* Astronomy

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.

Read more (54kb, PDF)

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.

Read more (106kb, PDF)

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.

Read more (76kb, PDF)

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.

Read more (637kb, PDF)