Evolution of the reverberation lag in GX 339–4 at the end of an outburst

Abstract

We studied X-ray reverberation lags in the BHXRB GX 339-4 at the end of the 2014-2015 outburst. We analysed data from a XMM-Newton campaign covering the end of the transition from the soft to the hard state, and the decrease of luminosity in the hard state. During all the observations we detected, at high frequencies, significant disc variability, responding to variations of the power law emission with an average time delay of ∼ 0.009 ± 0.002 s. These new detections of disc thermal reverberation add to those previously obtained and suggest the lag to be always present in hard and hard-intermediate states. Our study reveals a net decrease of lag amplitude as a function of luminosity. We ascribe this trend to variations of the inner flow geometry. A possible scenario implies a decrease of the inner disc truncation radius as the luminosity increases at the beginning of the outburst, followed by an increase of the inner disc truncation radius as the luminosity decreases at the end of the outburst. Finally, we found hints of FeK reverberation (∼ 3σ significance) during the best quality observation of the XMM monitoring. The lag at the FeK energy has similar amplitude as that of the thermally reprocessed component, as expected if the same irradiated region of the disc is responsible for producing both the thermalized and reflected component. This finding suggests FeK reverberation in BHXRBs to be at the reach of current detectors provided observations of sufficiently long exposure are available.

Physical Constraints from Near-infrared Fast Photometry of the Black Hole Transient GX 339–4

“…For geometrically thin disks, such relatively short (seconds) time-scales of large-amplitude (tens of per cent) disk continuum variability, presumably generated on the local viscous time-scale, would be difficult to explain if the disk emission originates at > 100 R G , as would be implied by the hot inflow model as presented in Veledina et al (2013). On the other hand, on even shorter variability time-scales (> 1 Hz), blackbody 'reverberation lags' are seen, some of which, if simply interpreted as light-travel times, could be consistent with such large radii (De Marco et al 2017), but interpretation of such lags is complex, as their combination with the continuum lags is not simply additive (e.g. see Mastroserio et al 2019 for the case of Fe K reverberation plus continuum lags in BHXRBs).…”

“…Casella). The full X-ray dataset has been already described in detail by De Marco et al (2017), who analyzed the evolution of the reverberation lag, by Stiele & Kong (2017), who reported timing and spectral analysis, and by Wang-Ji et al (2018), who focused on the spectral properties. For an overview of the whole X-ray dataset we refer the reader to these works.…”

Accretion Disks and Coronae in the X-Ray Flashlight

“…Studies of these time delays in BH LMXBs indicate that there is a significant change when moving from the soft to the hard state, which may indicate that the inner disk moves outwards in the hard state (e.g. De Marco et al 2015;De Marco and Ponti 2016;De Marco et al 2017). Reverberation can possibly be observed in NS LMXBs as well (Barret 2013;Cackett 2016), but needs to be further developed.…”

A public relativistic transfer function model for X-ray reverberation mapping of accreting black holes

“…In particular, signatures of thermal reverberation have been detected for a number of black hole X-ray binaries including GX 339-4 (Uttley et al 2011;De Marco et al 2017). In the 1 − 30 Hz frequency range, log-linear intrinsic lags are seen for E 1 keV and a turn up is seen for E 1 keV, which is attributed to thermal reverberation (see top right of Fig 7 in De Marco et al 2017). We investigate how this thermal reverberation signal may have been affected by the instrument response by first assuming that the intrinsic lag spectrum in the 1−30 Hz frequency range is simply log-linear for the full energy range (Fig 14, black line).…”

“…However, the observation of GX 339-4 that our model is based upon has a number of characteristics that convincingly point to the presence of thermal reverberation. In particular, De Marco et al (2017) (2017) and . The red line is the time lag that would be observed by XMM-Newton assuming that the intrinsic lag-energy spectrum is given by the black (log-linear) line.…”