A Disk--Jet interaction model for the X--Ray Variability in Microquasars

We propose a simple dynamical model that may account for the observed spectral and temporal properties of GRS 1915+105 and XTE J1550-5634. The model is based on the assumption that a fraction of the radiation emitted by a hot spot lying on the accreting disk is dynamically Comptonized by the relativistic jet that typically accompanies the microquasar phenomenon. We show that scattering by the jet produces a detectable modulation of the observed flux. In particular, we found that the phase lag between hard and soft photons depends on the radial position of the hot spot and, if the angle between the jet and the line of sight is sufficiently large, the lags of the fundamental and its harmonics may be either positive or negative.Comment: 14 pages, 4 figures, accepted for publication in ApJ Part

Detection of anti-correlated hard X-ray time lag in Cygnus X-3

The wide-band X-ray spectra of the high mass X-ray binary Cygnus X-3 exhibits a pivoting behavior in the `low' (as well as `hard') state, correlated to the radio emission. The time scale of the soft and hard X-rays' anti-correlation, which gave rise to the pivoting feature, was found to be less than a day from the monitoring observations by RXTE--ASM and CGRO--BATSE. In this Letter we report the detection of a lag of $\lesssim$ 1000s in the anti-correlation of the hard X-ray emission (20--50 keV) to that of the soft X-ray emission (2--7 keV), which may be attributed to the viscous time scale of flow of matter in the accretion disk. This suggests the geometrical picture of a truncated accretion disc with a Compton cloud inside the disc, the relative sizes of which determine the spectral shape. Any change in the disc structure will take place in a viscous time scale, with corresponding anti-correlated change in the Compton cloud. We also report the pivoting in the spectra in one span of a pointed observation when an episode of the rearranging of the accretion system is serendipitously observed. This is the first such observation of hard X-ray delay seen in the persistent Galactic microquasars, within the precincts of the hard state.Comment: Accepted in The Astrophysical Journal (Letters): in pres

Evidence For Advective Flow From Multi-Wavelength Observations Of Nova Muscae

We model the UV/optical spectrum of the black hole binary Nova Muscae as a sum of black body emissions from the outer region of an accretion disk. We show for self-consistency that scattering effects in this region are not important. The black hole mass ($M \approx 6 M_\odot$), the inclination angle ($\mu \approx 0.5$) and the distance to the source ($D \approx 5$ kpc) have been constrained by optical observations during quiescence (Orosz et al. 1996). Using these values we find that the accretion rate during the peak was ${\dot M} \approx 8 \times 10^{19}$ g sec$^{-1}$ and subsequently decayed exponentially. We define a radiative fraction ($f$) to be the ratio of the X-ray energy luminosity to the total gravitational power dissipated for a keplerian accretion disk. We find that $f \approx 0.1$ and remains nearly constant during the Ultra-soft and Soft spectral states. Thus for these states, the inner region of the accretion disk is advection dominated. $f$ probably increased to $\approx 0.5$ during the Hard state and finally decreased to $\approx 0.03$ as the source returned to quiescence.Comment: 5 figures. uses aasms4.sty, accepted by Ap

The Mixed State of Charge-Density-Wave in a Ring-Shaped Single Crystals

Charge-density-wave (CDW) phase transition in a ring-shaped crystals, recently synthesized by Tanda et al. [Nature, 417, 397 (2002)], is studied based on a mean-field-approximation of Ginzburg-Landau free energy. It is shown that in a ring-shaped crystals CDW undergoes frustration due to the curvature (bending) of the ring (geometrical frustration) and, thus, forms a mixed state analogous to what a type-II superconductor forms under a magnetic field. We discuss the nature of the phase transition in the ring-CDW in relation to recent experiments.Comment: 6 pages, 4 figure

Radiation mechanisms and geometry of Cygnus X-1 in the soft state

We present X-ray/gamma-ray spectra of Cyg X-1 observed during the transition from the hard to the soft state and in the soft state by ASCA, RXTE and OSSE in 1996 May and June. The spectra consist of a dominant soft component below ~2 keV and a power-law-like continuum extending to at least ~800 keV. We interpret them as emission from an optically-thick, cold accretion disc and from an optically-thin, non-thermal corona above the disc. A fraction f ~ 0.6 of total available power is dissipated in the corona. We model the soft component by multi-colour blackbody disc emission taking into account the torque-free inner-boundary condition. If the disc extends down to the minimum stable orbit, the ASCA/RXTE data yield the most probable black hole mass of about 10 solar masses and an accretion rate about 0.5 L_E/c^2, locating Cyg X-1 in the soft state in the upper part of the stable, gas-pressure dominated, accretion-disc solution branch. The spectrum of the corona is well modelled by repeated Compton scattering of seed photons from the disc off electrons with a hybrid, thermal/non-thermal distribution. The electron distribution can be characterized by a Maxwellian with an equilibrium temperature of kT ~ 30--50 keV and a Thomson optical depth of ~0.3 and a quasi-power-law tail. The compactness of the corona is between 2 and 7, and a presence of a significant population of electron-positron pairs is ruled out. We find strong signatures of Compton reflection from a cold and ionized medium, presumably an accretion disc, with an apparent reflector solid angle ~0.5--0.7. The reflected continuum is accompanied by a broad iron K-alpha line.Comment: 18 pages, 12 figures, 2 landscape tables in a separate file. Accepted to MNRA

Detection of Low-Hard State Spectral and Timing Signatures from the Black Hole X-Ray Transient XTE J1650-500 at Low X-Ray Luminosities

Using the Chandra X-ray Observatory and the Rossi X-ray Timing Explorer, we have studied the black hole candidate (BHC) X-ray transient XTE J1650-500 near the end of its 2001-2002 outburst after its transition to the low-hard state at X-ray luminosities down to L = 1.5E34 erg/s (1-9 keV, assuming a source distance of 4 kpc). Our results include a characterization of the spectral and timing properties. At the lowest sampled luminosity, we used an 18 ks Chandra observation to measure the power spectrum at low frequencies. For the 3 epochs at which we obtained Chandra/RXTE observations, the 0.5-20 keV energy spectrum is consistent with a spectral model consisting of a power-law with interstellar absorption. We detect evolution in the power-law photon index from 1.66 +/- 0.05 to 1.93 +/- 0.13 (90% confidence errors), indicating that the source softens at low luminosities. The power spectra are characterized by strong (20-35% fractional rms) band-limited noise, which we model as a zero-centered Lorentzian. Including results from an RXTE study of XTE J1650-500 near the transition to the low-hard state by Kalemci et al. (2003), the half-width of the zero-centered Lorentzian (roughly where the band-limited noise cuts off) drops from 4 Hz at L = 7E36 erg/s (1-9 keV, absorbed) to 0.067 +/- 0.007 Hz at L = 9E34 erg/s to 0.0035 +/- 0.0010 Hz at the lowest luminosity. While the spectral and timing parameters evolve with luminosity, it is notable that the general shapes of the energy and power spectra remain the same, indicating that the source stays in the low-hard state. This implies that the X-ray emitting region of the system likely keeps the same overall structure, while the luminosity changes by a factor of 470. We discuss how these results may constrain theoretical black hole accretion models.Comment: 11 pages, accepted by ApJ after minor revision