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The variability of timescales with regard to astronomical objects is explored. Variability is classified in length of time such as short vs. long or by repeatability. The article examines these timescales and how they relate to the variability of pulsars, binary stars and eclipses as well as other astronomical objects.

We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of the little-known southern nova-like cataclysmic variable, BB Doradus. The spectrum was obtained as part of our Cycle 8 FUSE survey of high-declination nova-like stars. The FUSE spectrum of BB Dor, observed in a high state, is modeled with an accretion disk with a very low inclination (possibly lower than 10deg). Assuming an average white dwarf (WD) mass of 0.8 M leads to a mass accretion rate of 10[?]9 M yr[?]1 and a distance on the order of ~650 pc, consistent with the extremely low Galactic reddening in the direction of BB Dor. The spectrum presents some broad and deep silicon and sulfur absorption lines, indicating that these elements are overabundant by 3 and 20 times solar, respectively.

SN 1988Z is the most luminous X-ray-emitting supernova, initially detected in 1995 using the ROSAT HRI with a luminosity of ~8 x 1040 ergs s-1 (Fabian & Terlevich 1996). Its high luminosity was ascribed to expansion of the blast wave into an especially dense circumstellar medium. In this paper, we describe a recent observation of SN 1988Z using the ACIS detector on Chandra. We readily detect SN 1988Z, obtaining ~30 net counts, which corresponds to a 0.2-2.0 keV luminosity of ~3.2 x 1039 ergs s-1. The calculated quantiles for the extracted counts allow a broad range of temperatures but require a temperature hotter than 5 keV if there is no intrinsic absorption. The long-term light curve (1995-2005) declines as t-2.6+-0.6. This is one of the steepest X-ray light curves. The X-ray luminosity indicates that the emitting region has a high density (>10 5 cm-3) and that the density profile is not consistent with a constant mass-loss stellar wind during the ~5000 yr before the explosion. If the circumstellar medium is due to progenitor mass loss, then the mass-loss rate is extremely high (~10-3 M yr-1 [vw/10 km s-1]). The X-ray results are compared with the predictions of models of SN 1988Z.

The low-resolution X-ray spectra around e Car covering Trumpler 16 and part of Trumpler 14 have been extracted from a Chandra CCD ACIS image. Various analysis techniques have been applied to the spectra based on their count rates. The spectra with the greatest number of counts (HD 93162 = WR 25, HD 93129 AB, and HD 93250) have been fitted with a wind model, which uses several components with different temperatures and depths in the wind. Weaker spectra have been fitted with Raymond-Smith models. The weakest spectra are simply intercompared with strong spectra. In general, fits produce reasonable parameters based on knowledge of the extinction from optical studies and on the range of temperatures for high- and low-mass stars. Direct comparisons of spectra confirm the consistency of the fitting results and also hardness ratios for cases of unusually large extinction in the clusters. The spectra of the low-mass stars are harder than the more massive stars. Stars in the sequence evolving from the main sequence (HD 93250) through the system containing the O supergiant (HD 93129 AB) and then through the Wolf-Rayet stage (HD 93162), presumably ending in the extreme example of e Car, share the property of being unusually luminous and hard in X-rays. For these X-ray-luminous stars, their high mass and evolutionary status (from the very last stages of the main sequence and beyond) is the common feature. Their binary status is mixed, and their magnetic status is still uncertain.

This paper describes the Chandra observation of the diffuse emission in the face-on spiral NGC 6946. Overlaid on optical and Ha images, the diffuse emission follows the spiral structure of the galaxy. An overlay on a 6 cm polarized radio intensity map confirms the phase offset of the polarized emission. We then extract and fit the spectrum of the unresolved emission with several spectral models. All model fits show a consistent continuum thermal temperature with a mean value of 0.25 +- 0.03 keV. Additional degrees of freedom are required to obtain a good fit, and any of several models satisfy that need; one model uses a second continuum component with a temperature of 0.70 +- 0.10 keV. An abundance measure of 3img1.gif for Si differs from the solar value at the 90% confidence level; the net diffuse spectrum shows that the line lies above the instrumental Si feature. For Fe the abundance measure of 0.67 +- 0.13 is significant at 99%. Multiple Gaussians also provide a good fit. Two of the fitted Gaussians capture the O VII and O VIII emission; the fitted emission is consistent with an XMM-Newton RGS spectrum of diffuse gas in M81. The ratio of the two lines is less than 0.6-0.7 and suggests the possibility of nonequilibrium ionization conditions existing in the interstellar medium of NGC 6946. An extrapolation of the point-source luminosity distribution shows that the diffuse component is not the sum of unresolved point sources; their contribution is at most 25%.

We present a deep (168 ks) Chandra observation of the nearby face-on spiral galaxy NGC 1637 and study the X-ray source population. A total of 32 X-ray sources are detected within the D25 ellipse of NGC 1637, each of which is found to be variable during the 629 day period covered by the observations. Deep Hubble Space Telescope WFPC2 observations of NGC 1637, nearly simultaneous with the Chandra data, were used to search for optical counterparts to the X-ray sources. An ultraluminous (LX = 2.2 x 1039 ergs s-1; 0.3-7 keV) and variable (factor of 1.4) X-ray source is found to positionally coincide with an optical faint and extended (~25 pc FWHM) counterpart in the HST images (mV ~ 22.8 mag, mI ~ 21.1 mag). Given the multiband properties of this source, it likely represents a stellar-mass (~8 M) black hole candidate located in an OB association in an outer spiral arm of NGC 1637. Our analysis further reveals large amounts of diffuse emission filling the entire extent of the galaxy. The spectrum of the inner [?]300 pc region is best described by a two-temperature thermal plasma emission, with a low-temperature/low-absorption component (kTlow ~ 0.3 keV) and a high-temperature/high-absorption component (kThigh ~ 0.7-0.8 keV), likely associated with an outflow of hot gas into the halo of NGC 1637. Comparison of the multiwavelength properties of a number of nearby galaxies shows that NGC 1637 has remarkably high LX/LFIR and LX/LB ratios, suggesting a recent epoch with an enhanced star formation rate.

The region around the e Carinae Nebula has three OB associations, which contain a Wolf-Rayet star and several massive O3 stars. An early Chandra ACIS-I image was centered on e Car and includes Trumpler 16 and part of Trumpler 14. The Chandra image confirms the well-known result that O and very early B stars are X-ray sources with LX [?] 10-7Lbol over an X-ray luminosity range of about 100. Two new, anomalously strong X-ray sources have been found among the hot star population: Tr 16-244, a heavily reddened O3 I star, and Tr 16-22, a heavily reddened O8.5 V star. Two stars have an unusually large LX/Lbol: HD 93162, a Wolf-Rayet star (and possible binary), and Tr 16-22, a possible colliding-wind binary. In addition, a population of sources associated with cool stars is detected. In the color-magnitude diagram, these X-ray sources sit above the sequence of field stars in the Carina arm. The OB stars are on average more X-ray-luminous than the cool star X-ray sources. X-ray sources among A stars have X-ray luminosities similar to those of cooler stars and may be due to cooler companions. Upper limits are presented for B stars that are not detected in X-rays. These upper limits are also the upper limits for any cool companions that the hot stars may have. Hardness ratios are presented for the most luminous sources in bands 0.5-0.9, 0.9-1.5, and 1.5-2.04 keV. The available information on the binary nature of the hot stars is discussed, but binarity does not correlate with X-ray strength in a simple way.

The cosmic diffuse X-ray background radiation has been almost completely resolved into point sources by Chandra and XMM. However, the nature and evolution of those sources are just beginning to be investigated. The answers are key to our understanding of the inter-relationship over cosmic time of (a) the formation and growth of black holes, (b) star formation, and (c) hierarchical structure formation. This requires multiwavelength followup of Chandra sources over a large area. The Chandra Multiwavelength Project (ChaMP) is a serendipitous survey of the X-ray sky, covering about 14 deg² of sky in the X-ray flux range of -13 to -16 (log 0.5-2 keV flux, cgs) and will include about 8000 sources, 70% of which are expected to be active galactic nuclei (AGN). Using Chandra archival data, we span flux limits between those reached by previous satellites and those of the deepest small-area Chandra and XMM surveys, allowing study of larger samples of rare objects like luminous absorbed AGN, groups and clusters of galaxies, and their spatial correlations. Optical imaging and spectroscopy of the ChaMP fields are underway at a variety of telescopes using Sloan g', r', and i' filters to a flux limit matched to the X-ray such that about 75% of the X-ray sources will be optically identified.We also plan for radio and near-IR observations of subsets of the ChaMP sample to expand our multi-wavelength coverage and increase the fraction of sources identified.

MS 1603.6+2600 is a high-latitude X-ray binary with a 111 minute orbital period, thought to be either an unusual cataclysmic variable or an unusual low-mass X-ray binary. In an ASCA observation in 1997 August, we find a burst, whose light curve suggests a type I (thermonuclear flash) origin. We also find an orbital X-ray modulation in MS 1603.6+2600, which is likely to be periodic dips, presumably due to azimuthal structure in the accretion disk. Both are consistent with this system being a normal low-mass X-ray binary harboring a neutron star, but at a great distance. We tentatively suggest that MS 1603.6+2600 is located in the outer halo of the Milky Way, perhaps associated with the globular cluster Palomar 14, 11deg away from MS 1603.6+2600 on the sky at an estimated distance of 73.8 kpc.

This paper reports the results of UV and optical spectroscopy of the nova-like variable UU Aqr. There is a decrease in the light flux of both the optical and UV emission lines at phase ~0.95. This appears to be due to an occultation of an asymmetric accretion disk. The emission-line profiles seem to show the first half of the rotational disturbance of the disk eclipse. There is a secondary eclipse at phase 0.4 in the optical continuum and the hydrogen emission lines. It is not present in the UV continuum or emission lines. This eclipse occurs when the bright spot is at superior conjunction. This raises the possibility that this is an occultation of the bright spot/s-wave region by material suspended above the central disk. The accretion disk emission in the Doppler tomogram of the Hb emission line was used to obtain the radial velocity amplitude of the white dwarf star. The mass ratio was estimated by matching the s-wave position with stream trajectories. The results were significantly different from that of Baptista et al. These authors used the eclipse phasing of the continuum hot spot and the assumption that the hot spot is located on the stream trajectory. The reasons for this discrepancy are unclear.

We report on a 1 day ASCAobservation and ROSATHRI observations of the eclipsing dwarf nova HT Cas. The presence of the X-ray eclipse, originally detected in a ROSATPSPC observation, is confirmed at a much higher significance level. The quality of the ASCAlight curve is high enough to allow detailed investigation of the eclipse depth, width, and shape. The eclipse is found to be deep, compatible with being total. The eclipse width is comparable to that of the white dwarf, as derived from optical light curves, and in fact may be narrower. The eclipse transition is also found to be short, which puts a limit of 1.15 times the white dwarf radius as the total size of the X-ray emission region. The out-of-eclipse spectrum of HT Cas is found to be consistent with a single temperature (kT~ 10 keV), absorbed (NH~ 3.3 × 1021cm-2), thermal model.

The quasispherical Szekeres metric is an exact solution to Einstein's equations describing an inhomogeneous and anisotropic cosmology. Though its governing equations are well known, there are subtle, often-overlooked details in how the model's functions relate to its physical layout, including the shapes and relative positions of structures. We present an illustrated overview of the quasispherical Szekeres models and show exactly how the model functions relate to the physical shape and distribution of matter. In particular, we describe a shell rotation effect that has not previously been fully understood. We show how this effect relates to other known properties, and lay out some mathematical tools useful for constructing models and picturing them accurately. [ABSTRACT FROM AUTHOR]

Presents updates on the Astronomical Society of the Pacific (ASP) as of September 2001. Redesign of the ASP Web site; Appointment of Mike Bennett as acting executive director of the ASP; Information on the Annual Meeting of the ASP held on July 13-17, 2001; Award given by the American Astronomical Society History of Astronomy Division to ASP member Donald E. Osterbrock; Death of John Gardner Phillips.