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Recent Science from NICER on the ISS

Ironing Out a Black Hole Binary

With millions of black holes and neutron stars throughout the galaxy that are only detectable when they are active, X-ray outbursts often mean a chance to discover a previously-unknown system. But occasionally, we discover outbursts from previously-detected systems. V4641 Sgr is one such source. Originally discovered during a bright, short outburst in 1999, V4641 is a black hole 6.4 times the mass of the sun in a 2.8 day orbit around a B star, roughly three times as massive as the sun. X-ray spectra of this outburst indicated a strong iron emission line, and since its discovery, 4641 has exhibited several small rebrightenings.

On October 12, MAXI detected a new outburst from V4641 Sgr, which was quickly confirmed by Swift, INTEGRAL, and NICER, which has been monitoring the source regularly ever since. Our data provide a sensitive new look at the iron line; these peaks in the energy spectrum can often be used to study effects of General Relativity, like the spin of the black hole. But the NICER spectrum shows a surprise: instead of the classic broadened iron line, we see two narrow lines at 6.7 and 7 keV (which indicates the presence of multi-million-degree iron). Comparing our NICER spectra to recent observations with the hard X-ray telescope NuSTAR may help determine the nature and location of this superheated iron gas (and what it tells us about the latest activity of V4641 Sgr).


 Average NICER spectrum of V4641 Sgr (red) with a continuum model (black) fitted to energies away from strong emission lines. The continuum may be a hard Comptonised spectrum or thermal emission from a hot accretion disk around the black hole. Bottom: Ratio of the spectrum to the continuum model, highlighting the appearance of the lines. The narrow lines near 7 and 8 keV and the broad bump near 1 keV may be signatures of X-rays reflecting off hot gas.



Figure: Average NICER spectrum of V4641 Sgr (red) with a continuum model (black) fitted to energies away from strong emission lines. The continuum may be a hard Comptonised spectrum or thermal emission from a hot accretion disk around the black hole. Bottom: Ratio of the spectrum to the continuum model, highlighting the appearance of the lines. The narrow lines near 7 and 8 keV and the broad bump near 1 keV may be signatures of X-rays reflecting off hot gas.

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