NICER / ISS Science Nugget for November 1, 2018

NICER watches a flare from a nearby stellar system

On February 9, 2018, an X-ray transient was detected by the MAXI all-sky monitor on-board ISS, consistent with the position of the nearby binary system HR 1099. The stars in this system, also known as V711 Tau, orbit one another every 2.8 days; the more massive of the two is magnetically active, exhibiting sunspots and flares.

In response to the detection alert from MAXI, NICER began observing the Feb 9 flare in its early decay phase, and made an extensive series of sensitive observations over the next four days. The NICER observations show a near-linear decline in X-ray flux for the first 1.5 days, followed by a fairly constant "plateau" level (with a small secondary flare) for the next 1.5 days, and then a resumption of the decline in the final day. At the end of this time, the flux was still a factor of 3-4 times brighter than the star's typical non-flaring level.

The high count rates recorded by NICER of 300-900 photons per second during the decay of this flare enabled time-resolved X-ray spectroscopy on timescales as short as 1 minute or less, yielding measurements of the flare plasma's properties such as temperatures, emission measures, and chemical abundances over the duration of these observations.

The decay phase of a stellar X-ray flare in HR 1099 as seen by NICER
Figure: The large Feb 9, 2018, flare from the binary star system HR 1099, as observed by MAXI (JAXA ISS payload) and NICER. The inset shows the MAXI-measured X-ray brightness, with the double-headed arrow indicating the dates of NICER's triggered observations during the decay phase (colored points in the larger panel). The initial linear decay becomes an extended plateau, during which a sub-flare is detected, on Modified Julian Day 58161. Spectroscopic analysis of the NICER data has revealed the time-evolution of the hot plasma in the flare with unprecedented detail.

Among other results, the NICER data provide evidence – increased absorption of X-rays by intervening gas –¬†for a "coronal mass ejection" by the star in the early hours of the flare.

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