arXiv subjects

Feryal Ozel

Publications and source records attributed to Feryal Ozel.

100 records · Page 6Linked to original sources

arXiv2002

The Amplitude Evolution and Harmonic Content of Millisecond Oscillations in Thermonuclear X-ray Bursts

We present a comprehensive observational and theoretical analysis of the amplitudes and profiles of oscillations that occur during thermonuclear X-ray bursts from weakly-magnetized neutron stars in low mass X-ray binaries. Our sample contains 59 oscillations from six sources observed with the Rossi X-ray Timing Explorer. The oscillations that we examined occurred primarily during the decaying portions of bursts, and lasted for several seconds each. We find that the oscillations are as large as 15% during the declines of the bursts, and they appear and disappear due to intrinsic variations in their fractional amplitudes. However, the maxima in the amplitudes are not related to the underlying flux in the burst. We derive folded profiles for each oscillation train to study the pulse morphologies. The mean rms amplitudes of the oscillations are 5%, although the eclipsing source MXB 1659-298 routinely produces 10% oscillations in weak bursts. We also produce combined profiles from all of the oscillations from each source. Using these pulse profiles, we place upper limits on the fractional amplitudes of harmonic and half-frequency signals of 0.3% and 0.6%, respectively (95% confidence). We then compare the pulse morphologies to theoretical profiles from models with one or two antipodal bright regions on the surface of a rotating neutron star. We find that if one bright region is present on the star, it must either lie near the rotational pole or cover nearly half the neutron star in order to be consistent with the observed lack of harmonic signals. If an antipodal pattern is present, the hot regions must form very near the rotational equator. We discuss how these geometric constraints challenge current models for the production of brightness variations on the surface of a neutron star. (abridged)

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arXiv2002

Spectral Lines from Rotating Neutron Stars

The line profiles from rotating neutron stars are affected by a number of relativistic processes such as Doppler boosts, strong self-lensing, frame-dragging, and the differential gravitational redshift arising from the stellar oblateness. In this Letter, we calculate line profiles taking into account the first two effects, which is accurate for rotation rates less than the breakup frequency. We show that the line profiles are not only broadened and weakened but are also significantly asymmetric, and allow for an independent measurement of both the mass and the radius of the neutron star. Furthermore, we investigate the case when a fraction of the neutron star surface contributes to the emission and find that the line profiles are typically doubly peaked. We discuss the implications of our results for searches for line features in the spectra of isolated neutron stars and X-ray bursters. We finally assess the systematic uncertainties introduced by the line asymmetry in inferring the compactness of neutron stars from the detection of redshifted lines.

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arXiv2002

The Effect of Vacuum Polarization and Proton Cyclotron Resonances on Photon Propagation in Strongly Magnetized Plasmas

We consider the effects of vacuum polarization and proton cyclotron resonances on the propagation of radiation through a strongly magnetized plasma. We analyze the conditions under which the photons evolve adiabatically through the resonant density and find that the adibaticity condition is satisfied for most photon energies of interest, allowing for a normal-mode treatment of the photon propagation. We then construct radiative equilibrium atmosphere models of strongly magnetized neutron stars that includes these effects, employing a new numerical method that resolves accurately the sharp changes of the absorption and mode-coupling cross sections at the resonant densities. We show that the resulting spectra are modified by both resonances and are harder at all field strengths than a blackbody at the effective temperature. We also show that the narrow absorption features introduced by the proton cyclotron resonance have small equivalent widths. We discuss the implications of our results for properties of thermal emission from the surfaces of young neutron stars.

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arXiv2002

Timing Properties of Magnetars

We study the pulse morphologies and pulse amplitudes of thermally emitting neutron stars with ultrastrong magnetic fields. The beaming of the radiation emerging from a magnetar was recently shown to be predominantly non-radial, with a small pencil and a broad fan component. We show that the combination of this radiation pattern with the effects of strong lensing in the gravitational field of the neutron star yields pulse profiles that show a qualitatively different behavior compared to that of the radially-peaked beaming patterns explored previously. Specifically, we find that: (i) the pulse profiles of magnetars with a single hot emission region on their surface exhibit 1-2 peaks, whereas those with an antipodal emission geometry have 1-4 peaks, depending on the neutron star compactness, the observer's viewing angle, and the size of the hot regions; (ii) the energy dependence of the beaming pattern may give rise to weakly or strongly energy-dependent pulse profiles and may introduce phase lags between different energy bands; (iii) the non-radial beaming pattern can give rise to high pulsed fractions even for very relativistic neutron stars; (iv) the pulsed fraction may not vary monotonically with neutron star compactness; (v) the pulsed fraction does not decrease monotonically with the size of the emitting region; (vi) the pulsed fraction from a neutron star with a single hot pole has, in general, a very weak energy dependence, in contrast to the case of an antipodal geometry. Comparison of these results to the observed properties of anomalous X-ray pulsars strongly suggests that they are neutron stars with a single hot region of ultrastrong magnetic field.

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arXiv2001

Constraints on Thermal Emission Models of Anomalous X-ray Pulsars

Thermal emission from the surface of an ultramagnetic neutron star is believed to contribute significantly to the soft X-ray flux of the Anomalous X-ray Pulsars. We compare the detailed predictions of models of the surface emission from a magnetar to the observed properties of AXPs. In particular, we focus on the combination of their luminosities and energy-dependent pulsed fractions. We use the results of recent calculations for strongly magnetized atmospheres to obtain the angle- and energy-dependence of the surface emission. We include in our calculations the effects of general relativistic photon transport and interstellar extinction. We find that the combination of the large pulsed fractions and the high luminosities of AXPs cannot be accounted for by surface emission from a magnetar with two antipodal hot regions or a temperature distribution characteristic of a magnetic dipole. This result is robust for reasonable neutron star radii, for the range of magnetic field strengths inferred from the observed spin down rates, and for surface temperatures consistent with the spectral properties of AXPs. Models with a single hot emitting region can reproduce the observations, provided that the distance to one of the sources is ~30% less than the current best estimate, and allowing for systematic uncertainties in the spectral fit of a second source. Finally, the thermal emission models with antipodal emission geometry predict a characteristic strong increase of the pulsed fraction with photon energy, which is apparently inconsistent with the current data. The energy-dependence of the pulsed fraction in the models with one hot region shows a wider range of behavior and can be consistent with the existing data. Upcoming high-resolution observations with Chandra and XMM-Newton will provide a conclusive test.

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arXiv2001

Surface Emission Properties of Strongly Magnetic Neutron Stars

We construct radiative equilibrium models for strongly magnetized (B > 10^13 G) neutron-star atmospheres taking into account magnetic free-free absorption and scattering processes computed for two polarization modes. We include the effects of vacuum polarization in our calculations. We present temperature profiles and the angle-, photon energy-, and polarization-dependent emerging intensity for a range of magnetic field strengths and effective temperatures of the atmospheres. We find that for B < 10^14 G, the emerging spectra are bluer than the blackbody corresponding to the effective temperature, T_eff, with modified Planckian shapes due to the photon-energy dependence of the magnetic opacities. However, vacuum polarization significantly modifies the spectra for B~10^15 G, giving rise to power-law tails at high photon energies. The angle-dependence (beaming) of the emerging intensity has two maxima: a narrow (pencil) peak at small angles (<5 degrees) with respect to the normal and a broad maximum (fan beam) at intermediate angles (~20-60 degrees). The relative importance and the opening angle of the radial beam decreases strongly with increasing magnetic field strength and decreasing photon energy. We finally compute a T_eff-T_c relation for our models, where T_c is the local color temperature of the spectrum emerging from the neutron star surface, and find that T_c/T_eff ranges between 1.1-1.8. We discuss the implications of our results for various thermally emitting neutron star models.

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arXiv2001

General Relativistic Effects on Magnetar Models of AXPs

General relativistic bending of light dramatically alters the variability of X-ray emission originating from the surfaces of ultramagnetic neutron stars. We construct radiative equilibrium models of such strongly magnetic cooling neutron stars with light-element atmospheres to compute the angle- and energy-dependent intensity emerging from their surfaces and find that the beaming of surface emission is predominantly non-radial. The combination of this radiation pattern with the calculations of light bending yields pulse amplitudes that vary non-monotonically with the neutron star compactness and the size of the emitting region. The significant suppression of the pulse amplitude for large emitting areas provides very strong constraints on the mechanisms that can simultaneously produce high periodic variability and X-ray luminosity. We apply these results to the thermally-emitting magnetar models of anomalous X-ray pulsars (AXPs), which are bright slowly-rotating X-ray sources with large pulse amplitudes. We use the observed fluxes and pulse amplitudes for all known AXPs and show that thermal emission from two antipodal regions on their surfaces, as predicted by some magnetar models, is inconsistent with these observed properties.

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arXiv2000

X-ray Images of Hot Accretion Flows

We consider the X-ray emission due to bremsstrahlung processes from hot, low radiative-efficiency accretion flows around supermassive and galactic black holes. We calculate surface brightness profiles and Michelson visibility functions for a range of density profiles, rho ~ r^(-3/2+p), with 0 < p < 1, to allow for the presence of outflows. We find that although the 1 keV emitting region in these flows can always extend up to 10^6 Schwarzschild radii (R_S), their surface brightness profiles and visibility functions are strongly affected by the specific density profile. The advection-dominated solutions with no outflows (p=0) lead to centrally peaked profiles with characteristic sizes of only a few tens of R_S. Solutions with strong outflows (p~1) lead to flat intensity profiles with significantly larger characteristic sizes of up to 10^6 R_S. This implies that low luminosity galactic nuclei, such as M87, may appear as extended X-ray sources when observed with current X-ray imaging instruments. We show that X-ray brightness profiles and their associated visibility functions may be powerful probes for determining the relevant mode of accretion and, in turn, the properties of hot accretion flows. We discuss the implications of our results for observations with the Chandra X-ray Observatory and the planned X-ray interferometer MAXIM.

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arXiv2000

Photon Propagation Around Compact Objects and the Inferred Properties of Thermally Emitting Neutron Stars

Anomalous X-ray pulsars, compact non-pulsing X-ray sources in supernova remnants, and X-ray bursters are three distinct types of sources for which there are viable models that attribute their X-ray emission to thermal emission from the surface of a neutron star. Inferring the surface area of the emitting regions in such systems is crucial in assessing the viability of different models and in providing bounds on the radii of neutron stars. We show that the inferred areas of the emitting regions may be over- or under-estimated by a factor of <=2, because of the geometry of the system and general relativistic light deflection, combined with the effects of phase averaging. Such effects make the determination of neutron-star radii uncertain, especially when compared to the ~5% level required for constraining the equation of state of neutron-star matter. We also note that, for a given spectral shape, the inferred source luminosities and pulse fractions are anticorrelated because they depend on the same properties of the emitting regions, namely their sizes and orientations, i.e., brighter sources have on average weaker pulsation amplitudes than fainter sources. We argue that this property can be used as a diagnostic tool in distinguishing between different spectral models. As an example, we show that the high inferred pulse fraction and brightness of the pulsar RXS J1708-40 are inconsistent with isotropic thermal emission from a neutron-star surface. Finally, we discuss the implication of our results for surveys in the soft X-rays for young, cooling neutron stars in supernova remnants and show that the absence of detectable pulsations from the compact source at the center of Cas A (at a level of >=30%) is not a strong argument againts its identification with a spinning neutron star.

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arXiv2000

Hybrid Thermal-Nonthermal Synchrotron Emission from Hot Accretion Flows

We investigate the effect of a hybrid electron population, consisting of both thermal and non-thermal 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: (i) a prominent shoulder at low (< 10^11 Hz) frequencies that is weakly dependent on the shape of the electron energy distribution, and (ii) an extended tail of emission at high (> 10^13 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 two 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 non-thermal 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 non-thermal 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 non-thermal 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 ~10^11 Hz and are accessible to radio interferometers.

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