290 research outputs found
Swift J1357.2-0933: the faintest black hole?
Swift J1357.2-0933 is the first confirmed very faint black hole X-ray
transient and has a short estimated orbital period of 2.8 hr. We observed Swift
J1357.2-0933 for ~50 ks with XMM-Newton in 2013 July during its quiescent
state. The source is clearly detected at a 0.5-10 keV unabsorbed flux of
~3x10^-15 erg cm-2 s-1. If the source is located at a distance of 1.5 kpc (as
suggested in the literature), this would imply a luminosity of ~8x10^29 erg
s-1, making it the faintest detected quiescent black hole LMXB. This would also
imply that there is no indication of a reversal in the quiescence X-ray
luminosity versus orbital period diagram down to 2.8 hr, as has been predicted
theoretically and recently supported by the detection of the 2.4 hr orbital
period black hole MAXI J1659-152 at a 0.5-10 keV X-ray luminosity of ~ 1.2 x
10^31 erg s-1. However, there is considerable uncertainty in the distance of
Swift J1357.2-0933 and it may be as distant as 6 kpc. In this case, its
quiescent luminosity would be Lx ~ 1.3 x 10^31 erg s-1, i.e., similar to MAXI
J1659-152 and hence it would support the existence of such a bifurcation
period. We also detected the source in optical at r' ~22.3 mag with the
Liverpool telescope, simultaneously to our X-ray observation. The X-ray/optical
luminosity ratio of Swift J1357.2-0933 agrees with the expected value for a
black hole at this range of quiescent X-ray luminosities.Comment: 5 pages, 3 figures, Accepted for publication in MNRA
Silo collapse under granular discharge
We investigate, at a laboratory scale, the collapse of cylindrical shells of
radius and thickness induced by a granular discharge. We measure the
critical filling height for which the structure fails upon discharge. We
observe that the silos sustain filling heights significantly above an
estimation obtained by coupling standard shell-buckling and granular stress
distribution theories. Two effects contribute to stabilize the structure: (i)
below the critical filling height, a dynamical stabilization due to granular
wall friction prevents the localized shell-buckling modes to grow irreversibly;
(ii) above the critical filling height, collapse occurs before the downward
sliding motion of the whole granular column sets in, such that only a partial
friction mobilization is at play. However, we notice also that the critical
filling height is reduced as the grain size, , increases. The importance of
grain size contribution is controlled by the ratio . We
rationalize these antagonist effects with a novel fluid/structure theory both
accounting for the actual status of granular friction at the wall and the
inherent shell imperfections mediated by the grains. This theory yields new
scaling predictions which are compared with the experimental results.Comment: Original work, 13 pages and 11 figure
New Insights into X-ray Binaries
X-ray binaries are excellent laboratories to study collapsed objects. On the
one hand, transient X-ray binaries contain the best examples of stellar-mass
black holes while persistent X-ray binaries mostly harbour accreting neutron
stars. The determination of stellar masses in persistent X-ray binaries is
usually hampered by the overwhelming luminosity of the X-ray heated accretion
disc. However, the discovery of high-excitation emission lines from the
irradiated companion star has opened new routes in the study of compact
objects. This paper presents novel techniques which exploits these irradiated
lines and summarises the dynamical masses obtained for the two populations of
collapsed stars: neutron stars and black holes.Comment: 12 pages, 5 figures, 2 tables, Invited review to plenary session in
"Highlights of Spanish Astrophysics V", Proceedings of the VIII Scientific
Meeting of the Spanish Astronomical Society (SEA) held in Santander, 7-11
July, 2008. Edited by J. Gorgas, L. J. Goicoechea, J. I. Gonzalez-Serrano, J.
M. Dieg
Inclination and relativistic effects in the outburst evolution of black hole transients
We have systematically studied the effect of the orbital inclination in the
outburst evolution of black hole transients. We have included all the systems
observed by the Rossi X-ray timing explorer in which the thermal, accretion
disc component becomes strongly dominant at some point of the outburst.
Inclination is found to modify the shape of the tracks that these systems
display in the colour/luminosity diagrams traditionally used for their study.
Black hole transients seen at low inclination reach softer spectra and their
accretion discs look cooler than those observed closer to edge-on. This
difference can be naturally explained by considering inclination dependent
relativistic effects on accretion discs.Comment: 8 pages, 4 figures, accepted for publication in MNRA
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