arXiv subjects

Ari Laor

Publications and source records attributed to Ari Laor.

At least 73 records · Page 4Linked to original sources

arXiv2007

What Can We Learn from the Smallest AGN?

Quite a few things. In particular, reverberation mapping of NGC 4395, the lowest luminosity type 1 Active Galactic Nucleus (AGN, L_bol~10^40 erg/s) revealed a size of only ~1 light hour for the C IV broad line region (BLR), which is by far the smallest BLR. This, together with a similar determination of a size of ~200 light days in a luminous quasar (Kaspi et al. 2007), suggests that the R_BLR\propto L^1/2 relation holds over a range of 10^7 in L. This relation was suggested to result from dust sublimation, which sets R_BLR. This suggestion was beautifully confirmed recently by the dust reverberation results of Suganuma et al. (2006). The R_BLR\propto L^1/2 relation implies that the broad lines width increases with decreasing luminosity according to v\propto L^-1/4. But, there is an observational cutoff at v~25,000 km/s, and thus below a certain threshold L the BLR would not be detectable. Such objects constitute the so-called "true type 2" AGN (e.g. most FR I radio galaxies). The physical origin of the BLR gas is not established yet, but high quality Keck spectra of the Halpha profile in NGC 4395 rule out a clumped distribution, and indicate that the gas resides in a smooth flow, most likely in a thick rotationally supported configuration. The Halpha line also reveals extended exponential wings, which are well modeled by electron scattering within the BLR emitting gas. Such wings can be used as a direct probe of the BLR temperature and optical depth.

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arXiv2006

Chemical Abundances in AGN Environment: X-Ray/UV Campaign on the MRK 279 Outflow

We present the first reliable determination of chemical abundances in an AGN outflow. The abundances are extracted from the deep and simultaneous FUSE and HST/STIS observations of Mrk 279. This data set is exceptional for its high signal-to-noise, unblended doublet troughs and little Galactic absorption contamination. These attributes allow us to solve for the velocity-dependent covering fraction, and therefore obtain reliable column densities for many ionic species. For the first time we have enough such column densities to simultaneously determine the ionization equilibrium and abundances in the flow. Our analysis uses the full spectral information embedded in these high-resolution data. Slicing a given trough into many independent outflow elements yields the extra constraints needed for a physically meaningful abundances determination. We find that relative to solar the abundances in the Mrk 279 outflow are (linear scaling): carbon 2.2+/-0.7, nitrogen 3.5+/-1.1 and oxygen 1.6+/-0.8. Our UV-based photoionization and abundances results are in good agreement with the independent analysis of the simultaneous Mrk 279 X-ray spectra. This is the best agreement between the UV and X-ray analyses of the same outflow to date.

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arXiv2006

Hubble Space Telescope Ultraviolet Spectroscopy of Fourteen Low-Redshift Quasars

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.

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arXiv2006

Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. III. Optical Variability and X-ray/UV/Optical Correlations

We present optical observations of the low-luminosity Seyfert 1 nucleus of NGC 4395, as part of a multiwavelength reverberation-mapping program. Observations were carried out over two nights in 2004 April at Lick, Wise, and Kitt Peak Observatories. We obtained V-band and B-band photometry, and spectra over the range 3500-6800 Angstroms. Simultaneous Hubble Space Telescope UV and Chandra X-ray observations are presented in companion papers. Even though NGC 4395 was in an extremely low state of activity, we detect significant continuum variability of 2-10%, increasing toward shorter wavelengths. The continuum light curves, both spectroscopic and photometric, are qualitatively similar to the simultaneous UV and X-ray light curves. Inter-band cross-correlations suggest that the optical continuum emission lags behind the UV continuum emission by 24 +7/-9 min, and that the optical continuum emission lags behind the X-ray continuum emission by 44 +/- 13 min, consistent with a reprocessing model for active galactic nucleus emission. There are also hints of Balmer emission lines lagging behind the optical continuum by an amount slightly larger than the emission-line lag detected in the UV. These results are all similar to those of other Seyfert 1 nuclei. The emission-line lag yields a mass measurement of the central black hole, which although not very significant, is consistent with the value derived from the simultaneous UV data.

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arXiv2006

Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. II. X-ray and Ultraviolet Continuum Variability

We report on two Chandra observations, and a simultaneous Hubble Space Telescope ultraviolet observation, of the dwarf Seyfert 1 galaxy NGC 4395. Each Chandra observation had a duration of ~30 ks, with a separation of ~50 ks. The spectrum was observed to harden between these observations via a scaling down of the soft-band flux. The inter-observation variability is in a different sense to the observed variability within each observation and is most likely the result of increased absorption. Spectral variations were seen during the first observation suggesting that the X-ray emission is produced in more than one disconnected region. We have also re-analyzed a ~17 ks Chandra observation conducted in 2000. During the three Chandra observations the 2-10 keV flux is about a factor of 2 lower than seen during an XMM-Newton observation conducted in 2003. Moreover, the fractional variability amplitude exhibited during the XMM-Newton observation is significantly softer than seen during the Chandra observations. A power-spectral analysis of the first of the two new Chandra observations revealed a peak at 341s with a formal detection significance of 99%. A similar peak was seen previously in the 2000 Chandra data. However, the detection of this feature is tentative given that it was found in neither the second of our two new Chandra observations nor the XMM-Newton data, and it is much narrower than expected. The Hubble Space Telescope observation was conducted during part of the second Chandra visit. A zero-lag correlation between the ultraviolet and X-ray fluxes was detected with a significance of about 99.5%, consistent with the predictions of the two-phase model for the X-ray emission from active galactic nuclei.

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arXiv2006

Evidence for Line Broadening by Electron Scattering in the Broad Line Region of NGC 4395

A high quality Keck spectrum of the Halpha line in NGC 4395 reveals symmetric exponential wings, fv\propto e^{-v/sigma}, with sigma~500km/s. The wings extend out to >2500km/s from the line core, and down to a flux density of <10^-3 of the peak flux density. Numerical and analytic calculations indicate that exponential wings are expected for optically thin, isotropic, thermal electron scattering. Such scattering produces exponential wings with sigma=1.1sigma_e(ln tau_e^-1)^0.45, where sigma_e is the electron velocity dispersion, and tau_e is the electron scattering optical depth. The Halpha wings in NGC 4395 are well fit by an electron scattering model with tau_e=0.34, and an electron temperature T_e=1.1x10^4K. Such conditions are produced in photoionized gas with an ionization parameter U=0.3, as expected in the broad line region (BLR). Similar analysis of the [O III] 5007 line yields tau_e<0.01, consistent with the lower ionization in the narrow line region. If the electron scattering interpretation is correct, there should be a tight correlation between tau_e and the ionizing flux on time scales shorter than the BLR dynamical time, or ~1 week for NGC 4395. In contrast, the value of sigma should remain nearly constant on these time scales. Such wings may be discernible in other objects with unusually narrow Balmer lines, and they can provide a useful direct probe of T_e and tau_e in the BLR.

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arXiv2005

Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. I. A Reverberation-Based Measurement of the Black Hole Mass

A reverberation-mapping program on NGC 4395, the least-luminous known Seyfert 1 galaxy, undertaken with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, yields a measurement of the mass of the central black hole of 360,000 solar masses. The observations consist of two visits of 5 orbits each, in 2004 April and July. During each of these visits, the UV continuum varied by at least 10% (rms) and only C IV 1549 showed corresponding variations large enough to reliably determine the emission-line lag, which was measured to be of order one hour for both visits. The size of the C IV-emitting region is about a factor of three smaller than expected if the slope of the broad-line region radius-luminosity relationship is identical to that for the H-beta emission line. NGC 4395 is underluminous even for its small black hole mass; the Eddington ratio of 0.0012 is lower than that of any other active galactic nucleus for which a black hole mass measurement has been made by emission-line reverberation.

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arXiv2005

Is the Broad-Line Region Clumped or Smooth? Constraints from the H alpha Profile in NGC 4395, the Least Luminous Seyfert 1 Galaxy

The origin and configuration of the gas which emits broad lines in Type I active galactic nuclei is not established yet. The lack of small-scale structure in the broad emission-line profiles is consistent with a smooth gas flow, or a clumped flow with many small clouds. An attractive possibility for the origin of many small clouds is the atmospheres of bloated stars, an origin which also provides a natural mechanism for the cloud confinement. Earlier studies of the broad-line profiles have already put strong lower limits on the minimum number of such stars, but these limits are sensitive to the assumed width of the lines produced by each cloud. Here we revisit this problem using high-resolution Keck spectra of the H alpha line in NGC 4395, which has the smallest known broad-line region (~10^14 cm). Only a handful of the required bloated stars (each having r~10^14 cm) could fit into the broad-line region of NGC 4395, yet the observed smoothness of the H alpha line implies a lower limit of ~10^4-10^5 on the number of discrete clouds. This rules out conclusively the bloated-stars scenario, regardless of any plausible line-broadening mechanisms. The upper limit on the size of the clouds is ~10^12 cm, which is comparable to the size implied by photoionization models. This strongly suggests that gas in the broad-line region is structured as a smooth rather than a clumped flow, most likely in a rotationally dominated thick disk-like configuration. However, it remains to be clarified why such a smooth, gravity-dominated flow generates double-peaked emission lines only in a small fraction of active galactic nuclei.

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arXiv2005

What controls the [O III] 5007 line strength in AGN?

AGN display an extreme range in the narrow emission line equivalent widths. Specifically, in the PG quasar sample the equivalent width of the narrow [O III] 5007 line has a range of >300, while the broad Hb line, for example, has a range of 10 only. The strength of [O III] 5007 is modulated by the covering factor, CF, of the narrow line region (NLR) gas, its density n_e, and ionization parameter U. To explore which of these factors produces the observed large range in [O III] 5007 strength, we measure the strength of the matching narrow Hb and [O III] 4363 lines, detected in 40 out of the 87 z<0.5 PG quasars in the Boroson & Green sample. The photoionization code CLOUDY is then used to infer CF, n_e, and U in each object, assuming a single uniform emitting zone. We find that the range of CF (~0.02-0.2) contributes about twice as much as the range in both n_e and U towards modulating the strength of the [O III] 5007 line. The CF is inversely correlated with luminosity, but it is not correlated with L_Edd as previously speculated. The single zone [O III] 5007 emitting region is rather compact, having R=40L_44^0.45 pc. These emission lines can also be fit with an extreme two zone model, where [O III] 4363 is mostly emitted by a dense (n_e=10^7) inner zone at R=L_44^0.5 pc, and [O III] 5007 by a low density (n_e=10^3) extended outer zone at R=750L_44^0.34 pc. Such an extended [O III] 5007 emission should be well resolved by HST imaging of luminous AGN. Further constraints on the radial gas distribution in the NLR can be obtained from the spectral shape of the IR continuum emitted by the associated dust.

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arXiv2005

X-ray/UV Observing Campaign on the Mrk 279 AGN Outflow: A Global Fitting Analysis of the UV Absorption

We present an analysis of the intrinsic UV absorption in the Seyfert 1 galaxy Mrk 279 based on simultaneous long observations with the Hubble Space Telescope (41 ks) and the Far Ultraviolet Spectroscopic Explorer (91 ks). To extract the line-of-sight covering factors and ionic column densities, we separately fit two groups of absorption lines: the Lyman series and the CNO lithium-like doublets. For the CNO doublets we assume that all three ions share the same covering factors. The fitting method applied here overcomes some limitations of the traditional method using individual doublet pairs; it allows for the treatment of more complex, physically realistic scenarios for the absorption-emission geometry and eliminates systematic errors that we show are introduced by spectral noise. We derive velocity-dependent solutions based on two models of geometrical covering -- a single covering factor for all background emission sources, and separate covering factors for the continuum and emission lines. Although both models give good statistical fits to the observed absorption, we favor the model with two covering factors because: (a) the best-fit covering factors for both emission sources are similar for the independent Lyman series and CNO doublet fits; (b) the fits are consistent with full coverage of the continuum source and partial coverage of the emission lines by the absorbers, as expected from the relative sizes of the nuclear emission components; and (c) it provides a natural explanation for variability in the Ly$α$ absorption detected in an earlier epoch. We also explore physical and geometrical constraints on the outflow from these results.

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arXiv2004

What controls the C IV line profile in active galactic nuclei?

The high ionization lines in active galactic nuclei (AGN), such as C IV, tend to be blueshifted with respect to the lower ionization lines, such as H beta, and often show a strong blue excess asymmetry not seen in the low ionization lines. There is accumulating evidence that the H beta profile is dominated by gravity, and thus provides a useful estimate of the black hole mass in AGN. The shift and asymmetry commonly seen in C IV suggest that non gravitational effects, such as obscuration and radiation pressure, may affect the line profile. We explore the relation between the H beta and C IV profiles using UV spectra available for 81 of the 87 z < 0.5 PG quasars in the Boroson & Green (BG92) sample. We find the following: (1) Narrow C IV lines (FWHM < 2000 km/s) are rare (~2 per cent occurrence rate) compared to narrow H beta lines (~20 per cent). (2) In most objects where the H beta FWHM <4000 km/s the C IV line is broader than H beta, but the reverse is true when the H beta FWHM > 4000 km/s. This argues against the view that C IV generally originates closer to the center, compared to H beta. (3) C IV appears to provide a significantly less accurate, and possibly biased estimate of the black hole mass in AGN, compared with H beta. (4) All objects where C IV is strongly blueshifted and asymmetric have a high L/L_Edd, but the reverse is not true. This suggests that a high L/L_Edd is a necessary but not sufficient condition for generating a blueshifted asymmetric C IV emission. (5) We also find indications for dust reddening and scattering in `normal' AGN. In particular, PG quasars with a redder optical-UV continuum slope show weaker C IV emission, stronger C IV absorption, and a higher optical continuum polarization.

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arXiv2004

On the origin of the C IV Baldwin effect in AGN

The origin of the luminosity dependence of the equivalent width (EW) of broad emission lines in AGN (the Baldwin effect) is not firmly established yet. We explore this question for the broad C IV λ1549 line using the Boroson & Green sample of the 87 z < 0.5 Bright Quasar Survey (BQS) quasars. Useful UV spectra of the C IV region are available for 81 of the objects, which are used to explore the dependence of the C IV EW on various emission properties. We confirm earlier results on the strong correlations of the C IV EW with some of the emission parameters which define the Boroson & Green Eigenvector 1, and with the optical to X-ray slope α_ox. In addition, we find a strong correlation of the C IV EW with the relative accretion rate, L/L_Edd. Since L/L_Edd drives some of the Eigenvector 1 correlations, it may be the primary physical parameter which drives the Baldwin effect for C IV.

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arXiv2003

Some Comments related to AGN Radio Loudness

The bimodality of the AGN radio loudness distribution, and the relation of radio loudness and black hole mass were recently disputed. A closer look at the existing data suggests possible resolutions of these disputes, as further described below.

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arXiv2003

What is the Origin of the Baldwin Effect in the BLR and the NLR?

The origin of the luminosity dependence of the strength of broad and narrow emission lines in AGN (i.e. the Baldwin effect) is not firmly established yet. We explore this question using the Boroson & Green sample of the 87 z<=0.5 PG quasars. UV spectra of the C IV region with a sufficient S/N are available for 81 of the objects. We use these spectra to explore the dependence of the C IV EW on M_BH and L_Edd, as deduced from optical spectra of the H beta region. We find a strong correlation of the C IV EW with L_Edd. This may be the primary correlation which drives the Baldwin effect for C IV. A similar correlation analysis using the C IV FWHM instead of the H beta FWHM, yields an insignificant correlation. This suggests that the C IV FWHM provides a significantly less accurate estimate of M_BH compared to the H Beta FWHM. We also measured the [O III] 4363 /[O III] 5007 line ratio for 72 objects in this sample, and use it to determine whether density or covering factor control the large range in [O III] 5007 strength in AGN.

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arXiv2003

What is the Broad Line Region?

What is the Broad Line Region (BLR) made of? What determines its location? Why is it sometimes missing? What controls its properties? Some recent results and new approaches which may shed light on these issues are briefly described.

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arXiv2003

On The Nature of Low Luminosity Narrow Line AGN

There is clear observational evidence that some narrow line (type 2) AGN have a hidden broad line region (BLR), and are thus intrinsically broad line (type 1) AGN. Does this AGN unification applies for all type 2 AGN? Indirect arguments suggest that some "true" type 2 AGN, i.e. AGN having no obscured BLR do exist, but it is not clear why the BLR is missing in these AGN. Here we point out a possible natural explanation. The observed radius-luminosity relation for the BLR implies an increasing line width with decreasing luminosity for a given black hole mass (Mbh). In addition, there appears to be an upper limit to the observed width of broad emission lines in AGN of Delta v_max~25,000 km/s, which may reflect a physical limit above which the BLR may not be able to survive. Thus, at a low enough luminosity the BLR radius shrinks below the Delta v_max radius, leaving no region where the BLR can exist, although the AGN may remain otherwise `normal'. The implied minimum bolometric luminosity required to sustain a BLR with Delta v<25,000 km/s is L_min~10^{41.8}(Mbh}/10^8M_sun)^2. All AGN with L<L_min are expected to be `true' type 2 AGN, i.e. narrow line AGN without a hidden BLR. Predictions for the true nature of low luminosity AGN in two samples of nearby galaxies are provided. These can be used to test the above L_min conjecture, and the predictions of other models for the size and origin of the BLR.

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arXiv2002

The Baldwin Effect and Black Hole Accretion: A Spectral Principal Component Analysis of a Complete QSO Sample

A unique set of UV-optical spectrograms of 22 low redshift QSOs are investigated using principal component analysis. We find three significant principal components over the broad wavelength range from Lyαto Hα. They together account for about 78% of the sample intrinsic variance. We present strong arguments that the first principal component represents the Baldwin effect, relating equivalent widths to the luminosity (i.e. accretion rate), but only emission-line cores are involved. The second component represents continuum variations, probably dominated by intrinsic reddening. The third principal component directly relates QSO UV properties to the optical principal component 1 found by Boroson & Green (1992). It is the primary cause of scatter in the Baldwin relationships. It is also directly related to broad emission-line width and soft X-ray spectral index, and therefore probably driven by Eddington accretion ratio. We demonstrate how Baldwin relationships can be derived using our first principal component, virtually eliminating the scatter caused by the third principal component. This rekindles the hope that the Baldwin relationships can be used for cosmological study.

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arXiv2001

HST STIS Observations of PG 0946+301: the Highest Quality UV Spectrum of a BALQSO

We describe deep (40 orbits) HST/STIS observations of the BALQSO PG 0946+301 and make them available to the community. These observations are the major part of a multi-wavelength campaign on this object aimed at determining the ionization equilibrium and abundances (IEA) in broad absorption line (BAL) QSOs. We present simple template fits to the entire data set, which yield firm identifications for more than two dozen BALs from 18 ions and give lower limits for the ionic column densities. We find that the outflow's metalicity is consistent with being solar, while the abundance ratio of phosphorus to other metals is at least ten times solar. These findings are based on diagnostics that are not sensitive to saturation and partial covering effects in the BALs, which considerably weakened previous claims for enhanced metalicity. Ample evidence for these effects is seen in the spectrum. We also discuss several options for extracting tighter IEA constraints in future analyses, and present the significant temporal changes which are detected between these spectra and those taken by the HST/FOS in 1992.

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