf novae in outburst by presenting FUSE FUV (904 -- 1187 A) spectroscopy of U Gem and SS Cyg. We observed each system four times during a single outburst. The outburst peak and early decline spectra of SS Cyg are well fit by models of a steady-state accretion disk and a biconical wind. A broad, blueshifted OVI wind-formed absorption line is the only strong spectral feature. In late outburst decline, OVI and CIII lines are seen as broad emission features and the continuum has flattened. In U Gem, the continua of the optical outburst plateau spectra are plausibly fit by accretion disk model spectra. The spectra also show numerous narrow, low-velocity absorption lines that do not originate in the inner accretion disk. We discuss the line spectra in the context of partial absorption of the FUV continuum by low-velocity, vertically-extended material located at large disk radii. The late outburst decline spectrum of U Gem is dominated by the white dwarf. WD model fits confirm the sub-solar C and super-solar N abundances found in earlier studies.

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arXiv2001

FUSE Observations of U Geminorum During Outburst and Decline

We have obtained FUV (904 -- 1187 Å) spectra of U Gem in outburst with FUSE. Three of the observations were acquired during the plateau phase of the outburst, while the fourth was obtained during late outburst decline. The plateau spectra have continuum shapes and fluxes that are approximated by steady-state accretion disk model spectra with $\dot{m} \simeq$ 7e-09 M_sun/yr. The spectra also show numerous absorption lines of H I, He II, and two- to five-times ionized transitions of C, N, O, P, S and Si. There are no emission features in the spectra, with the possible exception of a weak feature on the red wing of the O VI doublet. The absorption lines are narrow (FWHM ~ 500 km/s), too narrow to arise from the disk photosphere, and at low velocities (<= 700 km/s). The S VI and O VI doublets are optically thick. The absorption lines in the plateau spectra show orbital variability: in spectra obtained at orbital phases 0.53 -- 0.79, low-ionization absorption lines appear and the central depths of the pre-existing lines increase. The increase in line absorption occurs at the same orbital phases as previously observed EUV and X-ray light curve dips. If the absorbing material is in (near-) Keplerian rotation around the disk, it must be located at large disk radii. The final observation occurred when U Gem was about two magnitudes from optical quiescence. The spectra are dominated by emission from an $\simeq$43,000 K, metal-enriched white dwarf (WD). The inferred radius of the WD is 4.95e+08 cm, close to that observed in quiescence. Allowing for a hot heated region on the surface of the WD improves the fit to the spectrum at short (< 960 Å) wavelengths.

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arXiv2000

A Far-Ultraviolet Spectroscopic Survey of the Globular Cluster 47 Tuc

We present preliminary results from a FUV spectroscopic and photometric survey of the globular cluster 47 Tuc. Our main goal is to either confirm or rule out the existence of a large population of CVs in the core of this cluster. We have so far identified approximately 425 FUV sources, most of which are probably hot, young WDs. However, we have also found approximately 30 FUV sources whose position in a FUV-optical color-magnitude diagram makes them strong CV candidates. If most or all of these objects are eventually confirmed as CVs, the large CV population predicted by tidal capture theory has finally been found. Our data has also allowed us to resolve the long-standing puzzle surrounding AKO 9, a UV-bright and highly variable 1.1 day binary system in 47 Tuc. AKO 9 is the brightest FUV source in our data and presents a blue FUV spectrum with strong CIV and HeII emission lines. Its spectrum is similar to that of the long-period, DN-type field CV GK Per. Based on this similarity and other evidence, we suggest that AKO 9 is a long-period CV in which mass-transfer is driven by the nuclear evolution of a sub-giant donor star.

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arXiv2000

A Self-Occulting Accretion Disk in the SW Sex Star DW UMa

We present the ultraviolet spectrum of the SW Sex star and nova-like variable DW UMa in an optical low state, as observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST). The data are well described by a synthetic white dwarf (WD) spectrum with T_eff = 46,000 +/- 1000 K, log g = 7.60 +/- 0.15, v*sin(i) = 370 +/- 100 km/s and Z/Z_solar = 0.47 +/- 0.15. For this combination of T_eff and log g, WD models predict M_WD = 0.48 +/- 0.06 M_solar and R_WD = (1.27 +/- 0.18) * 10^9 cm. Combining the radius estimate with the normalization of the spectral fit, we obtain a distance estimate of d = 830 +/-150 pc. During our observations, DW UMa was approximately 3 magnitudes fainter in V than in the high state. A comparison of our low-state HST spectrum to a high-state spectrum obtained with the International Ultraviolet Explorer shows that the former is much bluer and has a higher continuum level shortward of 1450 A. Since DW UMa is an eclipsing system, this suggests that an optically thick accretion disk rim blocks our view of the WD primary in the high state. If self-occulting accretion disks are common among the SW Sex stars, we can account for (i) the preference for high-inclination systems within the class and (ii) their V-shaped continuum eclipses. Moreover, even though the emission lines produced by a self-obscured disk are generally still double-peaked, they are weaker and narrower than those produced by an unobscured disk. This may allow a secondary line emission mechanism to dominate and produce the single-peaked, optical lines that are a distinguishing characteristic of the SW Sex stars.

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arXiv2000

HST-STIS Observations of the Cygnus Loop: Spatial Structure of a Non-radiative Shock

We present a spatially-resolved ultraviolet spectrum of a non-radiative shock front in the Cygnus Loop, obtained with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). The spectrum covers the wavelength range 1118 - 1716 angstroms, with an effective spectral resolution of ~12 angstroms. The 0.1" spatial resolution of these data provides a huge improvement over earlier ultraviolet spectra, allowing us to study the spatial distribution of high ionization line emission directly behind the shock front. We are able to isolate individual shock features in our spectrum by comparing the STIS spectrum with a WFPC2 H-alpha image of the region. Isolating the brightest shock tangency, we identify lines of NV, CIV, HeII, OV, OIV] and SiIV, and NIV] as well as the hydrogen 2-photon continuum. The NV line peaks about 0.3" behind the CIV and HeII emission and is spatially broader. Also, the observed line ratios of CIV and HeII to NV are higher in our bright shock spectrum than in previous observations of the same filament obtained through much larger apertures, indicating that there must be a more widely distributed component of the NV emission. We calculate shock models and show that the observed separation between the CIV and NV emission zones and observed line intensities constrain the combinations of shock velocity and pre-shock density that are allowed.

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arXiv1999

The Compact UV Nucleus of M33

The most luminous X-ray source in the Local Group is associated with the nucleus of M33. This source, M33 X-8, appears modulated by ~20% over a ~106 day period, making it unlikely that the combined emission from unresolved sources could explain the otherwise persistent ~1e39 erg/s X-ray flux (Dubus et al. 1997, Hernquist et al. 1991). We present here high resolution UV imaging of the nucleus with the Planetary Camera of the HST undertaken in order to search for the counterpart to X-8. The nucleus is bluer and more compact than at longer wavelength images but it is still extended with half of its 3e38 erg/s UV luminosity coming from the inner 0.14". We cannot distinguish between a concentrated blue population and emission from a single object.

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arXiv1998

The eclipsing X-ray pulsar X-7 in M33

Using our extensive ROSAT X-ray observations of M33, we confirm a 3.45 day eclipse period for the Einstein source X-7 (Larson & Schulman, 1997) and discover evidence for a 0.31-s pulse period. The orbital period, pulse period and observed X-ray luminosity are remarkably similar to SMC X-1. We therefore suggest M33 X-7 is a neutron star high mass X-ray binary with a 15-40 Msol O/B companion and a binary separation of 25-33 Rsol if the companion is almost filling its Roche lobe.

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arXiv1998

HST spatially-resolved spectra of the accretion disc and gas stream of the nova-like variable UX Ursae Majoris

Time-resolved eclipse spectroscopy of the nova-like variable UX UMa obtained with the HST/FOS on 1994 August and November is analyzed with eclipse mapping techniques to produce spatially resolved spectra of its accretion disc and gas stream as a function of distance from disc centre. The inner accretion disc is characterized by a blue continuum filled with absorption bands and lines which cross over to emission with increasing disc radius, similar to that reported by Rutten et al (1994) at optical wavelengths. The comparison of spatially resolved spectra at different azimuths reveals a significant asymmetry in the disc emission at UV wavelengths, with the disc side closest to the secondary star showing pronounced absorption by an `iron curtain' and a Balmer jump in absorption. These results suggest the existence of an absorbing ring of cold gas whose density and/or vertical scale increase with disc radius. The spectrum of the infalling gas stream is noticeably different from the disc spectrum at the same radius suggesting that gas overflows through the impact point at disc rim and continues along the stream trajectory, producing distinct emission down to 0.1 RL1. The radial temperature profiles of the continuum maps are well described by a steady-state disc model in the inner and intermediate disc regions. There is evidence of an increase in the mass accretion rate from August to November (from Mdot= 10^{-8.3 +/- 0.1} to 10^{-8.1 +/- 0.1} Msun/yr), in accordance with the observed increase in brightness. Since the UX UMa disc seems to be in a high mass accretion, high-viscosity regime in both epochs, this result suggests that the mass transfer rate of UX UMa varies substantially (~ 50 per cent) on time scales of a few months.

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arXiv1998

HST/FOS Eclipse Observations of the Nova-like Cataclysmic Variable UX Ursae Majoris

[abridged abstract] We present and analyze Hubble Space Telescope observations of the eclipsing nova-like cataclysmic variable UX UMa obtained with the Faint Object Spectrograph. Two eclipses each were observed with the G160L grating (covering the ultraviolet waveband) in August of 1994 and with the PRISM (covering the near-ultraviolet to near-infrared) in November of the same year. The system was 50% brighter in November than in August, which, if due to a change in the accretion rate, indicates a fairly substantial increase in Mdot_acc by >~ 50%. Model disk spectra constructed as ensembles of stellar atmospheres provide poor descriptions of the observed post-eclipse spectra, despite the fact that UX UMa's light should be dominated by the disk at this time. Suitably scaled single temperature model stellar atmospheres with T_eff = 12,500-14,500 K actually provide a better match to both the ultraviolet and optical post-eclipse spectra. Evidently, great care must be taken in attempts to derive accretion rates from comparisons of disk models to observations. One way to reconcile disk models with the observed post-eclipse spectra is to postulate the presence of a significant amount of optically thin material in the system. Such an optically thin component might be associated with the transition region (``chromosphere'') between the disk photosphere and the fast wind from the system, whose presence has been suggested by Knigge & Drew (1997).

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arXiv1998

Recovery of 29-s Oscillations in HST/FOS Eclipse Observations of the Cataclysmic Variable UX Ursae Majoris

[abridged abstract] Low amplitude (~=0.5%) 29-s oscillations have been detected in HST/FOS eclipse observations of the nova-like cataclysmic variable UX UMa. These are the same dwarf nova-type oscillations that were originally discovered in this system by Warner & Nather in 1972. The oscillations are sinusoidal to within the small observational errors and undergo an approximately -360 degree phase shift during eclipses. Their amplitudes are highest at pre-eclipse orbital phases and exhibit a rather gradual eclipse whose shape is roughly similar to UX~UMa's overall light curve. Oscillation spectra derived from pre-eclipse data segments are extremely blue, whereas mid-eclipse oscillation spectra are much redder. We suggest that the ultimate source of the oscillations is a hot, compact region near disk center, but that a significant fraction of the observed, modulated flux is due to reprocessing of the light emitted by this source in the accretion disk atmosphere. The compact source is occulted at orbital phases near mid-eclipse, leaving only part of the more extended reprocessing region(s) to produce the weak oscillations that persist even at conjunction. The highly sinusoidal oscillation pulse shape does not permit the identification of the compact component in this model with emission produced by a rotating disturbance in the inner disk or in an equatorial boundary layer, and a standard intermediate polar model can also be ruled out. A model invoking magnetically controlled accretion onto differentially rotating WD surface layers may be viable, but needs more theoretical work.

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arXiv1997

A 106-d period in the nuclear source X-8 in M33

With an X-ray luminosity of about 10^39 erg/s, the source X-8 coincident with the optical center of M33 is the most luminous X-ray source in the Local Group. However, its nature remains a mystery. We present here new and archival ROSAT observations of X-8 spread over 6 years which show variability and a ~ 106-d periodicity. This implies that (most of) the emission from M33 X-8 arises from a single object, perhaps a binary system with a ~ 10 Msol black hole primary. We suggest that the companion is a giant orbiting with a ~ 10 day period, and that the observed modulation is ``super-orbital'', analogous to that seen in Cyg X-2 and X1820-30.

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arXiv1997

Hopkins Ultraviolet Telescope Observations of Her X-1

We have obtained a far-ultraviolet spectrum of the X-ray binary Hercules X-1/HZ Herculis using the Hopkins Ultraviolet Telescope aboard the Astro-1 space shuttle mission in 1990 December. This is the first spectrum of Her X-1 that extends down to the Lyman limit at 912 A. We observed emission lines of O VI, N V, and C IV, and the far UV continuum extending to the Lyman limit. We examine the conditions of the emitting gas through line strengths, line ratios, and doublet ratios. The UV flux is lower by about a factor of 2 than expected at the orbital phase of the observation. We model the UV continuum with a simple power-law and with a detailed model of an X-ray-illuminated accretion disk and companion star. The power-law provides a superior fit, as the detailed model predicts too little flux below 1200 A. We note, however, that there are uncertainties in the interstellar reddening, in the background airglow spectrum, and in the long-term phase of the accretion disk. We have searched the data for UV line and continuum pulsations near the neutron star spin period but found none at a detectable level.

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