arXiv subjects
Publications and source records attributed to Tolga Guver.
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 are 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.
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_sun and the radius to be R = 9.11 +/- 0.40 km.
Low mass X-ray binaries 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, prevents 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 low mass X-ray binary 4U 1608-52 that is based on the study of the interstellar extinction towards 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_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.
A linear relation between the hydrogen column density (N_H) and optical extinction (A_V) 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 N_H (cm^-2) = (2.21 \pm0.09) x10^{21} A_V (mag). We compare our result with the previous studies and assess any systematic uncertainties that may affect these results.
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_sun and R=11 km or around M=1.7 M_sun 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.
We measured the galactic hydrogen column densities to the neutron-star binaries GX 17+2, 4U 1705-44, and 4U 1728-34 by modeling the Mg and Si absorption edges found in high-resolution X-ray spectra obtained by the Chandra X-ray Observatory. We found for GX 17+2, N_H = (2.38 +/- 0.12) x 10^22 cm^-2, for 4U 1705-44, N_H = (2.44 +/- 0.09) x 10^22 cm^-2, and for 4U 1728-34, N_H = (2.49 +/- 0.14) x 10^22 cm^-2. These values are in reasonable agreement with the hydrogen column densities inferred earlier from modeling of the continuum spectra of the sources. Our results can be used to constrain the uncertainties of model parameters of the X-ray spectra of these sources that are correlated to the uncertainties of the hydrogen column density. In the case of continuum spectra obtained during thermonuclear X-ray bursts, they will significantly reduce the uncertainties in the spectroscopically measured masses and radii of the neutron stars.
Anomalous X-ray Pulsars (AXPs) belong to a class of neutron stars believed to harbor the strongest magnetic fields in the universe, as indicated by their energetic bursts and their rapid spindowns. We have developed a theoretical model that takes into account processes in the atmospheres and magnetospheres of ultramagnetic neutron stars, as well as the effects of their strong gravitational fields on the observable properties. Using this model, we have analyzed the X-ray spectra of a number of AXPs. We find that in all cases, the X-ray spectra are described very well with this emission model. The spectroscopically measured magnetic field strengths of these sources are in close agreement with the values inferred from their spindown properties and provide independent evidence for their magnetar nature. The analysis of spectral data using this physical model also sheds light on the long-term evolution of AXPs.
We analyze archival Chandra and XMM-Newton data of 4U 0142+61 within the context of the Surface Thermal Emission and Magnetospheric Scattering model. We show that 4U 0142+61 spectrum can be fit very well with this physical model that contains only four parameters. The system parameters can be tightly constrained from the fits, yielding a surface magnetic field strength of B=(4.75 \pm 0.02)x 10^{14} G, a surface temperature of kT = 0.309 \pm 0.001$ keV and a scattering optical depth of a few in the magnetosphere. These values do not vary between observations due to the stability of the source within the window of the observations. The detailed fits yield $χ^2$ values that are statistically much better than the traditionally employed blackbody+power-law and two blackbody fits. The spectroscopically measured surface magnetic field strength is higher than, but within, the theoretical uncertainties of the value inferred from the dipole spindown formula.
Anomalous X-ray Pulsars (AXPs) belong to a class of neutron stars believed to harbor the strongest magnetic fields in the universe, as indicated by their energetic bursts and their rapid spindowns. However, an unambiguous measurement of their surface field strengths has not been made to date. It is also not known whether AXP outbursts result from changes in the neutron star magnetic field or crust properties. Here we report a spectroscopic measurement of the surface magnetic field strength of an AXP, XTE J1810$-$197, and solidify its magnetar nature. The field strength obtained from detailed spectral analysis and modeling, B = (2.72$\pm$0.03)$\times10^{14}$ G, is remarkably close to the value inferred from the rate of spindown of this source and remains nearly constant during numerous observations spanning over an order of magnitude in source flux. The surface temperature, on the other hand, declines steadily and dramatically following the 2003 outburst of this source. Our findings demonstrate that heating occurs in the upper neutron star crust during an outburst and sheds light on the transient behaviour of AXPs.
We explore the hardness-intensity correlations observed in several AXPs and SGRs within the framework of a thermally emitting magnetar model. Using our detailed atmosphere models and taking into account reprocessing of the surface emission by the magnetosphere, we show that the hardness of the surface spectra increases with increasing temperature and hence the changes in the effective temperatures of the outer layers of the star alone can account for the observed correlations. We conclude that the slow release of the heat deposited in the deep crust during a magnetar burst naturally accounts for the spectral changes during the afterglow. The correlations are further enhanced by changes in the structures of the magnetic currents during or following a burst. However, the additional hardening produced by scattering of the surface photons off the magnetospheric charges saturates at moderate values of the scattering optical depth.
We present the first self-consistent theoretical models of magnetar spectra that take into account the combined effects of the stellar atmosphere and its magnetosphere. We find that the proton cyclotron lines that are already weakened by atmospheric effects become indistinguishable from the continuum for moderate scattering optical depths in the magnetosphere. Furthermore, the hard excess becomes more pronounced due to resonant scattering and the resulting spectra closely resemble the observed magnetar spectra. We argue that while the absence of proton cyclotron lines in the observed spectra are inconclusive about the surface field strengths of magnetars, the continuum carries nearly unique signatures of the field strength and can thus be used to infer this quantity. We fit our theoretical spectra with a phenomenological two-blackbody model and compare our findings to source spectra with existing two-blackbody fits. The field strengths that we infer spectroscopically are in remarkable agreement with the values inferred from the period derivative of the sources assuming a dipole spindown. These detailed predictions of line energies and equivalent widths may provide an optimal opportunity for measuring directly the surface field strengths of magnetars with future X-ray telescopes such as {\em Constellation}-X.