D. A. Leahy

Simultaneous X-Ray and Radio Monitoring of the Unusual Binary LS I +61?303: Measurements of the Light Curve and High-Energy Spectrum

The binary system, LS I +61°303, is unusual both because of the dramatic, periodic, radio outbursts, and because of its possible association with the 100 MeV gamma-ray source, 2CG 135+01. We have performed simultaneous radio and Rossi X-Ray Timing Explorer X-ray observations at 11 intervals over the 26.5 day orbit, and in addition searched for variability on timescales ranging from milliseconds to hours. We confirm the modulation of the X-ray emission on orbital timescales originally reported by Taylor et al., and in addition we find a significant offset between the peak of the X-ray and radio flux. We argue that based on these results, the most likely X-ray emission mechanism is inverse Compton scattering of stellar photons off of electrons accelerated at the shock boundary between the relativistic wind of a young pulsar and the Be star wind. In these observations we also detected 2-150 keV flux from the nearby low-redshift quasar QSO 0241+622. Comparing these measurements to previous hard X-ray and gamma-ray observations of the region containing both LS I +61°303 and QSO 0241+622, it is clear that emission from the QSO dominates.

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 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 on 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.

The distance of the SNR Kes 75 and PWN PSR J1846-0258 system

The supernova remnant (SNR) Kes 75/PSR J1846-0258 association can be regarded as a certainty due to the accurate location of young PSR J1846-0258 at the center of Kes 75 and the detected bright radio/X-ray synchrotron nebula surrounding the pulsar. We provide a new distance estimate to the SNR/pulsar system by analyzing the HI and 13 CO maps, the HI emission and absorption spectra, and the 13 CO emission spectrum of Kes 75. That there are no absorption features at negative velocities strongly argues against the widely-used large distance of 19 to 21 kpc for Kes 75, and shows that Kes 75 is within the Solar circle, i.e. a distance d < 13.2 kpc. Kes 75 is likely at a distance of 5.1 to 7.5 kpc because the highest HI absorption velocity is at 95 km s -1 , and no absorption is associated with a nearby HI emission peak at 102 km s -1 in the direction of Kes 75. This distance to Kes 75 gives a reasonable luminosity of PSR J1846-0258 and its PWN and also leads to a much smaller radius for Kes 75, so the age of the SNR is consistent with the spin-down age of PSR J1846-0258, confirming this pulsar as the second-youngest in the Galaxy.

Light Curves for Rapidly Rotating Neutron Stars

We present ray-tracing computations for light emitted from the surface of a rapidly rotating neutron star in order to construct light curves for X-ray pulsars and bursters. These calculations are for realistic models of rapidly rotating neutron stars that take into account both the correct exterior metric and the oblate shape of the star. We find that the most important effect arising from rotation comes from the oblate shape of the rotating star. Approximating a rotating neutron star as a sphere introduces serious errors in fitted values of the stars radius and mass if the rotation rate is very large. However, in most cases acceptable fits to the ratio M/R can be obtained with the spherical approximation.

The distance and age of the supernova remnants Kes 73 and AXP 1E 1841-045

We provide a new distance estimate to the supernova remnant (SNR) Kes 73 and its associated anomalous X-ray pulsar (AXP) 1E 1841-045. 21 cm H I images and H I absorption/emission spectra from new VLA observations, and 13CO emission spectra of Kes 73 and two adjacent compact H II regions (G27.276+0.148 and G27.491+0.189) are analyzed. The H I images show prominent absorption features associated with Kes 73 and the H II regions. The absorption appears up to the tangent point velocity giving a lower distance limit to Kes 73 of 7.5 kpc, which has previously been given as the upper limit. In addition, G27.276+0.148 and G27.491+0.189 are at the far kinematic distances of their radio recombination line velocities. There is prominent H I emission in the range 80-90 km s−1 for all three objects. The two H II regions show H I absorption at ~84 km s−1, but there is no absorption in the Kes 73 absorption spectrum. This implies an upper distance limit of ~9.8 kpc to Kes 73. This corrected larger distance to Kes 73/AXP 1E 1841-045 system leads to a refined age of the SNR of 500-1000 yr, and a ~50% larger AXP X-ray luminosity.

Discovery of the Radio and X-Ray Counterpart of TeV γ-Ray Source HESS J1731–347

We discover a faint shell-type radio and X-ray source, G353.6–0.7, associated with HESS J1731–347. G353.6–0.7 is likely an old supernova remnant (SNR), based on radio (0.8, 1.4, and 5 GHz), infrared (8 μm from the GLIMPSE Legacy Project and 21 μm from the Midcourse Space Experiment), and X-ray (0.1-2.4 keV from the ROSAT survey and 5-20 keV from the INTEGRAL survey) data. The SNR, centered at (l, b) = (353.55°, –0.65°) with a radius of ~0.25°, closely matches the outline of the recently discovered extended TeV source HESS J1731–347, which has no previously identified counterpart. A diffuse X-ray enhancement detected in the ROSAT all-sky survey is coincident with lower half-shell of the SNR. Therefore the SNR is the best radio counterpart of both the HESS source and the diffuse X-ray enhancement. G353.6–0.7 has an age of ~27,000 yr. Altogether, the new discovery provides the best case that an old SNR emits TeV γ-rays.

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 that needs to be accounted for when modeling light curves to extract constraints on the neutron stars 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 stars rotation. We also explain how the geometric oblateness effect can rival the Doppler effect for some emission geometries.

TYCHO SN 1572: A NAKED Ia SUPERNOVA REMNANT WITHOUT AN ASSOCIATED AMBIENT MOLECULAR CLOUD

The historical supernova remnant (SNR) Tycho SN 1572 originates from the explosion of a normal Type Ia supernova that is believed to have originated from a carbon-oxygen white dwarf in a binary system. We analyze the 21 cm continuum, H I, and 12CO-line data from the Canadian Galactic Plane Survey in the direction of SN 1572 and the surrounding region. We construct H I absorption spectra to SN 1572 and three nearby compact sources. We conclude that SN 1572 has no molecular cloud interaction, which argues against previous claims that a molecular cloud is interacting with the SNR. This new result does not support a recent claim that dust, newly detected by AKARI, originates from such an SNR-cloud interaction. We suggest that the SNR has a kinematic distance of 2.5-3.0 kpc based on a nonlinear rotational curve model. Very high energy γ-ray emission from the remnant has been detected by the VERITAS telescope, so our result shows that its origin should not be an SNR-cloud interaction. Both radio and X-ray observations support that SN 1572 is an isolated Type Ia SNR.

THE DISTANCES OF SNR W41 AND OVERLAPPING H II REGIONS

New H I images from the VLA Galactic Plane Survey show prominent absorption features associated with the supernovae remnant G23.3-0.3 (SNR W41). We highlight the H Iabsorption spectra and the 13CO emission spectra of eight small regions on the face of W41, including four H IIregions, three non-thermal emission regions, and one unclassified region. The maximum velocity of absorption for W41 is 78 ± 2 km s−1 and the CO cloud at radial velocity 95 ± 5 km s−1 is behind W41. Because an extended TeV source, a diffuse X-ray enhancement, and a large molecular cloud at radial velocity 77 ± 5 km s−1 overlap the center of W41, the kinematic distance is 3.9-4.5 kpc for W41. For the H IIregions, our analysis shows that both G23.42-0.21 and G23.07+0.25 are at the far kinematic distances (~9.9 kpc and ~10.6 kpc, respectively) of their recombination-line velocities (103 ± 0.5 km s−1 and 89.6 ± 2.1 km s−1, respectively), G23.07-0.37 is at the near kinematic distance (4.4 ± 0.3 kpc) of its recombination-line velocity (82.7 ± 2.0 km s−1), and G23.27-0.27 is probably at the near kinematic distance (4.1 ± 0.3 kpc) of its recombination-line velocity (76.1 ± 0.6 km s−1).

Limits on Mass and Radius for the Millisecond-Period X-Ray Pulsar SAX J1808.4–3658

SAX J1808.4–3658 has a 2.5 ms 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 J1808.4–3658 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 stars oblate shape. We also consider two different data sets, with different selections in time period (1 vs. 19 days of data combined) and different energy binning and time resolution. We find that including time delays and oblateness results in a 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.