NICER / ISS Science Nugget
for April 4, 2024

8-second pulses end 10-year slumber

Like a ringing alarm clock, the detection of X-ray pulsations signals the awakening of a neutron-star binary system in an accretion outburst. After an unpredictable amount of time in quiescence, most outbursts in such systems are short-lived and of modest brightness. But occasionally, transfer of matter from a companion star to the rapidly spinning neutron star is prolonged and prodigious, resulting in a giant outburst. If we are fortunate enough to catch the outburst's rising phase, the accretion process unfolds before our eyes: the flow of matter forms a disk - heated by turbulence to million-degree temperatures - around the neutron star; and, from the disk's inner edge, the flow continues to the neutron star's surface channeled by its powerful magnetic field. The resulting column of accreting material, swept around by the neutron star's rotation, emits X-rays that appear, to a distant observer, to be pulsed.

The known binary system RX J0520.5-6932 resides in the Large Magellanic Cloud, a dwarf galaxy gravitationally bound to our Milky Way approximately 165,000 light-years away. RX J0520 was last seen in outburst in 2014. This week, the Pavlinsky ART-XC telescope on-board Russia's SRG observatory detected RX J0520 at a higher-than-expected flux level. Given the brightness and the distance to the LMC, the ART-XC measurement suggested the beginning of a rare giant outburst. Based on that report, NICER received a target-of-opportunity request for observations to search for pulsations at the neutron star's known spin period of 8.2 seconds. In its first, brief observation, NICER readily detected those pulses (see Figure), confirming that RX J0520 is newly in outburst. This result was reported by R. Sharma (Raman Research Inst., India) and collaborators in Astronomer's Telegram #16569. The pulse profile is found to be asymmetric - slower rise, more rapid decline - while the spectrum is generally dominated by lower-energy X-rays, likely from the accretion disk. These details are important for understanding the physics of the accretion flow in the extreme strong-gravity, high density and temperature, and strong magnetic-field environment. Subsequent observations show the overall brightness of RX J0520 to be rising, and NICER is continuing to monitor this revived giant.