Adrienne Marie Juett

High-Resolution X-Ray Spectroscopy of the Interstellar Medium: Structure at the Oxygen Absorption Edge

We present high-resolution spectroscopy of the oxygen K-shell interstellar absorption edge in seven X-ray binaries using the High Energy Transmission Grating Spectrometer (HETGS) onboard the Chandra X-Ray Observatory. Using the brightest sources as templates, we found a best-fit model of two absorption edges and five Gaussian absorption lines. All of these features can be explained by the recent predictions of K-shell absorption from neutral and ionized atomic oxygen. We identify the Kα and Kβ absorption lines from neutral oxygen, as well as the S = 3/2 absorption edge. The expected S = 1/2 edge is not detected in these data because of overlap with instrumental features. We also identify the Kα absorption lines from singly and doubly ionized oxygen. The O I Kα absorption line is used as a benchmark with which to adjust the absolute wavelength scale for theoretical predictions of the absorption cross sections. We find that shifts of 30-50 mA are required, consistent with differences previously noticed from comparisons of the theory with laboratory measurements. Significant oxygen features from dust or molecular components, as suggested in previous studies, are not required by our HETGS spectra. With these spectra, we can begin to measure the large-scale properties of the interstellar medium (ISM). We place a limit on the velocity dispersion of the neutral lines of 200 km s-1, consistent with measurements at other wavelengths. We also make the first measurement of the oxygen ionization fractions in the ISM. We constrain the interstellar ratio of O II/O I to ≈0.1 and the ratio of O III/O I to 0.1. This work demonstrates the utility of X-ray spectroscopy for studies of the ISM. Future work will provide measurements of the relative abundances and ionization fractions for elements from carbon to iron.

High-energy particle acceleration at the radio-lobe shock of Centaurus A

We present new results on the shock around the southwest radio lobe of Centaurus A using data from the Chandra Very Large Programme observations (740 ks total observing time). The X-ray spectrum of the emission around the outer southwestern edge of the lobe is well described by a single power-law model with Galactic absorption ‐ thermal models are strongly disfavoured, except in the region closest to the nucleus. We conclude that a significant fraction of the X-ray emission around the southwest part of the lobe is synchrotron, not thermal. We infer that in the region where the shock is strongest and the ambient gas density lowest, the inflation of the lobe is accelerating particles to X-ray sync hrotron emitting energies, similar to supernova remnants such as SN1006. This interpretation resolves a problem of our earlier, purely thermal, interpretation for this emission, namely t hat the density compression across the shock was required to be much larger than the theoretically expected factor of 4. We describe a self-consistent model for the lobe dynamics and shock properties using the shell of thermal emission to the north of the lobe to estimate the lobe pressure. Based on this model, we estimate that the lobe is expanding to the southwest with a velocity of �2600 km s 1 , roughly Mach 8 relative to the ambient medium. We discuss the spatial variation of spectral index across the shock region, concluding that our observations constrain γmax for the accelerated particles to be �10 8 at the strongest part of the shock, consistent with expectat ions from diffusive shock acceleration theory. Finally, we consider the implications of these results for the production of ultra-high energy cosmic rays (UHECRs) and TeV emission from Centaurus A, concluding that the shock front region is unlikely to be a significant source of UHECRs, but that TeV emission from this region is expected at levels comparable to current limits at TeV energies, for plausible assumed magnetic field strength s.

The Low-Mass X-Ray Binary and Globular Cluster Connection in Virgo Cluster Early-Type Galaxies: Optical Properties

LMXBs form efficiently in GCs. By combining Chandra and HST ACS observations of 11 massive early-type galaxies in the Virgo Cluster, we use the most accurate identification of LMXBs and GCs to date to explore the optical properties of 270 GCs with LMXBs and 6488 GCs without detectable X-ray emission. More massive, redder, and more compact GCs are more likely to contain LMXBs. Unlike Galactic GCs, a large number of GCs with LMXBs have half-mass relaxation times >2.5 Gyr; GCs need not survive for more than five relaxation timescales to produce LMXBs. By fitting the dependence of the expected number of LMXBs per GC, λt, on the GC mass M, color (g - z), and half-mass radius rh,cor, we find that λt ∝ M1.24±0.08 × 10r. This rules out that the number of LMXBs per GC is linearly proportional to GC mass (99.89% confidence limit) and leads us to predict that most GCs with high X-ray luminosities contain a single LMXB. The detailed dependence of λt on GC properties appears mainly due to a dependence on the encounter rate Γh and the metallicity Z, λt ∝ ΓZ0.39±0.07. Our analysis provides strong evidence that dynamical formation and metallicity play the primary roles in determining the presence of an LMXB in extragalactic GCs. The shallower than linear encounter rate dependence requires an explanation by theories of dynamical binary formation. A metallicity-dependent variation in the number of neutron stars and black holes per unit GC mass, effects from irradiation-induced winds, or suppression of magnetic braking in metal-poor stars may all be consistent with our abundance dependence; all three scenarios require further development.

High-Resolution X-Ray Spectroscopy of the Interstellar Medium. II. Neon and Iron Absorption Edges

We present high-resolution spectroscopy of the neon K-shell and iron L-shell interstellar absorption edges in nine X-ray binaries using the High Energy Transmission Grating Spectrometer (HETGS) on board the Chandra X-Ray Observatory. We found that the iron absorption is well fit by an experimental determination of the cross section for metallic iron, although with a slight wavelength shift of ≈20 mA. The neon edge region is best fit by a model that includes the neutral neon edge and three Gaussian absorption lines. We identify these lines as due to the 1s-2p transitions from Ne II, Ne III, and Ne IX. As we found in our oxygen edge study, the theoretical predictions for neutral and low-ionization lines all require shifts of ≈20 mA to match our data. Combined with our earlier oxygen edge study, we find that a best-fit O/Ne ratio of 5.4 ± 1.6, consistent with standard interstellar abundances. Our best-fit Fe/Ne ratio of 0.20 ± 0.03 is significantly lower than the interstellar value. We attribute this difference to iron depletion into dust grains in the interstellar medium. We make the first measurement of the neon ionization fraction in the ISM. We find Ne /Ne ≈ 0.3 and Ne /Ne ≈ 0.07. These values are larger than is expected given the measured ionization of interstellar helium. For Ne IX, our results confirm the detection of the hot ionized interstellar medium of the Galaxy.

Ultracompact X-Ray Binaries with Neon-rich Degenerate Donors

There are three low-mass X-ray binaries (4U 0614+091, 2S 0918-549, and 4U 1543-624) for which broad-line emission near 0.7 keV was previously reported. A recent high-resolution observation of 4U 0614+091 with the Chandra Low-Energy Transmission Grating Spectrometer (LETGS) found evidence of an unusually high Ne/O abundance ratio along the line of sight but failed to detect the previously reported 0.7 keV feature. We have made a search of archival ASCA spectra and identified a fourth source with the 0.7 keV feature, the 20 minute ultracompact binary 4U 1850-087. In all four of these sources, the 0.7 keV residual is eliminated from the ASCA spectra by allowing excess photoelectric absorption due to a nonsolar relative abundance of neon, just as in the LETGS spectrum of 4U 0614+091. The optical properties of these systems suggest that all four are ultracompact (Porb < 80 minutes) binaries. We propose that there is excess neon local to each of these sources, as also found in the ultracompact binary pulsar 4U 1626-67. We suggest that the mass donor in these systems is a low-mass, neon-rich degenerate dwarf and that the binaries are all ultracompact.

New Results on Particle Acceleration in the Centaurus A Jet and Counterjet from a Deep Chandra Observation

We present new deep Chandra observations of the Centaurus A jet, with a combined on-source exposure time of 719 ks. These data allow detailed X-ray spectral measurements to be made along the jet out to its disappearance at 4.5 kpc from the nucleus. We distinguish several regimes of high-energy particle acceleration; while the inner part of the jet is dominated by knots and has properties consistent with local particle acceleration at shocks, the particle acceleration in the outer 3.4 kpc of the jet is likely to be dominated by an unknown distributed acceleration mechanism. In addition to several compact counterjet features, we detect probable extended emission from a counterjet out to 2.0 kpc from the nucleus and argue that this implies that the diffuse acceleration process operates in the counterjet as well. A preliminary search for X-ray variability finds no jet knots with dramatic flux density variations, unlike the situation seen in M87.

Chandra High-Resolution Spectrum of the Anomalous X-Ray Pulsar 4U 0142+61

We report on a 25 ks observation of the anomalous X-ray pulsar 4U 0142+61 with the High Energy Transmission Grating Spectrometer (HETGS) on the Chandra X-ray Observatory. The continuum spectrum is consistent with previous measurements and is well fit by an absorbed power-law + blackbody model with photon index Gamma=3.3+/-0.4 and blackbody temperature kT=0.418+/-0.013 keV. The pulsar frequency was 0.1150966+/-0.0000017 Hz and the pulse fractions were between 8.7% and 21%, which are also consistent with past measurements. No evidence was found for emission or absorption lines with an upper limit of ~50 eV on the equivalent width of a broad feature in the 2.5-13 A (0.95-4.96 keV) range. The absence of a proton cyclotron line strongly constrains magnetar atmosphere models and hence the magnetic field strength of the neutron star. For the energy range given above, the allowed magnetic field strengths of 4U 0142+61 are B 9.8x 10^14 G.

X-Ray Spectroscopy of the Low-Mass X-Ray Binaries 2S 0918?549 and 4U 1543?624: Evidence for Neon-rich Degenerate Donors

We present high-resolution spectroscopy of the neutron star/low-mass X-ray binaries 2S 0918-549 and 4U 1543-624 with the High Energy Transmission Grating Spectrometer on board the Chandra X-Ray Observatory and the Reflection Grating Spectrometer on board XMM-Newton. Previous low-resolution spectra of both sources showed a broad, linelike feature at 0.7 keV that was originally attributed to unresolved line emission. We recently showed that this feature could also be due to excess neutral Ne absorption, and this is confirmed by the new high-resolution Chandra and XMM spectra. The Chandra spectra are each well fitted by an absorbed-power-law+blackbody model with a modified Ne/O number ratio of 0.52 ± 0.12 for 2S 0918-549 and 1.5 ± 0.3 for 4U 1543-624, compared to the interstellar medium value of 0.18. The XMM spectrum of 2S 0918-549 is best fitted by an absorbed-power-law model with a Ne/O number ratio of 0.46 ± 0.03, consistent with the Chandra result. On the other hand, the XMM spectrum of 4U 1543-624 is softer and less luminous than the Chandra spectrum and has a best-fit Ne/O number ratio of 0.54 ± 0.03. The difference between the measured abundances and the expected interstellar ratio, as well as the variation of the column densities of O and Ne in 4U 1543-624, supports the suggestion that there is absorption local to these binaries. We propose that the variations in the O and Ne column densities of 4U 1543-624 are caused by changes in the ionization structure of the local absorbing material. It is important to understand the effect of ionization on the measured absorption columns before the abundance of the local material can be determined. This work supports our earlier suggestion that 2S 0918-549 and 4U 1543-624 are ultracompact binaries with Ne-rich companions.

Luminosity functions of LMXBs in Centaurus A: globular clusters versus the field

We study the X-ray luminosity function (XLF) of low-mass X-ray binaries (LMXB) in the nearby early-type galaxy Centaurus A, concentrating primarily on two aspects of binary populations: the XLF behavior at the low-luminosity limit and the comparison between globular cluster and field sources. The 800 ksec exposure of the deep Chandra VLP program allows us to reach a limiting luminosity of ∼8 × 10 35 erg s −1 , about ∼2–3 times deeper than previous investigations. We confirm the presence of the low-luminosity break of the overall LMXB XLF at log(LX) ≈ 37.2–37.6, below which the luminosity distribution follows a dN/d(ln L) ∼ const law. Separating globular cluster and field sources, we find a statistically significant difference between the two luminosity distributions with a relative underabundance of faint sources in the globular cluster population. This demonstrates that the samples are drawn from distinct parent populations and may disprove the hypothesis that the entire LMXB population in early-type galaxies is created dynamically in globular clusters. As a plausible explanation for this difference in the XLFs, we suggest an enhanced fraction of helium-accreting systems in globular clusters, which are created in collisions between red giants and neutron stars. Due to the four times higher ionization temperature of He, such systems are subject to accretion disk instabilities at ≈20 times higher mass accretion rate and, therefore, are not observed as persistent sources at low luminosities.

High-Resolution X-Ray Spectroscopy of the Ultracompact LMXB Pulsar 4U 1626–67

We report results from four recent observations of the ultracompact LMXB pulsar 4U 1626-67. These observations obtained high-resolution X-ray spectra with Chandra HETGS and XMM RGS, allowing us to study in detail the prominent Ne and O emission-line complexes. The observations were spaced over a period of 3 yr, enabling us to monitor the line regions as well as the overall spectral and timing properties. The structure of the emission lines and the helium-like Ne IX and O VII triplets support the hypothesis that they are formed in the high-density environment of an accretion disk. We do not find any significant variability in the line widths or ratios, though we note that the equivalent widths decrease. Using the most recent calibration products, we are able to place constraints on the strengths of the Ne K, Fe L, and O K photoelectric absorption edges. In contrast to our earlier analysis, the data do not require an overabundance of Ne or O in the system relative to the expected ISM values. We find that the pulsar is still spinning down but note that the pulse profile has changed significantly from what was found prior to the torque reversal in 1990, suggesting a change in the geometry of the accretion column. The flux of 4U 1626-67 continues to decrease, following the trend of the last ≈30 yr over which it has been observed. Taking into consideration current theory on disk stability, we expect that 4U 1626-67 will enter a period of quiescence in 2-15 yr.