498 research outputs found
Periodic Thermonuclear X-ray Bursts from GS 1826-24 and the Fuel Composition as a Function of Accretion Rate
We analyze 24 type I X-ray bursts from GS 1826-24 observed by the Rossi X-ray
Timing Explorer between 1997 November and 2002 July. The bursts observed
between 1997-98 were consistent with a stable recurrence time of 5.74 +/- 0.13
hr. The persistent intensity of GS 1826-24 increased by 36% between 1997-2000,
by which time the burst interval had decreased to 4.10 +/- 0.08 hr. In 2002
July the recurrence time was shorter again, at 3.56 +/- 0.03 hr. The bursts
within each epoch had remarkably identical lightcurves over the full approx.
150 s burst duration; both the initial decay timescale from the peak, and the
burst fluence, increased slightly with the rise in persistent flux. The
decrease in the burst recurrence time was proportional to Mdot^(-1.05+/-0.02)
(where Mdot is assumed to be linearly proportional to the X-ray flux), so that
the ratio alpha between the integrated persistent and burst fluxes was
inversely correlated with Mdot. The average value of alpha was 41.7 +/- 1.6.
Both the alpha value, and the long burst durations indicate that the hydrogen
is burning during the burst via the rapid-proton (rp) process. The variation in
alpha with Mdot implies that hydrogen is burning stably between bursts,
requiring solar metallicity (Z ~ 0.02) in the accreted layer. We show that
solar metallicity ignition models naturally reproduce the observed burst
energies, but do not match the observed variations in recurrence time and burst
fluence. Low metallicity models (Z ~ 0.001) reproduce the observed trends in
recurrence time and fluence, but are ruled out by the variation in alpha. We
discuss possible explanations, including extra heating between bursts, or that
the fraction of the neutron star covered by the accreted fuel increases with
Mdot.Comment: 9 pages, 6 figures, accepted by ApJ. Minor revisions following the
referee's repor
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
IGR J17544-2619: A new supergiant fast X-ray transient revealed by optical/infrared observations
One of the most recent discoveries of the INTEGRAL observatory is the
existence of a previously unknown population of X-ray sources in the inner arms
of the Galaxy. IGR J17544-2619, IGR J16465-4507 and XTE J1739-302 are among
these sources. Although the nature of these systems is still unexplained, the
investigations of the optical/NIR counterparts of the two last sources,
combined with high energy data, have provided evidence of them being highly
absorbed high mass X-ray binaries with blue supergiant secondaries and
displaying fast X-ray transient behaviour. In this work we present our
optical/NIR observations of IGR J17544-2619, aimed at identifying and
characterizing its counterpart. We show that the source is a high mass X-ray
binary at a distance of 2-4 kpc with a strongly absorbed O9Ib secondary, and
discuss the nature of the system.Comment: 7 pages, 4 figures. Accepted for publication in A&
The Evolution Of LMC X-4 Flares: Evidence For Super-Eddington Radiation Oozing Through Inhomogeneous Polar Cap Accretion Flows ?
We present the results of two extensive Rossi X-ray Timing Explorer
observations of large X-ray flaring episodes from the high-mass X-ray binary
pulsar LMC X-4. Light curves during the flaring episodes comprise bright peaks
embedded in relatively fainter regions, with complex patterns of recurrence and
clustering of flares. We identify precursors preceding the flaring activity.
Pulse profiles during the flares appear to be simple sinusoids, and pulsed
fractions are proportional to the flare intensities. We fit Gaussian functions
to flare peaks to estimate the mean full-width-half-maximum to be 68 s.
Significant rapid aperiodic variability exists up to a few hertz during the
flares, which is related to the appearance of narrow, spiky peaks in the light
curves. While spectral fits and softness ratios show overall spectral softening
as the flare intensity increases, the narrow, spiky peaks do not follow this
trend. The mean fluence of the flare peaks is (3.1 2.9)
10 ergs in the 2.5--25 keV energy range, with its maximum at 1.9
10 ergs. The flare peak luminosity reaches up to (2.1
0.2) 10 ergs s, far above the Eddington luminosity of a
neutron star. We discuss possible origins of the flares, and we also propose
that inhomogeneous accretion columns onto the neutron star polar caps are
responsible for the observed properties.Comment: 39 pages (including figures and tables), accepted for publication 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
SMC X-1 As An Intermediate-Stage Flaring X-ray Pulsar
We present Rossi X-ray Timing Explorer observations of the X-ray pulsar SMC
X-1. The source is highly variable on short time scales (< 1 h), exhibiting
apparent X-ray flares occupying a significant fraction (~3 %) of the total
observing time, with a recurrence time of ~100 s. The flares seem to occur over
all binary orbital phases, and correlate with the overall variability in the
light curve. We find a total of 323 discrete flares which have a mean full
width half maximum of ~18 s. The detailed properties of SMC X-1 do not vary
significantly between the flares and the normal state, suggesting that the
flare may be an extension of the normal state persistent emission with
increased accretion rates. The flares resemble Type II X-ray bursts from GRO
J1744--28. We discuss the origin of the SMC X-1 flares in terms of a viscous
instability near the inner edge of the accretion disk around a weakly
magnetized X-ray pulsar, and find this is consistent with the interpretation
that SMC X-1 is in fact an intermediate-stage source like GRO J1744--28.Comment: 14 pages (5 figures), To appear in ApJ Letter
X-Ray Observations of V4641 SGR (= SAX J1819.3-2525) During the Brief and Violent Outburst of 2003
We present the results of detailed analysis of pointed X-ray observations by
RXTE PCA/HEXTE of the black hole X-ray binary (BHXRB) system V4641 Sgr (= SAX
J1819.3-2525) during its outburst of August 2003. Soft X-ray (3-20 keV) flux
variations by factors of 10 or more on timescales of minutes or shorter were
seen. The rapid and strong variability of this source sets it apart from
typical XRBs. In spite of large luminosity fluctuations, the spectral state of
the source did not change significantly during the dwells which suggests that
the physical emission processes did not change much during the observations.
The energy spectra during the dwells were dominated by a hard Comptonized
powerlaw component, indicative of the canonical low/hard state observed in
other BHXRBs. No soft thermal component was found in three out of the four RXTE
pointings. However spectral deconvolution of the observation with largest
average luminosity suggests an obscured, hot accretion disk. During one of the
observations we detected a short term (about 100s) soft X-ray dropout which is
apparently due to variability in the observed column density. Strong Fe
K fluorescent emisssion line near 6.5 keV was detected with large
equivalent widths in the range of 700 - 1000eV. In the temporal domain, the
Fourier power spectra were dominated by red noise below a few Hz. Poisson noise
dominated at higher frequencies and no high frequency features were detected.
The strong Comptonized spectra, broad iron emission line, absence of disk
component in the spectra, absence of any timing variability above few Hz and
occasional large changes in the column density along the line-of-sight, all
support an enshrouded black hole with occasional outflow and a very dynamic
environment.Comment: 27 pages, 10 figures (1 color figure), accepted for publication in
the Astrophysical Journal. It is tentatively scheduled for the ApJ 01
February 2006, v637, 2 issu
Resolving the Composite Fe K-alpha Emission Line in the Galactic Black Hole Cygnus X-1 with Chandra
We observed the Galactic black hole Cygnus X-1 with the Chandra High Energy
Transmission Grating Spectrometer for 30 kiloseconds on 4 January, 2001. The
source was in an intermediate state, with a flux that was approximately twice
that commonly observed in its persistent low/hard state. Our best-fit model for
the X-ray spectrum includes narrow Gaussian emission line (E = 6.415 +/- 0.007
keV, FWHM = 80 (+28, -19) eV, W = 16 (+3, -2) eV) and broad line (E = 5.82
(+0.06, -0.07) keV, FWHM = 1.9 (+0.5, -0.3) keV, W = 140 (+70, -40) eV)
components, and a smeared edge at 7.3 +/- 0.2 keV (tau ~ 1.0). The broad line
profile is not as strongly skewed as those observed in some Seyfert galaxies.
We interpret these features in terms of an accretion disk with irradiation of
the inner disk producing a broad Fe K-alpha emission line and edge, and
irradiation of the outer disk producing a narrow Fe K-alpha emission line. The
broad line is likely shaped predominantly by Doppler shifts and gravitational
effects, and to a lesser degree by Compton scattering due to reflection. We
discuss the underlying continuum X-ray spectrum and these line features in the
context of diagnosing the accretion flow geometry in Cygnus X-1 and other
Galactic black holes.Comment: Accepted for publication in Ap
XTE J1739-302 as a Supergiant Fast X-ray Transient
XTE J1739-302 is a transient X-ray source with unusually short outbursts,
lasting on the order of hours. Here we give a summary of X-ray observations we
have made of this object in outburst with the Rossi X-ray Timing Explorer
(RXTE) and at a low level of activity with the Chandra X-ray Observatory, as
well as observations made by other groups. Visible and infrared spectroscopy of
the mass donor of XTE J1739-302 are presented in a companion paper. The X-ray
spectrum is hard both at low levels and in outburst, but somewhat variable, and
there is strong variability in the absorption column from one outburst to
another. Although no pulsation has been observed, the outburst data from
multiple observatories show a characteristic timescale for variability on the
order of 1500-2000 s. The Chandra localization (right ascension 17h 39m 11.58s,
declination -30o 20' 37.6'', J2000) shows that despite being located less than
2 degrees from the Galactic Center and highly absorbed, XTE J1739-302 is
actually a foreground object with a bright optical counterpart. The combination
of a very short outburst timescale and a supergiant companion is shared with
several other recently-discovered systems, forming a class we designate as
Supergiant Fast X-ray Transients (SFXTs). Three persistently bright X-ray
binaries with similar supergiant companions have also produced extremely short,
bright outbursts: Cyg X-1, Vela X-1, and 1E 1145.1-6141.Comment: 16 pages, 7 figures, 2 tables, in press in The Astrophysical Journal;
see also the companion paper by Negueruela et a
The bright Gamma-Ray Burst of February 10, 2000: a case study of an optically dark GRB
The gamma-ray burst GRB000210 had the highest gamma-ray peak flux of any
event localized by BeppoSAX as yet but it did not have a detected optical
afterglow. It is therefore one of the events recently classified as dark GRBs
or GHOST (GRB Hiding Optical Source Transient), whose origin is still unclear.
Chandra observations allowed us to localize this GRB within ~1" and a radio
transient was detected with the VLA. We identify the likely (P=0.01) host
galaxy of this burst at z=0.846. The X-ray spectrum of the afterglow shows
intrinsic absorption N_H=5x10**21 cm-2. The amount of dust needed to absorb the
optical flux of this object is consistent with the above HI column density,
given a dust-to-gas ratio similar to that of our Galaxy. We do not find
evidence for a partially ionized absorber expected if the absorption takes
place in a Giant Molecular Cloud. We therefore conclude that either the gas is
local to the GRB, but is condensed in small-scale high-density (n>~10**9 cm-3)
clouds, or that the GRB is located in a dusty, gas-rich region of the galaxy.
Finally, if GRB000210 lies at z>5, its X-ray absorbing medium would have to be
substantially different from that observed in GRBs with optical afterglows.Comment: 29 pages, 7 fig.s, some revisions, ApJ, in pres
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