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At least 1,135 records · Page 63
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

THE QUIESCENT X-RAY PROPERTIES OF THE ACCRETING MILLISECOND X-RAY PULSAR AND ECLIPSING BINARY SWIFT J1749.4-2807

Swift J1749.4-2807 is a transient neutron star low-mass X-ray binary that contains an accreting millisecond X-ray pulsar spinning at 518 Hz. It is the first of its kind that displays X-ray eclipses, which holds significant promise to precisely constrain the mass of the neutron star. We report on a {approx_equal} 105 ks long XMM-Newton observation performed when Swift J1749.4-2807 was in quiescence. We detect the source at a 0.5-10 keV luminosity of {approx_equal}1 Multiplication-Sign 10{sup 33}(D/6.7 kpc){sup 2} erg s{sup -1}. The X-ray light curve displays three eclipses that are consistent in orbital phase and duration with the ephemeris derived during outburst. Unlike most quiescent neutron stars, the X-ray spectrum can be adequately described with a simple power law, while a pure-hydrogen atmosphere model does not fit the data. We place an upper limit on the 0.01-100 keV thermal luminosity of the cooling neutron star of {approx}< 2 Multiplication-Sign 10{sup 33} erg s{sup -1} and constrain its temperature to be {approx}< 0.1 keV (for an observer at infinity). Timing analysis does not reveal evidence for X-ray pulsations near the known spin frequency of the neutron star or its first overtone with a fractional rms of {approx}< 34% and {approx}< 28%, respectively. We discuss the implications of our findings for dynamical mass measurements, the thermal state of the neutron star, and the origin of the quiescent X-ray emission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ORBITAL DECAY AND EVIDENCE OF DISK FORMATION IN THE X-RAY BINARY PULSAR OAO 1657-415

OAO 1657-415 is an eclipsing X-ray binary wind-fed pulsar that has exhibited smooth spin-up/spin-down episodes and has undergone several torque reversals throughout its long history of observation. We present a frequency history spanning nearly 19 years of observations from the Burst and Transient Source Experiment and from the Gamma-Ray Burst Monitor (Fermi/GBM). Our analysis suggests two modes of accretion: one resulting in steady spin-up correlated with flux during which we believe a stable accretion disk is present and one in which the neutron star is spinning down at a lesser rate which is uncorrelated with flux. Orbital elements of the pulsar system are determined at several intervals throughout this history. With these ephemerides, statistically significant orbital decay with a P-dot {sub orb}=(-9.74{+-}0.78) Multiplication-Sign 10{sup -8} is established.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SELF-CONSISTENT MAGNETIC STELLAR EVOLUTION MODELS OF THE DETACHED, SOLAR-TYPE ECLIPSING BINARY EF AQUARII

We introduce a new one-dimensional stellar evolution code, based on the existing Dartmouth code, that self-consistently accounts for the presence of a globally pervasive magnetic field. The methods involved in perturbing the equations of stellar structure, the equation of state, and the mixing-length theory of convection are presented and discussed. As a first test of the code's viability, stellar evolution models are computed for the components of a solar-type, detached eclipsing binary (DEB) system, EF Aquarii, shown to exhibit large disagreements with stellar models. The addition of the magnetic perturbation corrects the radius and effective temperature discrepancies observed in EF Aquarii. Furthermore, the required magnetic field strength at the model photosphere is within a factor of two of the magnetic field strengths estimated from the stellar X-ray luminosities measured by ROSAT and those predicted from Ca II K line core emission. These models provide firm evidence that the suppression of thermal convection arising from the presence of a magnetic field is sufficient to significantly alter the structure of solar-type stars, producing noticeably inflated radii and cooler effective temperatures. The inclusion of magnetic effects within a stellar evolution model has a wide range of applications, from DEBs and exoplanet host stars to the donor stars of cataclysmic variables.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

INTEGRAL AND SWIFT OBSERVATIONS OF THE Be X-RAY BINARY 4U 1036-56 (RX J1037.5-5647) AND ITS POSSIBLE RELATION WITH {gamma}-RAY TRANSIENTS

We present timing, spectral, and long-term temporal analysis of the high-mass X-ray binary 4U 1036-56 using INTEGRAL and Swift observations. We show that it is a weak hard X-ray source spending a major fraction of the time in quiescence and only occasionally characterized by X-ray outbursts. The outburst activity we report here lasts several days, with a dynamic range spanned by the luminosity in quiescence and in outburst as high as {approx}30. We report the detection of pulse period at 854.75 {+-} 4.39 s during an outburst, which is consistent with previous measurements. Finally, we analyze the possibility of the association of 4U 1036-56 with the unidentified transient {gamma}-ray sources AGL J1037-5708 and GRO J1036-55, as prompted by its positional correlation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

INVERSE COMPTON SCATTERING MODEL FOR X-RAY EMISSION OF THE GAMMA-RAY BINARY LS 5039

We propose a model for the gamma-ray binary LS 5039 in which the X-ray emission is due to the inverse Compton (IC) process instead of the synchrotron radiation. Although the synchrotron model has been discussed in previous studies, it requires a strong magnetic field which leads to a severe suppression of the TeV gamma-ray flux in conflict with H.E.S.S. observations. In this paper, we calculate the IC emission by low energy electrons ({gamma}{sub e} {approx}< 10{sup 3}) in the Thomson regime. We find that IC emission of the low energy electrons can explain the X-ray flux and spectrum observed with Suzaku if the minimum Lorentz factor of injected electrons {gamma}{sub min} is around 10{sup 3}. In addition, we show that the Suzaku light curve is well reproduced if {gamma}{sub min} varies in proportion to the Fermi flux when the distribution function of injected electrons at higher energies is fixed. We conclude that the emission from LS 5039 is well explained by the model with the IC emission from electrons whose injection properties are dependent on the orbital phase. Since the X-ray flux is primarily determined by the total number of cooling electrons, this conclusion is rather robust, although some mismatches between the model and observations at the GeV band remain in the present formulation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE Be/X-RAY BINARY SWIFT J1626.6-5156 AS A VARIABLE CYCLOTRON LINE SOURCE

Swift J1626.6-5156 is a Be/X-ray binary that was in outburst from 2005 December until 2008 November. We have examined Rossi X-ray Timing Explorer/Proportional Counter Array (PCA) and High Energy X-ray Timing Explorer spectra of three long observations of this source taken early in its outburst, when the PCA 2-20 keV count rate was >70 counts s{sup -1} PCU{sup -1}, as well as several combined observations from different stages of the outburst. The spectra are best fit with an absorbed cutoff power law with a {approx}6.4 keV iron emission line and a Gaussian optical depth absorption line at {approx}10 keV. We present strong evidence that this absorption-like feature is a cyclotron resonance scattering feature, making Swift J1626.6-5156 a new candidate cyclotron line source. The redshifted energy of {approx}10 keV implies a magnetic field strength of {approx}8.6(1 + z) Multiplication-Sign 10{sup 11} G in the region of the accretion column close to the magnetic poles where the cyclotron line is produced. Analysis of phase-averaged spectra spanning the duration of the outburst suggests a possible positive correlation between the fundamental cyclotron energy and source luminosity. Phase-resolved spectroscopy from a long observation reveals a variable cyclotron line energy, with phase dependence similar to a variety of other pulsars, as well as the first harmonic of the fundamental cyclotron line.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

SWIFT X-RAY TELESCOPE TIMING OBSERVATIONS OF THE BLACK HOLE BINARY SWIFT J1753.5-0127: DISK-DILUTED FLUCTUATIONS IN THE OUTBURST PEAK

After a careful analysis of the instrumental effects on the Poisson noise to demonstrate the feasibility of detailed stochastic variability studies with the Swift X-Ray Telescope (XRT), we analyze the variability of the black hole X-ray binary SWIFT J1753.5-0127 in all XRT observations during 2005-2010. We present the evolution of the power spectral components along the outburst in two energy bands: soft (0.5-2 keV) and hard (2-10 keV), and in the hard band we find results consistent with those from the Rossi X-Ray Timing Explorer (RXTE). The advantage of the XRT is that we can also explore the soft band not covered by RXTE. The source has previously been suggested to host an accretion disk extending down to close to the black hole in the low hard state, and to show low-frequency variability in the soft-band intrinsic to this disk. Our results are consistent with this, with stronger low-frequency variability at low intensities in the soft than in the hard band. From our analysis, we are able to present the first measurements of the soft-band variability in the peak of the outburst. We find the soft band to be less variable than the hard band, especially at high frequencies, opposite to what is seen at low intensity. Both results can be explained within the framework of a simple two emission-region model where the hot flow is more variable in the peak of the outburst and the disk is more variable at low intensities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

GALACTIC ULTRACOMPACT X-RAY BINARIES: DISK STABILITY AND EVOLUTION

We study the mass-transfer rates and disk stability conditions of ultracompact X-ray binaries (UCXBs) using empirical time-averaged X-ray luminosities from Paper I and compiled information from the literature. The majority of UCXBs are consistent with evolutionary tracks for white dwarf donors. Three UCXBs with orbital periods longer than 40 minutes have mass-transfer rates above 10{sup -10} M{sub Sun} yr{sup -1}, inconsistent with white dwarf donor tracks. We show that if helium star donors can retain their initial high entropy, they can explain the observed mass-transfer rates of these UCXBs. Several UCXBs show persistent luminosities apparently below the disk instability limit for irradiated He accretion disks. We point out that a predominantly C and/or O disk (as observed in the optical spectra of several) lowers the disk instability limit, explaining this disagreement. The orbital period and low time-averaged mass-transfer rate of 2S 0918-549 provide evidence that the donor star is a low-entropy C/O white dwarf, consistent with optical spectra. We combine existing information to constrain the masses of the donors in 4U 1916-053 (0.064 {+-} 0.010 M{sub Sun }) and 4U 1626-67 (<0.036 M{sub Sun} for a 1.4 M{sub Sun} neutron star). We show that 4U 1626-67 is indeed persistent, and not undergoing a transient outburst, leaving He star models as the best explanation for the donor.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Identification of five interacting binaries in the galactic bulge survey

We present optical light curves, spectroscopy, and classification of five X-ray sources in the Chandra Galactic Bulge Survey (CXOGBS J174009.1–284725 (CX5), CXOGBS J173935.7–272935 (CX18), CXOGBS J173946.9–271809 (CX28), CXOGBS J173729.1–292804 (CX37), CXOGBS J174607.6–261547 (CX561)). These objects were selected based on bright optical counterparts which were quickly found to have emission lines in their optical spectra. This paper presents an illustration of GBS optical follow-up, targeting emission line objects. Of these five objects, four exhibit photometric variability in the Sloan r' band. CX5 shows a tentative period of 2.1 hr and is clearly an intermediate polar (IP). CX28 and CX37 both exhibit flickering with no clear period. Both are also suggested to be IPs. CX18 was observed to undergo two dwarf nova outbursts. Finally, CX561 shows no detectable variability, although its characteristics would be consistent with either a quiescent low-mass X-ray binary or cataclysmic variable.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE AGES OF HIGH-MASS X-RAY BINARIES IN NGC 2403 AND NGC 300

We have examined resolved stellar photometry from HST imaging surrounding 18 high-mass X-ray binary (HMXB) candidates in NGC 300 and NGC 2403 as determined from combined Chandra/HST analysis. We have fit the color-magnitude distribution of the surrounding stars with stellar evolution models. All but one region in NGC 300 and two in NGC 2403 contain a population with an age between 20 and 70 Myr. One of the candidates is the ultraluminous X-ray source in NGC 2403, which we associate with a 60 {+-} 5 Myr old population. These age distributions provide additional evidence that 16 of these 18 candidates are HMXBs. Furthermore, our results suggest that the most common HMXB age in these galaxies is 40-55 Myr. This preferred age is similar to observations of HMXBs in the Small Magellanic Cloud, providing new evidence of this formation timescale, but in higher metallicity populations. We suggest that this preferred HMXB age is the result of the fortuitous combination of two physical effects. First, this is the age of a population when the greatest rate of core-collapse events should be occurring, maximizing neutron star production. Second, this is the age when B stars are most likely to be actively losing mass. We also discuss our results in the context of HMXB feedback in galaxies, confirming HMXBs as a potentially important source of energy for the interstellar medium in low-mass galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

MEASUREMENT OF THE RADIUS OF NEUTRON STARS WITH HIGH SIGNAL-TO-NOISE QUIESCENT LOW-MASS X-RAY BINARIES IN GLOBULAR CLUSTERS

This paper presents the measurement of the neutron star (NS) radius using the thermal spectra from quiescent low-mass X-ray binaries (qLMXBs) inside globular clusters (GCs). Recent observations of NSs have presented evidence that cold ultra dense matter-present in the core of NSs-is best described by ''normal matter'' equations of state (EoSs). Such EoSs predict that the radii of NSs, R{sub NS}, are quasi-constant (within measurement errors, of {approx}10%) for astrophysically relevant masses (M{sub NS}>0.5 M{sub Sun }). The present work adopts this theoretical prediction as an assumption, and uses it to constrain a single R{sub NS} value from five qLMXB targets with available high signal-to-noise X-ray spectroscopic data. Employing a Markov chain Monte-Carlo approach, we produce the marginalized posterior distribution for R{sub NS}, constrained to be the same value for all five NSs in the sample. An effort was made to include all quantifiable sources of uncertainty into the uncertainty of the quoted radius measurement. These include the uncertainties in the distances to the GCs, the uncertainties due to the Galactic absorption in the direction of the GCs, and the possibility of a hard power-law spectral component for count excesses at high photon energy, which are observed in some qLMXBs in the Galactic plane. Using conservative assumptions, we found that the radius, common to the five qLMXBs and constant for a wide range of masses, lies in the low range of possible NS radii, R{sub NS}=9.1{sup +1.3}{sub -1.5} km (90%-confidence). Such a value is consistent with low-R{sub NS} equations of state. We compare this result with previous radius measurements of NSs from various analyses of different types of systems. In addition, we compare the spectral analyses of individual qLMXBs to previous works.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

MASSES AND DISTANCE OF THE YOUNG BINARY NTTS 045251+3016

As part of our continuing campaign to measure the masses of pre-main-sequence (PMS) stars dynamically and thus to assess the reliability of the discrepant theoretical calculations of contraction to the main sequence, we present new results for NTTS 045251+3016, a visual and double-lined spectroscopic binary in the Taurus star-forming region (SFR). We obtained new high angular resolution astrometry and high spectral resolution spectroscopy at Keck Observatory. The new data lead to a significant revision of previously published orbital parameters. In particular, we find that the masses of the primary and secondary are 0.86 {+-} 0.11 and 0.55 {+-} 0.05 M{sub Sun }, respectively, smaller than previously reported, and that the system lies 158.7 {+-} 3.9 pc from the sun, further than previously reported.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

UPPER BOUNDS ON r-MODE AMPLITUDES FROM OBSERVATIONS OF LOW-MASS X-RAY BINARY NEUTRON STARS

We present upper limits on the amplitude of r-mode oscillations and gravitational-radiation-induced spin-down rates in low-mass X-ray binary neutron stars, under the assumption that the quiescent neutron star luminosity is powered by dissipation from a steady-state r-mode. For masses <2 M{sub Sun} we find dimensionless r-mode amplitudes in the range from about 1 Multiplication-Sign 10{sup -8} to 1.5 Multiplication-Sign 10{sup -6}. For the accreting millisecond X-ray pulsar sources with known quiescent spin-down rates, these limits suggest that {approx}< 1% of the observed rate can be due to an unstable r-mode. Interestingly, the source with the highest amplitude limit, NGC 6440, could have an r-mode spin-down rate comparable to the observed, quiescent rate for SAX J1808-3658. Thus, quiescent spin-down measurements for this source would be particularly interesting. For all sources considered here, our amplitude limits suggest that gravitational wave signals are likely too weak for detection with Advanced LIGO. Our highest mass model (2.21 M{sub Sun }) can support enhanced, direct Urca neutrino emission in the core and thus can have higher r-mode amplitudes. Indeed, the inferred r-mode spin-down rates at these higher amplitudes are inconsistent with the observed spin-down rates for some of the sources, such as IGR J00291+5934 and XTE J1751-305. In the absence of other significant sources of internal heat, these results could be used to place an upper limit on the masses of these sources if they were made of hadronic matter, or alternatively it could be used to probe the existence of exotic matter in them if their masses were known.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

MODELING X-RAY BINARY EVOLUTION IN NORMAL GALAXIES: INSIGHTS FROM SINGS

We present the largest-scale comparison to date between observed extragalactic X-ray binary (XRB) populations and theoretical models of their production. We construct observational X-ray luminosity functions (oXLFs) using Chandra observations of 12 late-type galaxies from the Spitzer Infrared Nearby Galaxy Survey. For each galaxy, we obtain theoretical XLFs (tXLFs) by combining XRB synthetic models, constructed with the population synthesis code StarTrack, with observational star formation histories (SFHs). We identify highest-likelihood models both for individual galaxies and globally, averaged over the full galaxy sample. Individual tXLFs successfully reproduce about half of the oXLFs, but for some galaxies we are unable to find underlying source populations, indicating that galaxy SFHs and metallicities are not well matched and/or that XRB modeling requires calibration on larger observational samples. Given these limitations, we find that the best models are consistent with a product of common envelope ejection efficiency and central donor concentration {approx_equal} 0.1, and a 50% uniform-50% ''twins'' initial mass-ratio distribution. We present and discuss constituent subpopulations of tXLFs according to donor, accretor, and stellar population characteristics. The galaxy-wide X-ray luminosity due to low-mass and high-mass XRBs, estimated via our best global model tXLF, follows the general trend expected from the L{sub X} -star formation rate and L{sub X} -stellar mass relations of Lehmer et al. Our best models are also in agreement with modeling of the evolution of both XRBs over cosmic time and of the galaxy X-ray luminosity with redshift.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

MEASUREMENTS OF CYCLOTRON FEATURES AND PULSE PERIODS IN THE HIGH-MASS X-RAY BINARIES 4U 1538–522 AND 4U 1907+09 WITH THE INTERNATIONAL GAMMA-RAY ASTROPHYSICS LABORATORY

We present a spectral and timing analysis of International Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of two high-mass X-ray binaries, 4U 1538–522 and 4U 1907+09. Our timing measurements for 4U 1538–522 find the pulse period to have exhibited a spin-up trend until approximately 2009, after which there is evidence for a torque reversal, with the source beginning to spin down to the most recently measured period of 525.407 ± 0.001 s. The most recent INTEGRAL observations of 4U 1907+09 are not found to yield statistically significant pulse periods due to the significantly lower flux from the source compared with 4U 1538–522. A spectral model consisting of a power-law continuum with an exponential cutoff and modified by two cyclotron resonance scattering features is found to fit both sources well, with the cyclotron scattering features detected at ∼22 and ∼49 keV for 4U 1538–522 and at ∼18 and ∼36 keV for 4U 1907+09. The spectral parameters of 4U 1538–522 are generally not found to vary significantly with flux and there is little to no variation across the torque reversal. Examining our results in conjunction with previous work, we find no evidence for a correlation between cyclotron line energy and luminosity for 4U 1538–522. 4U 1907+09 shows evidence for a positive correlation between cyclotron line energy and luminosity, which would make it the fourth, and lowest luminosity, cyclotron line source to exhibit this relationship.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Multiwavelength observations of the TeV binary LS I +61° 303 with Veritas, Fermi-LAT, and Swift/xrt during a TeV outburst

We present the results of a multiwavelength observational campaign on the TeV binary system LS I +61° 303 with the VERITAS telescope array (>200 GeV), Fermi-LAT (0.3-300 GeV), and Swift/XRT (2-10 keV). The data were taken from 2011 December through 2012 January and show a strong detection in all three wavebands. During this period VERITAS obtained 24.9 hr of quality selected livetime data in which LS I +61° 303 was detected at a statistical significance of 11.9σ. These TeV observations show evidence for nightly variability in the TeV regime at a post-trial significance of 3.6σ. The combination of the simultaneously obtained TeV and X-ray fluxes do not demonstrate any evidence for a correlation between emission in the two bands. For the first time since the launch of the Fermi satellite in 2008, this TeV detection allows the construction of a detailed MeV-TeV spectral energy distribution from LS I +61° 303. This spectrum shows a distinct cutoff in emission near 4 GeV, with emission seen by the VERITAS observations following a simple power-law above 200 GeV. This feature in the spectrum of LS I +61° 303, obtained from overlapping observations with Fermi-LAT and VERITAS, may indicate that there are two distinct populations of accelerated particles producing the GeV and TeV emission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Wind-driven evolution of white dwarf binaries to type Ia supernovae

In the single-degenerate scenario for the progenitors of Type Ia supernovae (SNe Ia), a white dwarf rapidly accretes hydrogen- or helium-rich material from its companion star and appears as a supersoft X-ray source. This picture has been challenged by the properties of the supersoft X-ray sources with very low mass companions and the observations of several nearby SNe Ia. It has been pointed out that the X-ray radiation or the wind from the accreting white dwarf can excite winds or strip mass from the companion star, thus significantly influencing the mass transfer processes. In this paper, we perform detailed calculations of the wind-driven evolution of white dwarf binaries. We present the parameter space for the possible SN Ia progenitors and for the surviving companions after the SNe. The results show that the ex-companion stars of SNe Ia have characteristics more compatible with the observations, compared with those in the traditional single-degenerate scenario.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Discovery of the third transient X-ray binary in the galactic globular cluster Terzan 5

We report and study the outburst of a new transient X-ray binary (XRB) in Terzan 5, the third detected in this globular cluster, Swift J174805.3-244637 or Terzan 5 X-3. We find clear spectral hardening in Swift/XRT data during the outburst rise to the hard state, thanks to our early coverage (starting at L{sub X} ∼ 4 × 10{sup 34} erg s{sup –1}) of the outburst. This hardening appears to be due to the decline in relative strength of a soft thermal component from the surface of the neutron star (NS) during the rise. We identify a Type I X-ray burst in Swift/XRT data with a long (16 s) decay time, indicative of hydrogen burning on the surface of the NS. We use Swift/BAT, MAXI/GSC, Chandra/ACIS, and Swift/XRT data to study the spectral changes during the outburst, identifying a clear hard-to-soft state transition. We use a Chandra/ACIS observation during outburst to identify the transient's position. Seven archival Chandra/ACIS observations show evidence for variations in Terzan 5 X-3's nonthermal component but not the thermal component during quiescence. The inferred long-term time-averaged mass accretion rate, from the quiescent thermal luminosity, suggests that if this outburst is typical and only slow cooling processes are active in the NS core, such outbursts should recur every ∼10 yr.

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