Lynn R. Cominsky

The Swift gamma-ray burst mission

The Swift mission, scheduled for launch in 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is a first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts yr � 1 and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to (1) determine the origin of GRBs, (2) classify GRBs and search for new types, (3) study the interaction of the ultrarelativistic outflows of GRBs with their surrounding medium, and (4) use GRBs to study the early universe out to z >10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a newgeneration wide-field gamma-ray (15‐150 keV) detector that will detect bursts, calculate 1 0 ‐4 0 positions, and trigger autonomous spacecraft slews; a narrow-field X-ray telescope that will give 5 00 positions and perform spectroscopy in the 0.2‐10 keV band; and a narrow-field UV/optical telescope that will operate in the 170‐ 600 nm band and provide 0B3 positions and optical finding charts. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of � 1m crab (� 2;10 � 11 ergs cm � 2 s � 1 in the 15‐150 keV band), more than an order of magnitude better than HEAO 1 A-4. A flexible data and operations system will allow rapid follow-up observations of all types of

The Nuclear Spectroscopic Telescope Array (NuSTAR) High-Energy X-Ray Mission

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a National Aeronautics and Space Administration (NASA) Small Explorer mission that carried the first focusing hard X-ray (6-79 keV) telescope into orbit. It was launched on a Pegasus rocket into a low-inclination Earth orbit on June 13, 2012, from Reagan Test Site, Kwajalein Atoll. NuSTAR will carry out a two-year primary science mission. The NuSTAR observatory is composed of the X-ray instrument and the spacecraft. The NuSTAR spacecraft is three-axis stabilized with a single articulating solar array based on Orbital Sciences Corporations LEOStar-2 design. The NuSTAR science instrument consists of two co-aligned grazing incidence optics focusing on to two shielded solid state CdZnTe pixel detectors. The instrument was launched in a compact, stowed configuration, and after launch, a 10-meter mast was deployed to achieve a focal length of 10.15 m. The NuSTAR instrument provides sub-arcminute imaging with excellent spectral resolution over a 12-arcminute field of view. The NuSTAR observatory will be operated out of the Mission Operations Center (MOC) at UC Berkeley. Most science targets will be viewed for a week or more. The science data will be transferred from the UC Berkeley MOC to a Science Operations Center (SOC) located at the California Institute of Technology (Caltech). In this paper, we will describe the mission architecture, the technical challenges during the development phase, and the post-launch activities.

A short γ-ray burst apparently associated with an elliptical galaxy at redshift z = 0.225

Gamma-ray bursts (GRBs) come in two classes: long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at high redshift (z ≈ 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars. In contrast, no short GRB had been accurately (< 10″) and rapidly (minutes) located. Here we report the detection of the X-ray afterglow from—and the localization of—the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect if the origin of this GRB is through the merger of neutron-star or black-hole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs.

The Nuclear Spectroscopic Telescope Array (NuSTAR)

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a NASA Small Explorer mission that will carry the first focusing hard X-ray (6 - 80 keV) telescope to orbit. NuSTAR will offer a factor 50 - 100 sensitivity improvement compared to previous collimated or coded mask imagers that have operated in this energy band. In addition, NuSTAR provides sub-arcminute imaging with good spectral resolution over a 12-arcminute eld of view. After launch, NuSTAR will carry out a two-year primary science mission that focuses on four key programs: studying the evolution of massive black holes through surveys carried out in fields with excellent multiwavelength coverage, understanding the population of compact objects and the nature of the massive black hole in the center of the Milky Way, constraining the explosion dynamics and nucleosynthesis in supernovae, and probing the nature of particle acceleration in relativistic jets in active galactic nuclei. A number of additional observations will be included in the primary mission, and a guest observer program will be proposed for an extended mission to expand the range of scientic targets. The payload consists of two co-aligned depth-graded multilayer coated grazing incidence optics focused onto a solid state CdZnTe pixel detectors. To be launched in early 2012 on a Pegasus rocket into a low-inclination Earth orbit, NuSTAR largely avoids SAA passage, and will therefore have low and stable detector backgrounds. The telescope achieves a 10.14-meter focal length through on-orbit deployment of an extendable mast. An aspect and alignment metrology system enable reconstruction of the absolute aspect and variations in the telescope alignment resulting from mast exure during ground data processing. Data will be publicly available at GSFCs High Energy Archive Research Center (HEASARC) following validation at the science operations center located at Caltech.

Detection of X-ray emission from the PSR 1259-63/SS 2883 binary system

Nonpulsed but variable X-ray emission has been detected from the binary system containing the radio pulsar PSR 1259-63 during two pointed ROSAT observations, taken 5 months apart. This 47.7 ms radio pulsar is in a highly eccentric (epsilon approximately 0.85) binary system with the 10-15 solar mass Be star SS 2883. It is the first radio pulsar found to be in a binary system with a massive main-sequence companion; it is also the most highly eccentric binary system known to contain a neutron star. The level of X-ray flux detected in the ROSAT observations has increased with orbital phase by a factor of at least 10 between 1992 February and 1993 February. The X-ray flux is significantly greater than expected from the Be stars corona and seems likely to originate either from low-level stellar wind accretion onto the neutron star or from the shock between the stellar wind and the relativistic pulsar wind. The system may be the progenitor of the more slowly rotating Be X-ray binary pulsar systems.

Phase-dependent Spectral Variability in 4U 1907+09

We report on ASCA, RXT E, and archival observations of the high-mass X-ray binary pulsar 4U 1907]09. Spectral measurements of the absorption and —ux were made at all phases of the X-ray pulsar orbit, including the —rst spectral measurements of an extended period of low —ux during two of the ASCA observations. We —nd that a simple spherical wind model can —t the time-averaged light curve as measured by the RXT E All-Sky Monitor but does not —t the observed changes in the absorption column or account for the existence of the phase-locked secondary —are. An additional model component consisting of a trailing stream can account for the variations in column depth. However, these models favor a high inclination angle for the system, suggesting a companion mass more consistent with an identi—cation as a Be star. In this case, an equatorially enhanced wind and inclined neutron star orbit may be a more appropriate interpretation of the data.

Swift observations of GRB 050904: The most distant cosmic explosion ever observed

Context. Swift discovered the high redshift (z = 6.29) GRB 050904 with the Burst Alert Telescope (BAT) and began observing with its narrow field instruments 161 s after the burst onset. This gamma-ray burst is the most distant cosmic explosion ever observed. Because of its high redshift, the X-ray Telescope (XRT) and BAT simultaneous observations provide 4 orders of magnitude of spectral coverage (0.2-150 keV; 1.4-1090 keV in the source rest frame) at a very early source-frame time (22 s). The X-ray emission was monitored by the XRT up to 10 days after the burst. Aims. We present the analysis of BAT and XRT observations of GRB 050904 and a complete description of its high energy phenomenology. Methods. We performed time resolved spectral analysis and light curve modeling. Results. GRB 050904 was a long, multi-peaked, bright GRB with strong variability during its entire evolution, The light curve observed by the XRT is characterized by the presence of a long flaring activity lasting up to 1-2 h after the burst onset in the burst rest frame, with no evidence of a smooth power-law decay following the prompt emission as seen in other GRBs. However, the BAT tail extrapolated to the XRT band joins the XRT early light curve and the overall behavior resembles that of a very long GRB prompt. The spectral energy distribution softens with time, with the photon index decreasing from -1.2 during the BAT observation to -1.9 at the end of the XRT observation. The dips of the late X-ray flares may be consistent with an underlying X-ray emission arising from the forward shock and with the properties of the optical afterglow reported by Tagliaferri et al. (2005b, AA very low metallicities of the progenitor at these epochs may provide an explanation.

A Multi-Institutional Investigation of Students' Preinstructional Ideas about Cosmology.

In order to improve instruction in introductory astronomy, we are investigating students’ preinstructional ideas about a number of cosmology topics. This article describes one aspect of this large research study in which 1270 students responded to a subset of three questions each from a larger set of questions about the following areas: definition of a light-year and the structure, composition, and evolution of the Universe. Within structure, we investigated students’ ideas about definitions or descriptions of Solar System, Galaxy, Universe, and the relationships among them. Composition included the formation of chemical elements, dark matter, and dark energy, while evolution focused on the Big Bang Theory, age of the Universe, and how the Universe changes over time. Responses were iteratively coded for common themes. Major findings demonstrate that students commonly misidentify the light-year as a measurement of time, and that they provide incomplete definitions of common objects (Solar System, Galaxy) and the Universe itself, often conflating the terms. Generally speaking, students have little understanding of dark matter or dark energy, providing definitions that are superficial or do not answer the question. Consistent with previous research, we found students view the Big Bang as an explosion. Students’ ideas about the age of the Universe range from millions to trillions of years, but some students believe the Universe to be infinitely old. For both the age of the Universe and the Big Bang Theory, students are not familiar with the scientific evidence that exists, and in some cases do not believe such evidence can exist. Finally, students’ understanding of how the Universe changes over time is based largely on smaller changes of objects within it (e.g., stellar evolution) or the motions of objects (e.g., planetary orbits). These and other ideas provide fodder—both scientifically accurate and inaccurate—on which to build effective instruction. Particular attention should be paid to areas in which words that are used differently between our everyday vernacular and scientific language can create or reinforce alternative conceptions.

Eclipse Timings of the Low-Mass X-Ray Binary EXO 0748–676. II. Detection of an Apparent Orbital Period Change and of Orbital Period Noise

The rare eclipsing low-mass X-ray binary EXO 0748-676 has now been monitored accurately for over a decade in the hope of detecting orbital period evolution. We report observations of 10 complete eclipses at resolution less than 0.1 s with the Rossi X-ray Timing Explorer during 1996 May and 1996 August. We have timed the eclipses with an accuracy limited by photon statistics to 0.2-0.5 s. When combined with historical measurements, the eclipse timings imply an unexpectedly large orbital period derivative, orb=3.3 × 10−11, and a rapid timescale for orbital evolution, τorb = 1.3 × 107 yr. The observations also require considerable intrinsic jitter in the mideclipse timings; a maximum likelihood estimate is 0.15 s per orbital cycle. We consider various models and scenarios that might account for these unexpected results.

Swift X-Ray Telescope Observations of the Deep Impact Collision

Comet 9P/Tempel 1 was observed by the Swift X-Ray Telescope (XRT) for a total of 250,024 s. Soft X-ray emission, 0.2-1.0 keV, was seen as a diffuse extended halo with an FWHM of 1.03 × 105 km centered on the comets nucleus. The X-ray light curve indicates that the comet exhibited a prolonged soft X-ray outburst just after impact of the NASA Deep Impact (DI) spacecraft and enhanced X-ray activity lasted for 12 days. The radial brightness distribution and X-ray spectrum are in excellent agreement with a model of X-ray production in which highly charged minor heavy ion species in the solar wind undergo charge exchange reactions with water group or carbon dioxide group molecules in the neutral coma of the comet. Using this model, we derive a simple expression for the X-ray emission and show that the X-ray flare is, in part, due to an increase in solar wind flux at the comet but is largely due to an enhanced molecule production rate. Assuming that the main outgassing constituent was water, the comet produced (2.9 ± 0.4) × 108 kg over the 12 day period postimpact. The quiescent water production was expected to inject ~1.0 × 108 kg into the coma over the same period so the observed X-ray flux indicates that an additional (1.9 ± 0.4) × 108 kg of water or, alternatively, (3.9 ± 0.5) × 108 kg of carbon dioxide were liberated by the DI impact.