Engineering topics

Biretta, J. A.

Publications and source records attributed to Biretta, J. A..

7 records

NASA NTRS2000

X-Ray Variability in M87

We present the evidence for X-ray variability from the core and from knot A in the M87 jet based on data from two observations with the Einstein Observatory High Resolution Imager (HRI) and three observations with the ROSAT HRI. The core intensity showed a 16% increase in 17 months ('79-'80); a 12% increase in the 3 years '92 to '95; and a 17% drop in the last half of 1995. The intensity of knot A appears to have decreased by 16% between 92Jun and 95Dec. Although the core variability is consistent with general expectations for AGB nuclei, the changes in knot A provide constraints on the x-ray emission process and geometry. Thus we predict that the x-ray morphology of knot A will differ significantly from the radio and optical structure.

Harris, Daniel E.↗

NASA NTRS2000

A Comparison of the Extra Nuclear X-ray and Radio Features in M87

ROSAT High Resolution Imager (HRI) data from eight observations have been co-added to obtain an effective exposure of 230 ksec. We have identified a number of features and regions with excess X-ray brightness over that from a circularly symmetric model of the well known hot gas component. A prominent 'spur' extends 4feet from the vicinity of knot A towards the south-west. The brightness to the south and east of this spur is significantly higher than that to the north and west. Excess brightness is also found to the East of the nucleus, with a local maximum centered on the eastern radio lobe 3 feet from the core. There are two well known relationships between radio and X-ray emission for radio galaxies in clusters: coincidence of emissions because the X-rays come from inverse Compton or synchrotron processes; and anti-coincidence caused by exclusion of hot gas from radio entities. We present a radio/X-ray comparison to determine if either of these relationships can be isolated in M87. The greatest obstacle we face is the unknown projection which affects both bands.

Harris, D. E.↗

NASA NTRS1998

The Large Scale X-Ray Emission from M87

We describe asymmetrical features in a long exposure X-ray map of M87 made with the ROSAT High Resolution Imager (HRI). A bright triangular region is marked by a linear 'spur' along one edge. The structure of this spur suggests an interpretation of a tangential view of a shock front 18 kpc long. None of the brighter features are spatially coincident with radio or optical structures so we concur with earlier investigators that most of the emission arises from thermal processes.

Harris, D. E.↗

NASA NTRS1998

X-Ray Variability in M87: 1992-1998

Beginning in 1995 June, we have obtained an observation of M 87 with the ROSAT High Resolution Imager (HRI) every 6 months. We present the measurements of X-ray intensity for the core and knot A through 1998 January. We find significant changes in both components. For the core, intensities measured in June 1995, December 1996, and December 1997 are roughly 30% higher than values obtained at three intervening times. For knot A, a secular decrease of approximately 15% is interrupted only by an intensity jump (3 sigma) in December 1997. Because the background used for subtraction is probably underestimated, we suspect the actual variation is somewhat greater than these values indicate.

Harris, D. E.↗

NASA NTRS1997

X-ray Variability in M87

We present evidence for X-ray variability from the core and from knot A in the M87 jet based on data from two observations with the Einstein Observatory High Resolution Imager (HRI) and three observations with the ROSAT HRI. The core intensity showed a 16 per cent increase in 17 months (1979-80); a 12 per cent increase in the 3 years 1992 to 1995; and a 17 per cent drop in the last half of 1995. The intensity of knot A appears to have decreased by 16 per cent between 1992 June and 1995 December. Although the core variability is consistent with general expectations for AGN, the changes in knot A provide constraints on the X-ray emission process and geometry. Thus we predict that the X-ray morphology of knot A will differ significantly from the radio and optical structure.

Harris, D. E.↗

NASA NTRS1991

The radio to X-ray spectrum of the M87 jet and nucleus

Einstein Observatory high-resolution imager data are used in conjunction with recent radio and optical observations to examine the morphology of the M87 jet; IR and UV data from the literature are then employed to construct X-ray spectra for the jet and nucleus, allowing the isolation of new features in the X-ray images which are associable with the radio/optical knots B and D. The synchrotron mechanism is suggested to be the originator of the jet X-ray emission, despite its entailment of very high energy electrons with short lifetimes. Optical data for the nucleus are consistent with the presence of an unresolved blue continuum source, implying a spectrum similar to that of the jet except at high RFs.

Biretta, J. A.↗

NASA NTRS1985

VLBI monitoring of the superluminal quasar 3C 273, 1977-1982

The compact ratio structure of the quasar 3C 273 has been monitored with a VLBI array at 5.0 and 10.7 GHz at six-month intervals during 1977-1982. THe VLBI monitoring observations, data reduction, calibration, and mapping techniques are described, and hybrid maps from the monitoring program are presented along with a brief discussion of the main features. Results on the source kinematics (superluminal motion of the knots and their time evolution) are given, and synchro-Compton models of the knots are presented and limits to their Doppler factors (delta) derived from X-ray measurements. The constraints on jet geometry provided by the combined measurements of delta and v/c are considered. For the core, the inhomogeneous jet model of Koenigl (1981) is compared with the available data. The relation between 3C 273 and the similar superluminal source 3C 345 is discussed.

Unwin, S. C.↗