Studying temporal variability of GRS1739-278 during the 2014 outburst

TL;DR

This study reports the detection and analysis of low-frequency QPOs in GRS1739-278 during its 2014 outburst, revealing spectral and timing evolution linked to accretion processes and suggesting a relatively massive black hole.

Contribution

The paper presents the first detection of low-frequency QPOs in GRS1739-278 during outburst and links spectral changes to QPO evolution, providing insights into black hole mass estimation.

Findings

Detected 0.3-0.7 Hz QPOs with harmonic structure.

Observed spectral softening with increasing QPO frequency.

Estimated black hole mass around 100 solar masses.

Abstract

We report a discovery of low-frequency quasi periodic oscillation at 0.3-0.7 Hz in the power spectra of the accreting black hole GRS1739-278 in the hard-intermediate state during its 2014 outburst based on the ${\it NuSTAR}$ and Swift/XRT data. The QPO frequency strongly evolved with the source flux during the NuSTAR observation. The source spectrum became softer with rising QPO frequency and simultaneous increasing of the power-law index and decreasing of the cut-off energy. In the power spectrum, a prominent harmonic is clearly seen together with the main QPO peak. The fluxes in the soft and the hard X-ray bands are coherent, however, the coherence drops for the energy bands separated by larger gaps. The phase-lags are generally positive (hard) in the 0.1-3 Hz frequency range, and negative below 0.1 Hz. The accretion disc inner radius estimated with the relativistic reflection spectral model appears to be $R_{\rm in} < 7.3 R_{\rm g}$. In the framework of the relativistic precession model, in order to satisfy the constraints from the observed QPO frequency and the accretion disc truncation radius, a massive black hole with $M_{\rm BH} \approx 100$M$_\odot$ is required.