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

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&

Hard X-ray Bursts Recorded by the IBIS Telescope of the INTEGRAL Observatory in 2003-2009

To find X-ray bursts from sources within the field of view of the IBIS/INTEGRAL telescope, we have analysed all the archival data of the telescope available at the time of writing the paper (the observations from January 2003 to April 2009). We have detected 834 hard (15-25 keV) X-ray bursts, 239 of which were simultaneously recorded by the JEM-X/INTEGRAL telescope in the standard X-ray energy range. More than 70% of all bursts (587 events) have been recorded from the well-known X-ray burster GX 354-0. We have found upper limits on the distances to their sources by assuming that the Eddington luminosity limit was reached at the brightness maximum of the brightest bursts.Comment: 18 pages, 2 figures, 2 table

Software framework architecture for programmable photonic chips

We introduce a software framework for design and programming of large scale programmable photonic chips. The framework facilitates design, simulation, configuration and measurements of reconfigurable waveguide meshes, managing hundreds of actuators and translate high-level user requirements into driving strategies of the electronic channels

Genetic polymorphisms of cytochrome P450 enzymes 2C9 and 2C19 in a healthy Iranian population

1. The genetically polymorphic cytochrome P450 enzymes 2C9 (CYP2C9) and 2C19 (CYP2C19) are involved in the metabolism and elimination of a number of widely used drugs. The polymorphisms give rise to substantial interindividual and interethnic variability in drug excretion rates and final serum concentrations. For this reason, therapeutic responses and adverse drug reactions may vary from one person to another. In the present study we determined CYP2C9 and CYP2C19 genotypes in a random Iranian population to compare allele frequencies with previous findings in other ethnic groups. 2. Allelic variants of CYP2C9 (*1/*2/*3) and CYP2C19 (*1/*2/*3) were determined in 200 unrelated healthy Iranian volunteers by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assays. 3. Fifteen subjects (7.5) were homozygous for the CYP2C9*2 allele, whereas 21 individuals (10.5) were heterozygous for this allele and 164 subjects (82) had the wild-type allele (CYP2C9*1). No CYP2C9*3 was detected in the population sampled. Six subjects (3) were homozygous for CYP2C19*2, whereas 44 individuals (22) were heterozygous for this allele. In the remaining subjects (75), no CYP2C19*2 was found. In addition, no CYP2C19*3 was detected in the population sampled. 4. Based on our data, the frequency of the CYP2C9*2 allelic variant in Iranians is similar to that in other Caucasian populations; however, the frequency of the CYP2C9*3 allele differed significantly (P 0.05). © 2007 Blackwell Publishing Asia Pty Ltd

Evidence for a Neutron Star in the non-pulsating massive X-ray binary 4U2206+54

We present an analysis of archival RXTE and BeppoSAX data of the X-ray source 4U2206+54 . For the first time, high energy data (> 30 kev) are analyzed for this source. The data are well described by comptonization models (CompTT and BMC) in which seed photons with temperatures between 1.1 kev and 1.5 kev are comptonized by a hot plasma at 50 kev thereby producing a hard tail which extends up to, at least, 100 kev. We offer a new method of identification of neutron star systems using a temperature - luminosity relation. If a given X-ray source is characterized by a low bolometric luminosity and a relatively high color blackbody temperature (>1 kev) it has necessarily to be a neutron star rather than a black hole. From these arguments it is shown that the area of the soft photon source must be small (r ~ 1 km) and that the accretion disk, if present, must be truncated very far from the compact object. Here we report on the possible existence of a cyclotron line around 30 kev. The presence of a neutron star in the system is strongly favored by the available data.Comment: Accepted for publication in A&A. 9 pages, 7 figures. Submitted to journal in November 200

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 $\sim$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 $\pm$ 2.9) $\times$ 10$^{40}$ ergs in the 2.5--25 keV energy range, with its maximum at $\sim$1.9 $\times$ 10$^{41}$ ergs. The flare peak luminosity reaches up to (2.1 $\pm$ 0.2) $\times$ 10$^{39}$ ergs s$^{-1}$, 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

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

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$\alpha$ 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