Hubble Space Telescope Ultraviolet Spectroscopy of Fourteen Low-Redshift Quasars

University of Toronto, Toronto, Ontario, Canada
The Astronomical Journal (Impact Factor: 4.02). 11/2006; 133(2). DOI: 10.1086/510129
Source: OAI


We present low-resolution ultraviolet spectra of 14 low redshift (z<0.8) quasars observed with HST/STIS as part of a Snap project to understand the relationship between quasar outflows and luminosity. By design, all observations cover the CIV emission line. Nine of the quasars are from the Hamburg-ESO catalog, three are from the Palomar-Green catalog, and one is from the Parkes catalog. The sample contains a few interesting quasars including two broad absorption line (BAL) quasars (HE0143-3535, HE0436-2614), one quasar with a mini-BAL (HE1105-0746), and one quasar with associated narrow absorption (HE0409-5004). These BAL quasars are among the brightest known (though not the most luminous) since they lie at z<0.8. We compare the properties of these BAL quasars to the z<0.5 Palomar-Green and z>1.4 Large Bright Quasar samples. By design, our objects sample luminosities in between these two surveys, and our four absorbed objects are consistent with the v ~ L^0.62 relation derived by Laor & Brandt (2002). Another quasar, HE0441-2826, contains extremely weak emission lines and our spectrum is consistent with a simple power-law continuum. The quasar is radio-loud, but has a steep spectral index and a lobe-dominated morphology, which argues against it being a blazar. The unusual spectrum of this quasar resembles the spectra of the quasars PG1407+265, SDSSJ1136+0242, and PKS1004+13 for which several possible explanations have been entertained.

Download full-text


Available from: Martin Gaskell, Nov 06, 2012
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The optical–UV continuum of quasars is broadly consistent with the emission from a geometrically thin optically thick accretion disc (AD). The AD produces the ionizing continuum which powers the broad and narrow emission lines. The maximum AD effective temperature is given by , where M is the black hole mass, the accretion rate and fmax is set by the black hole spin a*. For a low enough value of , the AD may become too cold to produce ionizing photons. Such an object will form a lineless quasar. This occurs for a local blackbody (BB) AD with a luminosity Lopt= 1046 erg s−1 for M > 3.6 × 109 M⊙, when a*= 0, and for M > 1.4 × 1010 M⊙, when a*= 0.998. Using the AD-based , derived from M and Lopt, and the reverberation-based M, derived from Lopt and the Hβ full width at half-maximum, v, gives Teff, max∝L−0.13optv−1.45. Thus, Teff, max is mostly set by v. Quasars with a local BB AD become lineless for v > 8000 km s−1, when a*= 0, and for v > 16 000 km s−1, when a*= 0.998. Higher values of v are required if the AD is hotter than a local BB. The AD becoming non-ionizing may explain why line-emitting quasars with v > 10 000 km s−1 are rare. Weak low-ionization lines may still be present if the X-ray continuum is luminous enough, and such objects may form a population of weak line quasars (WLQ). If correct, such WLQ should show a steeply falling spectral energy distribution (SED) at λ < 1000 Å. Such an SED was observed by Hryniewicz et al. in SDSS J094533.99+100950.1, a WLQ observed down to 570 Å, which is well modelled by a rather cold AD SED. UV spectroscopy of z∼ 1–2 quasars is required to eliminate potential intervening Lyman limit absorption by the intergalactic medium and to explore if the SEDs of lineless quasars and some additional WLQ are also well fitted by a cold AD SED.
    Preview · Article · Oct 2011 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Variability, both in X-ray and optical/UV, affects the well-known anti-correlation between the $\alpha_{ox}$ spectral index and the UV luminosity of active galactic nuclei, contributing part of the dispersion around the average correlation ("intra-source dispersion"), in addition to the differences among the time-average $\alpha_{ox}$ values from source to source ("inter-source dispersion"). We want to evaluate the intrinsic $\alpha_{ox}$ variations in individual objects, and their effect on the dispersion of the $\alpha_{ox}-L_{UV}$ anti-correlation. We use simultaneous UV/X-ray data from Swift observations of a low-redshift sample, to derive the epoch-dependent $\alpha_{ox}(t)$ indices. We correct for the host galaxy contribution by a spectral fit of the optical/UV data. We compute ensemble structure functions to analyse variability of multi-epoch data. We find a strong "intrinsic $\alpha_{ox}$ variability", which makes an important contribution ($\sim40%$ of the total variance) to the dispersion of the $\alpha_{ox}-L_{UV}$ anti-correlation ("intra-source dispersion"). The strong X-ray variability and weaker UV variability of this sample are comparable to other samples of low-z AGNs, and are neither due to the high fraction of strongly variable NLS1s, nor to dilution of the optical variability by the host galaxies. Dilution affects instead the slope of the anti-correlation, which steepens, once corrected, becoming similar to higher luminosity sources. The structure function of $\alpha_{ox}$ increases with the time lag up to $\sim$1 month. This indicates the important contribution of the intermediate-long timescale variations, possibly generated in the outer parts of the accretion disk.
    Full-text · Article · Dec 2012 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Based on a recently started programme, we report the first search for intranight optical variability (INOV) among radio-quiet ‘weak-line-quasars’ (RQWLQs). Eight members of this class were observed on 13 nights in the R band, such that each source was monitored continuously at least once for a minimum duration of about 3.5 h, using the recently installed 130-cm telescope at Devasthal, India. Statistical analysis of the differential light curves was carried out using two versions of the F test. Based on the INOV data acquired so far, the RQWLQ population appears to exhibit stronger INOV activity as compared to the general population of radio-quiet quasars, but similar to the INOV known for radio-loud quasars of non-blazar type. To improve upon this early result, as well as extend the comparison to blazars, a factor of ∼2 improvement in the INOV detection threshold would be needed. Such efforts are underway, motivated by the objective to search for the elusive radio-quiet blazars using INOV observations.
    Preview · Article · Dec 2012 · Monthly Notices of the Royal Astronomical Society
Show more