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NASA NTRS1997

ASCA Observations of the Barnard 209 Dark Cloud and an Intense X-Ray Flare on V773 Tauri

ASCA (Advanced Satellite for Cosmology and Astrophysics) detected an intense X-ray flare on the weak-lined T Tauri star V773 Tau (=HD 283447) during a 30 ks observation of the Barnard 209 dark cloud in 1995 September. This star is a spectroscopic binary and shows signs of strong magnetic surface activity including a spot-modulated optical light curve. The flare was seen only during its decay phase but is still one of the strongest ever recorded from a T Tauri star with a peak luminosity L(sub x) = 10(exp 32.4) ergs/s (0.5-10 keV), a maximum temperature of at least 42 million K, and energy release of greater than 10(exp 37) ergs. A shorter ASCA observation taken five months later showed V773 Tau in a quiescent state (L(sub x)= 10(exp 31.0) ergs/s) and detected variable emission from the infrared binary IRAS 04113+2758. The differential emission measure (DEM) distribution during the V773 Tau flare shows a bimodal temperature structure that is almost totally dominated by hot plasma at an average temperature of approx. 37 million K. Using information from time-resolved spectra, we examine the flare decay in terms of solar flare models (cooling loops and two-ribbon flares) and also consider possible nonsolar behavior (interbinary flares, star-disk flares, and rotational X-ray modulation). Solar models are unable to reproduce the unusual convex-shaped X-ray light curve, which decays slowly over a timespan of at least 1 day. However, the light curve decay is accurately modeled as a sinusoid with an inferred X-ray period of 2.97 days, which is nearly identical to the optical rotation period(s) of the two K-type components. This provides tantalizing evidence that the flaring region was undergoing rotational occultation, but periodic X-ray variability is not yet proven since our ASCA observation spans only one-third of a rotation cycle.

Skinner, Stephen L.↗
NASA NTRS1997

SS 433: Total Coverage of 162-Day Precession Phase in Four Years

The observations prior to AO-4 covered a number of precession phases, leaving a gap at phase 0.8. In addition, ASCA and previous observations of SS 433 did not observe the spectrum above approx. 10 keV, and consequently the continuum underlying the spectral lines was poorly constrained. Therefore RXTE observations were scheduled for April 1997 to extend the observed spectrum to higher energies; these observations were planned to sample the X-ray lightcurve during the 13.08 day binary period, concentrating on the eclipse of the compact object which emits the jets. We proposed and were awarded ASCA observations simultaneous with the RXTE observations; the purpose of the ASCA observations was to provide greater spectral resolution at the low end of the spectrum observed by RXTE, and to complete the phase coverage of SS 433. As a result of scheduling difficulties early in the mission the RXTE observations were confined to a much shorter time range than originally planned, April 18-91 1997. Optical observations of SS 433 were performed at a number of observatories. The ASCA observations occurred from April 18 13:10 (UT) to April 21 13:20 (UT) for a total effective exposure of 120 ks. The continuum X-ray light curve shows that the ASCA observations started shortly before the ingress into the X-ray partial eclipse, and ended approximately at the time of the egress. Light curves were also obtained for the prominent Fe emission lines in the blue-shifted frame (approaching jet), red-shifted frame (receding jet), and the stationary frame (fluorescent line from the ambient matter). Through the eclipse mapping technique using the light curves, the parameters of the jet emission model were constrained, showing that the kinetic power in the jet exceeds 104? erg s-l. If the energy source is gravitational accretion, as is commonly believed, the derived l;inetic power implies extremely supercritical accretion even for a black; hole with 10M. These results will be described more fully in a major presentation of all the ASCA observations of SS 433.

Band, David L.↗
NASA NTRS1998

Magnetospheric Accretion in PSR 1259-63

We report the results from X-ray and hard X-ray observations of the PSR B1259-63/SS 2883 system with the ASCA and the CGRO satellites, performed between 1995 February and 1996 January when the pulsar was near apastron. The system was clearly detected in each of the two ASCA observations with luminosity in the 1-10 keV band of L(sub X) = (9 +/- 3) x 10(exp 32) (d/2 kpc) erg/ s, while OSSE did not detect significant hard X-rays from the system. X-ray spectra obtained with ASCA show a single power-law spectrum with a photon index of 1.6 +/- 0.3. No pulsations were detected in either the ASCA nor the OSSE data. Combined with previous results from X-ray and hard X-ray observations, we obtained binary modulation in X-ray luminosity and photon index of high energy emission from the system for the entire orbit. The results are in agreement with predictions based on synchrotron emission model from relativistic particles in a shocked pulsar wind interacting with the gaseous outflow from the Be star.

Cominsky, Lynn R.↗
NASA NTRS1961

Earth Reflected Solar Radiation Input to Spherical Satellites

A general calculation is given for the earth's albedo input to a spherical satellite, with the assumption that the earth can be considered a diffusely reflecting sphere. The results are presented in general form so that appropriate values for the solar constant and albedo of the earth can be used as more accurate values become available. The results are also presented graphically; the incident power is determined on the assumption that the mean solar constant is 1.353 x 10( exp 6) erg/(sq cm.sec) and the albedo of the earth is 0.34.

Cunningham, F. G.↗
NASA NTRS1997

ROSAT Observations of a New X-ray Transient in the Small Magellanic Cloud

We describe ROSAT observations of a new X-ray transient and its probable optical companion in the Small Magellanic Cloud, The transient, designated RX J0117.6-7330, appeared at a position approx. 5 southeast of the X-ray pulsar, SMC X-1, in 1992 October 1-2 PSPC observations centered on the pulsar. It was detected again in a similar observation 246 days later at a counting rate diminished by a factor of 270, which corresponds to an average e-folding decay time of 44 days. No periodic pulsations have been detected. The average 1992 flux level would be produced by a source radiating isotropically with a luminosity of 1.6 x 10(exp 37) (D/50 kpc)(exp 2) ergs 1/s in the energy range 0.2-2.5 keV. The 5 radius positional error circle includes the probable optical counterpart in the form of a magnitude V = 14.2 star with the spectral characteristics of type Be.

Clark, George W.↗
NASA NTRS1998

ASCA Observations

This recently expired grant has supported the work of the PI, his students, and his collaborators on a variety of ASCA projects over the past four years. Annual reports have summarized much of the work accomplished; here we provide a brief review of the work resulting from this effort, and a summary of the personnel who have benefited from the grant's support. Starburst Galaxies with Extreme X-ray Luminosities This project began as a careful examination of the claims of Boller et al. (1992) that there were dozens of "normal" galaxies in the ROSAT All-Sky Survey that had X-ray luminosities in excess of 1042 erg sec, higher than that seen in the hundreds of non-AGN galaxies observed with Einstein. If true, this suggested that X-ray emission associated with star formation activity might have a significant contribution to make to the still unexplained cosmic X-ray background (XRB). Since some of our earlier work with the Einstein Observatory Deep Surveys had suggested a similar possibility and several sets of authors over the years had modelled the starburst XRB contribution, these claims were worth pursuing. Our work expanded the examination beyond the RASS to include earlier claims of high-luminosity galaxies powered by starburst emission (selected in this case on the basis of the far-IR luminosities). The result of extensive followup observations under several programs using ROSAT, ASCA, and ground-based facilities was to show that nearly all of these objects in fact have hidden AGN at their cores, and that their luminosities are not in any way extraordinary.

Helfand, David J.↗
NASA NTRS1997

Temperature, Size, and Energy of the Shoemaker-Levy 9 G-Impact Fireball

The fortunate position of the Galileo spacecraft provided us with a unique opportunity to directly observe the Shoemaker- Levy 9 impacts as they occurred on the far side of Jupiter, and we present observations of the G fireball obtained by the Near Infrared Mapping Spectrometer (NTMS). These measurements were performed using 10 spectral bands, 4 representing continua and spanning the wavelength range 1.84 to 4.38 mu. Fireball signals were evident for up to 80 sec, with the time of intensity maxima and duration being greater for longer wavelengths. Color temperatures and effective emitting areas were estimated by fitting blackbody functions at the four continuum wavelengths. Good blackbody fits were found, and their intensities at shorter wavelengths show excellent agreement with the Galileo Photopolarimeter/Radiometer measurements. Temperatures near the beginning are above 3000 K, decreasing to approximately 1000 K after 1 min. The corresponding areas range from 400 to 20,000 sq km. The effective diameter of the luminous fireball shows approximately linear time variation, at least for the first 45 sec. From the temperature-effective diameter relation we find an adiabatic coefficient of lambda = 1.2+/-0.1, much as expected from theoretical considerations. The luminosity, when integrated over the period of observations and assuming a Stephan-Boltzmann radiator, gives an above-cloud radiative energy loss of 0.48+/- 0.13 x 10(exp 25) erg.

Carlson, R. W.↗
NASA NTRS1998

ASCA Observations of the T Tauri Star SU Aurigae and the Surrounding L1517 Dark Cloud

We present the results of a approximately equals 40 ks pointed Advanced Satellite for Cosmology and Astrophysics (ASCA) observation of the L1517 star-forming region, centered on the X-ray-bright T Tauri star SU Aurigae. This star has the highest X-ray luminosity of any classical T Tauri star in the Taurus-Auriga region, and its optical spectra show evidence for both mass inflow and outflow. Strong X-ray emission was detected from SU Aur (L(sub x) = 10(exp 30.9) ergs s(exp -1)) as well as weaker emission from five other pre-main-sequence stars. Although no large-amplitude flares were detected, the X-ray emission of SU Aur showed clear variability in the form of a slow decline in count rate during the 1.3 day observation. We provide the first direct comparison of the coronal differential emission measure (DEM) distribution of a classical T Tauri star with that of a young main-sequence star of similar spectral type. The DEM distributions of SU Aur (G2; age 3 Myr) and the young solar-like star EK Draconis (GO V; age 70 Myr) are qualitatively similar, with both showing a bimodal temperature distribution characterized by a cool plasma component peaking at approximately 8-9 MK and a hot component peaking at approximately 20-21 MK. However, there is a striking difference in the relative proportion of plasma at high temperatures in the two stars, with hot plasma (>20 MK) accounting for approximately equals 80% of the volume emission measure of SU Aur, compared to only approximately equals 40% in EK Dra. These results provide new insight into the changes that will occur in the corona of a T Tauri star as it descends onto the main sequence. A sharp decline in the fraction of coronal plasma at flarelike temperatures will occur during the late-T Tauri and post-T Tauri phases, and other recent X-ray studies have shown that this decline will continue after the young solar-like star reaches the main sequence.

Skinner, Stephen L.↗
NASA NTRS1997

A Dayside Auroral Energy Desposition Case Study Using the Polar Ultraviolet Imager

In this letter, we report preliminary results from a study of dayside auroral energy deposition during quiet times using global auroral images acquired by the Ultraviolet Imager experiment on the Polar spacecraft. Solar wind plasma and interplanetary magnetic field measurements from the Wind spacecraft and kp values were used to characterize the state of the magnetosphere. The auroral oval was observed for a two- hour period in spring and summer during relatively quiet times (k(sub p) approx. 0 to 1). We find that, although the nightside energy deposition rate varied by an order of magnitude to as low as 1 x 10(exp 16) ergs per second (1 GW), the dayside was much less variable and remained between 4 and IO GW.

Brittnacher, M. J.↗
NASA NTRS1998

Low Luminosity States of the Black Hole Candidate GX 339-4: ASCA and Simultaneous Radio/RXTE Observations - 1

We discuss a series of observations of the black hole candidate GX 339-4 in low luminosity, spectrally hard states. We present spectral analysis of three separate archival Advanced Satellite for Cosmology and Astrophysics (ASCA) data sets and eight separate Rossi X-ray Timing Explorer (RXTE) data sets. Three of the RXTE observations were strictly simultaneous with 843 Mega Hertz and 8.3-9.1 Giga Hertz radio observations. All of these observations have (3-9 keV) flux approximately less than 10(exp-9) ergs s(exp-1) CM(exp -2). The ASCA data show evidence for an approximately 6.4 keV Fe line with equivalent width approximately 40 eV, as well as evidence for a soft excess that is well-modeled by a power law plus a multicolor blackbody spectrum with peak temperature approximately equals 150-200 eV. The RXTE data sets also show evidence of an Fe line with equivalent widths approximately equal to 20-1OO eV. Reflection models show a hardening of the RXTE spectra with decreasing X-ray flux; however, these models do not exhibit evidence of a correlation between the photon index of the incident power law flux and the solid angle subtended by the reflector. 'Sphere+disk' Comptonization models and Advection Dominated Accretion Flow (ADAF) models also provide reasonable descriptions of the RXTE data. The former models yield coronal temperatures in the range 20-50 keV and optical depths of r approximately equal to 3. The model fits to the X-ray data, however, do not simultaneously explain the observed radio properties. The most likely source of the radio flux is synchrotron emission from an extended outflow of extent greater than O(10 (exp7) GM/c2).

Wilms, Joern↗
NASA NTRS1998

The Nightside Auroral Gap: Implications for Magnetosphere-Ionosphere Coupling in the Midnight Auroral Zone

We present observations of a new synoptic scale feature of the nightside aurora identified by the Polar Ultraviolet Imager (UVI) in which a well defined segment of the pre-midnight auroral oval exhibits significantly reduced auroral luminosity in the atomic oxygen and Lyman-Birge-Hopfield (LBH) emission bands in comparison to adjacent auroral activity. This gap in the nightside aurora is typically observed in the 2200-2400 MLT sector and spans 1.0-1.5 hours in local time (710-1100 km). The energy flux ($\rm erg$ $\rm cm'{-2}$ $\rm s'{-1}$) is shown to be lower by as much as a factor of four in the nightside gap region in comparison to the adjacent auroral activity. The appearance of the nightside gap does not depend on substorm phase. However, substorm intensifications are often observed on either side of the nightside gap which remains relatively inactive. Moreover, auroral breakups sometimes develop within the nightside gap, indicating that this feature may have dynamical implications for substorm onset location.

Chua, D. H.↗
NASA NTRS1997

Torque Reversal and Spin-Down of the Accretion-Powered Pulsar 4U 1626-67

Over 5 yr of hard X-ray (20-60 keV) monitoring of the 7.66 s accretion-powered pulsar 4U 1626-67 with the Compton Gamma Ray Observatory/BATSE large-area detectors has revealed that the neutron star is now steadily spinning down, in marked contrast to the steady spin-up and spin-down torques differ by only 15% with the neutron star spin changing on a timescale |v/v| approximately equals 5000 yr in both states. The current spin-down rate is itself decreasing on a timescale |v/v| approximately equals 26 yr. The long-term timing history shows small-amplitude variations on a 4000 day timescale, which are probably due to variations in the mass transfer rate. The pulsed 20-60 keV emission from 4U 1626-67 is well-fitted by a power-law spectrum with photon index gamma = 4.9 and a typical pulsed intensity of 1.5 x 10(exp -10) ergs cm (exp -2)s(exp -1). The low count rates with BATSE prohibited us from constraining the reported 42 minute binary orbit, but we can rule out long-period orbits in the range 2 days < or = P(orb) < or = 900 days. We compare the long-term torque behavior of 4U 1626-67 to other disk-fed accreting pulsars and discuss the implications of our results for the various theories of magnetic accretion torques. The abrupt change in the sign of the torque is difficult to reconcile with the extremely smooth spin-down now observed. The strength of the torque noise in 4U 1626-67, approximately 10(exp -22) Hz(exp 2)s(exp -2) Hz(exp -1), is the smallest ever measured for an accreting X-ray pulsar, and it is comparable to the timing noise seen in young radio pulsars. We close by pointing out that the core temperature and external torque (the two parameters potentially relevant to internal sources of timing noise) of an accreting neutron star are also comparable to those of young radio pulsars.

Chakrabarty, Deepto L.↗
NASA NTRS1997

Gamma-Ray Bursts: Where are we Now?

There has been significant progress recently in our understanding of gamma-ray bursts. The long-sought counterparts at other wavelengths have finally been found for a few bursts. In one case, GRB970508, red-shifted absorption lines have been detected, finally settling the debate about the distance scale. The consensus is that the burst sources lie at cosmological distances, requiring at least about 10**51 ergs to be emitted in gamma rays in just a few seconds. Many mysteries remain. There is no consensus on the nature of the sources, although coalescing neutron stars are the leading candidate. I will describe recent GRB observations from a number of satellites, including the Compton Gamma Ray Observatory, the Hubble Space Telescope, and BeppoSAX, as well as ground-based optical and radio observations. I will relate these observations to our present picture of GRBs and describe how future observations might answer the remaining questions. Finally, I will describe how gamma-ray bursts may be able to tell us something about the early Universe.

Meegan, Charles A.↗
NASA NTRS1999

On Heating the Sun's Corona by Magnetic Explosions: Feasibility in Active Regions and prospects for Quiet Regions and Coronal Holes

We build a case for the persistent strong coronal heating in active regions and the pervasive quasi-steady heating of the corona in quiet regions and coronal holes being driven in basically the same way as the intense transient heating in solar flares: by explosions of sheared magnetic fields in the cores of initially closed bipoles. We begin by summarizing the observational case for exploding sheared core fields being the drivers of a wide variety of flare events, with and without coronal mass ejections. We conclude that the arrangement of an event's flare heating, whether there is a coronal mass ejection, and the time and place of the ejection relative to the flare heating are all largely determined by four elements of the form and action of the magnetic field: (1) the arrangement of the impacted, interacting bipoles participating in the event, (2) which of these bipoles are active (have sheared core fields that explode) and which are passive (are heated by injection from impacted active bipoles), (3) which core field explodes first, and (4) which core-field explosions are confined within the closed field of their bipoles and which ejectively open their bipoles. We then apply this magnetic-configuration framework for flare heating to the strong coronal heating observed by the Yohkoh Soft X-ray Telescope in an active region with strongly sheared core fields observed by the MSFC vector magnetograph. All of the strong coronal heating is in continually microflaring sheared core fields or in extended loops rooted against the active core fields. Thus, the strong heating occurs in field configurations consistent with the heating being driven by frequent core-field explosions that are smaller but similar to those in confined flares and flaring arches. From analysis of the thermal and magnetic energetics of two selected core-field microflares and a bright extended loop, we find that (1) it is energetically feasible for the sheared core fields to drive all of the coronal heating in the active region via a staccato of magnetic microexplosions, (2) the microflares at the feet of the extended loop behave as the flares at the feet of flaring arches in that more coronal heating is driven within the active bipole than in the extended loop, (3) the filling factor of the X-ray plasma in the core field microflares and in the extended loop is approximately 0.1, and (4) to release enough magnetic energy for a typical microflare (10^27 - 10^28 erg), a microflaring strand of sheared core field need expand and/or untwist by only a few percent at most. Finally, we point out that (1) the field configurations for strong coronal heating in our example active region (i.e., neutral-line core fields, many embedded in the feet of extended loops) are present in abundance in the magnetic network in quiet regions and coronal holes, and (2) it is known that many network bipoles do microflare and that many produce detectable coronal heating. We therefore propose that exploding sheared core fields are the drivers of most of the heating and dynamics of the solar atmosphere, ranging from the largest and most powerful coronal mass ejections and flares, to the vigorous microflaring and coronal heating in active regions, to the multitude of fine-scale explosive events in the magnetic network. The low-lysing exploding core fields in the network drive microflares, spicules, global coronal heating, and ,consequently, the solar wind.

Moore, Ronald L.↗
NASA NTRS1999

Major Optical Outburst of Two Blazars: 3C66A and OJ287

Differential CCD photometric observations of 3C66A and OJ287 were carried out on 16 and 35 nights between December 1995 and March 1996, at the Vainu Bappu Observatory, India, as part of the blazar monitoring program. During this period we detected major optical outbursts (brightness of 3C66A and OJ287 increased by 0.8 and 1 mag, respectively) of these two blazars on timescales of two months. Integrating the outburst profiles we find that both the blazars released around 10(exp 53) erg. Such large amount of energy may come from the release of binding energy of a compact star when tidally disrupted by a super-massive black hole at the center of these blazars. Also we extend this model to explain the observed multifrequency (radio through gamma-ray) outburst of these two blazars and show that this model will be able to explain the outburst phenomena of other blazars. These new results will be presented with a detailed discussion of the suggested model.

Ghosh, K. K.↗
NASA NTRS1999

Solar Flares and Their Prediction

Solar flares and coronal mass ejection's (CMES) can strongly affect the local environment at the Earth. A major challenge for solar physics is to understand the physical mechanisms responsible for the onset of solar flares. Flares, characterized by a sudden release of energy (approx. 10(exp 32) ergs for the largest events) within the solar atmosphere, result in the acceleration of electrons, protons, and heavier ions as well as the production of electromagnetic radiation from hard X-rays to km radio waves (wavelengths approx. = 10(exp -9) cm to 10(exp 6) cm). Observations suggest that solar flares and sunspots are strongly linked. For example, a study of data from 1956-1969, reveals that approx. 93 percent of major flares originate in active regions with spots. Furthermore, the global structure of the sunspot magnetic field can be correlated with flare activity. This talk will review what we know about flare causes and effects and will discuss techniques for quantifying parameters, which may lead to a prediction of solar flares.

Adams, Mitzi L.↗
NASA NTRS1999

Global Dynamics of Dayside Auroral Precipitation in Conjunction with Solar Wind Pressure Pulses

Global observation of the dayside auroral region by the Ultraviolet Imager (UVI) during transient solar wind pressure pulse events on October 1, 1997 has revealed unusual features in the auroral precipitation. The auroral arc structure on the dayside, possibly connected with the LLBL, split into 2 arc structures; one moving poleward and fading over a 5 min period, and the other stationary or slightly shifted equatorward (by changes in the x component). The y component was large and positive, and the z component was small and negative. The splitting of the arc structure extended from 9 to 15 MLT and was concurrent with an enhancement of the convection in the cusp region identified by SuperDARN observations. The convection reversal on the morningside was adjacent to and poleward of the weak lower latitude band of precipitation. The sensitivity of the UVI instrument enabled observation of arc structures down to about 0.2 erg electron energy flux, as confirmed by comparison with particle measurements from the FAST satellite for other dayside events. Removal of the spacecraft wobble by PIXON image reconstruction restored the original resolution of the UVI of about 40 km from apogee. This event is being analyzed in connection with a larger study of global dynamics of dayside energy and momentum transfer related to changes in IMF conditions using UVI images in conjunction with observations from FAST and SuperDARN.

Brittnacher, M.↗
NASA NTRS1996

Ion Acceleration in Solar Flares

Solar flares are among the most energetic and interesting phenomena in the Solar system, releasing up to 1032 ergs of energy on timescales of several tens of seconds to several tens of minutes. Much of this energy is in the form of suprathermal electrons and ions, which remain trapped at the Sun and produce a wide variety of radiations, as well as escape into interplanetary space, where they can be directly observed. The radiation from trapped particles consists in general of (1) continuum emission; (2) narrow gamma-ray nuclear deexcitation lines; and (3) high-energy neutrons observed in space or by ground-based neutron monitors. The particles that escape into space consist of both electrons and ions, which often have compositions quite different than that of the ambient solar atmosphere. Flares thus present many diagnostics of the particle acceleration mechanism(s), the identification of which is the ultimate goal of flare research. Moreover, flares in fact offer the only opportunity in astrophysics to study the simultaneous energization of both electrons and ions. Hopefully, an understanding of flares with their wealth of diagnostic data will lead to a better understanding of particle acceleration at other sites in the Universe. It is now generally accepted that flares are roughly divided into two classes: impulsive and gradual. Gradual events are large, occur high in the corona, have long-duration soft and hard X-rays and gamma rays, are electron poor, are associated with Type II radio emission and coronal mass ejections (CMEs), and produce energetic ions with coronal abundance ratios. Impulsive events are more compact, occur lower in the corona, produce short-duration radiation, and exhibit dramatic abundance enhancements in the energetic ions. Their He-3/He-4 ratio is - 1, which is a huge increase over the coronal value of about 5 x 10(exp -4), and they also posses smaller but still significant enhancements of Ne, Mg, Si, and Fe relative to He-4, C, N, and O. Specifically, above about 1 MeV nucleon(exp -1), the ratio of Fe to O is about 8 times larger than in the corona or in gradual flares, while the ratio of Ne, Mg, and Si to O is about 3 times higher; He-4, C, N, and 0 are not enchanced with respect to each other. In addition to these elemental enhancements, Ne and Mg have isotopic enhancements as well. The general scenario that has emerged from these (and other) observations is that energetic particles in gradual events are accelerated by a CME-driven shock, while those particles in impulsive events are accelerated by another mechanism(s).

Miller, James A.↗