Constraining the equation of state of supra-nuclear dense matter from XMM-Newton observations of neutron stars in globular clusters

We report on the detailed modelling of the X-ray spectra of three likely neutron stars. The neutron stars, observed with XMM-Newton are found in three quiescent X-ray binaries in the globular clusters: omega Cen, M 13 and NGC 2808. Whether they are accreting at very low rates or radiating energy from an accretion heated core, their X-ray spectra are expected to be those of a hydrogen atmosphere. We use and compare publicly available hydrogen atmosphere models, with constant and varying surface gravities to constrain the masses and radii of the neutron stars. Thanks to the high XMM-Newton throughput, and the accurate distances available for these clusters, using the latest science analysis software release and calibration of the XMM-Newton EPIC cameras, we derive the most stringent constraints on the masses and radii of the neutron stars obtained to date from these systems. A comparison of the models indicate that previously used hydrogen atmosphere models (assuming constant surface gravity) tend to underestimate the mass and overestimate the radius of neutron stars. Our data constrain the allowed equations of state to those which concern normal nucleonic matter and one possible strange quark matter model, thus constraining radii to be from 8 km and masses up to 2.4 M$_\odot$.Comment: 10 pages, 8 figures, accepted to be published in The Astrophysical Journa

Radii and Binding Energies of Nuclei in the Alpha-Cluster Model

The alpha-cluster model is based on two assumptions that the proton-neutron pair interactions are responsible for adherence between alpha-clusters and that the NN-interaction in the alpha-clusters is isospin independent. It allows one to estimate the Coulomb energy and the short range inter-cluster bond energy in dependence on the number of clusters. The charge radii are calculated on the number of alpha-clusters too. Unlike the Weizsacker formula in this model the binding energies of alpha-clusters and excess neutrons are estimated separately. The calculated values are in a good agreement with the experimental data.Comment: Latex2e 2.09, 13 pages, 4 figure

RXTE Studies of X-ray Spectral Variations with Accretion Rate in 4U 1915-05

We present the results of detailed spectral studies of the ultra-compact low mass X-ray binary (LMXB) 4U 1915-05 carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996. 4U 1915-05 is an X-ray burster (XRB) known to exhibit a ~199-day modulation in its 2--12 keV flux. Observations were performed with the PCA and HEXTE instruments on RXTE at roughly one-month intervals to sample this long-term period and study accretion rate-related spectral changes. We obtain good fits with a model consisting of a blackbody and an exponentially cut-off power law. The spectral parameters are strongly correlated with both the broad-band (2--50 keV) luminosity and the position in the color-color diagram, with the source moving from a low hard state to a high soft state as the accretion rate increases. The blackbody component appears to drive the spectral evolution. Our results are consistent with a geometry in which the soft component arises from an optically thick boundary layer and the hard component from an extended Comptonizing corona. Comparing our results with those of a similar study of the brighter source 4U 1820-30 (Bloser et al. 2000), we find that the two ultra-compact LMXBs occupy similar spectral states even though the transitions occur at very different total luminosities.Comment: 27 pages LaTeX, 8 figures, accepted to the Astrophysical Journa

ASCA Observations of GX 354-0 and KS 1731-260

We report on ASCA observations of the low mass X-ray binaries GX 354-0 and KS 1731-260. The spectrum of GX 354-0 is best described as a power-law or a Comptonized spectrum with tau ~ 5 and kT ~ 8 keV and a residual at ~6.5 keV. The residual may be a disk reflection or a Compton broadened Gaussian line from the hot inner ADAF-like coronal region. The absorption column density to the source is 2.9e22 cm^-2. No soft thermal component was detected. The spectrum from KS 1731-260 is softer and it is best fit with a two component model with a column density of 1.1e22 cm^-2. The likely interpretation is emission from a Comptonizing cloud with an optical depth tau>12 and either a neutron star or a disk blackbody emission. We discuss the likely location of the Comptonizing cloud for both sources within the context of several proposed emission models.Comment: Accepted for publication in the Astrophysical Journa

Morphological analysis on the coherence of kHz QPOs

We take the recently published data of twin kHz quasi-period oscillations (QPOs) in neutron star (NS) lowmass X-ray binaries (LMXBs) as the samples, and investigate the morphology of the samples, which focuses on the quality factor, peak frequency of kHz QPOs, and try to infer their physical mechanism. We notice that: (1) The quality factors of upper kHz QPOs are low (2 ~ 20 in general) and increase with the kHz QPO peak frequencies for both Z and Atoll sources. (2) The distribution of quality factor versus frequency for the lower kHz QPOs are quite different between Z and Atoll sources. For most Z source samples, the quality factors of lower kHz QPOs are low (usually lower than 15) and rise steadily with the peak frequencies except for Sco X-1, which drop abruptly at the frequency of about 750 Hz. While for most Atoll sources, the quality factors of lower kHz QPOs are very high (from 2 to 200) and usually have a rising part, a maximum and an abrupt drop. (3) There are three Atoll sources (4U 1728-34, 4U 1636-53 and 4U 1608-52) of displaying very high quality factors for lower kHz QPOs. These three sources have been detected with the spin frequencies and sidebands, in which the source with higher spin frequency presents higher quality factor of lower kHz QPOs and lower difference between sideband frequency and lower kHz QPO frequency.Comment: 8 pages, 8 figures, publishe

Effect of the Equivalence Between Topological and Electric Charge on the Magnetization of the Hall Ferromagnet

The dependence on temperature of the spin magnetization of a two-dimensional electron gas at filling factor unity is studied. Using classical Monte Carlo simulations we analyze the effect that the equivalence between topological and electrical charge has on the the behavior of the magnetization. We find that at intermediate temperatures the spin polarization increases in a thirty per cent due to the Hartree interaction between charge fluctuations.Comment: 4 pages. Submitted to Phys.Rev.

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

The correlations and anticorrelations in QPO data

Double peak kHz QPO frequencies in neutron star sources varies in time by a factor of hundreds Hz while in microquasar sources the frequencies are fixed and located at the line \nu_2 = 1.5 \nu_1 in the frequency-frequency plot. The crucial question in the theory of twin HFQPOs is whether or not those observed in neutron-star systems are essentially different from those observed in black holes. In black hole systems the twin HFQPOs are known to be in a 3:2 ratio for each source. At first sight, this seems not to be the case for neutron stars. For each individual neutron star, the upper and lower kHz QPO frequencies, \nu_2 and \nu_1, are linearly correlated, \nu_2=A \nu_1 + B, with the slope A < 1.5, i.e., the frequencies definitely are not in a 1.5 ratio. In this contribution we show that when considered jointly on a frequency-frequency plot, the data for the twin kHz QPO frequencies in several (as opposed to one) neutron stars uniquely pick out a certain preferred frequency ratio that is equal to 1.5 for the six sources examined so far.Comment: 3 pages, 1 figure, Astronomische Nachrichten, in pres

RXTE Studies of Long-Term X-ray Spectral Variations in 4U 1820-30

We present the results of detailed spectral studies of the ultra-compact low mass X-ray binary (LMXB) 4U 1820-30 carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996-7. 4U 1820-30 is an ``atoll'' source X-ray burster (XRB) located in the globular cluster NGC 6624. It is known to have an 11 minute binary period and a ~176 day modulation in its 2--12 keV flux. Observations were made with the PCA and HEXTE instruments on RXTE at roughly one-month intervals to sample this long-term period and study flux-related spectral changes. There are clear correlations between our fitted spectral parameters and both the broad-band (2--50 keV) flux and the position in the color-color diagram, as described by the parameter S_a introduced by Mendez et al. (1999). In addition, we find a strong correlation between the position in the color-color diagram and the frequencies of the kilohertz quasi-periodic oscillations (kHz QPOs) reported by Zhang et al. (1998). This lends further support to the notion that evidence for the last stable orbit in the accretion disk of 4U 1820-30 has been observed. For a model consisting of Comptonization of cool photons by hot electrons plus an additional blackbody component, we report an abrupt change in the spectral parameters at the same accretion rate at which the kHz QPOs disappear. For a model consisting of a multicolor disk blackbody plus a cut-off power law, we find that the inner disk radius reaches a minimum at the same accretion rate at which the kHz QPO frequency saturates, as expected if the disk reaches the last stable orbit. Both models face theoretical and observational problems when interpreted physically for this system.Comment: 39 pages, 11 figures, accepted to the Astrophysical Journa