J. Garrett Jernigan

The Planet around 51 Pegasi

Doppler measurements of 51 Pegasi have been made from 1995 October through 1996 August, with a precision of 5 m s-1. We find a period of 4.231 days, a velocity amplitude of 56 ± 1 m s-1, and a velocity curve that is essentially sinusoidal, all in excellent agreement with Mayor & Queloz. The only viable interpretation is a companion having minimum mass, m sin i = 0.45 MJupiter, in a circular orbit of radius of 0.051 AU, with an eccentricity less than 0.01. Alternative explanations involving stellar surface phenomena such as pulsation or spots are ruled out. The lack of tidal spin-up of the star constrains the mass of the companion to be less than 15 MJupiter. If the tidal Q-value is less than ~106 for the planet (close to Jupiters presumed value), then internal dissipation is adequate to circularize the orbit and synchronize the planets rotation. After subtracting the best-fit Keplerian velocity curve, the residuals exhibit no apparent variations at a level of 5 m s-1 during 10 months. The absence of further reflex motion along with limits from IR speckle observations rule out additional companions in a large portion of the parameter space of mass and orbital radius, including all masses greater than 1 MJupiter within 2.0 AU.

The X-Ray Afterglows of GRB 020813 and GRB 021004 with Chandra HETGS: Possible Evidence for a Supernova Prior to GRB 020813

We report on the detection of an emission line near 1.3 keV, which we associate with blueshifted hydrogen-like sulfur (S XVI), in a 76.8 ks Chandra HETGS spectrum of the afterglow of GRB 020813. The line is detected at 3.3 σ significance. We also find marginal evidence for a line possibly due to hydrogen-like silicon (Si XIV) with the same blueshift. A line from Fe is not detected, although a very low significance Ni feature may be present. A thermal model fits the data adequately, but a reflection model may provide a better fit. There is marginal evidence that the equivalent width of the S XVI line decreases as the burst fades. We infer from these results that a supernova likely occurred 2 months prior to the γ-ray burst. We find no discrete or variable spectral features in the Chandra HETGS spectrum of the GRB 021004 afterglow.

SILICON PIN DIODE ARRAY HYBRIDS FOR CHARGED PARTICLE DETECTION

We report on the design of silicon PIN diode array hybrids for use as charged particle detectors. A brief summary of the need for vertex detectors is presented. Circuitry, block diagrams and device specifications are included. 8 refs., 7 figs., 1 tab.

GRO J1744–28 and Scorpius X-1: First Evidence for Photon Bubble Oscillations and Turbulence

We discuss our recent Rossi X-Ray Timing Explorer (RXTE) observations of GRO J1744-28, which discovered quasi-periodic oscillations (QPOs) of the intensities in the energy band 3-12 keV during observations starting on 1996 January 18.77 UT. We have found that the power spectrum in the frequency band 5-1000 Hz consists of two red-noise components that can be characterized by two power laws, each with an index of -5/3 and a QPO peak centered at 40 Hz. We suggest that the power peak is due to a newly discovered form of turbulence in the accretion column of super-Eddington accretion-powered pulsars driven by photon bubble instabilities. These instabilities give rise to strong power peaks at frequencies characteristic of photon diffusion and bubble coalescence in the highly nonlaminar accretion column resulting in photon bubble oscillations (PBOs). The relationship between the rms amplitude of the PBOs and the intensity is in qualitative agreement with observations. Our calculations also suggest that the observed high-frequency red-noise component with a -5/3 power-law index from 40 to 600 Hz is the first evidence of photon bubble turbulence in the accretion column of an X-ray pulsar. Recent RXTE observations of Sco X-1 have found high-frequency QPOs at 1100 and 830 Hz. We show that PBOs at these frequencies are a natural consequence of photon bubble instabilities. We also show that the rms amplitudes of the calculated PBOs at these frequencies are consistent with the observations. We predict that further RXTE observations of Sco X-1 should reveal additional PBOs at 2000 and 2600 Hz as well as a broadband continuum spectrum with a -5/3 power law, extending from 3000 to several times 104 Hz.

Simulating the LSST system

Extracting science from the LSST data stream requires a detailed knowledge of the properties of the LSST catalogs and images (from their detection limits to the accuracy of the calibration to how well galaxy shapes can be characterized). These properties will depend on many of the LSST components including the design of the telescope, the conditions under which the data are taken and the overall survey strategy. To understand how these components impact the nature of the LSST data the simulations group is developing a framework for high fidelity simulations that scale to the volume of data expected from the LSST. This framework comprises galaxy, stellar and solar system catalogs designed to match the depths and properties of the LSST (to r=28), transient and moving sources, and image simulations that ray-trace the photons from above the atmosphere through the optics and to the camera. We describe here the state of the current simulation framework and its computational challenges.

Spectral-Lag Relations in GRB Pulses Detected with HETE-2

Using a pulse-fit method, we investigated the spectral lags between the traditional gamma-ray band (50–400 keV) and the X-ray band (6–25 keV) for 8 GRBs with known redshifts (GRB 010921, GRB 020124, GRB 020127, GRB 021211, GRB 030528, GRB 040924, GRB 041006, and GRB 050408), detected with the WXM and FREGATE instruments aboard the HETE-2 satellite. We found several relations for individual GRB pulses between the spectral lag and other observables, such as the luminosity, pulse duration, and peak energy, Epeak. The obtained results are consistent with those for BATSE, indicating that the BATSE correlations are still valid at lower energies (6–25 keV). Furthermore, we found that the photon energy dependence for the spectral lags can be reconciled with the simple curvature effect model. We discuss the implications of these results from various points of view.

An Optically Dark GRB Observed by HETE-2: GRB 051022

GRB 051022 was detected at 13:07:58 on 2005 October 22 UT by HETE-2. The location of GRB 051022 was determined immediately by the flight localization system. This burst contained multiple pulses, and had a rather long duration of about 190s. The detections of candidate X-ray and radio afterglows were reported, whereas no optical afterglow was found. Optical spectroscopic observations of the host galaxy revealed a redshift of z = 0.8. Using data derived by a HETE-2 observation of the prompt emission, we found absorption of NH = (8.8-2.9+3.1) × 1022 cm-2 and visual extinction of AV = 49-16+17 mag in the host galaxy. If this is the case, no detection of any optical transient would be quite reasonable. The absorption derived by Swift XRT observations of the afterglow is fully consistent with those obtained from an early HETE-2 observation of the prompt emission. Our analysis implies an interpretation that the absorbing medium may be outside the external shock at R ? 1016cm, which could be a dusty molecular cloud.

Development of an ultra-low-power x-ray-photon-resolving imaging detector array

We report on progress to develop and demonstrate CZT and Si hybrid detector arrays for future NASA missions in X-ray and Gamma-ray astronomy. The primary goal for these detectors is consistent with the design concept for the EXIST mission1 and will also be appropriate for other NASA applications and ground-based projects. In particular we target science instruments that have large aperture (multiple square meters) and therefore require a low power ROIC (readout integrated circuits) design (< 10 microwatt per pixel in quiescent mode). The design also must achieve good energy resolution for single photon detection for X rays in the range 5-600 keV with a CZT sense layer and 2-30 keV with a Si sense layer. The target CZT arrays are 2 cm × 2 cm with 600 micron square-shaped pixels. The low power smart pixel detects rare X-ray hits with an adjustable threshold setting. A test array of 7 × 5 pixels with a 5 mm thick CZT sense layer demonstrates that the low power pixel can successfully detect X-rays with ~50 readout noise electrons RMS.

HETE-2 observations of the X-ray flash XRF 040916

A long X-ray flash was detected and localized by instruments aboard the High Energy Transient Explorer II (HETE-2) at 00:03:30 UT on 2004 September 16. The position was reported to the GRB Coordinates Network (GCN) approximately 2 hours after the burst. This burst consisted of two peaks separated by ? 200 s, with durations of about 110 s and 60 s. We analyzed the energy spectra of the 1st and 2nd peaks observed with the Wide Field X-Ray Monitor (WXM) and the French Gamma Telescope (FREGATE). We discuss the origin of the 2nd peak in terms of the flux variabilities and timescales. We find that it is most likely to be part of prompt emission, and is explained by a long-acting engine model. This feature is similar to some bright X-ray flares detected in the early afterglow phase of bursts observed by the Swift satellite.

KiloHertz QPO and gravitational wave emission as the signature of the rotation and precession of a LMXB neutron star near breakup

The theory of torque free precession (TFP) of the outer crust of a neutron star (NS) as the signature of the approach to NS breakup is a viable explanation of the uniform properties of kHz Quasi-periodic Oscillations (QPO) observed in X-rays emitted by Low Mass X-ray Binary (LMXB) sources. The TFP model is in strong contrast to existing models which primarily relate the kHz QPO phenomenon to the physics of gas dynamics near the inner edge of the accretion disk and the transition flow onto the surface of the NS. We suggest the possibility of the direct detection of very low frequency (∼1 kHz) radio waves from magnetic dipole radiation and also predict kHz gravitational wave emission from the LMXB Sco X-1 that may be detectable by LIGO. The high accretion rates consistent with the predicted GW emission indicate the likely conversion of some LMXBs to maximally rotating Kerr black holes (BH) and further suggest that these systems are progenitors of some gamma-ray bursts (GRB).