Detection of X-ray spectral state transitions in mini-outbursts of black hole transient GRS 1739−278

Abstract

We report the detection of the state transitions and hysteresis effect in the two minioutbursts of the black hole (BH) transient GRS 1739−278 following its 2014 major outburst. The X-ray spectral evolutions in these two mini-outbursts are similar to the major outburst, in spite of their peak luminosities and the outburst durations are one order of magnitude lower. We found the L hard−to−soft and the L peak,soft of the mini-outbursts also follow the correlation previous found in other X-ray binaries. The L hard−to−soft of the mini-outbursts is still higher than that of the persistent BH binary Cyg X-1, which supports that there is a link between the maximum luminosity a source can reach in the hard state and the corresponding non-stationary accretion represented by substantial rate-of-change in the mass accretion rate during flares/outbursts. The detected luminosity range of these two mini-outbursts is roughly in 3.5×10The X-ray spectra of other BH transients at such low luminosities are usually dominated by a power-law component, and an anti-correlation is observed between the photon index and the X-ray luminosity below 1% L Edd . So the detection of X-ray spectral state transitions indicates that the accretion flow evolution in these two mini-outbursts of GRS 1739−278 are different from other BH systems at such low-luminosity regime.

Radio/X-Ray Correlation in the Mini-outbursts of Black Hole X-Ray Transient GRS 1739–278

“…As shown in the top panel of Figure 1, state transitions, typically observed in the main outbursts, are also observed in the first two mini-outbursts. Note that Yan & Yu (2017b) found that the much-fainter soft state in mini-outbursts likely follows the same tight L bol ∝ T 4 (bolometric luminosity L bol and representative temperature of the cold disk T ) relationship as determined by that in the major 2014 outburst, suggesting that the cold disk is also not truncated in these soft states of mini-outbursts.…”

“…21 (MJD 57194), that the system is in the soft state. 4 The X-ray observations show that this source enters into the soft state about 3 days ago, on MJD 57191 (more accurately, between MJD 57190.48 and MJD 57191.67, see Figure 1 and Yan & Yu 2017b.). As shown in Figure 3, the X-ray spectrum on MJD 57194 is wellfitted under the adopted model, where the disk component contributes ≈ 88% of total flux in 1-10 keV.…”

“…The X-ray lightcurve of GRS 1739-278 of the 2015-2016 mini-outbursts is shown in the top panel of Figure 1. The Xray properties are analyzed and discussed in detail in Yan & Yu (2017b). As shown in the top panel of Figure 1, state transitions, typically observed in the main outbursts, are also observed in the first two mini-outbursts.…”

“…For those with high count rate (> 0.6 counts s −1 for the photon counting mode, and > 150 counts s −1 for the 10 -2 10 -1 10 0 Unabsorbed X-ray flux windowed timing mode), the events in the central region that suffer pile-up effect are excluded. The details can be found in Yan & Yu (2017b). Below we provide a brief description on the spectral modeling.…”

“…more than one year. The peak X-ray luminosity of the 2014 outburst reaches ∼ 5 × 10 38 erg s −1 (Yan & Yu 2017b;Wang et al 2018). Thanks to the intense X-ray monitoring of Swift afterwards, a series of minioutbursts have been discovered, where state transitions are also detected in the first two mini-outbursts (see Yan & Yu 2017a,b and also Figure 1).…”