113 research outputs found
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 AGN 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.Comment: 9 pages latex plus 6 ps figs. To appear in Monthly Notices of the RA
Morphology of the Nuclear Disk in M87
A deep, fuly sampled diffraction limited (FWHM ~ 70 mas) narrow-band image of
the central region in M87 was obtained with the Wide Filed and Planetary Camera
2 of the Hubble Space Telescope using the dithering technique. The
H-alpha+[NII] continuum subtracted image reveals a wealth of details in the
gaseous disk structure described earlier by Ford et al. (1994). The disk
morphology is dominated by a well defined three-arm spiral pattern. In
addition, the major spiral arms contain a large number of small "arclets"
covering a range of sizes (0.1-0.3 arcsec = 10-30 pc). The overall surface
brightness profile inside a radius ~1.5" (100 pc) is well represented by a
power-law I(mu) ~ mu^(-1.75), but when the central ~40 pc are excluded it can
be equally well fit by an exponential disk. The major axis position angle
remains constant at about PA_disk ~ 6 deg for the innermost ~1", implying the
disk is oriented nearly perpendicular to the synchrotron jet (PA_jet ~ 291
deg). At larger radial distances the isophotes twist, reflecting the gas
distribution in the filaments connecting to the disk outskirts. The ellipticity
within the same radial range is e = 0.2-0.4, which implies an inclination angle
of i~35 deg. The sense of rotation combined with the dust obscuration pattern
indicate that the spiral arms are trailing.Comment: 5 pages, 3 postscript figures, to appear in the Proceedings of the
M87 Workshop, Ringberg castle, Germany, 15-19 Sep 1997, also available from
http://jhufos.pha.jhu.edu/~zlatan/papers.htm
Proper Motions of Ionized Gas at the Galactic Center: Evidence for Unbound Orbiting Gas
We present radio continuum observations of the spiral-shaped ionized feature
(Sgr A West) within the inner pc of the Galactic center at three epochs
spanning 1986 to 1995. The VLA A-configuration was used at 2cm
(resolution of 0\dasec10\dasec2). We detect proper motions of a number
of features in the Northern and Eastern Arms of Sgr A West including the
ionized gas associated with IRS 13 with V(RA)= 113 \pm 10, V(Dec)=150 \pm15
km/s, IRS 2 with V(RA)= 122 \pm 11, V(Dec)=24 \pm 34 km/s and the Norther Arm
V(RA)= 126 \pm 30, V(Dec)=--207 \pm 58 km/s. We also report the detection of
features having transverse velocities > 1000 km/s including a head-tail radio
structure, the ``Bullet'', northwest of Sgr A with V(RA)= 722
\pm 156, V(Dec)=832 \pm 203 km/s, exceeding the escape velocity at the Galactic
center.
The proper motion measurements when combined with previous H92 radio
recombination line data suggest an unambiguous direction of the flow of ionized
gas orbiting the Galactic center. The measured velocity distribution suggests
that the ionized gas in the Northern Arm is not bound to the Galactic center
assuming a 2.5 million solar mass of dark matter residing at the Galactic
center. This implies that the stellar and ionized gas systems are not
dynamically coupled, thus, supporting a picture in which the gas features in
the Northern Arm and its extensions are the result of an energetic phenomenon
that has externally driven a cloud of gas cloud into the Galactic center.Comment: 11 pages, three figures (one color) and one table. Astrophysical
Journal Letters in pres
Month-Timescale Optical Variability in the M87 Jet
A previously inconspicuous knot in the M87 jet has undergone a dramatic
outburst and now exceeds the nucleus in optical and X-ray luminosity.
Monitoring of M87 with the Hubble Space Telescope and Chandra X-ray Observatory
during 2002-2003, has found month-timescale optical variability in both the
nucleus and HST-1, a knot in the jet 0.82'' from the nucleus. We discuss the
behavior of the variability timescales as well as spectral energy distribution
of both components. In the nucleus, we see nearly energy-independent
variability behavior. Knot HST-1, however, displays weak energy dependence in
both X-ray and optical bands, but with nearly comparable rise/decay timescales
at 220 nm and 0.5 keV. The flaring region of HST-1 appears stationary over
eight months of monitoring. We consider various emission models to explain the
variability of both components. The flares we see are similar to those seen in
blazars, albeit on longer timescales, and so could, if viewed at smaller
angles, explain the extreme variability properties of those objects.Comment: 4 pages, 3 figures, ApJ Lett., in pres
HST Observations of the Optical Jets of PKS 0521-365, 3C371, and PKS 2201+044
HST observations have led to the discovery of the optical counterpart of the
radio jet of PKS 2201+044, and to a detailed analysis of the optical jets of
PKS 0521-365 and 3C371. At HST spatial resolution these jets are well resolved,
displaying knotty morphologies. When compared with radio maps of appropriate
resolution, a clear one-to-one correspondence between optical and radio
structures is found, showing that all detected optical structures are indeed
related to the radio synchrotron emission. Photometry of the brightest knots
shows that the radio-to-optical spectral index and the derived intensity of the
equipartition magnetic field are approximately constant along the jet. Thus,
present observations suggest that the electron energy distribution does not
change significantly all along the jet.Comment: Accepted for publications on the Astrphysical Journal. Contains 14
pages and 5 figure
- âŠ