The 2015 hard-state only outburst of GS 1354-64

TL;DR

This paper reports on the 2015 outburst of GS 1354-64, which remained in a hard state throughout, characteristic of a 'failed' outburst, with unique spectral and variability properties compared to typical black hole binaries.

Contribution

It provides the first detailed analysis of a 'failed' outburst in GS 1354-64, highlighting its persistent hard state and distinctive variability behavior.

Findings

GS 1354-64 remained in the hard state during the outburst

Covariance ratio decreased towards lower energies, unlike other black hole binaries

Both sources with 'failed' outbursts show harder photon indices

Abstract

Since its outburst in 1997 GS 135-64 stayed in quiescence. In June 2015 renewed activity of GS 1354-64 was observed. Based on our analysis of energy spectra and timing properties obtained from Swift/XRT monitoring data we found that GS 1354-64 stayed in the hard state during the entire outburst. Such a hard state only (or "failed" outburst) has also been observed in 1997. In addition, we analysed an XMM-Newton observation taken on August 6th. We compared variability on long and short time scales using covariance ratio and found that the ratio showed a decrease towards lower energies instead of the increase that has been found in other black hole X-ray binaries. There are now two sources (H 1743-322 and GS 1354-64) that do not show an increase towards lower energies in their covariance ratio. Both sources have been observed during "failed" outbursts and showed photon indices much harder than what is usually observed in black hole X-ray binaries.