Chandra Low Energy Transmission Grating Research Articles

Is RX J1856.5−3754 a Quark Star?

Deep Chandra Low Energy Transmission Grating and High Resolution Camera spectroscopic observations of the isolated neutron star candidate RX J1856.5-3754 have been analyzed to search for metallic and resonance cyclotron spectral features and for pulsation behavior. As found from earlier observations, the X-ray spectrum is well represented by an ~60 eV (7 × 105 K) blackbody. No unequivocal evidence of spectral line or edge features has been found, arguing against metal-dominated models. The data contain no evidence for pulsation, and we place a 99% confidence upper limit of 2.7% on the unaccelerated pulse fraction over a wide frequency range from 10-4 to 100 Hz. We argue that the derived interstellar medium neutral hydrogen column density of 8 × 1019 cm-2 ≤ NH ≤ 1.1 × 1020 cm-2 favors the larger distance from two recent Hubble Space Telescope parallax analyses, placing RX J1856.5-3754 at ~140 pc instead of ~60 pc and in the outskirts of the R CrA dark molecular cloud. That such a comparatively rare region of high interstellar matter (ISM) density is precisely where an isolated neutron star reheated by accretion of ISM would be expected is either entirely coincidental or current theoretical arguments excluding this scenario for RX J1856.5-3754 are premature. Taken at face value, the combined observational evidence—a lack of spectral and temporal features and an implied radius of R∞ = 3.8-8.2 km that is too small for current neutron star models—points to a more compact object, such as allowed for quark matter equations of state.

ITAL]Chandra[/ITAL] Detection of O [CSC]viii[/CSC] L[CLC]y[/CLC]α Absorption from an Overdense Region in the Intergalactic Medium

We report the first detection of an OVIII Ly-alpha absorption line associated with an overdense region in the intergalactic medium (IGM) along the sightline towards PKS 2155-304 with the Chandra Low Energy Transmission Grating Spectrometer (LETGS). The absorption line is detected at 4.5 sigma level with cz~16,600 km/s. At the same velocity Shull et al.(1998) detected a small group of spiral galaxies (with an overdensity of delta_gal ~ 100) and low metallicity HI Ly-alpha clouds. We constrain the intragroup gas that gives rise to the OVIII Ly-alpha line to a baryon density in the range 1e-5 < n_b < 7.5e-5 cm^-3 (50 < delta_b < 350) and a temperature of (4-5) x 10^6 K, assuming 0.1 solar abundance. These estimates are in accordance with those of the warm/hot intergalactic medium (WHIM) that are predicted from hydrodynamic simulations. Extrapolating from this single detection implies a large fraction of the ``missing baryons'' (~ 10%, or ~ 30-40% of the WHIM) are probed by the OVIII absorber.

Carbon and nitrogen abundances in the coronae of Algol B and other evolved stars: Evidence for CNO-cycle processed material

Using the Ly-lines of carbon and nitrogen measured in Chandra Low Energy Transmission Grating Spectrometer (LETGS) spectra we study the coronal abundances of these elements in a sample of late-type stars including dwarf stars, the prototypical eclipsing binary Algol, the single giant Cet, and three RS CVn binaries. In the main sequence stars of our sample the flux ratio RNC between the Ly-lines of nitrogen and carbon is below unity, while RNC is found to be8.5 for Cet and >23.3 for Algol. These values are more than an order of magnitude larger than expected from a solar abundance plasma in collisional equilibrium regardless of the chosen temperature structure. We therefore interpret the anomalously large RNC-ratios as being due to the exposure of CNO-cycle processed material at the surfaces and in the coronae of Algol and Cet.

Ultracompact X-Ray Binaries with Neon-rich Degenerate Donors

There are three low-mass X-ray binaries (4U 0614+091, 2S 0918-549, and 4U 1543-624) for which broad-line emission near 0.7 keV was previously reported. A recent high-resolution observation of 4U 0614+091 with the Chandra Low-Energy Transmission Grating Spectrometer (LETGS) found evidence of an unusually high Ne/O abundance ratio along the line of sight but failed to detect the previously reported 0.7 keV feature. We have made a search of archival ASCA spectra and identified a fourth source with the 0.7 keV feature, the 20 minute ultracompact binary 4U 1850-087. In all four of these sources, the 0.7 keV residual is eliminated from the ASCA spectra by allowing excess photoelectric absorption due to a nonsolar relative abundance of neon, just as in the LETGS spectrum of 4U 0614+091. The optical properties of these systems suggest that all four are ultracompact (Porb < 80 minutes) binaries. We propose that there is excess neon local to each of these sources, as also found in the ultracompact binary pulsar 4U 1626-67. We suggest that the mass donor in these systems is a low-mass, neon-rich degenerate dwarf and that the binaries are all ultracompact.

ITAL]Chandra[/ITAL] Grating Spectroscopy of the Seyfert Galaxy Ton S180

We present preliminary results from spectral observations of Ton S180 using Chandra and ASCA. The data confirm the presence of the soft excess, but the Chandra Low-Energy Transmission Grating spectrum reveals it to be broad and smooth rather than resolved into individual emission lines. This excess may represent either a primary or reprocessed continuum component or a blend of broad lines from an ionized accretion disk. The occurrence of a similar feature in five other narrow-line Seyfert 1 galaxies leads us to conclude that this soft X-ray component may be a characteristic of sources accreting at a high rate. The X-ray spectrum shows no evidence for absorption lines, indicating that if gas exists in the line of sight then it is in a very high ionization state or has an extreme velocity distribution. The new ASCA data confirm that the narrow component of the Fe Kα line peaks close to a rest energy of 7 keV, indicating the presence of a significant amount of highly ionized material in the nuclear environs.

Interstellar X‐Ray Absorption Spectroscopy of Oxygen, Neon, and Iron with theChandraLETGS Spectrum of X0614+091

We find resolved interstellar O K, Ne K, and Fe L absorption spectra in the Chandra Low Energy Transmission Grating Spectrometer spectrum of the low mass X-ray binary X0614+091. We measure the column densities in O and Ne, and find direct spectroscopic constraints on the chemical state of the interstellar O. These measurements probably probe a low-density line of sight through the Galaxy and we discuss the results in the context of our knowledge of the properties of interstellar matter in regions between the spiral arms.