The Physics of Disk Winds, Jets,and X-ray Variability in GRS 1915+105

We present new insights about accretion and ejection physics based on joint RXTE/Chandra HETGS studies of rapid X-ray variability in GRS 1915+105. For the first time, with fast phase-resolved spectroscopy of the rho state, we are able to show that changes in the broadband X-ray spectrum (RXTE) on timescales of seconds are associated with measurable changes in absorption lines (Chandra HETGS) from the accretion disk wind. Additionally, we make a direct detection of material evaporating from the radiation-pressure-dominated inner disk. Our X-ray data thus reveal the black hole as it ejects a portion of the inner accretion flow and then drives a wind from the outer disk, all in a bizarre cycle that lasts fewer than 60 seconds but can repeat for weeks. We find that the accretion disk wind may be sufficiently massive to play an active role in GRS 1915+105, not only in quenching the jet on long timescales, but also in possibly producing or facilitating transitions between classes of X-ray variability.Comment: 3 pages, 1 Figure. Proceedings of IAU Symposium 275 (Jets at all Scales), Buenos Aires, 13-17.09.2010; eds. G. Romero, R. Sunyaev, T. Bellon

The X-Ray Position and Infrared Counterpart of the Eclipsing X-Ray Pulsar OAO 1657-415

We have measured the precise position of the 38-s eclipsing X-ray pulsar OAO 1657-415 with the Chandra X-Ray Observatory: RA = 17h00m48.90s, Dec = -41d39m21.6s, equninox J2000, error radius = 0.5 arcsec. Based on the previously measured pulsar mass function and X-ray eclipse duration, this 10.4-d high-mass X-ray binary is believed to contain a B supergiant companion. Deep optical imaging of the field did not detect any stars at the Chandra source position, setting a limit of V>23. However, near-IR imaging revealed a relatively bright star (J=14.1, H=11.9, K_s=10.7) coincident with the Chandra position, and we identify this star as the IR counterpart of OAO 1657-415. The IR colors and magnitudes and the optical non-detections for this star are all consistent with a highly reddened B supergiant (A_V= 20.4 +/- 1.3) at a distance of 6.4 +/- 1.5 kpc. This implies an X-ray luminosity of 3e36 erg/s (2-10 keV). IR spectroscopy can verify the spectral type of the companion and measure its radial velocity curve, yielding a neutron star mass measurement.Comment: 4 pages. ApJ in press (Vol. 573, July 10 issue

Search for missing baryon resonances via associated strangeness photoproduction

Differential cross-section and single polarization observables in the process gamma p --> K^+ Lambda are investigated within a constituent quark model and a dynamical coupled-channel formalism. The effects of two new nucleon resonances and of the K*(892)- and K1(1270)-exchanges are briefly presented.Comment: Contributed paper to the IVth International Conference on Quarks and Nuclear Physics, Madrid June 5-10, 200

Determining the grain composition of the interstellar medium with high resolution X-ray spectroscopy

We investigate the ability of high resolution X-ray spectroscopy to directly probe the grain composition of the interstellar medium. Using iron K-edge experimental data of likely ISM dust candidates taken at the National Synchrotron Light Source at Brookhaven National Laboratory and the Advanced Photon Source at Argonne National Laboratory, we explore the prospects for determining the chemical composition of astrophysical dust and discuss a technique for doing so. Focusing on the capabilities of the AstroE2 XRS micro-calorimeters, we assess the limiting effects of spectral resolution and noise for detecting significant X-ray absorption fine structure signal in astrophysical environments containing dust. We find that given sufficient signal, the resolution of the XRS will allow us to definitively distinguish gas from dust phase absorption, and certain chemical compositions.Comment: 6 pages, 10 figures - XAFS spectra of Fe compounds; accepted for publication in Ap

Quark-Exchange Mechanism of γd→np\gamma d \to np Reaction At 2-6 GeV

Within the constituent quark model, we examine the extent to which the deuteron photo-disintegration at 2-6 GeV can be described by the quark-exchange mechanism. With the parameters constrained by the $np$ scattering, the calculated differential cross sections disagree with the data in both magnitude and energy-dependence. The results can be improved if we use a smaller size parameter for quark wavefunctions. We also find that the on-shell approximation used in a previous investigation is not accurateComment: To be published in the Proceeeding of Second Asia Pacific Conference on Few-Body Problems in Physics, Shanghai, China, August 27-30, 200

Dynamical Coupled-Channels Effects on Pion Photoproduction

The electromagnetic pion production reactions are investigated within the dynamical coupled-channels model developed in {\bf Physics Reports, 439, 193 (2007)}. The meson-baryon channels included in this study are $\gamma N$, $\pi N$, $\eta N$, and the $\pi\Delta$, $\rho N$ and $\sigma N$ resonant components of the $\pi\pi N$ channel. With the hadronic parameters of the model determined in a recent study of $\pi N$ scattering, we show that the pion photoproduction data up to the second resonance region can be described to a very large extent by only adjusting the bare $\gamma N \to N^*$ helicity amplitudes, while the non-resonant electromagnetic couplings are taken from previous works. It is found that the coupled-channels effects can contribute about 10 - 20 % of the production cross sections in the $\Delta$ (1232) resonance region, and can drastically change the magnitude and shape of the cross sections in the second resonance region. The importance of the off-shell effects in a dynamical approach is also demonstrated. The meson cloud effects as well as the coupled-channels contributions to the $\gamma N \to N^*$ form factors are found to be mainly in the low $Q^2$ region. For the magnetic M1 $\gamma N \to \Delta$ (1232) form factor, the results are close to that of the Sato-Lee Model. Necessary improvements to the model and future developments are discussed.Comment: Corrected version. 14 pages, 10 figure