Advanced Satellite For Cosmology And Astrophysics Research Articles

Recurring obscuration in NGC 3783

Context. Obscuration of the continuum emission from active galactic nuclei by streams of gas with relatively high velocity (>1000 km s−1) and column density (>3 × 1025 m−2) has been seen in a few Seyfert galaxies. This obscuration has a transient nature. In December 2016 we witnessed such an event in NGC 3783. Aims. The frequency and duration of these obscuration events is poorly known. Here we study archival data of NGC 3783 in order to constrain this duty cycle. Methods. We use archival Chandra/NuSTAR spectra taken in August 2016. We also study the hardness ratio of all Swift XRT spectra taken between 2008 and 2017. Results. In August 2016, NGC 3783 also showed evidence of obscuration. While the column density of the obscuring material is ten times lower than in December 2016, the opacity is still sufficient to block a significant fraction of the ionising X-ray and extreme ultraviolet photons. From the Swift hardness ratio behaviour we find several other epochs with obscuration. Obscuration with columns >1026 m−2 may take place about half of the time. Also, in archival X-ray data taken by the Advanced Satellite for Cosmology and Astrophysics (ASCA) in 1993 and 1996 we find evidence of obscuration. Conclusions. Obscuration of the ionising photons in NGC 3783 occurs more frequently than previously thought. This may not always have been recognised due to low-spectral-resolution observations, overly limited spectral bandwidth or confusion with underlying continuum variations.

Discovery of the Putative Pulsar and Wind Nebula Associated with the TeV Gamma‐Ray Source HESS J1813−178

We present a Chandra X-ray observation of G12.82-0.02, a shell-like radio supernova remnant coincident with the TeV gamma-ray source HESS J1813-178. We resolve the X-ray emission from the colocated Advanced Satellite for Cosmology and Astrophysics (ASCA) source into a point source surrounded by structured diffuse emission that fills the interior of the radio shell. The morphology of the diffuse emission strongly resembles that of a pulsar wind nebula. The spectrum of the compact source is well characterized by a power law with index Γ ≈ 1.3, typical of young and energetic rotation-powered pulsars. For a distance of 4.5 kpc, consistent with the X-ray absorption and an association with the nearby star formation region W33, the 2-10 keV X-ray luminosities of the putative pulsar and nebula are LPSR = 3.2 × 1033 ergs s-1 and LPWN = 1.4 × 1034 ergs s-1, respectively. Both the flux ratio of LPWN/LPSR = 4.3 and the total luminosity of this system predict a pulsar spin-down power of Ė > 1037 ergs s-1, placing it among the 10 most energetic young pulsars in the Galaxy. A deep search for radio pulsations using the Parkes telescope sets an upper limit of ≈0.07 mJy at 1.4 GHz for periods 50 ms. We discuss the energetics of this source and consider briefly the proximity of bright H II regions to this and several other High Energy Stereoscopic System (H.E.S.S.) sources, which may produce their TeV emission via inverse Compton scattering.

Cosmic-ray diffusion near the Bohm limit in the Cassiopeia A supernova remnant

The acceleration of cosmic rays in our Galaxy by means of diffusive shock (Fermi) acceleration is believed to occur primarily in supernova remnants (SNRs). Despite considerable theoretical work, the precise details are still unknown, in part because of the difficulty in directly observing nucleons that are accelerated to TeV energies in—and affect the structure of—SNR shocks. However, for the past ten years, X-ray observatories such as ASCA (Advanced Satellite for Cosmology and Astrophysics) and, more recently, Chandra, XMM-Newton and Suzaku, have made it possible to image the keV-scale synchrotron emission produced by cosmic-ray electrons accelerated in SNR shocks. Here, we describe a spatially resolved spectroscopic analysis of Chandra observations of the Galactic SNR Cassiopeia A to map the cutoff frequencies of electrons accelerated in the forward shock. We set upper limits on the diffusion coefficient and find locations where particles seem to be accelerated nearly as fast as theoretically possible (the Bohm limit).

Simultaneous Ultraviolet and X‐Ray Observations of Seyfert Galaxy NGC 4151. I. Physical Conditions in the X‐Ray Absorbers

We present a detailed analysis of the intrinsic X-ray absorption in the Seyfert 1 galaxy NGC 4151 using Chandra High Energy Transmission Grating Spectrometer data obtained in 2002 May as part of a program that included simultaneous ultraviolet (UV) spectra using the Hubble Space Telescope Space Telescope Imaging Spectrograph and the Far Ultraviolet Spectrographic Explorer. Previous studies, most recently using Advanced Satellite for Cosmology and Astrophysics (ASCA) spectra, revealed a large (>1022 cm-2) column of intervening gas, which has varied both in ionization state and total column density. NGC 4151 was in a relatively low flux state during the observations reported here (~25% of its historic maximum), although roughly 2.5 times as bright in the 2-10 keV band as during a Chandra observation in 2000. At both epochs, the soft X-ray band was dominated by emission lines, which show no discernible variation in flux between the two observations. The 2002 Chandra data show the presence of a very highly ionized absorber, in the form of H-like and He-like Mg, Si, and S lines, as well as lower ionization gas via the presence of inner-shell absorption lines from lower ionization species of these elements. The latter accounts for both the bulk of the soft X-ray absorption and the high covering factor UV absorption lines of O VI, C IV, and N V with outflow velocities ≈500 km s-1. The presence of high-ionization gas, which is not easily detected at low resolution (e.g., with ASCA), appears common among Seyfert galaxies. Since this gas is too highly ionized to be radiatively accelerated in sources such as NGC 4151, which is radiating at a small fraction of its Eddington Luminosity, it may be key to understanding the dynamics of mass outflow. We find that the deeper broadband absorption detected in the 2000 Chandra data is the result of both (1) lower ionization of the intervening gas due to the lower ionizing flux and (2) a factor of ~3 higher column density of the lower ionization component. To account for this bulk motion, we estimate that this component must have a velocity 1250 km s-1 transverse to our line of sight. This is consistent with the rotational velocity of gas arising from the putative accretion disk. While both thermal wind and magnetohydrodynamic models predict large nonradial motions, we suggest that the latter mechanism is more consistent with the results of the photoionization models of the absorbers

An X‐Ray Study of the Supernova Remnant G18.95−1.1

We present an analysis of data from both the Rontgen Satellite (ROSAT) and the Advanced Satellite for Cosmology and Astrophysics (ASCA) of the supernova remnant G18.95-1.1. We find that the X-ray emission from G18.95-1.1 is predominantly thermal, heavily absorbed with a column density around 1022 atoms cm-2, and can be best described by an nonequilibrium ionization (NEI) model with a temperature around 0.9 keV and an ionization timescale of 1.1 × 1010 cm-3 s-1. We find only marginal evidence for nonsolar abundances. Comparisons between 21 cm H I absorption data and derived parameters from our spectral analysis strongly suggest a relatively nearby remnant (a distance of about 2 kpc). Above 4 keV, we identify a small region of emission located at the tip of the central, flat-spectrum barlike feature in the radio image. We examine two possibilities for this emission region: a temperature variation within the remnant or a pulsar wind nebula (PWN). The current data do not allow us to distinguish between these possible explanations. In the scenario where this high-energy emission region corresponds to a PWN, our analysis suggests a rotational loss rate for the unseen pulsar of about 7 × 1035 ergs s-1 and a ratio Lr/LX of about 3.6 for the entire PWN, slightly above the maximum ratio (3.4 for Vela) measured in known PWNe.

Broadband Observations and Modeling of the Shell‐Type Supernova Remnant G347.3−0.5

The supernova remnant G347.3-0.5 emits a featureless power law in X-rays, thought to indicate shock acceleration of electrons to high energies. We here produce a broadband spectrum of the bright northwest limb of this source by combining radio observations from the Australia Telescope Compact Array (ATCA), X-ray observations from the Advanced Satellite for Cosmology and Astrophysics (ASCA), and TeV γ-ray observations from the CANGAROO imaging Cerenkov telescope. We assume that this emission is produced by an electron population generated by diffusive shock acceleration at the remnant forward shock. The nonlinear aspects of the particle acceleration force a connection between the widely different wavelength bands and between the electrons and the unseen ions, presumably accelerated simultaneously with the electrons. This allows us to infer the relativistic proton spectrum and estimate ambient parameters such as the supernova explosion energy, magnetic field, matter density in the emission region, and efficiency of the shock acceleration process. We find convincing evidence that the shock acceleration is efficient, placing greater than 25% of the shock kinetic energy flux into relativistic ions. Despite this high efficiency, the maximum electron and proton energies, while depending somewhat on assumptions for the compression of the magnetic field in the shock, are well below the observed "knee" at ~1015 eV in the Galactic cosmic-ray spectrum.

X-ray telescope onboard Astro-E: optical design and fabrication of thin foil mirrors

X-ray telescopes (XRT's) of nested thin foil mirrors are developed for Astro-E, the fifth Japanese x-ray astronomy satellite. Although the launch was not successful, the design concept, fabrication, and alignment procedure are summarized. The main purpose of the Astro-E XRT is to collect hard x rays up to 10 keV with high efficiency and to provide medium spatial resolution in limited weight and volume. Compared with the previous mission, Advanced Satellite for Cosmology and Astrophysics (ASCA), a slightly longer focal length of 4.5-4.75 m and a larger diameter of 40 cm yields an effective area of 1750 cm2 at 8 keV with five telescopes. The image quality is also improved to 2-arc min half-power diameter by introduction of a replication process. Platinum is used instead of gold for the reflectors of one of the five telescopes to enhance the high-energy response. The fabrication and alignment procedure is also summarized. Several methods for improvement are suggested for the reflight Astro-E II mission and for other future missions. Preflight calibration results will be described in a forthcoming second paper, and a detailed study of images will be presented in a third paper.

Discovery of Non-Thermal X-Rays from the Shell of RCW 86

Abstract We report the ASCA (Advanced Satellite for Cosmology and Astrophysics) results of RCW 86, a shell-like supernova remnant (SNR). The bright region in the X-ray band traces the radio clumpy shell, although details of the structure are different. The X-ray spectrum from each part of the shell can not be fitted to a thin thermal plasma model, but requires at least three components: a low-temperature plasma of 0.3 keV, a high-temperature plasma of ≥ several keV, and a power-law component with a photon index of ∼ 3. The abundances of O, Ne, Mg, and Si are significantly higher than that of Fe, indicating that RCW 86 is a type II SNR. The absorption column of ∼ 3×1021 H cm-2 indicates the distance to the SNR to be several kpc. The power-law component can be interpreted to be synchrotron radiation of high-energy electrons. Assuming an energy density equipartition between the magnetic field and the electrons, and using the radio and X-ray spectra, we argue that high-energy electrons are accelerated up to 20 TeV. The acceleration efficiency is, however, different from shell to shell.

Restoration of theASCASource Position Accuracy

We present a calibration of the absolute pointing accuracy of the Advanced Satellite for Cosmology and Astrophysics (ASCA) which allows us to compensate for a large error (up to 1') in the derived source coordinates. We parameterize a temperature dependent deviation of the attitude solution which is responsible for this error. By analyzing ASCA coordinates of 100 bright active galactic nuclei, we show that it is possible to reduce the uncertainty in the sky position for any given observation by a factor of 4. The revised 90% error circle radius is then 12'', consistent with preflight specifications, effectively restoring the full ASCA pointing accuracy. Herein, we derive an algorithm which compensates for this attitude error and present an internet-based table to be used to correct post facto the coordinate of all ASCA observations. While the above error circle is strictly applicable to data taken with the on-board Solid-state Imaging Spectrometers (SISs), similar coordinate corrections are derived for data obtained with the Gas Imaging Spectrometers (GISs), which, however, have additional instrumental uncertainties. The 90% error circle radius for the central 20' diameter of the GIS is 24''. The large reduction in the error circle area for the two instruments offers the opportunity to greatly enhance the search for X-ray counterparts at other wavelengths. This has important implications for current and future ASCA source catalogs and surveys.

ASCAObservation of the New Transient X‐Ray Pulsar XTE J0111.2−7317 in the Small Magellanic Cloud

The new transient X-ray pulsar XTE J0111.2-7317 was observed with Advanced Satellite for Cosmology and Astrophysics (ASCA) on 1998 November 18, a few days after its discovery with the proportional counter array on board the Rossi X-ray Timing Explorer. The source was detected at a flux level of 3.6 × 10-10 ergs cm-2 s-1 in the 0.7-10.0 keV band, which corresponds to the X-ray luminosity of 1.8 × 1038 ergs s-1, if a distance of 65 kpc for this pulsar in the Small Magellanic Cloud is assumed. Nearly sinusoidal pulsations with a period of 30.9497 ± 0.0004 s were unambiguously detected during the ASCA observation. The pulsed fraction is low and slightly energy dependent with an average value of ~27%. The energy spectrum shows a large soft excess below ~2 keV when fitted to a simple power-law-type model. The soft excess is eliminated if the spectrum is fitted to an "inversely broken power-law" model, in which photon indices below and above a break energy of 1.5 keV are 2.3 and 0.8, respectively. The soft excess can also be described by a blackbody or a thermal bremsstrahlung when the spectrum above ~2 keV is modeled by a power law. In these models, however, the thermal soft component requires a very large emission zone, and hence it is difficult to explain the observed pulsations at energies below 2 keV. A bright state of the source enables us to identify a weak iron line feature at 6.4 keV with an equivalent width of 50 ± 14 eV. Pulse phase-resolved spectroscopy revealed a slight hardening of the spectrum and marginal indication of an increase in the iron line strength during the pulse maximum.

The Flared Disc Project: RXTE and ASCA observations of X 1822--371

We present archival Rossi X-ray Timing Explorer (RXTE) and simultaneous Advanced Satellite for Cosmology and Astrophysics (ASCA) data of the eclipsing low mass X-ray binary (LMXB) X 1822-371. Our spectral analysis shows that a variety of simple models can fit the spectra relatively well. Of these models, we explore two in detail through phase-resolved fits. These two models represent the case of a very optically thick and a very optically thin corona. While systematic residuals remain at high energies, the overall spectral shape is well approximated. The same two basic models are fitted to the X-ray light curve, which shows sinusoidal modulations interpreted as absorption by an opaque disc rim of varying height. The geometry we infer from these fits is consistent with previous studies: the disc rim reaches out to the tidal truncation radius, while the radius of the corona (approximated as spherical) is very close to the circularization radius. Timing analysis of the RXTE data shows a time-lag from hard to soft consistent with the coronal size inferred from the fits. Neither the spectra nor the light curve fits allow us to rule out either model, leaving a key ingredient of the X 1822-371 puzzle unsolved. Furthermore, while previous studies were consistent with the central object being a 1.4 M⊙ neutron star, which has been adopted as the best guess scenario for this system, our light curve fits show that a white dwarf or black hole primary can work just as well. Based on previously published estimates of the orbital evolution of X 1822-371, however, we suggest that this system contains either a neutron star or a low mass (≲2.5 M⊙) black hole and is in a transitional state of duration shortward of 107 yr.

ITAL]ROSAT[/ITAL] HRI and [ITAL]ASCA[/ITAL] Observations of the Spiral Galaxy NGC 6946 and Its Northeast Complex of Luminous Supernova Remnants

Analysis of 80 ks ASCA (Advanced Satellite for Cosmology and Astrophysics) and 60 ks ROSAT HRI (High Resolution Image) observations of the face-on spiral galaxy NGC 6946 are presented. The ASCA image is the first observation of this galaxy above approximately 2 keV. Diffuse emission may be present in the inner approximately 4' extending to energies above approximately 2-3 keV. In the HRI data, 14 pointlike sources are detected, the brightest two being a source very close to the nucleus and a source to the northeast that corresponds to a luminous complex of interacting supernova remnants (SNRs). We detect a point source that lies approximately 30 west of the SNR complex but with a luminosity -1115 of the SNR complex. None of the point sources show evidence of strong variability; weak variability would escape our detection. The ASCA spectrum of the SNR complex shows evidence for an emission line at approximately 0.9 keV that could be either Ne IX at approximately 0.915 keV or a blend of ion stages of Fe L-shell emission if the continuum is fitted with a power law. However, a two-component, Raymond-Smith thermal spectrum with no lines gives an equally valid continuum fit and may be more physically plausible given the observed spectrum below 3 keV. Adopting this latter model, we derive a density for the SNR complex of 10-35 cm(exp -3), consistent with estimates inferred from optical emission-line ratios. The complex's extraordinary X-ray luminosity may be related more to the high density of the surrounding medium than to a small but intense interaction region where two of the complex's SNRs are apparently colliding.

Implications of the X-Ray Variability for the Mass of MCG -6-30-15.

The bright Seyfert 1 galaxy MCG -6-30-15 shows large variability on a variety of timescales. We study the less, similar3 day timescale variability using a set of simultaneous archival observations that were obtained from the Rossi X-Ray Timing Explorer (RXTE) and the Advanced Satellite for Cosmology and Astrophysics (ASCA). The RXTE observations span nearly 106 s and indicate that the X-ray Fourier power spectral density has an rms variability of 16%, is flat from approximately 10-6 to 10-5 Hz, and then steepens into a power law proportional to f-alpha with alpha greater, similar1. A further steepening to alpha approximately 2 occurs between 10-4 and 10-3 Hz. The shape and rms amplitude are comparable to what has been observed in NGC 5548 and Cyg X-1, albeit with break frequencies that differ by a factor of 10-2 and 104, respectively. If the break frequencies are indicative of the central black hole mass, then this mass may be as low as 106 M middle dot in circle. An upper limit of approximately 2 ks for the relative lag between the 0.5-2 keV ASCA band compared to the 8-15 keV RXTE band was also found. Again by analogy with NGC 5548 and Cyg X-1, this limit is consistent with a relatively low central black hole mass.

A Study of the Populations of X‐Ray Sources in the Small Magellanic Cloud withASCA

The Advanced Satellite for Cosmology and Astrophysics (ASCA) has made multiple observations of the Small Magellanic Cloud (SMC). X-ray mosaic images in the soft (0.7--2.0 keV) and hard (2.0--7.0 keV) bands are separately constructed, and the latter provides the first hard X-ray view of the SMC. We extract 39 sources from the two-band images with a criterion of S/N>5, and conduct timing and spectral analyses for all of these sources. Coherent pulsations are detected from 12 X-ray sources; five of which are new discoveries. Most of the 12 X-ray pulsars are found to exhibit long-term flux variabilities, hence they are likely to be X-ray binary pulsars (XBPs). On the other hand, we classify four supernova remnants (SNRs) as thermal SNRs, because their spectra exhibit emission lines from highly ionized atoms. We find that XBPs and thermal SNRs in the SMC can be clearly separated by their hardness ratio (the ratio of the count rate between the hard and soft bands). Using this empirical grouping, we find many XBP candidates in the SMC, although no pulsations have yet been detected from these sources. Possible implications on the star-formation history and evolution of the SMC are presented by a comparison of the source populations in the SMC and our Galaxy.

ASCAObservations of the Young Rotation‐powered Pulsars PSR B1046−58 and PSR B1610−50

We present X-ray observations of two young energetic radio pulsars, PSRs B1046-58 and B1610-50, and their surroundings, using archival data from the Advanced Satellite for Cosmology and Astrophysics (ASCA). The energetic pulsar PSR B1046-58 is detected in X-rays with a significance of 4.5 sigma. The unabsorbed flux, estimated assuming a power-law spectrum and a neutral hydrogen column density N_H of 5E21 cm^-2 is (2.5 +/- 0.3) x 10E-13 ergs/cm^2/s in the 2-10 keV band. Pulsed emission is not detected; the pulsed fraction is less than 31% at the 90% confidence level for a 50% duty cycle. We argue that the emission is best explained as originating from a pulsar-powered synchrotron nebula. The X-ray counterpart of the pulsar is the only hard source within the 95% error region of the previously unidentified gamma-ray source 3EG J1048-5840. This evidence supports the results of Kaspi et al. (1999), who in a companion paper, suggest that PSR B1046-58 is the counterpart to 3EG J1048-5840. X-ray emission from PSR B1610-50 is not detected. Using similar assumptions as above, the derived 3 sigma upper limit for the unabsorbed 2-10 keV X-ray flux is 1.5E-13 ergs/cm^2/s. We use the flux limit to estimate the pulsar's velocity to be less than ~170 km/s, casting doubt on a previously reported association between PSR B1610-50 and supernova remnant Kes 32. Kes 32 is detected, as is evident from the correlation between X-ray and radio emission. The ASCA images of PSR B1610-50 are dominated by mirror-scattered emission from the X-ray-bright supernova remnant RCW 103, located 33' away. We find no evidence for extended emission around either pulsar, in contrast to previous reports of large nebulae surrounding both pulsars.

Large X‐Ray Flare from the Herbig Be Star MWC 297

Hard X-ray emissions from the Herbig Be star MWC 297 were discovered in three separate observations spanning 5 days in 1994 April with the Advanced Satellite for Cosmology and Astrophysics (ASCA). An X-ray flare was found at the beginning of the second observation with a maximum luminosity of ≈4.9 × 1032 ergs s-1, which is 5 times larger than that of the quiescent phase (the first observation). It then declined with an e-folding time of ≈5.6 × 104 s to the preflare level in the third observation. The X-ray spectra are explained by absorbed thin thermal plasma models. The temperature of ≈2.7 keV in the quiescent phase is significantly higher than those of main-sequence OB stars and similar to those of low-mass young stellar objects (YSOs) and other Herbig Ae/Be stars observed with ASCA. The temperature increased in the flare phase to about 6.7 keV at the flux maximum, then decreased to 3.2 keV in the decay phase. These facts strongly suggest that X-rays from Herbig Ae/Be stars, at least for MWC 297, are attributable to magnetic activity similar to that in low-mass YSOs. Since no theory predicts surface convection zones in massive stars like MWC 297, our results may require a mechanism other than the conventional stellar dynamo theory. Possible magnetic activity could be either the stellar interior shear or the inherited magnetic field from the parent molecular cloud.

Evidence for Merging in the Centaurus Cluster

We present a two-dimensional map of the gas temperature distribution in the Centaurus cluster, based on Advanced Satellite for Cosmology and Astrophysics (ASCA) observations derived using a novel approach to account for the energy-dependent point-spread function. Along with a cool region centered on NGC 4696, asymmetric temperature variations of moderate amplitude are detected. The hottest region roughly coincides with the position of the second-brightest galaxy, NGC 4709, known to be the dominant galaxy of one of the subgroups in the Centaurus cluster. ROSAT images show faint surface brightness emission also centered on this galaxy. The imaging and spectral results suggest that a subcluster centered on NGC 4709 is merging with the main cluster centered on NGC 4696, in agreement with earlier suggestion that these two systems are located at the same distance, despite their different line-of-sight velocities.

Flaring and Quiescent Coronae of UX Arietis: Results fromASCAandEUVECampaigns

The RS CVn binary star UX Ari was observed for 14 hr with all four detectors onboard the Advanced Satellite for Cosmology and Astrophysics (ASCA), and for 135 ks with the spectrometers onboard the Extreme-Ultraviolet Explorer (EUVE). During the ASCA observations, the X-ray emission was at a constant, quiescent level during the first 12 hr, after which time a powerful flare with a peak luminosity of 1.4×1032 ergs s-1 started. The flare was observed until shortly after its peak. The EUVE observations were obtained on two different days when the star was in a quiescent phase. We present a spectral and temporal analysis of the UX Ari observations and interpret the ASCA flare data with a two-ribbon flare model including estimates for cooling losses. The quiescent emission measure (EM) distributions derived independently from ASCA and EUVE data agree remarkably. The distribution increases up to a peak around 25 MK. We find elemental abundances that are significantly subsolar, in particular for Fe (≈17%). A time-dependent reconstruction of the flare EM distribution shows that two separate plasma components evolve during the flare (one being identified with the quiescent EM). Most of the flare EM reaches temperatures between 50 and 100 MK or more. Magnetic confinement requires the loop arcade to be geometrically large, with length scales on the order of one stellar radius. The electron densities inferred from the model decrease from initial values around 1012 cm-3 early in the flare to about 1011 cm-3 at the flare peak. The best-fit models require surface magnetic field strengths of a few hundred G, compatible with the maximum photospheric fields expected from equipartition. The flare parameters imply a (conductive and radiative) cooling loss time of less than 1 hr at flare peak. The elemental abundances increase significantly during the flare rise, with the abundances of the low first ionization potential (FIP) elements Fe, Mg, Si, and Ni typically increasing to higher levels than the high-FIP elements, such as S or Ne. The Fe abundance increases from 17%±4% of the solar photospheric value during quiescence up to 89%±18% at flare peak. A fractionation process that occurs during the chromospheric evaporation phase may selectively enrich low-FIP elements as in the solar corona; alternatively, the chromospheric evaporation may itself bring metal-rich chromospheric plasma into the metal-poor corona.

Accurate Position of SGR 1900+14 by Bursts and Changes in Pulse Period and Folded Pulse Profile with [ITAL]ASCA[/ITAL

The Advanced Satellite for Cosmology and Astrophysics (ASCA) observed the soft gamma repeater SGR 1900+14 on 1998 April 30-May 1 and discovered a pulsar with a period of 5.1589715(8) s from the known X-ray source of RX J190714.2+0919.3. Four months later, on September 16-17, ASCA observed SGR 1900+14 again just after the giant burst on 1998 August 27. Comparing the observations in September with those in April, there are several changes in characteristics. The pulse period changed to 5.160295(3) s, and thus the long-term period derivative is 1.1 × 10-10 s s-1. This strongly supports a magnetar model. The folded pulse profile in 2-10 keV largely changed from three peaks in April to one simple peak, while the steady intensity increased by a factor of 2. Finally, we successfully determined the accurate location of SGR 1900+14 by the bursts with an accuracy of 15 in diameter.

ASCAObservations 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.