arXiv subjects

E. P. Liang

Publications and source records attributed to E. P. Liang.

26 records · Page 2Linked to original sources

arXiv1998

Radiation Feedback in Hot Accretion-Disk Corona Models

We present a detailed study of the observable effects of photoionization and Comptonization of line and continuum radiation from a cold accretion disk with a thin, warm, photoionized transition layer in the framework of self-consistent accretion-disk corona models for Galactic black-hole candidates. We use an iterative method to treat the non-linear radiation feedback between the transition layer and the hot corona numerically using a Monte-Carlo Comptonization code in combination with the photoionization and line transfer code XSTAR. The subset of the parameter space allowed in self-consistent accretion-disk corona systems on energetic grounds is explored, checking for the emergence of emission lines and/or absorption edges, with special emphasis on the spectral range $E \lesssim 1$ keV where such features might become observable with the advent of the AXAF satellite. Comparing our model calculations to the broadband spectrum of GX~339-4, we find good agreement with the observed spectral features and discuss how the future detection of the predicted features at lower energies can be used to constrain parameters.

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arXiv1998

Comptonization signatures in the rapid aperiodic variability of Galactic black-hole candidates

We investigate the effect of inverse-Compton scattering of flares of soft radiation in different geometries of a hot, Comptonizing region and a colder accretion disk around a solar-mass black hole. The photon-energy dependent light curves, their Fourier transforms, power spectra and Fourier-period dependent time lags of hard photons with respect to softer photons are discussed. On the basis of a comparison with existing data we find arguments against Comptonization of external soft radiation as well as Comptonization in a homogeneous medium as dominant mechanisms for the rapid aperiodic variability in Galactic black-hole candidates. Possible further observational tests for the influence of Comptonization on the rapid aperiodic variability of Galactic black-hole candidates are suggested.

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arXiv1998

Spectral Properties of a Hybrid Thermal-Nonthermal Isotropic Plasma

The spectral properties of a hybrid steady thermal-nonthermal isotropic plasma are studied. The involved emission mechanisms are thermal bremsstrahlung, nonthermal bremsstrahlung, cyclotron and nonthermal synchrotron. Formulas are derived to calculate the emissivities and absorption coefficients of all these processes. Calculations show that the nonthermal synchrotron and the thermal bremsstrahlung dominate over most of the frequency range and that the cyclotron and the nonthermal bremsstrahlung are important only at the two ends of the spectrum. The combination of the two dominant processes could generate the synchrotron self-absorbed spectrum, the Rayleigh-Jeans spectrum and an absorbed flatter spectrum which is seen in some solar flare radio spectra. Absorbed spectra steeper than the three types of spectra are rare. The conditions for these three possible spectral types are given. Cyclotron harmonic features are seen in the emergent spectra from the hybrid plasmas. These harmonic features, useful in determining the magnetic field, exist only under certain conditions which are explored in this paper.

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arXiv1997

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 not significantly alter the emission pattern. Compton cooling is more effective in changing emission and absorption coefficients when the photon energy density is high.

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arXiv1997

Confronting Synchrotron Shock and Inverse Comptonization Models with GRB Spectral Evolution

The time-resolved spectra of gamma-ray bursts (GRBs) remain in conflict with many proposed models for these events. After proving that most of the bursts in our sample show evidence for spectral "shape-shifting", we discuss what restrictions that BATSE time-resolved burst spectra place on current models. We find that the synchrotron shock model does not allow for the steep low-energy spectral slope observed in many bursts, including GRB 970111. We also determine that saturated Comptonization with only Thomson thinning fails to explain the observed rise and fall of the low-energy spectral slope seen in GRB 970111 and other bursts. This implies that saturated Comptonization models must include some mechanism which can cause the Thomson depth to increase intially in pulses.

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arXiv1997

Testing the Invariance of Cooling Rate in Gamma-Ray Burst Pulses

Recent studies have found that the spectral evolution of pulses within gamma-ray bursts (GRBs) is consistent with simple radiative cooling. Perhaps more interesting was a report that some bursts may have a single cooling rate for the multiple pulses that occur within it. We determine the probability that the observed "cooling rate invariance" is purely coincidental by sampling values from the observed distribution of cooling rates. We find a 0.1-26% probability that we would randomly observe a similar degree of invariance based on a variety of pulse selection methods and pulse comparison statistics. This probability is sufficiently high to warrant skepticism of any intrinsic invariance in the cooling rate.

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arXiv1997

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.

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arXiv1996

Evolution of the Low-Energy Photon Spectra in Gamma-Ray Bursts

We report evidence that the asymptotic low-energy power law slope alpha (below the spectral break) of BATSE gamma-ray burst photon spectra evolves with time rather than remaining constant. We find a high degree of positive correlation exists between the time-resolved spectral break energy E_pk and alpha. In samples of 18 "hard-to-soft" and 12 "tracking" pulses, evolution of alpha was found to correlate with that of the spectral break energy E_pk at the 99.7% and 98% confidence levels respectively. We also find that in the flux rise phase of "hard-to-soft" pulses, the mean value of alpha is often positive and in some bursts the maximum value of alpha is consistent with a value > +1. BATSE burst 3B 910927, for example, has a alpha_max equal to 1.6 +/- 0.3. These findings challenge GRB spectral models in which alpha must be negative of remain constant.

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