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Post Info TOPIC: LMXBs


L

Posts: 131433
Date:
Low Mass X-ray Binary
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NuSTAR Helps Solve 'Rapid Burster' Mystery

Scientists observing a neutron star in the "Rapid Burster" system may have solved a 40-year-old mystery surrounding its puzzling X-ray bursts.
Discovered in the 1970s, the Rapid Burster is a binary system comprising a low-mass star in its prime and a neutron star -- the compact remnant of a massive star's demise. The gravitational pull of the neutron star strips its companion of some of its gas, which then forms an accretion disk and spirals toward the neutron star.

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L

Posts: 131433
Date:
LMXBs
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Title: Cosmic recycling of millisecond pulsars
Authors: Wynn C. G. Ho, Thomas J. Maccarone, Nils Andersson (University of Southampton)

We compare the rotation rate of neutron stars in low-mass X-ray binaries (LMXBs) with the orbital period of the binaries. We find that, while short orbital period LMXBs span a range of neutron star rotation rates, all the long period LMXBs have fast rotators. We also find that the rotation rates are highest for the systems with the highest mean mass accretion rates, as can be expected if the accretion rate correlates with the orbital period. We show that these properties can be understood by a balance between spin-up due to accretion and spin-down due to gravitational radiation. Our scenario indicates that the gravitational radiation emitted by these systems may be detectable by future ground-based gravitational wave detectors.

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Posts: 131433
Date:
EXO 0748-676
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Title: The distance and internal composition of the neutron star in EXO 0748-676 with XMM-Newton
Authors: Guobao Zhang (1), Mariano Mendez (1), Peter Jonker (2), Beike Hiemstra (1) ((1) Groningen, (2) SRON)

Recently, the neutron star X-ray binary EXO 0748-676 underwent a transition to quiescence. We analysed an XMM-Newton observation of this source in quiescence, where we fitted the spectrum with two different neutron-star atmosphere models. From the fits we constrained the allowed parameter space in the mass-radius diagram for this source for an assumed range of distances to the system. Comparing the results with different neutron-star equations of state, we constrained the distance to EXO 0748-676. We found that the EOS model 'SQM1' is rejected by the atmosphere model fits for the known distance, and the 'AP3' and 'MS1' is fully consistent with the known distance.

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L

Posts: 131433
Date:
EXO0748-676
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Title: Eclipse Timings of the Transient Low Mass X-ray Binary EXO0748-676. IV. The Rossi X-Ray Timing Explorer Eclipses
Authors: Michael T. Wolff, Paul S. Ray, Kent S. Wood (NRL), Paul L. Hertz (NASA)
(Version v2)

We report our complete database of X-ray eclipse timings of the low mass X-ray binary EXO0748-676 observed by the Rossi X-Ray Timing Explorer (RXTE) satellite. As of this writing we have accumulated 443 full X-ray eclipses, 392 of which have been observed with the Proportional Counter Array on RXTE. These include both observations where an eclipse was specifically targeted and those eclipses found in the RXTE data archive. Eclipse cycle count has been maintained since the discovery of the EXO0748-676 system in February 1985. We describe our observing and analysis techniques for each eclipse and describe improvements we have made since the last compilation by Wolff et al. (2002). The principal result of this paper is the database containing the timing results from a seven-parameter fit to the X-ray light curve for each observed eclipse along with the associated errors in the fitted parameters. Based on the standard O-C analysis, EXO0748-676 has undergone four distinct orbital period epochs since its discovery. In addition, EXO0748-676 shows small-scale events in the O-C curve that are likely due to short-lived changes in the secondary star.

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L

Posts: 131433
Date:
4U 1624-490
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Title: The accretion disk corona and disk atmosphere of 4U 1624-490 as viewed by the Chandra-HETGS
Authors: Jingen Xiang, Julia C. Lee, Michael A. Nowak, Jörn Wilms, Norbert S. Schulz

We present a detailed spectral study (photoionisation modelling and variability) of the "Big Dipper" 4U 1624-490 based on a chandra-High Energy Transmission Gratings Spectrometer (HETGS) observation over the ~76 ks binary orbit of 4U 1624-490. While the continuum spectrum can be modelled using a blackbody plus power-law, a slightly better fit is obtained using a single \Gamma=2.25 power-law partially (71%) covered by a local absorber of column density N_{
m H, Local}=8.1_{-0.6}^{+0.7} x 10^{22}
m cm^{-2}. The data show a possible quasi-sinusoidal modulation with period 43_{-9}^{+13} ks that might be due to changes in local obscuration. Photoionisation modelling with the {\sc xstar} code and variability studies of the observed strong \ion{Fe}{25} and \ion{Fe}{26} absorption lines point to a two-temperature plasma for their origin: a highly ionised component of ionisation parameter \xi_{
m hot} \approx 10^{4.3} {
m ergs cm s^{-1}} (T ~ 3.0 x 10^{6} K) associated with an extended accretion disk corona of radius R ~3 x 10^{10} cm, and a less ionised more variable component of \xi \approx 10^{3.4} {
m ergs cm s^{-1}} (T ~ 1.0 x 10^{6} K) and
m \xi \approx 10^{3.1} ergs cm s^{-1} (T ~ 0.9 x 10^{6} K) coincident with the accretion disk rim. We use this, with the observed \ion{Fe}{25} and \ion{Fe}{26} absorption line variations (in wavelength, strength, and width) to construct a viewing geometry that is mapped to changes in plasma conditions over the 4U 1624-490 orbital period.

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L

Posts: 131433
Date:
LMXBs
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Title: A study of the low-mass X-ray binary dip sources XB 1916-053, XB 1323-619, X 1624-490 and 4U 1746-371 observed with INTEGRAL
Authors: Solen Balman (METU)

We detect dipping activity/modulations in the light curve of the four LMXBs in the 3--10 keV and 20--40 keV energy ranges. The spectral parameters derived from the fits to the INTEGRAL data are consistent with hot coronal structures in these systems where we find a range of plasma temperatures 3.0--224.9 keV. The unabsorbed X-ray to soft Gamma-ray flux between 4--200 keV are 5.9 x 10^{-10} erg s^{-1} cm^{-2} for XB 1916-053, 3.3 x 10^{-10} erg s^{-1} cm^{-2} for XB 1323-619, 21.6 x 10^{-10} erg s^{-1} cm^{-2} for X 1624-490 and 11.0 x 10^{-10} erg s^{-1} cm^{-2} for 4U 1746-371. The optical depth to Compton scattering, \tau, varies in a range 4.4--0.002 consistent with electron densities n_e < 1.4 x 10^{15} cm^{-3}. In general, we find no significant difference in the dip and non-dip spectra in the ISGRI energy range (above 20 keV) for all the four sources. We only detect absorption differences between dipping and non-dipping intervals for XB 1916-053 and X 1624-490 in the JEM-X energy range. Fits in the 4--200 keV range including an additional photo-ionised absorber model for the two sources show that XB 1916-053 has the highest ionised absorber among the two.

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L

Posts: 131433
Date:
EXO0748-676
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Title: Eclipse Timings of the Transient Low Mass X-ray Binary EXO0748-676. IV. The Rossi X-Ray Timing Explorer Eclipses
Authors: Michael T. Wolff, Paul S. Ray, Kent S. Wood (NRL), Paul L. Hertz (NASA)

We report our complete database of X-ray eclipse timings of the low mass X-ray binary EXO0748-676 observed by the Rossi X-Ray Timing Explorer (RXTE) satellite. As of this writing we have accumulated 441 full X-ray eclipses, 390 of which have been observed with the Proportional Counter Array on RXTE. These include both observations where an eclipse was specifically targeted and those eclipses found in the RXTE data archive. Eclipse cycle count has been maintained since the discovery of the EXO0748-676 system in February 1985. We describe our observing and analysis techniques for each eclipse and describe improvements we have made since the last compilation by Wolff et al. (2002). The principal result of this paper is the database containing the timing results from a seven-parameter fit to the X-ray light curve for each observed eclipse along with the associated errors in the fitted parameters. Based on the standard O-C analysis, EXO0748-676 has undergone four distinct orbital period epochs since its discovery. In addition, EXO0748-676 shows small-scale events in the O-C curve that are likely due to short-lived changes in the secondary star.

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L

Posts: 131433
Date:
Super-Orbital X-ray Binaries
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Title: A Swift BAT Look at Super-Orbital X-ray Binaries
Authors: Sean A. Farrell, Didier Barret, Gerald Skinner

We present the results of a study with the Swift Burst Alert Telescope in the 14 - 195 keV range of the long-term variability of 5 low mass X-ray binaries with reported or suspected super-orbital periods -- 4U 1636-536, 4U 1820-303, 4U 1916-053, Cyg X-2 and Sco X-1. No significant periodic modulation was detected around the previously reported values in the 4U 1916-053, Cyg X-2 or Sco X-1 light curves. The ~170 d period of 4U 1820-303 was detected up to 24 keV, consistent with the proposed triple system model. The ~46 d period in 4U 1636-536 was detected up to 100 keV, clearly inconsistent with variable photoelectric absorption via a warped precessing disc. We speculate that the appearance of this modulation after 4U 1636-536 entered the low/hard state indicates that this variability could be linked to jet precession such as observed in SS 433.

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L

Posts: 131433
Date:
RE: LMXBs
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Title: Deep Chandra X-ray Observations of Low Mass X-ray Binary Candidates in the Early-Type Galaxy NGC 4697
Authors: Gregory R. Sivakoff (1,2), Andrés Jordán (3,4), Adrienne M. Juett (5), Craig L. Sarazin (1), Jimmy A. Irwin (6) ((1) University of Virginia, (2) The Ohio State University, (3) Harvard-Smithsonian Centre for Astrophysics, (4) Departamento de Astronomía y Astrofísica, (5) NASA Goddard Space Flight Centre, (6) University of Michigan)

Chandra X-ray observations routinely resolve tens to hundreds of low-mass X-ray binaries (LMXBs) per galaxy in nearby massive early-type galaxies. These studies have raised important issues regarding the behaviour of this population of remnants of the once massive stars in early-type galaxies, namely the connection between LMXBs and globular clusters (GCs) and the nature of the LMXB luminosity function (LF). In this paper, we combine five epochs of Chandra observations and one central field Hubble Space Telescope Advance Camera for Surveys observation of NGC 4697, one of the nearest, optically luminous elliptical (E6) galaxies, to probe the GC-LMXB connection and LMXB-LF down to a detection/completeness limit of (0.6/1.4) x 10^{37} ergs/s. We detect 158 sources, present their luminosities and hardness ratios, and associate 34 LMXBs with GCs. We confirm that GCs with higher encounter rates (\Gamma_h) and redder colours (higher metallicity Z) are more likely to contain GCs, and find that the expected number of LMXBs per GC is proportional to \Gamma_h^{0.79+0.18/-0.15} Z^{0.50+0.20/-0.18}, consistent with fainter X-ray sources in Galactic GCs and LMXBs in Virgo early-type galaxies. Approximately 11±2% / 8 ±2% of GCs in NGC 4697 contain an LMXB at the detection/completeness limit. We propose that the larger proportion of metal-rich GCs in NGC 4697 compared to the Milky Way explains why these fractions are much higher than those of the Milky Way at similar luminosities. We confirm that a broken power-law is the best fit to the LMXB-LF, although we cannot rule out a cutoff power-law, and argue that this raises the possibility that there is no universal form for the LMXB-LF in early-type galaxies. We find marginal evidence for different LFs of LMXBs in GCs and the field and different spectra of GC-LMXBs and Field-LMXBs.

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Posts: 131433
Date:
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Title: Measurements of Variability of Low Mass X-ray Binary Candidates in the Early-Type Galaxy NGC 4697 from Multi-Epoch Chandra X-ray Observations
Authors: Gregory R. Sivakoff (1,2), Andrés Jord'an (3,4), Adrienne M. Juett (5), Craig L. Sarazin (1), Jimmy A. Irwin (6) ((1) University of Virginia, (2) The Ohio State University, (3) Harvard-Smithsonian Centre for Astrophysics, (4) Departamento de Astronomía y Astrofísica, (5) NASA Goddard Space Flight Center, (6) University of Michigan)

Multi-epoch Chandra X-ray observations of nearby massive early-type galaxies open up the study of an important regime of low-mass X-ray binary (LMXB) behaviour -- long term variability. In a companion paper, we report on the detection of 158 X-ray sources down to a detection/completeness limit of 0.6/1.4 x 10^{37} ergs/s using five Chandra observations of NGC 4697, one of the nearest (11.3 Mpc), optically luminous (M_B < -20), elliptical (E6) galaxy. In this paper, we report on the variability of LMXB candidates measured on timescales from seconds to years. At timescales of seconds to hours, we detect five sources with significant variability. Approximately 7% of sources show variability between any two observations, and 16±4% of sources do not have a constant luminosity over all five observations. Among variable sources, we identify eleven transient candidates, with which we estimate that if all LMXBs in NGC 4697 are long-term transients then they are on for ~ 100 yr and have a 7% duty cycle. These numbers are consistent with those found for brighter LMXBs in M87 and NGC 1399, which suggests that there does not appear to be a measurable difference between the outburst durations of long-term transient neutron star LMXBs and black hole LMXBs. We discuss in detail a transient supersoft source, whose properties are not easily explained by standard explanations for supersoft sources.

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