1,085 research outputs found
Models for gamma-ray production in low-mass microquasars
Unlike high-mass gamma-ray binaries, low-mass microquasars lack external
sources of radiation and matter that could produce high-energy emission through
interactions with relativistic particles. In this work we consider the
synchrotron emission of protons and leptons that populate the jet of a low-mass
microquasar. In our model photohadronic and inverse Compton (IC) interactions
with synchrotron photons produced by both protons and leptons result in a
high-energy tail of the spectrum. We also estimate the contribution from
secondary pairs injected through photopair production. The high-energy emission
is dominated by radiation of hadronic origin, so we can call these objects
proton microquasars.Comment: 4 pages, 2 figures, accepted for publication in the International
Journal of Modern Physics D, proceedings of HEPRO meeting, held in Dublin, in
September 200
Correlated radio--X-ray variability of Galactic Black Holes: A radio--X-ray flare in Cygnus X-1
We report on the first detection of a quasi-simultaneous radio-X-ray flare of
Cygnus X-1. The detection was made on 2005 April 16 with pointed observations
by the Rossi X-ray Timing Explorer and the Ryle telescope, during a phase where
the black hole candidate was close to a transition from the its soft into its
hard state. The radio flare lagged the X-rays by approximately 7 minutes,
peaking at 3:20 hours barycentric time (TDB 2453476.63864). We discuss this lag
in the context of models explaining such flaring events as the ejection of
electron bubbles emitting synchrotron radiation.Comment: 4 pages, 4 figure
Steady jets from radiatively efficient hard states in GRS 1915+105
Recent studies of different X-ray binaries (XRBs) have shown a clear
correlation between the radio and X-ray emission. We present evidence of a
close relationship found between the radio and X-ray emission at different
epochs for GRS1915+105, using observations from the Ryle Telescope and Rossi
X-ray Timing Explorer satellite. The strongest correlation was found during the
hard state (also known as the `plateau' state), where a steady AU-scale jet is
known to exist. Both the radio and X-ray emission were found to decay from the
start of most plateau states, with the radio emission decaying faster. An
empirical relationship of was then
fitted to data taken only during the plateau state, resulting in a power-law
index of , which is significantly higher than in other black
hole XRBs in a similar state. An advection-flow model was then fitted to this
relationship and compared to the universal XRB relationship as described by
Gallo et al. (2003). We conclude that either (I) the accretion disk in this
source is radiatively efficient, even during the continuous outflow of a
compact jet, which could also suggest a universal turn-over from radiatively
inefficient to efficient for all stellar-mass black holes at a critical mass
accretion rate ( g/s); or (II) the X-rays in
the plateau state are dominated by emission from the base of the jet and not
the accretion disk (e.g. via inverse Compton scattering from the outflow).Comment: 9 pages, 7 figures, accepted in A&
Millimeter Light Curves of Sagittarius A* Observed during the 2017 Event Horizon Telescope Campaign
Radio and X-ray observations during the outburst decay of the Black Hole Candidate XTE J1908+094
Obtaining simultaneous radio and X-ray data during the outburst decay of soft
X-ray transients is a potentially important tool to study the disc - jet
connection. Here we report results of the analysis of (nearly) simultaneous
radio (VLA or WSRT) and Chandra X-ray observations of XTE J1908+094 during the
last part of the decay of the source after an outburst. The limit on the index
of a radio - X-ray correlation we find is consistent with the value of 0.7
which was found for other black hole candidates in the low/hard state.
Interestingly, the limit we find seems more consistent with a value of 1.4
which was recently shown to be typical for radiatively efficient accretion flow
models. We further show that when the correlation-index is the same for two
sources one can use the differences in normalisation in the radio - X-ray flux
correlation to estimate the distance towards the sources if the distance of one
of them is accurately known (assuming black hole spin and mass and jet Lorentz
factor differences are unimportant or minimal). Finally, we observed a strong
increase in the rate of decay of the X-ray flux. Between March 23, 2003 and
April 19, 2003 the X-ray flux decayed with a factor ~5 whereas between April
19, 2003 and May 13, 2003, the X-ray flux decreased by a factor ~750. The
source (0.5-10 keV) luminosity at the last Chandra observation was L~3x10^32
(d/8.5 kpc)^2 erg s^-1.Comment: 7 pages, 3 figures, accepted for publication by MNRA
Jet-lag in Sgr A*: What size and timing measurements tell us about the central black hole in the Milky Way
The black hole at the Galactic Center, Sgr A*, is the prototype of a galactic
nucleus at a very low level of activity. Its radio through submm-wave emission
is known to come from a region close to the event horizon, however, the source
of the emission is still under debate. A successful theory explaining the
emission is based on a relativistic jet model scaled down from powerful
quasars. We want to test the predictive power of this established jet model
against newly available measurements of wavelength-dependent time lags and the
size-wavelength structure in Sgr A*. Using all available closure amplitude VLBI
data from different groups, we again derived the intrinsic wavelength-dependent
size of Sgr A*. This allowed us to calculate the expected frequency-dependent
time lags of radio flares, assuming a range of in- and outflow velocities.
Moreover, we calculated the time lags expected in the previously published
pressure-driven jet model. The predicted lags are then compared to radio
monitoring observations at 22, 43, and 350 GHz. The combination of time lags
and size measurements imply a mildly relativistic outflow with bulk outflow
speeds of gamma*beta ~ 0.5-2. The newly measured time lags are reproduced well
by the jet model without any major fine tuning. The results further strengthen
the case for the cm-to-mm wave radio emission in Sgr A* as coming from a mildly
relativistic jet-like outflow. The combination of radio time lag and VLBI
closure amplitude measurements is a powerful new tool for assessing the flow
speed and direction in Sgr A*. Future VLBI and time lag measurements over a
range of wavelengths will reveal more information about Sgr A*, such as the
existence of a jet nozzle, and measure the detailed velocity structure of a
relativistic jet near its launching point for the first time.Comment: Latex, 7 pages, accepted for publication in Astronomy & Astrophysic
Wide-field LOFAR imaging of the field around the double-double radio galaxy B1834+620:A fresh view on a restarted AGN and doubeltjes
Gravitational Test beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole
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