Engineering topics

Edelson, R. A.

Publications and source records attributed to Edelson, R. A..

15 records

NASA NTRS2000

The Origin of the X-Ray and Ultraviolet Emission in NGC 7469

We present a spectral analysis of a approx 30 day, near-continuous observation of the Seyfert 1 galaxy NGC 7469 with RXTE. Daily integrations show strong spectral changes during the observation. Our main result is that we find the X-ray spectral index to be correlated with the UV flux. Furthermore, the broadband X-ray photon flux is also correlated with the UV continuum. These correlations point toward a model in which the X-rays originate via thermal Comptonization of UV seed photons. Furthermore, the UV is also correlated with the extrapolation of the X-ray power law into the soft X-ray/EUV region. Our data analysis therefore reopens the possibility that the UV photons and their variability arise from reprocessing, as long as the primary source of heating is photoelectric absorption in the re-processor, rather than Compton down-scattering. A coherent picture of the X-ray/UV variability can therefore be constructed whereby absorption and reprocessing of EUV/soft X-rays in a standard accretion disk produce a variable seed photon distribution, which is in turn up-scattered into the X-ray band. We also find a significant correlation between the 2-10 keV flux and the 6.4 keV iron K-alpha line, suggesting that at least some portion of the line originates within approx. 1 light day of the X-ray continuum source. Neither the power-law photon index nor the Compton reflection component are correlated with the 2-10 keV flux. The latter is not correlated with the iron K-alpha line flux either. We do find an apparent correlation between the X-ray spectral index and the strength of the Compton reflection component. In an Appendix we show, however, that this can be produced by a combination of statistical and systematic errors. We conclude that the apparent variations in the Compton reflection component may be an artifact of these effects.

Nandra, K.↗

NASA NTRS1991

Rapid ultraviolet variability in the BL Lacertae object PKS 2155 - 304

This letter reports the first clear evidence of a rapid change in the ultraviolet flux of a BL Lac object. Sophisticated analysis of 57 closely spaced IUE spectra yielded clear evidence of an about 12 percent drop in the ultraviolet flux of PKS 2155 - 304 over a 2 day period in October 1989 and a strong indication of a similar drop in a 5 hr period some three weeks later. These are near the fastest variations which could have been detected with these data, but they are still much slower (or weaker) variations than have been observed in the X-rays. However, they are orders of magnitude fater than Seyfert 1 ultraviolet variations. This suggests that a 'standard' Seyfert 1 accretion disk may not be responsible for the ultraviolet emission from PKS 2155 - 304.

Edelson, R. A.↗

NASA NTRS1991

Ultraviolet variability of NGC 5548 - Dynamics of the continuum production region and geometry of the broad-line region

Data from the 1989-1990 IUE monitoring of the Seyfert galaxy NGC 5548 are used here to analyze the continuum variability properties of the galaxy and to derive the structure or its emission-line region. The mean shape of the UV continuum is well fit by an accretion disk model with a given black hole mass and an additional component required to reproduce the observed soft X-ray flux. The continuum fluctuation power spectrum is very steep, with most of the variance coming from about 1 yr time scales. The entire optical/UV continuum rises and falls almost simultaneously, so that the logarithmic slope of the power spectrum is nearly the same for all bands, but the flux at higher photon frequencies varies with larger amplitude. The emission-line material around the nucleus may best be described by a highly ionized inner zone of high and nearly constant pressure that stretches about 4-14 light-days from the center and an outer, more weakly ionized zone of considerably lower ionization at least 20-30 light-days out.

Krolik, J. H.↗

NASA NTRS1990

Broad-band properties of the CfA Seyfert galaxies. III - Ultraviolet variability

A total of 657 archived IUE spectra are used to study the UV variability properties of six members of the CfA Seyfert I galaxy sample. All show strong evidence for continuum and line variations and a tendency for less luminous objects to be more strongly variable. Most objects show a clear correlation at zero lag between UV spectral index and luminosity, evidence that the variable component is an accretion disk around a black hole which is systematically smaller in less luminous sources. No correlation is seen between the continuum luminosity and equivalent width of the C IV, Mg II, and semiforbidden C III emission lines when the entire sample is examined, but a clear anticorrelation is present when only repeated observations of individual objects are considered. This is due to a combination of light-travel time effects in the broad-line region and the nonlinear responses of lines to continuum fluctuations.

Edelson, R. A.↗

NASA NTRS1990

The mass function of Seyfert 1 nuclei

The first mass function of Seyfert 1 nuclei is derived from optical spectra of the complete CfA sample of Seyfert galaxies by estimating the mass for each object from a dynamical relation. An independent estimate is also derived using a complete infrared-selected sample. The two mass functions are indistinguishable. The mean mass of Seyfert 1 nuclei is about 2 x 10 to the 7th solar masses, and the integrated mass density is about 6 x 10 to the 11th solar masses/cu Gpc. This is approximately two orders of magnitude less than the value inferred from the energetics associated with quasar counts. A careful analysis of the various parameters and assumptions involved suggests that this large difference is not due to systematic errors in the determinations. Therefore, the bulk of mass related to the accretion processes connected with past quasar activity does not reside in Seyfert 1 nuclei. Instead, the remnants of past activity must be present in a much larger number of galaxies, and a one-to-one relation between distant and local active galactic nuclei seems then to be excluded.

Padovani, P.↗

NASA NTRS1989

The complex X ray spectra of AGN

X-ray spectral surveys of large samples of Seyfert galaxies are discussed. The spectral shape in the 0.1 to 20 keV energy range is considered. Two new spectral survey are undertaken, one involving 105 Imaging Proportional Counter (IPC) observations of 75 Seyfert galaxies, the other using IPC and Monitor Proportional Counter (MPC) data from 28 observations of 23 Seyfert galaxies. The X-ray spectra of active galactic nuclei (AGN) are complex, with in most cases considerable steepening at the lowest energies. At higher energies (2 to 20 keV), the existence of a universal, canonical power law is confirmed, independent of X-ray luminosity over four orders of magnitude.

Urry, C. M.↗

NASA NTRS1988

Evidence from sub-millimetre observations for thermal dust emission in NGC4151

Observations of the Seyfert 1 galaxy NGC4151 using the UKT14 bolometer on the James Clerk Maxwell Telescope are reported. A 5 sigma upper limit of 200 mJy in an 11 arcsec aperture is found. Comparison with an earlier measurement at 155 microns indicates that at least half of the 155-micron flux is due to thermal dust emission. The remainder may be from a synchrotron source which becomes self-absorbed at wavelengths of less than 80 microns.

Edelson, R. A.↗

NASA NTRS1988

The discrete correlation function - A new method for analyzing unevenly sampled variability data

A method for measuring correlation functions without interpolating in the temporal domain is proposed which provides an assumption-free representation of the correlation measured in the data and allows meaningful error estimates. Physical interpretation of the cross-correlation function of two series believed to be related by a convolution is shown to require knowledge of the input function's fluctuation power spectrum. Application of the method to two systems reveals no correlation for the optical data of Akn 120, but a strong correlation for the UV data of NGC 4151, placing bounds of between 1.2 and 20 light days on the size of the line-emitting region.

Edelson, R. A.↗

NASA NTRS1988

The discrete correlation function: A new method for analyzing unevenly sampled variability data

A method of measuring correlation functions without interpolating in the temporal domain, the discrete correlation function, is introduced. It provides an assumption-free representation of the correlation measured in the data, and allows meaningful error estimates. This method does not produce spurious correlations at zero lag due to correlated errors. It is shown that physical interpretation of active galactic nuclei cross-correlation functions requires knowledge of the input function's fluctuation power spectrum, involves model-dependence in the form of symmetry assumptions, and must take into account intrinsic scale bias. This technique was used to find a correlation in published IUE data for NGC 4151, which indicates that the broad C IV feature emanates from a shell 15 to 75 light-days in radius, assuming spherical symmetry.

Edelson, R. A.↗

NASA NTRS1987

Far-infrared variability in active galactic nuclei

Pointed IRAS observations of 20 active galaxies were examined in the first detailed search for far-infrared variability in a large sample of active galaxies. Less extensive survey data were also checked for evidence of strong 6-month variability in 45 active galaxies. The far-infrared fluxes of three highly polarized objects ('blazars') appeared to vary by up to a factor of 2 on time scales of a few months. No convincing cases of variability greater than about 15 percent (rms) were found in any of the normal quasars or Seyfert galaxies studied. It appears that blazars tend to be variable at all wavelengths, but most active galaxies may be systematically less variable in the far-infrared than at higher frequencies.

Edelson, R. A.↗

NASA NTRS1987

Millimeter-wave spectra and variability of bright, compact radio sources

Observations at 2.7 mm and at 1.5 cm were used to study the millimeter spectra and variability of 176 bright, compact radio sources. More than 20 percent of the flat-spectrum sources, but none of the steep-spectrum sources, were seen to vary at 1.5 cm by at least 30 percent over ten months. This is consistent with the hypothesis that flat-spectrum sources are compact and possibly beamed, while steep-spectrum sources are not. These data can also be used to choose sources for VLBI observations and for calibration of millimeter-wave observations.

Edelson, R. A.↗

NASA NTRS1987

Broad-band properties of the CfA Seyfert Galaxies. II - Infrared to millimeter properties

IR and mm observations of the 48 Seyfert 1 and 2 galaxies (SG1s and SG2s) of the CfA sample (Huchra and Berg, 1987) are reported. Data obtained (1) in the NIR using the 1.55-m reflector at Stewart Observatory and the 3-m IRTF during 1984-1986, (2) in the FIR with IRAS, and (3) at 1.3 mm using the 12-m NRAO telescope at KPNO in June 1984 are presented in extensive tables and graphs and characterized in detail. None of the objects was detected at 1.3 mm, and the IR spectra of the SG2s are found to be significantly steeper (indicating thermal emission) than those of SG1s and QSOs (nonthermal emission). Turnover in the IR emission below 100 microns (in half of the objects detected at three or more IRAS wavelengths) is shown to be consistent with an accretion disk in dust-free SG1s and with unusually warm (35-65 K) dust in SG2s. It is inferred that a 60-100-micron cool excess is masking turnover in the other SGs, so that a general association of SG nuclei with strong star formation can be confirmed.

Edelson, R. A.↗

NASA NTRS1987

Far-infrared properties of optically-selected quasars and Seyfert galaxies

Pointed IRAS observations and ground based observations are used to determine the infrared properties of optically selected galaxies and quasars. The use of complete, unbiased, optically selected samples means that statistical tests can be applied to probe the underlying properties of active galactic nuclei (AGNs). The near infrared to millimeter spectral energy distributions (SEDs) were studied of the CfA Seyfert galaxies, a well defined, unbiased sample of 25 Type 1 and 23 Type 2 Seyfert galaxies selected by optical spectroscopy. Data given show strong trends in the infrared SEDs. Strong evidence is also given that the infrared spectra of Seyfert 2 galaxies are dominated by thermal emission from warm dust, while nonthermal emission is more important in the spectra of quasars and luminous Seyfert 1 nuclei.

Edelson, R. A.↗

NASA NTRS1986

Far-infrared properties of optically selected quasars

The far-infrared properties of 10, optically selected quasars were studied on the basis of pointed IRAS observations and ground-based near-infrared and radio measurements. Nine of these quasars were detected in at least three IRAS bands. The flat spectral energy distributions characterizing these optically selected quasars together with large 60-100-micron luminosities suggest that the infrared emission is dominated by nonthermal radiation. Seven of the nine quasars with far-infrared detections were found to have low-frequency turnovers.

Edelson, R. A.↗

NASA NTRS1986

Spectral energy distributions of active galactic nuclei between 0.1 and 100 microns

The 0.1-100-micron line-free spectral energy distributions of 29 AGN are characterized by fitting simple models to data from IRAS and IUE observations and ground-based NIR and optical spectrophotometry. The data are presented in tables and graphs and discussed in detail. It is found that nuclear dust does not play a major role in most of the Seyfert 1 galaxies and QSOs, which can be described by a model with four spectral components.

Edelson, R. A.↗