NICER X-ray activity indicates GX 339-4 rising in hard state

ATel #14384; 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), for the NICER team
on 10 Feb 2021; 21:04 UT
Credential Certification: Javier Garcia (javier@caltech.edu)

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GX 339-4 is a low-mass X-ray binary that goes into outburst cycles typically every 2-3 years. X-ray observations with Swift/BAT, MAXI/GSC (ATel #14351, #14367), NuSTAR (ATel #14352) and INTEGRAL (ATel #14354) suggested that the source entered a new outburst during the last week of January 2021 and is brightening.

We triggered NICER Target of Opportunity observations with a near-daily cadence starting on 2021-01-20 (MJD 59234). By 2021-02-07 (MJD 59252) we had obtained a total exposure of ~35 ks. During this period the source brightened rapidly, with the total NICER mean count rate increasing from ~40 counts/s/52FPM to ~520 counts/s/52FPM. The energy spectra in the 0.5-10 keV band can be well approximated by a disk blackbody with Compton up-scattering into a continuum power-law (modeled by simplcut*diskbb; Steiner et al. 2017) and a weak Fe K line feature (modeled by the laor model), absorbed by a moderate Galactic column density. The fitted column density, using Wilms et al. 2000 abundances, is (6.46+/-0.03)E21 cm$-2$, which agrees well with previous X-ray spectroscopy results (e.g., Tomsick et al. 2009). The modeled flux increases from 2.2E-10 to 2.2E-9 erg cm$-2$ s$-1$, corresponding to 7 and 73 mCrab, respectively. Our preliminary fit suggests the disk temperature increased from kT~0.1 to ~0.25 keV, with the disk emission flux increasing from 2.3E-13 to 1.6E-10 erg cm$-2$ s$-1$. The fraction of the disk blackbody flux to the total flux increases from 0.1% to 8%. The source slightly softened as the hardness ratio ([4-12]/[2-4] keV) steadily dropped from 0.4 to 0.36, and the photon index Gamma increased from 1.54$$±0.03 to 1.66$$±0.01.

The light curve and hardness ratio show extreme variability, with large amplitude flaring on timescales of tens of seconds. The power spectra show an integrated 0.1-64 Hz fractional rms of ~40%, dropping from 43% to 35%. Combined with spectral features including the detection of relatively cool thermal disk emission of growing prominence and the hard power-law spectrum with a Gamma ~1.6, the X-ray activity indicates that GX 339-4 is rising in the canonical hard state, and is already twice as bright as it was in the previous two hard-only outbursts (Wang et al. 2020).

NICER will keep monitoring the source on a near-daily cadence. We encourage further multi-wavelength observations as a brightening hard state and possible state transition are in the offing.

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.

Evolution of flux, hardness, and disk emission; fast variability, for an observation in ObsID 3133010113