Extracting Neutron Star Properties from X-ray Burst Oscillations

Many thermonuclear X-ray bursts exhibit brightness oscillations. The brightness oscillations are thought to be due to the combined effects of non-uniform nuclear burning and rotation of the neutron star. The waveforms of the oscillations contain information about the size and number of burning regions. They also contain substantial information about the mass and radius of the star, and hence about strong gravity and the equation of state of matter at supranuclear densities. We have written general relativistic ray-tracing codes that compute the waveforms and spectra of rotating hot spots as a function of photon energy. Using these codes, we survey the effect on the oscillation waveform and amplitude of parameters such as the compactness of the star, the spot size, the surface rotation velocity, and whether there are one or two spots. We also fit phase lag versus photon energy curves to data from the millisecond X-ray pulsar, SAX J1808--3658.Comment: To appear in Proc. of the 10th Annual October Astrophysics Conference in Maryland: Cosmic Explosions, 4 page

The Origin of Double-Peaked Narrow Lines in Active Galactic Nuclei III: Feedback from Biconical AGN Outflows

We apply an analytic Markov Chain Monte Carlo model to a sample of 18 AGN-driven biconical outflows that we identified from a sample of active galaxies with double-peaked narrow emission lines at z < 0.1 in the Sloan Digital Sky Survey. We find that 8/18 are best described as asymmetric bicones, 8/18 are nested bicones, and 2/18 are symmetric bicones. From the geometry and kinematics of the models, we find that these moderate-luminosity AGN outflows are large and energetic. The biconical outflows axes are randomly oriented with respect to the photometric major axis of the galaxy, implying a randomly oriented and clumpier torus to collimate the outflow, but the torus also allows some radiation to escape equatorially. We find that 16/18 (89%) outflows are energetic enough to drive a two-staged feedback process in their host galaxies. All of these outflows geometrically intersect the photometric major axis of the galaxy, and 23% of outflow host galaxies are significantly redder or have significantly lower specific star formation rates when compared to a matched sample of active galaxies.Comment: 32 pages, 11 figures, accepted for publication in MNRAS; See Figure 7 for a summary of the finding

Carbon Detonation and Shock-Triggered Helium Burning in Neutron Star Superbursts

The strong degeneracy of the 12C ignition layer on an accreting neutron star results in a hydrodynamic thermonuclear runaway, in which the nuclear heating time becomes shorter than the local dynamical time. We model the resulting combustion wave during these superbursts as an upward propagating detonation. We solve the reactive fluid flow and show that the detonation propagates through the deepest layers of fuel and drives a shock wave that steepens as it travels upward into lower density material. The shock is sufficiently strong upon reaching the freshly accreted H/He layer that it triggers unstable 4He burning if the superburst occurs during the latter half of the regular Type I bursting cycle; this is likely the origin of the bright Type I precursor bursts observed at the onset of superbursts. The cooling of the outermost shock-heated layers produces a bright, ~0.1s, flash that precedes the Type I burst by a few seconds; this may be the origin of the spike seen at the burst onset in 4U 1820-30 and 4U 1636-54, the only two bursts observed with RXTE at high time resolution. The dominant products of the 12C detonation are 28Si, 32S, and 36Ar. Gupta et al. showed that a crust composed of such intermediate mass elements has a larger heat flux than one composed of iron-peak elements and helps bring the superburst ignition depth into better agreement with values inferred from observations.Comment: 11 pages, 11 figures, accepted to ApJ; discussion about onset of detonation discussed in new detail, including a new figur

Stellar Dynamics at the Galactic Center with an Extremely Large Telescope

We discuss experiments achievable via monitoring of stellar dynamics near the massive black hole at the Galactic center with a next generation, extremely large telescope (ELT). Given the likely observational capabilities of an ELT and current knowledge of the stellar environment at the Galactic center, we synthesize plausible samples of stellar orbits around the black hole. We use the Markov Chain Monte Carlo method to evaluate the constraints that orbital monitoring places on the matter content near the black hole. Results are expressed as functions of the number N of stars with detectable orbital motions and the astrometric precision dtheta and spectroscopic precision dv at which stellar proper motions and radial velocities are monitored. For N = 100, dtheta = 0.5 mas, and dv = 10 km/s -- a conservative estimate of the capabilities of a 30 meter telescope -- the extended matter distribution enclosed by the orbits will produce measurable deviations from Keplerian motion if >1000 Msun is enclosed within 0.01 pc. The black hole mass and distance to the Galactic center will be measured to better than ~0.1%. Lowest-order relativistic effects, such as the prograde precession, will be detectable if dtheta < 0.5 mas. Higher-order effects, including frame dragging due to black hole spin, requires dtheta < 0.05 mas, or the favorable discovery of a compact, highly eccentric orbit. Finally, we calculate the rate at which monitored stars undergo detectable nearby encounters with background stars. Such encounters probe the mass function of stellar remnants that accumulate near the black hole. We find that ~30 encounters will be detected over a 10 yr baseline for dtheta = 0.5 mas.Comment: 14 pages, 5 figures; discussion no longer aperture-specific (TMT -> ELT), matches ApJ versio

The Origin of Double-peaked Narrow Lines in Active Galactic Nuclei. IV. Association with Galaxy Mergers

Double-peaked narrow emission lines in active galactic nucleus (AGN) spectra can be produced by AGN outflows, rotation, or dual AGNs, which are AGN pairs in ongoing galaxy mergers. Consequently, double-peaked narrow AGN emission lines are useful tracers of the coevolution of galaxies and their supermassive black holes, as driven by AGN feedback and AGN fueling. We investigate this concept further with follow-up optical longslit observations of a sample of 95 Sloan Digital Sky Survey (SDSS) galaxies that have double-peaked narrow AGN emission lines. Based on a kinematic analysis of the longslit spectra, we confirm previous work that finds that the majority of double-peaked narrow AGN emission lines are associated with outflows. We also find that eight of the galaxies have companion galaxies with line-of-sight velocity separations < 500 km/s and physical separations <30 kpc. Since we find evidence of AGNs in both galaxies, all eight of these systems are compelling dual AGN candidates. Galaxies with double-peaked narrow AGN emission lines occur in such galaxy mergers at least twice as often as typical active galaxies. Finally, we conclude that at least 3% of SDSS galaxies with double-peaked narrow AGN emission lines are found in galaxy mergers where both galaxies are resolved in SDSS imaging.Comment: 14 pages, 2 figures, ApJ in pres

Viability of laser cleaning of papyrus: Conservation and scientific assessment

There has been a growing interest in laser cleaning applications for a variety of organic materials such as paper, parchment, textiles, and leather during the last decade. However, archaeological organic materials, notably papyrus, have rarely been investigated. This contribution examines whether removal of burial encrustation can be justified in view of its short-term and long-term effects on the substrate. To examine this, tests using mock objects have been performed. Using artificially soiled and archaeological papyrus samples, optimization of laser cleaning parameters using a picosecond laser (1064 nm, various operating conditions) was attempted. Optimization was based on colorimetry, optical microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, and cellulose degree of polymerization data, both before and after accelerated degradation. In papyrus, there is no clear damage threshold, and substrate degradation can always be observed and is comparable in treated (cleaned) and untreated (soiled) objects. Therefore, the decision on whether to clean papyrus using lasers is predominantly based on aesthetic and treatability (e.g. need for consolidation) criteria