1,225 research outputs found
Chandra imaging spectroscopy of 1E 1740.7 - 2942
We have observed the black hole candidate 1E 1740.7 - 2942, the brightest
persistent hard X-ray source within a few degrees of the Galactic centre, for
10 ksec with Chandra (ACIS-I) on August 2000. Attempting to compensate for
pile-up effects we found the spectra were well-fit by an absorbed power law,
with photon indices Gamma = 1.54 ^{+0.42}_{-0.37} (readout streak) and Gamma =
1.42^{+0.14}_{-0.14} (annulus), consistent with a black hole low/hard state. We
have analysed a public observation performed by Chandra which utilised short
frames in order to avoid severe pile-up effects: subtracting the core point
spread function from the whole image, we did not find evidence for any
elongated feature perpendicular to the radio jet axis, as reported in a recent
analysis of the same data. Moreover, comparing the radial profiles with those
of an unscattered X-ray point source, we found indication of an extended,
previously undetected, X-ray scattering halo. The measured halo fractional
intensity at 3 keV is between 30 and 40 percent within 40 arcsec but drops
below detectable levels at 5 keV. Finally, by placing a limit on the X-ray flux
from the radio emitting lobe which has been identified as the hot spot at the
end of the northern jet of 1E 1740.7 - 2942, we are able to constrain the
magnetic energy density in that region.Comment: Accepted for publication in MNRA
On the correlation between radio and X-ray flux in Low/Hard state Black Holes
Radio emission from X-ray binary systems (XRBs) has developed in recent years
from being peculiar phenomenon to being recognised as an ubiquitous property of
several classes of XRBs. In this scenario the synchrotron emission is
interpreted as the radiative signature of jet-like outflows, some or all of
which may possess relativistic bulk motion. We have analysed a collection of
quasi-simultaneous radio/X-ray observations of Black Holes in the Low/Hard
X-ray state, finding evidence of a clear correlation between their fluxes over
many orders of magnitude in luminosity. Given that the correlation extends down
to GX 339-4 and V404 Cyg in quiescence, we can confidently assert that even at
accretion rates as low as ~ 10^{-5} dot{m}_{Edd} a powerful jet is being
formed. The normalisation of the correlation is very similar across a sample of
nine sources, implying that it is nearly independent of jet inclination angle.
Remarkably, V 404 Cyg is the second source (after GX 339-4) to show the
correlation S_{radio} proportional to S_{X}^{+0.7} from quiescent level up to
close to the High/Soft state transition. Moreover, assuming the same physics
and accretion:outflow coupling for all of these systems, the simplest
interpretation for the observed scenario is that outflows in Low/Hard state do
not have large bulk Lorentz factors.Comment: 4 pages, 3 figures, Proceedings of the 4th Microquasar Workshop, eds.
Ph Durouchoux, Y. Fuchs and J. Rodriguez, published by the Center for Space
Physics: Kolkat
An attempt to identify the extended synchrotron structure associated with the micro-quasar GRS 1915+105
The energy ejected from the galaxy micro-quasar GRS1915+105 in the form of
jets is expected to lead to formation of an extended double lobe/hot-spot
structure with the energy content comparable to an average supernova remnant.
We used the Effelsberg 100m telescope at 10.45 GHz in attempt to identify such
structures. For this distant galactic plane source any definite identification
was not possible due to high confusion by numerous background sources, however,
a few suspect structures were pointed out.Comment: LaTeX uses a new 2001 A&A macro, 4 pages, 2 figures, A&A accepte
Radio-loudness in black hole transients: evidence for an inclination effect
Accreting stellar-mass black holes appear to populate two branches in a
radio:X-ray luminosity plane. We have investigated the X-ray variability
properties of a large number of black hole low-mass X-ray binaries, with the
aim of unveiling the physical reasons underlying the radio-loud/radio-quiet
nature of these sources, in the context of the known accretion-ejection
connection. A reconsideration of the available radio and X-ray data from a
sample of black hole X-ray binaries confirms that being radio-quiet is the more
normal mode of behaviour for black hole binaries. In the light of this we chose
to test, once more, the hypothesis that radio loudness could be a consequence
of the inclination of the X-ray binary. We compared the slope of the
`hard-line' (an approximately linear correlation between X-ray count rate and
rms variability, visible in the hard states of active black holes), the orbital
inclination, and the radio-nature of the sources of our sample. We found that
high-inclination objects show steeper hard-lines than low-inclination objects,
and tend to display a radio-quiet nature (with the only exception of V404 Cyg),
as opposed to low-inclination objects, which appear to be radio-loud(er). While
in need of further confirmation, our results suggest that - contrary to what
has been believed for years - the radio-loud/quiet nature of black-hole low
mass X-ray binaries might be an inclination effect, rather than an intrinsic
source property. This would solve an important issue in the context of the
inflow-outflow connection, thus providing significant constraints to the models
for the launch of hard-state compact jets.Comment: 16 pages, 6 figures, accepted for pubblication on MNRA
Multiple relativistic outbursts of GRS 1915+105: radio emission and internal shocks
We present 5-GHz MERLIN radio images of the microquasar GRS 1915+105 during
two separate outbursts in 2001 March and 2001 July, following the evolution of
the jet components as they move outwards from the core of the system. Proper
motions constrain the intrinsic jet speed to be >0.57c, but the uncertainty in
the source distance prevents an accurate determination of the jet speed. No
deceleration is observed in the jet components out to an angular separation of
about 300mas. Linear polarisation is observed in the approaching jet component,
with a gradual rotation in position angle and a decreasing fractional
polarisation with time. Our data lend support to the internal shock model
whereby the jet velocity increases leading to internal shocks in the
pre-existing outflow before the jet switches off. The compact nuclear jet is
seen to re-establish itself within two days, and is visible as core emission at
all epochs. The energetics of the source are calculated for the possible range
of distances; a minimum power of 1-10 per cent of the Eddington luminosity is
required to launch the jet.Comment: 18 pages, 14 figures, accepted for publication in MNRAS. For
higher-resolution versions of Figures 3, 5, and 12, see
http://remote.science.uva.nl/~jmiller/grs1915/figures.htm
On the peak radio and X-ray emission from neutron star and black hole candidate X-ray transients
We have compiled and analysed reports from the literature of
(quasi-)simultaneous observations of X-ray transients at radio and X-ray
wavelengths and compared them with each other and with more unusual
radio-bright sources such as Cygnus X-3, GRS 1915+105 and Circinus X-1. There
exists a significant (>97% likelihood) positive (rank) correlation between the
peak X-ray flux P_X and radio flux density P_R for the black hole candidate
(BHC) systems, and a marginally significant positive (rank) correlation for the
neutron star (NS) systems. This is further evidence for a coupling between
accretion and outflows in X-ray binary systems, in this case implying a
relation between peak disc-accretion-rate and the number of synchroton-emitting
electrons ejected. However, we also show that the distribution of `radio
loudness', P_R/P_X, is significantly different for the two samples, in the
sense that the BHCs generally have a higher ratio of P_R/P_X. The origin of
this discrepancy is uncertain, but probably reflects differences in the
energetics and/or radiative efficiency of flows around the neutron stars and
black holes; we briefly discuss some of these possibilities. We conclude that
these data point to the formation of a mildly relativistic jet whose luminosity
is a function of the accretion rate, in the majority, if not all, of X-ray
transient outbursts, but whose relation to the observed X-ray emission is
dependent on the nature of the accreting compact object. (Abridged).Comment: Accepted for publication in MNRA
Universal shapes formed by two interacting cracks
We investigate the origins of the widely-observed "en passant" crack pattern
which forms through interactions between two approaching cracks. A rectangular
elastic plate is notched on each long side and then subjected to quasistatic
uniaxial strain from the short side. The two cracks propagate along
approximately straight paths until they pass each other, after which they curve
and release a lenticular fragment. We find that for materials with diverse
mechanical properties, the shape of this fragment has an aspect ratio of 2:1,
with the length scale set by the initial crack offset and the time scale
set by the ratio of to the pulling velocity. The cracks have a universal
square root shape which we understand using a simple geometric model of the
crack-crack interaction
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