Chandra's First Decade of Discovery

Stellar Spectroscopy

High-resolution X-ray Spectra Of The Symbiotic Star SS73 17

Romanus Eze, SAO
Randall Smith (SAO) and Juan Luna (SAO)

SS73 17 was an innocuous Mira-type symbiotic star until INTEGRAL and Swift discovered its bright hard X-ray emission. Suzaku observations then showedit emits three bright iron lines as well, with almost no emission in the 0.5-2 keV bandpass. We detected and confirmed strong emission lines of Fe K fluorescence, Fe XXV & Fe XXVI. We believe these thermal emissions lines are from the boundary layer of the accreting white dwarf. There is also Si XVI emission linewhich appears brighter than other lines which includes S XVI & Mg XII at the soft x-ray spectrum. We attribute the origin of these soft emission line to collisional ionization of the H-like particles falling into the potential well of the white dwarf. The soft x-ray emission lines could also be coming from leaking radiations from the absorber, which have gone through several scattering and re-absorptions before it get to our line of sight. Both observations show that the system is moderately variable as opposed to earlier result by Smith et al (2008).This result confirms the variability of the partial covering absorber campaign. We also found the source to be highly absorbed (NH = 14.65 x 1022 cm-2) for the partial covering absorber and (NH = 1.59 x 1022 cm-2) for the full covering absorber.

Summing Flares From EV Lac

David Huenemoerder, MIT
Paola Testa (CfA), Norbert Schulz (MIT), Jeremy Drake (CfA), Fabio Reale (Palermo), Claude Canizares (MIT), Rachel Osten (STScI)

We present preliminary results of a study of the spectral and temporal behavior of flares from the active M-dwarf EV Lac. We will apply three powerful techniques: high-resolution spectroscopic diagnostics to infer flaring loop conditions (temperature, density, dynamics); measurement of flare scale heights from Fe K fluorescence; and state-of-the-art hydrodynamic flare models. The high rate of short flares, the X-ray signature of high density, and the evidence for opacity in X-ray resonance lines make EV Lac an ideal target. A deep flare exposure obtained by summing short events will provide some of the best high-resolution time-resolved spectra possible for constraining physical models of M-dwarf coronae and flares with detail which can only be achieved through high resolution X-ray spectroscopy.

X-ray Jets in the CH Cyg Symbiotic System

Margarita Karovska, Harvard-Smithsonian Center for Astrophysics
M. Karovska (SAO), T. Gaetz(SAO), N. Lee(SAO), J. Raymond (SAO), W. Hack (STScI), C. Carilli (NRAO)

Symbiotic binaries are interacting systems in which a compact stellar source accretes matter from the wind of the cool evolved companion. There are a few hundred symbiotic systems known today, but jet activity has been detected in only a few of them, including in CH Cyg. CH Cyg is a symbiotic system that has shown significant activity since the mid 1960s. Jets have been detected in optical and radio since 1984, and more recently in 2001 in X-rays using Chandra observations.In 2008 we carried out coordinated multi-wavelength observations of the CH Cyg system with Chandra, HST, and the VLA, in order to study the propagation and interaction with the circumbinary medium of the jet detected in 2001. We report here on the detection of the 2001 SE jet which has expanded in seven years from ~350AU to ~1400 AU. The apex of the loop delineating the region of interaction with the circumbinary matter is moving with a speed of ~700 km/s. Assuming a linear expansion, the jet was launched during the 1999-2000 active phase. We also report on a detection of a powerful new jet in the SW direction, observed in X-ray, optical and radio wavelengths. The new jet has a multi-component structure including an inner jet and counter jet, and a SW component ending in several clumps extending up to a distance of about 750AU.

Distant Suns: Solar Flares as Proxies for Stellar Flares

Vinay Kashyap, SAO
Kathy Reeves (SAO), Brad Wargelin (SAO)

The solar corona has been a Rosetta stone directing our knowledge and understanding of stellar coronae. Because of its proximity, the Sun can be observed in great detail, and detailed physical models derived from such observations are often used to explain stellar phenomena. Large stellar flares are traditionally modeled as a single coronal loop evolving hydrodynamically. However, such a picture has no counterpart on the Sun, where flares are observed to be complex events, generally affecting large areas of an active region and resulting in post-flare loop arcades that have a different magnetic topology compared to the pre-flare region. Here we discuss a large flare on a dM3.5Ve dwarf, Ross 154, that was observed with Chandra. The flare shows a pronounced dual-decay structure. We model the event as a set of cascading loops in an arcade that are sequentially energized, leading to predicted light curves that also exhibit similar behavior. This work was supported by CXC NASA contract NAS8-39073 (VLK, BW) and XRT NASA contract NNM07AB07C (KR).

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The outbursts of R Aquarii - a symbiotic star, an X-ray binary, or a nanoquasar?

Edwin Kellogg, CfA
Joy Nichols (CfA)

Chandra is an unparalleled facility for resolving the behavior of R Aquarii. Chandra's sub arc second angular resolution allow us to resolve the X-ray jets and lobes, and its energy resolution lets us delineate regions of thermal and nonthermal emission from this variegated system. Chandra and other major observatories have observed R Aquarii over the Chandra decade, recording outbursts across the spectrum from radio to X-rays. We review the observations and discuss thermal and non thermal emission mechanisms in the jets, lobes and central source. The central engine is known to contain a Mira-type variable red giant star and a putative white dwarf. R Aquarii has refused so far to yield information on its binary period, or compelling evidence for an accretion disk. There is tantalizing evidence for a rotation period in the central source of 29 min on one observing occasion, that tempts us to believe the WD is magnetic. The many facets of R Aquarii's emission invite comparisons with extragalactic quasars and galactic microquasars, with the difference that observed velocities in the R Aquarii system are generally nonrelativistic, even comparable to Solar wind speeds in some cases. All in all, R Aquarii is a very versatile laboratory for study of a wide range of high energy phenomena, and being only 200 pc distant can be studied in great detail.

Two Types of X-ray Spectra in Catalysmic Variables Revisited

Koji Mukai, NASA/GSFC and UMBC

In a 2003 paper, Mukai et al. presented the Chandra HETG spectra of 7 cataclysmic variables (CVs) then available in the public archive, and classified them into “cooling flow” and “photoionized” types. In this presentation, I will revisit this classification. It is clear that multi-temperature plasma exists in the post-shock region of accreting white dwarfs; I will discuss why the cooling flow model works well, and what its limitations are when applied to CVs. It is also clear that even the “photoionized” CVs are also powered by the same multi-temperature plasma. I will discuss how the appearance of a much harder continuum of these “photoionized” CVs is created by the complex intrinsic absorber. I will show that there is an additional soft emission in these systems that the multi-temperature plasma models cannot explain, and discuss what evidence there is for the photoionization origin of this component.

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Serendipitous O star ACIS HETG spectra

J. Nichols, Smithsonian Astrophysical Observatory
A. W. Mitschang (SAO), N. Walborn (STSci)

O stars have been frequent targets for ACIS HETG grating spectra. The resulting spectra from the primary targets of these observatiions are available in TGCat and XAtlas. However, a number of additional O stars, many of which have dispersed spectra, fall in the fields of view of the ACIS HETG grating observations. The spectra from these additional O stars have not previously been extracted. We extracted all O star spectra projected on the field of view of any ACIS HETG grating observation that are within 2 arcmin of the aim point. While some of these new O star spectra have insufficient S/N for analysis, there are several spectra that are useful for evaluating the detailed evolution of O stars and O star wind parameters. The newly extracted O stars are compared to the available grid of O star spectral morphology with respect to spectral class and luminosity class. The new data presented here represent one of the unexplored benefits of the Chandra dataset.

The X-ray Variability of Capella

Jennifer Posson-Brown, SAO
Vinay Kashyap (SAO)

We have analyzed a decade of Chandra grating and imaging observations of the active binary Capella to characterize its X-ray variability at various timescales. Examining 205 ks of HRC-I imaging data taken over a time period of ~14 months (Dec 2005 to Jan 2007), we find short term broadband variability at the 2-7% level at timescales < 20 ks. At a scale of months to years, we see variations of 3-10% in the HRC-I data. Twelve HRC-S/LETG observations taken over the ten year period since launch, for a total of ~408 ks, indicate that the overall flux was steady until 2004 but has increased by roughly 50% since that time.In addition to examining broad band flux changes, we have also used the HRC-S/LETG observations to analyze the line emission in detail for several lines. After correcting for the broad spectrum flux increase, we determine how the line flux variability changes at different temperatures in the coronae. Variability is present at all temperatures, and we find that the high temperature plasma is more strongly variable than the low temperature plasma. We explore this variability for spectral lines of different elements at different temperatures. The emission measure structure of Capella undergoes changes more extensive than hitherto suspected.

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X-rays From The Exciting Region Of Cepheus A

Steven Pravdo, Jet Propulsion Laboratory/Caltech
Yohko Tsuboi (Chuo University)

We report an observation of X-ray emission from the exciting region of Cepheus A with the Chandra/ACIS instrument. What had been an unresolved X-ray source comprising the putative power sources is now resolved into at least 3 point-like sources, each with similar X-ray properties and differing radio and submillimeter properties. They each have inferred X-ray luminosities ≥ 1031 erg s-1 with hard spectra, T ≥ 107 K, and high low-energy absorption equivalent to tens to as much as a hundred magnitudes of visual absorption. The star usually assumed to be the most massive and energetic, HW2, is not detected with an upper limit about 7 times lower than the detections. The X-rays may arise via thermal bremsstrahlung in diffuse emission regions associated with a gyrosynchrotron source for the radio emission, or they could arise from powerful stellar winds. We also analyzed the Spitzer/IRAC mid-IR observation from this star-formation region and present the X-ray results and mid-IR classifications of the nearby stars. The X-ray emission from HH 168 is confirmed and it is not as underluminous in X-rays as previously believed. Copyright 2009 California Institute of Technology. Government sponsorship acknowledged.

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Chandra's First Decade Observing AR Lac

Peter Ratzlaff, SAO
Jeremy J. Drake (SAO), R. Nicholas Durham (SAO), Vinay Kashyap (SAO), Jennifer Posson-Brown (SAO), Bradford J. Wargelin (SAO)

X-ray observations of the eclipsing RS CVn-type binary AR Lacertae have been obtained every year from 1999 to 2008 with the Chandra High Resolution Camera imaging and spectroscopic detectors (HRC-I, HRC-S) as part of their gain and point spread function calibration. These represent the best quality data yet obtained on the long term variability of the X-ray emission of an RS CVn star, and are rendered especially valuable for the multi-epoch coverage of the AR Lac eclipses. The data are characterised by stochastic variability by factors of ~2 on timescales of one to several ks, and by minor flaring events in which count rates are observed to be elevated by slightly larger factors. During primary eclipse, the X-ray count rate is generally observed at approximately 60% of its value outside of eclipse and during periods of relative quiescence. Little evidence for secondary eclipses is present in the data, reminiscent of earlier X-ray and EUV observations. The X-ray count rate modulation through the eclipses allow us to place an upper limit on the extent of a spherically symmetric coronae of about two stellar radii, the exact limit depending on the details of the coronal models and partition of emission between the component stars. We compare the observed Chandra count rates to earlier EUVE, EINSTEIN, EXOSAT and ROSAT observations and comment on the apparent lack of cyclic coronal activity on RS CVn-type binaries.

X-Ray and Infrared Emission from Young Stellar Objects near LkHα101

Scott Wolk, Harvard-Smithsonian Center for Astrophysics
Elaine Winston (Exeter), Tyler L. Bourke (CfA), S. Thomas Megeath (Toledo), Robert Gutermuth (Smith), Bradley D. Spitzbart(CfA) and Rachel Osten (STSCI)

We report on a multiwavelength study of a partially embedded region of star formation centered on the Herbig Be star LkHα101. Using two 40 ks Chandra observations, we detect 213 X-ray sources in the field. We combine the X-ray data with 2MASS near-IR observations and Spitzer IRAC and MIPS 24 micron observations to obtain a complete picture of the cluster. A total of 158 of the X-ray sources have infrared counterparts. Of these, we find 9 protostars, 48 Class II objects, 5 transition objects and 72 Class III objects. From the Spitzer data we identify an additional 10 protostars, 53 Class II objects and 4 transition disk candidates which are not detected by Chandra. We obtained optical spectra of a sample of these objects. Combining the X-ray, Spitzer and spectral data, we obtain independent estimates of cluster distance and the total cluster size - excluding protostars. We obtain consistent distance estimates of 510+100-40pc and a total cluster size of 255+50-25 stars. We find the Class II:III ratio is about 5:7 with some evidence that the Class III sources are spatially more dispersed. The cluster appears very young with three sites of active star formation and a median age of about 1 Myr.