The outburst of the X-ray nova GRS 1739-278 in September, 2016

TL;DR

This paper reports the detection and analysis of a failed outburst of the X-ray nova GRS 1739-278 in 2016, characterized by a low/hard spectral state and insufficient accretion to reach a high/soft state.

Contribution

First detailed broadband spectral and temporal analysis of the 2016 outburst of GRS 1739-278, identifying it as a failed outburst in the low/hard state.

Findings

Flux increased from 11 to 30 mCrab in 20-60 keV band.

Spectrum fitted by absorbed powerlaw with photon index ~1.86.

Outburst was 'failed', not reaching high/soft state.

Abstract

During the scanning observations of the Galactic Center region in August - September 2016 we detected the new outburst of the historical X-ray nova GRS 1739-278, the black hole candidate LMXB system. In this letter we present results of INTEGRAL and Swift-XRT observations taken during the outburst. In hard X-ray band (20-60 keV) the flux from the source raised from $\sim$11 to $\sim$30 mCrab between 3 and 14 of September. For nearly 8 days the source has been observed at this flux level and then faded to $\sim$15 mCrab. The broadband quasi-simultaneous spectrum obtained during the outburst is well described by the absorbed powerlaw with the photon index $\Gamma=1.86\pm0.07$ in broad energy range 0.5-150 keV, with absorption corresponding to ${N_{H}}=2.3\times10^{22}$ cm$^{-2}$ assuming solar abundance. Based on this we can conclude that the source was in the low/hard state. From the lightcurve and spectra we propose that this outburst was `failed', i.e. amount of accreted matter was not sufficient to achieve the high/soft spectral state with dominant soft blackbody component as seen in normal outbursts of black hole candidates.