Gamma-Ray Burst Spectral Features: Interpretation as X-ray Emission From A Photoionized Plasma

Numerous reports have been made of features, either in emission or absorption, in the 10 - 1000 keV spectra of some gamma-ray bursts. Originally interpreted in the context of Galactic neutron star models as cyclotron line emission and $e^+ - e^-$ annihilation features, the recent demonstration that the majority of GRBs lie at cosmological distances make these explanations unlikely. In this letter, we adopt a relativistic fireball model for cosmological GRBs in which dense, metal rich blobs or filaments of plasma are entrained in the relativistic outflow. In the context of this model, we investigate the conditions under which broadband features, similar to those detected, can be observed. We find a limited region of parameter space capable of reproducing the observed GRB spectra. Finally, we discuss possible constraints further high-energy spectral observations could place on fireball model parameters.Comment: Accepted for publication in Astrophysical Journal Letters Four pages, 2 figure

An imaging gas scintillation proportional counter for the detection of subkiloelectron-volt X-rays

A large area imaging gas scintillation proportional counter (IGSPC) was constructed for use in X-ray astronomy. The IGSPC consists of a gas scintillation proportional counted (GSPC) with a micron polyprotylene window coupled to a multiwire proportional counter (MWPC) via a calcium fluoride window. Over a sensitive area of 21 cu cm the instrument has a measured energy resolution of 17.5% (FWHM) and 1.9 mm (FWHM) spatial resolution at 1.5 keV

Experimental Verification of Attitude Control Techniques for Slew Maneuvers of Flexible Spacecraft

The article of record as published may be found at http://dx.doi.org/10.2514/6.1992-4456This paper presents experimental verification of modern and classical control laws on flexible spacecraft structures. The Flexible Spacecraft Simulator at the Naval Postgraduate School is designed to test a variety of control theory on a two-dimensional representation of an antenna at the end of a low-frequency astromast. The Simulator represents motion about the pitch axis and is restricted to rotatation only. Control laws are implemented through a momentum wheel mounted on the rigid main body. Feedback is obtained through a rotary variable differential transformer (RVDT) which senses the body's rotation angle and a rate-gyro giving body rate. The analytical model contains the linearized equations of motion accounting for the flexible dynamics. Slewing maneuvers are conducted for positioning the main body by using proportional- derivative (PD), torque profiles and optimal controllers. No active control is applied to the flexible structure. A new technique for state estimation is developed for the optimal controller since the standard estimation methods prove to be unsatisfactory. In all cases, the experimental results are in close agreement with the analytical predictions. This paper presents experimental verification of modern and classical control laws on flexible spacecraft structures. The Flexible Spacecraft Simulator at the Naval Postgraduate School is designed to test a variety of control theory on a two-dimensional representation of an antenna at the end of a low-frequency astromast. The Simulator represents motion about the pitch axis and is restricted to rotatation only. Control laws are implemented through a momentum wheel mounted on the rigid main body. Feedback is obtained through a rotary variable differential transformer (RVDT) which senses the body's rotation angle and a rate-gyro giving body rate. The analytical model contains the linearized equations of motion accounting for the flexible dynamics. Slewing maneuvers are conducted for positioning the main body by using proportional- derivative (PD), torque profiles and optimal controllers. No active control is applied to the flexible structure. A new technique for state estimation is developed for the optimal controller since the standard estimation methods prove to be unsatisfactory. In all cases, the experimental results are in close agreement with the analytical predictions. This paper presents experimental verification of modern and classical control laws on flexible spacecraft structures. The Flexible Spacecraft Simulator at the Naval Postgraduate School is designed to test a variety of control theory on a two-dimensional representation of an antenna at the end of a low-frequency astromast. The Simulator represents motion about the pitch axis and is restricted to rotatation only. Control laws are implemented through a momentum wheel mounted on the rigid main body. Feedback is obtained through a rotary variable differential transformer (RVDT) which senses the body's rotation angle and a rate-gyro giving body rate. The analytical model contains the linearized equations of motion accounting for the flexible dynamics. Slewing maneuvers are conducted for positioning the main body by using proportional- derivative (PD), torque profiles and optimal controllers. No active control is applied to the flexible structure. A new technique for state estimation is developed for the optimal controller since the standard estimation methods prove to be unsatisfactory. In all cases, the experimental results are in close agreement with the analytical predictions. This paper presents experimental verification of modern and classical control laws on flexible spacecraft structures. The Flexible Spacecraft Simulator at the Naval Postgraduate School is designed to test a variety of control theory on a two-dimensional representation of an antenna at the end of a low-frequency astromast. The Simulator represents motion about the pitch axis and is restricted to rotatation only. Control laws are implemented through a momentum wheel mounted on the rigid main body. Feedback is obtained through a rotary variable differential transformer (RVDT) which senses the body's rotation angle and a rate-gyro giving body rate. The analytical model contains the linearized equations of motion accounting for the flexible dynamics. Slewing maneuvers are conducted for positioning the main body by using proportional- derivative (PD), torque profiles and optimal controllers. No active control is applied to the flexible structure. A new technique for state estimation is developed for the optimal controller since the standard estimation methods prove to be unsatisfactory. In all cases, the experimental results are in close agreement with the analytical predictions

Mid-J CO Emission From NGC 891: Microturbulent Molecular Shocks in Normal Star Forming Galaxies

We have detected the CO(6-5), CO(7-6), and [CI] 370 micron lines from the nuclear region of NGC 891 with our submillimeter grating spectrometer ZEUS on the CSO. These lines provide constraints on photodissociation region (PDR) and shock models that have been invoked to explain the H_2 S(0), S(1), and S(2) lines observed with Spitzer. We analyze our data together with the H_2 lines, CO(3-2), and IR continuum from the literature using a combined PDR/shock model. We find that the mid-J CO originates almost entirely from shock-excited warm molecular gas; contributions from PDRs are negligible. Also, almost all the H_2 S(2) and half of the S(1) line is predicted to emerge from shocks. Shocks with a pre-shock density of 2x10^4 cm^-3 and velocities of 10 km/s and 20 km/s for C-shocks and J-shocks, respectively, provide the best fit. In contrast, the [CI] line emission arises exclusively from the PDR component, which is best parameterized by a density of 3.2x10^3 cm^-3 and a FUV field of G_o = 100 for both PDR/shock-type combinations. Our mid-J CO observations show that turbulence is a very important heating source in molecular clouds, even in normal quiescent galaxies. The most likely energy sources for the shocks are supernovae or outflows from YSOs. The energetics of these shock sources favor C-shock excitation of the lines.Comment: 18 pages, 2 figures, 6 tables, accepted by Ap

Evidence for a Mid-Atomic-Number Atmosphere in the Neutron Star 1E1207.4-5209

Recently Sanwal et al. (2002) reported the first clear detection of absorption features in an isolated neutron star, 1E1207.4-5209. Remarkably their spectral modeling demonstrates that the atmosphere cannot be Hydrogen. They speculated that the neutron star atmosphere is indicative of ionized Helium in an ultra-strong (~1.5x10^{14} G) magnetic field. We have applied our recently developed atomic model (Mori & Hailey 2002) for strongly-magnetized neutron star atmospheres to this problem. We find that this model, along with some simp le atomic physics arguments, severely constrains the possible composition of the atmosphere. In particular we find that the absorption features are naturally associated with He-like Oxygen or Neon in a magnetic field of ~10^{12} G, comparable to the magnetic field derived from the spin parameters of the neutron star. This interpretation is consistent with the relative line strengths and widths and is robust. Our model predicts possible substructure in the spectral features, which has now been reported by XMM-Newton (Mereghetti et al. 2002). However we show the Mereghetti et al. claim that the atmosphere is Iron or some comparable high-Z element at ~ 10^{12} G is easily ruled out by the Chandra and XMM-Newton data.Comment: 5 pages, AASTeX, Revised version. Accepted for publication in ApJ Letter

Development of Aluminum LEKIDs for Balloon-Borne Far-IR Spectroscopy

We are developing lumped-element kinetic inductance detectors (LEKIDs) designed to achieve background-limited sensitivity for far-infrared (FIR) spectroscopy on a stratospheric balloon. The Spectroscopic Terahertz Airborne Receiver for Far-InfraRed Exploration (STARFIRE) will study the evolution of dusty galaxies with observations of the [CII] 158 $\mu$m and other atomic fine-structure transitions at $z=0.5-1.5$, both through direct observations of individual luminous infrared galaxies, and in blind surveys using the technique of line intensity mapping. The spectrometer will require large format ($\sim$1800 detectors) arrays of dual-polarization sensitive detectors with NEPs of $1 \times 10^{-17}$ W Hz$^{-1/2}$. The low-volume LEKIDs are fabricated with a single layer of aluminum (20 nm thick) deposited on a crystalline silicon wafer, with resonance frequencies of $100-250$ MHz. The inductor is a single meander with a linewidth of 0.4 $\mu$m, patterned in a grid to absorb optical power in both polarizations. The meander is coupled to a circular waveguide, fed by a conical feedhorn. Initial testing of a small array prototype has demonstrated good yield, and a median NEP of $4 \times 10^{-18}$ W Hz$^{-1/2}$.Comment: accepted for publication in Journal of Low Temperature Physic