arXiv subjects

Shai Kaspi

Publications and source records attributed to Shai Kaspi.

83 records · Page 5Linked to original sources

arXiv2001

High-Resolution X-ray Spectroscopy and Modeling of the Absorbing and Emitting Outflow in NGC 3783

(abridged) The high-resolution X-ray spectrum of NGC 3783 shows several dozen absorption lines and a few emission lines from the H-like and He-like ions of O, Ne, Mg, Si, and S as well as from Fe XVII - Fe XXIII L-shell transitions. We have reanalyzed the Chandra HETGS spectrum using better flux and wavelength calibrations along with more robust methods. Combining several lines from each element, we clearly demonstrate the existence of the absorption lines and determine they are blueshifted relative to the systemic velocity by -610+/-130 km/s. We find the Ne absorption lines in the High Energy Grating spectrum to be resolved with FWHM=840{+490}{-360} km/s. We have used regions in the spectrum where no lines are expected to determine the X-ray continuum, and we model the absorption and emission lines using photoionized-plasma calculations. The model consists of two absorption components which have an order of magnitude difference in their ionization parameters. The two components are spherically outflowing from the AGN and thus contribute to both the absorption and the emission via P Cygni profiles. The model also clearly requires O VII and O VIII absorption edges. The low-ionization component of our model can plausibly produce UV absorption lines with equivalent widths consistent with those observed from NGC 3783. However, we note that this result is highly sensitive to the unobservable UV-to-X-ray continuum, and the available UV and X-ray observations cannot firmly establish the relationship between the UV and X-ray absorbers. We find good agreement between the Chandra spectrum and simultaneous ASCA and RXTE observations. We set an upper limit on the FWHM of the narrow Fe Kαemission line of 3250 km/s. This is consistent with this line originating outside the broad line region, possibly from a torus.

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arXiv2000

Reverberation Measurements of Quasars and the Size-Mass-Luminosity Relations in Active Galactic Nuclei

A 7.5 years spectrophotometric monitoring program of 28 Palomar-Green quasars to determine the size of their broad emission line region (BLR) is reviewed. We find both the continuum and the emission line fluxes of all quasars to vary during this period. Seventeen objects has adequate sampling for reverberation mapping and in all of them we find the Balmer line variations to lag those of the continuum by ~100 days. This study increases the available luminosity range for studying the size-mass-luminosity relations in active galactic nuclei (AGNs) by two orders of magnitude and doubles the number of objects suitable for such studies. Combining our results with data available for Seyfert 1 galaxies, we find the BLR size to scale with the rest-frame 5100 A luminosity as L^{0.70 +/- 0.03}. This result is different from previous studies, and suggests that the effective ionization parameter in AGNs may be a decreasing function of luminosity. We are also able to constrain, subject to the assumption that gravity dominates the motions of the BLR gas, the scaling relation between the mass of the central black hole and the AGN's luminosity. We find that the central mass scales with the 5100 A luminosity as M\proptoL^{0.5 +/- 0.1}. A program to monitor 11 high-luminosity quasars is presented here for the first time. Preliminary results from this program indicate continuum variation of order of 0.1 mag in all objects. We illustrate the importance and feasibility of monitoring those objects spectrophotometrically. When this program will be completed reverberation mapping studies will cover the entire AGNs luminosity range from 10^{41} to 10^{48} ergs/s.

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arXiv2000

Discovery of Narrow X-ray Absorption Lines from NGC 3783 with the Chandra HETGS

We present the first grating-resolution X-ray spectra of the Seyfert 1 galaxy NGC 3783, obtained with the High Energy Transmission Grating Spectrometer on the Chandra X-ray Observatory. These spectra reveal many narrow absorption lines from the H-like and He-like ions of O, Ne, Mg, Si, S and Ar, as well as FeXVII-FeXXI L-shell lines. We have also identified several weak emission lines, mainly from O and Ne. The absorption lines are blueshifted by a mean velocity of approximately 440\pm200 km/s and are not resolved, indicating a velocity dispersion within the absorbing gas of a few hundred km/s or less. We measure the lines' equivalent widths and compare them with the predictions of photoionization models. The best-fitting model has a microturbulence velocity of 150 km/s and a hydrogen column density of 1.3\times10^22 cm^-2. The measured blueshifts and inferred velocity dispersions of the X-ray absorption lines are consistent with those of the strongest UV absorption lines observed in this object. However, simple models that propose to strictly unify the X-ray and UV absorbers have difficulty explaining simultaneously the X-ray and UV absorption line strengths.

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arXiv2000

The X-ray Properties of z>4 Quasars

We report on a search for X-ray emission from quasars with redshifts greater than four using the ROSAT public database. Our search has doubled the number of z>4 quasars detected in X-rays from 6 to 12. Most of those known prior to this work were radio-loud and X-ray selected sources; our study increases the number of X-ray detected, optically selected z>4 quasars from one to seven. We present their basic X-ray properties and compare these to those of lower redshift quasars. We do not find any evidence for strong broad-band spectral differences between optically selected z>4 quasars and those at lower redshifts.

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arXiv1999

Reverberation Measurements for 17 Quasars and the Size-Mass-Luminosity Relations in Active Galactic Nuclei

(abridged) We have spectrophotometrically monitored a well-defined sample of 28 Palomar-Green quasars in order to obtain measurements of their BLRs and to investigate the relationships between quasar luminosity, central black hole mass, and broad emission line region (BLR) size in active galactic nuclei (AGN). Spectrophotometry was obtained every 1-4 months for 7.5 years, yielding 20-70 observing epochs per object. Both the continuum and emission line fluxes of all of the quasars were observed to change during the duration of the observing program. Seventeen of the 28 objects were observed with adequate sampling to search for correlated variations between the Balmer emission lines and the continuum flux. For each of these 17 objects, a significant correlation was observed, with the Balmer line variations lagging those of the continuum by \~100 days (rest frame). Our work increases the available luminosity range for studying the size-mass-luminosity relations in AGN by two orders of magnitude and doubles the number of objects suitable for such studies. Combining our results with comparable published data available for Seyfert 1 galaxies, we find the BLR size scales with the rest-frame 5100 A luminosity as L^{0.70+-0.03}. This determination of the scaling of the size of the BLR as a function of luminosity is significantly different from those previously published, and suggests that the effective ionization parameter in AGN may be a decreasing function of luminosity. We are also able to constrain, subject to our assumption that gravity dominates the motions of the BLR gas, the scaling relationship between the mass of the central black holes and the luminosity in AGN: M \propto L^{0.5+-0.1}. This is inconsistent with all AGN having optical luminosity that is a constant fraction of the Eddington luminosity.

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arXiv1999

Modeling Variable Emission Lines in AGNs: Method and Application to NGC 5548

We present a new scheme for modeling the broad line region in active galactic nuclei (AGNs). It involves photoionization calculations of a large number of clouds, in several pre-determined geometries, and a comparison of the calculated line intensities with observed emission line light curves. Fitting several observed light curves simultaneously provides strong constraints on model parameters such as the run of density and column density across the nucleus, the shape of the ionizing continuum, and the radial distribution of the emission line clouds. When applying the model to the Seyfert 1 galaxy NGC 5548, we were able to reconstruct the light curves of four ultraviolet emission-lines, in time and in absolute flux. This has not been achieved by any previous work. We argue that the Balmer lines light curves, and possibly also the MgII2798 light curve, cannot be tested in this scheme because of the limitations of present-day photoionization codes. Our fit procedure can be used to rule out models where the particle density scales as r^{-2}, where r is the distance from the central source. The best models are those where the density scales as r^{-1} or r^{-1.5}. We can place a lower limit on the column density at a distance of 1 ld, of N_{col}(r=1) >~ 10^{23} cm^{-2} and limit the particle density to be in the range of 10^{12.5}>N(r=1)>10^{11} cm^{-3}. We have also tested the idea that the spectral energy distribution (SED) of the ionizing continuum is changing with continuum luminosity. None of the variable-shape SED tried resulted in real improvement over a constant SED case although models with harder continuum during phases of higher luminosity seem to fit better the observed spectrum. Reddening and/or different composition seem to play a minor role, at least to the extent tested in this work.

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arXiv1999

Long-term optical variability properties of the Palomar-Green quasars

We present results from a monitoring program of 42 quasars from the Palomar Green sample. The objects were observed for 7 years at the Wise Observatory, as part of a long term effort to monitor AGN of various types. This is the most extensive program of its kind carried out to date on a well-defined optically selected quasar sample. The typical sampling interval is ~40 days. One third of the quasars were observed at ~60 epochs and the rest at ~30 epochs in two bands (B&R) with photometric accuracy of ~0.01 mag. We present lightcurves for all of the sources and discuss the sample variability properties. All of the quasars in the sample varied during the campaign with intrinsic rms amplitudes of 5%<sigma_B<34% and 4%<sigma_R<26%. The rms amplitude and colour for the entire sample are sigma_B=14%, sigma_R=12%, and sigma_{B-R}=5%. On time scales of 100- 1000 days the power spectra of the sources have a power-law shape with index approximately 2.0 and a spread <0.6. At least half of the quasars, particularly those that are most variable, become bluer when they brighten, and the rest do not show this behaviour. We quantify this phenomenon, which has been observed previously mainly in Seyfert galaxies. The quasars which are most variable tend also to exhibit asymmetry in their variations, in the sense that the brightening phases last longer than the fading phases. We have searched for correlations between the measured variability properties and other parameters of the quasars, such as luminosity, redshift, radio loudness, and X-ray slope. We find several new correlations, and reproduce some of the correlations reported by previous studies.

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arXiv1998

BLR Modeling: A New Approach

We present a new scheme for modeling the broad line region in active galactic nuclei. It involves photoionization calculations applied to a number of variable emission lines at {\it all times}. We demonstrate how fitting all lines simultaneously provide strong constraints on several of the more important parameters, such as the density and column density, and the radial distribution of the emission line clouds. When applying the model to the Seyfert 1 galaxy NGC 5548, we are able to reconstruct the light curves of four emission-lines, in time and in absolute flux. We argue that the Balmer line light curves, and possibly also the MgII light curve, do not fit this scheme because of the limitations of present-day photoionization codes. We rule out models where the particle density scales as r^{-2} and favor models where it scales as r^{-(1-1.5)}. We can place lower limits on the column density at a distance of 10 ld, of N_{col}(r=10)>10^{22} cm^{-2}, and limit the particle density to be in the range of 10^{11}>N(r=10)>10^{9.5} cm^{-3}.

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arXiv1998

The Discovery of a Nearby M Dwarf

We report the discovery of a nearby M dwarf star, found accidentally while observing the old nova DN Gem at the Wise Observatory. The star is designated 1200-05296925 in the PMM USNO-A1.0 catalogue and its coordinates, calculated for 1997 November 27 are: RA=6 55 05.13 Dec = +32 09 54.1 (Equinox J2000, Epoch J1997.90). Astrometric measurements for the star yielded a yearly proper motion rate of 0.155$\pm$0.002 arcseconds in right ascension and negligible yearly proper motion rate in declination. The apparent V magnitude of the star was measured as m_V=13.87$\pm$0.16 mag and spectral identification yielded a spectral type of M3.5Ve$\pm$0.5 subclasses. Using relations between spectral type and absolute V magnitude in M dwarfs, we arrive at an absolute magnitude of M_V=12.3^{+1.2}_{-1.1} mag, which corresponds to a distance of 21^{+15}_{-10} pc.

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arXiv1998

On Uncertainties in Cross-Correlation Lags and the Reality of Wavelength-Dependent Continuum Lags in Active Galactic Nuclei

We describe a model-independent method of assessing the uncertainties in cross-correlation lags determined from AGN light curves, and use this method to investigate the reality of lags between UV and optical continuum variations in well-studied AGNs. Our results confirm the existence of such lags in NGC 7469. We find that the continuum variations at 1825 A, 4845 A, and 6962 A follow those at 1315A by 0.22^{+0.12}_{-0.13} days, 1.25^{+0.48}_{-0.35} days, and 1.84^{+0.93}_{-0.94} days, respectively, based on the centroids of the cross-correlation functions; the error intervals quoted correspond to 68% confidence levels, and each of these lags is greater than zero at no less than 97% confidence. We do not find statistically significant interband continuum lags in NGC 5548, NGC 3783, or Fairall 9. Wavelength-dependent continuum lags may be marginally detected in the case of NGC 4151. However, on the basis of theoretical considerations, wavelength-dependent continuum lags in sources other than NGC 7469 are not expected to have been detectable in previous experiments. We also confirm the existence of a statistically significant lag between X-ray and UV continuum variations in the blazar PKS 2155-304.

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arXiv1996

Measurement of the Broad Line Region Size in Two Bright Quasars

We present 4 years of spectrophotometric monitoring data for two radio-quiet quasars, PG 0804+762 and PG 0953+414, with typical sampling intervals of several months. Both sources show continuum and emission line variations. The variations of the H$β$ line follow those of the continuum with a time lag, as derived from a cross-correlation analysis, of 93$\pm$30 days for PG 0804+762 and 111$\pm$55 days for PG 0953+414. This is the first reliable measurement of such a lag in active galactic nuclei with luminosity $L>10^{45}$ erg s$^{-1}$. The broad line region (BLR) size that is implied is almost an order of magnitude larger than that measured in several Seyfert 1 galaxies and is consistent with the hypothesis that the BLR size grows as $L^{0.5}$.

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