The Reflection Component from Cygnus X-1 in the Soft State Measured by NuSTAR and Suzaku

The black hole binary Cygnus X-1 was observed in late-2012 with the Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku, providing spectral coverage over the ~1-300 keV range. The source was in the soft state with a multi-temperature blackbody, power-law, and reflection components along with absorption from highly ionized material in the system. The high throughput of NuSTAR allows for a very high quality measurement of the complex iron line region as well as the rest of the reflection component. The iron line is clearly broadened and is well-described by a relativistic blurring model, providing an opportunity to constrain the black hole spin. Although the spin constraint depends somewhat on which continuum model is used, we obtain a*>0.83 for all models that provide a good description of the spectrum. However, none of our spectral fits give a disk inclination that is consistent with the most recently reported binary values for Cyg X-1. This may indicate that there is a >13 degree misalignment between the orbital plane and the inner accretion disk (i.e., a warped accretion disk) or that there is missing physics in the spectral models.

Using the X-ray Dust Scattering Halo of Cygnus X-1 to determine distance and dust distributions

We present a detailed study of the X-ray dust scattering halo of the black hole candidate cygx1 based on two chandra HETGS observations. Using 18 different dust models, including one modified by us (dubbed XLNW), we probe the interstellar medium between us and this source. A consistent description of the cloud properties along the line of sight that describes at the same time the halo radial profile, the halo lightcurves, and the column density from source spectroscopy is best achieved with a small subset of these models. Combining the studies of the halo radial profile and the halo lightcurves, we favor a geometric distance to cygx1 of $d=1.81pm{0.09}$,kpc. Our study also shows that there is a dense cloud, which contributes $sim$50% of the dust grains along the line of sight to cygx1, located at $sim1.6$ kpc from us. The remainder of the dust along the line of sight is close to the black hole binary.

The accretion disk corona and disk atmosphere of 4U 1624-490 as viewed by the Chandra-HETGS

We present a detailed spectral study (photoionization modelling and variability) of the Big Dipper 4U 1624-490 based on a chandra-High Energy Transmission Gratings Spectrometer (HETGS) observation over the $sim76$ ks binary orbit of 4U 1624-490. While the continuum spectrum can be modeled using a blackbody plus power-law, a slightly better fit is obtained using a single $Gamma=2.25$ power-law partially (71%) covered by a local absorber of column density $N_{rm H, Local}=8.1_{-0.6}^{+0.7}times 10^{22} rm cm^{-2}$. The data show a possible quasi-sinusoidal modulation with period $43_{-9}^{+13}$ ks that might be due to changes in local obscuration. Photoionization modeling with the {sc xstar} code and variability studies of the observed strong ion{Fe}{25} and ion{Fe}{26} absorption lines point to a two-temperature plasma for their origin: a highly ionized component of ionization parameter $xi_{rm hot} approx 10^{4.3} {rm ergs cm s^{-1}}$ ($Tsim 3.0times 10^{6}$ K) associated with an extended accretion disk corona of radius $R sim3times10^{10}$ cm, and a less ionized more variable component of $xi approx 10^{3.4} {rm ergs cm s^{-1}}$ ($Tsim 1.0times 10^{6}$ K) and $rm xi approx 10^{3.1} ergs cm s^{-1}$ ($Tsim 0.9times 10^{6}$ K) coincident with the accretion disk rim. We use this, with the observed ion{Fe}{25} and ion{Fe}{26} absorption line variations (in wavelength, strength, and width) to construct a viewing geometry that is mapped to changes in plasma conditions over the 4U 1624-490 orbital period.

Chandra X-ray spectroscopy of the focused wind in the Cygnus X-1 system. I. The non-dip spectrum in the low/hard state

We present analyses of a 50 ks observation of the supergiant X-ray binary system Cygnus X-1/HDE 226868 taken with the Chandra High Energy Transmission Grating Spectrometer (HETGS). Cyg X-1 was in its spectrally hard state and the observation was performed during superior conjunction of the black hole, allowing for the spectroscopic analysis of the accreted stellar wind along the line of sight. A significant part of the observation covers X-ray dips as commonly observed for Cyg X-1 at this orbital phase, however, here we only analyze the high count rate non-dip spectrum. The full 0.5-10 keV continuum can be described by a single model consisting of a disk, a narrow and a relativistically broadened Fe Kalpha line, and a power law component, which is consistent with simultaneous RXTE broad band data. We detect absorption edges from overabundant neutral O, Ne and Fe, and absorption line series from highly ionized ions and infer column densities and Doppler shifts. With emission lines of He-like Mg XI, we detect two plasma components with velocities and densities consistent with the base of the spherical wind and a focused wind. A simple simulation of the photoionization zone suggests that large parts of the spherical wind outside of the focused stream are completely ionized, which is consistent with the low velocities (<200 km/s) observed in the absorption lines, as the position of absorbers in a spherical wind at low projected velocity is well constrained. Our observations provide input for models that couple the wind activity of HDE 226868 to the properties of the accretion flow onto the black hole.

Probing the accretion disk and central engine structure of NGC4258 with Suzaku and XMM-Newton observations

[abridged] We present an X-ray study of the low-luminosity active galactic nucleus (AGN) in NGC4258 using data from Suzaku, XMM-Newton, and the Swift/BAT survey. We find that signatures of X-ray reprocessing by cold gas are very weak in the spectrum of this Seyfert-2 galaxy; a weak, narrow fluorescent-Kalpha emission line of cold iron is robustly detected in both the Suzaku and XMM-Newton spectra but at a level much below that of most other Seyfert-2 galaxies. We conclude that the circumnuclear environment of this AGN is very clean and lacks the Compton-thick obscuring torus of unified Seyfert schemes. From the narrowness of the iron line, together with evidence for line flux variability between the Suzaku and XMM-Newton observations, we constrain the line emitting region to be between $3times 10^3r_g$ and $4times 10^4r_g$ from the black hole. We show that the observed properties of the iron line can be explained if the line originates from the surface layers of a warped accretion disk. In particular, we present explicit calculations of the expected iron line from a disk warped by Lens-Thirring precession from a misaligned central black hole. Finally, the Suzaku data reveal clear evidence for large amplitude 2-10keV variability on timescales of 50ksec as well as smaller amplitude flares on timescales as short as 5-10ksec. If associated with accretion disk processes, such rapid variability requires an origin in the innermost regions of the disk ($rapprox 10r_g$ or less).

Suzaku Observations of Cyg X-1

We present highlights from a series of four simultaneous Suzaku/RXTE observations of the black hole candidate Cyg X-1. We briefly summarize several key results from our decade long RXTE monitoring campaign. We then comment on challenges of analyzing the Suzaku data, i.e., improving the aspect correction beyond that of the existing tools, and quantitatively assessing pileup. All of our Suzaku observations (one, by design) occurred at or very near orbital phase 0 (superior conjunction), and hence show evolution in color-color diagrams due to X-ray absorption by material from the wind of the secondary. We present simple partial absorption models for this evolution. We then compare the Suzaku and RXTE data, and explicitly divide the Fe line region into narrow and broad components. Both are required for the Suzaku data, and are seen to be consistent with the RXTE data. These Suzaku observations occurred near historically hard, low flux states. We present fits of the broad band spectra with a simple phenomenological broken powerlaw model, as well as a more physically motivated Comptonization model. Whereas the former class of models described nearly all of the RXTE campaign better than any physical model, here the latter model is slightly more successful. The Comptonization model, however, exhibits little evidence for a soft disk component, which formally corresponds to a small, inner disk radius. Whether this is physical, due to unmodeled absorption, or is a calibration issue, remains an open question.

Disk Dominated States of 4U 1957+11: Chandra, XMM, and RXTE Observations of Ostensibly the Most Rapidly Spinning Galactic Black Hole

We present simultaneous Chandra-HETG and RXTE observations of a moderate flux `soft state of the black hole candidate 4U1957+11. These spectra, having a minimally discernible hard X-ray excess, are an excellent test of modern disk atmosphere models that include the effects of black hole spin. The HETG data show that the soft disk spectrum is only very mildly absorbed with N_H =1-2 X 10^{21} cm^-2. These data additionally reveal 13.449 A NeIX absorption consistent with the warm/hot phase of the interstellar medium. The fitted disk model implies a highly inclined disk around a low mass black hole rapidly rotating with normalized spin a*~1. We show, however, that pure Schwarzschild black hole models describe the data extremely well, albeit with large disk atmosphere ``color-correction factors. Standard color-correction factors can be attained if one additionally incorporates mild Comptonization. We find that the Chandra observations do not uniquely determine spin. Similarly, XMM/RXTE observations, taken only six weeks later, are equally unconstraining. This lack of constraint is partly driven by the unknown mass and unknown distance of 4U1957+11; however, it is also driven by the limited bandpass of Chandra and XMM. We therefore present a series of 48 RXTE observations taken over the span of several years and at different brightness/hardness levels. These data prefer a spin of a*~1, even when including a mild Comptonization component; however, they also show evolution of the disk atmosphere color-correction factors. If the rapid spin models with standard atmosphere color-correction factors of h_d=1.7 are to be believed, then the RXTE observations predict that 4U1957+11 can range from a 3 M_sun black hole at 10 kpc with a*~0.83 to a 16 M_sun black hole at 22 kpc with a* ~ 1, with the latter being statistically preferred.

Limits on Hot Galactic Halo Gas from X-ray Absorption Lines

Although the existence of large-scale hot gaseous halos around massive disk galaxies have been theorized for a long time, there is yet very little observational evidence. We report the Chandra and XMM-Newton grating spectral detection of OVII and NeIX Kalpha absorption lines along the sight-line of 4U 1957+11. The line absorption is consistent with the interstellar medium in origin. Attributing these line absorptions to the hot gas associated with the Galactic disk, we search for the gaseous halo around the Milky Way by comparing this sight-line with more distant ones (toward X-ray binary LMC X-3 and the AGN Mrk 421). We find that all the line absorptions along the LMC X-3 and Mrk 421 sight-lines are attributable to the hot gas in a thick Galactic disk, as traced by the absorption lines in the spectra of 4U~1957+11 after a Galactic latitude dependent correction. We constrain the OVII column density through the halo to be N(OVII) < 5E15 cm^{-2} (95% confidence limit), and conclude that the hot gas contribution to the metal line absorptions, if existing, is negligible.