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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1986

Variability studies of 3C 371

The compact extragalactic radio source, 3C 371, was observed with the X-ray detectors of the EXOSAT satellite for 19.5 hours in 2 observing periods separated by 18 days. This resulted in the discovery of X-ray variability of the source on time scales between 8 hours and about 25 minutes and the confirmation of earlier reports of variability on longer time scales. The short time scale variability agrees remarkably well with earlier predictions based on fitting multifrequency data from the source to a relativistically beamed inhomogeneous synchrotron self-Compton (SSC) jet model. This SSC model is frequently applied to BL Lac objects and related sources, such as 3C 371. However, the number of free parameters in model fits tends to be large, and so independent support such as from this work is important. The X-ray spectral results for 3C 371 also may provide qualitative support for the SSC model in that the spectrum probably consists of two components, with the steeper one at low photon energies. In terms of the model, the low-energy spectrum would be dominated by synchrotron emission extending down to radio energies, whereas the higher energy X-rays would be dominated by Compton radiation.

Worrall, D. M.↗
NASA NTRS1986

Continuum radiation from active galactic nuclei: A statistical study

The physics of the continuum spectrum of active galactic nuclei (AGNs) was examined using a large data set and rigorous statistical methods. A data base was constructed for 469 objects which include radio selected quasars, optically selected quasars, X-ray selected AGNs, BL Lac objects, and optically unidentified compact radio sources. Each object has measurements of its radio, optical, X-ray core continuum luminosity, though many of them are upper limits. Since many radio sources have extended components, the core component were carefully selected out from the total radio luminosity. With survival analysis statistical methods, which can treat upper limits correctly, these data can yield better statistical results than those previously obtained. A variety of statistical tests are performed, such as the comparison of the luminosity functions in different subsamples, and linear regressions of luminosities in different bands. Interpretation of the results leads to the following tentative conclusions: the main emission mechanism of optically selected quasars and X-ray selected AGNs is thermal, while that of BL Lac objects is synchrotron; radio selected quasars may have two different emission mechanisms in the X-ray band; BL Lac objects appear to be special cases of the radio selected quasars; some compact radio sources show the possibility of synchrotron self-Compton (SSC) in the optical band; and the spectral index between the optical and the X-ray bands depends on the optical luminosity.

Isobe, T.↗
NASA NTRS1986

New synchrotron luminosity distance limit for the 1979 March 5 gamma-ray event

New synchrotron luminosity estimates are provided for the March 5, 1979 gamma-ray event, based on the lack of low-frequency self-absorption cutoff. This leads to distance limits much lower than 55 kpc for a neutron-star emission area even in the most conservative case. The source is likely Galactic. If in addition the thermal self-Compton interpretation is invoked for the hard tail above 300 keV in the burst spectrum, then the source distance lies in the range 0.1-2 kpc for extreme ranges of the neutron-star emission area. This result strongly impacts current observational programs for this source.

Liang, E. P.↗
NASA NTRS1986

The synchrotron-self-Compton process in spherical geometries. II - Application to active galactic nuclei

Observational implications and constraints on spherical nonthermal synchrotron self-Compton models with applications to AGNs are discussed. The Compton optical depth of the source and the synchrotron optical depth are considered, and plausible electron distributions are discussed. The alpha(0x), alpha(rx), and alpha(0r) system of comparing the fluxes in different energy bands is related to the nonthermal models, and sample spectra are presented that show the spectral features the models can produce. The models are applied to radio-quiet AGNs. It is shown that the infrared and X-ray continua of the AGNs can be produced by nonthermal processes.

Band, D. L.↗
NASA NTRS1986

BL Lac objects and relativistic beaming

General arguments for relativistic beaming in BL Lac objects are reviewed. These include overproduction of X-rays and fast time variability. Comments are made about the relationship of the X-ray continuum to that at lower frequencies, and observational evidence for and against continuum radiation being relativistically beamed is discussed. Finally, there is discussion of the influence of geometrical effects on predictions for time variability as a function of frequency in the context of inhomogeneous synchrotron self-Compton jet models.

Worrall, Diana M.↗
NASA NTRS1987

Simultaneous multifrequency observations of the BL Lacertae object Markarian 421

Simultaneous multifrequency observations of the BL Lac object Mrk 421 covering radio through X-ray wavelengths were performed on two occasions separated by 5 weeks in January and March 1984, and each observation was coordinated for about 1 week. Composite multifrequency spectra of the central nonthermal component were obtained at the two epochs after subtracting the optical and infrared light of the underlying galaxy. The spectra show the gradual steepening toward high frequency; the power law indices are about 0.1, about 0.6, and about 1.0 for radio, infrared-optical, and UV bands, respectively. The UV and optical-infrared fluxes decreased by about 20 percent in 5 weeks, while the radio flux remained stable. The X-ray flux decreased by a factor of about 2, and the change was more pronounced at hard X-rays, which suggest that X-ray emission possibly consists of two components. The degree of polarization at the optical band varied on the time scale of a few days, while the position angle remained unchanged. Physical parameters of Mrk 421 are discussed in terms of the synchrotron self-Compton model.

Makino, F.↗
NASA NTRS1987

Luminosity dependence in the ratio of X-ray to infrared emission of QSOs

The correlation of X-ray and near-infrared luminosity is studied for a sample of radio-quiet QSOs. The X-ray to infrared ratio is found to decrease as the infrared luminosity increases. No preference is found between the correlations of X-ray luminosity with optical or infrared luminosity. This implies that optical and infrared emission are equally good predictors of X-ray emission. Source models which directly link infrared and X-ray emission are discussed, and a preference is found for a specific synchrotron self-Compton model. This model predicts the correct luminosity dependence of the X-ray to infrared ratio if certain conditions apply.

Worrall, D. M.↗
NASA NTRS1988

Relativistic jet models for the BL Lacertae object Mrk 421 during three epochs of observation

Coordinated observation of the nearby BL Lacertae object Mrk 421 obtained during May 1980, January 1984, and March 1984 are described. These observations give a time-frozen picture of the continuous spectrum of Mrk 421 at X-ray, ultraviolet, optical, and radio wavelengths. The observed spectra have been fitted to an inhomogeneous relativistic jet model. In general, the models reproduce the data well. Many of the observed differences during the three epochs can be attributed to variations in the opening angle of the jet and in the angle that the jet makes to the line of sight. The jet models obtained here are compared with the homogeneous, spherically symmetric, synchrotron self-Compton models for this source. The models are also compared with the relativistic jet models obtained for other active galactic nuclei.

Mufson, S. L.↗
NASA NTRS1987

Superluminal radio sources - What does X-ray emission tell us?

In a study on superluminal radio sources, statistical relationships between X-ray, optical, and radio luminosities among different categories of active galactic nuclei are compared to search for common energy mechanisms. The X-ray versus optical and X-ray versus radio correlations of radio-loud QSOs and superluminal radio sources are found to be similar, arguing against a model in which the emission in only one or two of the three wave bands is relativistically boosted. A regression analysis shows that highly polarized QSOs and optically violently variable QSOs are more similar to other flat-spectrum, radio-loud QSOs than to BL Lac objects, and it is reasonable to assume that self-Compton emission dominates the X-ray emission from at least half of the sources in this class. The X-ray versus radio correlation for BL Lac objects is poor, and there is support for the hypothesis that their X-ray emission is dominated by an isotropic component which is not directly related to relativistically boosted radio emission.

Worrall, Diana M.↗
NASA NTRS1987

Synchro-Compton emission from superluminal sources

The application of synchro-Compton theory to real compact radio sources, the question of a self-Compton origin of the X-rays in radio-loud quasars and active galactic nuclei, and the phenomenology of superluminal motion are discussed in a review of research concerning synchro-Compton emission from superluminal sources. After examining the basic synchro-Compton theory of ideal sources, applications of the theory to real sources is discussed. It is concluded that the Compton problem and total energy requirements are not substantially mitigated by considering source structures more complicated than the multiple, uniform-component model used by most investigators. Also, alternatives to the standard model of superluminal motion are discussed, focusing on the assumptions usually made when interpreting superluminal sources.

Marscher, Alan P.↗
NASA NTRS1988

Studies of BL Lacertae objects with the Einstein Observatory - The soft X-ray spectra of OJ 287 and PKS 0735+178

Accurate, soft X-ray spectra of two BL Lac objects, OJ 287 and PKS 0735+178, are presented. The X-ray spectra are well described by a power-law model with a low-energy cutoff consistent with photoelectric absorption within the Galaxy. The best-fit values of the energy spectral index in the 0.2-4.0 keV band are 0.91 and 0.76 respectively. The X-ray flux from OJ 287 is variable by a ratio of three from low to high state; PKS 0735+178 shows no indication of X-ray variability. The X-ray emission in OJ 287 is interpreted to be due to the synchrotron process from a volume common with either a beamed radio component or a stationary optical component. In PKS 0735+178, where the X-ray emission is most likely due to the Compton process operating in one of the VLBI radio components. The synchrotron self-Compton process with modest kinematic Doppler factors predicts the measured X-ray flux from PKS 0735+178 and lower than the measured flux in OJ 287.

Madejski, Greg M.↗
NASA NTRS1988

Active galactic nuclei

The astrophysical processes in AGN are examined, reviewing the results of ground-based and space observations obtained in different regions of the electromagnetic spectrum. Topics addressed include blazars and blazar variability, the apparently nonthermal broadband continuum emission of blazars, simple synchrotron self-Compton models, inhomogeneous models, and the need for multiwavelength spectra of blazars. Consideration is given to nonblazar emission-line AGN, the radio/IR power law, dust emission in the IR, the NIR excess, host-galaxy starlight, 'blue bumps', the X-ray power law, the big blue bump, broadband X-ray emission, and emission lines. The great potential value of multiwavelength variability studies of AGN is stressed.

Urry, C. Megan↗
NASA NTRS1989

A search for gamma rays of 1.5-20.0 MeV from Centaurus A

A search for a gamma-ray continuum and lines from Centaurus A is reported. No measurable gamma rays are found in the 1.5-20.0 MeV and 3 sigma upper limits are reported for various energies which are about a factor of two lower than the results reported by von Ballmoos et al. (1987). These upper limits, along with those of SAS 2 and COS B, suggest that the energy distribution turns over in the energy range of 500 keV to a few MeV. A high source temperature of 5 MeV or synchrotron self-Compton models for producing an excess of gamma rays of a few MeV do not appear necessary.

O'Neill, Terrence↗
NASA NTRS1989

On the ratio of the infrared-to-ultraviolet continuum to the X-rays in quasars and active galaxies

A sample of 122 quasars and Seyfert galaxies was analyzed and it was found that the NIR 7500 A rest frame luminosity is more linearly related to the X-ray luminosity than is the optical luminosity. It is also found that the mean spectral slope between 7500 and 4200 A in the rest frame monotonically flattens with luminosity over more than three orders of magnitude. The present results support models which attribute the IR and X-ray continua to linked nonthermal processes such as synchrotron self-Compton, while the blue-UV bump comes from a different source, such as an accretion disk.

Mushotzky, R. F.↗
NASA NTRS1990

Multifrequency observations of BL Lacertae

Optical, IR, and radio monitoring data covering 20 years are presented for BL Lac. Also, four simultaneous multifrequency spectra are given, covering the range 10 to the 9th - 10 to the 18th Hz. These spectra include ground-based observations and data from the Einstein, Exosat, IUE, and IRAS satellites. From the monitoring data, the character of emission variability is determined using structure functions. The multifrequency spectra are used to study the origin of X-ray emission and to determine the physical parameters of emitting regions. The data show that the emitting region is inhomogeneous, with a small inner region of high density and magnetic field that eventually becomes a large, partially opaque radio emitting region with a low density and magnetic fields. Also, the two simple predictions of the synchrotron self-Compton model are reproduced by the multifrequency spectra.

Bregman, Joel N.↗
NASA NTRS1990

An evolving relativistic jet model for the BL Lacertae object Markarian 421

Coordinated observations of the nearby BL Lacertae object Mrk 421 obtained during May 1980, January 1984 and March 1984, are described. These observations give a time-frozen picture of the continuous spectrum of Mrk 421 at X-ray, ultraviolet, optical, and radio wavelengths. The observed spectra have been fitted to an inhomogeneous relativistic jet model described by Hutter and Mufson. The studies show that it is difficult to describe the data by a single jet whose properties vary as a function of time in a self-consistent manner unless the flat spectrum, hard X-ray tail present in January 1984 is due to an additional short-lived component of emission. These jet models are compared with homogeneous, spherically symmetric, synchrotron self-Compton models for this source.

Mufson, S. L.↗
NASA NTRS1990

X-ray spectra of compact extragalactic radio sources

A comparison is conducted of the 0.1-3.5 keV mean power law energy spectral indices (alpha) measured by the Einstein Observatory IPC for (1) 19 low polarization, flat radio spectrum (FRS) core-dominant QSOs; (2) 12 highly polarized QSOs (HPQs); and (3) 24 radio-selected BL Lac objects. Individual spectral fits are presented for several sources for which results are unavailable elsewhere. A maximum-likelihood analysis was used to ascertain the mean power-law index and the standard deviation for each class, under the assumption of a Gaussian intrinsic distribution. The distributions for HPQ and FRS QSOs are similar, in contrast with a small sample of the X-ray-selected BL Lac objects. Implications of these findings for synchrotron self-Compton models are considered.

Worrall, D. M.↗
NASA NTRS1992

A survey of ultraviolet variability in blazars

This is the first comprehensive, consistent survey of the ultraviolet variability properties of a large sample of blazars. All of the 14 blazars with five or more observations in a single IUE camera showed significant variability. The strength of the ultraviolet variability is positively correlated with both the apparent ultraviolet luminosity and the degree of optical polarization of the source. This is the first clear indication that the ultraviolet continua of blazars are beamed. A search for rapid variability yielded nine 'events' with doubling time scales of 0.5-7 days. These rapid flux variations and correlations argue for the standard synchrotron self-Compton model, and against the accretion disk model, which would predict slower variations and the opposite sense of correlation.

Edelson, Rick↗