Unveiling Secrets of Black Hole Transient GX 339-4 Revealed

In the vast expanse of space, a binary system known as GX 339-4 has captivated astronomers for decades with its transient nature. Located approximately 8 kiloparsecs from Earth, this black hole X-ray transient is characterized by periods of inactivity punctuated by occasional bright outbursts, marked by rapid fluctuations in spectral and temporal properties.

The study of GX 339-4 has provided valuable insights into the behavior of black hole X-ray binaries (BHXBs), classified into two primary categories: transients and persistent systems. Transient BHXBs like GX 339-4 predominantly remain in a quiescent state, with occasional bright outbursts, whereas persistent systems consistently maintain an active state throughout their observed lifespan.

Researchers have used simultaneous observations from AstroSat and NICER to analyze the combined spectrum obtained from these instruments, revealing that the source is in a low hard state, with a photon index of 1.64. This suggests that the corona is hot and optically thin, producing a dominant hard spectrum. The power density spectra obtained from both observatories exhibit two prominent broad features at 0.22 Hz and 2.94 Hz, indicative of variability in the system.

Further analysis has revealed energy-dependent time lag and fractional root mean square (rms) spectra, indicating that the system is experiencing changes in accretion rate, inner disk radius, coronal heating rate, and scattering fraction. The study of GX 339-4 has provided valuable insights into the behavior of BHXBs, which can be used to understand the behavior of these systems.
## What is a Black Hole X-ray Binary?

A black hole X-ray binary (BHXB) is a type of binary system that consists of a black hole and a companion star. These systems are classified into two primary categories: transient and persistent. Transient BHXBs predominantly remain in a quiescent, low-flux state, while persistent systems consistently maintain an active state throughout their observed lifespan.

The majority of BHXBs are considered to be transients and exhibit phases of quiescence along with occasional bright outbursts, characterized by rapid fluctuations in their spectral and temporal properties. During the outburst, a BHXB transient typically transits through four different spectral states: the hard state (HS), the hard intermediate state (HIMS), the soft intermediate state (SIMS), and the soft state (SS).

The HS is characterized by a dominant hard spectrum with a power-law nature, where nonthermal emission produced by a hot and optically thin plasma known as the corona dominates the X-ray spectra of BHXBs. On the other hand, the SS is characterized by the predominance of soft photons originating from the geometrically thin and optically thick accretion disk.

GX 339-4 is a black hole X-ray transient that was observed during its 2021 outburst period using simultaneous observations from AstroSat and the Neutron Star Interior Composition Explorer (NICER). The combined spectrum obtained from NICER’s Large Area X-ray Proportional Counter and SXT data is effectively described by a model comprising a thermal disk component, hard Comptonization component, and reflection component with an edge.

The analysis of the AstroSat and NICER spectra indicates that GX 339-4 was in a low-hard state with a photon index of 1.64. The power density spectra obtained from both observations exhibit two prominent broad features at 0.22 Hz and 2.94 Hz, which were used to generate energy-dependent time lag and fractional root mean square (rms) at both frequencies.

The power density spectra obtained from AstroSat and NICER observations exhibit two prominent broad features at 0.22 Hz and 2.94 Hz, which are indicative of the variability in GX 339-4’s X-ray emission. The energy-dependent time lag and fractional rms were generated at both frequencies in a broad energy range of 0.5-30 keV.

The presence of hard lags along with a decrease in variability at higher energy levels was observed, indicating that the variability in GX 339-4 is not uniform across all energies. Additionally, the correlated variations in accretion rate, inner disk radius, coronal heating rate, and scattering fraction, along with a delay between them, can explain the observed rms and lag spectra for both features.

## What are the Implications of the Energy-dependent Time Lag?

The energy-dependent time lag is an important aspect of understanding the variability in GX 339-4’s X-ray emission. The presence of hard lags indicates that the variability in the high-energy band (0.5-30 keV) is delayed compared to the low-energy band.

This delay can be attributed to the different timescales associated with the accretion disk and corona, which are responsible for producing the soft and hard X-ray emission, respectively. The energy-dependent time lag provides valuable insights into the physical processes occurring in GX 339-4 during its outburst period.

The accretion rate and inner disk radius play a crucial role in understanding the variability in GX 339-4’s X-ray emission. The correlated variations in these parameters, along with a delay between them, can explain the observed rms and lag spectra for both features.

The accretion rate is responsible for producing the soft X-ray emission from the accretion disk, while the inner disk radius determines the size of the accretion disk. The energy-dependent time lag indicates that the variability in the high-energy band (0.5-30 keV) is delayed compared to the low-energy band.

The study of GX 339-4 provides valuable insights into understanding black hole X-ray binaries, particularly during their outburst period. The energy-dependent time lag and fractional rms at both frequencies in a broad energy range of 0.5-30 keV are indicative of the variability in GX 339-4’s X-ray emission.

The correlated variations in accretion rate, inner disk radius, coronal heating rate, and scattering fraction, along with a delay between them, can explain the observed rms and lag spectra for both features. This study highlights the importance of considering the energy-dependent time lag when understanding the variability in black hole X-ray binaries.

The study of GX 339-4 provides a foundation for future research into understanding black hole X-ray binaries, particularly during their outburst period. The energy-dependent time lag and fractional rms at both frequencies in a broad energy range of 0.5-30 keV are indicative of the variability in GX 339-4’s X-ray emission.

Future studies should focus on investigating the physical processes responsible for producing the energy-dependent time lag and fractional rms, as well as exploring the implications of these findings for understanding black hole X-ray binaries.