An Update on the X-Ray Transient Narrow-Line Seyfert 1 Galaxy WPVS 007:SwiftObservations of UV Variability and Persistence of X-Ray Faintness

The faintest object which can be observed with the Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) is set by the detector cosmic-ray background and not by object flux. We use data from Beaver and Lyons to show that 48% of the background counts are from cosmic rays which each generate a near instantaneous burst of two or more counts. Setting the FOS threshold parameter REJLIM = 1, which rejects all frames with more than one count increases the ratio of signal-to-dark counts (S/D) by a factor of 1.94, regardless of the frame time or the object signal, because half of the dark counts which arrive in single counts (from either cosmic-ray bursts or thermal dark current photoemission) are rejected at the same rate as the object signal. But the signal-to-noise ratio (SNR proportional to S/square root of S + D) increases by at most a factor of 1.35, and for realistic parameters and taking into account dead time, we expect a gain in SNR of only 1.18. If a diode has failed and now emits noise, no data at all will be recorded. The chance of this occurring is approximately = 10% , and for this reason we do not recommend the use of REJLIM. The two-point correlation function of dark counts per pixel has strong peaks every four pixels, caused by the action of quarter-stepping on the counts from large bursts. The counts from such bursts spread over at least 80-100 diodes, and we show that such bursts can be rejected during data reduction if the data are recorded in RAPID mode with individual exposures of about 35 seconds. The SNR of the spectrum can also be improved by weighting each exposure by its SNR (a function ofthe mean dark count rate at that time), since dark varies by a factor of 2 around an orbit. These two procedures together increase the SNR by a factor of 1.1.3 (a 28% gain in exposure time) in regions of a spectrum where the object is much fainter than the background. We find that the Ly-alpha and O I sky emission lines give at most 4 (counts/array/s/sq arcsec), and typically half this number. With a small frame time and a small entrance aperture, these counts cause REJLIM = 1 to reject only a few percent of the data. We discuss only the HST FOS blue side detector, but the same ideas apply to any photon counter which records bursts of noise, and the calculations resemble those for the loss of signal due to coincidences in photon counters. In the ideal photon counter the dark counts are readily distinguished from photons. One way to do this is to add detectors which records only dark current, so that every cosmic ray produces more than one count and the instrument can be operated in an anti-coincidence mode.

Despite the contributions of the Hubble Space Telescope Faint Object Spectrograph (FOS) to the archive of UV observations of active galactic nuclei, the vast majority of UV reference data were obtained using the International Ultraviolet Explorer (IUE) satellite. These data remain important since they provide historical information about the intensities of the UV continua and emission lines that is needed to constrain models of the active nucleus. A detailed comparison of the FOS and IUE data is critical to understanding how the measurable quantities depend on the individual instrumental calibrations, and how any conclusions derived from modeling the observations may vary depending on the source of the UV data. Rigorous comparison of FOS and IUE spectra have so far been performed only for spectrophotometric standard star observations that are acquired accurately and have high signal-to-noise ratios. We compare typical FOS spectra that were not acquired and observed with the strict regimen that is used for standard-star observations, especially in the pre-COSTAR era. All nonproprietary UV FOS spectrophotometric archival data for the Seyfert 1 galaxies Mrk 509, NGC 3783, and NGC 5548 that have near-simultaneous (within 24 hr) IUE observations are used in the analysis. These data demonstrate that the absolute photometric calibrations of the FOS and IUE agree within ~5% in absolute flux for two of the objects. For NGC 5548, the FOS and IUE flux data disagree by ~50% in the 1200-2000 A region. In this object there may be evidence for flux nonlinearity of the IUE detector and a contribution from the host galaxy redward of 2800 A. Cross-correlation of the FOS and IUE spectra reveals no zero-point wavelength shift larger than the IUE wavelength calibration errors. Comparison of line flux measurements from both the FOS and IUE spectra show that for strong emission lines (e.g., Lyα, C IV, and Mg II) the measured intensities always agree within 15%, while for moderately strong lines (e.g., N V, Si IV/O IV, He II, and C III]) the agreement is ~30% (1 σ). Weak lines (e.g., O I, C II, N IV], O III], and N III]) may not even be detected in the IUE spectra, and when they are detected the disagreement between the measured fluxes can be very large.

Ultraviolet spectroscopic study of EU UMa and ST LMi from HST and IUE observations

We report the results of spectral and photometric observations of DQ Her. Available spectra from International Ultraviolet Explorer (IUE), Hubble Space Telescope Faint Object Spectrograph (HST - FOS) and CCD photometry of one night were used. Some profiles revealing the variations of some spectral lines at different times are presented. There is variation with time for photometric observations and the brightness of DQ Her is changed from 14 mag. to 17.7 mag with clear display of burst. The ultraviolet luminosity for emitting region is in the range of (1.9x1031 erg s −1). The mass accretion rate is in the range of (3.5x10−12 M⊙yr −1). The line flux modulations at different times can be explained in terms of the accretion curtain model for intermediate polars, Kim & Beuermann (1996).

We have produced an archive of the ultraviolet low-dispersion spectra for the full set of white dwarf stars observed with the International Ultraviolet Explorer (IUE) satellite over the course of its 18 yr mission. This archive contains the spectra of 322 individual degenerate stars which have been processed to optimize the signal-to-noise for each star. In particular, all spectra have been corrected for residual temporal and thermal effects and placed on the Hubble Space Telescope Faint Object Spectrograph absolute flux scale using procedures described by Massa & Fitzpatrick. Wherever possible, multiple observations of individual stars have been co-added to further enhance signal-to-noise and have been combined into a single spectrum

view Abstract Citations (3) References (1) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hubble Space Telescope Faint Object Spectrograph and Ground-based Observations of the Broad Absorption Line Quasar 0226-1024: Erratum Korista, Kirk T. ; Weymann, Ray J. ; Morris, Simon L. ; Kopko, Michael, Jr. ; Turnshek, David A. ; Hartig, George F. ; Foltz, Craig B. ; Burbidge, E. Margaret ; Junkkarinen, Vesa T. Abstract In the paper `Hubble Space Telescope Faint Object spectrograph and Ground-based Observations of the Broad Absorption Line Quasar 0226 - 1024" by Kirk T. Korista et al. (ApJ, 401,529 [1992]), two minor errors appear. The first appears in Table 1: the logarithmic relative number abundance of argon should be -5.440, not -4.440. This error occurs only in the table and has no effect on the subsequent analysis. The second appears in Section 3.3 on page 537. The wavelength for the Si II transition mentioned in the text should be 1260 A, not 1263 A. Publication: The Astrophysical Journal Pub Date: August 1993 DOI: 10.1086/173013 Bibcode: 1993ApJ...413..445K Keywords: ERRATA; ADDENDA full text sources ADS | Related Materials (1) Original Paper: 1992ApJ...401..529K

As a step towards clarifying the ionization mechanism of LINERs, we have used the Hubble Space Telescope Faint Object Spectrograph to obtain an ultraviolet spectrum of the nucleus of NGC 6500. Compared with more luminous Seyfert nuclei, the emission-line spectrum of NGC 6500 is remarkable for its low-excitation character: C III], C II], and Mg II are the strongest collisionally excited lines in the UV, and C IV is not detected. The fast shock models of Dopita & Sutherland (1995, 1996) are a poor match to the observed spectrum, as they predict much stronger emission than is observed for high-excitation lines such as C IV. Photoionization by an obscured nonstellar continuum source, or possibly ionization by slower (100 km/s) shocks, are more likely explanations for the emission-line ratios. The origin of the UV continuum is unclear; its overall spectral shape is reasonably well matched by spectral evolution models for single-burst populations with ages in the range 70-100 Myr, but no definite absorption features from young stars are present. Alternately, the UV continuum (or some fraction of it) may be scattered radiation from a hidden active nucleus. We tentatively detect a broad He II 4686 emission feature, which may be due to Wolf-Rayet stars, although the UV spectrum does not show any clear Wolf-Rayet signatures. We also find that NGC 6500 is weakly detected in an archival ROSAT HRI image with a luminosity of 5 x 10^40 erg/s in the 0.1-2.4 keV band, within the normal range for LINERs.

The narrow absorption lines (NALs) that are seen in the rest-frame ultraviolet near the systemic redshift of active galactic nuclei (AGNs) are not always intrinsic to the near-nuclear region but may originate in the host galaxy or in neighboring galaxies intervening along the line of sight. A variety of criteria have been sought—and several identified—as evidence of an intrinsic origin. We have measured both emission and absorption lines in a Hubble Space Telescope Faint Object Spectrograph sample of objects with both C IV and N V NALs within ±5000 km s-1 of the systemic redshift. We find a strong (>99.5% confidence) linear correlation between the N V/C IV ratio in broad emission lines and that in NALs. A control sample of AGNs with NALs separated by larger velocities shows no such correlation. Our finding thus identifies an additional test for the intrinsic nature of NALs in any given object. The correlation shows that the chemical-enrichment histories and/or ionization parameters of the NAL clouds are closely related to those of clouds that produce the broad emission lines.

We present an analysis of the blank-sky spectra observed with the Faint Object Spectrograph on board the Hubble Space Telescope. We study the diffuse sky emission from ultraviolet to optical wavelengths, which is composed of zodiacal light (ZL), diffuse Galactic light (DGL), and residual emission. The observations were performed towards 54 fields distributed widely over the sky, with spectral coverage from 0.2 to 0.7 μm. In order to avoid contaminating light from earthshine, we use the data collected only in orbital nighttime. The observed intensity is decomposed into the ZL, DGL, and residual emission, in eight photometric bands spanning our spectral coverage. We found that the derived ZL reflectance spectrum is flat in the optical, which indicates major contribution of C-type asteroids to the interplanetary dust (IPD). In addition, the ZL reflectance spectrum has an absorption feature at ∼0.3 μm. The shape of the DGL spectrum is consistent with those found in earlier measurements and model predictions. While the residual emission contains a contribution from the extragalactic background light, we found that the spectral shape of the residual looks similar to the ZL spectrum. Moreover, its optical intensity is much higher than that measured from beyond the IPD cloud by Pioneer 10/11, and also than that of the integrated galaxy light. These findings may indicate the presence of an isotropic ZL component, which is missed in the conventional ZL models.

Ultraviolet spectra of two intermediate polars (IPs), PQ Gem and V405 Aur, observed with Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph and Faint Object Spectrograph and International Ultraviolet Explorer (IUE) satellites were analyzed during the period between 1994–2000. We estimated the reddening of the two systems from the 2200 Å feature. Six spectra of the two systems revealing modulations of line fluxes at different times are presented. PQ Gem and V405 Aur are featured by spectral lines in different ionization states. This paper focuses on the third ionized carbon emission line at 1550 Å and the first ionized helium emission line at 1640 Å produced in the optically thin outer region of the accretion curtain for the two systems by calculating spectral line fluxes. From HST and IUE data, we deduced ultraviolet luminosities and ultraviolet accretion rates for the two binary stars. The average temperature of the accretion streams for PQ Gem and V405 Aur are ∼4500 K and 4100 K, respectively. The results reveal that there are modulations in fluxes of spectral lines, ultraviolet luminosities, and ultraviolet accretion rates with time for both systems. These modulations are referred to the changes of both density and temperature as a result of the variations of mass transfer rate from the secondary star to the primary star. The current results are consistent with an accretion curtain model for IPs.

We obtained a spectrum of Europa from 2224 to 3302 Å with high signal to noise and a resolution of 6.3 Å with the Hubble space telescope faint object spectrograph. We detect a broad absorption feature centered at 2800 Å, seen previously at lower signal to noise in International Ultraviolet Explorer spectra of Europa. No additional absorption features are detected. We compare the observed spectrum to laboratory spectra of sulfur ions in water ice and sulfur dioxide ice on water ice. The absorption in Europa is a closer match to sulfur dioxide ice. The poor fit of ice irradiated by sulfur ions may indicate that a direct source of SO2 is required to explain the observed absorption.

Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) spectra are presented for three bright Seyfert galaxies including one (PG 1351+64) which possesses blue-displaced absorption features in C IV, Si IV, N V, and Ly-alpha (but not in Mg II) similar to those seen at high redshift in the broad-absorption-line (BAL) quasi-stellar objects (QSOs). Several features of the absorptions in PG 1351+64, including variability seen in archival International Ultraviolet Explorer (IUE) data, confirm their similarity to the BAL clouds rather than to the 'associated absorber' phenomenon which they superficially resemble. In PG 1351+64 'satellite' emission lines (called herein E1 and E2) have been detected nearly symmetrically placed at +/- 4000 km/s around the Mg II emission line; this velocity is just larger than the most blue-displaced of the BALs, suggesting that these two phenomena are related. The satellite line luminosity, L(E1) approximately = (3 x 10(exp 41) erg/s)/sq. h(sub 75), requires a cloud emission measure n(sub e)(exp 2)V(sub c) approximately = (1 x 10(exp 64)/cc)/sq. h(sub 75) at T approximately = 30,000 K. We believe the Mg II lines are produced by 50-90 km/s shocks driven into dense (approximately 10(exp 6)/cc, pre-shock) clouds by the ram pressure of a 0.1 solar mass/yr wind leaving the nucleus at velocity 4000 km/s. The detection of satellite lines in Mg II, the detection of the blue-shifted cloud in H-alpha and H-beta and the nondetection of this cloud in C IV restricts the cloud shocks to velocities v(sub c) less than or = 90 km/s and requires a cloud/wind density contrast greater than or = 10(exp 3). In this model, the emitting clouds are located at distances of R(sub eff) approximately = 10(exp 18-19) cm from the nucleus and are entrained and shock-accelerated to approximately 4000 km/s. The possible detection of similar 'satellite' emission features in the non-BAL Seyfert, Ton 951, suggests that the subrelativistic wind that accelerates BAL clouds is a physical feature of many radio-quiet active galactic nuclei (AGN).

Just before earth passed through Saturn's ring plane on 10 August 1995, the Hubble Space Telescope Faint Object Spectrograph detected ultraviolet fluorescent emissions from a tenuous atmosphere of OH molecules enveloping the rings. Brightnesses decrease with increasing distance above the rings, implying a scale height of about 0.45 Saturn radii (Rs). A spatial scan 0.28Rs above the A and B rings indicates OH column densities of about 10(13) cm(-2) and number densities of up to 700 cm(-3). Saturn's rings must produce roughly 10(25) to 10(29) OH molecules per second to maintain the observed OH distribution.

We present Far Ultraviolet Spectroscopic Explorer spectroscopy of Sand 2, an LMC WO-type Wolf-Rayet star, revealing the O VI resonance P Cygni doublet at 1032-1038 A. These data are combined with Hubble Space Telescope Faint Object Spectrograph ultraviolet and Mount Stromlo 2.3 m optical spectroscopy and analyzed using a spherical, non-LTE, line-blanketed code. Our study reveals exceptional stellar parameters: T* ~ 150,000 K, v∞ = 4100 km s-1, log(L/L☉) = 5.3, and = 1 × 10-5 M☉ yr-1, if we adopt a volume filling factor of 10%. Elemental abundances of C/He ~ 0.7 ± 0.2 and O/He ~ 0.15 by number qualitatively support previous recombination line studies. We confirm that Sand 2 is more chemically enriched in carbon than LMC WC stars and that it is expected to undergo a supernova explosion within the next 5 × 104 yr.

view Abstract Citations (20) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hubble Space Telescope FOS Observations of KPD 0005+5106: A Subluminous WN-WC Descendant with Ongoing Mass Outflow? Sion, Edward M. ; Downes, Ronald A. Abstract The results are reported of spectroscopic observations of the ultrahigh excitation, helium-rich, prewhite-dwarf KPD 0005 + 5106 obtained with the HST Faint Object Spectrograph (FOS) in 1991 May and July with the red (FOS/RD) and blue (FOS/BL) Digicon detectors. The data reveal a rich line spectrum both in absorption and in emission with ultra-high excitation species present including O VIII, N V, possibly C V, Fe VI, Fe VII, and numerous weaker high n, low l, transitions of C IV and O VI as well as the predominant He II (3 yields n) and He II (Balmer alpha) absorption lines. There is a strong emission complex at 2981 A which is identified as primarily due to three transitions of N V commonly seen in WN Wolf-Rayet spectra. Evidence is presented that high-ionization species in emission (O VIII, N V, C IV, Si IV) and in absorption (He II, Fe VI, Fe VIII) are longward-shifted relative to the far-UV resonance (circumstellar) absorption lines by 25-50 km/s. Publication: The Astrophysical Journal Pub Date: September 1992 DOI: 10.1086/186521 Bibcode: 1992ApJ...396L..79S Keywords: Abundance; Hubble Space Telescope; Stellar Spectra; Stellar Spectrophotometry; White Dwarf Stars; Carbon; Faint Objects; Iron; Line Spectra; Nitrogen; Oxygen Spectra; Wolf-Rayet Stars; Astrophysics; STARS: ABUNDANCES; STARS: EVOLUTION; STARS: MASS LOSS; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (4) MAST (2) INES (1) ESA (1)