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NICER / ISS Science Nugget
for May 12, 2022



NICER Disentangles Adjacent X-ray Emitters

On April 28, the JAXA MAXI payload team alerted the NICER team to excess X-ray emission from the direction of the known black-hole binary system EXO 1846-031, last seen in outburst in the summer of 2019 (see Weekly Science Summary for 16 January, 2020). Its previous outburst was in 1985, so a new outburst was somewhat surprising. Closer inspection of the MAXI data suggested the possibility that another known source might be responsible for the emission: the soft, faint X-ray transient (SFXT) IGR J18483-0311, a likely neutron star system 0.27 degrees away from EXO 1846 and present within the localization uncertainty region derived by MAXI.

Within a few hours, NICER observed EXO 1846 and saw no evidence of recent brightening. A repointing to IGR J18483 was promptly commanded in real time, and here NICER recorded an elevated countrate (see Figure), confirming the association of the original MAXI detection with the SFXT source. Additional exposure accumulated on subsequent orbits yielded the X-ray spectrum of IGR J18483 and an estimate of its luminosity in the rapidly fading outburst, two orders of magnitude higher than a measurement in quiescence made by ESA's XMM-Newton telescope in 2015.

Figure: Top: Measured X-ray lags (colored points) are shown in the context of theNICER X-ray detection count rate during an observation of two adjacent targets, the black-hole system EXO 1846 and the neutron-star system IGR J18483, with a brief slew between them (at approx. 270 seconds in this plot). The initial rate of a few counts per second while pointed at EXO 1846 is consistent with background in NICER's detectors, while the higher ~8 cts/sec rate during the IGR J18483 pointing indicated renewed activity of this faint transient. (The data dropout at ~500 sec due to a real-time telemetry gap was later filled in with data recovered from NICER's onboard recorder.)



Figure: NICER X-ray detection count rate during an observation of two adjacent targets, the black-hole system EXO 1846 and the neutron-star system IGR J18483, with a brief slew between them (at approx. 270 seconds in this plot). The initial rate of a few counts per second while pointed at EXO 1846 is consistent with background in NICER's detectors, while the higher ~8 cts/sec rate during the IGR J18483 pointing indicated renewed activity of this faint transient. (The data dropout at ~500 sec due to a real-time telemetry gap was later filled in with data recovered from NICER's onboard recorder.)



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