Kilohertz QPO Frequency and Flux Decrease in AQL X-1 and Effect of Soft X-ray Spectral Components

We report on an RXTE/PCA observation of Aql X-1 during its outburst in March 1997 in which, immediately following a Type-I burst, the broad-band 2-10 keV flux decreased by about 10% and the kilohertz QPO frequency decreased from 813+-3 Hz to 776+-4 Hz. This change in kHz QPO frequency is much larger than expected from a simple extrapolation of a frequency-flux correlation established using data before the burst. Meanwhile a very low frequency noise (VLFN) component in the broad-band FFT power spectra with a fractional root-mean-square (rms) amplitude of 1.2% before the burst ceased to exist after the burst. All these changes were accompanied by a change in the energy spectral shape. If we characterize the energy spectra with a model composed of two blackbody (BB) components and a power law component, almost all the decrease in flux was in the two BB components. We attribute the two BB components to the contributions from a region very near the neutron star or even the neutron star itself and from the accretion disk, respectively.Comment: 12 pages with 4 figures, accepted for publication in ApJ Letters, typos corrected and references update

The 1996 Soft State Transitions of Cygnus X-1

We report continuous monitoring of Cygnus X-1 in the 1.3 to 200 keV band using ASM/RXTE and BATSE/CGRO for about 200 days from 1996 February 21 to 1996 early September. During this period Cygnus X-1 experienced a hard-to-soft and then a soft-to-hard state transition. The low-energy X-ray (1.3-12 keV) and high-energy X-ray (20-200 keV) fluxes are strongly anti-correlated during this period. During the state transitions flux variations of about a factor of 5 and 15 were seen in the 1.3-3.0 keV and 100-200 keV bands, respectively, while the average 4.8-12 keV flux remains almost unchanged. The net effect of this pivoting is that the total 1.3-200 keV luminosity remained unchanged to within about 15%. The bolometric luminosity in the soft state may be as high as 50-70% above the hard state luminosity, after color corrections for the luminosity below 1.3 keV. The blackbody component flux and temperature increase in the soft state is probably caused by a combination of the optically thick disk mass accretion rate increase and a decrease of the inner disk radius.Comment: 18 pages, 1 PostScript figure. Accepted for ApJ

The Plasma Structure of the Cygnus Loop from the Northeastern Rim to the Southwestern Rim

The Cygnus Loop was observed from the northeast to the southwest with XMM-Newton. We divided the observed region into two parts, the north path and the south path, and studied the X-ray spectra along two paths. The spectra can be well fitted either by a one-component non-equilibrium ionization (NEI) model or by a two-component NEI model. The rim regions can be well fitted by a one-component model with relatively low \kTe whose metal abundances are sub-solar (0.1--0.2). The major part of the paths requires a two-component model. Due to projection effects, we concluded that the low kTe (about 0.2 keV) component surrounds the high kTe (about 0.6 keV) component, with the latter having relatively high metal abundances (about 5 times solar). Since the Cygnus Loop is thought to originate in a cavity explosion, the low-kTe component originates from the cavity wall while the high-kTe component originates from the ejecta. The flux of the cavity wall component shows a large variation along our path. We found it to be very thin in the south-west region, suggesting a blowout along our line of sight. The metal distribution inside the ejecta shows non-uniformity, depending on the element. O, Ne and Mg are relatively more abundant in the outer region while Si, S and Fe are concentrated in the inner region, with all metals showing strong asymmetry. This observational evidence implies an asymmetric explosion of the progenitor star. The abundance of the ejecta also indicates the progenitor star to be about 15 M_sun.Comment: 24 pages, 9 figures, Astrophysical Journal in pres

Multi-Temperature Blackbody Spectra of Thin Accretion Disks With and Without a Zero-Torque Inner Boundary Condition

The standard spectral model for analyzing the soft component of thermal emission from a thin accretion disk around a black hole is the multi-temperature blackbody (MTB) model. The widely used implementation of this model, which is known as ``diskbb,'' assumes nonzero torque at the inner edge of the accretion disk. This assumption is contrary to the classic and current literature on thin-disk accretion, which advocates the use of a zero-torque boundary condition. Consequently, we have written code for a zero-torque model, ``ezdiskbb,'' which we compare to the nonzero-torque model diskbb by fitting RXTE spectra of three well-known black hole binaries: 4U 1543-47, XTE J1550-564, and GRO J1655-40. The chief difference we find is that the zero-torque model gives a value for the inner disk radius that is about 2.2 times smaller than the value given by diskbb. This result has important implications, especially for the determination of black-hole angular momentum and mass accretion rate.Comment: 27 pages, 9 figures, submitted to Ap

Spin-Wave Spectrum in `Single-Domain' Magnetic Ground State of Triangular Lattice Antiferromagnet CuFeO2

By means of neutron scattering measurements, we have investigated spin-wave excitation in a collinear four-sublattice (4SL) magnetic ground state of a triangular lattice antiferromagnet CuFeO2, which has been of recent interest as a strongly frustrated magnet, a spin-lattice coupled system and a multiferroic. To avoid mixing of spin-wave spectrum from magnetic domains having three different orientations reflecting trigonal symmetry of the crystal structure, we have applied uniaxial pressure on [1-10] direction of a single crystal CuFeO2. By elastic neutron scattering measurements, we have found that only 10 MPa of the uniaxial pressure results in almost 'single domain' state in the 4SL phase. We have thus performed inelastic neutron scattering measurements using the single domain sample, and have identified two distinct spin- wave branches. The dispersion relation of the upper spin-wave branch cannot be explained by the previous theoretical model [R. S. Fishman: J. Appl. Phys. 103 (2008) 07B109]. This implies the importance of the lattice degree of freedom in the spin-wave excitation in this system, because the previous calculation neglected the effect of the spin-driven lattice distortion in the 4SL phase. We have also discussed relationship between the present results and the recently discovered "electromagnon" excitation.Comment: 5 pages, 3 figures, accepted for publication in J. Phys. Soc. Jp

Variable-Frequency QPOs from the Galactic Microquasar GRS 1915+105

We show that the galactic microquasar GRS 1915+105 exhibits quasi-periodic oscillations (QPOs) whose frequency varies continuously from 1-15 Hz, during spectrally hard dips when the source is in a flaring state. We report here analyses of simultaneous energy spectra and power density spectra at 4 s intervals. The energy spectrum is well fit at each time step by an optically thick accretion disk plus power law model, while the power density spectrum consists of a varying red noise component plus the variable frequency QPO. The features of both spectra are strongly correlated with one another. The 1-15 Hz QPOs appear when the power law component becomes hard and intense, and themselves have an energy spectrum consistent with the power law component (with root mean square amplitudes as high as 10%). The frequency of the oscillations, however, is most strikingly correlated with the parameters of the thermal disk component. The tightest correlation is between QPO frequency and the disk X-ray flux. This fact indicates that the properties of the QPO are not determined by solely a disk or solely a corona.Comment: Accepted to ApJ Letters, 12 pages, 3 figures, AASTEX forma

Multiple Components of the Luminous Compact X-ray Source at the Edge of Holmberg II observed by ASCA and ROSAT

We report the results of the analysis of ASCA/ROSAT observations of the compact luminous X-ray source found at the edge of the nearby star-forming dwarf galaxy Holmberg II (UGC 4305).Our ASCA spectrum revealed that the X-ray emission extends to the hard band and can be best described by a power-law with a photon spectral index of 1.9. The ASCA spectrum does not fit with a multi-color disk blackbody. The joint ASCA-ROSAT spectrum suggests two components to the spectrum: the hard power-law component and a warm thermal plasma kT~0.3[keV]. An additional absorption over that of our galaxy is required. The wobble correction of the ROSAT HRI image has clearly unveiled the existence of an extended component which amounts to 27+/-5% of the total X-ray emission. These observations indicate that there are more than one component in the X-ray emission. The properties of the point-like component is indicative of an accretion onto an intermediate mass blackhole, unless a beaming is taking place. We argue that the extended component does not come from electron scattering and/or reflection by scattered optically-thick clouds of the central radiation. Possible explanations of this X-ray source include multiple supernova remnants feeding an intermediate-mass blackhole. (abridged)Comment: 12 pages, 6 figures accepted to Astronomical Journa

Boundary layer on the surface of a neutron star

In an attempt to model the accretion onto a neutron star in low-mass X-ray binaries, we present two-dimensional hydrodynamical models of the gas flow in close vicinity of the stellar surface. First we consider a gas pressure dominated case, assuming that the star is non-rotating. For the stellar mass we take M_{\rm star}=1.4 \times 10^{-2} \msun and for the gas temperature $T=5 \times 10^6$ K. Our results are qualitatively different in the case of a realistic neutron star mass and a realistic gas temperature of $T\simeq 10^8$ K, when the radiation pressure dominates. We show that to get the stationary solution in a latter case, the star most probably has to rotate with the considerable velocity.Comment: 7 pages, 7 figure

Towards a New Standard Theory for Astrophysical Disk Accretion

We briefly review recent developments in black hole accretion disk theory, placing new emphasis on the vital role played by magnetohydrodynamic (MHD) stresses in transporting angular momentum. The apparent universality of accretion-related outflow phenomena is a strong indicator that vertical transport of angular momentum by large-scale MHD torques is important and may even dominate radial transport by small-scale MHD turbulence. This leads to an enhanced overall rate of angular momentum transport and allows accretion of matter to proceed at an interesting rate. Furthermore, we argue that when vertical transport is important, the radial structure of the accretion disk is modified and this affects the disk emission spectrum. We present a simple model demonstrating that energetic, magnetically-driven outflows give rise to a disk spectrum that is dimmer and redder than a standard accretion disk accreting at the same rate. We briefly discuss the implications of this key result for accreting black holes in different astrophysical systems.Comment: Accepted for publication as brief review in Mod. Phys. Let.

Advection-Dominated Accretion and Black Hole Event Horizons

The defining characteristic of a black hole is that it possesses an event horizon through which matter and energy can fall in but from which nothing escapes. Soft X-ray transients (SXTs), a class of X-ray binaries, appear to confirm this fundamental property of black holes. SXTs that are thought to contain accreting black holes display a large variation of luminosity between their bright and faint states, while SXTs with accreting neutron stars have a smaller variation. This difference is predicted if the former stars have horizons and the latter have normal surfaces.Comment: 11 pages, including 2 tables and 2 figures. To appear in The Astrophysical Journal Letter