NICER X-ray activity shows GX 339-4 undergoing a rapid state transition

ATel #14490; Jingyi Wang (MIT), James F. Steiner (SAO, CfA), Erin Kara (MIT), Andy Fabian (University of Cambridge), Keith Gendreau, Zaven Arzoumanian (NASA/GSFC), Jeroen Homan (Eureka Scientific/SRON), Javier Garcia (Caltech), Phil Uttley (University of Amsterdam), Tomaso Belloni (INAF-OAB), John Tomsick (UC Berkeley), Poshak Gandhi (Southampton) for the NICER team
on 28 Mar 2021; 14:22 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jingyi Wang (jingyiw@mit.edu)

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GX 339-4 is a black hole X-ray binary that started a new outburst in the last week of January 2021 (ATel #14351, #14367, #14352, #14354). After that, the source slowly brightened for 2 months while remaining in the hard state, as suggested by X-ray spectral and timing properties (ATel # 14384, #14400, #14440, #14455, #14484), and optical monitoring (ATel #14419).

NICER observed GX 339-4 with a near-daily cadence from 2021-01-20 (MJD 59234) to 2021-03-20 (MJD 59293), by which time it reached a mean NICER count rate of ~1,200 counts/s/52 FPMs, and a model-determined absorbed flux in 0.5-10 keV of 4.5E-9 erg cm$-2$ s$-1$ (0.15 Crab) using the model Tbabs\*(simplcut\*diskbb+laor). The fitted column density, using Wilms et al. 2000 abundances, is (5.77+/-0.02)E21 cm$-2$. The disk temperature was ~0.3 keV with a thermal flux - to - bolometric fraction of 0.14, and a photon index of ~1.74, confirming the source was still in a bright hard state. After a 6-day visibility gap, NICER monitoring resumed. From the first of these observations on 2021-03-26 (MJD 59299) to 2021-03-28 (MJD 59301.42), the count rate exhibited a dramatic increase at ~2,500 counts/s/52 FPMs, and rose rapidly reaching ~4,600 counts/s/52 FPMs. With the same model, we found the associated increase in the modeled absorbed flux (7.9E-9 to 1.1E-8 erg cm$-2$ s$-1$, i.e., 0.26 to 0.36 Crab), the disk temperature (0.46 to 0.61 keV), the disk flux fraction (0.40 to 0.63), and also the photon index (~2.22 to ~2.79). The dramatic evolution of these spectral parameters all suggest that GX 339-4 has gone into the hard intermediate state and is evolving rapidly towards the soft-intermediate/soft state.

From the power spectra, we also see an evolution in the QPO frequencies and the variability level. Before the visibility gap, the integrated 0.1-64 Hz fractional rms in 0.4- 10 keV was 27% , and a strong QPO at 0.51 Hz ( rms 9%) was present. After the coverage gap, the total fractional rms dropped from 11% to 6%; the QPO moved to higher frequencies (~2.58 Hz to ~5.29 Hz) with lower rms (4.0% to 2.1%). The QPO became more prominent and narrower until MJD 59300.8 (Q factor 8 to 9), and became weaker and blunter at MJD 59301.4 (Q factor of 3). In 4-10 keV, the integrated 0.1-64 Hz fractional rms was 31% before the gap, and dropped slightly from 21% to 18% after the gap. These changes also indicate that the source entered the hard-to-soft state transition during the gap in visibility.

NICER will continue to observe nearly every ~90 minutes for the next couple of weeks, unless we determine it is not worth covering at this cadence. We encourage further multi-wavelength observations as the source has entered a hard-to-soft state transition and is approaching the soft intermediate state, where both QPO type transitions and transient ballistic jets are commonly observed.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.

The hardness-intensity diagram, power spectra, and parameter evolution.