NICER / ISS Science Nugget
for May 27, 2021
Successful Application of NICER's New Sky Scanning Capability
In October 2020, a NICER flight-software (FSW) update took an important initial step toward implementation of the new OHMAN project (the On-Orbit Hookup of MAXI and NICER). JAXA's payload MAXI, on JEM-EF, offers a wide field-of-view look at the X-ray sky, scanning 95% of the celestial sphere every ISS orbit, with the objective of finding transient and variable objects. NICER, by contrast, has a much smaller field of view but offers higher sensitivity, as well as energy and time resolution. MAXI has historically triggered many "Targets of Opportunity" (TOOs) for NICER, which have represented scientific windfalls. One significant upgrade in NICER FSW was the ability to scan NICER's small FOV around a patch of the sky consistent with MAXI's error box on localizations of new X-ray transients.
On May 2nd, 2021, MAXI detected a new X-ray source that is now called MAXI J1803-298. MAXI's localization was only good to about +/-0.3 degrees, large compared to NICER's FOV of 0.1 deg diameter. The NICER team employed the new FSW scanning capability and accurately determined the position of MAXI J1803-298 on the sky to better than 3 arcminutes (see figure). This was the first successful application of the new scanning capability in a real-world test with a completely unknown target. NICER communicated this result in an Astronomers Telegram. Subsequently, the X-ray Telescope on NASA's Swift observatory refined the position to better than 1 arcsecond.
NICER's ability to scan is important for the rapid follow up of new transients, which may only last for a short time. This new capability will be used frequently when the full OHMAN project comes online in March 2022. OHMAN will be fully realized when an Express Rack Laptop Computer in the JEM pressurized segment will automatically find new TOOs in raw MAXI data collected live. This laptop computer will communicate the TOO's approximate coordinates to NICER via the Unique Ancillary Data (UAD) channel on ISS, and NICER will quickly home in on the target.
Figure: When NICER followed up on the MAXI J1803-298 TOO, it executed a grid search in Right Ascension and Declination coordinates on the sky, indicated by the exposure map panel A. Raw X-ray counts mapped to this raster scan are seen in panel B. Panel C shows the exposure-corrected count rates, from which the NICER team could determine the source position. Finally, panel D is a smoothed image of this result. In the last two panels, the black point and circle represent the best-fit NICER position and its uncertainty; the star indicates the position later determined by the Swift X-ray Telescope. Figure credit: T. Enoto (RIKEN, Japan).