arXiv subjects

J. J. Drake

Publications and source records attributed to J. J. Drake.

At least 127 records · Page 7Linked to original sources

arXiv2008

A planetary nebula around nova V458 Vul undergoing flash ionization

Nova V458 Vul erupted on 2007 August 8th and reached a visual magnitude of 8.1 a few days later. H$α$ images obtained six weeks before the outburst as part of the IPHAS galactic plane survey reveal an 18th magnitude progenitor surrounded by an extended nebula. Subsequent images and spectroscopy of the nebula reveal an inner nebular knot increasing rapidly in brightness due to flash ionization by the nova event. We derive a distance of 13 kpc based on light travel time considerations, which is supported by two other distance estimation methods. The nebula has an ionized mass of 0.2 M$_{\odot}$ and a low expansion velocity: this rules it out as ejecta from a previous nova eruption, and is consistent with it being a ~14,000 year old planetary nebula, probably the product of a prior common envelope (CE) phase of evolution of the binary system. The large derived distance means that the mass of the erupting WD component of the binary is high. We identify two possible evolutionary scenarios, in at least one of which the system is massive enough to produce a Type Ia supernova on merging.

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arXiv2008

Swift Observations of Shock Evolution in RS Ophiuchi

Our \textit{Swift} observations of RS Oph form an unprecedented X-ray dataset to undertake investigations of both the central source and the interaction of the outburst ejecta with the circumstellar environment. Over the first month, the XRT data are dominated by emission from rapidly evolving shocks. We discuss the differences in derived parameters from those found for \textit{RXTE} at early times and the evolution of the X-ray emission to much later times. It is apparent that at late times several emission components are present. We find no strong evidence of the proposed shock break-out in our data.

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arXiv2008

Coronal Structure and Abundances in Young Fast Rotators

AB Dor, Speedy Mic and Rst137B are in their early post-T Tauri evolutionary phase (<100Myr), at the age of fastest rotation in the life of late-type stars. They straddle the coronal saturation-supersaturation boundary first defined by young stars in open clusters. High resolution Chandra X-ray spectra have been analysed to study their coronal properties as a function of coronal activity parameters Rossby number, $L_X/L_{bol}$ and a coronal temperature index. Plasma emission measure distributions as a function of temperature show broad peaks at T~10e7K. Differences between stars suggest that as supersaturation is reached the DEM slope below the temperature of peak DEM becomes shallower, while the DEM drop-off above this temperature becomes more pronounced. A larger sample comprising our three targets and 22 active stars studied in the recent literature reveals a general increase of plasma at T>10e7 toward the saturated-supersaturated boundary but a decline beyond this among supersaturated stars. All three of the stars studied in detail here show lower coronal abundances of the low FIP elements Mg, Si and Fe, relative to the high FIP elements S, O and Ne, as compared to the solar mixture. The coronal Fe abundances of the stellar sample are inversely correlated with Lx/Lbol, declining slowly with rising Lx/Lbol, but with a much more sharp decline at Lx/Lbol>3x10e-4. For dwarfs the Fe abundance is also well-correlated with Rossby number. The coronal O/Fe ratios for dwarfs show a clear increase with decreasing Rossby number, apparently reaching saturation at [O/Fe]=0.5 at the coronal supersaturation boundary. Similar increases in O/Fe with increasing coronal temperature and $L_X/L_{bol}$ are seen.

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arXiv2008

Geometry Diagnostics of a Stellar Flare from Fluorescent X-rays

We present evidence of Fe fluorescent emission in the Chandra HETGS spectrum of the single G-type giant HR 9024 during a large flare. In analogy to solar X-ray observations, we interpret the observed Fe K$α$ line as being produced by illumination of the photosphere by ionizing coronal X-rays, in which case, for a given Fe photospheric abundance, its intensity depends on the height of the X-ray source. The HETGS observations, together with 3D Monte Carlo calculations to model the fluorescence emission, are used to obtain a direct geometric constraint on the scale height of the flaring coronal plasma. We compute the Fe fluorescent emission induced by the emission of a single flaring coronal loop which well reproduces the observed X-ray temporal and spectral properties according to a detailed hydrodynamic modeling. The predicted Fe fluorescent emission is in good agreement with the observed value within observational uncertainties, pointing to a scale height $\lesssim 0.3$\rstar. Comparison of the HR 9024 flare with that recently observed on II Peg by Swift indicates the latter is consistent with excitation by X-ray photoionization.

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arXiv2007

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 L_X = 1.8x10^30 ergs/s) 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 non-flaring 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.

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arXiv2007

Silicate dust in the environment of RS Ophiuchi following the 2006 eruption

We present further Spitzer Space Telescope observations of the recurrent nova RS Ophiuchi, obtained over the period 208-430 days after the 2006 eruption. The later Spitzer IRS data show that the line emission and free-free continuum emission reported earlier is declining, revealing incontrovertible evidence for the presence of silicate emission features at 9.7 and 18microns. We conclude that the silicate dust survives the hard radiation impulse and shock blast wave from the eruption. The existence of the extant dust may have significant implications for understanding the propagation of shocks through the red giant wind and likely wind geometry.

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arXiv2007

An investigation of Fe XVI emission lines in solar and stellar EUV and soft X-ray spectra

New fully relativistic calculations of radiative rates and electron impact excitation cross sections for Fe XVI are used to determine theoretical emission-line ratios applicable to the 251 - 361 A and 32 - 77 A portions of the extreme-ultraviolet (EUV) and soft X-ray spectral regions, respectively. A comparison of the EUV results with observations from the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS) reveals excellent agreement between theory and experiment. However, for emission lines in the 32 - 49 A portion of the soft X-ray spectral region, there are large discrepancies between theory and measurement for both a solar flare spectrum obtained with the X-Ray Spectrometer/Spectrograph Telescope (XSST) and observations of Capella from the Low Energy Transmission Grating Spectrometer (LETGS) on the Chandra X-ray Observatory. These are probably due to blending in the solar flare and Capella data from both first order lines and from shorter wavelength transitions detected in second and third order. By contrast, there is very good agreement between our theoretical results and the XSST and LETGS observations in the 50 - 77 A wavelength range, contrary to previous results. In particular, there is no evidence that the Fe XVI emission from the XSST flare arises from plasma at a much higher temperature than that expected for Fe XVI in ionization equilibrium, as suggested by earlier work.

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arXiv2007

The SSS phase of RS Ophiuchi observed with Chandra and XMM-Newton I.: Data and preliminary Modeling

The phase of Super-Soft-Source (SSS) emission of the sixth recorded outburst of the recurrent nova RS Oph was observed twice with Chandra and once with XMM-Newton. The observations were taken on days 39.7, 54.0, and 66.9 after outburst. We confirm a 35-sec period on day 54.0 and found that it originates from the SSS emission and not from the shock. We discus the bound-free absorption by neutral elements in the line of sight, resonance absorption lines plus self-absorbed emission line components, collisionally excited emission lines from the shock, He-like intersystem lines, and spectral changes during an episode of high-amplitude variability. We find a decrease of the oxygen K-shell absorption edge that can be explained by photoionization of oxygen. The absorption component has average velocities of -1286+-267 km/s on day 39.7 and of -771+-65 km/s on day 66.9. The wavelengths of the emission line components are consistent with their rest wavelengths as confirmed by measurements of non-self absorbed He-like intersystem lines. We have evidence that these lines originate from the shock rather than the outer layers of the outflow and may be photoexcited in addition to collisional excitations. We found collisionally excited emission lines that are fading at wavelengths shorter than 15A that originate from the radiatively cooling shock. On day 39.5 we find a systematic blue shift of -526+-114 km/s from these lines. We found anomalous He-like f/i ratios which indicates either high densities or significant UV radiation near the plasma where the emission lines are formed. During the phase of strong variability the spectral hardness light curve overlies the total light curve when shifted by 1000sec. This can be explained by photoionization of neutral oxygen in the line of sight if the densities of order 10^{10}-10^{11} cm^{-3}.

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arXiv2006

Soft X-ray Spectroscopy of the Hot DA White Dwarf LB1919 and the PG1159 Star PG1520+525

We have performed soft X-ray spectroscopy of two hot white dwarfs with the Chandra observatory using the Low Energy Transmission Grating. The first target is the hot DA white dwarf LB1919 (Teff=69000 K). This star is representative of a small group of hot DAs whose metallicities lie well below predictions from radiative levitation theory. The Chandra spectrum shows a rich absorption line spectrum which may allow to find the origin of the low-metallicity nature of these DAs. The second target is PG1520+525, a very hot non-pulsating PG1159 star. We find that it is hotter (Teff=150 000 K) than the pulsating prototype PG1159-035 (Teff=140 000) and conclude that both stars confine the blue edge of the GW Vir instability strip.

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arXiv2006

Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: I. Early X-ray emission from the shocked ejecta and red giant wind

RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts have indicated that high velocity ejecta interact with a pre-existing red giant wind, setting up shock systems analogous to those seen in Supernova Remnants. However, in the previous outburst in 1985, X-ray observations did not commence until 55 days after the initial explosion. Here we report on Swift observations covering the first month of the 2006 outburst with the Burst Alert (BAT) and X-ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25 keV band from t=0 to $t\sim6$ days. XRT observationsfrom 0.3-10 keV, started at 3.17 days after outburst. The rapidly evolving XRT spectra clearly show the presence of both line and continuum emission which can be fitted by thermal emission from hot gas whose characteristic temperature, overlying absorbing column, $[N_H]_W$, and resulting unabsorbed total flux decline monotonically after the first few days. Derived shock velocities are in good agreement with those found from observations at other wavelengths. Similarly, $[N_H]_W$ is in accord with that expected from the red giant wind ahead of the forward shock. We confirm the basic models of the 1985 outburst and conclude that standard Phase I remnant evolution terminated by $t\sim10$ days and the remnant then rapidly evolved to display behaviour characteristic of Phase III. Around t=26 days however, a new, luminous and highly variable soft X-ray source began to appear whose origin will be explored in a subsequent paper.

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arXiv2006

On the Relevance of Compton Scattering for the Soft X-ray Spectra of Hot DA White Dwarfs

We re-examine the effects of Compton scattering on the emergent spectra of hot DA white dwarfs in the soft X-ray range. Earlier studies have implied that sensitive X-ray observations at wavelengths $λ< 50$ Åmight be capable of probing the flux deficits predicted by the redistribution of electron-scattered X-ray photons toward longer wavelengths. We adopt two independent numerical approaches to the inclusion of Compton scattering in the computation of pure hydrogen atmospheres in hydrostatic equilibrium. One employs the Kompaneets diffusion approximation formalism, while the other uses the cross-sections and redistribution functions of Guilbert. Models and emergent spectra are computed for stellar parameters representative of HZ 43 and Sirius B, and for models with an effective temperature $T_{\rm eff} = 100 000$ K. The differences between emergent spectra computed for Compton and Thomson scattering cases are completely negligible in the case of both HZ 43 and Sirius B models, and are also negligible for all practical purposes for models with temperatures as high as $T_{\rm eff} = 100 000$ K. Models of the soft X-ray flux from these stars are instead dominated by uncertainties in their fundamental parameters.

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arXiv2006

Soft X-ray emission lines of Fe XV in solar flare observations and the Chandra spectrum of Capella

Recent calculations of atomic data for Fe XV have been used to generate theoretical line ratios involving n = 3-4 transitions in the soft X-ray spectral region (52-83 A), for a wide range of electron temperatures and densities applicable to solar and stellar coronal plasmas. A comparison of these with solar flare observations from a rocket-borne spectrograph (XSST) reveals generally good agreement between theory and experiment. In particular, the 82.76 A emission line in the XSST spectrum is identified, for the first time to our knowledge in an astrophysical source. Most of the Fe XV transitions which are blended have had the species responsible clearly identified, although there remain a few instances where this has not been possible. The line ratio calculations are also compared with a co-added spectrum of Capella obtained with the Chandra satellite, which is probably the highest signal-to-noise observation achieved for a stellar source in the 25-175 A soft X-ray region. Good agreement is found between theory and experiment, indicating that the Fe XV lines are reliably detected in Chandra spectra, and hence may be employed as diagnostics to determine the temperature and/or density of the emitting plasma. However the line blending in the Chandra data is such that individual emission lines are difficult to measure accurately, and fluxes may only be reliably determined via detailed profile fitting of the observations. The co-added Capella spectrum is made available to hopefully encourage further exploration of the soft X-ray region in astronomical sources.

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arXiv2005

Evidence of the FIP effect in the coronae of late-type giants

$β$ Cet, 31 Com and $μ$ Vel represent the main stages through which late-type giants evolve during their lifetime (the Hertzsprung gap (31 Com), the rapid braking zone ($μ$ Vel) and the core helium burning ``clump'' phase ($β$ Cet)). An analysis of their high resolution {\it Chandra} X-ray spectra reveals similar coronal characteristics in terms of both temperature structure and element abundances for the more evolved stars ($μ$ Vel and $β$ Cet) with slight differences for the `younger' giant (31 Com). The coronal temperature structure of 31 Com is significantly hotter showing a clear peak while $β$ Cet and $μ$ Vel show a plateau. $β$ Cet and $μ$ Vel show evidence for a FIP effect in which coronae are depleted in high FIP elements relative to their photospheres by a factor of $\sim 2$. In contrast, 31 Com is characterized by a lack of FIP effect. In other words, neither depletion nor enhancement relative to stellar photospheric values is found. We conclude that the structural changes during the evolution of late-type giants could be responsible for the observed differences in coronal abundances and temperature structure. In particular, the size of the convection zone coupled with the rotation rate seem obvious choices for playing a key role in determining coronal characteristics.

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arXiv2005

An Overview of the Performance of the Chandra X-Ray Observatory

The Chandra X-ray Observatory is the X-ray component of NASA's Great Observatory Program which includes the recently launched Spitzer Infrared Telescope, the Hubble Space Telescope (HST) for observations in the visible, and the Compton Gamma-Ray Observatory (CGRO) which, after providing years of useful data has reentered the atmosphere. All these facilities provide, or provided, scientific data to the international astronomical community in response to peer-reviewed proposals for their use. The Chandra X-ray Observatory was the result of the efforts of many academic, commercial, and government organizations primarily in the United States but also in Europe. NASA's Marshall Space Flight Center (MSFC) manages the Project and provides Project Science; Northrop Grumman Space Technology (NGST -- formerly TRW) served as prime contractor responsible for providing the spacecraft, the telescope, and assembling and testing the Observatory; and the Smithsonian Astrophysical Observatory (SAO) provides technical support and is responsible for ground operations including the Chandra X-ray Center (CXC). Telescope and instrument teams at SAO, the Massachusetts Institute of Technology (MIT), the Pennsylvania State University (PSU), the Space Research Institute of the Netherlands (SRON), the Max-Planck Institut fur extraterrestrische Physik (MPE), and the University of Kiel also provide technical support to the Chandra Project. We present here a detailed description of the hardware, its on-orbit performance, and a brief overview of some of the remarkable discoveries that illustrate that performance.

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arXiv2005

Detection of pulsations and a spectral feature in the X-ray emission of the isolated neutron star 1RXS J214303.7+065419/RBS 1774

We report on the results of a deep XMM-Newton observation of RBS 1774, the most recent dim isolated neutron star candidate found in the ROSAT archive data. Spectral and timing analysis of the high-quality PN and MOS data confirm the association of this source with an isolated neutron star. The spectrum is thermal and blackbody-like, and there is evidence at a significance level > 4sigma that the source is an X-ray pulsar, with spin period of 9.437 s. Spectral fitting reveils the presence of an absorption feature at ~0.7 keV, but at this level data do not have enough resolution to allow us to discriminate between an absorption line or an edge. We compare the newly measured properties of RBS 1774 with those of other known dim isolated neutron stars, and discuss possible interpretations for the absorption feature.

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arXiv2005

An Assessment of the Fe XVIII and Fe XIX Line Ratios from the Chandra Grating Observations of Capella

Observations of Fe XVIII and Fe XIX X-ray, EUV, and FUV line emission, formed at the peak of Capella's (alpha Aurigae) emission measure distribution and ubiquitous in spectra of many cool stars and galaxies, provide a unique opportunity to test the robustness of Fe XVIII and Fe XIX spectral models. The Astrophysical Plasma Emission Code (APEC) is used to identify over 35 lines from these two ions alone, and to compare model predictions with spectra obtained with the Chandra Low Energy Transmission Grating and High Energy Transmission Grating Spectrometers, the Far Ultraviolet Spectroscopic Explorer, and the Extreme Ultraviolet Explorer. Some flux discrepancies larger than factors of two are found between observations of Fe XVIII and Fe XIX lines and predictions by APEC and other models in common usage. In particular the X-ray resonance lines for both ions are stronger than predicted by all models relative to the EUV resonance lines. The multiwavelength observations demonstrate the importance of including dielectronic recombination and proton impact excitation, and of using accurate wavelengths in spectral codes. These ions provide important diagnostic tools for 10^7 K plasmas currently observed with Chandra, XMM-Newton, and FUSE.

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arXiv2005

Line identification in soft X-ray spectra of stellar coronae by comparison with the hottest white dwarf's photosphere: Procyon, alpha Cen A+B, and H1504+65

H1504+65 is a young white dwarf with an effective temperature of 200,000 K and is the hottest post-AGB star ever analysed with detailed model atmospheres. Chandra LETG+HRC-S spectra have revealed the richest X-ray absorption line spectrum recorded from a stellar photosphere to date. The line forming regions in this extremely hot photosphere produce many transitions in absorption that are also observed in emission in cool star coronae. We have performed a detailed comparison of Chandra spectra of H1504+65 with those of Procyon and alpha Cen A and B. State of the art non-LTE model spectra for the hot white dwarf have enabled us to identify a wealth of absorption lines from highly ionized O, Ne and Mg. In turn, these features have allowed us to identify coronal lines whose origins were hitherto unknown.

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arXiv2004

The Coronae of AB Dor and V471 Tau: Primordial angular momentum vs tidal spin-up

The zero-age main-sequence star AB Dor and the K dwarf component of the V471 Tau close binary have essentially identical rotation rates and spectral types. An analysis of their high resolution {\it Chandra} X-ray spectra reveals remarkably similar coronal characteristics in terms of both temperature structure and element abundances. Both stars show depletions of low FIP elements by factors of $\sim 3$, with higher FIP elements showing more mild depletions. No evidence for enhancements of very low FIP ($< 7$ eV) elements, such as Na, Al and Ca, as compared to other low FIP elements was found. The abundance anomaly pattern for AB Dor and V471 Tau is similar to, though less extreme than, the abundance anomalies exhibited by active RS-CVn-type binaries. While we find statistically significant structure in the underlying differential emission measure distributions of these stars over narrow temperature intervals, this structure is strongly dependent on the lines used in the analysis and is probably spurious. Based on their X-ray similarities, we conclude that the exact evolutionary state of a star has little effect on coronal characteristics, and that the parameters that dominate coronal structure and composition are simply the rotation rate and spectral type.

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