International ultraviolet explorer observing program: Ultraviolet observations of selected astronomical sources

Progress is described for the following topics: rapid variations in the far ultraviolet spectrum of the cool Ap star 21 Com; elliptical Seyfert galaxies; variations of cool Ap stars with strong magnetic fields; and X-ray selected BL Lac Objects

Imaging the Nearby Seyfert 2 Galaxy NGC 1068, and Spectrum and Variability of Geminga

The results of the research for NASA Grant NAG5-1606 are summarized in the following publications: (1) A ROSAT high resolution image of NGC 1068; (2) Discovery of soft x-ray pulsations from the gamma-ray source Geminga; and (3) Soft x-ray properties of the Geminga pulsar

Observations and Modeling of the Companions of Short Period Binary Millisecond Pulsars: Evidence for High-Mass Neutron Stars

We present observations of fields containing eight recently discovered binary millisecond pulsars using the telescopes at MDM Observatory. Optical counterparts to four of these systems are detected, one of which, PSR J2214+3000, is a novel detection. Additionally, we present the fully phase-resolved B, V, and R light curves of the optical counterparts to two objects, PSR J1810+1744 and PSR J2215+5135 for which we employ model fitting using the ELC model of Orosz & Hauschildt (2000) to measure the unknown system parameters. For PSR J1810+1744 we find that the system parameters cannot be fit even assuming that 100% of the spin-down luminosity of the pulsar is irradiating the secondary, and so radial velocity measurements of this object will be required for the complete solution. However, PSR J2215+5135 exhibits light curves that are extremely well constrained using the ELC model and we find that the mass of the neutron star is constrained by these and the radio observations to be greater than 1.75 solar masses at the 3-sigma level. We also find a discrepancy between the model temperature and the measured colors of this object which we interpret as possible evidence for an additional high-temperature source such as a quiescent disk. Given this and the fact that PSR J2214+5135 contains a relatively high mass companion (greater than 0.1 solar masses), we propose that similar to the binary pulsar systems PSR J1023+0038 and IGR J18245-2452, the pulsar may transition between accretion- and rotation-powered modes.Comment: Submitted to ApJ, 12 pages, 5 figures, 2 table

Optical and X-ray Studies of Ten X-ray Selected Cataclysmic Binaries

We report on ground-based optical observations of ten cataclysmic binaries that were discovered through their X-ray emission. Time-resolved radial velocity spectroscopy yields unambiguous orbital periods for eight objects and ambiguous results for the remaining two. The orbital periods range from 87 min to 9.38 hr. We also obtained time-series optical photometry for six targets, four of which have coherent pulsations. These periods are 1218 s for 1RXS J045707.4+452751, 628 s for AX J1740.2-2903, 477 s for AX J1853.3-0128, and 935 s for IGR J19267+1325. A total of seven of the sources have coherent oscillations in X-rays or optical, indicating that they are intermediate polars (DQ Herculis stars). Time-resolved spectroscopy of one object, Swift J2218.4+1925, shows that it is an AM Herculis star, or polar, and IGR J19552+0044 may also be in that class. For another object, Swift J0476.2-1611, we find an orbital period of 9.384 hr and detect the spectrum of the secondary star. The secondary's spectral contribution implies a distance of 900 (+190, -150) pc, where the error bars are estimated using a Monte Carlo technique to account for correlated uncertainties.Comment: Accepted for publication in The Astronomical Journal. 38 pages, 16 figures Revised to include a correct finding chart for RX J0457+4

Optical Counterparts of Two Fermi Millisecond Pulsars: PSR J1301+0833 and PSR J1628-3205

Using the 1.3m and 2.4m telescopes of the MDM Observatory, we identified the close companions of two eclipsing millisecond radio pulsars discovered by the Green Bank Telescope in searches of Fermi Gamma-ray Space Telescope sources, and measured their light curves. PSR J1301+0833 is a black widow pulsar in a 6.5 hr orbit whose companion star is strongly heated on the side facing the pulsar. It varies from R = 21.8 to R > 24 around the orbit. PSR J1628-3205 is a "redback," a nearly Roche-lobe filling system in a 5.0 hr orbit whose optical modulation in the range 19.0 < R < 19.4 is dominated by strong ellipsoidal variations, indicating a large orbital inclination angle. PSR J1628-3205 also shows evidence for a long-term variation of about 0.2 mag, and an asymmetric temperature distribution possibly due to either off-center heating by the pulsar wind, or large starspots. Modelling of its light curve restricts the inclination angle to i > 55 degrees, the mass of the companion to 0.16 < M_c < 0.30 M_sun, and the effective temperature to 3560 < T_eff < 4670 K. As is the case for several redbacks, the companion of PSR J1628-3205 is less dense and hotter than a main-sequence star of the same mass.Comment: 9 pages, 5 figures, accepted for publication in Ap

Multiwavelength observations of unidentified high energy gamma-ray sources

As was the case for COS B, the majority of high-energy (greater than 100 MeV) gamma-ray sources detected by the EGRET instrument on GRO are not immediately identifiable with catalogued objects at other wavelengths. These persistent gamma-ray sources are, next to the gamma-ray bursts, the least understood objects in the universe. This two year investigation is intended to support the analysis, correlation, and theoretical interpretation of data that we are obtaining at x-ray, optical, and radio wavelengths in order to render the gamma-ray data interpretable. This second year was devoted to studies of unidentified gamma-ray sources from the first EGRET catalog, similar to previous observations. Efforts have concentrated on the sources at low and intermediate Galactic latitudes, which are the most plausible pulsar candidates