NICER / ISS Science Nugget
for February 25, 2021

NICER TOO Observation of 2S 1553-542

On February 23, 2021, NICER executed a Target of Opportunity (TOO) observation of the high mass X-ray binary system known as 2S 1553-542, which consists of a neutron star and a 'Be'-type star (a hot, massive star that exhibits emission lines from a gaseous disk). 2S 1553 is located more than 15 kpc (about 288,000 trillion miles) from the Earth, making it one of the most distant X-ray binaries known within our Milky Way Galaxy. The pulsar in 2S 1553 is a mostly quiescent accretor and only rarely shows bright outbursts. The outburst currently in progress, the first since NICER's launch, offers an opportunity to compare previous findings with newer data in NICER's soft X-ray window, sampling a different accretion regime. Moreover, the GBM instrument onboard NASA's Fermi Gamma-Ray Space Telescope has found that 2S 1553 is showing unusual behavior: the pulsed flux recently measured by GBM is comparable to the last outburst in 2015, but the spin period change is almost an order of magnitude lower - these properties are expected to be correlated.

NICER observations this week were done in coordination with India's ASTROSAT X-ray observatory. NICER observations will help to constrain the soft X-ray spectrum and explore the energy-resolved pulse profile behavior. The accompanying figure shows a few hundred seconds of NICER data collected on 2S 1553. NICER clearly detected periodic X-ray emission every 9.3 seconds, as the neutron star's rotation brings into (and out of) view the hot accretion stream funneled into the star's magnetic field. The ongoing outburst represents a phase of active mass transfer from the companion star, with this accretion gradually (at a level of milliseconds over a few weeks) spinning up the neutron star.