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NASA NTRS1996

The Supernova Remnant CTA 1

The supernova remnants G327.1-1.1 and G327.4+0.4 (Kes 27) are located 1.5 deg apart in the constellation Norma. In 1980, Einstein IPC observations discovered that both were irregular filled-center X-ray sources with possible point sources superposed. This paper describes new ROSAT position sensitive proportional counter (PSPC) observations which both map the diffuse structure and clearly show several unresolved sources in each field. Both remnants have bright emitting regions inside the limb which might indicate the presence of high energy electrons accelerated by a pulsar. The interior region is more prominent in G327.1-1.1 than in Kes 27. The spectra are relatively strongly absorbed, as expected from distant remnants close to the galactic plane. Comparison of the X-ray and radio maps of each remnant allows us to attribute some emission to a shell and some to the interior. With this information, a blast-wave model is used to derive approximate ages and energy release. Indications are that the Kes 27 supernova deposited approximately 10(exp 51) ergs in the surrounding medium. The G327.1-1.1 event probably deposited a factor of 3-10 less.

Seward, Frederick D.↗
NASA NTRS1995

The Distance to the Heliospheric VLF Emission Region

Two major episodes of heliospheric VLF emissions near 3 kHz have been observed by the Voyager spacecraft in 1983-1984 and 1992-1993. This higher-frequency component is apparently triggered by solar wind transients with sufficiently large spatial extents and energies to continue to propagate as shocks in the heliosheath. Entrainment of previously unshocked material and changed flow conditions in the heliosheath both tend to slow the shock propagation. The shock evolution is not self-similar. Rather, it is intermediate to two blast-wave similarity solutions in the moving solar wind frame. In one solution the shock moves as time to the 2/3 power and in the other as time to the 4/5 power. Using these models, the shock/Forbush decrease observed at Voyager 2 in September, 1991 and the turn-on of the 1992 emission is consistent with an emission region distance of approx. 130 AU (assuming no additional slowing of the shock in the heliosheath). If the termination shock was at approx. 70 AU when the transient shock collided with it, the true distance to the source region was probably closer to approx. 115 AU.

McNutt, R. L., Jr.↗
NASA NTRS1997

A Study of Supernova Remnants with Center-Filled X-Ray Morphology

CTA 1 is a center-filled supernova remnant (SNR) whose morphology and spectrum indicate the presence of a central pulsar, a synchrotron nebula, and a thermal component associated with the expansion of the blast wave into the interstellar medium. The centrally bright emission surrounds the position of a faint point source of x-rays observed with the ROSAT PSPC. Here we report on ASCA observations that confirm the nonthermal nature of the diffuse emission from the central regions of the remnant. We also present evidence for weak thermal emission that appears to increase in strength toward the outer boundary of the SNR. Thus, CTA 1 appears to be an x-ray composite remnant. Both the aftermath of the explosive supernova event and the energetic compact core are observable.

Slane, Patrick O.↗
NASA NTRS1961

Analytic Study of Induced Pressure on Long Bodies of Revolution with Varying Nose Bluntness at Hypersonic Speeds

Pressure distributions and shock shapes for a series of cylindrical afterbodies having nose fineness ratios from 0.4 to 4 have been calculated by using the method of characteristics for a perfect gas. The fluid mediums investigated were air and helium and the Mach number range was from 5 to 40. Flow parameters obtained from blast-wave analogy gave good correlations of blunt-nose induced pressures and shock shapes. Experimental results are found to be in good agreement with the characteristic calculations. The concept of hypersonic similitude enables good correlation of the results with respect to body shape, Mach number, and ratio of specific heats.

VanHise, Vernon↗
NASA NTRS1960

The Blunt Plate In Hypersonic Flow

The sonic-wedge characteristics method has been used to obtain the shock shapes and surface pressure distributions on several blunt two-dimensional shapes in a hypersonic stream for several values of the ratio of specific heats. These shapes include the blunt slab at angle of attack and power profiles of the form yb = a)P, where 0 les than m less than 1, Yb and x are coordinates of the body surface, and a is a constant. These numerical results have been compared with the results of blast-wave theory, and methods of predicting the pressure distributions and shock shapes are proposed in each case. The effects of a free-stream conical-flow gradient on the pressure distribution on a blunt slab in hypersonic flow were investigated by the sonic-wedge characteristics method and were found to be sizable in many cases. Procedures which are satisfactory for reducing pressure data obtained in conical flows with small gradients are presented.

Baradell, Donald L.↗
NASA NTRS1959

The Effect of Leading-Edge Sweep and Surface Inclination on the Hypersonic Flow Field Over a Blunt Flat Plate

An investigation of the effects of variation of leading-edge sweep and surface inclination on the flow over blunt flat plates was conducted at Mach numbers of 4 and 5.7 at free-stream Reynolds numbers per inch of 6,600 and 20,000, respectively. Surface pressures were measured on a flat plate blunted by a semicylindrical leading edge over a range of sweep angles from 0 deg to 60 deg and a range of surface inclinations from -10 deg to +10 deg. The surface pressures were predicted within an average error of +/- 8 percent by a combination of blast-wave and boundary-layer theory extended herein to include effects of sweep and surface inclination. This combination applied equally well to similar data of other investigations. The local Reynolds number per inch was found to be lower than the free-stream Reynolds number per inch. The reduction in local Reynolds number was mitigated by increasing the sweep of the leading edge. Boundary-layer thickness and shock-wave shape were changed little by the sweep of the leading edge.

Creager, Marcus O.↗
NASA NTRS1998

Radio to Gamma-Ray Emission from Shell-Type Supernova Remnants: Predictions from Non-Linear Shock Acceleration Models

Supernova remnants (SNRs) are widely believed to be the principal source of galactic cosmic rays, produced by diffusive shock acceleration in the environs of the remnant's expanding blast wave. Such energetic particles can produce gamma-rays and lower energy photons via interactions with the ambient plasma. The recently reported observation of TeV gamma-rays from SN1006 by the CANGAROO Collaboration, combined with the fact that several unidentified EGRET sources have been associated with known radio/optical/X-ray-emitting remnants, provides powerful motivation for studying gamma-ray emission from SNRs. In this paper, we present results from a Monte Carlo simulation of non-linear shock structure and acceleration coupled with photon emission in shell-like SNRs. These non-linearities are a by-product of the dynamical influence of the accelerated cosmic rays on the shocked plasma and result in distributions of cosmic rays which deviate from pure power-laws. Such deviations are crucial to acceleration efficiency considerations and impact photon intensities and spectral shapes at all energies, producing GeV/TeV intensity ratios that are quite different from test particle predictions.

Baring, Matthew G.↗
NASA NTRS1998

The 1.4 GHz Light Curve of GRB 970508

We report on Westerbork 1.4 GHz radio observations of the radio counterpart to gamma-ray burst GRB 970508, between 0.80 and 138 days after this event. The 1.4 GHz light curve shows a transition from optically thick to thin emission between 39 and 54 days after the event. We derive the slope rho of the spectrum of injected electrons (dN/d(gamma(sub e) proportional to (gamma(sub e)exp -p)) in two independent ways, which yield values very close to p = 2.2. This is in agreement with a relativistic dynamically near-adiabatic blast-wave model whose emission is dominated by synchrotron radiation and in which a significant fraction of the electrons cool fast.

Galama, T. J.↗
NASA NTRS1998

The Radio-to-X-Ray Spectrum of GRB 970508 on 1997 May 21.0 UT

We have reconstructed the spectrum of the afterglow of GRB 970508 on 1997 May 21.0 UT (12.1 days after the gamma-ray burst) on the basis of observations spanning the X-ray-to-radio range. The low-frequency power-law index of the spectrum, alpha = 0.44 +/- 0.07 (F(sub nu) proportional to nu(exp alpha)), is in agreement with the expected value alpha = 1/3 for optically thin synchrotron radiation. The 1.4 GHz emission is self-absorbed. We infer constraints on the break frequencies nu(sub c) and nu(sub m) on 1997 May 21.0 UT from a spectral transition from F(sub nu) approx. nu(exp -0.6) to F(sub nu) approx. nu(exp -1.1) in the optical passband around 1.4 days. A model of an adiabatically expanding, blast wave emitting synchrotron radiation, in which a significant fraction of the electrons cool rapidly, provides a successful and consistent description of the afterglow observations over nine decades in frequency, ranging in time from trigger until several months later.

Galama, T. J.↗
NASA NTRS1998

The 1.4 GHz Light Curve of GRB 970508

We report on Westerbork 1.4 GHz radio observations of the radio counterpart to gamma-ray burst GRB 970508 between 0.80 and 138 days after this event. The 1.4 GHz light curve shows a transition from optically thick to thin emission between 39 and 54 days after the event We derive the slope p of the spectrum of injected electrons (dN/dgamma(sub e) proportional to gamma(sup -p, sub e) in two independent ways which yield values very close to p = 2.2. This is in agreement with a relativistic dynamically near-adiabatic blast wave model whose emission is dominated by synchrotrons radiation and in which a significant fraction of The electrons cool fast.

Galama, T.↗
NASA NTRS1998

The Radio-to-X-Ray Spectrum of GRB 970508 on 1997 May 21.0 UT

We have reconstructed the spectrum of the afterglow of GRB 970508 on 1997 May 21.0 UT (12.1 days after the gamma-ray burst burst) on the basis of observations spanning, the X-ray-to-radio range. The low-frequency power-law index of the spectrum, alpha = 0.44 +/- 0.07 (F, proportional to nu(sup alpha)) is in agreement with the expected value alpha = 1/3 for optically thin synchrotron radiation. The 1.4 emission is self-absorbed. We infer constraints on the break frequencies nu(sub c) and nu(sub m) on 1997 May 21.0 UT from a spectral transition from F, approximately nu(sup -0.6) to F, approximately nu(sup -1.1) in the optical passband around 1.4 days. A model of an adiabatically expanding blast wave emitting synchrotrons radiation, in which a significant fraction of the electrons cool rapidly, provides a successful and consistent description of the afterglow observations over nine decades in frequency, ranging in time from trigger until several months later.

Galama, T. J.↗
NASA NTRS1986

A Conservation Treatment of Zonal Boundaries for Euler Equation Calculations

Finite-difference calculations require the generation of a grid for the region of interest. A zonal approach, wherein the given region is subdivided into zones and the grid for each zone is generated independently, makes the grid-generation process for complicated topologies and for regions requiring selective grid refinement a fairly simple task. This approach results in new boundaries within the given region, that is, zonal boundaries at the interfaces of the various zones. The zonal-boundary scheme (the integration scheme used to update the points on the zonal boundary) for the Euler equations must be conservative, accurate, stable, and applicable to general curvilinear coordinate systems. A zonal-boundary scheme with these desirable properties is developed in this study. The scheme is designed for explicit, first-order-accurate integration schemes but can be modified to accommodate second-order-accurate explicit and implicit integration schemes. Results for inviscid flow, including supersonic flow over a cylinder, blast-wave diffraction by a ramp, and one-dimensional shock-tube flow are obtained on zonal grids. The conservative nature of the zonal-boundary scheme permits the smooth transition of the discontinuities associated with these flows from one zone to another. The calculations also demonstrate the continuity of contour lines across zonal boundaries that can be achieved with the present zonal scheme.

Rai, Man Mohan↗
NASA NTRS2000

GRB970228: A Supernova Gamma-Ray Burst

We present B-, V-, R(sub c)-, I(sub c)-, J-, H-, and K-band observations of the optical transient (OT) associated with GRB970228 based on a reevaluation of published data. and present yet unpublished data. In order to minimize small calibration differences we collected and analyzed most of the photometry and determined the magnitude of the OT relative to a set of secondary field stars. We confirm that the early decay of the light curves (before March 6. 1997) was faster than that at later times (between March 6 and April 7. 1997). The early-time observations of GRB 970228 are consistent with relativistic blast wave models but the late-time observations are hard to understand in this framework. The observations are well explained by an initial power law decay with alpha = -1.46 +/- 0.33 modified at later times by a type-I(sub c) supernova light curve. together with the evidence for GRB980326 and GRB 980425 this is further evidence that at least some GRBs are associated with an unusual class of core-collapse supernovae.

Galama, T. J.↗
NASA NTRS1999

"Driverless" Shocks in the Interplanetary Medium

Many interplanetary shocks have been detected without an obvious driver behind them. These shocks have been thought to be either blast waves from solar flares or shocks due to sudden increase in solar wind speed caused by interactions between large scale open and closed field lines of the Sun. We investigated this problem using a set of interplanetary shock detected {\it in situ} by the Wind space craft and tracing their solar origins using low frequency radio data obtained by the Wind/WAVES experiment. For each of these "driverless shocks" we could find a unique coronal mass ejections (CME) event observed by the SOHO (Solar and Heliospheric Observatory) coronagraphs. We also found that these CMEs were ejected at large angles from the Sun-Earth line. It appears that the "driverless shocks" are actually driver shocks, but the drivers were not intercepted by the spacecraft. We conclude that the interplanetary shocks are much more extended than the driving CMEs.

Gopalswamy, N.↗
NASA NTRS2000

Interferometry for X-Ray Astronomy

With direct imaging, the nature of distant astronomical objects and the physical mechanisms that control them can be constrained and understood. From Galileo's observations of the solar system, to Hubble Space Telescope's imaging of distant galaxies, improved astronomical imaging has always brought scientific understanding. The x-ray band of the spectrum, where exotic objects can have extremely high surface brightness, is ideally suited for high resolution imaging, but has lacked ultra-high quality telescopes. We report a practical x-ray interferometer that features high efficiency, affordable mirrors, adjustable baseline, and can be scaled to a full size observatory. Our prototype system, with just under one millimeter of baseline, created fringes at 1.25 keV with angular resolution of 100 milli-arcseconds. With a larger version of this interferometer in orbit it will be possible to resolve stars, black holes and other compact constituents of the universe. We can study the environments of pulsars, image and then model relativistic blast waves, study the space-time metric near the surface of a black hole, watch the physical formation of astrophysical jets, and study the dynamos of stellar coronae.

Cash, Webster↗
NASA NTRS2000

A Newly Discovered Supernova Remnant and MSH 11-62 and 3C58

CTA 1 is a center-filled supernova remnant (SNR) whose morphology and spectrum indicate the presence of a central pulsar, a synchrotron nebula, and a thermal component associated with the expansion of the blast wave into the interstellar medium. The centrally bright emission surrounds the position of a faint point source of X-rays observed with the ROSAT Position Sensitive Proportional Counter (PSPC). Here we report on Advanced Spacecraft for Cosmology Astrophysics (ASCA) observations that confirm the nonthermal nature of the diffuse emission from the central regions of the remnant. We also present evidence for weak thermal emission that appears to increase in strength toward the outer boundary of the SNR. Thus, CTA 1 appears to be an X-ray composite remnant. Both the aftermath of the explosive supernova event and the energetic compact core are observable.

Slane, Patrick O.↗
NASA NTRS2001

Jets in Gamma-Ray Bursts

In the afterglows of several gamma-ray bursts (GRBs), rapid temporal decay, which is inconsistent with spherical (isotropic) blast-wave models, is observed. In particular, GRB 980519 had the most rapidly fading of the well-documented GRB afterglows, with t(sup -2.05 +/- 0.04) in optical as well as in X-rays. We show that such temporal decay is more consistent with the evolution of a jet after it slows down and spreads laterally, for which t(sup -P) decay is expected (where p is the index of the electron energy distribution). Such a beaming model would relax the energy requirements on some of the more extreme GRBs by a factor of several hundred. It is likely that a large fraction of the weak- (or no-) afterglow observations are also due to the common occurrence of beaming in GRBs and that their jets have already transitioned. to the spreading phase before the first afterglow observations were made. With this interpretation, a universal value of p approx. = 2.4 is consistent with all data.

Sari, R.↗
NASA NTRS2001

The Rapidly Fading Afterglow of GRB 980519

GRB 980519 had the most rapidly fading of the well-documented GRB afterglows, consistent with t(sup -2.05 +/- 0.04) in BVRI as well as in X-rays during the two days in which observations were made. We report VRI observations from the MDM 1.3m and WIYN 3.5m telescopes, and we synthesize an optical spectrum from all of the available photometry. The optical spectrum alone is well fitted by a power law of the form nu(sup - 1.20 +/- 0.25), with some of the uncertainty due to the significant Galactic reddening in this direction. The optical and X-ray spectra together are adequately fitted by a single power law nu(sup 1.05 +/- 0.10). This combination of steep temporal decay and flat broad-band spectrum places a severe strain on the simplest afterglow models involving spherical blast waves in a homogeneous medium. Instead, the rapid observed temporal decay is more consistent with 2 models of expansion into a medium of density n(r) proportional to r(sup -2), or with predictions of the evolution of a jet after it slows down and spreads laterally. The jet model would relax the energy requirements on some of the more extreme GRBs, of which GRB 980519 is likely to be an example because of its large gamma-ray fluence and faint host galaxy.

Halpern, Jules P.↗