Optical Modulation in the X-Ray Binary 4U 1543-624 Revisited

The X-ray binary 4U 1543$-$624 has been provisionally identified as an ultracompact system with an orbital period of $\simeq$18~min. We have carried out time-resolved optical imaging of the binary to verify the ultra-short orbital period. Using 140\,min of high-cadence $r'$-band photometry we recover the previously-seen sinusoidal modulation and determine a period $P=18.20\pm0.09$\,min. In addition, we also see a 7.0$\times 10^{-4}$\,mag\,min$^{-1}$ linear decay, likely related to variations in the source's accretion activity. Assuming that the sinusoidal modulation arises from X-ray heating of the inner face of the companion star, we estimate a distance of 6.0--6.7\,kpc and an inclination angle of 34$^{\circ}$--61$^{\circ}$ (90\% confidence) for the binary. Given the stability of the modulation we can confirm that the modulation is orbital in origin and 4U 1543$-$624 is an ultracompact X-ray binary.Comment: 6 pages, 3 figures, accepted for publication in Publications of the Astronomical Society of Australia (PASA

Spectral Softening Between Outburst and Quiescence In The Neutron Star Low-Mass X-Ray Binary SAX J1750.8-2900

Tracking the spectral evolution of transiently accreting neutron stars between outburst and quiescence probes relatively poorly understood accretion regimes. Such studies are challenging because they require frequent monitoring of sources with luminosities below the thresholds of current all-sky X-ray monitors. We present the analysis of over 30 observations of the neutron star low-mass X-ray binary SAX J1750.8-2900 taken across four years with the X-ray telescope aboard Swift. We find spectral softening with decreasing luminosity both on long ($\sim$1 year) and short ($\sim$days to week) timescales. As the luminosity decreases from $4\times10^{36}$ erg s$^{-1}$ to $\sim1\times10^{35}$ erg s$^{-1}$ (0.5-10 keV), the power law photon index increases from from 1.4 to 2.9. Although not statistically required, our spectral fits allow an additional soft component that displays a decreasing temperature as the luminosity decreases from $4 \times 10^{36}$ to $6 \times 10^{34}$ erg s$^{-1}$. Spectral softening exhibited by SAX J1750.8-2900 is consistent both with accretion emission whose spectral shape steepens with decreasing luminosity and also with being dominated by a changing soft component, possibly associated with accretion onto the neutron star surface, as the luminosity declines.Comment: 11 pages, 9 figures, 3 Tables. Resubmitted to The Astrophysical Journal after incorporating comments from the refere

A Debris Disk Around An Isolated Young Neutron Star

Pulsars are rotating, magnetized neutron stars that are born in supernova explosions following the collapse of the cores of massive stars. If some of the explosion ejecta fails to escape, it may fall back onto the neutron star or it may possess sufficient angular momentum to form a disk. Such 'fallback' is both a general prediction of current supernova models and, if the material pushes the neutron star over its stability limit, a possible mode of black hole formation. Fallback disks could dramatically affect the early evolution of pulsars, yet there are few observational constraints on whether significant fallback occurs or even the actual existence of such disks. Here we report the discovery of mid-infrared emission from a cool disk around an isolated young X-ray pulsar. The disk does not power the pulsar's X-ray emission but is passively illuminated by these X-rays. The estimated mass of the disk is of order 10 Earth masses, and its lifetime (at least a million years) significantly exceeds the spin-down age of the pulsar, supporting a supernova fallback origin. The disk resembles protoplanetary disks seen around ordinary young stars, suggesting the possibility of planet formation around young neutron stars.Comment: 5 pages, 3 figures. To appear in Nature (6 Apr 2006

Effect of hyperon-hyperon interaction on bulk viscosity and r-mode instability in neutron stars

We investigate the effect of hyperon matter including hyperon-hyperon interaction on bulk viscosity. Equations of state are constructed within the framework of a relativistic field theoretical model where baryon-baryon interaction is mediated by the exchange of scalar and vector mesons. Hyperon-hyperon interaction is also taken into account by the exchange of two strange mesons. This interaction results in a smaller maximum mass neutron star compared with the case without the interaction. The coefficient of bulk viscosity due to the non-leptonic weak process is determined by these equations of state. The interacting hyperon matter reduces the bulk viscosity coefficient in a neutron star interior compared with the no interaction case. The r-mode instability is more effectively suppressed in hyperon-hyperon interaction case than that without the interaction.Comment: 25 pages, 10 figures; two new figures added and results and discussion section revised; final version to appear in PR

X-Ray Spectroscopy of the Low-Mass X-ray Binaries 2S 0918-549 and 4U1543-624: Evidence for Neon-Rich Degenerate Donors

We present high-resolution spectroscopy of the neutron-star/low-mass X-ray binaries 2S 0918-549 and 4U 1543-624 with the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory and the Reflection Grating Spectrometer onboard XMM-Newton. Previous low-resolution spectra of both sources showed a broad line-like feature at 0.7 keV that was originally attributed to unresolved line emission. We recently showed that this feature could also be due to excess neutral Ne absorption, and this is confirmed by the new high-resolution Chandra spectra. The Chandra spectra are each well fit by an absorbed power-law + blackbody model with a modified Ne/O number ratio of 0.52+/-0.12 for 2S 0918-549 and 1.5+/-0.3 for 4U 1543-624, compared to the interstellar-medium value of 0.18. The XMM spectrum of 2S 0918-549 is best fit by an absorbed power-law model with a Ne/O number ratio of 0.46+/-0.03, consistent with the Chandra result. On the other hand, the XMM spectrum of 4U 1543-624 is softer and less luminous than the Chandra spectrum and has a best-fit Ne/O number ratio of 0.54+/-0.03. The difference between the measured abundances and the expected interstellar ratio, as well as the variation of the column densities of O and Ne in 4U 1543-624, supports the suggestion that there is absorption local to these binaries. We propose that the variations in the O and Ne column densities of 4U 1543-624 are caused by changes in the ionization structure of the local absorbing material. It is important to understand the effect of ionization on the measured absorption columns before the abundance of the local material can be determined. This work supports our earlier suggestion that 2S 0918-549 and 4U 1543-624 are ultracompact binaries with Ne-rich companions.Comment: 11 pages, 5 figures, major revisions including addition of XMM spectral analysis, accepted for publication in the Astrophysical Journal, vol. 59

Discovery of a 270 Hz X-Ray Burst Oscillation in the X-Ray Dipper 4U 1916-053

We report the discovery of a highly coherent oscillation in a type-I X-ray burst observed from 4U 1916-053 by the Rossi X-ray Timing Explorer (RXTE). The oscillation was most strongly detected approx. 1 s after the burst onset at a frequency of 269.3 Hz, and it increased in frequency over the following 4 seconds of the burst decay to a maximum of around 272 Hz. The total measured drift of 3.58 +/- 0.41 Hz (1 sigma) represents the largest fractional change in frequency (1.32 +/- 0.15 %) yet observed in any burst oscillation. If the asymptotic frequency of the oscillation is interpreted in terms of a decoupled surface burning layer, the implied neutron star spin period is around 3.7 ms. However, the expansion of the burning layer required to explain frequency drift during the burst is around 80 m, substantially larger than expected theoretically (assuming rigid rotation). The oscillation was not present in the persistent emission before the burst, nor in the initial rise. When detected its amplitude was 6-12% (RMS) with a roughly sinusoidal profile. The burst containing the oscillation showed no evidence for photospheric radius expansion, while at least 5 of the other 9 bursts observed from the source by RXTE during 1996 and 1998 did. No comparable oscillations were detected in the other bursts. A pair of kilohertz quasi-periodic oscillations (QPOs) has been previously reported from this source with a mean separation of 348 +/- 12 Hz. 4U 1916-053 is the first example of a source where the burst oscillation frequency is significantly smaller than the frequency separation of the kHz QPOs.Comment: 8 pages, 2 figures, 2 tables; accepted for ApJ Letter

Instability of Quark Matter Core in a Compact Newborn Neutron Star With Moderately Strong Magnetic Field

It is explicitly shown that if phase transition occurs at the core of a newborn neutron star with moderately strong magnetic field strength, which populates only the electron's Landau levels, then in the $\beta$-equilibrium condition, the quark core is energetically much more unstable than the neutron matter of identical physical condition.Comment: Six pages REVTEX file, one .eps file (included

Two Party Non-Local Games

In this work we have introduced two party games with respective winning conditions. One cannot win these games deterministically in the classical world if they are not allowed to communicate at any stage of the game. Interestingly we find out that in quantum world, these winning conditions can be achieved if the players share an entangled state. We also introduced a game which is impossible to win if the players are not allowed to communicate in classical world (both probabilistically and deterministically), yet there exists a perfect quantum strategy by following which, one can attain the winning condition of the game.Comment: Accepted in International Journal of Theoretical Physic