# Re-awakening of GRS 1716--249 after 23 years, observed by {\it Swift}/XRT and {\it NuSTAR}

## TL;DR

This study reports the first detection of a type-C QPO in GRS 1716--249 during its 2016-2017 outburst, revealing spectral-timing correlations and constraining the inner disk radius after 23 years of quiescence.

## Contribution

It provides the first detailed spectral and timing analysis of GRS 1716--249 during its recent outburst, including the detection of a QPO and disk radius estimation.

## Key findings

- Detection of a 1.20 Hz QPO drifting to 1.55 Hz.
- Spectral analysis indicating cold disk and thermal Comptonization.
- Inner disk radius constrained to about 10-12 R_ISCO.

## Abstract

In this work, we present a spectral and temporal analysis of {\it Swift}/XRT and {\it NuSTAR} observations of GRS 1716--249 during its recent 2016--2017 outburst. This low mass X-ray binary underwent an extraordinary outburst after a long quiescence of 23 years, since its last major outburst in 1993. The source was observed over two different epochs during 2017 April, 07 and 10. The best fit joint spectral fitting in the energy range 0.5 $-$ 79.0 keV indicates that the spectrum is best described by relatively cold, weak disk blackbody emission, dominant thermal Comptonization emission, and a relativistically broadened fluorescent iron K$\alpha$ emission line. We observed a clear indication of a Compton hump around 30 keV. We also detected an excess feature of $\sim1.3$ keV. Assuming a lamp-post geometry of the corona, we constrained the inner disk radius for both observations to 11.92$^{+8.62}_{-11.92}$ R$_{ISCO}$ (i.e., an upper limit) and 10.39$^{+9.51}_{-3.02}$ R$_{ISCO}$ (where R$_{ISCO}\equiv$ radius of the innermost stable circular orbit) for the first epoch (E1) and second epoch (E2), respectively. A significant ($\sim5\sigma$) type$-$C quasi-periodic oscillation (QPO) at $1.20\pm0.04$ Hz is detected for the first time for GRS 1716--249, which drifts to $1.55\pm0.04$ Hz ($\sim6\sigma$) at the end of the second observation. The derived spectral and temporal properties show a positive correlation between the QPO frequency and the photon index.