Karen T. Lewis

Black Hole Masses of Active Galaxies with Double-peaked Balmer Emission Lines*

We have obtained near-IR spectra of five AGNs that exhibit double-peaked Balmer emission lines (NGC 1097, Pictor A, PKS 0921-213, 1E 0450.30-1817, and IRAS 0236.6-3101). The stellar velocity dispersions of the host galaxies were measured from the Ca II λλ8494, 8542, 8662 absorption lines and were found to range from 140 to 200 km s-1. Using the well-known correlation between the black hole mass and the stellar velocity dispersion, the black hole masses in these galaxies were estimated to range from 4 × 107 to 1.2 × 108 M☉. We supplement the observations presented here with estimates of the black holes masses for five additional double-peaked emitters (Arp 102B, 3C 390.3, NGC 4579, NGC 4203, and M81) obtained by other authors using similar methods. Using these black hole masses, we infer the ratio of the bolometric luminosity to the Eddington luminosity, (Lbol/LEdd). We find that two objects (Pictor A and PKS 0921-213) have Lbol/LEdd ~ 0.2, whereas the other objects have Lbol/LEdd 10-2 (nearby, low-luminosity double-peaked emitters are the most extreme, with Lbol/LEdd 10-4). The physical timescales in the outer regions of the accretion disks (at r ~ 103GM/c2) in these objects were also estimated and range from a few months for the dynamical timescale to several decades for the sound crossing timescale. The profile variability in these objects is typically an order of magnitude longer than the dynamical time, but we note that variability occurring on the dynamical timescale has not been ruled out by the observations.

THE RADIAL STRUCTURE OF SUPERNOVA REMNANT N103B

We report on the results from a Chandra ACIS observation of the young, compact, supernova remnant N103B. The unprecedented spatial resolution of Chandra reveals subarcsecond structure, in both the brightness and spectral variations. Underlying these small-scale variations is a surprisingly simple radial structure in the equivalent widths of the strong Si and S emission lines. We investigate these radial variations through spatially resolved spectroscopy, using a plane-parallel, nonequilibrium ionization model with multiple components. The majority of the emission arises from components with a temperature of 1 keV: a fully ionized hydrogen component; a high ionization timescale (net > 1012 s cm-3) component containing Si, S, Ar, Ca, and Fe; and a low ionization timescale (net ~ 1011 s cm-3) O, Ne, and Mg component. To reproduce the strong Fe Kα line, it is necessary to include additional Fe in a hot (> 2 keV), low ionization timescale (net ~ 1010.8 s cm-3) component. This hot Fe might be in the form of hot Fe bubbles, formed in the radioactive decay of clumps of 56Ni. We find no radial variation in the ionization timescales or temperatures of the various components. Rather, the Si and S equivalent widths increase at large radii because these lines, as well as those of Ar and Ca, are formed in a shell occupying the outer half of the remnant. A shell of hot Fe is located interior to this, but there is a large region of overlap between these two shells. In the inner 30% of the remnant, there is a core of cooler, 1 keV Fe. We find that the distribution of the ejecta and the yields of the intermediate-mass species are consistent with model prediction for Type Ia events.

A Simultaneous RXTE and XMM-Newton Observation of the Broad-Line Radio Galaxy 3C 111

We present the results of simultaneous XMM-Newton and RXTE observations of the broad-line radio galaxy 3C 111. We find that the Compton reflection bump is extremely weak; however, broad residuals are clearly present in the spectrum near the Fe Kα emission line region. When fitted with a Gaussian emission line, the feature has an equivalent width of 40-100 eV and FWHM of greater than 20,000 km s-1; however, the exact properties of this weak line are highly dependent on the chosen continuum model. The width of the line suggests an origin in the inner accretion disk, which is, however, inconsistent with the lack of Compton reflection. We find that much of the broad residual emission can be attributed to continuum curvature. The data are consistent with a model in which the primary power-law continuum is reprocessed by an accretion disk that is truncated at small radii. Alternatively, the primary source could be partially covered by a dense absorber. The latter model is less attractive than the former because of the small inclination angle of the jet of 3C 111 to the line of sight. We consider it likely that the curved continuum of the partial covering model is fortuitously similar to the continuum shape of the reprocessing model. In both models, the fit is greatly improved by the addition of an unresolved Fe Kα emission line, which could arise in either a Compton-thin obscuring torus or dense clouds lying along the line of sight. We also find that there are unacceptable residuals at low energies in the MOS data in particular, which were modeled as a Gaussian with an energy of ~1.5 keV; we attribute these residuals to calibration uncertainties of the MOS detectors.

Emission-Line Diagnostics of the Central Engines of Weak-Line Radio Galaxies

A handful of well-studied weak-line radio galaxies (WLRGs) have been traditionally classified as low-ionization nuclear emission-line regions (LINERs), suggesting that these two groups of active galactic nuclei (AGNs) might be related. In this paper, we present new optical emission-line measurements for 20 WLRGs, which we supplement with measurements for an additional four from the literature. Classifying these objects by their emission-line ratios, we find that 50% of the objects are robustly classified as LINERs, while an additional 25% are likely to be LINERs. Photoionization calculations show that the spectral energy distribution of the well-studied WLRG 3C 270 (NGC 4261) is able to produce the observed emission-line ratios, but only if the UV emission seen by the narrow emission line gas is significantly higher than that observed, implying AV = 2.5-4.2 mag along our line of sight to the nucleus. From the photoionization calculations, we find that the emission-line gas must have an ionization parameter between 10-3.5 and 10-4.0 and a wide range in hydrogen density (102-106 cm-3) to reproduce the measured emission-line ratios, similar to the properties inferred for the emission-line gas in LINERs. Thus, we find that properties of the emission-line gas as well as the underlying excitation mechanism are indeed similar in LINERs and WLRGs. By extension, the central engines of accretion-powered LINERs and WLRGs, which do host an accreting black hole, may be qualitatively similar.

Early results from Chandra observations of supernova remnants

We present preliminary results from observations of supernova remnants by the Chandra X-ray Observatory. The data include imaging spectroscopy from objects observed with both GTO and GO data. The high spatial resolution of Chandra is revealing a wealth of small-scale structure in these remnants. Specifically, we have resolved the remnant of SN1987A, and have discovered fine-scale structure in N103B and G292.0+1.8.

Long-term profile variability of double-peaked emission lines in AGNs

An increasing number of AGNs exhibit broad, double-peaked Balmer emission lines, which are thought to arise from the outer regions of the accretion disk which fuels the AGN. The line profiles are observed to vary on a characteristic timescales of 5-10 years. The variability is not a reverberation effect; it is a manifestation of physical changes in the disk. Our group has monitored a set of 20 double-peaked emitters for the past 8 years (longer for some objects). Here, we characterize the variability of the double-peaked H alpha line profiles in five objects from our sample. By experimenting with simple models, we find that disks with a single precessing spiral arm are able to reproduce many of the variability trends that are seen in the data.

Exploring the small scale structure of N103B

We present the preliminary results of a 40.8 ks Chandra ACIS observation of the young supernova remnant (SNR) N103B located in the Large Magellanic Cloud. The image reveals structure at the sub-arcsecond level, including several bright knots and filamentary structures. The remnant has the characteristic spectrum of a Type Ia SNR, containing strong lines of Fe, He- and H-like Si and S, Ar, and Ca. Narrow band images reveal non-uniformities in the remnant.