NICER / ISS Science Nugget
for July 13, 2023

Good news from XB-NEWS

Understanding accretion -- the transfer of matter from one star to another -- is important because it is common to so many astrophysical systems and a key driver of their long-term evolution. Complicating the study of accretion is the fact that most mass-transfer processes are episodic rather than persistent or long-lived, especially where neutron stars and black holes are the recipients of their companion stars' matter. Especially important is understanding how accretion begins: what triggers it and how the flow of matter organizes itself. At X-ray wavelengths, it's common to catch an accreting system in full outburst with all-sky monitoring instrumentation (such as JAXA's MAXI payload), but by the time X-ray emission is typically detected, the accretion flow is already well established. To address this challenge, teams of astronomers have set up a global network of ground-based, robotic telescopes to regularly observe a "usual suspects" list of known X-ray binaries for signs of brightening at visible-light wavelengths as an indicator of incipient accretion activity. One such effort is XB-NEWS, the X-ray Binary New Early Warning System, an automated data-analysis pipeline that uses the Faulkes Telescopes in Hawaii and Australia (for northern- and southern-sky coverage) in partnership with other telescopes of the Las Cumbres Observatory (LCO) network.

On July 2nd, XB-NEWS reported optical brightening of the transient low-mass X-ray binary MAXI J1807+132, originally discovered in a 2017 outburst. NICER observations were requested on July 5; those initial data revealed no obvious X-ray emission. But by the 8th of July, MAXI J1807 began to brighten rapidly, with NICER's continued coverage providing a valuable record of the onset of accretion. In recent days, MAXI J1807's brightness appears to have peaked and may already have begun its decline, but historically MAXI J1807 has exhibited brightness fluctuations on sub-day timescales, so its immediate future is unclear. NICER previously observed this system during its 2019 outburst and revealed that it harbors a neutron star by detecting thermonuclear explosions on its surface -- the "Type I" X-ray bursts that result from these detonations are never seen from black holes, which lack surfaces onto which accreted fuel can accumulate.

The renewed activity of MAXI J1807 and the triggering of NICER observations by the LCO optical monitoring campaign were reported by G. Illiano (Italian National Institute for Astrophysics) and collaborators via Astronomer's Telegram #16125.