NICER / ISS Science Nugget
for February 11, 2021

NICER studies a Black Hole Binary in Outburst

GX 339-4 is a low-mass X-ray binary (LMXB), a system comprising a black hole and a low-mass companion star. Since its discovery in 1973, GX 339-4 has been through more than a dozen outburst cycles, typically every 2-3 years. In an outburst, GX 339-4 rises from the quiescent state, when the accretion rate onto the black hole is low and thus the X-ray emission is weak, into the hard state, when the X-ray spectrum is dominated by a high-energy component resulting from the collision of fast-moving electrons with X-ray photons from the accretion disk. The source luminosity increases until it transitions to the soft state when the thermal disk emission dominates. The luminosity then drops, and the system makes its transition back to the hard state and then quiescence, completing an outburst cycle. The variety of accretion states comes with a high level of multiwavelength activity and phenomenology, in one single system, and on a human timescale. NICER's large collecting area and superior timing resolution enable measurements through many observations in a single outburst, to track key physical parameters. This makes NICER observations of LMXBs like GX 339-4 important for studying accretion and ejection physics in the strong gravity regime around black holes.

During the last week of January 2021, GX 339-4 started to show flaring in X-rays, following activity in the radio band. We triggered NICER Target of Opportunity observations with a near-daily cadence starting on 2021-01-20. By 2021-02-07, NICER obtained a total exposure of approximately 35 ksec. During this period, the source brightened rapidly, with the NICER mean count rate increasing from ~40 counts/s to ~520 counts/s, and the source flux increasing from 7 to 73 mCrab. The modeled disk temperature increased up to approximately 2.5 million degrees Kelvin, with an increasing contribution to the total flux.

The NICER data show extreme variability in the light curve and hardness ratio (count rate ratio in hard energy band to soft band) of GX 339-4, with large-amplitude flaring on timescales of tens of seconds. The power spectra show an integrated 0.1-64 Hz fractional root-mean-square deviation of ~40%, dropping from 43% to 35% over time. Combined with spectral features including the detection of relatively cool thermal disk emission of growing prominence and the hard power-law spectrum, the X-ray activity indicates that GX 339-4 is rising in the traditional hard state, and is already twice as bright as it was in the previous two hard-state-only outbursts. NICER will keep monitoring the source on a near-daily cadence, as a brightening hard state and possible soft-state transition are in the offing. These initial NICER observations were reported by Wang et al. in Astronomer's Telegram #14384.