arXiv subjects

Bradley M. Peterson

Publications and source records attributed to Bradley M. Peterson.

At least 55 records · Page 3Linked to original sources

arXiv2007

The Masses of Black Holes in Active Galactic Nuclei

Reverberation mapping methods have been used to measure masses in about three dozen AGNs. The consistency of the virial masses computed from line widths and time delays, the relationship between black hole mass and host-galaxy stellar bulge velocity dispersion, and the consistency with black hole masses estimated from stellar dynamics in the two cases in which such determinations are possible all indicate that reverberation mass measurements are robust and are accurate to typically a factor of a few. The reverberation-mapped AGNs are of particular importance because they anchor the scaling relationships that allow black hole mass estimation based on single spectra. We discuss potential sources of systematic error, particularly with regard to how the emission line widths are measured.

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arXiv2007

Refining the Radius-Luminosity Relationship for Active Galactic Nuclei

We have measured the host-galaxy starlight contribution to four lower-luminosity AGNs (NGC 3516, NGC 4593, IC 4329A, and NGC 7469). We include these objects with new broad line region measurements for NGC 4151 and NGC 4593 to present a revised version of the radius-luminosity relationship for AGNs.

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arXiv2007

NGC 5548 in a Low-Luminosity State: Implications for the Broad-Line Region

We describe results from a new ground-based monitoring campaign on NGC 5548, the best studied reverberation-mapped AGN. We find that it was in the lowest luminosity state yet recorded during a monitoring program, namely L(5100) = 4.7 x 10^42 ergs s^-1. We determine a rest-frame time lag between flux variations in the continuum and the Hbeta line of 6.3 (+2.6/-2.3) days. Combining our measurements with those of previous campaigns, we determine a weighted black hole mass of M_BH = 6.54 (+0.26/-0.25) x 10^7 M_sun based on all broad emission lines with suitable variability data. We confirm the previously-discovered virial relationship between the time lag of emission lines relative to the continuum and the width of the emission lines in NGC 5548, which is the expected signature of a gravity-dominated broad-line region. Using this lowest luminosity state, we extend the range of the relationship between the luminosity and the time lag in NGC 5548 and measure a slope that is consistent with alpha = 0.5, the naive expectation for the broad line region for an assumed form of r ~ L^alpha. This value is also consistent with the slope recently determined by Bentz et al. for the population of reverberation-mapped AGNs as a whole.

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arXiv2006

The Mass of the Black Hole in the Seyfert 1 Galaxy NGC 4593 from Reverberation Mapping

We present new observations leading to an improved black hole mass estimate for the Seyfert 1 galaxy NGC 4593 as part of a reverberation-mapping campaign conducted at the MDM Observatory. Cross-correlation analysis of the H_beta emission-line light curve with the optical continuum light curve reveals an emission-line time delay of 3.73 (+-0.75) days. By combining this time delay with the H_beta line width, we derive a central black hole mass of M_BH = 9.8(+-2.1)x10^6 M_sun, an improvement in precision of a factor of several over past results.

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arXiv2006

A Reverberation-Based Mass for the Central Black Hole in NGC 4151

We have undertaken a new ground-based monitoring campaign to improve the estimates of the mass of the central black hole in NGC 4151. We measure the lag time of the broad H beta line response compared to the optical continuum at 5100 A and find a lag of 6.6 (+1.1/-0.8) days. We combine our data with the recent reanalysis of UV emission lines by Metzroth et al. to calculate a weighted mean of the black hole mass, M_BH = 4.57 (+0.57/-0.47) x 10^7 M_sun. The absolute calibration of the black hole mass is based on normalization of the AGN black hole mass - stellar velocity dispersion (M_BH - sigma_*) relationship to that of quiescent galaxies by Onken et al. The scatter in the M_BH - sigma_* relationship suggests that reverberation-mapping based mass measurements are typically uncertain by a factor of 3-4.

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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

The Mass of the Central Black Hole in the Seyfert Galaxy NGC 4151

In order to improve the reverberation-mapping based estimate of the mass of the central supermassive black hole in the Seyfert 1 galaxy NGC 4151, we have reanalyzed archival ultraviolet monitoring spectra from two campaigns undertaken with the International Ultraviolet Explorer. We measure emission-line time delays for four lines, C IV 1549, He II 1640, C III] 1909, and Mg II 2798, from both campaigns. We combine these measurements with the dispersion of the variable part of each respective emission line to obtain the mass of the central object. Despite the problematic nature of some of the data, we are able to measure a mass of 41.1 (+/- 7.3) million solar masses, although this, like all reverberation-based masses, is probably systematically uncertain by a factor of 3-4.

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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

The Radius-Luminosity Relationship for Active Galactic Nuclei: The Effect of Host-Galaxy Starlight on Luminosity Measurements

We have obtained high resolution images of the central regions of 14 reverberation-mapped active galactic nuclei (AGN) using the Hubble Space Telescope Advanced Camera for Surveys High Resolution Camera to account for host-galaxy starlight contamination of measured AGN luminosities. We measure the host-galaxy starlight contribution to the continuum luminosity at 5100 A through the typical ground-based slit position and geometry used in the reverberation-mapping campaigns. We find that removing the starlight contribution results in a significant correction to the luminosity of each AGN, both for lower luminosity sources, as expected, but also for the higher luminosity sources such as the PG quasars. After accounting for the host galaxy starlight, we revisit the well-known broad-line region radius--luminosity relationship for nearby AGN. We find the power-law slope of the relationship for the H beta line to be 0.518 +/- 0.039, shallower than previously reported and consistent with the slope of 0.5 expected from the naive theoretical assumption that all AGN have, on average, the same ionizing spectrum and the same ionization parameter and gas density in the H beta line-emitting region.

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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

The Relationship Between Luminosity and Broad-Line Region Size in Active Galactic Nuclei

We reinvestigate the relationship between the characteristic broad-line region size (R_blr) and the Balmer emission-line, X-ray, UV, and optical continuum luminosities. Our study makes use of the best available determinations of R_blr for a large number of active galactic nuclei (AGNs) from Peterson et al. Using their determinations of R_blr for a large sample of AGNs and two different regression methods, we investigate the robustness of our correlation results as a function of data sub-sample and regression technique. Though small systematic differences were found depending on the method of analysis, our results are generally consistent. Assuming a power-law relation R_blr \propto L^α, we find the mean best-fitting αis about 0.67+/-0.05 for the optical continuum and the broad Hβluminosity, about 0.56+/-0.05 for the UV continuum luminosity, and about 0.70+/-0.14 for the X-ray luminosity. We also find an intrinsic scatter of about 40% in these relations. The disagreement of our results with the theoretical expected slope of 0.5 indicates that the simple assumption of all AGNs having on average same ionization parameter, BLR density, column density, and ionizing spectral energy distribution, is not valid and there is likely some evolution of a few of these characteristics along the luminosity scale.

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arXiv2004

Supermassive Black Holes in Active Galactic Nuclei. II. Calibration of the M-sigma Relationship for AGNs

We calibrate reverberation-based black hole masses in active galactic nuclei (AGNs) by using the correlation between black hole mass, M, and bulge/spheroid stellar velocity dispersion, sigma. We use new measurements of sigma for 6 AGNs and published velocity dispersions for 10 others, in conjunction with improved reverberation mapping results, to determine the scaling factor required to bring reverberation-based black hole masses into agreement with the quiescent galaxy M-sigma relationship. The scatter in the AGN black hole masses is found to be less than a factor of 3. The current observational uncertainties preclude use of the scaling factor to discriminate between broad-line region models.

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arXiv2004

Black Hole Masses from Reverberation Measurements

We have reanalyzed in a consistent way existing reverberation data for 35 AGNs for the purpose of refining the black hole masses derived from these data. 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. As discussed in this volume by Onken et al., we have established an absolute calibration for AGN reverberation-based masses by assuming that AGNs and quiescent galaxies follow an identical 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 three. 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 source inclination.

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arXiv2003

Correlated long-term optical and X-ray variations in NGC 5548

We combine the long-term optical light curve of the Seyfert 1 galaxy NGC5548 with the X-ray light curve measured by the Rossi X-ray Timing Explorer over 6 years, to determine the relationship between the optical and X-ray continua. The X-ray light curve is strongly correlated with the optical light curve on long (~year) time-scales. The amplitude of the long-term optical variability in NGC5548 is larger than that of the X-ray variability (after accounting for the host galaxy contribution), implying that X-ray reprocessing is not the main source of the optical/X-ray correlation. The correlated X-ray and optical variations in NGC5548 may be caused by instabilities in the inner part of the accretion flow, where both the X-ray and optical emission regions may be located.

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arXiv2002

An Intrinsic Baldwin Effect in the H-beta Broad Emission Line in the Spectrum of NGC 5548

We investigate the possibility of an intrinsic Baldwin Effect (i.e.,nonlinear emission-line response to continuum variations) in the broad H-beta emission line of the active galaxy NGC 5548 using cross-correlation techniques to remove light travel-time effects from the data. We find a nonlinear relationship between the H-beta emission line and continuum fluxes that is in good agreement with theoretical predictions. We suggest that similar analysis of multiple lines might provide a useful diagnostic of physical conditions in the broad-line region.

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arXiv2002

Masses of Black Holes in Active Galactic Nuclei

We present a progress report on a project whose goal is to improve both the precision and accuracy of everberation-based black-hole masses. Reverberation masses appear to be accurate to a factor of about three, and the black-hole mass/bulge velocity dispersion relationship appears to be the same in active and quiescent galaxies.

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arXiv2002

The Ionized Gas and Nuclear Environment in NGC 3783 II. Averaged HST/STIS and FUSE Spectra

We present observations of the intrinsic absorption in the Seyfert 1 galaxy NGC 3783 obtained with the STIS/HST and FUSE. We have coadded multiple STIS and FUSE observations to obtain a high S/N averaged spectrum spanning 905-1730 A. The averaged spectrum reveals absorption in O VI, N V, C IV, N III, C III and the Lyman lines up to LyE in the three blueshifted kinematic components previously detected in the STIS spectrum (at radial velocities of -1320, -724, and -548 km/s). The highest velocity component exhibits absorption in Si IV. We also detect metastable C III* in this component, indicating a high density in this absorber. We separate the individual covering factors of the continuum and emission-line sources as a function of velocity in each kinematic component using the LyA and LyB lines. Additionally, we find that the continuum covering factor varies with velocity within the individual kinematic components, decreasing smoothly in the wings of the absorption by at least 60%. The covering factor of Si IV is found to be less than half that of H I and N V in the high velocity component. Additionally, the FWHM of N III and Si IV are narrower than the higher ionization lines in this component. These results indicate there is substructure within this absorber. We derive a lower limit on the total column (N_H>=10^{19}cm^{-2}) and ionization parameter (U>=0.005) in the low ionization subcomponent of this absorber. The metastable-to-total C III column density ratio implies n_e~10^9 cm^{-3} and an upper limit on the distance of the absorber from the ionizing continuum of R<=8x10^{17} cm.

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arXiv2002

The Structure and Energetics of Active Galactic Nuclei

The black-hole/accretion-disk paradigm for active galactic nuclei (AGNs) is now reasonably secure, but there are still important unresolved issues, some of which will require the capabilities of an 8 to 10-m class UV/optical space-based telescope. Imaging spectroscopy with a diffraction-limited large telescope will be required to measure AGN black-hole masses from stellar dynamics for direct comparison with reverberation mapping-based masses. High spectral resolution in the UV is required to determine the mass and kinetic energy of the outflows observed in the absorption spectra of AGNs and to understand the energetics of the accretion process. As with ground-based astronomy, however, effective use of a large UV/optical space telescope requires complementary smaller facility instruments; a meter-class UV spectroscopic telescope, for example, can fit into a Medium Explorer budget.

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