D. Lin

The Energy Dependence of the Aperiodic Variability for Cygnus X-1, GX 339–4, GRS 1758–258, and 1E 1740.7–2942

Using the data from the Rossi X-Ray Timing Explorer (RXTE), we report the different energy dependence of the variability of the four hard X-ray sources in the hard/low state: Cygnus X-1, GX 339-4, GRS 1758-258, and 1E 1740.7-2942. Cygnus X-1 is found to have a flatter power density spectrum (PDS) shape at higher energies. The other three sources have energy-independent PDS shapes. The energy dependence of the overall variability (the integrated rms amplitude) varies from source to source and from observation to observation. 1E 1740.7-2942, for example, has a variability generally increasing with energy while GX 339-4 has a decreasing variability. A general trend is found in the four sources that the integrated rms amplitude anticorrelates with the X-ray flux. We compare these distinct energy-dependent behaviors with several emission models. None of the models can fully explain all the features that we have found.

Simultaneous Observations of GRS 1758–258 in 1997 by VLA, IRAM, SEST, RXTE, and OSSE: Spectroscopy and Timing

We report the results of our multiwavelength observations of GRS 1758-258 made in 1997 August. The energy bands include radio, millimeter, X-ray, and gamma-ray. The observations enable us to obtain a complete spectrum of the source over an energy range of 2-500 keV. The spectrum shows that GRS 1758-258 was in its hard state. It is well fit by the Sunyaev-Titarchuk (ST) Compton scattering model with a plasma temperature of 45 keV and a Thomson depth of 3.3. Taking relativistic effects into account, we get a little higher plasma temperature (52 keV) by using the improved version of the ST model (HT model) plus a soft blackbody component. The spectrum is also fit by a power law with an exponential cutoff (PLE) plus a soft blackbody component. The temperature of the soft components in both models is about 1.2 keV, and the energy flux is less than 1.5% of the total X-ray and gamma-ray flux. The deduced hydrogen column density is in the range of (0.93-2.0) × 1022 cm-2. No significant iron lines are detected. The radio emission has a flat energy spectrum. The daily radio, X-ray, and gamma-ray light curves show that GRS 1758-258 was stable during the observation period but was highly variable on smaller timescales in X-rays and gamma rays. The power density spectra are typical for the low state, but we find the photon flux for the 5-10 keV band to be more variable than that in the other two energy bands (2-5 keV and 10-40 keV). Harmonically spaced quasi-periodic oscillations are observed in the power spectra. The phase lags between the hard photons and the soft photons have a flat distribution over a wide range of frequencies. A high coherence of about 1.0 (0.01-1 Hz) between the hard photons and the soft photons is also obtained in our observations. We compare these results with two variation models. Our millimeter observations did not reveal any conclusive signatures of an interaction between the jet from GRS 1758-258 and the molecular cloud that lies in the direction of GRS 1758-258.

Complex Phase Lag Behaviors of the 0.5-10 Hz Quasi-periodic Oscillations in GRS 1915+105

Through studying the hard lags between the soft (3.3-5.8 keV) and hard (13.0-41.0 keV) photons of the 0.5-10 Hz quasi-periodic oscillations (QPOs) in GRS 1915+105, we have classified them into three types: 0.5-2.0, 2.0-4.5, and 4.5-10 Hz QPOs. They are closely related to different temporal and spectral states. The first type of QPO (0.5-2 Hz) has positive hard lags at both the QPO fundamental and first harmonic frequencies. These QPOs were observed in the quiescent soft state. The second type of QPO (2-4.5 Hz) was also detected in the quiescent soft state; these QPOs have opposite signs of hard lags at the QPO fundamental and first harmonic frequencies. The third type (4.5-10 Hz), which showed up in medium soft quiescent/outburst states, does not have significant higher harmonic peaks. There is a smooth transition between these three types of QPO behaviors. We did not detect 0.5-10 Hz QPOs in the very soft state. We discuss some of the implications of these results.

A thermal-nonthermal inverse Compton model for Cyg X-1

Using Monte Carlo methods to simulate the inverse Compton scattering of soft photons, we model the spectrum of the Galactic black hole candidate Cyg X-1, which shows evidence of a nonthermal tail extending beyond a few hundred keV. We assume an ad hoc sphere of leptons, whose energy distribution consists of a Maxwellian plus a high energy power-law tail, and inject 0.5 keV blackbody photons. The spectral data is used to constrain the nonthermal plasma fraction and the power-law index assuming a reasonable Maxwellian temperature and Thomson depth. A small but non-negligible fraction of nonthermal leptons is needed to explain the power-law tail.

Saturated Compton scattering models for the soft gamma-ray repeater bursts

The Soft Gamma-Ray Repeaters (SGR) are sources of brief intense outbursts of low energy gamma rays. Most likely they are a new manifestation of neutron stars. In this paper, we explore Compton scattering models for the bursts. We use a BATSE SGR 1900+14 burst to show that Comptonization by a thermal plasma alone is unable to explain the shape of the burst spectrum, but that adding a small but significant fraction of non-thermal plasma produces an acceptable fit.

Emission and cooling processes in a hybrid thermal-nonthermal plasma

In a hybrid thermal-nonthermal plasma, we find that the dominant emission and absorption mechanisms are synchrotron by nonthermal electrons and bremsstrahlung by thermal electrons. These two processes significantly change the spectrum from inverse Compton scatterings at low energies. We also find that Coulomb collisions are effective in cooling down the lower energy electrons but do no significantly alter the emission pattern. Compton cooling is more effective in changing emission and absorption coefficients when the photon energy density is high.