38 research outputs found
On the Nature of the X-ray Emission from the Accreting Millisecond Pulsar SAX J1808.4-3658
The pulse profiles of the accreting X-ray millisecond pulsar SAX J1808.4-3658
at different energies are studied. The two main emission component, the black
body and the Comptonized tail that are clearly identified in the time-averaged
spectrum, show strong variability with the first component lagging the second
one. The observed variability can be explained if the emission is produced by
Comptonization in a hot slab (radiative shock) of Thomson optical depth ~0.3-1
at the neutron star surface. The emission patterns of the black body and the
Comptonized radiation are different: a "knife"- and a "fan"-like, respectively.
We construct a detailed model of the X-ray production accounting for the
Doppler boosting, relativistic aberration and gravitational light bending in
the Schwarzschild spacetime. We present also accurate analytical formulae for
computations of the light curves from rapidly rotating neutron stars using
formalism recently developed by Beloborodov (2002). Our model reproduces well
the pulse profiles at different energies simultaneously, corresponding phase
lags, as well as the time-averaged spectrum. We constrain the compact star mass
to be bounded between 1.2 and 1.6 solar masses. By fitting the observed
profiles, we determine the radius of the compact object to be R~11 km if M=1.6
M_sun, while for M=1.2 M_sun the best-fitting radius is ~6.5 km, indicating
that the compact object in SAX J1808.4-3658 can be a strange star. We obtain a
lower limit on the inclination of the system of 65 degrees.Comment: 11 pages, 7 figures, submitted to MNRA
X-ray spectral transitions of black holes from RXTE All-Sky Monitor
We have analysed X-ray outbursts from several Galactic black hole (GBH)
transients, as seen by the ASM on board RXTE. We have used the best estimates
of distance and black hole mass to find their luminosity (scaled to the
Eddington limit), which allowed for direct comparison of many sources. We have
found two distinct hard-to-soft state transitions in the initial part of the
outburst. The distinction is made on the basis of the transition luminosity,
its duration, the shape of the track in the hardness-luminosity diagram, and
evolution of the hardness ratio. The bright/slow transition occurs at ~30 per
cent of Eddington (estimated bolometric) luminosity and takes >~30 days, during
which the source quickly reaches the intermediate/very high state and then
proceeds to the soft state at much slower pace. The dark/slow transition is
less luminous (<~10 per cent of Eddington), shorter (<~15 days) and the source
does not slow its transition rate before reaching the soft state. We speculate
that the distinction is due to irradiation and evaporation of the disc, which
sustains the Comptonizing corona in the bright intemediate/very high state.Comment: Revised version, accepted for publication in MNRA
Broad-band X-ray/gamma-ray spectra and binary parameters of GX 339-4 and their astrophysical implications
We present X-ray/gamma-ray spectra of the binary GX 339-4 observed in the
hard state simultaneously by Ginga and CGRO OSSE during an outburst in 1991
September. The Ginga spectra are well represented by a power law with a photon
spectral index of 1.75 and a moderately-strong Compton reflection component
with a fluorescent Fe K alpha line. The OSSE data require a sharp high-energy
cutoff in the power-law spectrum. The broad-band spectra are very well modelled
by repeated Compton scattering in a thermal plasma with tau=1 and kT=50 keV. We
also find the distance to the system to be > 3 kpc, ruling out earlier
determinations of 1.3 kpc. Using this limit, the observed reddening and the
orbital period, we find the allowed range of the mass of the primary is
consistent with it being a black hole. The data are inconsistent with models of
either homogenous or patchy coronae above the surface of an accretion disc.
Rather, they are consistent with the presence of a hot inner hot disc accreting
at a rate close to the maximum set by advection and surrounded by a cold outer
disc. The seed photons for Comptonization are supplied by the outer cold disc
and/or cold clouds within the hot disc. Pair production is negligible if
electrons are thermal. The hot disc model, which scaled parameters are
independent of the black-hole mass, is supported by the similarity of the
spectrum of GX 339-4 to those of other black-hole binaries and Seyfert 1s. On
the other hand, their spectra in the soft gamma-ray regime are significantly
harder than those of weakly-magnetized neutron stars. Based on this difference,
we propose that the presence of broad-band spectra corresponding to thermal
Comptonization with kT of 50 keV or more represents a black-hole signature.Comment: 17 pages, 9 figures, accepted to MNRA