3,077 research outputs found
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
Flaring X-ray Emission from HST-1, a Knot in the M87 Jet
We present Chandra X-ray monitoring of the M87 jet in 2002, which shows that
the intensity of HST-1, an optical knot 0.8" from the core, increased by a
factor of two in 116 days and a factor of four in 2 yrs. There was also a
significant flux decrease over two months, with suggestive evidence for a
softening of the spectrum. From this variability behavior, we argue that the
bulk of the X-ray emission of HST-1 comes from synchrotron emission. None of
the other conceivable emission processes can match the range of observed
characteristics. By estimating synchrotron model parameters for various bulk
relativistic velocities, we demonstrate that a model with a Doppler factor,
delta, in the range 2 to 5 fits our preliminary estimates of light travel time
and synchrotron loss timescales.Comment: 4 pages with 3 embedded figures; 1 of which is color but prints ok in
b/w. Accepted for publication in the ApJ
Murine cytomegalovirus genomic material in marrow cells; relation to altered leukocyte counts during sublethal infection of mice
The Event Horizon of M87
The 6 billion solar mass supermassive black hole at the center of the giant
elliptical galaxy M87 powers a relativistic jet. Observations at millimeter
wavelengths with the Event Horizon Telescope have localized the emission from
the base of this jet to angular scales comparable to the putative black hole
horizon. The jet might be powered directly by an accretion disk or by
electromagnetic extraction of the rotational energy of the black hole. However,
even the latter mechanism requires a confining thick accretion disk to maintain
the required magnetic flux near the black hole. Therefore, regardless of the
jet mechanism, the observed jet power in M87 implies a certain minimum mass
accretion rate. If the central compact object in M87 were not a black hole but
had a surface, this accretion would result in considerable thermal
near-infrared and optical emission from the surface. Current flux limits on the
nucleus of M87 strongly constrain any such surface emission. This rules out the
presence of a surface and thereby provides indirect evidence for an event
horizon.Comment: 9 pages, 2 figures, submitted to Ap
Large-Scale Regular Morphological Patterns in the Radio Jet of NGC 6251
We report on large-scale, regular morphological patterns found in the radio
jet of the nearby radio galaxy NGC 6251. Investigating morphological properties
of this radio jet from the nucleus to a radial distance of 300 arcsec
( 140 kpc) mapped at 1662 MHz and 4885 MHz by Perley, Bridle, &
Willis, we find three chains, each of which consists of five radio knots. We
also find that eight radio knots in the first two chains consist of three small
sub-knots (the triple-knotty substructures). We discuss the observational
properties of these regular morphological patterns.Comment: 8 figures, 15 pages, accepted for publication in A
Ultraviolet HST Observations of the Jet in M87
We present new ultraviolet photometry of the jet in M87 obtained from HST
WFPC2 imaging. We combine these ultraviolet data with previously published
photometry for the knots of the jet in radio, optical, and X-ray, and fit three
theoretical synchrotron models to the full data set. The synchrotron models
consistently overpredict the flux in the ultraviolet when fit over the entire
dataset. We show that if the fit is restricted to the radio through ultraviolet
data, the synchrotron models can provide a good match to the data. The break
frequencies of these fits are much lower than previous estimates. The implied
synchrotron lifetimes for the bulk of the emitting population are longer than
earlier work, but still much shorter than the estimated kinematic lifetimes of
the knots. The observed X-ray flux cannot be successfully explained by the
simple synchrotron models that fit the ultraviolet and optical fluxes. We
discuss the possible implications of these results for the physical properties
of the M87 jet. We also observe increased flux for the HST-1 knot that is
consistent with previous results for flaring. This observation fills in a
significant gap in the time coverage early in the history of the flare, and
therefore sets constraints on the initial brightening of the flare.Comment: 14 pages, 2 figures, Accepted for publication in ApJ, changed
lightcurve and caption in Figure
X-ray Emission Properties of Large Scale Jets, Hotspots and Lobes in Active Galactic Nuclei
We examine a systematic comparison of jet-knots, hotspots and radio lobes
recently observed with Chandra and ASCA. This report will discuss the origin of
their X-ray emissions and investigate the dynamics of the jets. The data was
compiled at well sampled radio (5GHz) and X-ray frequencies (1keV) for more
than 40 radio galaxies. We examined three models for the X-ray production:
synchrotron (SYN), synchrotron self-Compton (SSC) and external Compton on CMB
photons (EC). For the SYN sources -- mostly jet-knots in nearby low-luminosity
radio galaxies -- X-ray photons are produced by ultrarelativistic electrons
with energies 10-100 TeV that must be accelerated in situ. For the other
objects, conservatively classified as SSC or EC sources, a simple formulation
of calculating the ``expected'' X-ray fluxes under an equipartition hypothesis
is presented. We confirmed that the observed X-ray fluxes are close to the
expected ones for non-relativistic emitting plasma velocities in the case of
radio lobes and majority of hotspots, whereas considerable fraction of
jet-knots is too bright at X-rays to be explained in this way. We examined two
possibilities to account for the discrepancy in a framework of the
inverse-Compton model: (1) magnetic field is much smaller than the
equipartition value, and (2) the jets are highly relativistic on kpc/Mpc
scales. We concluded, that if the inverse-Compton model is the case, the X-ray
bright jet-knots are most likely far from the minimum-power condition. We also
briefly discuss the other possibility, namely that the observed X-ray emission
from all of the jet-knots is synchrotron in origin.Comment: 20 pages, 10 figures, accepted for publication in the Astrophysical
Journal, vol.62
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