NICER / ISS Science Nugget for November 15, 2018

Observations of new X-ray transient show intense flaring episodes

Swift J1858.6-0814 was discovered with the Burst Alert Telescope on NASA's Swift observatory as a new, flaring X-ray source first detected on 2018 October 25 (Krimm et al. 2018, ATel #12151). This alert led to observations from both space and ground-based facilities.

NICER first observed Swift J1858.6-0814 on November 1 and confirmed prominent X-ray flaring on a timescale of seconds (Ludlam et al. 2018, ATel #12158). A radio counterpart was identified (Bright et al. 2018, ATel #12184), indicating the presence of a jet. Optical observations show complex, fast flaring at both blue and red wavelengths (Paice et al. 2018, ATel #12197).

All of these properties suggest that the source is an accreting compact object, but the accretor may be either a black hole or a neutron star. There is only one precedent for this kind of behavior, and that is the transient black hole binary known as V404 Cygni (e.g., Kimura et al. 2016, Nature 529, 54; Gandhi et al. 2016, MNRAS 459, 554).

During the first 11 days of November 2018, NICER observed Swift J1858.6-0814 for 48 intervals, with exposures lasting 90 to 1900 s. Three sample light curves are plotted in the figure. The apparent prototype for "crazy flares," V404 Cyg, did not produce outbursts while fast X-ray timing instruments, such as NICER or RXTE, were operating. We therefore have a special opportunity with NICER and the new X-ray source, Swift J1858.6-0814, to find the underlying cause of these remarkable X-ray flares.

Figure: The NICER count rate (0.4–12 keV) versus time, in 1-second bins for three sample NICER observations. These data (observations 3, 7, and 8) illustrate the intense flaring that is characteristic of this source.

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