Alberto Sadun - Academia.edu (original) (raw)
Papers by Alberto Sadun
Journal of the American College of Cardiology, Jul 1, 1993
Journal of the American College of Cardiology, Feb 1, 1991
Galaxies, 2020
We report on seven nights of optical observation taken over a two-week period, and the resultant ... more We report on seven nights of optical observation taken over a two-week period, and the resultant analysis of the intermediate-frequency peaked BL Lac object (IBL), BL Lac itself, at redshift z = 0.069. The microvariable behavior can be confirmed over the course of minutes for each night. A relativistic beaming model was used in our analysis, to infer changes to the line of sight angles for the motion of the different relativistic components. This model has very few free parameters. The light curves we generated show both high and moderate frequency cadence to the variable behavior of BL Lac itself, in addition to the well documented long-term variability.
Multi-Wavelength Continuum Emission of AGN, 1994
The Astronomical Journal, 1991
ABSTRACT
The Astronomical Journal, 1990
The Astrophysical Journal, 2010
Monthly Notices of the Royal Astronomical Society
The bright blazar OJ 287 routinely parades high brightness bremsstrahlung flares, which are expla... more The bright blazar OJ 287 routinely parades high brightness bremsstrahlung flares, which are explained as being a result of a secondary supermassive black hole (SMBH) impacting the accretion disc of a more massive primary SMBH in a binary system. The accretion disc is not rigid but rather bends in a calculable way due to the tidal influence of the secondary. Below we refer to this phenomenon as a variable disc level. We begin by showing that these flares occur at times predicted by a simple analytical formula, based on general relativity inspired modified Kepler equation, which explains impact flares since 1888. The 2022 impact flare, namely flare number 26, is rather peculiar as it breaks the typical pattern of two impact flares per 12-year cycle. This is the third bremsstrahlung flare of the current cycle that follows the already observed 2015 and 2019 impact flares from OJ 287. It turns out that the arrival epoch of flare number 26 is sensitive to the level of primary SMBH’s accre...
Bulletin of the American Astronomical Society, Oct 1, 2000
Her A is a powerful double-lobe radio source that shows unusual features, from a double optical c... more Her A is a powerful double-lobe radio source that shows unusual features, from a double optical core to a string of radio intensity rings in one of its two lobes. Among scores of nearby strong radio sources, Her A is neither a hotspot FR II source nor a weak plume-lobed FR I, but rather shows strong resemblances to both types. We model the development of Her A and its curious combination of features as follows. A rough billion years ago, at an epoch when little magnetized lobe plasma was in place, small clouds of denser gas from the double core reached the galaxy halo boundary, there to expand and radiate picohertz spherical acoustic wave trains. These grew as they drifted out in the wind, both the speed of sound and the bulk drift speed nearly uniform. Two small clouds near the core have been seen in optical absorption that fully resemble the radio features at the present lobe root. The simple kinematics of uniform acoustic drift shaped the old line of enlarging wave trains, now seen as remarkably circular radio features, their emitting plasma only some ten million years old. The AGN jet has only recently filled in the pre-existing minima of the moving acoustic spheres. The periodic pattern was timed by successive orbital tidal excitations of the central AGN by its lighter companion galaxy. The eastern lobe can be understood as roughly symmetrical with the western lobe only because it is viewed earlier in the process of magnetic plasma infill that built the radio-emitting lobes, evolving by dilution and decay. The eastern radio image is seen somewhat delayed by projection; the plasma we see there is still spreading. Recent X-ray observations have shown features around Her A whose explanation is not inconsistent with the aforementioned model.
AIP Conference Proceedings, 1992
ABSTRACT
Her A (3C 348) is one of a few strong radio sources which shows unusually simple intensity featur... more Her A (3C 348) is one of a few strong radio sources which shows unusually simple intensity features within its extended lobes. Such objects do not easily fit the Fanaroff-Riley scheme for jet and lobe sources. In the plane of the sky these features appear as circular rings, but they are in fact thin edge-brightened spherical shells. Her A displays a remarkably simple and uniform kinematics. We model the source by a two-stage account of its dynamics. First, acoustic waves were excited to form sphere after sphere drifting with a pre-existing thermal galactic wind. They arise at one spot along the jet axis at each edge of the galaxy, expanding uniformly at sound speed while they drift axially out at Mach 6. In a much later second stage, a faster flow of relativistic plasma forms the radio lobe and infuses it with radio electrons. These electrons decorate the pre-existing shells, now in steady motion. (Morrison and Sadun, 1996.) Although the two lobes do not look at all the same, they appear quite similar in structure after taking into account plausible orientations and the transit time delays they cause. Two small absorption rings have been found near the core with the HST (Baum, et al. 1996), apparently just what is needed to generate the initiating acoustic waves at the galaxy boundary. This source has an optically double active nucleus (Sadun and Hayes, 1993). Perhaps periodic tidal forces acting on the AGN core winds determine the modulations that gave repeated rise to the acoustic waves during the early outflow.
Journal of the American College of Cardiology, Jul 1, 1993
Journal of the American College of Cardiology, Feb 1, 1991
Galaxies, 2020
We report on seven nights of optical observation taken over a two-week period, and the resultant ... more We report on seven nights of optical observation taken over a two-week period, and the resultant analysis of the intermediate-frequency peaked BL Lac object (IBL), BL Lac itself, at redshift z = 0.069. The microvariable behavior can be confirmed over the course of minutes for each night. A relativistic beaming model was used in our analysis, to infer changes to the line of sight angles for the motion of the different relativistic components. This model has very few free parameters. The light curves we generated show both high and moderate frequency cadence to the variable behavior of BL Lac itself, in addition to the well documented long-term variability.
Multi-Wavelength Continuum Emission of AGN, 1994
The Astronomical Journal, 1991
ABSTRACT
The Astronomical Journal, 1990
The Astrophysical Journal, 2010
Monthly Notices of the Royal Astronomical Society
The bright blazar OJ 287 routinely parades high brightness bremsstrahlung flares, which are expla... more The bright blazar OJ 287 routinely parades high brightness bremsstrahlung flares, which are explained as being a result of a secondary supermassive black hole (SMBH) impacting the accretion disc of a more massive primary SMBH in a binary system. The accretion disc is not rigid but rather bends in a calculable way due to the tidal influence of the secondary. Below we refer to this phenomenon as a variable disc level. We begin by showing that these flares occur at times predicted by a simple analytical formula, based on general relativity inspired modified Kepler equation, which explains impact flares since 1888. The 2022 impact flare, namely flare number 26, is rather peculiar as it breaks the typical pattern of two impact flares per 12-year cycle. This is the third bremsstrahlung flare of the current cycle that follows the already observed 2015 and 2019 impact flares from OJ 287. It turns out that the arrival epoch of flare number 26 is sensitive to the level of primary SMBH’s accre...
Bulletin of the American Astronomical Society, Oct 1, 2000
Her A is a powerful double-lobe radio source that shows unusual features, from a double optical c... more Her A is a powerful double-lobe radio source that shows unusual features, from a double optical core to a string of radio intensity rings in one of its two lobes. Among scores of nearby strong radio sources, Her A is neither a hotspot FR II source nor a weak plume-lobed FR I, but rather shows strong resemblances to both types. We model the development of Her A and its curious combination of features as follows. A rough billion years ago, at an epoch when little magnetized lobe plasma was in place, small clouds of denser gas from the double core reached the galaxy halo boundary, there to expand and radiate picohertz spherical acoustic wave trains. These grew as they drifted out in the wind, both the speed of sound and the bulk drift speed nearly uniform. Two small clouds near the core have been seen in optical absorption that fully resemble the radio features at the present lobe root. The simple kinematics of uniform acoustic drift shaped the old line of enlarging wave trains, now seen as remarkably circular radio features, their emitting plasma only some ten million years old. The AGN jet has only recently filled in the pre-existing minima of the moving acoustic spheres. The periodic pattern was timed by successive orbital tidal excitations of the central AGN by its lighter companion galaxy. The eastern lobe can be understood as roughly symmetrical with the western lobe only because it is viewed earlier in the process of magnetic plasma infill that built the radio-emitting lobes, evolving by dilution and decay. The eastern radio image is seen somewhat delayed by projection; the plasma we see there is still spreading. Recent X-ray observations have shown features around Her A whose explanation is not inconsistent with the aforementioned model.
AIP Conference Proceedings, 1992
ABSTRACT
Her A (3C 348) is one of a few strong radio sources which shows unusually simple intensity featur... more Her A (3C 348) is one of a few strong radio sources which shows unusually simple intensity features within its extended lobes. Such objects do not easily fit the Fanaroff-Riley scheme for jet and lobe sources. In the plane of the sky these features appear as circular rings, but they are in fact thin edge-brightened spherical shells. Her A displays a remarkably simple and uniform kinematics. We model the source by a two-stage account of its dynamics. First, acoustic waves were excited to form sphere after sphere drifting with a pre-existing thermal galactic wind. They arise at one spot along the jet axis at each edge of the galaxy, expanding uniformly at sound speed while they drift axially out at Mach 6. In a much later second stage, a faster flow of relativistic plasma forms the radio lobe and infuses it with radio electrons. These electrons decorate the pre-existing shells, now in steady motion. (Morrison and Sadun, 1996.) Although the two lobes do not look at all the same, they appear quite similar in structure after taking into account plausible orientations and the transit time delays they cause. Two small absorption rings have been found near the core with the HST (Baum, et al. 1996), apparently just what is needed to generate the initiating acoustic waves at the galaxy boundary. This source has an optically double active nucleus (Sadun and Hayes, 1993). Perhaps periodic tidal forces acting on the AGN core winds determine the modulations that gave repeated rise to the acoustic waves during the early outflow.