NICER / ISS Science Nugget
for April 9, 2020

NICER observes an outburst of the Rapid Burster

The Rapid Burster (also known as MXB 1730-335) is an accreting neutron star in the globular cluster Liller 1. The source is well known for its unique bursting behavior. In addition to (type I) X-ray bursts due to thermonuclear explosions on the surface of the neutron star, the Rapid Burster also shows a type of X-ray bursts (type II) that is caused by a sudden increase in the amount of matter falling onto the neutron star surface. It is one of only two sources known to show these type II bursts. The origin of the type II bursts is still not fully understood, although it is suspected that interaction of the accretion flow with the neutron-star magnetic field plays an important role.

In March of this year the Rapid Burster, which is a transient system, went into outburst again. NICER observed the source for a total exposure of ~20 ks, covering various phases of its month-long outburst. In Figure 1 the full light curve of the outburst can be seen, showing hundreds of type II bursts. The type II bursts are not present at the highest and lowest mass accretion rates (i.e. beginning and end of our coverage), and their strength and rate of occurrence evolves throughout the outburst.

In Figure 2 we show a single NICER snapshot from March 2020. It reveals one of the most interesting aspects of the type II burst behavior: the longer/stronger the burst the longer the wait time for the next burst. This hints at the presence of a reservoir from which matter can be released (creating a type II burst) and then replenished at a constant rate, so that the strength of the burst determines the time to refill the reservoir.

Using NICER's low-energy band, large collecting area, and high time-resolution, we will be able to better study the evolution of the accretion flow during the type II bursts and hope to improve our understanding of what makes the Rapid Burster burst.