Joseph M. DePasquale

Outer Jet X-Ray and Radio Emission in R Aquarii: 1999.8 to 2004.0

Chandra and VLA observations of the symbiotic star R Aqr in 2004 reveal significant changes over the 3-4 year interval between these observations and previous observations taken in with the VLA in 1999 and with Chandra in 2000. This paper reports on the evolution of the outer thermal X-ray lobe jets and radio jets. The emission from the outer X-ray lobe jets lies farther away from the central binary than the outer radio jets and comes from material interpreted as being shock-heated to 106 K, a likely result of collision between high-speed material ejected from the central binary and regions of enhanced gas density. Between 2000 and 2004, the northeast (NE) outer X-ray lobe jet moved out, away from the central binary, with an apparent projected motion of 580 km s-1. The southwest (SW) outer X-ray lobe jet almost disappeared between 2000 and 2004, presumably due to adiabatic expansion and cooling. The NE radio-bright spot also moved away from the central binary between 2000 and 2004, but with a smaller apparent velocity than the NE X-ray-bright spot. The SW outer lobe jet was not detected in the radio in either 1999 or 2004. The density and mass of the X-ray-emitting material is estimated. Cooling times, shock speeds, pressure, and confinement are discussed.

Discovery of Rapid Hard X-Ray Variability and New Jet Activity in the Symbiotic Binary R Aquarii

Two Chandra observations of the R Aqr symbiotic binary system taken 3.3 yr apart show dramatic changes in the X-ray morphology and spectral characteristics in the inner 500 AU of this system. The morphology of the soft X-ray emission has evolved from a nearly circular region centered on the binary system to an hourglass shape that indicates the formation of a new southwest jet. Synchrotron radiation from the new jet in contemporaneous VLA radio spectra implies the physical conditions in the early stages of jet development are different from those in the more extended outer thermal jets known to exist for decades in this system. The central binary source has two X-ray spectral components in each of the two epochs, a soft component and a highly absorbed hard component characterized by T ~ 108 K if fit with a thermal plasma model. The spectrum hardened considerably between 2000.7 and 2004.0, primarily due to increased flux above 5 keV, suggesting a change in the accretion activity of the white dwarf on a timescale of a few years or less. Point-source Fe K emission is detected at the position of the central binary system in both observations. While the earlier observation shows evidence of only a single emission peak near Fe Kα at 6.4 keV, the later observation shows a more complex emission structure between 6 and 7 keV. Finally, we have discovered a modulation in the hard X-ray flux with a period of 1734 s at a 95% confidence level in the 2004 observation only. The modulation potentially arises from standing shocks in an accretion column, and we have explored the possibility that the white dwarf in R Aqr is analogous to the magnetic white dwarfs in Intermediate Polars.

Flight spectral response of the ACIS instrument

We discuss the flight calibration of the spectral response of the Advanced CCD Imaging Spectrometer (ACIS) on-board the Chandra X-ray Observatory (CXO). The spectral resolution and sensitivity of the ACIS instrument have both been evolving over the course of the mission. The spectral resolution of the frontside-illuminated (FI) CCDs changed dramatically in the first month of the mission due to radiation damage. Since that time, the spectral resolution of the FI CCDs and the Backside-illuminated (BI) CCDs have evolved gradually with time. We demonstrate the efficacy of charge-transfer inefficiency (CTI) correction algorithms which recover some of the lost performance. The detection efficiency of the ACIS instrument has been declining throughout the mission, presumably due to a layer of contamination building up on the filter and/or CCDs. We present a characterization of the energy dependence of the excess absorption and demonstrate software which models the time dependence of the absorption from energies of 0.4 keV and up. The spectral redistribution function and the detection efficiency are well-characterized at energies from 1.5 to 8.0~keV primarily due to the existence of strong lines in the ACIS calibration source in that energy range. The calibration at energies below 1.5 keV is challenging because of the lack of strong lines in the calibration source and also because of the inherent non-linear dependence with energy of the CTI and the absorption by the contamination layer. We have been using data from celestial sources with relatively simple spectra to determine the quality of the calibration below 1.5 keV. We have used observations of 1E0102.2-7219 (the brightest supernova remnant in the SMC), PKS2155-304 (a bright blazar), and the pulsar PSR~0656+14 (nearby pulsar with a soft spectrum), since the spectra of these objects have been well-characterized by the gratings on the CXO. The analysis of these observations demonstrate that the CTI correction recovers a significant fraction of the spectral resolution of the FI CCDs and the models of the time-dependent absorption result in consistent measurements of the flux at low energies for data from a BI (S3) CCD.

An evaluation of a bake-out of the ACIS instrument on the Chandra X-Ray Observatory

The sensitivity of the Advanced CCD Imaging Spectrometer (ACIS) instrument on the Chandra X-ray Observatory (CXO) to low-energy X-rays (0.3 - 2.0 keV) has been declining throughout the mission. The most likely cause of this degradation is the growth of a contamination layer on the cold (-60 C) filter which attenuates visible and near-visible light incident on the CCDs. The contamination layer is still increasing 4 years after launch, but at a significantly lower rate than initially. We have determined that the contaminant is composed mostly of C with small amounts of O and F. We have conducted ground experiments to determine the thermal desorption properties of candidate materials for the contaminant. We have conducted experiments to determine the robustness of the thin filter to the thermal cycling necessary to remove the contaminant. We have modeled the migration of the contaminant during this bake-out process to ensure that the end result will be a reduction in the thickness of the contamination layer. We have considered various profiles for the bake-out consisting of different temperatures for the ACIS focal plane and detector housing and different dwell times at these temperatures. The largest uncertainty which affects our conclusions is the volatility of the unknown contaminants. We conclude that bakeout scenarios in which the focal plane temperature and the detector housing temperature are raised to +20~C are the most likely to produce a positive outcome.

Cross-calibration of the x-ray instruments onboard the Chandra, Suzaku, Swift, and XMM-Newton Observatories using the SNR 1E 0102.2-7219

We report on our continuing efforts to compare the absolute effective areas of the current generation of CCD instruments onboard the active observatories, specifically: Chandra ACIS, XMM-Newton EPIC (MOS and pn), Suzaku XIS, and Swift XRT, using 1E 0102.2-7219, the brightest supernova remnant in the Small Magellanic Cloud. 1E 0102.2-7219 has strong lines of O, Ne, and Mg below 1.5 keV and little Fe emission to complicate the spectrum. The spectrum of 1E 0102.2-7219 has been well-characterized using the RGS grating instrument on XMM-Newton and the HETG grating instrument on Chandra. We have developed an empirical model that includes Gaussians for the identified lines, an absorption component in the Galaxy, another absorption component in the SMC, and two continuum components with different temperatures. In our fits, the model is highly constrained in that only the normalizations of the four brightest line complexes (the OVII triplet, OVIII Lyα line, the NeIX triplet, and the NeX Lyα) and an overall normalization are allowed to vary, while all other components are fixed. We adopted this approach to provide a straightforward comparison of the measured line fluxes at these four energies. We find that the measured fluxes of the OVII triplet, the OVIII Lyαline, the NeIX triplet, and the NeX Lyαline generally agree to within ±10% for all instruments, with the exception of the OVII triplet and the OVIII Lyαline normalizations for the Suzaku XIS1, XIS2, & XIS3, and the Swift XRT, which can be up to 20%lower compared to the reference model.

The SMC SNR 1E0102.2-7219 as a calibration standard for x-ray astronomy in the 0.3-2.5 keV bandpass

The flight calibration of the spectral response of CCD instruments below 1.5 keV is difficult in general because of the lack of strong lines in the on-board calibration sources typically available. We have been using E0102, the brightest supernova remnant in the Small Magellanic Cloud, to evaluate the response models of the ACIS CCDs on the Chandra X-ray Observatory (CXO), the EPIC CCDs on the XMM-Newton Observatory, the XIS CCDs on the Suzaku Observatory, and the XRT CCD on the Swift Observatory. E0102 has strong lines of O, Ne, and Mg below 1.5 keV and little or no Fe emission to complicate the spectrum. The spectrum of E0102 has been well characterized using high-resolution grating instruments, namely the XMM-Newton RGS and the CXO HETG, through which a consistent spectral model has been developed that can then be used to fit the lower-resolution CCD spectra. Fits with this model are sensitive to any problems with the gain calibration and the spectral redistribution model of the CCD instruments. We have also used the measured intensities of the lines to investigate the consistency of the effective area models for the various instruments around the bright O (570 eV and 654 eV) and Ne (910 eV and 1022 eV) lines. We find that the measured fluxes of the O VII triplet, the O VIII Ly-a line, the Ne IX triplet, and the Ne X Ly-a line generally agree to within ±10% for all instruments, with 28 of our 32 fitted normalizations within ±10% of the RGS-determined value. The maximum discrepancies, computed as the percentage difference between the lowest and highest normalization for any instrument pair, are 23% for the O VII triplet, 24% for the O VIII Ly-a line, 13% for the Ne IX~triplet, and 19% for the Ne X Ly-a line. If only the CXO and XMM are compared, the maximum discrepancies are 22% for the O VII triplet, 16% for the O VIII Ly-a line, 4% for the Ne IX triplet, and 12% for the Ne X Ly-a line.

The Secret XUV Lives of Cepheids: FUV/X‐ray observations of Polaris and β Dor

We report on the surprising recent discovery of strong FUV emissions in two bright, nearby Classical Cepheids from analyses of FUSE archival observations and one of our own approved observations just prior to the failure of the satellite. Polaris and β Dor are currently the only two Cepheids to have been observed with FUSE, and β Dor is the only one to have multiple spectra. Both Cepheids show strong C III (977 A, 1176 A) and O VI (1032 A, 1038 A) emissions, indicative of 50,000–500,000 K plasma, well above the photospheric temperatures of the stars. More remarkably, β Dor displays variability in the FUV emission strengths which appears to be correlated to its 9.84‐d pulsation period. This phenomenon has never before been observed in Cepheids. The FUV studies are presented along with our recent Chandra/XMM X‐ray observations of Polaris and β Dor, in which X‐ray detections were found for both stars. Further X‐ray observations have been proposed to unambiguously determine the origin and nature of the observ...

Verifying the ACIS contamination model with 1E0102.2-7219

The low-energy sensitivity of the Advanced CCD Imaging Spectrometer (ACIS) instrument on board the Chandra X-ray Observatory (CXO) has been continuously degrading since launch due to the accumulation of a layer of contamination on the ACIS optical blocking filter (OBF). This contamination layer, the result of out-gassing and off-gassing within the observatory, introduces a new, energy dependent absorption into the ACIS response. The thickness of this layer has been increasing with time and its spatial distribution across the OBF has been continually changing with time. We utilize multiple observations of the SMC Supernova Remnant 1E0102.2-7219 to verify the models for the spectral, temporal, and spatial dependence of the contamination layer. We also use this source to investigate cross-calibration between the front illuminated (FI) and back illuminated (BI) CCDs. 1E0102.2-7219 has a soft, line-dominated spectrum which is very sensitive to the additional absorption of the contamination layer. The extensive calibration observations of 1E0102.2-7219 over the course of the mission at several different locations on the ACIS Imaging (I) and Spectroscopy (S) arrays allows for a verification of the temporal and spatial dependence of the contamination model.

X-RAY DETECTION OF THE CLUSTER CONTAINING THE CEPHEID S MUS

The galactic Cepheid S Muscae has recently been added to the important list of Cepheids linked to open clusters, in this case the sparse young cluster ASCC 69. Low-mass members of a young cluster are expected to have rapid rotation and X-ray activity, making X-ray emission an excellent way to discriminate them from old field stars. We have made an XMM-Newton observation centered on S Mus and identified a population of X-ray sources whose near-IR Two Micron All Sky Survey counterparts lie at locations in the J, (J – K) color-magnitude diagram consistent with cluster membership at the distance of S Mus. Their median energy and X-ray luminosity are consistent with young cluster members as distinct from field stars. These strengthen the association of S Mus with the young cluster, making it a potential Leavitt law (period-luminosity relation) calibrator.

Changing software philosophy for ACIS operations as Chandra ages

The Chandra X-Ray Observatory is about to start its 10th year of operations. Over the time of the mission, the Science Operations Team, ACIS (Advanced CCD Imaging Spectrometer) Operations group, has participated in spacecraft command load reviews. These reviews ensure the spacecraft commanding is safe for the instrument and the ACIS configuration matches the planned observation. The effectiveness of spacecraft command load reviews for ACIS depends on the ability to adapt the software as operations change in response to the aging of the spacecraft. We have recently rewritten this software to start incorporating other spacecraft subsystems, including maneuvers and hardware commanding, to ensure the safety of ACIS. In addition, operational changes that optimize the science return of the spacecraft have created new constraints on commanding. This paper discusses the reorganization of the code and the multiple changes to the philosophy of the code. The result is stronger, more flexible software that will continue to assist us in protecting ACIS throughout the Chandra mission.