891 research outputs found
Near-infrared synchrotron emission from the compact jet of GX339-4
We have compiled contemporaneous broadband observations of the black hole
candidate X-ray binary GX 339-4 when in the low/hard X-ray state in 1981 and
1997. The data clearly reveal the presence of two spectral components, with
thermal and non-thermal spectra, overlapping in the optical -- near-infrared
bands. The non-thermal component lies on an extrapolation of the radio spectrum
of the source, and we interpret it as optically thin synchrotron emission from
the powerful, compact jet in the system. Detection of this break from
self-absorbed to optically thin synchrotron emission from the jet allows us to
place a firm lower limit on the ratio of jet (synchrotron) to X-ray
luminosities of %. We further note that extrapolation of the optically
thin synchrotron component from the near-infrared to higher frequencies
coincides with the observed X-ray spectrum, supporting models in which the
X-rays could originate via optically thin synchrotron emission from the jet
(possibly instead of Comptonisation).Comment: Accepted for publication in ApJ Lette
On the Origin of Radio Emission in the X-ray States of XTE J1650-500 during the 2001-2002 Outburst
We report on simultaneous radio and X-ray observations of the black hole
candidate XTE J1650-500 during the course of its 2001-2002 outburst. The
scheduling of the observations allowed us to sample the properties of XTE
J1650-50 in different X-ray spectral states, namely the hard state, the steep
power-law state and the thermal dominant state, according to the recent
spectral classification of McClintock & Remillard. The hard state is consistent
with a compact jet dominating the spectral energy distribution at radio
frequencies; however, the current data suggest that its contribution as direct
synchrotron emission at higher energies may not be significant. In that case,
XTE J1650-50 may be dominated by Compton processes (either inverse
Comptonization of thermal disk photons and/or SSC from the base of the compact
jet) in the X-ray regime. We, surprisingly, detect a faint level of radio
emission in the thermal dominant state that may be consistent with the emission
of previously ejected material interacting with the interstellar medium,
similar (but on a smaller angular scale) to what was observed in XTE J1550-564
by Corbel and co-workers. Based on the properties of radio emission in the
steep power-law state of XTE J1650-50, and taking into account the behavior of
other black hole candidates (namely GX 339-4, XTE J1550-564, and XTE J1859+226)
while in the intermediate and steep power-law states, we are able to present a
general pattern of behavior for the origin of radio emission in these two
states that could be important for understanding the accretion-ejection
coupling very close to the black hole event horizon.Comment: Accepted for publication in The Astrophysical Journal. 33 pages, 10
figure
Opening angles, Lorentz factors and confinement of X-ray binary jets
We present a collation of the available data on the opening angles of jets in
X-ray binaries, which in most cases are small (less than 10 degrees). Under the
assumption of no confinement, we calculate the Lorentz factors required to
produce such small opening angles via the transverse relativistic Doppler
effect. The derived Lorentz factors, which are in most cases lower limits, are
found to be large, with a mean greater than 10, comparable to those estimated
for AGN and much higher than the commonly-assumed values for X-ray binaries of
2 to 5. Jet power constraints do not in most cases rule out such high Lorentz
factors. The upper limits on the opening angles show no evidence for smaller
Lorentz factors in the steady jets of Cygnus X-1 and GRS 1915+105. In those
sources in which deceleration has been observed (notably XTE J1550-564 and
Cygnus X-3), some confinement of the jets must be occurring, and we briefly
discuss possible confinement mechanisms. It is however possible that all the
jets could be confined, in which case the requirement for high bulk Lorentz
factors can be relaxed.Comment: 11 pages, 4 figures (2 colour), accepted for publication in MNRA
X-ray states and radio emission in the black hole candidate XTE J1550-564
We report on radio and X-ray observations of the black hole candidate (BHC)
XTE J1550-564 performed during its 2000 X-ray outburst. Observations have been
conducted with the Australia Telescope Compact Array (ATCA) and have allowed us
to sample the radio behavior of XTE J1550-564 in the X-ray Low Hard and
Intermediate/Very High states. We observed optically thin radio emission from
XTE J1550-564 five days after a transition to an Intermediate/Very High state,
but we observed no radio emission six days later, while XTE J1550-564 was still
in the Intermediate/Very High state. In the Low Hard state, XTE J1550-564 is
detected with an inverted radio spectrum. The radio emission in the Low Hard
state most likely originates from a compact jet; optical observations suggest
that the synchrotron emission from this jet may extend up to the optical range.
The total power of the compact jet might therefore be a significant fraction of
the total luminosity of the system. We suggest that the optically thin
synchrotron radio emission detected five days after the transition to the
Intermediate/Very High state is due to a discrete ejection of relativistic
plasma during the state transition. Subsequent to the decay of the optically
thin radio emission associated with the state transition, it seems that in the
Intermediate/Very High state the radio emission is quenched by a factor greater
than 50, implying a suppression of the outflow. We discuss the properties of
radio emission in the X-ray states of BHCs.Comment: 15 pages, including 3 figures. Accepted for publication in ApJ,
scheduled for the vol. 553 Jun 1, 2001 issu
Positron trapping in semiconductors
Positron trapping into vacancies in semiconductors is studied on the basis of Fermiâs golden-rule calculations. The emphasis is put on the comparison of the trapping properties into defects in different charge states. In particular, the temperature dependences are investigated. Important features for vacancy-type defects in semiconductors are the localized electron states within the forbidden energy gap and (in the case of negatively charged defects) the weakly bound Rydberg states for positrons. Compared to vacancy-type defects in metals, these features make possible new kinds of trapping mechanisms with electron-hole and phonon excitations. For charged defects the Coulomb wave character of the delocalized positron states before trapping determines the amplitude of the wave function at the defect and thereby strongly affects the magnitude of the trapping rate. As a result, trapping into positively charged defects is effectively forbidden while negatively charged defects show remarkable properties which differ from the picture established for positron trapping in metals. The trapping rate into negative defects increases strongly with decreasing temperature and at very low temperatures ââgiganticââ values may resultPeer reviewe
A decelerating jet observed by the EVN and VLBA in the X-ray transient XTE J1752-223
The recently discovered Galactic X-ray transient XTE J1752-223 entered its
first known outburst in 2010, emitting from the X-ray to the radio regimes. Its
general X-ray properties were consistent with those of a black hole candidate
in various spectral states, when ejection of jet components is expected. To
verify this, we carried out very long baseline interferometry (VLBI)
observations. The measurements were carried out with the European VLBI Network
(EVN) and the Very Long Baseline Array (VLBA) at four epochs in 2010 February.
The images at the first three epochs show a moving jet component that is
significantly decelerated by the last epoch, when a new jet component appears
that is likely to be associated with the receding jet side. The overall picture
is consistent with an initially mildly relativistic jet, interacting with the
interstellar medium or with swept-up material along the jet. The brightening of
the receding ejecta at the final epoch can be well explained by initial Doppler
deboosting of the emission in the decelerating jet.Comment: Accepted for publication in MNRAS Letters. 5 pages, 2 figure
The "universal" radio/X-ray flux correlation : the case study of the black hole GX 339-4
The existing radio and X-ray flux correlation for Galactic black holes in the
hard and quiescent states relies on a sample which is mostly dominated by two
sources (GX 339-4 and V404 Cyg) observed in a single outburst. In this paper,
we report on a series of radio and X-ray observations of the recurrent black
hole GX 339-4 with the Australia Telescope Compact Array, the Rossi X-ray
Timing Explorer and the Swift satellites. With our new long term campaign, we
now have a total of 88 quasi-simultaneous radio and X-ray observations of GX
339-4 during its hard state, covering a total of seven outbursts over a
15--year period. Our new measurements represent the largest sample for a
stellar mass black hole, without any bias from distance uncertainties, over the
largest flux variations and down to a level that could be close to quiescence,
making GX 339-4 the reference source for comparison with other accreting
sources (black holes, neutrons stars, white dwarfs and active galactic nuclei).
Our results demonstrate a very strong and stable coupling between radio and
X-ray emission, despite several outbursts of different nature and separated by
a period of quiescence. The radio and X-ray luminosity correlation of the form
L_X ~L_Rad^0.62 +/-0.01 confirms the non-linear coupling between the jet and
the inner accretion flow powers and better defines the standard correlation
track in the radio-X-ray diagram for stellar mass black holes. We further note
epochs of deviations from the fit that significantly exceed the measurement
uncertainties, especially during the formation and destruction of the compact
jets ...[abridged]. We incorporated our new data in a more global study of
black hole candidates strongly supporting a scale invariance in the
jet-accretion coupling of accreting black holes, and confirms the existence of
two populations of sources in the radio/X-ray diagram.Comment: Paper accepted in MNRAS. 18 pages, 9 figure
The Search for Signatures Of Transient Mass Loss in Active Stars
The habitability of an exoplanet depends on many factors. One such factor is
the impact of stellar eruptive events on nearby exoplanets. Currently this is
poorly constrained due to heavy reliance on solar scaling relationships and a
lack of experimental evidence. Potential impacts of Coronal Mass Ejections
(CMEs), which are a large eruption of magnetic field and plasma from a star,
are space weather and atmospheric stripping. A method for observing CMEs as
they travel though the stellar atmosphere is the type II radio burst, and the
new LOw Frequency ARray (LOFAR) provides a means for detection. We report on 15
hours of observation of YZ Canis Minoris (YZ CMi), a nearby M dwarf flare star,
taken in LOFAR's beam-formed observation mode for the purposes of measuring
transient frequency-dependent low frequency radio emission. The observations
utilized Low-Band Antenna (10-90 MHz) or High-Band Antenna (110-190 MHz) for
five three-hour observation periods. In this data set, there were no confirmed
type II events in this frequency range. We explore the range of parameter space
for type II bursts constrained by our observations Assuming the rate of shocks
is a lower limit to the rate at which CMEs occur, no detections in a total of
15 hours of observation places a limit of shocks/hr for YZ CMi due to the stochastic nature of the events and
limits of observational sensitivity. We propose a methodology to interpret
jointly observed flares and CMEs which will provide greater constraints to CMEs
and test the applicability of solar scaling relations
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