Emission-line Lags Research Articles

Space Telescope and Optical Reverberation Mapping Project. V. Optical Spectroscopic Campaign and Emission-line Analysis for NGC 5548

Abstract We present the results of an optical spectroscopic monitoring program targeting NGC 5548 as part of a larger multiwavelength reverberation mapping campaign. The campaign spanned 6 months and achieved an almost daily cadence with observations from five ground-based telescopes. The Hβ and He ii λ4686 broad emission-line light curves lag that of the 5100 Å optical continuum by and , respectively. The Hβ lag relative to the 1158 Å ultraviolet continuum light curve measured by the Hubble Space Telescope is ∼50% longer than that measured against the optical continuum, and the lag difference is consistent with the observed lag between the optical and ultraviolet continua. This suggests that the characteristic radius of the broad-line region is ∼50% larger than the value inferred from optical data alone. We also measured velocity-resolved emission-line lags for Hβ and found a complex velocity-lag structure with shorter lags in the line wings, indicative of a broad-line region dominated by Keplerian motion. The responses of both the Hβ and He ii emission lines to the driving continuum changed significantly halfway through the campaign, a phenomenon also observed for C iv, Lyα, He ii(+O iii]), and Si iv(+O iv]) during the same monitoring period. Finally, given the optical luminosity of NGC 5548 during our campaign, the measured Hβ lag is a factor of five shorter than the expected value implied by the R BLR–L AGN relation based on the past behavior of NGC 5548.

SPACE TELESCOPE AND OPTICAL REVERBERATION MAPPING PROJECT. I. ULTRAVIOLET OBSERVATIONS OF THE SEYFERT 1 GALAXY NGC 5548 WITH THE COSMIC ORIGINS SPECTROGRAPH ONHUBBLE SPACE TELESCOPE

We describe the first results from a six-month long reverberation-mapping experiment in the ultraviolet based on 170 observations of the Seyfert 1 galaxy NGC 5548 with the Cosmic Origins Spectrograph on the Hubble Space Telescope. Significant correlated variability is found in the continuum and broad emission lines, with amplitudes ranging from ~30% to a factor of two in the emission lines and a factor of three in the continuum. The variations of all the strong emission lines lag behind those of the continuum, with He II 1640 lagging behind the continuum by ~2.5 days and Lyman alpha 1215, C IV 1550, and Si IV 1400 lagging by ~5-6 days. The relationship between the continuum and emission lines is complex. In particular, during the second half of the campaign, all emission-line lags increased by a factor of 1.3-2 and differences appear in the detailed structure of the continuum and emission-line light curves. Velocity-resolved cross-correlation analysis shows coherent structure in lag versus line-of-sight velocity for the emission lines; the high-velocity wings of C IV respond to continuum variations more rapidly than the line core, probably indicating higher velocity BLR clouds at smaller distances from the central engine. The velocity-dependent response of Lyman alpha, however, is more complex and will require further analysis.

REVERBERATION MAPPING OF THE SEYFERT 1 GALAXY NGC 7469

A large reverberation mapping study of the Seyfert 1 galaxy NGC 7469 has yielded emission-line lags for Hbeta 4861 and He II 4686 and a central black hole mass measurement of about 10 million solar masses, consistent with previous measurements. A very low level of variability during the monitoring campaign precluded meeting our original goal of recovering velocity-delay maps from the data, but with the new Hbeta measurement, NGC 7469 is no longer an outlier in the relationship between the size of the Hbeta-emitting broad-line region and the AGN luminosity. It was necessary to detrend the continuum and Hbeta and He II 4686 line light curves and those from archival UV data for different time-series analysis methods to yield consistent results.

REVERBERATION MAPPING OF THEKEPLERFIELD AGN KA1858+4850

KA1858+4850 is a narrow-line Seyfert 1 galaxy at redshift 0.078 and is among the brightest active galaxies monitored by the Kepler mission. We have carried out a reverberation mapping campaign designed to measure the broad-line region size and estimate the mass of the black hole in this galaxy. We obtained 74 epochs of spectroscopic data using the Kast Spectrograph at the Lick 3-m telescope from February to November of 2012, and obtained complementary V-band images from five other ground-based telescopes. We measured the H-beta light curve lag with respect to the V-band continuum light curve using both cross-correlation techniques (CCF) and continuum light curve variability modeling with the JAVELIN method, and found rest-frame lags of lag_CCF = 13.53 (+2.03, -2.32) days and lag_JAVELIN = 13.15 (+1.08, -1.00) days. The H-beta root-mean-square line profile has a width of sigma_line = 770 +/- 49 km/s. Combining these two results and assuming a virial scale factor of f = 5.13, we obtained a virial estimate of M_BH = 8.06 (+1.59, -1.72) x 10^6 M_sun for the mass of the central black hole and an Eddington ratio of L/L_Edd ~ 0.2. We also obtained consistent but slightly shorter emission-line lags with respect to the Kepler light curve. Thanks to the Kepler mission, the light curve of KA1858+4850 has among the highest cadences and signal-to-noise ratios ever measured for an active galactic nucleus; thus, our black hole mass measurement will serve as a reference point for relations between black hole mass and continuum variability characteristics in active galactic nuclei.

THE LICK AGN MONITORING PROJECT: REVERBERATION MAPPING OF OPTICAL HYDROGEN AND HELIUM RECOMBINATION LINES

We have recently completed a 64-night spectroscopic monitoring campaign at the Lick Observatory 3-m Shane telescope with the aim of measuring the masses of the black holes in 12 nearby (z < 0.05) Seyfert 1 galaxies with expected masses in the range ~10^6-10^7M_sun and also the well-studied nearby active galactic nucleus (AGN) NGC 5548. Nine of the objects in the sample (including NGC 5548) showed optical variability of sufficient strength during the monitoring campaign to allow for a time lag to be measured between the continuum fluctuations and the response to these fluctuations in the broad Hbeta emission, which we have previously reported. We present here the light curves for the Halpha, Hgamma, HeII 4686, and HeI 5876 emission lines and the time lags for the emission-line responses relative to changes in the continuum flux. Combining each emission-line time lag with the measured width of the line in the variable part of the spectrum, we determine a virial mass of the central supermassive black hole from several independent emission lines. We find that the masses are generally consistent within the uncertainties. The time-lag response as a function of velocity across the Balmer line profiles is examined for six of the AGNs. Finally we compare several trends seen in the dataset against the predictions from photoionization calculations as presented by Korista & Goad. We confirm several of their predictions, including an increase in responsivity and a decrease in the mean time lag as the excitation and ionization level for the species increases. Further confirmation of photoionization predictions for broad-line gas behavior will require additional monitoring programs for these AGNs while they are in different luminosity states. [abridged]

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- and B-band photometry, and spectra over the range 3500-6800 Å. 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. Interband cross-correlations suggest that the optical continuum emission lags behind the UV continuum emission by 24 minutes, and that the optical continuum emission lags behind the X-ray continuum emission by 44 ± 13 minutes, 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.

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 MBH = (3.6 ± 1.1) × 105 M☉. The observations consist of two visits of five 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 λ1549 showed corresponding variations large enough to reliably determine the emission-line lag, which was measured to be of order 1 hr for both visits. The size of the C IV-emitting region is about a factor of 3 smaller than expected if the slope of the broad-line region radius-luminosity relationship is identical to that for the Hβ emission line. NGC 4395 is underluminous even for its small black hole mass; the Eddington ratio of ~1.2 × 10-3 is lower than that of any other active galactic nucleus for which a black hole mass measurement has been made by emission-line reverberation.

Variability of FeiiEmission Features in the Seyfert 1 Galaxy NGC 5548

We study the low-contrast Fe ii emission blends in the ultraviolet (1250–2200 8) and optical (4000–6000 8) spectra of the Seyfert 1 galaxy NGC 5548 and show that these features vary in flux and that these variations are correlated with those of the optical continuum. The amplitude of variability of the optical Fe ii emission is 50%– 75% that of H� , and the ultraviolet Fe iiemission varies with an even larger amplitude than H� .H owever, accurate measurement of the flux in these blends proves to be very difficult even using excellent Fe ii templates to fit the spectra. We are able to constrain only weakly the optical Fe ii emission-line response timescale to a value of less than several weeks; this upper limit exceeds all the reliably measured emission-line lags in this source so it is not particularly meaningful. Nevertheless, the fact that the optical Fe ii and continuum flux variations are correlated indicates that line fluorescence in a photoionized plasma, rather than collisional excitation, is responsible for the Fe ii emission. The iron emission templates are available upon request. Subject headingg galaxies: active — galaxies: nuclei — galaxies: Seyfert — quasars: emission lines

Central Masses and Broad‐Line Region Sizes of Active Galactic Nuclei. II. A Homogeneous Analysis of a Large Reverberation‐Mapping Database

We present improved black hole masses for 35 active galactic nuclei (AGNs) based on a complete and consistent reanalysis of broad emission-line reverberation-mapping data. From objects with multiple line measurements, we find that the highest precision measure of the virial product cτΔV2/G, where τ is the emission-line lag relative to continuum variations and ΔV is the emission-line width, is obtained by using the cross-correlation function centroid (as opposed to the cross-correlation function peak) for the time delay and the line dispersion (as opposed to FWHM) for the line width and by measuring the line width in the variable part of the spectrum. Accurate line-width measurement depends critically on avoiding contaminating features, in particular the narrow components of the emission lines. We find that the precision (or random component of the error) of reverberation-based black hole mass measurements is typically around 30%, comparable to the precision attained in measurement of black hole masses in quiescent galaxies by gas or stellar dynamical methods. Based on results presented in a companion paper by Onken et al., we provide a zero-point calibration for the reverberation-based black hole mass scale by using the relationship between black hole mass and host-galaxy bulge velocity dispersion. The scatter around this relationship implies that the typical systematic uncertainties in reverberation-based black hole masses are smaller than a factor of 3. We present a preliminary version of a mass-luminosity relationship that is much better defined than any previous attempt. Scatter about the mass-luminosity relationship for these AGNs appears to be real and could be correlated with either Eddington ratio or object inclination.

Steps toward Determination of the Size and Structure of the Broad‐Line Region in Active Galactic Nuclei. IX. Ultraviolet Observations of Fairall 9

An 8 month monitoring campaign on the Seyfert 1 galaxy Fairall 9 has been conducted with the International Ultraviolet Explorer in an attempt to obtain reliable estimates of continuum-continuum and continuum-emission-line delays for a high-luminosity active galactic nucleus (AGN). While the results of this campaign are more ambiguous than those of previous monitoring campaigns on lower luminosity sources, we find general agreement with the earlier results: (1) there is no measurable lag between ultraviolet continuum bands, and (2) the measured emission-line time lags are very short. It is especially notable that the Ly? + N V emission-line lag is about 1 order of magnitude smaller than determined from a previous campaign by Clavel, Wamsteker, & Glass (1989) when Fairall 9 was in a more luminous state. In other well-monitored sources, specifically NGC 5548 and NGC 3783, the highest ionization lines are found to respond to continuum variations more rapidly than the lower ionization lines, which suggests a radially ionization-stratified broad-line region. In this case, the results are less certain, since none of the emission-line lags are very well determined. The best-determined emission line lag is Ly? + N V, for which we find that the centroid of the continuum-emission-line cross-correlation function is ?cent ? 14-20 days. We measure a lag ?cent 4 days for He II ?1640; this result is consistent with the ionization-stratification pattern seen in lower luminosity sources, but the relatively large uncertainties in the emission-line lags measured here cannot rule out similar lags for Ly? + N V and He II ?1640 at a high level of significance. We are unable to determine a reliable lag for C IV ?1550, but we note that the profiles of the variable parts of Ly? and C IV ?1550 are not the same, which does not support the hypothesis that the strongest variations in these two lines arise in the same region.

A month in the life of NGC 4151: velocity-delay maps of the broad-line region

We analyse continuum and emission-line variability in NGC 4151 based on IUE spectra taken with nearly complete 1-d sampling over a 35-d interval during 1991 November–December. The 1320-Å continuum undertook a bumpy exponential decline by a factor ∼3 over 25 d, and then made a dramatic recovery in ∼2 d near the end of the campaign. The same pattern of variations occurred in all continuum bands with an amplitude that decreases with wavelength out to 3000 Å. Cross-correlations reveal emission-line variations lagging behind the continuum variations. The red wing, core and blue wing of the C iv emission line lag by 1.8, 4.4 and 3.2 d respectively. Maximum entropy fits to continuum light curves and line profile variations yield more detailed velocity-delay maps of the C iv and He ii responses at delays from 0 to 20 d. The line maps generally decline from a peak at 0 delay, implying high-velocity gas inside 1 light-day. The velocity-delay structure of C iv is asymmetric, with a strong red wing at τ ≲ 2 d, and a faint blue wing at τ ≲ 10 d. This suggests inflow in the high-velocity inner regions, but might also be interpreted as outflow combined with an inward anisotropy. The C iv velocity range decreases with delay in a way that suggests virial motions around a mass of ∼107 M⊙.

Optical Continuum and Emission‐Line Variability of the Seyfert 1 Galaxy Markarian 509

We report on the results of a 5 year coordinated program of spectroscopic monitoring of the luminous Seyfert 1 galaxy Markarian 509. The Hβ and He II λ4686 emission lines are found to respond to continuum variations with time lags of ~80 and ~60 days, respectively, considerably longer than the emission-line lags measured for other Seyfert galaxies.

COMMENTS ON CROSS-CORRELATION METHODOLOGY IN VARIABILITY STUDIES OF ACTIVE GALACTIC NUCLEI

ABSTRACT We discuss two separate cross-correlation methdologies, the interpolation method of Gaskell and Sparkle and the discrete correlation function of Edelson and Krolik, that are commonly used to quantify the lags between continuum and emission-line flux variations in active galactic nuclei (AGNs). We show that if similar assumptions are made to normalize the cross-correlation functions, the two methods are in good agreement for well-sampled AGN light curves. We also investigate the performance of cross-correlation methodology for less well-sampled data sets through Monte Carlo simulations that employ realistic models of the continuum behavior (based on well-observed Seyfert galaxies) and typical emission-line response times. We find that the interpolation method fairly accurately recovers the emission-line lags as the sampling is degraded (i.e., as the number of observed points is reduced). We find that for the cases investigated, the emission-line lags can be determined with reasonable accuracy even with mean sampling intervals as large as around two weeks.

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. III - Further observations of NGC 5548 at optical wavelengths

The results of the second year of an intensive ground-based spectroscopic and photometric study of variability in the bright Seyfert 1 galaxy NGC 5548 are reported in order to study the relationship between continuum and emission-line variability. Relative to the first year of the monitoring program, the nucleus of NGC 5548 was considerably fainter and the continuum variations slower during the second year, but the continuum H-beta cross-correlation results for the two years are nearly identical. The variations in the broad H-beta emission-line lag behind those in the continuum by somewhat less than 20 days, as concluded from the first year's data.

On the emission-line response to continuum variations in the Seyfert galaxy NGC 5548

view Abstract Citations (52) References (4) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Emission-Line Response to Continuum Variations in the Seyfert Galaxy NGC 5548 Netzer, Hagai ; Maoz, Dan Abstract Recently, two intensive optical monitoring projects were carried out, with the aim of measuring the continuum and emission line variations in the Seyfert galaxy NGC 5548. The two groups, observing the galaxy during two different periods, reported apparently contradicting results for the delay of Hβ variations with respect to the continuum. Using the UV measurements of Clavel et al., obtained the same year as one of the above data sets, we show that the emission-line lag behind continuum variations depends on the continuum variability time scale in this object, in the sense that continuum variations with larger time scales give larger emission-line lags. Since the continuum variability time scales are different in the data sets obtained by the two groups, so are the derived lags. Such an effect can arise in some thick BLR geometries, especially if the line response to the ionizing flux changes is nonlinear. Using Monte Carlo simulations, we show that even without resorting to a nonlinear response, there is at least one possible model that could reproduce the two differing delays. In view of the effect, we note that previously reported differences in the lags of various emission lines are uncertain. In particular, we find that, using only the largest outburst in the simultaneous optical and UV light curves, Lyα, C IV λ1549, and Hβ all have similar lags behind the continuum. Publication: The Astrophysical Journal Pub Date: December 1990 DOI: 10.1086/185875 Bibcode: 1990ApJ...365L...5N Keywords: Continuous Radiation; Emission Spectra; Line Spectra; Seyfert Galaxies; H Beta Line; Light Curve; Monte Carlo Method; Ultraviolet Radiation; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 5548; GALAXIES: SEYFERT full text sources ADS | data products SIMBAD (2) NED (1) MAST (1)