arXiv subjects

J. C. Houck

Publications and source records attributed to J. C. Houck.

14 recordsLinked to original sources

arXiv2014

On the Expansion Rate, Age, and Distance of the Supernova Remnant G266.2-1.2 (Vela Jr.)

An analysis of Chandra ACIS data for two relatively bright and narrow portions of the northwestern rim of G266.2-1.2 (a.k.a. RX J0852.0-4622 or Vela Jr.) reveal evidence of a radial displacement of 2.40 +/- 0.56 arcsec between 2003 and 2008. The corresponding expansion rate (0.42 +/- 0.10 arcsec/yr or 13.6 +/- 4.2%/kyr) is about half the rate reported for an analysis of XMM-Newton data from a similar, but not identical, portion of the rim over a similar, but not identical, time interval (0.84 +/- 0.23 arcsec/yr, Katsuda et al. 2008a). If the Chandra rate is representative of the remnant as a whole, then the results of a hydrodynamic analysis suggest that G266.2-1.2 is between 2.4 and 5.1 kyr old if it is expanding into a uniform ambient medium (whether or not it was produced by a Type Ia or Type II event). If the remnant is expanding into the material shed by a steady stellar wind, then the age could be as much as 50% higher. The Chandra expansion rate and a requirement that the shock speed be greater than or equal to 1000 km/s yields a lower limit on the distance of 0.5 kpc. An analysis of previously-published distance estimates and constraints suggests G266.2-1.2 is no further than 1.0 kpc. This range of distances is consistent with the distance to the nearer of two groups of material in the Vela Molecular Ridge (0.7 +/- 0.2 kpc, Liseau et al. 1992) and to the Vel OB1 association (0.8 kpc, Eggen 1982).

astro-ph.HE↗

arXiv2013

The 3 Ms Chandra Campaign on Sgr A*: A Census of X-ray Flaring Activity from the Galactic Center

Over the last decade, X-ray observations of Sgr A* have revealed a black hole in a deep sleep, punctuated roughly once per day by brief flares. The extreme X-ray faintness of this supermassive black hole has been a long-standing puzzle in black hole accretion. To study the accretion processes in the Galactic Center, Chandra (in concert with numerous ground- and space-based observatories) undertook a 3 Ms campaign on Sgr A* in 2012. With its excellent observing cadence, sensitivity, and spectral resolution, this Chandra X-ray Visionary Project (XVP) provides an unprecedented opportunity to study the behavior of the closest supermassive black hole. We present a progress report from our ongoing study of X-ray flares, including the brightest flare ever seen from Sgr A*. Focusing on the statistics of the flares and the quiescent emission, we discuss the physical implications of X-ray variability in the Galactic Center.

astro-ph.HE↗

arXiv2010

The Three-Dimensional Structure of Cassiopeia A

We used the Spitzer Space Telescope's Infrared Spectrograph to map nearly the entire extent of Cassiopeia A between 5-40 micron. Using infrared and Chandra X-ray Doppler velocity measurements, along with the locations of optical ejecta beyond the forward shock, we constructed a 3-D model of the remnant. The structure of Cas A can be characterized into a spherical component, a tilted thick disk, and multiple ejecta jets/pistons and optical fast-moving knots all populating the thick disk plane. The Bright Ring in Cas A identifies the intersection between the thick plane/pistons and a roughly spherical reverse shock. The ejecta pistons indicate a radial velocity gradient in the explosion. Some ejecta pistons are bipolar with oppositely-directed flows about the expansion center while some ejecta pistons show no such symmetry. Some ejecta pistons appear to maintain the integrity of the nuclear burning layers while others appear to have punched through the outer layers. The ejecta pistons indicate a radial velocity gradient in the explosion. In 3-D, the Fe jet in the southeast occupies a "hole" in the Si-group emission and does not represent "overturning", as previously thought. Although interaction with the circumstellar medium affects the detailed appearance of the remnant and may affect the visibility of the southeast Fe jet, the bulk of the symmetries and asymmetries in Cas A are intrinsic to the explosion.

astro-ph.GA↗

arXiv2008

Evidence of a Curved Synchrotron Spectrum in the Supernova Remnant SN 1006

A joint spectral analysis of some Chandra ACIS X-ray data and Molonglo Observatory Synthesis Telescope radio data was performed for 13 small regions along the bright northeastern rim of the supernova remnant SN 1006. These data were fitted with a synchrotron radiation model. The nonthermal electron spectrum used to compute the photon emission spectra is the traditional exponentially cut off power law, with one notable difference: The power-law index is not a constant. It is a linear function of the logarithm of the momentum. This functional form enables us to show, for the first time, that the synchrotron spectrum of SN 1006 seems to flatten with increasing energy. The effective power-law index of the electron spectrum is 2.2 at 1 GeV (i.e., radio synchrotron-emitting momenta) and 2.0 at about 10 TeV (i.e., X-ray synchrotron-emitting momenta). This amount of change in the index is qualitatively consistent with theoretical models of the amount of curvature in the proton spectrum of the remnant. The evidence of spectral curvature implies that cosmic rays are dynamically important instead of being "test" particles. The spectral analysis also provides a means of determining the critical frequency of the synchrotron spectrum associated with the highest-energy electrons. The critical frequency seems to vary along the northeastern rim, with a maximum value of 1.1e17 (0.6e17 - 2.1e17) Hz. This value implies that the electron diffusion coefficient can be no larger than a factor of ~4.5-21 times the Bohm diffusion coefficient if the velocity of the forward shock is in the range 2300-5000 km/s. Since the coefficient is close to the Bohm limit, electrons are accelerated nearly as fast as possible in the regions where the critical frequency is about 1.0e17 Hz.

astro-ph↗

arXiv2007

Nonthermal Bremsstrahlung vs. Synchrotron Radiation: Cas A

We performed a spectral analysis of some Chandra ACIS and RXTE PCA data for the supernova remnant Cas A. A very large (1.1 Ms) ACIS data set is used to identify regions dominated by synchrotron radiation. The best-fit spectral models for these regions are combined to obtain a composite synchrotron model for the entire remnant. The difference between this model and the observed PCA flux is fitted with a nonthermal bremsstrahlung model. The results of this analysis suggest that (1) the ratio of the nonthermal bremsstrahlung to synchrotron radiation varies from about 2:1 to 4:1 in the 10-32 keV energy band, (2) the electron spectrum is significantly steeper at 10-32 keV than it is at 1 GeV, (3) about 5% of the electrons are nonthermal and (4) about 30% of the energy in the electron distribution is in nonthermal electrons.

astro-ph↗

arXiv2006

X-ray Supercavities in the Hydra A Cluster and the Outburst History of the Central Galaxy's Active Nucleus

A 227 ksec Chandra Observatory X-ray image of the hot plasma in the Hydra A cluster has revealed an extensive cavity system. The system was created by a continuous outflow or a series of bursts from the nucleus of the central galaxy over the past 200-500 Myr. The cavities have displaced 10% of the plasma within a 300 kpc radius of the central galaxy, creating a swiss-cheese-like topology in the hot gas. The surface brightness decrements are consistent with empty cavities oriented within 40 degrees of the plane of the sky. The outflow has deposited upward of 10^61 erg into the cluster gas, most of which was propelled beyond the inner ~100 kpc cooling region. The supermassive black hole has accreted at a rate of approximately 0.1-0.25 solar masses per year over this time frame, which is a small fraction of the Eddington rate of a ~10^9 solar mass black hole, but is dramatically larger than the Bondi rate. Given the previous evidence for a circumnuclear disk of cold gas in Hydra A, these results are consistent with the AGN being powered primarily by infalling cold gas. The cavity system is shadowed perfectly by 330 MHz radio emission. Such low frequency synchrotron emission may be an excellent proxy for X-ray cavities and thus the total energy liberated by the supermassive black hole.

astro-ph↗

arXiv2005

Using the Parallel Virtual Machine for Everyday Analysis

A review of the literature reveals that while parallel computing is sometimes employed by astronomers for custom, large-scale calculations, no package fosters the routine application of parallel methods to standard problems in astronomical data analysis. This paper describes our attempt to close that gap by wrapping the Parallel Virtual Machine (PVM) as a scriptable S-Lang module. Using PVM within ISIS, the Interactive Spectral Interpretation System, we've distributed a number of representive calculations over a network of 25+ CPUs to achieve dramatic reductions in execution times. We discuss how the approach applies to a wide class of modeling problems, outline our efforts to make it more transparent for common use, and note its growing importance in the context of the large, multi-wavelength datasets used in modern analysis.

astro-ph↗

arXiv2005

AWM 4 - an isothermal cluster observed with XMM-Newton

We present analysis of an XMM observation of the poor cluster AWM 4. The cluster is relaxed and its X-ray halo is regular with no apparent substructure. Azimuthally averaged radial spectral profiles suggest that the cluster is isothermal to a radius of at least 160 kpc, with no evidence of a central cooling region. Spectral mapping shows some significant temperature and abundance substructure, but no evidence of strong cooling in the cluster core. Abundance increases in the core, but not to the extent expected, and we find some indication of gas mixing. Modeling the three dimensional properties of the system, we show that ongoing heating by an AGN in the dominant elliptical, NGC 6051, is likely to be responsible for the lack of cooling. We also compare AWM 4 to MKW 4, a cluster of similar mass observed recently with XMM. While the two systems have similar gravitational mass profiles, MKW 4 has a cool core and somewhat steeper gas density profile, which leads to a lower core entropy. AWM 4 has a considerably larger gas fraction at 0.1 R200, and we show that these differences result from the difference in mass between the two dominant galaxies and the activity cycles of their AGN. We estimate the energy required to raise the temperature profile of MKW 4 to match that of AWM 4 to be 9x10^58 erg, or 3x10^43 erg/s for 100 Myr, comparable to the likely power output of the AGN in AWM 4.

astro-ph↗

arXiv2003

Chandra High-Resolution X-Ray Spectrum of Supernova Remnant 1E0102.2-7219

Chandra High Energy Transmission Grating Spectrometer observations of the supernova remnant 1E0102.2-7219 in the Small Magellanic Cloud reveal a spectrum dominated by X-ray emission lines from hydrogen-like and helium-like ions of oxygen, neon, magnesium and silicon, with little iron. The dispersed spectrum shows a series of monochromatic images of the source in the light of individual spectral lines. Detailed examination of these dispersed images reveals Doppler shifts within the supernova remnant, indicating bulk matter velocities on the order of 1000 km/s. These bulk velocities suggest an expanding ring-like structure with additional substructure, inclined to the line of sight. A two-dimensional spatial/velocity map of the SNR shows a striking spatial separation of redshifted and blueshifted regions, and indicates a need for further investigation before an adequate 3D model can be found. The radii of the ring-like images of the dispersed spectrum vary with ionization stage, supporting an interpretation of progressive ionization due to passage of the reverse shock through the ejecta. Plasma diagnostics with individual emission lines of oxygen are consistent with an ionizing plasma in the low density limit, and provide temperature and ionization constraints on the plasma. Assuming a pure metal plasma, the mass of oxygen is estimated at ~6 solar masses, consistent with a massive progenitor.

astro-ph↗

arXiv2002

The shape of the relativistic iron K-alpha line from MCG6--6-30-15 measured with the Chandra HETGS and RXTE

We confirm the detection of the relativistically broadened iron K-alpha emission at 6.4 keV with simultaneous Chandra HETGS and RXTE PCA observations. Heavily binned HETGS data show a disk line profile with parameters very similar to those previously seen by ASCA. We observe a resolved narrow component with a velocity width ~4700 km/s (FWHM ~ 11,000 km/s), that is most prominent, and narrower (FWHM ~ 3600 km/s) when the continuum flux is high. It plausibly is just the blue wing of the broad line. We obtain a stringent limit on the equivalent width of an intrinsically narrow line in the source of 16 eV, indicating little or no contribution due to fluoresence from distant material such as the molecular torus. Variability studies of the narrow component show a constant iron line flux and variable width indicating the line may be originating from different kinematic regions of the disk.

astro-ph↗

arXiv2001

Ionization and Velocity Structure in the Supernova Remnant E0102-72

The High Energy Transmission Grating (HETG) Spectrometer aboard the Chandra X-Ray Observatory was used to observe E0102-72, a ~1000 year old, oxygen rich supernova in the Small Magellanic Cloud. The HETG disperses the image of the remnant into a spectrum of images in the light of individual X-ray emission lines. Doppler shifts in the strongest lines of oxygen and neon reveal bulk motions of up to 2000 km/sec with a complex morphology. Comparison of progressive ionization stages of magnesium, neon, oxygen and silicon provide new insights into the mechanism of the `reverse shock' that heats the stellar ejecta.

astro-ph↗

arXiv2001

High Resolution Spectroscopy of Two Oxygen-Rich SNRs with the Chandra HETG

The HETG can be used to obtain spatially resolved spectra of moderately extended sources. We present preliminary results for two well studied, oxygen rich supernova remnants in the Magellanic clouds, E0102-72 and N132D. The dispersed spectrum of E0102-72 shows images of the remnant in the light of individual emission lines from H-like and He-like ions of O, Mg, Ne and He-like Si with no evidence of Fe. The diameters of the images for various ions, measured in the cross-dispersion direction, increase monotonically with the ionization age for the given ion. This shows in detail the progression of the reverse shock through the expanding stellar ejecta. We see clear evidence for asymmetric Doppler shifts across E0102-72 of ~2000 km/s. These can be modelled approximately by a partially-filled, expanding shell inclined to the line of sight. The dispersed spectrum of N132D is more affected by spatial/spectral overlap but also shows monochromatic images in several strong lines. Preliminary spectra have been extracted for several bright knots. Some regions of oxygen-rich material, presumably stellar ejecta, are clearly identified. Additional details on E0102-72 are presented by Flanagan et al. and Davis et al. in these proceedings, and further analysis is in progress.

astro-ph↗

arXiv2001

Ionization Structure and the Reverse Shock in E0102-72

The young oxygen-rich supernova remnant E0102-72 in the Small Magellanic Cloud has been observed with the High Energy Transmission Grating Spectrometer of Chandra. The high resolution X-ray spectrum reveals images of the remnant in the light of individual emission lines of oxygen, neon, magnesium and silicon. The peak emission region for hydrogen-like ions lies at larger radial distance from the SNR center than the corresponding helium-like ions, suggesting passage of the ejecta through the "reverse shock". We examine models which test this interpretation, and we discuss the implications.

astro-ph↗

arXiv2001

Spectral Line Imaging Observations of 1E0102.2-7219

E0102-72 is the second brightest X-ray source in the Small Magellanic Cloud and the brightest supernova remnant in the SMC. We observed this SNR for ~140 ksec with the High Energy Transmission Gratings (HETG) aboard the Chandra X-ray Observatory. The small angular size and high surface brightness make this an excellent target for HETG and we resolve the remnant into individual lines. We observe fluxes from several lines which include O VIII Ly$α$, Ly$β$, and O VII along with several lines from Ne X, Ne IX and Mg XII. These line ratios provide powerful constraints on the electron temperature and the ionization age of the remnant.

astro-ph↗