Feryal Ozel

The relation between optical extinction and hydrogen column density in the Galaxy

A linear relation between the hydrogen column density (NH) and optical extinction (AV) in the Galaxy has long been observed. A number of studies found differing results in the slope of this relation. Here, we utilize the data on 22 supernova remnants that have been observed with the latest generation X-ray observatories and for which optical extinction and/or reddening measurements have been performed and find NH(cm ) = (2.21 ± 0.09) × 10 AV(mag). We compare our result with the previous studies and assess any systematic uncertainties that may affect these results.

The Black Hole Mass Distribution in the Galaxy

We use dynamical mass measurements of 16 black holes in transient low-mass X-ray binaries to infer the stellar black hole mass distribution in the parent population. We find that the observations are best described by a narrow mass distribution at 7.8 {+-} 1.2 M{sub sun}. We identify a selection effect related to the choice of targets for optical follow-ups that results in a flux-limited sample. We demonstrate, however, that this selection effect does not introduce a bias in the observed distribution and cannot explain the absence of black holes in the 2-5 M{sub sun} mass range. On the high-mass end, we argue that the rapid decline in the inferred distribution may be the result of the particular evolutionary channel followed by low-mass X-ray binaries. This is consistent with the presence of high-mass black holes in the persistent, high-mass X-ray binary sources. If the paucity of low-mass black holes is caused by a sudden decrease of the supernova explosion energy with increasing progenitor mass, this would have observable implications for ongoing transient surveys that target core-collapse supernovae. Our results also have significant implications for the calculation of event rates from the coalescence of black hole binaries for gravitational wave detectors.

Soft equations of state for neutron-star matter ruled out by EXO 0748-676

The interiors of neutron stars contain matter at very high densities, in a state that differs greatly from those found in the early universe or achieved at terrestrial experiments. Matter in these conditions can only be probed through astrophysical observations that measure the mass and radius of neutron stars with sufficient precision. Here I report for the first time a unique determination of the mass and radius of the neutron star EXO 0748-676, which appears to rule out all the soft equations of state of neutron star matter. If this object is typical, then condensates and unconfined quarks do not exist in the centers of neutron stars.The interiors of neutron stars contain matter at very high densities, in a state that differs greatly from those found in the early Universe or achieved in terrestrial experiments. Matter in these conditions can only be probed through astrophysical observations that measure the mass and radius of neutron stars with sufficient precision. Here I report a determination of the mass and radius of the neutron star EXO 0748 - 676 that appears to rule out all the soft equations of state of neutron-star matter. If this object is typical, then condensates and unconfined quarks do not exist in the centres of neutron stars.

Astrophysical measurement of the equation of state of neutron star matter

We present the first astrophysical measurement of the pressure of cold matter above nuclear saturation density, based on recently determined masses and radii of three neutron stars. The pressure at higher densities is below the predictions of equations of state that account only for nucleonic degrees of freedom, and thus present a challenge to the microscopic theory of neutron star matter.

Masses, Radii, and the Equation of State of Neutron Stars

We summarize our current knowledge of neutron-star masses and radii. Recent instrumentation and computational advances have resulted in a rapid increase in the discovery rate and precise timing of radio pulsars in binaries in the past few years, leading to a large number of mass measurements. These discoveries show that the neutron-star mass distribution is much wider than previously thought, with three known pulsars now firmly in the 1.9–2.0-M⊙ mass range. For radii, large, high-quality data sets from X-ray satellites as well as significant progress in theoretical modeling led to considerable progress in the measurements, placing them in the 10–11.5-km range and shrinking their uncertainties, owing to a better understanding of the sources of systematic errors. The combination of the massive-neutron-star discoveries, the tighter radius measurements, and improved laboratory constraints of the properties of dense matter has already made a substantial impact on our understanding of the composition and bulk p...

The Distance, Mass, and Radius of the Neutron Star in 4U?1608?52

Low-mass X-ray binaries (LMXBs) that show thermonuclear bursts are ideal sources for constraining the equation of state of neutron star matter. The lack of independent distance measurements for most of these sources, however, prevent a systematic exploration of the masses and radii of the neutron stars, hence limiting the equation-of-state studies. We present here a measurement of the distance to the LMXB 4U 1608-52 that is based on the study of the interstellar extinction toward the source. We first model the individual absorption edges of the elements Ne and Mg in the high-resolution X-ray spectrum obtained with XMM-Newton. We then combine this information with a measurement of the run of reddening with distance using red clump stars and determine a minimum distance to the source of 3.9 kpc, with a most probable value of 5.8 kpc. Finally, we analyze time-resolved X-ray spectra of Type I X-ray bursts observed from this source to measure the mass and the radius of the neutron star. We find a mass of M = 1.74 +- 0.14 M{sub sun} and a radius of R = 9.3 +- 1.0 km, respectively. This mass and radius can be achieved by several multi-nucleon equations of state.

ON THE MASS DISTRIBUTION AND BIRTH MASSES OF NEUTRON STARS

We investigate the distribution of neutron star masses in different populations of binaries, employing Bayesian statistical techniques. In particular, we explore the differences in neutron star masses between sources that have experienced distinct evolutionary paths and accretion episodes. We find that the distribution of neutron star masses in non-recycled eclipsing high-mass binaries as well as of slow pulsars, which are all believed to be near their birth masses, has a mean of 1.28 M ☉ and a dispersion of 0.24 M ☉. These values are consistent with expectations for neutron star formation in core-collapse supernovae. On the other hand, double neutron stars, which are also believed to be near their birth masses, have a much narrower mass distribution, peaking at 1.33 M ☉, but with a dispersion of only 0.05 M ☉. Such a small dispersion cannot easily be understood and perhaps points to a particular and rare formation channel. The mass distribution of neutron stars that have been recycled has a mean of 1.48 M ☉ and a dispersion of 0.2 M ☉, consistent with the expectation that they have experienced extended mass accretion episodes. The fact that only a very small fraction of recycled neutron stars in the inferred distribution have masses that exceed ~2 M ☉ suggests that only a few of these neutron stars cross the mass threshold to form low-mass black holes.

THE MASS AND RADIUS OF THE NEUTRON STAR IN EXO 1745–248

Bursting X-ray binaries in globular clusters are ideal sources for measuring neutron star masses and radii, and hence, for determining the equation of state of cold, ultradense matter. We use time-resolved spectroscopic data from EXO 1745–248 during thermonuclear bursts that show strong evidence for photospheric radius expansion to measure the Eddington flux and the apparent surface area of the neutron star. We combine this with the recent measurement of the distance to the globular cluster Terzan 5, where this source resides, to measure the neutron star mass and radius. We find tightly constrained pairs of values for the mass and radius, which are centered around M = 1.4 M ☉ and R = 11 km or around M = 1.7 M ☉ and R = 9 km. These values favor nucleonic equations of state with symmetry energy that is relatively low and has a weak dependence on density.

The Mass and Radius of the Neutron Star in 4U 1820-30

We report on the measurement of the mass and radius of the neutron star in the low-mass X-ray binary 4U 1820-30. The analysis of the spectroscopic data on multiple thermonuclear bursts yields well-constrained values for the apparent emitting area and the Eddington flux, both of which depend in a distinct way on the mass and radius of the neutron star. The distance to the source is that of the globular cluster NGC 6624, where the source resides. Combining these measurements, we uniquely determine the probability density over the stellar mass and radius. We find the mass to be M = 1.58 {+-} 0.06 M{sub sun} and the radius to be R = 9.1 {+-} 0.4 km.

Hybrid Thermal-Nonthermal Synchrotron Emission from Hot Accretion Flows

We investigate the effect of a hybrid electron population, consisting of both thermal and nonthermal particles, on the synchrotron spectrum, image size, and image shape of a hot accretion flow onto a supermassive black hole. We find two universal features in the emitted synchrotron spectrum: (1) a prominent shoulder at low (1011 Hz) frequencies that is weakly dependent on the shape of the electron energy distribution, and (2) an extended tail of emission at high (1013 Hz) frequencies whose spectral slope depends on the slope of the power-law energy distribution of the electrons. In the low-frequency shoulder, the luminosity can be up to 2 orders of magnitude greater than with a purely thermal plasma even if only a small fraction (<1%) of the steady state electron energy is in the nonthermal electrons. We apply the hybrid model to the Galactic center source, Sgr A*. The observed radio and IR spectra imply that at most 1% of the steady state electron energy is present in a power-law tail in this source. This corresponds to no more than 10% of the electron energy injected into the nonthermal electrons and hence 90% into the thermal electrons. We show that such a hybrid distribution can be sustained in the flow because thermalization via Coulomb collisions and synchrotron self-absorption are both inefficient. The presence of nonthermal electrons enlarges the size of the radio image at low frequencies and alters the frequency dependence of the brightness temperature. A purely thermal electron distributions produces a sharp-edged image, while a hybrid distribution causes strong limb brightening. These effects can be seen up to frequencies ~1011 Hz and are accessible to radio interferometers.