1,319 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
Proper Motion of the Irradiated Jet HH 399 in the Trifid Nebula
HH 399 is one of the first Herbig Haro flows recognized to be irradiated by
the UV radiation of the massive O7.5 star in the Trifid nebula. We present the
proper motion of the first irradiated jet based on two epochs of HST
observations of HH 399 separated nearly by five years using H and [SII]
line filters. High proper motion with continuous velocities between 20055
and 528 \kms are detected in both lines along the 18 extent of the
jet axis. The irradiated fully-ionized jet consists of numerous knots along the
jet but also shows the evidence for a number of isolated blob-like structures
running immediately outside the jet with lower transverse velocities. The
transverse velocities combined with radial velocity measurements indicate that
the jet axis lies away from the plane of the sky by only few degrees. We argue
that the jet is fully ionized based on [SII]/H line ratio as well as
radio continuum emission detected from the full extent of the jet at 3.6cm
wavelength. The stellar mass-loss rate producing HH 399 is estimated to be
\approx 2\times10^{-6} \msol yr.Comment: 14 pages, 6 figures, ApJ (in press
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
Probing the origin of VHE emission from M 87 with MWL observations in 2010
The large majority of extragalactic very high energy (VHE; E>100 GeV) sources
belongs to the class of active galactic nuclei (AGN), in particular the BL Lac
sub-class. AGNs are characterized by an extremely bright and compact emission
region, powered by a super-massive black hole (SMBH) and an accretion disk, and
relativistic outflows (jets) detected all across the electro-magnetic spectrum.
In BL Lac sources the jet axis is oriented close to the line of sight, giving
rise to a relativistic boosting of the emission. In radio galaxies, on the
other hand, the jet makes a larger angle to the line of sight allowing to
resolve the central core and the jet in great details. The giant radio galaxy M
87 with its proximity (1 6Mpc) and its very massive black hole ((3-6) x 10^9
M_solar) provides a unique laboratory to investigate VHE emission in such
objects and thereby probe particle acceleration to relativistic energies near
SMBH and in jets. M 87 has been established as a VHE emitter since 2005. The
VHE emission displays strong variability on time-scales as short as a day. It
has been subject of a large joint VHE and multi-wavelength (MWL) monitoring
campaign in 2008, where a rise in the 43 GHz VLBA radio emission of the
innermost region (core) was found to coincide with a flaring activity at VHE.
This had been interpreted as a strong indication that the VHE emission is
produced in the direct vicinity of the SMBH black hole. In 2010 again a flare
at VHE was detected triggering further MWL observations with the VLBA, Chandra,
and other instruments. At the same time M 87 was also observed with the
Fermi-LAT telescope at GeV energies and the European VLBI Network (EVN). In
this contribution preliminary results from the campaign will be presented.Comment: 5 pages, 2 figures, in the proceedings of the "International Workshop
on Beamed and Unbeamed Gamma-Rays from Galaxies" 11-15 April 2011, Lapland
Hotel Olos, Muonio, Finland, Journal of Physics: Conference Series Volume
355, 201
A Magnetohydrodynamic Model of the M87 Jet I: Superluminal Knot Ejections from HST-1 as Trails of Quad Relativistic MHD Shocks
This is the first in a series of papers that introduces a new paradigm for
understanding the jet in M87: a collimated relativistic flow in which strong
magnetic fields play a dominant dynamical role. Here wefocus on the flow
downstream of HST-1 - an essentially stationary flaring feature that ejects
trails of superluminal components. We propose that these components are quad
relativistic magnetohydrodynamic shock fronts (forward/reverse fast and slow
modes) in a narrow jet with a helically twisted magnetic structure. And we
demonstrate the properties of such shocks with simple one-dimensional numerical
simulations. Quasi-periodic ejections of similar component trails may be
responsible for the M87 jet substructures observed further downstream on 100 -
1,000 pc scales. This new paradigm requires the assimilation of some new
concepts into the astrophysical jet community, particularly the behavior of
slow/fast-mode waves/shocks and of current-driven helical kink instabilities.
However, the prospects of these ideas applying to a large number of other jet
systems may make this worth the effort.Comment: 7 pages, 4 figures, Accepted for Publication in Ap
Optical and Radio Polarimetry of the M87 Jet at 0.2" Resolution
We discuss optical (HST/WFPC2 F555W) and radio (15 GHz VLA) polarimetry
observations of the M87 jet taken during 1994-1995. Many knot regions are very
highly polarized (, approaching the theoretical maximum for
optically thin synchrotron radiation), suggesting highly ordered magnetic
fields. High degrees of polarization are also observed in interknot regions.
While the optical and radio polarization maps share many similarities, we
observe significant differences between the radio and optical polarized
structures, particularly for bright knots in the inner jet, giving us important
insight into the jet's radial structure. Unlike in the radio, the optical
magnetic field position angle becomes perpendicular to the jet at the upstream
ends of knots HST-1, D, E and F. Moreover, the optical polarization decreases
markedly at the position of the flux maxima in these knots. In contrast, the
magnetic field position angle observed in the radio remains parallel to the jet
in most of these regions, and the decreases in radio polarization are smaller.
More minor differences are seen in other jet regions. Many of the differences
between optical and radio polarimetry results can be explained in terms of a
model whereby shocks occur in the jet interior, where higher-energy electrons
are concentrated and dominate both polarized and unpolarized emissions in the
optical, while the radio maps show strong contributions from lower-energy
electrons in regions with {\bf B} parallel, near the jet surface.Comment: 28 pages, 7 figures; accepted for publication in AJ (May 1999
Milli-arcsecond--scale Spectral Properties and Jet Motions in M87
We have combined high resolution VLBI Space Observatory Programme (VSOP) data
at 1.6 and 4.8 GHz with Very Long Baseline Array (VLBA) data at higher
frequencies and with similar resolutions to study the spectral properties of
the core of M87 with milliarcsecond resolution. The VSOP data allow a more
accurate measurement of the turn-over frequency, and hence more reliable
determination of associated physical parameters of the source. Comparison of
the images with previously published images yields no evidence for significant
motion of components in the parsec-scale jet. In addition, the brightness
temperatures obtained from model-fits to the core are well below the inverse
Compton limit, suggesting the radio emission we are observing is not strongly
Doppler boosted.
Colour version on http://www.vsop.isas.jaxa.jp/survey/publications/m87.ps.gzComment: To appear in PASJ VSOP special issue. Minor correction
A Modified Synchrotron Model for Knots in the M87 Jet
For explaining the broadband spectral shape of knots in the M87 jet from
radio through optical to X-ray, we propose a modified synchrotron model that
considers the integrated effect of particle injection from different
acceleration sources in the thin acceleration region. This results in two break
frequencies at two sides of which the spectral index of knots in the M87 jet
changes. We discuss the possible implications of these results for the physical
properties in the M87 jet. The observed flux of the knots in the M87 jet from
radio to X-ray can be satisfactorily explained by the model, and the predicted
spectra from ultraviolet to X-ray could be further tested by future
observations. The model implies that the knots D, E, F, A, B, and C1 are
unlikely to be the candidate for the TeV emission recently detected in M87.Comment: 12 pages, 1 figure, 2 tables, Accepted for publication in ApJ Letter
- …