arXiv subjects

Denis A. Leahy

Publications and source records attributed to Denis A. Leahy.

25 records · Page 2Linked to original sources

arXiv2007

Limits on Mass and Radius for the ms-Period X-ray Pulsar SAX J1808.4-3658

SAX J1808.4-3658 has a 2.5 millisecond neutron star rotation period and exhibits X-ray pulsations due to its rotating hot spot. Here we present an analysis of the pulse shapes of SAX J1808 during its 1998 outburst. The modeling of the pulse shape includes several effects, including gravitational light-bending, doppler effects and two spectral components with different emissivity. In addition we include the new effects of light-travel time-delays and the neutron star's oblate shape. We also consider two different data sets, with different selection in time period (1 day versus 19 days of data combined) and different energy binning and time resolution. We find that including time-delays and oblateness results in stronger restriction on allowed masses and radii. A second result is that the choice of data selection strongly affects the allowed masses and radii. Overall, the derived constraints on mass and radius favor compact stars and a soft equation of state.

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arXiv2007

The Oblate Schwarzschild Approximation for Light Curves of Rapidly Rotating Neutron Stars

We present a simple method for including the oblateness of a rapidly rotating neutron star when fitting X-ray light curves. In previous work we showed that the oblateness induced by rotation at frequencies above 300 Hz produces a geometric effect which needs to be accounted for when modelling light curves to extract constraints on the neutron star's mass and radius. In our model X-rays are emitted from the surface of an oblate neutron star and propagate to the observer along geodesics of the Schwarzschild metric for a spherical neutron star. Doppler effects due to rotation are added in the same manner as in the case of a spherical neutron star. We show that this model captures the most important effects due to the neutron star's rotation. We also explain how the geometric oblateness effect can rival the Doppler effect for some emission geometries.

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arXiv2006

Radio Observations and Spectrum of the SNR G127.1+0.5 and its Central Source 0125+628

We present new images of the Supernova Remnant (SNR) G127.1+0.5 (R5), based on the 408 MHz and 1420 MHz continuum emission and the HI-line emission data of the Canadian Galactic Plane Survey (CGPS). The radio spectrum of the central compact source (0125+628) is analyzed in the range 178 MHz - 8.7 GHz, indicating a flat spectrum with synchrotron self-absorption below 800 MHz. The SNR's flux density at 408 MHz is 17.1$\pm$1.7 Jy and at 1420 MHz is 10.0$\pm$0.8 Jy, corrected for flux densities from compact sources within the SNR. The SNR's integrated flux density based spectral index (S$_ν$$\propto$$ν$$^{-α}$) is 0.43$\pm$0.10. The respective T-T plot spectral index (derived from the relative size of brightness temperature variations between two frequencies, see text for details) is 0.46$\pm$0.01. There is no evidence at 1$σ$ for spatial variations in spectral index within G127.1+0.5. In particular, we compared the northern shell, southern shell and central diffuse region. HI observations show structures associated with the SNR in the radial velocity range of -12 to -16 km$/$s, suggesting G127.1+0.5's distance is 1.15 kpc. The estimated Sedov age is 2 - 3 $\times$$10^{4}$ yr.

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arXiv2005

Radio observations and spectral index study of SNR G126.2+1.6

We present new images of the low radio surface brightness Supernova Remnant (SNR) G126.2+1.6, based on the 408 MHz and 1420 MHz continuum emission and the HI-line emission data of the Canadian Galactic Plane Survey (CGPS). {\bf We find the SNR's flux densities at 408 MHz (9.7$\pm$3.9 Jy) and 1420 MHz (6.7$\pm$2.1 Jy) which have been} corrected for flux densities from compact sources within the SNR. The integrated flux density based spectral index (S$_ν$$\propto$$ν$$^{-α}$) is 0.30$\pm$0.41. The respective T-T plot spectral index is 0.30 $\pm$0.08. We also find spatial variations of spectral index within the SNR{\bf:0.2-0.6.} HI observations show structures probably associated with the SNR, i.e, features associated with the SNR's southeastern filaments in the radial velocity range of -33 to -42 km$/$s, and with its northwestern filaments in -47 to -52 km$/$s. This association suggests a distance of 5.6 kpc for SNR G126.2+1.6. The estimated Sedov age for G126.2+1.6 is less than 2.1$\times10^{5}$ yr.

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arXiv2004

Pulse Shapes From Rapidly-Rotating Neutron Stars: Equatorial Photon Orbits

We demonstrate that fitted values of stellar radius obtained by fitting theoretical light curves to observations of millisecond period X-ray pulsars can significantly depend on the method used to calculate the light curves. The worst-case errors in the fitted radius are evaluated by restricting ourselves to the case of light emitted and received in the equatorial plane of a rapidly-rotating neutron star. First, using an approximate flux which is adapted to the one-dimensional nature of such an emission region, we show how pulse shapes can be constructed using an exact spacetime metric and fully accounting for time-delay effects. We compare this to a method which approximates the exterior spacetime of the star by the Schwarzschild metric, inserts special relativistic effects by hand, and neglects time-delay effects. By comparing these methods, we show that there are significant differences in these methods for some applications, for example pulse timing and constraining the stellar radius. In the case of constraining the stellar radius, we show that fitting the approximate pulse shapes to the full calculation yields errors in the fitted radius of as much as about +/- 10%, depending on the rotation rate and size of the star as well as the details describing the emitting region. However, not all applications of pulse shape calculations suffer from significant errors: we also show that the calculation of the soft-hard phase lag for a 1 keV blackbody does not strongly depend on the method used for calculating the pulse shapes.

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arXiv2001

Phase Dependent Spectral Variability of 4U1907+09

We report on ASCA, RXTE, and archival observations of the high mass X-ray binary pulsar 4U1907+09. Spectral measurements of the absorption and flux were made at all phases of the X-ray pulsar orbit, including the first spectral measurements of an extended period of low flux during two of the ASCA observations. We find that a simple spherical wind model can fit the time averaged light curve as measured by the RXTE ASM, but does not fit the observed changes in the absorption column or account for the existence of the phase-locked secondary flare. An additional model component consisting of a trailing stream can account for the variations in column depth. However, these models favor a high inclination angle for the system, suggesting a companion mass more consistent with an identification as a Be-star. In this case an equatorially enhanced wind and inclined neutron star orbit may be a more appropriate interpretation of the data.

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arXiv2000

The 35-Day Evolution of the Hercules X-1 Pulse Profile: Evidence For A Resolved Inner Disk Occultation of the Neutron Star

Ginga and Rossi X-ray Timing Explorer (RXTE) observations have allowed an unprecedented view of the recurrent systematic pulse shape changes associated with the 35-day cycle of Hercules X-1, a phenomenon currently unique among the known accretion-powered pulsars. We present observations of the pulse shape evolution. An explanation for the pulse evolution in terms of a freely precessing neutron star is reviewed and shown to have several major difficulties in explaining the observed pulse evolution pattern. Instead, we propose a phenomenological model for the pulse evolution based upon an occultation of the pulse emitting region by the tilted, inner edge of a precessing accretion disk. The systematic and repeating pulse shape changes require a resolved occultation of the pulse emission region. The observed pulse profile motivates the need for a pulsar beam consisting of a composite coaxial pencil and fan beam but the observed evolution pattern requires the fan beam to be focused around the neutron star and beamed in the antipodal direction. The spectral hardness of the pencil beam component suggests an origin at the magnetic polar cap, with the relatively softer fan beam emission produced by backscattering from within the accretion column, qualitatively consistent with several theoretical models for X-ray emission from the accretion column of an accreting neutron star.

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