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  • 1
    Electronic Resource
    Electronic Resource
    The broad emission-line region in AGNs and quasars: The impact of variability studies (1996)
    Springer
    Astrophysics and space science 237 (1996), S. 207-240 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Coordinated observations of variability of the continuum and the emission-line luminosities (reverberation mapping) in AGNs and quasars have fundamentally altered our understanding of the broad-line regions in active galaxies. The constraints these observations impose on the models of the BLRs have been demonstrated here by an attempt to model the BLR of NGC5548, the most intensively monitored AGN. Two models of a BLR, with one having a power law radial distribution of density and the other a gaussian radial distribution, are described and the modelled line luminosities and centroid of the line response functions are presented. A self-consistent model is presented for the change in theCiv/Lyα ratio as the continuum luminosity changes. It is shown that BLR gas must be composed of a mixture of optically thin and optically thick gas and the proportion of thick and thin gas alters with the luminosity of the ionizing continuum. The observed centroid or the lag of a line, can be a function of the continuum luminosity. The variability of the profile of theCiv line in the spectrum of NGC5548 is investigated. This profile is extremely robust and is not significantly affected by changes in the ionizing continuum. Future models of the kinematics of the BLR clouds will have to be based on very stable cloud motions and include anisotropic line emission.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02424433
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  • 2
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    Interpreting broad emission-line variations - II. Tensions between luminosity, characteristic size, and responsivity (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-11
    Description: We investigate the variability behaviour of the broad H β emission-line to driving continuum variations in the best-studied AGN NGC 5548. For a particular choice of broad emission-line region (BLR) geometry, H β surface emissivity based on photoionization models, and using a scaled version of the 13-yr optical continuum light-curve as a proxy for the driving ionizing continuum, we explore several key factors that determine the broad emission-line luminosity L , characteristic size R RW , and variability amplitude (i.e. responsivity) , as well as the interplay between them. For fixed boundary models which extend as far as the hot dust the predicted delays for H β are on average too long. However, the predicted variability amplitude of H β provides a remarkably good match to observations except during low-continuum states. We suggest that the continuum flux variations which drive the redistribution in H β surface emissivity F ( r ) do not on their own lead to large enough changes in R RW or eff . We thus investigate dust-bounded BLRs for which the location of the effective outer boundary is modulated by the continuum level and the dust-sublimation and dust-condensation time-scales. We find that in order to match the observed variability amplitude of broad H β in NGC 5548 a rather static outer boundary is preferred. Intriguingly, we show that the most effective way of reducing the H β delay, while preserving its responsivity and equivalent width, is to invoke a smaller value in the incident ionizing photon flux H for a given ionizing source–cloud radial distance r , than is normally inferred from the observed UV continuum flux and typical models of the continuum spectral energy distribution.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Physical properties and the variability mechanism of the He I outflow in NGC 4151 (2016)
    Oxford University Press
    Details
    Publication Date: 2016-07-13
    Description: We report on variable helium absorption lines in NGC 4151 observed across six epochs of quasi-simultaneous near-infrared and optical data. These observations cover the transitions from the metastable 2 3 S state at 3889 Å and 10 830 Å, and from the 2 1 S state at 20 587 Å. This is the first active galactic nuclei absorption line variability study to include measurements of the 20 587 Å line. The physical properties of the absorber recorded at the fifth observational epoch are relatively well constrained by the presence of absorption in both the optical and near-infrared components, with the 10 830 Å line likely saturated. The observations suggest variations in this absorber's strength are best explained by ionization changes in response to a variable incident continuum. Photoionization simulations constrain the total hydrogen number density of the epoch 5 absorber to 7.1 ≤ log( n H /cm –3 ) ≤ 8.8, the hydrogen column density to 21.2 ≤ log( N H /cm –2 ) ≤ 23.3 and the ionization parameter range to –1.9 ≤ log U ≤ 0.4. The simulations also suggest the absorber is located between 0.03 and 0.49 pc from the continuum emission region. This range in physical properties is consistent with an absorber of similar velocity seen in NGC 4151 from previous ultraviolet and optical studies, but with high column density X-ray absorbing components not present. The mass outflow rate due to the fifth epoch absorber is in the range 0.008 to 0.38 M  yr –1 , too low to contribute to galaxy feedback effects.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    Interpreting broad emission-line variations - II. Tensions between luminosity, characteristic size, and responsivity (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-27
    Description: We investigate the variability behaviour of the broad H β emission-line to driving continuum variations in the best-studied AGN NGC 5548. For a particular choice of broad emission-line region (BLR) geometry, H β surface emissivity based on photoionization models, and using a scaled version of the 13-yr optical continuum light-curve as a proxy for the driving ionizing continuum, we explore several key factors that determine the broad emission-line luminosity L , characteristic size R RW , and variability amplitude (i.e. responsivity) , as well as the interplay between them. For fixed boundary models which extend as far as the hot dust the predicted delays for H β are on average too long. However, the predicted variability amplitude of H β provides a remarkably good match to observations except during low-continuum states. We suggest that the continuum flux variations which drive the redistribution in H β surface emissivity F ( r ) do not on their own lead to large enough changes in R RW or eff . We thus investigate dust-bounded BLRs for which the location of the effective outer boundary is modulated by the continuum level and the dust-sublimation and dust-condensation time-scales. We find that in order to match the observed variability amplitude of broad H β in NGC 5548 a rather static outer boundary is preferred. Intriguingly, we show that the most effective way of reducing the H β delay, while preserving its responsivity and equivalent width, is to invoke a smaller value in the incident ionizing photon flux H for a given ionizing source–cloud radial distance r , than is normally inferred from the observed UV continuum flux and typical models of the continuum spectral energy distribution.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Interpreting broad emission-line variations - I. Factors influencing the emission-line response (2014)
    Oxford University Press
    Details
    Publication Date: 2014-08-14
    Description: We investigate the sensitivity of the measured broad emission-line responsivity dlog f line /dlog f cont to continuum variations in the context of straw-man broad emission-line region (BLR) geometries of varying size with fixed BLR boundaries, and for which the intrinsic emission-line responsivity is known a priori. We find for a generic emission line that the measured responsivity eff , delay and maximum of the cross-correlation function are correlated for characteristic continuum variability time-scales T char less than the maximum delay for that line max (line) for a particular choice of BLR geometry and observer orientation. The above correlations are manifestations of geometric dilution arising from reverberation effects within the spatially extended BLR. When present, geometric dilution reduces the measured responsivity, delay and maximum of the cross-correlation function. Conversely, geometric dilution is minimized if T char  ≥ max (line). We also find that the measured responsivity and delay show a strong dependence on light-curve duration, with shorter campaigns resulting in smaller than expected values, and only a weak dependence on sampling rate (for irregularly sampled data). The observed strong negative correlation between continuum level and line responsivity found in previous studies cannot be explained by differences in the sampling pattern, light-curve duration or in terms of purely geometrical effects. To explain this and to satisfy the observed positive correlation between continuum luminosity and BLR size in an individual source, the responsivity-weighted radius must increase with increasing continuum luminosity. For a BLR with fixed inner and outer boundaries this requires radial surface emissivity distributions which deviate significantly from a simple power law, and in such a way that the intrinsic emission-line responsivity increases towards larger BLR radii, in line with photoionization calculations.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    A bright ultraluminous X-ray source in NGC 5907 (2013)
    Oxford University Press
    Details
    Publication Date: 2013-08-18
    Description: We present a multimission X-ray analysis of a bright (peak observed 0.3–10 keV luminosity of ~6  x 10 40 erg s –1 ), but relatively highly absorbed ultraluminous X-ray source (ULX) in the edge-on spiral galaxy NGC 5907. The ULX is spectrally hard in X-rays ( ~ 1.2–1.7, when fitted with an absorbed power law), and has a previously reported hard spectral break consistent with it being in the ultraluminous accretion state. It is also relatively highly absorbed for a ULX, with a column of ~0.4–0.9  x 10 22 atom cm –2 in addition to the line-of-sight column in our Galaxy. Although its X-ray spectra are well represented by accretion disc models, its variability characteristics argue against this interpretation. The ULX spectra instead appear dominated by a cool, optically thick Comptonizing corona. We discuss how the measured 9 per cent rms variability and a hardening of the spectrum as its flux diminishes might be reconciled with the effects of a very massive, radiatively driven wind and subtle changes in the corona, respectively. We speculate that the cool disc-like spectral component thought to be produced by the wind in other ULXs may be missing from the observed spectrum due to a combination of a low temperature (~0.1 keV), and the high column to the ULX. We find no evidence, other than its extreme X-ray luminosity, for the presence of an intermediate mass black hole (MsBHs, ~ 10 2 –10 4 M ) in this object. Rather, the observations can be consistently explained by a massive ( 20 M ) stellar remnant black hole in a super-Eddington accretion state.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    Quasar broad absorption line variability measurements using reconstructions of unabsorbed spectra (2013)
    Oxford University Press
    Details
    Publication Date: 2013-12-18
    Description: We present a two-epoch Sloan Digital Sky Survey and Gemini/GMOS+William Herschel Telescope/ISIS variability study of 50 broad absorption line (BAL) quasars of redshift range 1.9 〈 z  〈 4.2, containing 38 Si iv and 59 C iv BALs and spanning rest-frame time intervals of 10 months to 3.7 years. We find that 35/50 quasars exhibit one or more variable BALs, with 58 per cent of Si iv and 46 per cent of C iv BALs showing variability across the entire sample. On average, Si iv BALs show larger fractional change in BAL pseudo-equivalent width than C iv BALs, as referenced to an unabsorbed continuum+emission line spectrum constructed using non-negative matrix factorization. No correlation is found between BAL variability and quasar luminosity, suggesting that ionizing continuum changes do not play a significant role in BAL variability (assuming the gas is in photoionization equilibrium with the ionizing continuum). A subset of 14 quasars have one variable BAL from each of Si iv and C iv with significant overlap in velocity space and for which variations are in the same sense (strengthening or weakening) and which appear to be correlated (98 per cent confidence). We find examples of both appearing and disappearing BALs in weaker/shallower lines with disappearance rates of 2.3 per cent for C iv and 5.3 per cent for Si iv , suggesting average lifetimes of 142 and 43 years, respectively. We identify five objects in which the BAL is coincident with the broad emission line, but appears to cover only the continuum source. Assuming a clumpy inhomogeneous absorber model and a typical size for the continuum source, we infer a maximum cloud radius of 10 13 to 10 14  cm, assuming Eddington limited accretion.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    Accurate early positions forSwiftGRBs: enhancing X-ray positions with UVOT astrometry (2007)
    EDP Sciences
    Details
    Publication Date: 2007-10-23
    Print ISSN: 0004-6361
    Electronic ISSN: 1432-0746
    Topics: Physics
    Published by EDP Sciences
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  • 9
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    Panchromatic study of GRB 060124: from precursor to afterglow (2006)
    EDP Sciences
    Details
    Publication Date: 2006-09-01
    Print ISSN: 0004-6361
    Electronic ISSN: 1432-0746
    Topics: Physics
    Published by EDP Sciences
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  • 10
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    The outburst of Nova CSS 081007:030559+054715 (HV Ceti) (2012)
    EDP Sciences
    Details
    Publication Date: 2012-09-01
    Print ISSN: 0004-6361
    Electronic ISSN: 1432-0746
    Topics: Physics
    Published by EDP Sciences
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