7 research outputs found
State transitions in the 2001/2002 outburst of XTE J1650-500
We present a study of the X-ray transient and black hole candidate XTE
J1650-500 during its 2001/2002 outburst. The source made two state transitions
between the hard and soft states, at luminosity levels that differed by a
factor of ~5-10. The first transition, between hard and soft, lasted for ~30
days and showed two parts; one part in which the spectral properties evolve
smoothly away from the hard state and another that we identify as the 'steep
power law state'. The two parts showed different behavior of the Fe K emission
line and QPO frequencies. The second transition, from soft to hard, lasted only
\~15 days and showed no evidence of the presence of the 'steep power law
state'. Comparing observations from the early rise and the decay of the
outburst, we conclude that the source can be in the hard state in a range of
more 10^4 in luminosity. We briefly discuss the state transitions in the
framework of a two-flow model.Comment: 4 pages, 5 figures. To appear in Proc. of the II BeppoSAX Meeting:
"The Restless High-Energy Universe" (Amsterdam, May 5-8, 2003), E.P.J. van
den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers Ed
A Compton reflection dominated spectrum in a peculiar accreting neutron star
We report on a puzzling event occurred during a long BeppoSAX observation of
the slow-rotating binary pulsar GX 1+4. During this event, lasting about 1 day,
the source X-ray flux was over a factor 10 lower than normal. The low-energy
pulsations disappeared while at higher energies they were shifted in phase. The
spectrum taken outside this low-intensity event was well fitted by an absorbed
cut-off power law, and exhibited a broad iron line at ~6.5 keV probably due to
the blending of the neutral (6.4 keV) and ionised (6.7 keV) K_alpha iron lines.
The spectrum during the event was Compton reflection dominated and it showed
two narrow iron lines at ~6.4 keV and ~7.0 keV, the latter never revealed
before in this source. We also present a possible model for this event in which
a variation of the accretion rate thickens a torus-like accretion disc which
hides for a while the direct neutron star emission from our line of sight. In
this scenario the Compton reflected emission observed during the event is well
explained in terms of emission reflected by the side of the torus facing our
line of sight.Comment: 10 pages; to be published in MNRA
The Energy Spectrum of Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters
Assuming that AXPs and SGRs accrete matter from a fallback disk, we attempt
to explain both the soft and the hard X-ray emission as the result of the
accretion process. We also attempt to explain their radio emission or the lack
of it. We test the hypothesis that the power-law, hard X-ray spectra are
produced in the accretion flow mainly by bulk-motion Comptonization of soft
photons emitted at the neutron star surface. Fallback disk models invoke
surface dipole magnetic fields of G, which is what we
assume here. Unlike normal X-ray pulsars, for which the accretion rate is
highly super-Eddington, the accretion rate is approximately Eddington in AXPs
and SGRs and thus the bulk-motion Comptonization operates efficiently. As an
illustrative example we reproduce both the hard and the soft X-ray spectra of
AXP 4U 0142+61 well using the XSPEC package compTB. Our model seems to explain
both the hard and the soft X-ray spectra of AXPs and SGRs, as well as their
radio emission or the lack of it, in a natural way. It might also explain the
short bursts observed in these sources. On the other hand, it cannot explain
the giant X-ray outbursts observed in SGRs, which may result from the
conversion of magnetic energy in local multipole fields.Comment: 6 pages, 2 figures, minor corrections, accepted for publication in
A&
Bursts, eclipses, dips and a refined position for the luminous low-mass X-ray binary in the globular cluster Terzan 6
GRS 1747-312 is a bright transient X-ray source in the globular cluster
Terzan 6 with quasi-periodic outbursts approximately every 4.5 months. We
carried out 2-60 keV target-of-opportunity observations during eight outbursts
with the Proportional Counter Array on the RXTE satellite, for a total exposure
time of 301 ks, and detect the first unambiguous thermonuclear X-ray bursts
from this source. This identifies the compact accretor in this binary as a
neutron star. The neutron star identification implies that twelve out of
thirteen luminous (above 10^36 erg/s X-ray sources in Galactic globular
clusters harbor neutron stars, with AC211's nature (in M15) remaining elusive.
We observed 24 transitions of eclipses of the X-ray emitting region by the
companion star and are able to improve the accuracy of the orbital period by a
factor of 10^4. The period is P=0.514980303(7) d. We do not detect a period
derivative with an upper limit of |\dot{P}/P|=3X10^-8 yr^{-1}. Archival Chandra
data were analyzed to further refine the X-ray position, and the cluster's
center of gravity was re-determined from optical data resulting in a correction
amounting to 2 core radii. We find that GRS 1747-312 is 0.2+/-0.2 core radii
from the cluster center.Comment: Accepted by A&
INTEGRAL/IBIS survey of the Sagittarius Arm Tangent region: A source catalog
Analysis of 18-120 keV images of the Sagittarius Arm Tangent region (SATR)
obtained by IBIS telescope onboard INTEGTRAL observatory during the spring of
2003 is performed. In the 18-60 keV energy range, 28 sources have been detected
with a flux level above 1.4 mCrab. Of these sources, 16 were identified earlier
as galactic X-ray binary systems, 3 as extragalactic objects, 2 as pulsars
inside supernova remnants, and 7 has unknown nature. The analysis revealed the
presence of three previously unknown sources. Fourteen sources show significant
flux in the 60-120 keV energy rangeComment: 5 pages, 2 figures. Accepted for publication in Astronomy Letter