Linqing Wen

Understanding the Long-Term Spectral Variability of Cygnus X-1 with Burst and Transient Source Experiment and All-Sky Monitor Observations

We present an analysis of all observations of Cygnus X-1 by the BATSE (20-300 keV) and by ASM (1.5-12 keV) until 2002 June, including 1200 days of simultaneous data. We find a number of correlations between fluxes and hardnesses in different energy bands. In the hard state, the variability can be explained by softening the overall spectrum with a pivot at 50 keV and another, independent variability pattern where the spectral shape does not change when the luminosity changes. In the soft state, the variability is caused by a variable hard tail of a constant shape superimposed on a constant soft component. These variability patterns are in agreement with the energy-dependent rms variability in the two states. We also study in detail recent soft states in 2000-02. The last of them has lasted so far for >200 days. Their spectra are harder in the 1.5-5 keV band but similar or in the 3-12 keV band than those of the 1996 soft state whereas the rms variability is stronger in all the ASM bands. On the other hand, the 1994 soft state transition observed by BATSE appears very similar to the 1996 one. We interpret the variability patterns by theoretical Comptonization models. In the hard state, the variability appears to be driven mostly by changing flux in seed photons Comptonized in a hot thermal plasma cloud with an approximately constant power supply. In the soft state, the variability is consistent with flares of hybrid, thermal/nonthermal, plasma with variable power above a stable cold disk. The spectral and timing differences between the 1996 and 2000-02 soft states are explained by a decrease of the color disk temperature. Also, based on broad-band pointed observations, we find the intrinsic bolometric luminosity increases by a factor of 3-4 from the hard state to the soft one.We present a comprehensive analysis of all observations of Cyg X-1 by the Compton Gamma Ray Observatory Burst and Transient Source Experiment (BATSE; 20-300 keV) and by the Rossi X-Ray Timing Explorer all-sky monitor (ASM; 1.5-12 keV) until 2002 June, including ~1200 days of simultaneous data. We find a number of correlations between fluxes and hardnesses in different energy bands. In the hard (low) spectral state, there is a negative correlation between the ASM 1.5-12 keV flux and the hardness at any energy. In the soft (high) spectral state, the ASM flux is positively correlated with the ASM hardness but uncorrelated with the BATSE hardness. In both spectral states, the BATSE hardness correlates with the flux above 100 keV, while it shows no correlation with the 20-100 keV flux. At the same time, there is clear correlation between the BATSE fluxes below and above 100 keV. In the hard state, most of the variability can be explained by softening the overall spectrum with a pivot at ~50 keV. There is also another, independent variability pattern of lower amplitude where the spectral shape does not change when the luminosity changes. In the soft state, the variability is mostly caused by a variable hard (Comptonized) spectral component of a constant shape superposed on a constant soft blackbody component. These variability patterns are in agreement with the dependencies of the rms variability on the photon energy in the two states. We also study in detail recent soft states from late 2000 until 2002. The last of them has lasted thus far for more than 200 days. Their spectra are generally harder in the 1.5-5 keV band and similar or softer in the 3-12 keV band than the spectra of the 1996 soft state, whereas the rms variability is stronger in all the ASM bands. On the other hand, the 1994 soft state transition observed by BATSE appears very similar to the 1996 one. We interpret the variability patterns in terms of theoretical Comptonization models. In the hard state, the variability appears to be driven mostly by changing flux in seed photons Comptonized in a hot thermal plasma cloud with an approximately constant power supply. In the soft state, the variability is consistent with flares of hybrid, thermal/nonthermal, plasma with variable power above a stable cold disk. The spectral and timing differences between the 1996 and 2000-2002 soft states are explained by a decrease of the color disk temperature. Also, on the basis of broadband pointed observations simultaneous with those of the ASM and BATSE, we find the intrinsic bolometric luminosity increases by a factor of ~3-4 from the hard state to the soft one, which supports models of the state transition based on a change of the accretion rate.

Orbital Modulation of X-Rays from Cygnus X-1 in its Hard and Soft States

We have analyzed over 2 yr of RXTE/All-Sky Monitor data for Cygnus X-1. We have detected the 5.6 day orbital period in Lomb-Scargle periodograms of both light curves and hardness ratios when Cyg X-1 was in the hard state. This detection was made with improved sensitivity and temporal coverage compared with previous detections by other X-ray missions. The folded light curves and hardness ratios show a broad intensity dip accompanied by spectral hardening centered on superior conjunction of the X-ray source. The dip has a duration of about 27% of the orbital period and depth ranging from 8% to 23% of the nondip intensities in three energy bands. Variability on timescales of hours is often evident within the broad dip in the unfolded data. In contrast, no feature at the orbital period is evident in the periodograms or folded light curves for the soft state. Absorption of X-rays by a stellar wind from the companion star can reproduce the observed X-ray orbital modulations in the hard state. To explain the low orbital modulation in the soft-state data, a reduction of the wind density during the soft state would be required. As an alternative, a partial covering scenario is described that could also account for the lack of the orbital modulation in the soft state.

X1908+075: An X-Ray Binary with a 4.4 Day Period

X1908+075 is an optically unidentified and highly absorbed X-ray source that appeared in early surveys such as Uhuru, OSO 7, Ariel 5, HEAO-1, and the EXOSAT Galactic Plane Survey. These surveys measured a source intensity in the range 2-12 mcrab at 2-10 keV, and the position was localized to ~05. We use the Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor (ASM) to confirm our expectation that a particular Einstein/IPC detection (1E 1908.4+0730) provides the correct position for X1908+075. The analysis of the coded mask shadows from the ASM for the position of 1E 1908.4+0730 yields a persistent intensity ~8 mcrab (1.5-12 keV) over a 3 yr interval beginning in 1996 February. Furthermore, we detect a period of 4.400 ± 0.001 days with a false-alarm probability less than 10-7. The folded light curve is roughly sinusoidal, with an amplitude that is 26% of the mean flux. The X-ray period may be attributed to the scattering and absorption of X-rays through a stellar wind combined with the orbital motion in a binary system. We suggest that X1908+075 is an X-ray binary with a high-mass companion star.

The Correlated Intensity and Spectral Evolution of Cygnus X-1 During State Transitions

Using data from the All-Sky Monitor (ASM) aboard the Rossi X-Ray Timing Explorer (RXTE), we found that the 1.5-12 keV X-ray count rate of Cygnus X-1 is, on timescales from 90 s to at least 10 days, strongly correlated with the spectral hardness of the source in the soft state but is weakly anticorrelated with the latter in the hard state. The correlation shows an interesting evolution during the 1996 spectral state transition. The entire episode can be roughly divided into three distinct phases: (1) a 20 day transition phase from the hard state to the soft state, during which the correlation changes from being negative to positive, (2) a 50 day soft state with a steady positive correlation, and (3) a 20 day transition back to the hard state. The pointed RXTE observations confirmed the ASM results but revealed new behaviors of the source at energies beyond the ASM passband. We discuss the implications of our findings.

Gamma-ray bursts in the All-Sky Monitor on RXTE

The All-Sky Monitor (ASM) on RXTE can provide useful scientific data to the astrophysics community for the investigation of gamma-ray bursts. We are developing two complementary methods by which the ASM searches for the X-ray counterparts to gamma-ray bursts: (1) we determine whether or not a BATSE location as delivered through the GCN is scanned by the ASM at or shortly after the BATSE trigger time, and (2) we search for transient events in ASM time-series data that correspond to new sources in ASM position histogram data. On 1997 August 15, we were able to determine an arcminute position within 12 h of trigger. On 1997 August 28, we released a position in 2 h, enabling the RXTE/PCA, 4-m Herschel telescope, and the VLA to be on target within 4 h of the burst. We present position and timing information on GRB970815 and GRB970828, comparing the ASM and BATSE data.

A New Indicator for the Spectral States and State Transitions in Cygnus X-1

Based on data from the RXTE All-Sky Monitor, we found that, on time scales of days to weeks, the X-ray intensity of Cyg X-1 is correlated with the spectral hardness of the source in the soft state, but is anti-correlated with the latter in the hard state. The correlation shows interesting evolution during the state transition of 1996 May-August. The entire episode consists of three distinct phases: (1) a ~ 20-d transition phase from the hard state to the soft state, where the correlation changes from negative to positive, (2) a 50-d soft state with a steady positive correlation, and (3) a 20-d transition back to the hard state. The existence and time scales of these three phases are consistent with the timing results from the RXTE/PCA observations. This result confirms the conclusion that the soft X-ray flux alone is not a reliable indicator for a true soft state. We argue that the flux-hardness correlation likely provides a better indicator for defining the spectral states of Cyg X-1.

The superorbital variability and triple nature of the X‐ray source 4U 1820–303

In Zdziarski et al. (2007a), we have performed a comprehensive analysis of the superorbital modulation in the ultracompact X‐ray source 4U 1820–303, consisting of a white dwarf accreting onto a neutron star. Based on RXTE data, we measured the fractional amplitude of the source superorbital variability (with a ∼170‐d quasi‐period) in the folded and averaged light curves, and found it to be by a factor of ∼2. As proposed before, that variability can be explained by oscillations of the binary eccentricity. In that work, we presented detailed calculations of the eccentricity‐dependent flow through the inner Lagrangian point, and found a maximum of the eccentricity of ≃0.004 is sufficient to explain the observed fractional amplitude. We then studied hierarchical triple models yielding the required quasi‐periodic eccentricity oscillations through the Kozai process. We found the resulting theoretical light curves to match well the observed ones. We constrained the ratio of the semimajor axes of the outer and in...