NICER / ISS Science Nugget
for December 3, 2020
On November 23rd, 2020, the NICER team executed its first payload Flight Software (FSW) update since launch in 2017. The update, which does not affect any interfaces to ISS, enables streamlined Target of Opportunity (TOO) processing. Normally, the NICER operations team prepares a command load for the payload twice a week. These command loads direct NICER to follow a sequence of celestial pointings over a 1-2 week period - NICER will track one target over a specific time interval and then slew to the next target. These plans are optimized to maximize time on science targets (with typically 85 - 90% efficiency), minimizing slewing and idle time between targets. NICER is always moving when in science mode. These target observation plans fulfill requirements from scientists around the world, providing data to address their peer-reviewed science objectives. Occasionally, these plans need to be interrupted for TOOs.
TOOs come from a variety of sources. Typically, a wide-field-of-view astrophysics observatory detects new transient behavior from either a known target or a previously unknown X-ray source in the sky. Investigators interested in understanding these transient phenomena suggest NICER follow-up observations using an online request form, and the ops team evaluates the feasibility of the request. Oftentimes, the follow-up must happen quickly; to accomplish this, we interrupt the existing onboard science plan and insert the TOO observation.
The recent FSW update greatly facilitates the process of inserting TOO observations in an efficient way, to maintain maximum NICER science productivity. In addition, the update is capable of executing a local grid search on the sky (see Figure) whenever the reported TOO's location is imprecisely known. The FSW update also lays the groundwork for a future, more automated process, in which NICER will automatically follow up triggers from the ISS/JAXA payload Monitor of All-sky X-ray Image (MAXI) on JEM-EF. MAXI offers a wide field of view and has provided a great number of fruitful TOO triggers - activated from the group - to NICER. The NICER and MAXI teams are working together toward implementing this new no-ground-intervention triggering capability as part of a project called "OHMAN," the On-orbit Hookup of MAXI and NICER.
Figure: Sky pointing offset angles (in radians) forming a grid over which NICER will search for precise localization of a poorly known target-of-opportunity source of X-ray emission.