NICER / ISS Science Nugget for April 11, 2019

NICER Observations of the Recurrent Black Hole Transient, XTE J1908+094

In Astronomers Telegram #12652, the NICER team confirms that the black hole in the binary system known as XTE J1908+094 is actively accreting again in a "soft" X-ray state. Renewed X-ray emission from this source was first reported by the INTEGRAL telescope (ESA) on 2019 April 1. The identification of the source was confirmed with a precise X-ray position from NASA's Neil Gehrels Swift Observatory on April 4 (ATel #12632; Miller et al. 2019).

NICER observed the source on April 6 (576 sec exposure) and April 9 (2555 sec). A detailed spectral fit indicates the presence of an accretion disk with an inner disk temperature of 0.62 keV. What NICER measures in these black hole binaries is the integrated spectrum from an X-ray ring that would look very much like the event-horizon ring that was recently imaged by radio astronomers with the Event Horizon Telescope, and reported widely in the press. While we cannot image the ring in X-rays, the NICER spectrum does convey the temperature and size of the inner rim of the accretion disk.

In the case of XTE J1908, the spectrum indicates a soft X-ray state. This distinction is significant because the soft state is dominated in X-rays by the glow of the accretion disk, while the hard state indicates the additional presence of bipolar jets of outflowing hot gas. One of the unique capabilities of NICER is the opportunity to track the accretion disk properties (i.e., size and temperature), even when jets form and the disk temperature recedes to lower values, e.g., 0.1-0.2 keV. What the disk size does across the hard/soft transition is a matter of intense debate. Does the inner disk rim expand when the jet is operating, or does the jet feed from the disk right at the innermost edge allowed in General Relativity?

Accreting Black Hole XTE J1908+094 spectrum and light curve

Figures: (Left) The NICER spectrum of XTE J1908+094, fit with a simple accretion disk model plus interstellar absorption. The spectrum and fit are in the top panel and the fit residuals are shown in the bottom panel. (Right) The light curve of XTE J1908, in 1-second bins, shows the gentle flickering of a quasi-steady source, typical for short observations of accretion disk radiation.

This is the 25th black-hole binary observed with NICER, for a mission that is not yet two years old! Conventional wisdom would have expected that NICER would have the chance to observe the six known persistent black-hole binaries (Milky Way and Large Magellanic Cloud), plus 2–3 new transients per year. The yield shows just how fantastically rich the transient sky has been since June 2017.

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