Saeqa Dil Vrtilek

A catalog of intracluster gas temperatures

We have searched the Einstein Monitor Proportional Counter (MPC) data base for observations of clusters of galaxies. By coadding the MPC spectra obtained during all pointed observations of clusters with IPC count rates greater than 0.1 counts/s, we have obtained sufficient photon statistics to estimate the X-ray temperature of 84 clusters. Combining the MPC results with EXOSAT and Ginga results reported in the literature yields a combined sample of 104 clusters with known X-ray temperatures. One of the best studied X-ray correlations between clusters is that between their X-ray luminosity and gas temperature. We show that the best-fit power-law relation for our combined cluster sample can be explained by the observed increase in the gas-to-stellar mass ratio between low- and high-temperature clusters. The statistical significance of any evolution in our combined X-ray sample has been examined and compared with the statistical properties of clusters culled from optical catalogs. We find that there is strong evidence for a decrease in the X-ray luminosity of optically rich clusters beyond z approximately about 0.06. This result is used to estimate the normalization of the primordial power spectrum of density fluctuations.

Complete and simultaneous spectral observations of the black hole X-Ray nova XTE J1118+480

The X-ray nova XTE J1118+480 suffers minimal extinction (b = 62 degrees) and therefore represents an outstanding opportunity for multiwavelength studies. Hynes et al. (2000) conducted the first such study, which was centered on 2000 April 8 using UKIRT, EUVE, HST and RXTE. On 2000 April 18, the Chandra X-ray Observatory obtained data coincident with a second set of observations using all of these same observatories. A 30 ks grating observation using Chandra yielded a spectrum with high resolution and sensitivity covering the range 0.24-7 keV. Our near-simultaneous observations cover approximately 80% of the electromagnetic spectrum from the infrared to hard X-rays. The UV/X-ray spectrum of XTE J1118+480 consists of two principal components. The first of these is an approximately 24 eV thermal component which is due to an accretion disk with a large inner disk radius: > 35 Schwarzschild radii. The second is a quasi power-law component that was recorded with complete spectral coverage from 0.4-160 keV. A model for this two-component spectrum is presented in a companion paper by Esin et al. (2001).

Rotational Modulation of X-Ray Emission in Orion Nebula Young Stars

Weinvestigatethespatialdistributionof X-ray–emittingplasmainasampleof youngOrionNebulaClusterstars by modulation of their X-ray light curves due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project (COUP), is made possible by the exceptional length of the observation: 10 days of ACIS integration during atimespanof13days,yieldingatotalof1616detectedsourcesinthe17 0 ;17 0 fieldofview.Weherefocusonasubsample of 233 X-ray–bright stars with known rotational periods. We search for X-ray modulation using the Lomb NormalizedPeriodogrammethod.X-raymodulationrelatedtotherotationperiodisdetectedinatleast23starswith periodsbetween2and12daysandrelativeamplitudesrangingfrom20%to70%.In16cases,theX-raymodulation periodissimilartothestellarrotationperiod, while in 7casesitisabouthalfthatvalue,possiblydueto thepresence of X-ray–emitting structures at opposite stellar longitudes. These results constitute the largest sample of low-mass stars in which X-ray rotational modulation has been observed. The detection of rotational modulation indicates that the X-ray–emitting regions are distributed inhomogeneneously in longitude and do not extend to distances significantly larger than the stellar radius. Modulation is observed in stars with saturated activity levels (LX/Lbol � 10 � 3 ) showing that saturation is not due to the filling of the stellar surface with X-ray–emitting regions. Subject headings: open clusters and associations: individual (Orion Nebula Cluster) — stars: activity — stars: low-mass, brown dwarfs — stars: pre–main-sequence — X-rays: stars

Chandra X-Ray Observatory Detection of Extended X-Ray Emission from the Planetary Nebula BD +30°3639

We report the detection of well-resolved, extended X-ray emission from the young planetary nebula BD +30°3639 using the Advanced CCD Imaging Spectrometer (ACIS) aboard the Chandra X-ray Observatory. The X-ray emission from BD +30°3639 appears to lie within, but is concentrated to one side of, the interior of the shell of ionized gas seen in high-resolution optical and IR images. The relatively low X-ray temperature (TX ~ 3 × 106 K) and asymmetric morphology of the X-ray emission suggest that conduction fronts are present and/or mixing of shock-heated and photoionized gas has occurred, and furthermore hints at the presence of magnetic fields. The ACIS spectrum suggests that the X-ray-emitting region is enriched in the products of nuclear burning. Our detection of extended X-ray emission from BD +30°3639 demonstrates the power and utility of Chandra imaging as applied to the study of planetary nebulae.

A Compact X-ray source and possible X-ray jets within the planetary nebula Menzel 3

We report the discovery, by the Chandra X-Ray Observatory, of X-ray emission from the bipolar planetary nebula Menzel 3. In Chandra CCD imaging, Mz 3 displays hot [~(3-6) × 106 K] gas within its twin, coaxial bubbles of optical nebulosity as well as a compact X-ray source at the position of its central star(s). The brightest diffuse X-ray emission lies along the polar axis of the optical nebula, suggesting a jetlike configuration. The observed combination of an X-ray-emitting point source and possible X-ray jet(s) is consistent with models in which accretion disks and, potentially, magnetic fields shape bipolar planetary nebulae via the generation of fast, collimated outflows.

Multiwavelength observations of Herculis X-1/HZ Herculis

We present first results from a multiwavelength campaign to observe the binary pulsar system Her X-1/ HZ Her. The campaign spanned 4 days during 1993 August: observations were taken with five satellites (IUE, EUVE, ROSAT, ASCA, and CGRO/BATSE) and several ground-based facilities. A substantial, unexpected drop in flux at X-ray energies, with no change in absorbing column density, was observed. The pulse period has increased from the previous measurement, contrary to the usual spin-up, and no pulsed emission is detected above 0.9 keV in the low state. The optical and UV fluxes continued to show 1.7 day modulation attributed to X-ray heating of the companion star. However, the significant reduction in UV flux observed around eclipse implies an absence of the normally observed excess attributed to X-ray heating of the disk. We conclude that we have observed an anomalous low state, seen only once before (Parmar et al. 1985), in which the X-ray flux is not redistributed but obscured. We suggest explanations for the behavior of the flux at different wavelengths.

The Einstein objective grating spectrometer survey of galactic binary X-ray sources

The results of observations of 22 bright Galactic X-ray point sources are presented, and the most reliable measurements to date of X-ray column densities to these sources are derived. The results are consistent with the idea that some of the objects have a component of column density intrinsic to the source in addition to an interstellar component. The K-edge absorption due to oxygen is clearly detected in 10 of the sources and the Fe L and Ne K edges are detected in a few. The spectra probably reflect emission originating in a collisionally excited region combined with emission from a photoionized region excited directly by the central source. 64 refs.

The eccentric accretion disc of the black hole A0620−00★

We present spectroscopic observations of the quiescent black hole binary A0620-00 with the 6.5-m Magellan Clay telescope at Las Campanas Observatory. We measure absorption-line radial velocities of the secondary and make the most precise determination to date (K-2 = 435.4 +/- 0.5 km s(-1)). By fitting the rotational broadening of the secondary, we refine the mass ratio to q = 0.060 +/- 0.004; these results, combined with the orbital period, imply a minimum mass for the compact object of 3.10 +/- 0.04 M-circle dot. Although quiescence implies little accretion activity, we find that the disc contributes 56 +/- 7 per cent of the light in B and V, and is subject to significant flickering. Doppler maps of the Balmer lines reveal bright emission from the gas stream-disc impact point and unusual crescent-shaped features. We also find that the disc centre of symmetry does not coincide with the predicted black hole velocity. By comparison with smoothed particle hydrodynamics (SPH) simulations, we identify this source with an eccentric disc. With high signal-to-noise ratio (S/N), we pursue modulation tomography of H alpha and find that the aforementioned bright regions are strongly modulated at the orbital period. We interpret this modulation in the context of disc precession, and discuss cases for the accretion disc evolution.

Doppler and modulation tomography of XTE J1118+480 in quiescence

We present Doppler and modulation tomography of the X-ray nova XTE J1118+480 with data obtained during quiescence using the 10-m Keck II telescope. The hotspot where the gas stream hits the accretion disc is seen in H alpha, H beta, He i lambda 5876 and Ca ii lambda 8662, thus verifying the presence of continued mass transfer within the system. The disc is clearly seen in H alpha and Ca ii lambda 8662. We image the mass-donor star in narrow absorption lines of Na i lambda lambda 5890, 5896, 8183, 8195 and Ca ii lambda 8662, implying an origin from the secondary itself rather than the interstellar medium. We also detect deviations in the centroid of the double peak of H alpha akin to those found by Zurita et al. suggesting disc eccentricity.

On the Asymmetries of Extended X-ray Emission from Planetary Nebulae

Chandra X-Ray Observatory (CXO) images have revealed that the X-ray-emitting regions of the molecule-rich young planetary nebulae (PNe) BD +30°3639 and NGC 7027 are much more asymmetric than their optical nebulosities. To evaluate the potential origins of these X-ray asymmetries, we analyze X-ray images of BD +30°3639, NGC 7027, and another PN resolved by CXO, NGC 6543, within specific energy bands. Image resolution has been optimized by subpixel repositioning of individual X-ray events. The resulting subarcsecond resolution images reveal that the soft (E < 0.7 keV) X-ray emission from BD +30°3639 is more uniform than the harder emission, which is largely confined to the eastern rim of the optical nebula. In contrast, soft X-rays from NGC 7027 are highly localized and this PN is more axially symmetric in harder emission. The broadband X-ray morphologies of BD +30°3639 and NGC 7027 are highly anticorrelated with their distributions of visual extinction, as determined from high-resolution, space- and ground-based optical and infrared imaging. Hence, it is likely that the observed X-ray asymmetries of these nebulae are due in large part to the effects of nonuniform intranebular extinction. However, the energy-dependent X-ray structures in both nebulae and NGC 6543—which is by far the least dusty and molecule-rich of the three PNe and displays very uniform intranebular extinction—suggest that other mechanisms, such as the action of collimated outflows and heat conduction, are also important in determining the detailed X-ray morphologies of young PNe.