1,287 research outputs found
Hubble Space Telescope Observations of the Black Hole X-ray Transient GRO J0422+32 Near Quiescence
We present HST/FOS ultraviolet and optical spectroscopy of the black hole
X-ray transient GRO J0422+32 shortly before the system reached quiescence. We
find that the accretion spectrum from 2500-9000A can be very well fit by a
self-absorbed synchrotron model, with superposed HI and MgII emission lines.
The explanations we suggest for this spectrum are that it is either due to
active coronal regions above a geometrically thin accretion disc, or that the
disc is evaporated into an advective flow.Comment: 7 pages with 7 postscript figures included, uses mn.sty. Accepted for
publication in MNRA
SPH simulations of irradiation-driven warped accretion discs and the long periods in X-ray binaries
We present three dimensional smoothed particle hydrodynamics (SPH)
calculations of irradiation-driven warping of accretion discs. Initially
unwarped planar discs are unstable to the radiation reaction when the disc is
illuminated by a central radiation source. The disc warps and tilts and
precesses slowly in a retrograde direction; its shape continuously flexes in
response to the changing orientation of the Roche potential. We simulate ten
systems: eight X-ray binaries, one cataclysmic variable (CV), and a `generic'
low mass X-ray binary (LMXB). We adopt system parameters from observations and
tune a single parameter: our model X-ray luminosity () to reproduce the
observed or inferred super-orbital periods. Without exception, across a wide
range of parameter space, we find an astonishingly good match between the
observed and the model . We conclude irradiation-driven warping
is the mechanism underlying the long periods in X-ray binaries. Our Her X-1
simulation simultaneously reproduces the observed , the "main-" and
"short-high" X-ray states and the orbital inclination. Our simulations of SS
433 give a maximum warp angle of , a good match to the cone
traced by the jets, but this angle is reached only in the outer disc. In all
cases, the overall disc tilt is less than \degrees{13} and the maximum disc
warp is less than and or equal to \degrees{21}.Comment: 17 pages, 14 figures, shorter abstract (24 lines limit
Simulations of spectral lines from an eccentric precessing accretion disc
Two dimensional SPH simulations of a precessing accretion disc in a q=0.1
binary system (such as XTE J1118+480) reveal complex and continuously varying
shape, kinematics, and dissipation. The stream-disc impact region and disc
spiral density waves are prominent sources of energy dissipation.The dissipated
energy is modulated on the period P_{sh} = ({P_{orb}}^{-1}-{P_{prec}}^{-1}^{-1}
with which the orientation of the disc relative to the mass donor repeats. This
superhump modulation in dissipation energy has a variation in amplitude of ~10%
relative to the total dissipation energy and evolves, repeating exactly only
after a full disc precession cycle. A sharp component in the light curve is
associated with centrifugally expelled material falling back and impacting the
disc. Synthetic trailed spectrograms reveal two distinct "S-wave" features,
produced respectively by the stream gas and the disc gas at the stream-disc
impact shock. These S-waves are non-sinusoidal, and evolve with disc precession
phase. We identify the spiral density wave emission in the trailed spectrogram.
Instantaneous Doppler maps show how the stream impact moves in velocity space
during an orbit. In our maximum entropy Doppler tomogram the stream impact
region emission is distorted, and the spiral density wave emission is
uppressed. A significant radial velocity modulation of the whole line profile
occurs on the disc precession period. We compare our SPH simulation with a
simple 3D model: the former is appropriate for comparison with emission lines
while the latter is preferable for skewed absorption lines from precessing
discs.Comment: See http://physics.open.ac.uk/FHMR/ for associated movie (avi) files.
The full paper is in MNRAS press. Limited disk space limit of 650k, hence low
resolution figure file
Superhumps in Low-Mass X-Ray Binaries
We propose a mechanism for the superhump modulations observed in optical
photometry of at least two black hole X-ray transients (SXTs). As in extreme
mass-ratio cataclysmic variables (CVs), superhumps are assumed to result from
the presence of the 3:1 orbital resonance in the accretion disc. This causes
the disc to become non-axisymmetric and precess. However the mechanism for
superhump luminosity variations in low mass X-ray binaries (LMXBs) must differ
from that in CVs, where it is attributed to a tidally-driven modulation of the
disc's viscous dissipation, varying on the beat between the orbital and disc
precession period. By contrast in LMXBs, tidal dissipation in the outer
accretion disc is negligible: the optical emission is overwhelming dominated by
reprocessing of intercepted central X-rays. Thus a different origin for the
superhump modulation is required. Recent observations and numerical simulations
indicate that in an extreme mass-ratio system the disc area changes on the
superhump period. We deduce that the superhumps observed in SXTs arise from a
modulation of the reprocessed flux by the changing area. Therefore, unlike the
situation in CVs, where the superhump amplitude is inclination-independent,
superhumps should be best seen in low-inclination LMXBs, whereas an orbital
modulation from the heated face of the secondary star should be more prominent
at high inclinations. Modulation at the disc precession period (10s of days)
may indicate disc asymmetries such as warping. We comment on the orbital period
determinations of LMXBs, and the possibility and significance of possible
permanent superhump LMXBs.Comment: 6 pages, 1 encapsulated figure. MNRAS in press; replaced to correct
typographical error
Black hole hunting in the Andromeda Galaxy
We present a new technique for identifying stellar mass black holes in low
mass X-ray binaries (LMXBs), and apply it to XMM-Newton observations of M31. We
examine X-ray time series variability seeking power density spectra (PDS)
typical of LMXBs accreting at a low accretion rate (which we refer to as Type A
PDS); these are very similar for black hole and neutron star LMXBs. Galactic
neutron star LMXBs exhibit Type A PDS at low luminosities (~10^36--10^37 erg/s)
while black hole LMXBs can exhibit them at luminosities >10^38 erg/s. We
propose that Type A PDS are confined to luminosities below a critical fraction
of the Eddington limit, that is constant for all LMXBs; we have examined
asample of black hole and neutron star LMXBs and find they are all consistent
with = 0.10+/-0.04 in the 0.3--10 keV band. We present luminosity and PDS
data from 167 observations of X-ray binaries in M31 that provide strong support
for our hypothesis. Since the theoretical maximum mass for a neutron star is
\~3.1 M_Sun, we therefore assert that any LMXB that exhibits a Type A PDS at a
0.3--10 keV luminosity greater than 4 x 10^37 erg/s is likely to contain a
black hole primary. We have found eleven new black hole candidates in M31 using
this method. We focus on XMM-Newton observations of RX J0042.4+4112, an X-ray
source in M31 and find the mass of the primary to be 7+/-2 M_Sun, if our
assumptions are correct. Furthermore, RX J0042.4+4112 is consistently bright in
\~40 observations made over 23 years, and is likely to be a persistently bright
LMXB; by contrast all known Galactic black hole LMXBs are transient. Hence our
method may be used to find black holes in known, persistently bright Galactic
LMXBs and also in LMXBs in other galaxies.Comment: 6 Pages, 6 figures. To appear in the conference proceedings of
"Interacting Binaries: Accretion, Evolution and Outcomes" (Cefalu, July 4-10
2004
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