Peter W. Ratzlaff

First Light Measurements of Capella with the Low-Energy Transmission Grating Spectrometer aboard the Chandra X-Ray Observatory

We present the first X-ray spectrum obtained by the Low-Energy Transmission Grating Spectrometer (LETGS) aboard the Chandra X-Ray Observatory. The spectrum is of Capella and covers a wavelength range of 5-175 Å (2.5-0.07 keV). The measured wavelength resolution, which is in good agreement with ground calibration, is Deltalambda approximately 0.06 Å (FWHM). Although in-flight calibration of the LETGS is in progress, the high spectral resolution and unique wavelength coverage of the LETGS are well demonstrated by the results from Capella, a coronal source rich in spectral emission lines. While the primary purpose of this Letter is to demonstrate the spectroscopic potential of the LETGS, we also briefly present some preliminary astrophysical results. We discuss plasma parameters derived from line ratios in narrow spectral bands, such as the electron density diagnostics of the He-like triplets of carbon, nitrogen, and oxygen, as well as resonance scattering of the strong Fe xvii line at 15.014 Å.

Chandra multiwavelength project. II. First results of X-ray source properties

The Chandra Multiwavelength Project (ChaMP) is a wide-area (~14 deg2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed active galactic nuclei [AGNs] and high-z clusters of galaxies) at flux levels (fX ~ 10-15 to 10-14 ergs s-1 cm-2) intermediate between the Chandra Deep Field surveys and previous missions. We present the first results of X-ray source properties obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. Utilizing only near-on-axis X-ray-bright sources (to avoid problems caused by incompleteness and the Eddington bias), we derive the log N- log S relation in soft (0.5-2 keV) and hard (2-8 keV) energy bands. The ChaMP data are consistent with previous results of ROSAT, ASCA, and Chandra Deep Field surveys. In particular, our data nicely fill in the flux gap in the hard band between the Chandra Deep Field data and the previous ASCA data. We check whether there is any systematic difference in the source density between cluster and noncluster fields and also search for field-to-field variation, both of which have been previously reported. We found no significant field-to-field cosmic variation in either test within the statistics (~1 σ) across the flux levels included in our sample. In the X-ray color-color plot, most sources fall in the location characterized by photon index = 1.5-2 and NH = a few × 1020 cm2, suggesting that they are typical broadline AGNs. There also exist a considerable number of sources with peculiar X-ray colors (e.g., highly absorbed, very hard, very soft). We confirm a trend that on average the X-ray color hardens as the count rate decreases. Since the hardening is confined to the softest energy band (0.3-0.9 keV), we conclude that it is most likely due to absorption. We cross-correlate the X-ray sources with other catalogs and describe their properties in terms of optical color, X-ray-to-optical luminosity ratio, and X-ray colors.

Description and performance of the low-energy transmission grating spectrometer on board Chandra

The Chandra spacecraft has been launched successfully on July 23, 1999. The payload consists of a high resolution X- ray telescope, two imaging detector systems in the focal plane and two transmission gratings. Each one of the two gratings can be put in the beam behind the telescope and the grating spectrometers are optimized for high and low energy, respectively. The Low Energy Transmission Grating Spectrometer consists of three parts: the high-resolution telescope, the transmission grating array and the detector, to read-out the spectral image.

Doppler Shifts and Broadening and the Structure of the X-Ray Emission from Algol

In a study of Chandra High Energy Transmission Grating spectra of Algol, we clearly detect Doppler shifts caused by the orbital motion of Algol B. These data provide the first definitive proof that the X-ray emission of Algol is dominated by the secondary, in concordance with expectations that the primary B8 component should be X-ray-dark. However, the measured Doppler shifts are slightly smaller than might be expected, implying an effective orbital semimajor axis of about 10 R☉ instead of 11.5 R☉ for the Algol B center of mass. This could be caused by a small contribution of Algol A, possibly through accretion, to the observed X-ray flux, in which case such a contribution does not exceed 10%-15%. We suggest that the more likely explanation is an asymmetric corona biased toward the system center of mass by the tidal distortion of the surface of Algol B. A detailed analysis of the profiles of the strongest lines indicates the presence of excess line broadening amounting to approximately 150 km s-1 above that expected from thermal motion and surface rotation. Possible explanations for this additional broadening include turbulence, flows or explosive events, or rotational broadening from a radially extended corona. We favor the latter scenario and infer that a significant component of the corona at temperatures less than 107 K has a scale height of order the stellar radius. This interpretation is supported by the shape of the X-ray light curve and tentative detection of a shallow dip at secondary eclipse. We also examine the O VII intercombination and forbidden lines in a Low Energy Transmission Grating Spectrograph observation and find no change in their relative line fluxes as the system goes from quadrature to primary eclipse. Since these lines appear to be strongly affected by UV irradiation from Algol A through radiative excitation of the 2 3S → 2 3P transition, this supports the conjecture that the corona of Algol B at temperatures of several million kelvins must be significantly extended and/or located toward the poles to avoid being shadowed from Algol A during primary eclipse.

An Absence of X-ray Accretion Shock Instability Signatures in TW Hydrae

Gas accreting onto T Tauri stars should form shocks that are susceptible to the classical radiative shock instability. The instability should give rise to strong periodic modulation in the X-ray emission from the shock-heated plasma. Time series analysis of soft X-rays thought to arise predominantly in an accretion shock on the classical T Tauri star TW Hydrae reveals no periodic variations and a 99% confidence pulsed fraction limit of 5% over the frequency range 0.0001-6.81 Hz. We find no clear explanation for the absence of X-ray instability signatures, but suggest that existing one-dimensional models are too simple to explain the three-dimensional shock structure, or that preheating and deceleration of the accretion stream by the damping of magnetohydrodynamic waves excited either by the shock itself, or more deeply in the stellar envelope, could ameliorate the instability in the likely case of a sub-Alfvenic shock.

X-Ray Flaring on the dMe Star, Ross 154

We present results from two Chandra imaging observations of Ross 154, a nearby flaring M dwarf star. During a 61 ks ACIS-S exposure, a very large flare occurred (the equivalent of a solar X3400 event, with -->LX = 1.8 ? 1030 ergs s?1) in which the count rate increased by a factor of over 100. The early phase of the flare shows evidence for the Neupert effect, followed by a further rise and then a two-component exponential decay. A large flare was also observed at the end of a later 48 ks HRC-I observation. Emission from the nonflaring phases of both observations was analyzed for evidence of low-level flaring. From these temporal studies we find that microflaring probably accounts for most of the quiescent emission and that, unlike for the Sun and the handful of other stars that have been studied, the distribution of flare intensities does not appear to follow a power law with a single index. Analysis of the ACIS spectra, which was complicated by exclusion of the heavily piled-up source core, suggests that the quiescent Ne/O abundance ratio is enhanced by a factor of ~2.5 compared to the commonly adopted solar abundance ratio and that the Ne/O ratio and overall coronal metallicity during the flare appear to be enhanced relative to quiescent abundances. Based on the temperatures and emission measures derived from the spectral fits, we estimate the length scales and plasma densities in the flaring volume and also track the evolution of the flare in color-intensity space. Lastly, we searched for a stellar wind charge exchange X-ray halo around the star but without success; because of the relationship between mass-loss rate and the halo surface brightness, not even an upper limit on the stellar mass-loss rate can be determined.

Monte Carlo processes for including Chandra instrument response uncertainties in parameter estimation studies

Instrument response uncertainties are almost universally ignored in current astrophysical X-ray data analyses. Yet modern X-ray observatories, such as Chandra and XMM-Newton, frequently acquire data for which photon counting statistics are not the dominant source of error. Including allowance for performance uncertainties is, however, technically challenging in terms of both understanding and specifying the uncertainties themselves, and in employing them in data analysis. Here we describe Monte Carlo methods developed to include instrument performance uncertainties in typical model parameter estimation studies. These methods are used to estimate the limiting accuracy of Chandra for understanding typical X-ray source model parameters. The present study indicates that, for ACIS-S3 observations, the limiting accuracy is reached for ~ 104 counts.

Low-energy effective area of the Chandra low-energy transmission grating spectrometer

The Chandra X-ray Observatory was successfully launched on July 23, 1999, and subsequently began an intensive calibration phase. We present preliminary results from in- flight calibration of the low energy response of the High Resolution Camera Spectroscopic readout (HRC-S) combined with the Low Energy Transmission Grating (LETG) aboard Chandra. These instruments comprise the Low Energy Transmission Grating Spectrometer (LETGS). For this calibration study, we employ a pure hydrogen non-LTE white dwarf emission model (Teff equals 25000 K and log g equals 9.0) for comparison with the Chandra observations of Sirius B. Pre-flight calibration of the LETGS effective area was conducted only at wavelengths shortward of 45 angstroms (E > 0.277 keV). Our Sirius B analysis shows that the HRC-S quantum efficiency (QE) model assumed for longer wavelengths overestimates the effective area on average by a factor of 1.6. We derive a correction to the low energy HRC-S QE model to match the predicted and observed Sirius B spectra over the wavelength range of 45 - 185 angstroms. We make an independent test of our results by comparing a Chandra LETGS observation of HZ 43 with pure hydrogen model atmosphere predictions and find good agreement.

In-flight performance and calibration of the Chandra high-resolution camera spectroscopic readout (HRC-S)

The High Resolution Camera (HRC) is one of two focal plane instruments on the NASA Chandra X-ray Observatory which was successfully launched on July 23, 1999. The Chandra X-ray Observatory was designed to perform high resolution spectroscopy and imaging in the X-ray band of 0.07 to 10 keV. The HRC instrument consists of two detectors, HRC-I for imaging and HRC-S for spectroscopy. Each HRC detector consists of a thin aluminized polyimide blocking filter, a chevron pair of microchannel plates and a crossed grid charge readout. The HRC-I is an approximately 100 X 100 mm detector optimized for high resolution imaging and timing, the HRC-S is an approximately 20 X 300 detector optimized to function as the readout for the Low Energy Transmission Grating. In this paper we discuss the in-flight performance of the HRC-S, and present preliminary analysis of flight calibration data and compare it with the results of the ground calibration and pre-flight predictions. In particular we will compare ground data and in-flight data on detector background, quantum efficiency, spatial resolution, pulse height resolution, and point spread response function.

AXAF grating efficiency measurements with calibrated nonimaging detectors

In Phase 1 of AXAF testing at the X-Ray Calibration Facility (XRCF), calibrated flow proportional counters (FPCs) and solid-state detectors were used both in the focal plane and as beam-normalization detectors. This use of similar detectors in the beam and focal plane combined with detailed fitting of their pulse-height spectra allowed accurate measurements of the HRMA absolute effective area with minimum influence of source and detector effects. This paper describes the application of these detectors and fitting techniques to the analysis of effective area and efficiency measurements of the AXAF transmission gratings, the High Energy Transmission Grating (HETG) and the Low Energy Transmission Grating. Because of the high dispersion of these gratings the analysis must be refined. Key additional ingredients are the inclusion of detailed x-ray source models of the K and L lines based on companion High-Speed Imager microchannel-plate data and corrections to the data based on high-fidelity ray-trace simulations. The XRCF- measured efficiency values that result from these analyses have systematic errors estimated in the 10-20 percent range. Within these errors the measurements agree with the pre-XRCF laboratory-based efficiency models of the AXAF grating diffraction efficiencies.