88 research outputs found
Relativistic outflow from two thermonuclear shell flashes on neutron stars
We study the exceptionally short (32-41 ms) precursors of two
intermediate-duration thermonuclear X-ray bursts observed with RXTE from the
neutron stars in 4U 0614+09 and 2S 0918-549. They exhibit photon fluxes that
surpass those at the Eddington limit later in the burst by factors of 2.6 to
3.1. We are able to explain both the short duration and the super-Eddington
flux by mildly relativistic outflow velocities of 0.1 to 0.3 subsequent
to the thermonuclear shell flashes on the neutron stars. These are the highest
velocities ever measured from any thermonuclear flash. The precursor rise times
are also exceptionally short: about 1 ms. This is inconsistent with predictions
for nuclear flames spreading laterally as deflagrations and suggests
detonations instead. This is the first time that a detonation is suggested for
such a shallow ignition column depth ( = 10 g cm).
The detonation would possibly require a faster nuclear reaction chain, such as
bypassing the alpha-capture on C with the much faster
C(p,)N(,p)O process previously proposed.
We confirm the possibility of a detonation, albeit only in the radial
direction, through the simulation of the nuclear burning with a large nuclear
network and at the appropriate ignition depth, although it remains to be seen
whether the Zel'dovich criterion is met. A detonation would also provide the
fast flame spreading over the surface of the neutron star to allow for the
short rise times. (...) As an alternative to the detonation scenario, we
speculate on the possibility that the whole neutron star surface burns almost
instantly in the auto-ignition regime. This is motivated by the presence of 150
ms precursors with 30 ms rise times in some superexpansion bursts from 4U
1820-30 at low ignition column depths of ~10 g cm.Comment: 11 pages, 6 figures, accepted by Astronomy and Astrophysic
The Low-Mass X-ray Binary X1822-330 in the Globular Cluster NGC 6652: A Serendipitous ASCA Observation
The Low Mass X-ray Binary (LMXB) X1822-330 in NGC 6652 is one of 12 bright,
or transient, X-ray sources to have been discovered in Globular Clusters. We
report on a serendipitous ASCA observation of this Globular Cluster LMXB,
during which a Type I burst was detected and the persistent, non-burst emission
of the source was at its brightest level recorded to date. No orbital
modulation was detected, which argues against a high inclination for the
X1822-330 system. The spectrum of the persistent emission can be fit with a
power law plus a partial covering absorber, although other models are not ruled
out. Our time-resolved spectral analysis through the burst shows, for the first
time, clear evidence for spectral cooling from kT=2.4+/-0.6 keV to kT=1.0+/0.1
keV during the decay. The measured peak flux during the burst is ~10% of the
Eddington luminosity for a 1.4 Msun neutron star. These are characteristic of a
Type I burst, in the context of the relatively low quiescent luminosity of
X1822-330.Comment: 9 pages, 5 figures, accepted for Ap
Identification of the LMXB and Faint X-ray Sources in NGC 6652
We have detected three new x-ray point sources, in addition to the known
low-mass x-ray binary (LMXB) X1832-330, in the globular cluster NGC 6652 with a
Chandra 1.6 ksec HRC-I exposure. Star 49 (M_{V}~4.7), suggested by Deutsch et
al.(1998) as the optical candidate for the LMXB, is identified (<0.3") not with
the LMXB, but with another, newly detected source (B). Using archival HST
images, we identify (<0.3") the LMXB (A) and one of the remaining new sources
(C) with blue variable optical counterparts at M_{V}~3.7 and 5.3 respectively.
The other new source (D) remains unidentified in the crowded cluster core. In
the 0.5-2.5 keV range, assuming a 5 keV thermal bremsstrahlung spectrum and
N_{H}=5.5*10^{20}, source A has intrinsic luminosity L_{X}~5.3*10^{35} ergs/s.
Assuming a 1 keV thermal bremsstrahlung spectrum, B has L_{X}~4.1*10^{33}
ergs/s, while C and D have L_{X}~8*10^{32}$ ergs/s. Source B is probably a
quiescent LMXB, while source C may be either a luminous CV or quiescent LMXB.Comment: 14 pages, 3 figures, accepted by Astrophysical Journa
The Zoo of X-ray sources in the Galactic Center Region: Observations with BeppoSAX
We report the results of a survey of the Galactic Center region (|l| < 2deg,
|b| < 0.5deg) performed with the BeppoSAX satellite. The flux from the center
of our Galaxy corresponds to a luminosity of ~3 10^{35} erg/s in the 2-10 keV
range. Due to the limited angular resolution (\gsim 1') only part of it is
supposed to come from Sagittarius A*, the non-thermal radio source which is
believed to mark the dynamical center of the Galaxy. In addition to the diffuse
emission, several bright (L_X\gsim10^{36} ergs/s) point sources have been
observed, both persistent (A 1742-294, SLX 1744-299, SLX 1744-300, 1E
1743.1-2843, 1E 1740.7-2942) and transient (XTE J1748-288, SAX J1747.0-2853 and
KS 1741-293). The Low Mass X-ray Binary AX J1745.6-2901, discovered with ASCA
at only 1.3'$ from SgrA* was detected in a low luminosity state in August
1997. Two fainter sources are very likely associated with young neutron stars:
the (possibly diffuse) X-ray source at the center of the composite supernova
remnant G0.9+0.1, and the "head" of the axially symmetric radio source
G359.23-0.92. The latter has been detected above 6 keV, supporting a
non-thermal emission mechanism.Comment: 34 pages, 11 figures and 3 tables included, accepted for publication
in Ap
Helium-rich thermonuclear bursts and the distance to the accretion-powered millisecond pulsar SAX J1808.4-3658
We analysed Rossi X-ray Timing Explorer observations of the accretion-powered
401 Hz pulsar SAX J1808.4-3658, in order to precisely determine the source
distance. While the fluences for the five transient outbursts observed from
1996 were constant to within the uncertainties, the outburst interval varied
signficantly, so that the time-averaged flux (and accretion rate) decreased by
around 40%. By equating the time-averaged X-ray flux with the expected mass
transfer rate from gravitational radiation, we derived a lower limit on the
distance of 3.4 kpc. Combined with an upper limit from assuming that the four
radius-expansion thermonuclear bursts observed during the 2002 October outburst
reached at most the Eddington limit for a pure He atmosphere, we found that the
probable distance range for the source is 3.4-3.6 kpc. The implied inclination,
based on the optical/IR properties of the counterpart, is i<~30 degrees.
We compared the properties of the bursts with an ignition model. The time
between bursts was long enough for hot CNO burning to significantly deplete the
accreted hydrogen, so that ignition occurred in a pure helium layer underlying
a stable hydrogen burning shell. This is the first time that this burning
regime has been securely observationally identified. The observed energetics of
the bursts give a mean hydrogen fraction at ignition of approx. 0.1, and
require that the accreted hydrogen fraction X_0 and the CNO metallicity Z_CNO
are related by Z_CNO approx. 0.03(X_0/0.7)^2. We show that in this burning
regime, a measurement of the burst recurrence time and energetics allows the
local accretion rate onto the star to be determined independently of the
accreted composition, giving a new method for estimating the source distance
which is in good agreement with our other estimates.Comment: 10 pages, 6 figures, accepted by Ap
The Reappearance of the Transient Low Mass X-ray Binary X1658-298
In April 1999 the transient low mass X-ray binary X1658-298 resumed its
strong and persistent X-ray emission after a 21-year interval of quiescence. We
present RXTE data obtained soon after the reappearance, including four eclipses
with a mean duration of 901.9 +/- 0.8 sec and ingress/egress times of 6-13 sec.
Our updated ephemeris for the source indicates that the 7.1-hr orbital period
of the system is decreasing with a timescale of 10^7 yr. Contemporaneous
optical observations provide the first-ever lightcurve of V2134 Oph, the
optical counterpart of X1658-298. The optical modulation is highly variable
from night to night and exhibits a distinct, narrow eclipse feature of about
0.2 mag superposed on a gradual brightness variation with ~0.7-0.8 mag
amplitude. Our data indicate that there is no significant offset between the
time of mid-eclipse in the X-ray and optical and that the narrow optical
eclipse feature is of the same duration as the X-ray eclipse. This implies an
accretion disk structure characterized by enhanced optical emission coincident
with the central X-ray emitting area.Comment: 13 pages including 4 figures and 3 tables; Accepted for publication
in The Astrophysical Journa
The discovery of quiescent X-ray emission from SAX J1808.4-3658, the transient 2.5ms pulsar
We report on a 20 ks BeppoSAX observation of the transient 2.5 ms X-ray
pulsar SAX J1808.4-3658 in its quiescent state. A source at a level of ~ 3 x
10^-3 c/s was detected with the BeppoSAX/MECS at a position consistent with
that of SAX J1808.4-3658. The inferred 0.5-10 keV luminosity was to that
measured from other quiescent low mass X-ray transients hosting an old neutron
star. This result is discussed in the light of the propeller and radio pulsar
shock emission models for the quiescent emission of these systems.Comment: 5 pages plus 1 ps figure. Accepted for publication on ApJ Letter
Partially Absorbed Comptonization Spectrum from the Nearly Edge-on Source X 1822-371
We report the results of a spectral analysis over the range 0.1-200 keV
performed on the dipping source X 1822-371 observed by BeppoSAX. We find the
best fit to the continuum using a partially covered Comptonization model, due
to scattering off soft seed photons by electrons at a temperature of ~4.8 keV,
without the presence of any soft blackbody emission. The equivalent hydrogen
column obtained for the absorbed component is ~4.5 10^{22} cm^{-2}, an order of
magnitude larger than the Galactic absorption for this source, and the covering
fraction is ~71%. Because the inclination angle of X 1822-371 to the line of
sight is ~85^\circ, this model gives a reasonable scenario for the source: the
Comptonized spectrum could come from an extended accretion disk corona (ADC),
probably the only region that can be directly observed due to the high
inclination. The excess of matter producing the partial covering could be close
to the equatorial plane of the system, above the outer disk, occulting the
emission from the inner disk and the inner part of the ADC. An iron emission
line is also present at ~6.5 keV with an equivalent width of ~150 eV. We argue
that this strong iron line cannot be explained as reflection of the Comptonized
spectrum by the accretion disk. It is probably produced in the ADC. An emission
line at ~1.9 keV (with an equivalent width of ~54 eV) and an absorption edge at
\~8.7 keV (with an optical depth of ~0.1) are also required to fit this
spectrum. These features are probably produced by highly ionized iron (Fe XXIV)
present in the outer part of the ADC, where the plasma density is
\~10^{11}-10^{12} cm^{-3} and ionized plasma is present.Comment: 15 pages, including 3 figures. Accepted by ApJ. Corrected typos and
Figure
Detection of a Hard Tail in the X-ray Spectrum of the Z Source GX 349+2
We present the results of a BeppoSAX observation of the Z source GX 349+2
covering the energy range 0.1-200 keV. The presence of flares in the light
curve indicates that the source was in the flaring branch during the BeppoSAX
observation. We accumulated energy spectra separately for the non-flaring
intervals and the flares. In both cases the continuum is well described by a
soft blackbody ( keV) and a Comptonized spectrum
corresponding to an electron temperature of keV, optical depth
(for a spherical geometry), and seed photon temperature of keV. All temperatures tend to increase during the flares. In the
non-flaring emission a hard tail dominates the spectrum above 30 keV. This can
be fit by a power law with photon index , contributing of the
total source luminosity over the BeppoSAX energy range. A comparison with hard
tails detected in some soft states of black hole binaries suggests that a
similar mechanism could originate these components in black hole and neutron
star systems.Comment: 15 pages, including 8 figures, to appear in Ap
Outburst of the X-ray transient SAX J1818.6-1703 detected by INTEGRAL in September 2003
During the observation of the Galactic-center field by the INTEGRAL
observatory on September 9, 2003, the IBIS/ISGRI gamma-ray telescope detected a
short (several-hours-long) intense (~380 mCrab at the peak) outburst of hard
radiation from the X-ray transient SAX J1818.6-1703. Previously, this source
was observed only once in 1998 during a similar short outburst. We present the
results of our localization, spectral and timing analyses of the object and
briefly discuss the possible causes of the outburst. The release time of the
bulk of the energy in such an outburst is appreciably shorter than the
accretion (viscous) time that characterizes the flow of matter through a
standard accretion disk.Comment: 16 pages, 7 figures, to be published in Astronomy Letters, v. 31, n.
10, p. 672 (2005
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