Patricia Therese Boyd

The Swift Ultra-Violet/Optical Telescope

The Ultra-Violet/Optical Telescope (UVOT) is one of three instruments flying aboard the Swift Gamma-ray Observatory. It is designed to capture the early (∼1 min) UV and optical photons from the afterglow of gamma-ray bursts in the 170–600 nm band as well as long term observations of these afterglows. This is accomplished through the use of UV and optical broadband filters and grisms. The UVOT has a modified Ritchey–Chrétien design with micro-channel plate intensified charged-coupled device detectors that record the arrival time of individual photons and provide sub-arcsecond positioning of sources. We discuss some of the science to be pursued by the UVOT and the overall design of the instrument.

Very Early Optical Afterglows of Gamma-Ray Bursts: Evidence for Relative Paucity of Detection

Very early observations with the Swift satellite of γ-ray burst (GRB) afterglows reveal that the optical component is not detected in a large number of cases. This is in contrast to the bright optical flashes previously discovered in some GRBs (e.g., GRB 990123 and GRB 021211). Comparisons of the X-ray afterglow flux to the optical afterglow flux and prompt γ-ray fluence is used to quantify the seemingly deficient optical, and in some cases X-ray, light at these early epochs. This comparison reveals that some of these bursts appear to have higher than normal γ-ray efficiencies. We discuss possible mechanisms and their feasibility for explaining the apparent lack of early optical emission. The mechanisms considered include, foreground extinction, circumburst absorption, Lyα blanketing and absorption due to high-redshift, low-density environments, rapid temporal decay, and intrinsic weakness of the reverse shock. Of these, foreground extinction, circumburst absorption, and high redshift provide the best explanations for most of the nondetections in our sample. There is tentative evidence of suppression of the strong reverse shock emission. This could be because of a Poynting flux-dominated flow or a pure nonrelativistic hydrodynamic reverse shock.

GRB 060313: A New Paradigm for Short-Hard Bursts?

We report the simultaneous observations of the prompt emission in the gamma-ray and hard X-ray bands by the Swift BATand the Konus-Wind instruments of the short-hard burst, GRB 060313. The observations reveal multiple peaks in both the gamma-ray and hard X-ray bands suggesting a highly variable outflow from the central explosion. We also describe the early-time observations of the X-ray and UV/optical afterglows by the Swift XRT and UVOT instruments. The combination of the X-ray and UV/optical observations provides the most comprehensive light curves to date of a short-hard burst at such an early epoch. The afterglows exhibit complex structure with different decay indices and flaring. This behavior can be explained by the combination of a structured jet, radiative loss of energy, and decreasing microphysics parameters occurring in a circumburst medium with densities varying by a factor of approximately two on a length scale of 10 17 cm. These density variations are normally associated with the environment of a massive star and inhomogeneities in its windy medium. However, the mean density of the observed medium (n � 10 � 4 cm 3 ) is much less than that expected for a massive star. Although the collapse of a massive star as theoriginofGRB060313isunlikely,themergerofacompactbinaryalsoposesproblemsforexplainingthebehavior of this burst. Two possible suggestions for explaining this scenario are that some short bursts may arise from a mechanism that does not invoke the conventional compact binary model, or that soft late-time central engine activity is producing UV/optical but no X-ray flaring.

The Swift ultra-violet/optical telescope

The UV/optical telescope (UVOT) is one of three instruments flying aboard the Swift Gamma-ray Observatory. It is designed to capture the early (~1 minute) UV and optical photons from the afterglow of gamma-ray bursts as well as long term observations of these afterglows. This is accomplished through the use of UV and optical broadband filters and grisms. The UVOT has a modified Ritchey-Chretien design with micro-channel plate intensified charged-coupled device detectors that provide sub-arcsecond imaging. Unlike most UV/optical telescopes the UVOT can operate in a photon-counting mode as well as an imaging mode. We discuss some of the science to be pursued by the UVOT and the overall design of the instrument.

Swift Observations of GRB 050603: An Afterglow with a Steep Late-Time Decay Slope

We report the results of Swift observations of the gamma-ray burst GRB 050603. With a V magnitude V ¼ 18:2 about 10 hr after the burst, the optical afterglow was the brightest thus far detected bySwiftand one of the brightest optical afterglows ever seen. The Burst Alert Telescope (BAT) light curves show three fast-rise exponential-decay spikeswithT90 ¼ 12 s and afluence of 7:6 ;10 � 6 ergscm � 2 in the 15‐150 keV band. With E�; iso ¼1:26 ; 10 54 ergs, it was also one of the most energetic bursts of all times. The Swift spacecraft began observation of the afterglow with the narrow-field instruments about 10 hr after the detection of the burst. The burst was bright enough to be detectedbytheSwiftUV/Opticaltelescope(UVOT)foralmost3daysandbytheX-RayTelescope(XRT)foraweek after the burst. The X-ray light curve shows a rapidly fading afterglow with a decay index � ¼ 1:76 þ0:15 � 0:07 . The X-ray

The Spectrum of the Large Magellanic Cloud Pulsar B0540–69*

A prism spectrum of PSR B0540-69, the 50 ms pulsar in the Large Magellanic Cloud, was obtained with the Faint Object Spectrograph on the Hubble Space Telescope. This is only the second pulsar spectrum that has been observed in the optical. The 2500-5500 A spectrum shows a smooth continuum with several broad emission lines. The lines are emitted by the extended nebula surrounding B0540-69. Less than 30% of the total flux in the spectrum can be contributed by the synchrotron nebula surrounding the pulsar; the time-averaged emission from the pulsar must be the dominant source of the continuum. The observed spectral index, α, of the continuum, where flux density Sν = Kνα μJy, is -2.5 ± 0.2 between 2500 and 4500 A. Using E(B - V) = 0.20 ± 0.05 gives a de-extincted spectral index α0 = -1.6 ± 0.4. This intrinsic spectral index is significantly different from that of the Crab pulsar in the optical α0 = 0.11 ± 0.13. Unless E(B - V) ~ 0.5, the intrinsic spectral index of time-averaged pulsar radiation must vary as much from pulsar to pulsar in the high-frequency regime as it does in the radio.

THE Be/X-RAY BINARY SWIFT J1626.6-5156 AS A VARIABLE CYCLOTRON LINE SOURCE

Swift J1626.6−5156 is a Be/X-ray binary that was in outburst from 2005 December until 2008 November. We have examined Rossi X-ray Timing Explorer/Proportional Counter Array (PCA) and High Energy X-ray Timing Explorer spectra of three long observations of this source taken early in its outburst, when the PCA 2–20 keV count rate was >70 counts s −1 PCU −1 , as well as several combined observations from different stages of the outburst. The spectra are best fit with an absorbed cutoff power law with a ∼6.4 keV iron emission line and a Gaussian optical depth absorption line at ∼10 keV. We present strong evidence that this absorption-like feature is a cyclotron resonance scattering feature, making Swift J1626.6−5156 a new candidate cyclotron line source. The redshifted energy of ∼10 keV implies a magnetic field strength of ∼8.6(1 + z) × 10 11 G in the region of the accretion column close to the magnetic poles where the cyclotron line is produced. Analysis of phase-averaged spectra spanning the duration of the outburst suggests a possible positive correlation between the fundamental cyclotron energy and source luminosity. Phase-resolved spectroscopy from a long observation reveals a variable cyclotron line energy, with phase dependence similar to a variety of other pulsars, as well as the first harmonic of the fundamental cyclotron line.

Fine-Tuning the Accretion Disk Clock in Hercules X-1

Rossi X-Ray Timing Explorer all-sky monitor count rates from the X-ray pulsar Her X-1 began falling consistently during the late months of 2003. The source is undergoing another state transition similar to the anomalous low state of 1999. This new event has triggered observations from both space- and ground-based observatories. In order to aid data interpretation and telescope scheduling and to facilitate the phase connection of cycles before and after the state transition, we have recalculated the precession ephemeris using cycles over the last 3.5 yr. We report that the source has displayed a different precession period since the last anomalous event. Additional archival data from the Compton Gamma Ray Observatory suggests that each low state is accompanied by a change in precession period and that the subsequent period is correlated with accretion flux. Consequently our analysis reveals long-term accretion disk behavior that is predicted by theoretical models of radiation-driven warping.

Discovery of Millihertz ULTRAVIOLET Quasi-periodic Oscillations in Hercules X-1

Observations of the ultraviolet continuum of the X-ray binary system Her X-1/HZ Herculis with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST) show quasi-periodic oscillations (QPOs) at frequencies of 8 ± 2 and 43 ± 2 mHz, with rms amplitudes of 2% and 4% of the steady flux. Observations with the Keck Telescope confirm the presence of the higher frequency QPO in the optical continuum, with a rms amplitude of 1.6% ± 0.2%. The QPOs are most prominent in the HST data near = 0.5 (where = 0 is the middle of the X-ray eclipse), suggesting that they arise not in the accretion disk but on the X-ray-heated face of the companion star. We discuss scenarios in which the companion star reprocesses oscillations in the disk which are caused by either Keplerian rotation or a beat frequency between the neutron star spin and Keplerian rotation at some radius in the accretion disk.

GRB 060607A: a gamma-ray burst with bright asynchronous early X-ray and optical emissions

The early optical emission of the moderately high redshift (