NICER / ISS Science Nugget
for August 25, 2022

A New Outburst from a Rosetta Stone Pulsar

The accreting millisecond pulsar known as SAX J1808.4-3658 has been described as a Rosetta Stone for pulsar astrophysics - it brings together, in a single object, multiple phenomena that had been seen previously only across multiple unrelated systems. Thus, SAX J1808 offers the prospect of understanding relationships between accretion processes (outburst evolution, matter flows in disks and channeled onto magnetic poles, "reflection" of X-rays from accretion disks, winds and other outflows) and the physics of and around the central neutron star (persistent pulsations from surface hotspots, thermonuclear X-ray bursts, burst oscillations, etc.).

Discovered in 1996, SAX J1808 is generally quiescent and faint, but goes into outburst approximately every 3 years for several weeks. Following a hint of excess brightness from the direction of SAX J1808 seen by JAXA's MAXI payload this week, NICER confirmed with a brief snapshot observation that SAX J1808 has begun its 10th known outburst (see Figure), and reported this finding in an Astronomers Telegram (https://www.astronomerstelegram.org/?read=15559). NICER is continuing to monitor the system intensively. Similar monitoring of the 2019 outburst resulted in at least three peer-reviewed publications, with additional in-depth analysis of those earlier data still in work.

Among many questions about SAX J1808 is the nature of its long-term orbital evolution. Observations over the past decade show a general trend of contraction of the 2-hour orbit, but earlier data suggested expansion. Preliminary measurements of the neutron star's orbital phase in the current outburst show that it reached ascending node 15 sec later than predicted, suggesting oscillatory long-term behavior that will be fully explored in this and future outbursts.