A XMM-Newton observation during the 2000 outburst of SAX J1808.4-3658

I present a XMM-Newton observation of the accretion driven millisecond X-ray pulsar SAX J1808.4-3658 during its 2000 outburst. The source was conclusively detected, albeit at a level of only ~2 x 10^{32} erg/s. The source spectrum could be fitted with a power-law model (with a photon index of ~2.2), a neutron star atmosphere model (with a temperature of ~0.2 keV), or with a combination of a thermal (either a black-body or an atmosphere model) and a power-law component. During a XMM-Newton observation taken approximately one year later, the source was in quiescence and its luminosity was a factor of ~4 lower. It is possible that the source spectrum during the 2000 outburst was softer than its quiescent 2001 spectrum, however, the statistics of the data do not allow to make a firm conclusion. The results obtained are discussed in the context of the 2000 outburst of SAX J1808.4-3658 and the quiescent properties of the source.Comment: Accepted for publication in ApJ, 15 January 200

Coupled-Cluster Approach to Electron Correlations in the Two-Dimensional Hubbard Model

We have studied electron correlations in the doped two-dimensional (2D) Hubbard model by using the coupled-cluster method (CCM) to investigate whether or not the method can be applied to correct the independent particle approximations actually used in ab-initio band calculations. The double excitation version of the CCM, implemented using the approximate coupled pair (ACP) method, account for most of the correlation energies of the 2D Hubbard model in the weak ($U/t \simeq 1$) and the intermediate $U/t$ regions ($U/t \simeq 4$). The error is always less than 1% there. The ACP approximation gets less accurate for large $U/t$ ($U/t \simeq 8$) and/or near half-filling. Further incorporation of electron correlation effects is necessary in this region. The accuracy does not depend on the system size and the gap between the lowest unoccupied level and the highest occupied level due to the finite size effect. Hence, the CCM may be favorably applied to ab-initio band calculations on metals as well as semiconductors and insulators.Comment: RevTeX3.0, 4 pages, 4 figure

Quiescent Thermal Emission from the Neutron Star in Aql X-1

We report on the quiescent spectrum measured with Chandra/ACIS-S of the transient, type-I X-ray bursting neutron star Aql X-1, immediately following an accretion outburst. The neutron star radius, assuming a pure hydrogen atmosphere and hard power-law spectrum, is $R_\infty$=13.4{+5}{-4} (d/5 \kpc) km. Based on the historical outburst record of RXTE/ASM, the quiescent luminosity is consistent with that predicted by Brown, Bildsten and Rutledge from deep crustal heating, lending support to this theory for providing a minimum quiescent luminosity of transient neutron stars. While not required by the data, the hard power-law component can account for 18+/-8% of the 0.5-10 keV thermal flux. Short-timescale intensity variability during this observation is less than 15% rms (3 sigma; 0.0001-1 Hz, 0.2-8 keV). Comparison between the Chandra spectrum and three X-ray spectral observations made between Oct 1992 and Oct 1996 find all spectra consistent with a pure H atmosphere, but with temperatures ranging from 145--168 eV, spanning a factor of 1.87+/-0.21 in observed flux. The source of variability in the quiescent luminosity on long timescales (greater than years) remains a puzzle. If from accretion, then it remains to be explained why the quiescent accretion rate provides a luminosity so nearly equal to that from deep crustal heating.Comment: 15 pages, 1 figure, 2 tables; ApJ, accepte

An Accretion Model for Anomalous X-Ray Pulsars

We present a model for the anomalous X-ray pulsars (AXPs) in which the emission is powered by accretion from a fossil disk, established from matter falling back onto the neutron star following its birth. The time-dependent accretion drives the neutron star towards a ``tracking'' solution in which the rotation period of the star increases slowly, in tandem with the declining accretion rate. For appropriate choices of disk mass, neutron star magnetic field strength and initial spin period, we demonstrate that a rapidly rotating neutron star can be spun down to periods characteristic of AXPs on timescales comparable to the estimated ages of these sources. In other cases, accretion onto the neutron star switches off after a short time, and the star becomes an ordinary radio pulsar. Thus, in our picture, radio pulsars and AXPs are drawn from the same underlying population, in contrast to models involving neutron stars with ultrastrong magnetic fields, which require a new population of stars with very different properties.Comment: 15 pages and 3 Postscript figure

Crustal Heating and Quiescent Emission from Transiently Accreting Neutron Stars

Nuclear reactions occurring deep in the crust of a transiently accreting neutron star efficiently maintain the core at a temperature >5e7 K. When accretion halts, the envelope relaxes to a thermal equilibrium set by the flux from the hot core, as if the neutron star were newly born. For the time-averaged accretion rates typical of low-mass X-ray transients, standard neutrino cooling is unimportant and the core thermally re-radiates the deposited heat. The resulting luminosity has the same magnitude as that observed from several transient neutron stars in quiescence. Confirmation of this mechanism would strongly constrain rapid neutrino cooling mechanisms for neutron stars. Thermal emission had previously been dismissed as a predominant source of quiescent emission since blackbody spectral fits implied an emitting area much smaller than a neutron star's surface. However, as with thermal emission from radio pulsars, fits with realistic emergent spectra will imply a substantially larger emitting area. Other emission mechanisms, such as accretion or a pulsar shock, can also operate in quiescence and generate intensity and spectral variations over short timescales. Indeed, quiescent accretion may produce gravitationally redshifted metal photoionization edges in the quiescent spectra (detectable with AXAF and XMM). We discuss past observations of Aql~X-1 and note that the low luminosity X-ray sources in globular clusters and the Be star/X-ray transients are excellent candidates for future study.Comment: 5 pages, 2 ps figures, uses AASTEX macros. To appear in ApJ letters, 10 September 1998. Revised to conform with journal; minor numerical correction

A Chandra observation of the long-duration X-ray transient KS 1731-260 in quiescence: too cold a neutron star?

After more than a decade of actively accreting at about a tenth of the Eddington critical mass accretion rate, the neutron-star X-ray transient KS 1731-260 returned to quiescence in early 2001. We present a Chandra/ACIS-S observation taken several months after this transition. We detected the source at an unabsorbed flux of ~2 x 10^{-13} erg/cm^2/s (0.5-10 keV). For a distance of 7 kpc, this results in a 0.5-10 keV luminosity of ~1 x 10^{33} erg/s and a bolometric luminosity approximately twice that. This quiescent luminosity is very similar to that of the other quiescent neutron star systems. However, if this luminosity is due to the cooling of the neutron star, this low luminosity may indicate that the source spends at least several hundreds of years in quiescence in between outbursts for the neutron star to cool. If true, then it might be the first such X-ray transient to be identified and a class of hundreds of similar systems may be present in the Galaxy. Alternatively, enhanced neutrino cooling could occur in the core of the neutron star which would cool the star more rapidly. However, in that case the neutron star in KS 1731-260 would be more massive than those in the prototypical neutron star transients (e.g., Aql X-1 or 4U 1608-52).Comment: Accepted for publicaton in ApJ letters, 13 September 200

The Burst Spectra of EXO 0748-676 during a Long 2003 XMM-Newton Observation

Gravitationally redshifted absorption lines from highly ionized iron have been previously identified in the burst spectra of the neutron star in EXO 0748-676. To repeat this detection we obtained a long, nearly 600 ks observation of the source with XMM-Newton in 2003. The spectral features seen in the burst spectra from the initial data are not reproduced in the burst spectra from this new data. In this paper we present the spectra from the 2003 observations and discuss the sensitivity of the absorption structure to changes in the photospheric conditions.Comment: 18 Pages, 3 Figures. Accepted for publication in Ap

Advection-Dominated Accretion and Black Hole Event Horizons

The defining characteristic of a black hole is that it possesses an event horizon through which matter and energy can fall in but from which nothing escapes. Soft X-ray transients (SXTs), a class of X-ray binaries, appear to confirm this fundamental property of black holes. SXTs that are thought to contain accreting black holes display a large variation of luminosity between their bright and faint states, while SXTs with accreting neutron stars have a smaller variation. This difference is predicted if the former stars have horizons and the latter have normal surfaces.Comment: 11 pages, including 2 tables and 2 figures. To appear in The Astrophysical Journal Letter

Spectroscopic Analysis of an EIT Wave/Dimming Observed by Hinode/EIS

EIT waves are a wavelike phenomenon propagating outward from the coronal mass ejection (CME) source region, with expanding dimmings following behind. We present a spectroscopic study of an EIT wave/dimming event observed by Hinode/EIS. Although the identification of the wave front is somewhat affected by the pre-existing loop structures, the expanding dimming is well defined. We investigate the line intensity, width, and Doppler velocity for 4 EUV lines. In addition to the significant blue shift implying plasma outflows in the dimming region as revealed in previous studies, we find that the widths of all the 4 spectral lines increase at the outer edge of the dimmings. We illustrate that this feature can be well explained by the field line stretching model, which claims that EIT waves are apparently moving brightenings that are generated by the successive stretching of the closed field lines.Comment: 11 pages, 7 figure

Evolution of shocks and turbulence in major cluster mergers

We performed a set of cosmological simulations of major mergers in galaxy clusters to study the evolution of merger shocks and the subsequent injection of turbulence in the post-shock region and in the intra-cluster medium (ICM). The computations were done with the grid-based, adaptive mesh refinement hydro code Enzo, using an especially designed refinement criteria for refining turbulent flows in the vicinity of shocks. A substantial amount of turbulence energy is injected in the ICM due to major merger. Our simulations show that the shock launched after a major merger develops an ellipsoidal shape and gets broken by the interaction with the filamentary cosmic web around the merging cluster. The size of the post-shock region along the direction of shock propagation is about 300 kpc h^-1, and the turbulent velocity dispersion in this region is larger than 100 km s^-1. Scaling analysis of the turbulence energy with the cluster mass within our cluster sample is consistent with M^(5/3), i.e. the scaling law for the thermal energy in the self-similar cluster model. This clearly indicates the close relation between virialization and injection of turbulence in the cluster evolution. We found that the ratio of the turbulent to total pressure in the cluster core within 2 Gyr after the major merger is larger than 10%, and it takes about 4 Gyr to get relaxed, which is substantially longer than typically assumed in the turbulent re-acceleration models, invoked to explain the statistics of observed radio halos. Striking similarities in the morphology and other physical parameters between our simulations and the "symmetrical radio relics" found at the periphery of the merging cluster A3376 are finally discussed. In particular, the interaction between the merger shock and the filaments surrounding the cluster could explain the presence of "notch-like" features at the edges of the double relics.Comment: 16 pages, 19 figures, Published in Astrophysical Journal (online) and printed version will be published on 1st January, 201