X-ray Spectrum and Pulsations of the Vela Pulsar

We report the results of the spectral and timing analysis of observations of the Vela pulsar with the Chandra X-ray Observatory. The spectrum shows no statistically significant spectral lines in the observed 0.25--8.0 keV band. It consists of two distinct continuum components. The softer component can be modeled as either a magnetic hydrogen atmosphere spectrum with kT = 59 +- 3 eV, R = 15.5 +- 1.5 km, or a standard blackbody with kT = 129 +- 4 eV, R = 2.1 +- 0.2 km (the radii are for a distance of 250 pc). The harder component, modeled as a power-law spectrum, gives photon indices depending on the model adopted for the soft component: gamma = 1.5 +- 0.3 for the magnetic atmosphere soft component, and gamma = 2.7 +- 0.4 for the blackbody soft component. Timing analysis shows three peaks in the pulse profile, separated by about 0.3 in phase. Energy-resolved timing provides evidence for pulse profile variation with energy. The higher energy (E > 1.8 keV) profile shows significantly higher pulsed fraction.Comment: 4 pages, 2 figures, To appear in "Neutron Stars in Supernova Remnants" (ASP Conference Proceedings), eds P. O. Slane and B. M. Gaensler Corrected TYPO

The Compact Central Object in the Supernova Remnant G266.2-1.2

We observed the compact central object CXOU J085201.4--461753 in the supernova remnant G266.2--1.2 (RX J0852.0--4622) with the Chandra ACIS detector in timing mode. The spectrum of this object can be described by a blackbody model with the temperature kT=404 eV and radius of the emitting region R=0.28 km, at a distance of 1 kpc. Power-law and thermal plasma models do not fit the source spectrum. The spectrum shows a marginally significant feature at 1.68 keV. Search for periodicity yields two candidate periods, about 301 ms and 33 ms, both significant at a 2.1 sigma level; the corresponding pulsed fractions are 13% and 9%, respectively. We find no evidence for long-term variability of the source flux, nor do we find extended emission around the central object. We suggest that CXOU J085201.4--461753 is similar to CXOU J232327.9+584842, the central source of the supernova remnant Cas A. It could be either a neutron star with a low or regular magnetic field, slowly accreting from a fossil disk, or, more likely, an isolated neutron star with a superstrong magnetic field. In either case, a conservative upper limit on surface temperature of a 10 km radius neutron star is about 90 eV, which suggests accelerated cooling for a reasonable age of a few thousand years.Comment: Accepted to ApJ, 13 pages, 1 figur

The Chemical Compositions of the Type II Cepheids -- The BL Her and W Vir Variables

Abundance analyses from high-resolution optical spectra are presented for 19 Type II Cepheids in the Galactic field. The sample includes both short-period (BL Her) and long-period (W Vir) stars. This is the first extensive abundance analysis of these variables. The C, N, and O abundances with similar spreads for the BL Her and W Vir show evidence for an atmosphere contaminated with $3\alpha$-process and CN-cycling products. A notable anomaly of the BL Her stars is an overabundance of Na by a factor of about five relative to their presumed initial abundances. This overabundance is not seen in the W Vir stars. The abundance anomalies running from mild to extreme in W Vir stars but not seen in the BL Her stars are attributed to dust-gas separation that provides an atmosphere deficient in elements of high condensation temperature, notably Al, Ca, Sc, Ti, and $s$-process elements. Such anomalies have previously been seen among RV Tau stars which represent a long-period extension of the variability enjoyed by the Type II Cepheids. Comments are offered on how the contrasting abundance anomalies of BL Her and W Vir stars may be explained in terms of the stars' evolution from the blue horizontal branch.Comment: 41 pages including 11 figures and 4 tables; Accepted for publication in Ap

The Luminosities and Distance Scales of Type II Cepheid and RR Lyrae variables

Infrared and optical absolute magnitudes are derived for the type II Cepheids kappa Pav and VY Pyx from revised Hipparcos parallaxes and for kappa Pav, V553 Cen and SW Tau from pulsation parallaxes. Phase-corrected JHK mags are given for 142 RR Lyrae variables based on 2MASS data. RR Lyrae itself is overluminous compared with LMC RR Lyraes at the classical Cepheid modulus (18.39) consistent with a prediction of Catalan and Cortes. V553 Cen and SW Tau deviate by only 0.02 mag in the mean from the Matsunaga PL(K) relation for globular cluster type II Cepheids with a zero-point based on the same LMC modulus. Comparing directly these two stars with type II Cepheids in the LMC and in the Galactic Bulge leads to an LMC modulus of 18.37\pm0.09 and a distance to the Galactic Centre of 7.64\pm 0.21kpc. Kappa Pav may be a binary. V553 Cen and SW Tau show that at optical wavelengths PL relations are wider for field stars than for those in globular clusters (abridged).Comment: 29 pages, 13 figures, accepted for MNRA

The Tensor-Vector-Scalar theory and its cosmology

Over the last few decades, astronomers and cosmologists have accumulated vast amounts of data clearly demonstrating that our current theories of fundamental particles and of gravity are inadequate to explain the observed discrepancy between the dynamics and the distribution of the visible matter in the Universe. The Modified Newtonian Dynamics (MOND) proposal aims at solving the problem by postulating that Newton's second law of motion is modified for accelerations smaller than ~10^{-10}m/s^2. This simple amendment, has had tremendous success in explaining galactic rotation curves. However, being non-relativistic, it cannot make firm predictions for cosmology. A relativistic theory called Tensor-Vector-Scalar (TeVeS) has been proposed by Bekenstein building on earlier work of Sanders which has a MOND limit for non-relativistic systems. In this article I give a short introduction to TeVeS theory and focus on its predictions for cosmology as well as some non-cosmological studies.Comment: 44 pages, topical review for Classical and Quantum Gravit

Six Years of Chandra Observations of Supernova Remnants

We present a review of the first six years of Chandra X-ray Observatory observations of supernova remnants. From the official "first-light" observation of Cassiopeia A that revealed for the first time the compact remnant of the explosion, to the recent million-second spectrally-resolved observation that revealed new details of the stellar composition and dynamics of the original explosion, Chandra observations have provided new insights into the supernova phenomenon. We present an admittedly biased overview of six years of these observations, highlighting new discoveries made possible by Chandra's unique capabilities.Comment: 82 pages, 28 figures, for the book Astrophysics Update

Dark Energy from Mass Varying Neutrinos

We show that mass varying neutrinos (MaVaNs) can behave as a negative pressure fluid which could be the origin of the cosmic acceleration. We derive a model independent relation between the neutrino mass and the equation of state parameter of the neutrino dark energy, which is applicable for general theories of mass varying particles. The neutrino mass depends on the local neutrino density and the observed neutrino mass can exceed the cosmological bound on a constant neutrino mass. We discuss microscopic realizations of the MaVaN acceleration scenario, which involve a sterile neutrino. We consider naturalness constraints for mass varying particles, and find that both ev cutoffs and ev mass particles are needed to avoid fine-tuning. These considerations give a (current) mass of order an eV for the sterile neutrino in microscopic realizations, which could be detectable at MiniBooNE. Because the sterile neutrino was much heavier at earlier times, constraints from big bang nucleosynthesis on additional states are not problematic. We consider regions of high neutrino density and find that the most likely place today to find neutrino masses which are significantly different from the neutrino masses in our solar system is in a supernova. The possibility of different neutrino mass in different regions of the galaxy and the local group could be significant for Z-burst models of ultra-high energy cosmic rays. We also consider the cosmology of and the constraints on the ``acceleron'', the scalar field which is responsible for the varying neutrino mass, and briefly discuss neutrino density dependent variations in other constants, such as the fine structure constant.Comment: 26 pages, 3 figures, refs added, typos corrected, comment added about possible matter effect

Constraining the Evolution of Zz Ceti

We report our analysis of the stability of pulsation periods in the DAV star (pulsating hydrogen atmosphere white dwarf) ZZ Ceti, also called R548. On the basis of observations that span 31 years, we conclude that the period 213.13 s observed in ZZ Ceti drifts at a rate dP/dt ≤ (5:5 ± 1:9) x 10-15 s s-1, after correcting for proper motion. Our results are consistent with previous Ṗ values for this mode and an improvement over them because of the larger time base. The characteristic stability timescale implied for the pulsation period is ⎸P / Ṗ ⎸=⎹≥ 1:2 Gyr, comparable to the theoretical cooling timescale for the star. Our current stability limit for the period 213.13 s is only slightly less than the present measurement for another DAV, G117-B15A, for the period 215.2 s, establishing this mode in ZZ Ceti as the second most stable optical clock known, comparable to atomic clocks and more stable than most pulsars. Constraining the cooling rate of ZZ Ceti aids theoretical evolutionary models and white dwarf cosmochronology. The drift rate of this clock is small enough that we can set interesting limits on reflex motion due to planetary companions