Bram Seth Boroson

CLASSIFYING X-RAY SOURCES IN EXTERNAL GALAXIES FROM X-RAY COLORS

The X-ray populations of Local Group galaxies have been classified in detail by Einstein, ROSAT, and ASCA, revealing a mix of binaries, supernova remnants (SNRs), and H II regions. However, these observatories were unable to resolve X-ray sources in galaxies beyond the Local Group. With Chandras exquisite spatial resolution, we are able to resolve sources in a sample of nearby galaxies. We show that there are highly significant differences in the X-ray colors of sources in bulge and disk systems. In particular, we find that there is a population of X-ray-soft, faint sources in disk galaxies not seen in bulges and a smaller population of hard sources also seen preferentially in disk systems. These differences can be used as a basis to classify sources as low- and high-mass X-ray binaries, SNRs, and supersoft sources. We suggest that the soft sources seen preferentially in disks are probably dominated by SNRs, although we cannot rule out the possibility that they are a new population of absorbed, faint, supersoft accretion sources associated with the young stellar population. The hard sources seen in disks but not bulges we identify as high-mass X-ray binaries. While it is impossible to classify any individual source on the basis of X-ray color alone, the observations presented here suggest that it is possible to separate sources into groups dominated by one or two source types. This classification scheme is likely to be very useful in population studies, where it is crucial to distinguish between different classes of objects.The X-ray populations of Local group galaxies have been classified in detail by Einstein, ROSAT and ASCA revealing a mix of binaries, supernova remnants and HII regions. However, these observatories were unable to resolve X-ray sources in galaxies beyond the local group. With Chandras exquisite spatial resolution we are able to resolve sources in a sample of nearby galaxies. We show that there are highly significant differences in the X-ray colors of sources in bulge and disk systems. These differences can be used as a basis to classify sources as low and high mass X-ray binaries, supernova remnants and supersoft sources. While it is impossible to identify any individual source on the basis of X-ray color alone, the observations presented here suggest that it is possible to separate sources into groups dominated by a single type of source. This classification scheme is likely to be very useful in population studies, where it is crucial to distinguish between different classes of objects.

Chandra and XMM Observations of the ADC Source 0921-630

We analyze observations of the low-mass X-ray binary (LMXB) 2S 0921-63 obtained with the gratings and CCDs on Chandra and XMM. This object is a high-inclination system showing evidence for an accretion disk corona (ADC). Such a corona has the potential to constrain the properties of the heated accretion disk in this system and other LMXBs by extension. We find evidence for line emission that is generally consistent with that found by previous experiments, although we are able to detect more lines. For the first time in this source, we find that the iron K line has multiple components. We set limits on the line widths and velocity offsets, and we fit the spectra to photoionization models and discuss the implications for ADC models. For the first time in any ADC source we use these fits, together with density constraints based on the O VII line ratio, in order to constrain the flux in the medium-ionization region of the ADC. Under various assumptions about the source luminosity this constrains the location of the emitting region. These estimates, together with estimates for the emission measure, favor a scenario in which the intrinsic luminosity of the source is comparable to what we observe.

Multiwavelength Studies of Hercules X-1 during Short High and Anomalous Low States: On-again, Off-again

We present results from multiwavelength observations of the Hercules X-1 system during a short high state (SHS) and during an anomalous low state (ALS). The magnitude of deviation from spin-up appears to be positively correlated with duration of the ALS. Such a correlation is consistent with an interpreta- tion of the ALS in terms of a change in mass accretion rate that causes the disk to tilt and twist beyond the normal deviations that cause the 35 day cycle. A larger deviation from the average results in a M0 stronger disruption of the disk and causes the disk to take longer to settle back to its ii normal ˇˇ 35 day behavior. Our modelwhich includes X-ray heating of the disk and companion star, shadowing of the X-ray —ux by the disk, and a contribution to the continuum emission from the accretion stream or hot spotcan consistently explain the observed changes in X-ray, ultraviolet (UV), and optical continuum light curves for both the SHS and ALS. The Hubble Space Telescope (HST ) and Space Telescope Imaging Spectrograph (STIS) observations presented here are the —rst UV observations of sufficient spectral and temporal resolution to construct Doppler tomograms of the line emission. Doppler tomo- grams of the UV emission lines during SHS and ALS show the majority of the emission coming from the surface of the companion star rather than from the accretion disk. Tomograms made after separating the N V emission lines into broad and narrow components suggests that while the narrow component is associated with emission from the companion star, the broad component may be associated with emis- sion from a distorted disk. The Doppler maps also show that heating over the inner face of HZ Her is not uniform and imply partial eclipse of the UV line emission by an accretion stream and/or hot spot. Subject headings: accretion, accretion disksbinaries: closepulsars: individual (Hercules X-1) ¨ ultraviolet: starsX-rays: stars On-line material: color —gures

Pulsations and Line Profile Changes in the Ultraviolet Spectrum of Hercules X‐1: Results from a Multiwavelength Campaign

We report simultaneous X-ray and ultraviolet observations of the X-ray binary pulsar Hercules X-1, obtained over nearly half of a 1.7 day binary orbit using the ASCA X-ray telescope and the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope. The GHRS resolved, for the first time, two moving emission components. One feature in the N V λλ1238.8, 1242.8 doublet is broad (FWHM ≈ 1000 km s–1) and the other is narrow (FWHM ≈ 150 km s–1). We attribute the broad emission line to the X-ray—illuminated surface of a Keplerian accretion disk and the narrow line to the X-ray-heated atmosphere of the companion star, HZ Her. Near orbital phase orb = 0.80 (simultaneous with a pre-eclipse X-ray dip), the N V λ1242.8 component became stronger than the N V λ1238.8 component. Later in the orbit, the blue edge of the broad lines diminished as the approaching side of the accretion disk was occulted by HZ Her. UV continuum pulsations were detected for the first time, with an amplitude ≈ 0.5% of the steady flux. The pulse shapes of the UV continuum and hard X-rays were similar at orb = 0.83 but not at orb = 0.56. We found evidence for a stronger pulsation (amplitude ≈ 15%) within a narrow (0.25 A) segment of the broad 1242.8 A N v component, only at W erb = 0.80. We briefly discuss some implications of these observations for the gas flows in the Her X-1 system.

Discovery of Millihertz ULTRAVIOLET Quasi-periodic Oscillations in Hercules X-1

Observations of the ultraviolet continuum of the X-ray binary system Her X-1/HZ Herculis with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST) show quasi-periodic oscillations (QPOs) at frequencies of 8 ± 2 and 43 ± 2 mHz, with rms amplitudes of 2% and 4% of the steady flux. Observations with the Keck Telescope confirm the presence of the higher frequency QPO in the optical continuum, with a rms amplitude of 1.6% ± 0.2%. The QPOs are most prominent in the HST data near = 0.5 (where = 0 is the middle of the X-ray eclipse), suggesting that they arise not in the accretion disk but on the X-ray-heated face of the companion star. We discuss scenarios in which the companion star reprocesses oscillations in the disk which are caused by either Keplerian rotation or a beat frequency between the neutron star spin and Keplerian rotation at some radius in the accretion disk.

Simultaneous Hubble Space Telescope and ASCA Observations of LMC X-4: X-Ray Ionization Effects on a Stellar Wind

We present first results from simultaneous ultraviolet (HST/GHRS) and X-ray (ASCA) observations of the 13.5 s pulsar LMC X-4, taken in 1996 May. The ASCA observations covered 1.12 binary orbits (1.58 days), and the Hubble Space Telescope observations were centered on this for roughly 0.4 orbital phase coverage (0.56 days). The Goddard High Resolution Spectograph data are the highest resolution (both temporal and spectral) ultraviolet spectra ever taken of LMC X-4. With generally accepted parameters for the source, fits to the UV continuum, using a model that incorporates X-ray heating of the companion star and the accretion disk, yield a mass accretion rate =4.0×10−8 M☉ yr-1; the X-ray luminosity implied by this value is consistent with the X-ray flux measured during simultaneous observations (3.2 × 10-10 ergs cm-2 s-1). The model accurately predicts observed B-magnitude and ultraviolet variations over both orbital and long-term periods. The ultraviolet P Cygni lines show dramatic changes with orbital phase, with strong broad absorption near X-ray eclipse and narrow absorption when the X-ray source is in the line of sight. We interpret this as a result of X-ray photoionization of the stellar wind; when the neutron star is in front of the normal star, the wind absorption disappears and mainly the photospheric absorption lines are visible. The X-ray pulse period measured during our observations, 13.5090 ± 0.0002 s, is consistent with steady spin-down over the past 10 years. No pulsations were detected in the ultraviolet observations, with upper limits to the pulsed fraction around N V and C IV of 1.8% and 2.7% in the continuum and 7% and 12% in the absorption troughs, respectively.

Simultaneous [ITAL]Chandra[/ITAL] and [ITAL]Hubble Space Telescope[/ITAL] Observations of SMC X-1

We present first results from simultaneous ultraviolet (Hubble Space Telescope/STIS) and X-ray (Chandra/ACIS) observations of the SMC X-1/SK 160 eclipsing binary system. Observations covering four orbital phases during each of the X-ray high and X-ray low states were taken in 2000 October-November and 2001 April. The ultraviolet P Cygni lines show strong broad absorption near X-ray eclipse and narrow absorption when the X-ray source is in the line of sight. The effect is visible during both the X-ray high and X-ray low states; the UV continuum flux remains roughly constant in spite of more than an order-of-magnitude reduction in X-ray flux, as expected if the X-ray flux reduction is due to occultation of the X-ray source by a precessing disk rather than an intrinsic change in X-ray luminosity. The X-ray spectra are dominated by continuum emission in the X-ray high state. Occultation of the neutron star by the disk during the low state also implies that X-ray emission from the disk surface should be present, and the low-state spectra do show strong emission lines. During eclipse and during the X-ray low state, the continuum emission largely disappears, and we see line emission from O, Ne, Mg, and Fe and possibly from Si and S. The emission lines are consistent with recombination lines from mostly hydrogenic and helium-like species, which could be produced by photoionization in an extended stellar wind.

RXTE Observations of Hercules X-1 during the 1998 July Short High State

We present RXTE monitoring of the eclipsing X-ray binary Hercules X-1 conducted over the short high state of 1998 July. This was one of the last major short high states before the source entered an anomalous low state of activity. A comparison with previous epochs finds no evidence for special behavior during these observations. We determine orbital and pulsar spin periods to facilitate measurements of spin and orb during the subsequent anomalous low state and the next epoch of high-state activity. Spectrally, the decay of the short high state and concurrent pre-eclipse dips are consistent with obscuration of a central X-ray source by a cloud of nonuniform column density. The standard model of a warped accretion disk of finite vertical scale height fits the characteristics of this absorber well. Pre-eclipse dips have durations a factor of a few longer than the characteristic durations of dips during main high states. Pulse profile structure increases in complexity toward the tail of the short high state, suggesting changes in accretion curtain geometry.

Hercules X-1: Empirical Models of Ultraviolet Emission Lines

The UV emission lines of Hercules X-1, resolved with the Hubble Space Telescope (HST) Goddard High Resolution Spectrograph and Space Telescope Imaging Spectrograph, can be divided into broad (FWHM ? 750 km s-1) and narrow (FWHM ? 150 km s-1) components. The broad lines can be unambiguously identified with emission from an accretion disk which rotates prograde with the orbit. The narrow lines, previously identified with the X-ray-illuminated atmosphere of the companion star, are blueshifted at both = 0.2 and = 0.8, and the line flux at = 0.2 is ?0.2 of the flux at = 0.8. Line ratio diagnostics show that the density of the narrow-line region is log ne = 13.4 ? 0.2, and Te = 1.0 ? 0.2 ? 105 K. The symmetry of the eclipse ingress suggests that the line emission on the surface of the disk is left-right symmetric relative to the orbit. Model fits to the O V, Si IV, and He II line profiles agree with this result, but fits to the N V lines suggest that the receding side of the disk is brighter. We note that there are narrow absorption components in the N V lines with blueshifts of ?500 km s-1.

Models of X-Ray Photoionization in LMC X-4: Slices of a Stellar Wind

We show that the orbital variation in the UV P Cygni lines of the X-ray binary LMC X-4 results when X-rays photoionize nearly the entire region outside of the X-ray shadow of the normal star. We fit models to Goddard High Resolution Spectrograph (GHRS) observations of N V and C IV P Cygni line profiles. Analytic methods assuming a spherically symmetric wind show that the wind velocity law is well fitted by v (1 - 1/r)?, where ? is likely ?1.4-1.6 and definitely less than 2.5. Escape probability models can fit the observed P Cygni profiles and provide measurements of the stellar wind parameters. The fits determine LX/ = 2.6 ? 0.1 ? 1043 ergs s-1 M -1? yr, where LX is the X-ray luminosity and is the mass-loss rate of the star. Allowing an inhomogeneous wind improves the fits. IUE spectra show greater P Cygni absorption during the second half of the orbit than during the first. We discuss possible causes of this effect.